The two most common myths on the subject of septic system function are that the septic tank treats the sewage, and that the soil filters the remaining particles creating pure water underground.

In fact the septic tank is merely a concrete box that holds roughly two days of sewage. In the calm environment of the tank, dirt and solids settle out and fall to the bottom. Grease and lighter particles from the sewage float to the top. The septic tank works like a gravy boat delivering liquid sewage effluent to the drainfield or leach field and storing solids and indigestible bits for pumping out every few years. Numerous anaerobic bacteria (water breathing bugs like those living in the human gut) continue working to reduce some of the strength of the sewage, but not much treatment happens in the septic tank.

The separated liquid containing only water and the dissolved sewage solids, ("effluent") flows out of the septic tank through a pipe into the drainfield. Here it spreads over the floor of the drainfield trench. Now the real treatment takes place. Millions of aerobic (air breathing) bacteria live in the soil (30 million or so organisms live in a teaspoon of soil). The aerobic bacteria thrive in the area of the trenches and await the sewage effluent, their food source. These necessary creatures will eventually consume all organic material in the sewage, and everything in sewage is organic. Nature is cool.

In a municipal sewage treatment plant, these same basic bacteria are doing the work of municipal sewage treatment.

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Old school drainfields are built using traditional gravel. However today most new septic systems use the plastic vault technology to create the drainfield (as long as the vaults are allowed in the jurisdiction.) Construction process is almost the same for each type with less shoveling with the vaults. Once excavators, contractors and health departments try vaults, they seldom go back to using drainrock due to ease of construction and favorable public acceptance. This picture to the right shows how the vaults look under construction on a standard home-site.  Click on the link to view our page that explains how to build a septic system using the new vault technology, although many of the points apply to the traditional drainrock (described below) systems too.

The traditional gravity drainfield employs long trenches filled with special gravel (inch-and-a-half, round, uniform and washed clean) with perforated pipes running down the center of the trench to spread the effluent into the soil.

The vault technology uses no drainrock and no center pipe. The floor of the trench is used to distribute the effluent to the soil. The vaults, besides providing much greater effluent storage capacity than traditional drainrock, also have the advantage of being easy to transport and place in the trench compared with tons of drainrock. Some county health departments will allow a reduced drainfield size if vaults are used. However, the cost of vaults will be higher than drainrock.

The image to the left shows a standard two trench gravity drainfield using a new stronger PVC vault design with a central post cast in.

This new design was selected because the spot would occasionally be driven over with farm equipment. Careful hand compaction of select backfill is a must. Don't forget to allow surface drainage away from the drainfield. 

Almost all pipes used in septic systems are now plastic, although older systems used clay or concrete pipes. Plastic is the best available material for construction of septic system pipes and components to resist corrosion. However, concrete septic tanks are still much preferred over fiberglass and particularly polyethylene tanks because of structural and cost reasons.

In the standard gravel or drainrock type drainfield, the trenches are dug three feet wide (beds are dug 10 feet wide.) The maximum depth for trenches and beds is usually three feet deep. The distribution pipe in each trench of a gravity drainrock system is 4 inch diameter plastic, PVC (polyvinyl chloride)  pipe with holes along the sides (perforations) and capped ends.

The picture to the right shows a close-up of drainrock known sometimes as drainfield gravel. This special washed and graded stone is bought from a gravel crushing operation, trucked to the site, stockpiled on the site and eventually poured carefully into the trench with the front bucket of the backhoe. This picture shows a pressure lateral with orifice shields at each orifice to prevent the holes from being blocked by the drainrock. Pressure systems are described below in detail. The drainrock in this image is mediocre quality. Uniform size and shape is superior, and very clean (washed.)  

Building the Gravity Drainfield: In gravity systems the trench is 3 feet wide and no deeper than three feet from finish grade. In the picture below the trenches are placed between the tree rows in an orchard. The drainfield media instead of the usual drainrock shown above is a product known as EZlay or EZflow. This product consists of a one foot tube consisting of mesh bags of expanded foam beads surrounding a flexible perforated pipe. Three tubes are laid in the bottom of the trench and covered with the black fabric (filter fabric) to keep the dirt out of the system. The inset photo shows how the sections are connected through the center pipe. Some people feel that the expanded foam beads are not as reliable as good-old drainrock or vaults. I have found that the expanded foam media is as reliable as drainrock.

For drainfields constructed of drainrock start with a 6 inch layer of rock placed in the floor of the trench and rake the center by hand to dead level. The perforated pipe (ASTM 2729 perf) placed over the rock and the joints are glued. The perforated pipe is then covered with an additional 6 inch layer of rock leaving the 4 inch diameter pipe covered with 2 inches of drainrock.

Gravity trenches can also be built by staking a 1 x 6 plank down the center of the trench and wiring the perforated pipe to the top. This way the drainrock can be placed in one step and the the drainfield will be dead level without requiring further adjustment.

In gravity drainfields with gravity distribution, the drainfield is supposed to distribute the effluent evenly from all the holes in the 4 inch perforated pipe (1/2 inch dia holes every foot or so along both sides of the pipe) as the effluent ripples up and down inside. Common sense and numerous demonstrations have confirmed that it is impossible to lay the pipe perfectly level. Health rules usually give the installer one inch of latitude up or down. Further, even a level pipe will spill effluent from only a few holes. A researcher Bomblat reports: "in a laboratory study by Machmeier and Anderson (1987) 84% of the gravity fed wastewater drained from the pipe from the first hole. Yet, septic system designs that fail to distribute wastewater uniformly within and among filter trenches do not take full advantage of the regenerative capacity of the soil."(Field Performance of Conventional and Low Pressure Distribution Septic Systems University of Arkansas 1994.)  These older articles have influenced design of systems profoundly. The center pipe has been eliminated from the gravity vault design and pressure distribution is increasing in popularity.

In finer soils, loamy sands and finer, the floor of the trench will wind up being the distribution system.  This requires that the backhoe operator be highly skilled to provide a flat trench without over excavating. Most jurisdictions used to assume that pressure distribution was of limited additional value in soils that do not end with the word sand. In some more populated counties and municipalities, gravity systems are no longer approved regardless of soil.

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They are sometimes called low pressure systems. They are required in coarse sand soils and gravels. We now know that gravity distribution can allow streams of untreated sewage to sink underground unchecked. Pressure distribution of the effluent works like an underground sprinkler system and is the best type of system in gravelly soils. The drainfield can be "lined" underneath with 1 or 2 feet of concrete sand to slow the drainage even further as shown below.

Poor and shallow soils will require pressure distribution systems in most counties nowadays too. When good soil is scarce, all of the drainfield area must be used. Almost all pretreatment devices, mounds and sand filters use pressure distribution as well.   

In the pressure septic system, the drainfield is soaked and let rest with two to 6 or more doses per day depending on the receiving media. Drainfields are built of black plastic vaults (seen in the view to the right foreground) or drainrock as shown above. The PVC Vault Technology is now allowed in most areas. Infiltrator® and Hancor® are two companies who supply this product. The vaults hook together like a freight train and are very easy to transport and build. Excavators and installers usually prefer the vaults over drainrock once they have tried them. Most health jurisdictions are recognizing the value and efficiency of the vault technology and the use of vaults is becoming widespread. Gravel is still used for geometric reasons and can be cheaper.

Some health jurisdictions give "credit" for the increased storage capacity of the vaults over gravel systems. Each linear foot of standard PVC vault yields around 10 gallons of storage. A linear foot of 3 ft wide gravel trench yields less than half of that even with the highest quality drainrock. The vaults or the drainrock have only one function - to provide storage space for the effluent while it slowly seeps into the dirt below the drainfield.

In pressure systems each row of vaults has a small 1 inch diameter center pipe (three pictures above.) When the system calls for a dose, effluent is pumped into these pipes which are drilled with several dozen small holes (one eighth inch diameter or smaller.) Pressure is provided by the pump sitting in the bottom of the concrete dosing tank or pump chamber. The pump chamber is located between the septic tank and the drainfield. The white transport line in the picture directly above can be seen coming out of the pump chamber and connecting to the end of the drainfield (end manifold configuration.) The 1000 gallon septic tank is behind the pump chamber and has a separate lid and green riser. The sewer line from the house enters the septic tank from the far end and can't be seen in this view.

Pressure systems have many varieties of layout compared to gravity systems. For instance, pumps work best when the drainfield is upslope of the pump chamber. If the drainfield is downslope of the pump chamber, the design must somehow prevent siphoning of the effluent into the drainfield once the pump starts the flow - much smaller pumps are needed too. Our plans contain 95% or more of all of the possible situations you will find on most sites - we have seen it all. When you start laying out your site - study the designs and find the situation that most matches your site conditions. This is how to avoid missing an important part like a check-valve or a disconnect switch. All pressure systems require design calculations be performed.

The quarter horsepower electric pump is about the size of a skill saw motor, and runs for three minutes per dose six times a day.  Everything will be covered with dirt and leveled so that nothing will be seen in the yard except a few lids level with the grass.

The pump chamber holds onto the effluent until pre measured doses are built up. Gravity-fed drainfields without pressure dosing are randomly flooded with wastewater whenever water is used inside the house. In gravity designs, this leads to random saturation of the soil. Bomblat states "Resting between doses prevents saturated conditions in the drainfield."

The picture to the left shows a standard hyper compact GTO pumping system for a lakefront home on a very tight site. The septic tank can be seen behind the pump chamber with the usual 2 ft gap reduced to 6 inches and sand filled. For the picture the pump has been pulled from the pump chamber by breaking the hand tight union at the top of the pump discharge line. The green cast-iron centrifugal pump draws less power than a skill saw and runs around 20 minutes per day total during 6 pumping events each day. The PVC float tree is attached to the discharge line of the pump. The bottom of the three floats is the redundant off to prevent the water cooled pump from running dry. The next float up turns the pump on and off within a range controlled by adjusting the "tether length" of the float. This length is determined by the normal pumping range - usually between 4 to 8 inches. Smaller or larger doses would require a 4 float tree and/ or a timed dosing panel (most pressure systems are demand dosing systems that will accept sewage overloads into the drainfield.) The top float is a high water alarm.

The floats are clustered as close to the bottom of the float tree to make sure the pump is submerged all the time but still allow emergency space at the top of the tank above the level of the high level alarm. This could allow some limited use of the system during repairs or loss of electric power. Septic pumps range between 1/4 and 2 horsepower for 99 percent of pressurized septic systems.

Septic systems use one of only two electric pump types, centrifugal above and turbines to the left. The left image shows a duplex pumping system under construction. One of the two 3+ inch diameter high head turbine pumps is setting on the top of the pump chamber and is about to be inserted into its tube in the blue pump basin next to its twin. The upper part of the stainless steel pump body contains the stack of turbine disks tuned to the requirements of the system. The lower part of the pump is the electric motor with the black plastic 1/8 mesh intake between the two halves. All pumps are water cooled and must remain submerged at all times to the top of the metal. Turbines can be rebuilt with a kit containing rotors, bearings and seals. Centrifugal pumps usually can not.

Turbines are both more reliable and more expensive than the centrifugal type pump in the upper picture. They are available in a range of sizes and have taken over the commercial market. Turbines are designed to cycle hundreds of thousands of times and will generally outlast centrifugal pumps at least 2 to 1. Orenco® and Hydromatic® are two of many companies who supply good quality pumps these days.

The superior performance of pressure distribution systems is described in detail in many studies such as Anderson of the Florida Department of Health (In-Situ Lysimeter Investigation of Pollutant Attenuation in the Vadose Zone of a Fine Sand 1994). In this study, tunnels were dug next to working drainfields. Samples of effluent were recovered from various places throughout the system and the surrounding yard using probes called lysimeters. The effluent was tested for the presence of bacteria, nitrate, BOD (biochemical oxygen demand, a measure of sewage strength) and other tests.

"Results of the lysimeter facility monitoring after nine months of operation indicated substantial attenuation of key pollutants in the unsaturated zone of fine sand soils. Biochemical oxygen demand (BOD) reductions were in excess of 98 percent, total organic carbon (TOC) reductions were more than 90 percent. Total Kjeldahl nitrogen (TKN) reductions were in excess of 97 percent. Nitrate-nitrogen (NO₃-N) generated from nitrification was transported to both the two and 4 foot depths, but at lower concentrations than the total nitrogen applied to the soil, indicating some reduction of total nitrogen concentrations within the soil system. Phosphorus attenuation was variable, but averaged more than a 90 percent reduction during the first nine months of operation. No positive sample results were obtained for fecal coliform or fecal streptococcus bacteria below the infiltration system at any of the variable levels, indicating that significant attenuation of these fecal indicators also occurred in the sandy soil."

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A Sand Lining of One or Two feet like the one going in to the left may be required under trenches and beds in very gravelly and extremely gravelly soils. This sand lining under a conventional bed type drainfield (or leachfield) is known as a "bottomless sand filter." This design allows slower percolation rates in areas with excessively coarse soils (gravelly) with too rapid drainage into lower layers. This imported type of washed and graded sand slows the sewage effluent from percolating too quickly through excessively coarse soils. The depth of excavation for this design (the bottomless sand filter) is around four to five feet including a 2 ft sand lining instead of the usual three feet total trench depth. The sand being placed is ASTM C-33 concrete sand. This is about the only universally available media that works in the place of soil. Regular soil if moved and placed in a trench would fail after a few weeks due to a lack of soil structure. This mystery of soil is why drainfields and replacement areas should be treated carefully and for instance not be driven on with any vehicle. In the image above, excavation is being performed from the end of the system with a large bucket excavator rather than a rubber tired backhoe.

The excavation must be done by scooping material from the sides and end of the pit and not by running equipment in the floor of the drainfield excavation. Placing any tracked or wheeled equipment into the excavation is sometimes common but always inferior excavation practice even though it may save much time on the job.

The Pressure System Squirt Test:  Upon completion and before backfilling, each system is field tested and mapped with an as-built drawing. The squirt test is done by the excavator and attended by the local heath inspector, the designer, or sometimes both. Among other tests such as checking the depth of pipes and parts in the system, the "squirt height" is measured in several places in the drainfield to make sure the designer has done the job.  This is done by filling the system with water and running the pump after exposing several of the small holes in the piping where the effluent is forced into the drainfield. 

The squirt test shown here to the right is probably the easiest way to assess pressure septic design and construction quality. Remove the threaded caps from the swept up ends of the laterals in the 6 inspection ports on both ends of the drainfield.  Replace the caps with test caps that have had 1/8th inch holes drilled in them (or whatever the drainfield orifice size is - the 1/8th inch holes in the lateral being the orifices).  Run the pump and measure the height of the squirt.  Remember to measure the squirt height from the lateral pipe (not the top of the threaded cap), to the top of each of the fountains. When the drainfield is backfilled, the inspection ports will be cut flush with the lawn and be capped for future inspection of squirt height and effluent depth inside the drainfield. Read this article about ports (long download time.)

The two systems shown above and right are bed designs. In a bed system a rectangular area no wider than 10 feet is used for the drainfield because it is more compact. The trench system below shows typical 3 foot wide trenches that can be built with gravel (drainrock) or vaults as a drainfield storage media. Because trenches can be no wider than 3 feet and because trenches must be separated one to another by at least 4 or 5 feet (in most areas) and more normally 7 or 10 feet, trench systems can take up a lot more space than beds.

The system below shows a typical shallow trench system for a large rural house in quite shallow soil. Why not always use beds instead of trenches to save space? Beds are only generally allowed in soil with good drainage. By referring to the soil chart on the left column, any soil that ends with the word sand is suitable for bed designs. All other soils including loams, silt loams, clay loams and some clays will require trench type drainfields like the one shown here.

Uneven distribution or too low or too high squirt means the designer miscalculated the values, the contractor did the plumbing wrong or the supplier sent the wrong pump. A well designed and properly built pressure system will evenly distribute and treat the sewage from the building project for many years with minor maintenance and safe operation.  Remember, the squirt height on a well designed system may vary from high squirt to low squirt by as much as 20% (as much as a foot difference from high to low.)  This does not mean a 20% variation in flow.  A squirt height difference of 20% represents a flow difference of closer to 10%. However, the ideal condition is to have equal squirt from all points. Equal squirt height is best achieved with a center rather than end manifold and connect the transport line as close to the center of the drainfield if possible, regardless of what the calculations show.

Local health may require exposure of the entire piping network for the test with another inspection of the vaults or drainrock at a later time with everything ready to bury.  Each county health department may inspect finished septic systems in slightly different ways.  Designers get to know the different counties and how to accommodate their concerns, particularly at the time of final inspection when everyone wants things to go smoothly.

This squirt height test (with the pump run time) is how the system can be checked over the years for best operation. When the system is new, record the squirt height for each corner of the drainfield.  In the future, a higher squirt, an extra two or three feet means the system is getting plugged up and needs cleaning.  A low squirt means a worn pump, or a leak in the system.

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The site evaluation has replaced the traditional "perc test" in most states as a way to demonstrate the treatment qualities of your property to the local health department. The traditional perc test is done by pouring a bucket of water into a prepared hole and timing the sinking water level with a stop-watch as it soaks into the soil. The site evaluation is done down in a 6 foot deep pit. The perc test only tells you about a small spot in the yard. The site evaluation is a much wider look across the soil face (see picture directly below and the video clip to the left.)

The size of the drainfield is determined by two factors: the soil type, and the expected daily amount of sewage to be drained from the tank. Check this link to see a chart. This daily amount is usually determined by local health based on the number of bedrooms in a house or the seats in a restaurant etc. These sizes and volumes are usually arrived at using state and federal guidelines. However, the most common accepted volume of sewage from a three bedroom house in the US is 360 gallons per day (480 gallons for a four bedroom).

For a site evaluation, the homeowner hires an excavator with a backhoe to dig from two to six pits (usually two) five to ten feet deep (usually six), or whatever is customary in your county.  The soil scientist or designer in most cases will be on the site during the test hole digging to tell the excavator where the holes should be dug. Some subdivisions have uniform soils that are well known by local health and digging test holes on every lot in a subdivision can be skipped. Such breaks are lately getting rare in most areas.

The Test Pit to the Right shows a classic soil known as Extremely Gravelly Soil (greater than 60% rock fragments.)  Any soil that is extremely gravelly or Very Gravelly (35% to 60% rock fragments by volume) may require a special drainfield design. Note the sandy loam soil in the top 18 to 24 inches. This drainfield if built in this spot will drain into the lower coarser soil and will not be able to take advantage of the superior treatment characteristics of the upper soil layer. This upper layer is mostly unusable  except if the drainfield was designed as a mound system although such an expensive design as a mound can be avoided here by using a cheaper solution - the bottomless sand filter.

Notice that the coarse sand is darker in colour, has cobbles and a large range of grain sizes and does not form straight walls in the trench. This sloughing behavior is because this soil has little clay or silt content to bind it together unlike the upper loamy soil. Also notice that the upper soil has many more plant roots than the lower coarse sand showing how the coarse sand is so well drained that almost no moisture is present to allow the formation of any plant roots.

Sewage treatment in the soil takes time. The aerobic (air breathing) bacteria need a day or so to work to destroy the anaerobic bacteria (water breathing germs in sewage.) If these germs are allowed to penetrate too quickly because of excessively gravelly soil, the water aquifer can become contaminated with persistent germs such as viruses. The usual answer is to "sand line" the drainfield creating a septic design known as a bottomless sand filter. This design is simply a pressurized drainfield built over a 2 foot deep bed of ASTM C-33 concrete sand which is available at all gravel crushing operations. Imported soil and unwashed natural soils will not work for this.

Getting the Soil Right:

The designer must chose the system that best matches the soil and other site conditions. The soil type may not be obvious. The soil scientist, engineer or designer who is in charge of the design is trained and experienced to identify the texture or coarseness of the soil by feel. Check this video which shows how this is done in the field. Sometimes water is added to the soil to help determine the texture. This process determines such things as the proportion of sand, silt and clay content which all affect the ability of the soil to absorb moisture from the septic system.

The texture and structure of the soil is critical in determining the amount of drainfield required. In the case of disputes, or when the soil is unusual with complex results, the texture can be determined by drying the sample and running it through a set of sieves like the one shown to the right. The soil trapped on each sieve is weighed and the proportion by weight of different grades of sand plus the amount of very fine soil (silt and clay) is determined accurately.

The exact location of the future drainfield and therefore the test pits is found mostly by experience.  Of course, the location of the drainfield, down-slope from the proposed house site is the obvious spot.   If the soil there is not suitable, the hunt begins for a better spot.  Besides soils, the designer's knowledge must include the true slope of the ground, plant growth and type, local geology, groundwater plus pumping strategies, technical details and associated costs to find the location of the best spot for the septic drainfield.

The designer should be aware of the other excavation required for the house besides the septic system.  This other excavation includes trenching for footings, the power/  water lines and the location of driveways and accessory buildings.   Sewer lines and effluent lines for instance can sometimes share a trench with power lines, reducing trenching costs depending on layout and hard digging such as a rocky site.

How long should it take? A site evaluation/meeting with the homeowner, the local health inspector and the excavator will usually solve all of the various issues in an hour or less.  The septic designer should coordinate this meeting and keep control of events if poor soil is discovered.  The designer should have a plan B & C at all times to avoid an expensive complicated system being required by local health.   Prior discussion between the designer and the homeowner will determine if, for instance, another house location would work.

A first look, with the future home or business owner at a known difficult site should probably exclude the local health inspector.  Long discussions with an owner over alternative plans, costs and systems can bog down the approval process.  If an unfavorable pit is dug, it should be filled in and a more suitable one prepared in a different area. Once the site characteristics are known and a strategy is devised to meet regulations, a meeting with the health inspector should go smoothly.  Expert knowledge of soils, health regulations and construction methods are required by the site assessor to ensure an eco-nomical result on a difficult site.  A couple of hours with a back-hoe and a soil expert may be the difference between a simple gravity system and an expensive mound on your new house lot.

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A Word About Waste Strength Note: For more detail see the article below What is in Wastewater Anyway? 

There are 4 basic tests for sewage to determine its strength both raw and after treatment; 

1. BOD (or BOD5 biochemical oxygen demand) The organic material in sewage will deplete the oxygen in water as it decays inhibiting treatment and general water quality. The average 5 day BOD strength of domestic crude sewage is about 100 mg/l to 300 mg/l, although this can easily vary up to 500 mg/l (strong sewage.)

2. TSS (total suspended solids) The amount of total suspended solids will indicate the insoluble content of the sewage. TSS causes cloudiness in water or turbidity. SS is caused by the presence of suspended and dissolved matter, such as clay, silt, finely divided organic matter, and other microscopic organisms, organic acids, and dyes. The average for domestic crude sewage is about 200 mg/l of TSS with spikes to 400 mg/l.

3. FOG (fats, oils and greases) is mostly a product of cooking and cleaning in the kitchen. FOG can have negative impacts on wastewater collection and treatment systems. Most wastewater collection system blockages can be traced to FOG. Blockages in the wastewater collection system are serious, causing sewage spills, manhole overflows, or sewage backups in homes and businesses.

High FOG is almost always associated with restaurants and commercial food service. It can cause clogging of the drainfield. The impact of grease in sewage cannot be underestimated. BOD5 in excess of 300 mg/L and FOG in excess of 30 mg/L would shorten the life of drainfields, mounds, sandfilters, and some aerobic treatment devices without pretreatment. Normal residential strength waste is generally believed to have a FOG level in the low 20’s mg/l or lower.

Standard Raw Sewage Strength and Total Nitrogen: Most researchers apply a range to quantify concentrations of BOD5, TSS, and FOG for raw sewage influent. The most commonly used range is 100-300 mg/l BOD5 and 100-350 mg/l TSS (NSF Standard No. 40). FOG ranges typically between 20 and 150 mg/l with over 150 mg/l being identified as “strong”.

Sewage effluent is almost 100% organic. It degrades quickly inside the active drainfield to become water vapor, heat and carbon dioxide.  Small amounts of chemical residue are deposited in the soil around the drainfield in the forms of phosphorus and nitrate. The phosphorus as phosphate from soap is in very limited use anymore. The breakdown of sewage results in small amounts of phosphate, but this substance binds to the soil surrounding the drainfield more or less permanently. In municipal treatment, phosphate is generally discharged to surface or ground water. On-site septic systems do not discharge phosphate. The only other inorganic waste from the breakdown of sewage is nitrogen.  

4. N or NH3 (Ammoniacal Nitrogen) The nitrogen in sewage is assessed as ammoniacal nitrogen. This indicates the amount of nitrogenous organic matter which has been converted to ammonia and potentially converts to nitrogen as nitrate. According to the EPA experts in a 3 county study known as the Wekivia Study Florida, Feb 2006 by D. L. Anderson et al, the researchers determined that the average amount of nitrogen in untreated domestic sewage contributed by each person in a home was 11.2 grams per person per day or around 22 pounds per year per each household of 2.5 people. The percentage of this nitrogen from the sewage that is converted to nitrogen gas and expelled to the atmosphere from the septic system drainfield has never been officially quantified. Several qualified researchers have put this amount of nitrogen eliminated from concern in the high 90% range. If most of the nitrogen produced by septic systems is converted naturally in the drainfield to nitrogen gas, then requiring expensive technologies to remove it before it even gets there would be a waste of money.

Some health departments disagree that nitrogen in septic systems is of limited concern. Nitrogen is a pollutant by itself and is associated with Methemoglobinemia, a rare disease of infants. Nitrogen (Nitrate) is like salt and once dissolved in water can not be easily filtered out. Although farmland and home landscaping can contribute hundreds of pounds of nitrogen per acre per year to the land, septic tanks are often pointed to as a serious source of nitrate in groundwater. In reality each home produces very little nitrogen compared with popular application rates of nitrogen fertilizer on lawns and farms even if all of the nitrogen from the sewage winds up in the groundwater. Any urgent concern over widespread nitrogen "pollution" from septic tanks has no science behind it.

SO. . . . . If septic systems are safe and effective and even cheaper by far than municipal sewage treatment, why are their dangers brought up all the time in public meetings? Both individuals and public officials bring up the dangers of septic tanks polluting our water wells more than all other environmental dangers combined. This is in spite of almost no recent evidence anywhere in the USA of this type of pollution having happened?

Treatment of Industrial or Non-Residential Wastewater:

For non-residential sewage, the first step is to determine the sewage strength of the proposed wastewater and identify specific chemicals and substances that could be found in the sewage not customary in residential sewage. Wastewater from a truck wash for instance would likely contain fuel, oil, brake, tire and clutch residue from the vehicles and from the roads. Some of these industrial residues would be broken down in a septic system and some perhaps not. Traces of these materials will certainly wind up in the sludge in the bottom of the tank and possibly in the soil surrounding the drainfield. Municipal sludge (excuse me "bio-solids") containing the same traces and even nastier stuff like fly ash are usually permitted for spreading on farm fields and marketed as a soil supplement.

A septic system as a truck wash for a cattle operation is no different from any commercial truck wash connected to a municipal treatment plant. Except municipal sewage treatment is usually at the end of a long, winding, old and possibly leaking sewer ( the collection system.) The traces of toxic residue in all sludge and in the discharge pipe of the treatment plant where ever it is will have no reduced effect on the environment simply because the system is owned by a municipality. In fact many existing municipal treatment plants are obsolete, in need of upgrades or regularly exceed their capacity. Modern septic systems provide as good or superior treatment of organic and many chemical substances than large sewage treatment plants. Everything goes somewhere after all.

The Difference between Agricultural and Industrial Wastewater : In reality there may not be much difference. Regulations about industrial wastewater are strict and thorough and they usually blanket any non-residential/ domestic operation. However when one examines the details of the rules, agricultural type operations are usually exempted or simply not mentioned. Small wineries of say 5000 or fewer cases per year are not worth the state requiring municipal designs with full time certified plant operators.

The small non-domestic septic systems we are talking about here are not intended for true industrial operations. Electroplating, painting and chemical processing have stricter waste rules as do very large scale food processors. Also remember that handling of hazardous materials usually means that the operation may not be suitable for either onsite or municipal treatment. Some processes may require holding tanks or transportation off-site for treatment or recycling.   

In any case, whether you are treating smaller volumes of agricultural or food processing waste, car wash water, kennel waste or other common wastewater, septic systems are generally safe and cost effective whether sewers are available or not. Your local health department must be consulted anyway so when you discuss it with them emphasize the small scale and agricultural character of your proposal if you can. 

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Any system other than a simple gravity design is known as an alternative septic system design, or an alternative system (sometimes called alternate).  These special systems usually have electric pumps and are used when shallow or poor soil is found on the property.  Requirements for alternative systems vary widely from place to place.  Some counties have no pressure systems while some have nothing but. This may show simply how up-to-date the county is, or how widely spaced neighbors are (generally an indication of potential public health concerns).  In some places, state septic regulations will uniformly require alternative designs in certain soil conditions regardless of the local lot sizes, or low overall population density.  An expensive pressure system may be needed on a large lot because uniform state regulations require it based on soil type alone.  Neighbors with older homes may have standard gravity designs in the same soil conditions.

See the paragraphs below on Failure of the Septic System to understand why alternatives may be required in certain areas.

1 - Mound Systems: The mound system pictured to the right is on a flat site with 15 inches of shallow fine soil over solid rock.  The "mound" is the drainfield,  a shallow pyramid of C-33 Concrete Sand with the drainfield gravel (or in newer designs, vaults) and distribution piping set in the top.  This raises the drainfield above the ground level to provide the vertical separation that is otherwise not available due to a high water table, a restrictive soil layer or shallow rock.  The original ground must be plowed and the sod turned up-slope with a moldboard type of plow blade before the sand is placed to provide flow into the ground.  In the picture left the moldboard is no7. The teeth of the backhoe should never be used if a plough is not available. Excavators who build a lot of mounds will sometimes weld a mount to the backhoe bucket to hold a plough. Instead a tractor or backhoe can be used to pull a farm plough through the soil in a few passes. Any more driving than this over the area of the mound is unacceptable.        

The reason mounds cost so much is that the sand placement uses a lot of hand shoveling. Driving any equipment over the mound area or replacement area will disqualify the system on final inspection. Construction equipment or trucks are kept off the mound area, so construction must happen from the side and ends. Next the pressure drainfield vaults or gravel is built on the flat top of the sculpted pile of sand (the mound), and a special fabric (filter fabric) to keep dirt out of the drainfield gravel is placed over the top.  Then a cap of finer soil, 9 inches or so is placed over the whole thing to finish and to shed rainwater.  The side slope is no steeper than 3 to 1, gentle enough to mow the lawn that will be planted over top.  Mounds, due to the sloping sides can be over 50 or more feet wide and over 90 feet long at the base, depending on lot slope, bedrooms in the house, and soil type on the lot.  This example is 27 feet wide, 90 feet long at the base, and 3 feet 8 inches high.  It was built in 1991 in the back yard of the 4 bedroom house shown, and has been working fine ever since with a minimum of maintenance.

2 - Filtering Systems: Sand filters use the same principle as the mound, and the same type of sand, but the sides are straight like a box about 4 feet deep.  The sand filter takes a lot less space than a mound.  The sand filter can be built above the ground in a concrete frame or set in the ground flush with the surface within a treated plywood frame.  The filter may drain into the ground like a mound. This is called a bottomless sand filter. The image to the right is this type. 

On poorer soils or protected areas, intermittent sand filters are designed with a thick, 30 mil PVC or vinyl liner under the whole thing. This design pre-treats the sewage effluent before it hits the drainfield. Clean coarse gravel with a long under drain along the bottom of the filter drains the treated sewage effluent out of the filter downhill to a gravity drainfield. The drain is directed out of the filter through the vinyl liner. A special "boot" is factory built at one end at the bottom of the liner. Otherwise the filter would have a pump inside a vault inside the sand in the middle of the filter. This pump can send the treated effluent over the top of the liner or through a boot into a small disposal drainfield nearby.  Some newer sand filters use a coil of tubing in the sand at the bottom of the filter to allow the connection of the filter to a small electric air pump. The air pump blows oxygen into the system to help the aerobic action of the filter if there is a problem with the filter. Of course the sand filter is really a treatment system and not a filter at all. It merely acts like a filter.

Recirculating gravel filters use a coarser sand media, and recover the effluent from the bottom of the filter and pass it five or more times through the gravel by splitting the output flow and redirecting most of it back through the system.   This type of design is extremely reliable and is often chosen for homes or small communities. Gravel filters use an extra recirculation tank and more complicated panels and controls.  Communities that choose recirculating gravel filters and effluent pumping systems can in most cases provide community plants to treat their sewage for 25% or less than the cost of lagoons, oxidation ditches or other standard methods.

Following a sand filter or the gravel filter, the effluent looks like clear water and has no sewage odor.   The disposal drainfield can be very much smaller than a conventional drainfield in the same soil because there are no solid particles left in the effluent to form a "clogging mat."  There are bacteria in the effluent that tends to enhance the ability of the soil to absorb water.  In highly sensitive areas, and because of this bacteria, local health may require a mound following the sand filter to get rid of the treated effluent.  In this case your septic system can begin to cost big money. In a difficult soil area, to reduce this cost or to improve reliability, a community system may be indicated. Often, a public entity such as a PUD, or a municipality may be designated to oversee the management of the system. Unlike municipal systems onsite septic systems do not require a licensed operator. A maintenance guy with a pickup with an understanding of the system can easily manage even larger systems as part of other duties.

3- STEP Systems: Septic-Tank-Effluent-Pumping Systems are community systems. The sewage collection system is the STEP part.  STEP Systems require septic tanks at each house.  Instead of a drainfield to treat the effluent near the house, community drainfields are used with gravel filters or other similar small treatment plants at the end of the pipeline. Small pumps in each septic tank are all the force needed to move the effluent to the treatment plants located underground in community parks. The collection lines, generally 2 inch diameter Schedule 40 PVC pipe, avoid the need for vast large deep gravity sewers. The effluent collection lines can be placed above the water system because the sewer system is less prone to freezing. The collection lines can follow natural ground slope. Trenching can often be done with ditch-witches. This way, an existing town water system can be left undisturbed when the sewer system is built over-top. This is impossible with conventional gravity sewers due to the size and depth of required excavation. STEP systems are not associated with any higher risk of spills or failures than conventional sewer systems.  STEP systems are operating in most states from California to Alaska and Florida to Maine.

Community septic systems served by recirculating gravel filter technology are also cheaper for small towns to operate. This is because they do not require a licensed operator on site.  A reasonably handy maintenance person in a pick-up can generally handle all maintenance tasks and repairs for a community of several hundred homes.  There is some resistance to the use of this alternative type of system for small communities, but I can find no reasonable explanation for it.  Perhaps the low initial price alone raises suspicions from the orthodox community and conservative state regulators. The engineering fees are proportionally smaller and many engineers who specialize in municipal treatment plants scoff at alternatives like STEP and other onsite technologies.   

4 - Black Boxes: In areas with difficult soils, developers and home owners faced with the cost of large, complex septic systems are always hoping for a cheap, small mechanical filter system that will turn sewage into clear water.  The industry calls this the black box septic system.  No such system exists.  People are continually asking me to check out packaged systems, usually a green box, that is claimed to provide excellent treatment.   Explanations in a glossy brochure include lots of glowing testimonials.  These systems, usually sloughing filter designs, are probably from Europe or Scandinavia and may have no proper support in this country.  They almost always require a regular maintenance contract from the company that imports them.  Packaged systems must be approved by each state and local jurisdiction.  This is a tremendous hurdle that few packaged systems are able to get over and maintain service in multiple jurisdictions.  Most of the packaged systems that were advertised six years ago are nowhere to be found today.

Unlike the packaged systems, the standard designs described in this article use off-the-shelf parts.  Any good local excavator can make repairs, and the track record of each type of system in an area can be found in the public record of the offices of state and local health.

There are many innovative systems that have been built over the years and work well under certain conditions.  The Wisconsin at-grade system lays the lateral distribution lines more or less on the plowed ground and covers them with with gravel, filter fabric and dirt.  If the vertical separation is the same as the available soil depth, a standard pressure system would not work because the pressure drainfield must be set into the ground at least a foot.  A mound system would provide more separation than needed, so at-grade would save the cost of the mound sand.  In the evapotranspiration system (sometimes evapotransporation), no contact with the native soil is allowed.  This type of system is used as a last resort in impossible soils or very high water tables.  A large pit with a vinyl liner is required.  The pit is about 3 feet deep, filled with dirt, gravel at the bottom and plants to help evaporate all of the sewage. The evapotranspiration or ET system is only allowed in areas where the annual evaporation rate exceeds the annual rainfall, even in wet years. This restricts the ET system to very dry (treeless) areas only.

The textile filter or packed bed system is a super compact design that uses a fluffy material instead of soil or sand as a filter medium and has an area of a tenth or so of that required by a sand filter.  A recirculation tank is required here. The filter material is patented and must be bought from a Canadian source. Peat is also used as a filter medium. This recirculating filter with special medium is a promising design. Some new models have proven robust and efficient. They knock waste strength and even reduce nitrogen strength by as much as 60% or more. However these filtering systems can double the cost of the entire system regardless of the size.  

5 - Experimental Septic Systems: If you want to apply for an experimental system of any type, even if the design has a track record in some other state or country, you will have to get your designer or engineer to present the plans to a state review committee.  The resulting experimental system may require expensive monitoring and sampling of effluent from several places in the system at several times throughout a period of several years, usually at least three.  It can take several months to get approval to construct and there must be something in it for the state, usually advanced knowledge about the system in your setting.

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Because a septic system is a construction project, you might think that the building department should oversee the design and construction like they do with houses. However septic systems can represent a hazard to public health. Health officials must ensure that a homeowner or developer will not allow sewage to escape from the treatment process to contaminate any water well, surface water, public or private area. Health departments administer the rules that can require alternative systems in sensitive areas, or when soil conditions are less than ideal (difficult sites.)

In poor soil conditions, the more expensive and complicated systems may be required. Local health rules and officials can sometimes seize on these difficult sites and set costly restrictions on homeowners or developers with the full backing of an uninvolved board of health and flimsy science, if any. Illogical rules and inconsistent enforcement can happen to any project. Attempting to prove it is usually not worth it because it would halt progress on the building project.

Research has proven for instance that more expensive pressure distribution provides more reliable treatment with fewer disease causing micro-organisms able to escape the treatment process in excessively coarse (gravelly) soils. Once imposed in an area, proving that these more expensive systems are helping with groundwater problems through widespread sanitary surveys is rarely required or attempted. The law, however usually leans toward public health agencies once the rules are in place.

This is where a homeowner or excavator can get into trouble with the local health inspector.  If the site evaluation of a proposed development is not performed with skill and experience, local health may consider the site more difficult than it is and require more system than is really needed. Neighbors who think they have endured any extra costs for their system will offer little support for any relaxations of the rules on yours. It is seldom simple to compare the potential for septic system problems on two properties. Side-by-side homes can require vastly different systems. Your neighbors may offer advice that may not apply to your property.

Remember, the site evaluation and soil assessment points you down a road to a certain type of design. Simply missing an area of good soil on an otherwise poor lot can require a costly alternative septic system with little benefit to the owner or to public health.

Read the section on Site Evaluation to find out how to maximize your property for the septic system.

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The only way to get a true estimate is to circulate an approved design to several licensed system installers.  The following costs should be considered "ballpark" only.

The cost of a standard gravity system for a three bedroom house on a level site in sandy soil can vary widely from place to place but it should be roughly between $2,500 to $4,500 complete, to a county licensed excavator plus state or local taxes. Permit fees and design fees are extra and covered later. If plastic vaults are used, the cost will be on the higher side of this range. Vault systems will usually be allowed smaller than gravel designs depending on soil type, but the cost of the vaults is still more than the relatively inexpensive drainrock. It is still possible in some areas to get prices lower than this range.

Availability and prices for tanks, drainrock, vaults and pipe doesn't vary a whole lot from place to place, but poor soils and extra bedrooms will simply cost you more. Fine silty soils require more drainfield and are more risky to build due to slightly higher failure rates. The drainfield vaults these days are around $25 to $35 each and good quality clean drainrock is around $8 - $12 per ton if the pit is nearby. Concrete tanks are the only practical choice, and these run around $500-&800 for a 1000 gallon tank delivered within 50 miles of the yard. A 1250 gallon tank  for an extra hundred or two is preferred by some homeowners, and minimum tank size is determined by state and local rules. I recommend a 1000 gal tank up to 4 bedrooms if allowed because pumping trucks are usually 1000 gallons and can pump your system in one trip.  The larger tank for an average house offers little advantage except for homeowners who put off maintenance. Some experts disagree about this. A new rule-of-thumb for tank size is 2 x daily flow instead of the traditional 1.5 x daily flow.

The number of excavators licensed in the area and how busy they are will have a huge affect on septic system costs.  The strictness of rules of local health in the design of systems and the toughness of the installer's test can encourage or discourage the number of contractors competing for the construction of systems.  Excavators are already mobile and they can enter a market area looking for better prospects.  Hungry new excavators in an area can drive down prices and put pressure on established contractors to lower rates.

Pressure systems will cost a lot more in places where they are new or scarce with only a few excavators choosing to put them in.  As pressure systems become more popular in an area, the prices slowly come down.  $4,000 to $6,500 is an average range for a small, simple 3 bedroom pressure system or bottomless sand filter (no vinyl liner, concrete or plywood walls needed).  Some electrical inspectors allow the excavator to wire the pressure system into the house wiring.  More commonly, an electrician will be required as an extra sub on pressure systems. (septic system costs continued below. . . . )

Insist on getting a set of the plans from the designer to check yourself before work is started, that all the above items are covered. Good designers will be glad to give you model numbers if these things are missing from the plans. Go over these details as the system is built. Be nosy. Everything will be buried eventually, and excavators are aware that some homeowners just don't want to know the details about the septic system.

Mound systems can be as much as $9,000 - $20,000 depending on the site, with sand filters sometimes called intermittent sand filters to distinguish them from recirculating sand filters costing about the same due to the need for a disposal drainfield and the 30 mil custom welded vinyl liner but avoiding the cost of placing and shaping the mound sand. The wide range of costs for the mound is also due to the sensitivity of this design to lot slope. Steeper sites compound the construction problems, and require much larger mounds. Again, if everyone in the area has a mound or a sand filter, market competition will lower the cost for these usually special systems.

Recirculating sand or gravel filters have about the same components as intermittent sand filters plus a recirculation tank and extra parts such as a recirculation valve and a flow splitter. Costs should be in the $9,000 to $15,000 zone and higher if these designs are relatively rare in your area. If the site is difficult because of high ground water, local health may insist on a mound to dispose of the effluent once the sand filter has pre-treated it. The effluent from the sand filter still contains harmful bacteria although it will look drinkable.

Bottomless Sand Filters as noted above can cost a lot less than the other "filters" between $4,500 - $7,500 because the wall of the excavation is the only formwork or containment required to form the drainfield. Bottomless sand filters require an excavation depth of 4 to 5 feet minimum on a level site, and most of these systems are built in areas that would otherwise qualify as a gravel pit (where drainage is too fast to allow the time for treatment of the sewage within the oxygenated zone of the soil.) Either one or two feet of ASTM C-33 concrete sand forming what can be called in some places a liner or a sand lining (not to be confused with a vinyl liner pictured in this link as a tree root barrier.) The sand liner is occasionally used on the bottom of the drainfield trenches (max 3 ft wide) or more often under drainfield beds (max 10 ft wide.) Above  the sand lining a conventional pressurized drainfield of vaults or drainrock is built. A foot of backfill covers the whole thing to finish.

Compared with a conventional pressure system, the only features of the bottomless sand filter are the extra excavation and the sand liner - a very inexpensive (relatively) cost for a huge public health benefit by providing protection for shallow aquifers in gravelly soils. In the past, no certain protection was possible. If the soil has large boulders in with the gravel, or if the soil contains too little loam to hold together, the excavations can become huge and strenuous, particularly for the tank, and costs will rise accordingly. Difficult digging, frozen ground, large boulders and deep hard pan can add more than $1,000 - $2,000 to the job.     

A Wisconsin At-Grade-System should run about $5,000 to $7,500 and is only required in rare cases where the soil depth over rock or groundwater is the same as the required vertical separation. This would require a mound that sits right on the surface of the ground which technically is an at-grade system except that certain subtle construction differences apply.

Evapotranspiration Systems should start about $18,000 except that many counties will not be familiar with this extreme example of limited popularity. This system is only suitable when annual pan evaporation grossly exceeds rainfall (treeless or desert locations.) The ET system is only suitable on lots large enough to contain the system but without sufficient soil depth to provide for any of the standard alternative systems. The cautions involving pressure systems above (good, bad or ugly,) apply to all pressurized systems including mounds and sand filters as well.

Experimental systems aside from having no guarantee that the system will work at all, may have a stunning price tag.  Much expense is required before it is known if the regulatory authority will even accept the design for consideration. Be prepared to argue your case knowing that the regulators will only allow rules to be radically twisted when some public benefit is possible too. Design costs for these systems can be high because many designers and excavators will not qualify or be interested in submitting a bid.  Before construction, the designer or engineer will attend many meetings with boards and regulators.  Complete drawings and carefully documented research will be required at these meetings. Certain properties may only be buildable with strenuous engineering needed such as the experimental system.

Owners of difficult lots usually consider experimental systems when all other options have been exhausted. Local health may have already condemned the property as unbuildable.

Each one of the above systems will be more if the site is cramped with trees, existing buildings or structures.  Large houses, duplexes or homes with extra bedrooms will add to the cost too.

As a general rule, difficult sites (high water tables, lousy soil, tiny lots and remote or environmentally sensitive locations) can double costs or more.  Some very attractive sites are difficult sites.  If a beautiful area has few homes, look at the property carefully for septic problems, or poorly accessible water or power.

Septic System Design fees and permit fees represent the only other costs associated with this work.  Septic permits run from a nominal $100 or less in remote places to a whopping $3,000 for a permit in built-up areas where the county doesn't really want any more growth.  A quick call to the local health department will clear this up.  An average permit in a state with good public health service will be around $500 for a standard 3 bedroom house system permit. Alternative designs may require additional inspections during construction, and a higher permit fee than the standard system. The site evaluation may have a separate fee to local health of $200 - $400 or more. Your designer or soil scientist will likely charge a separate fee for providing this service for you (usually between $200 - $400) or more depending on the market as well and a backhoe is required to dig the inspection pits. This takes usually an hour or so including unloading and loading for a simple site with two to four holes and should cost $150 - $400 for the backhoe operator and his equipment. Installers will generally not bill the homeowner for digging the test holes if it leads to the job of building the system. Note: In some areas backhoe services and design and permit fees are 5 or 10 times these guidelines and I can see no reason for this other than extremely restrictive rules on who is allowed to perform these tasks and unsympathetic elected officials. It will not hurt to ask for justifications for high fees from your local elected politicians.     

However, in some places, local health inspectors even design the system as part of a low permit fee of $100 - $300. It used to be this way across much of the country in the past. Very low fees serve the interests of public health in very low density areas and where resources are strained and experts are few. Some of these counties and states allow excavators to design simple systems for the homeowner. A few far flung counties have no regulations and no requirements for formal design and everyone just does their own thing.

With increasing national concern for public health and loads of new technology available, this is changing. Counties are being encouraged in most states to create standard rules and regulations for the design of septic systems.  This often allows for the licensing of designers who specialize in septic design and hold state or county licenses.  In some places on the other hand only professional engineers are allowed to submit designs.

Professionally licensed designers charge from $350 to $800 for a simple pressure design and perhaps less for a gravity system including a documented site evaluation.  In counties that require only engineers to do the designs, costs can run as high as several thousand dollars.  If system designs are routine and the site is not classified as difficult, the design cost should be about $450 to $700 for a 3 bedroom house with costs increasing up to several hundred dollars more for a site with difficult soil.

Complex and alternative designs involve more work, and some designers prefer to work by the day or hour ($60-$90 per hour is a fair range.) Designers of anything who have a consistent track record, are entitled to at least ten percent of construction costs if they consistently solve the problems of their clients, and their clients are fair.

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The vault type drainfield shown here to the left is totally failed. The water has risen to the top of the inspection ports and is forming puddles on the surface of the ground. If operating normally, the soil surrounding and below the healthy drainfield remains moist and not wet. Aerobic bacteria dominate healthy drainfields. If the soil becomes saturated with sewage effluent or water, anaerobic (water breathing) bacteria from the effluent will take over the drainfield. Then the treatment stops. The soil surrounding the drainfield becomes abnormally clogged with organic particles.  The normal dark bacterial film (biomat) found below and around normally operating drainfields, becomes thick and totally waterproof. The soil stops absorbing effluent and ponding above ground usually occurs. Failure is also often accompanied by a "sewage odor."

So what is the Fix for a failed system? A completely failed drainfield generally must be replaced in a fresh location.  If the failed drainfield is abandoned for a year, it probably will recover on its own (usually an impractical plan in a busy house.) Everything causing the failure is organic and will degrade when aerobic conditions return.  In replacing a failed gravity drainfield, it is wise to put in a valve (bull run valve) to allow switching back to the old drainfield later as a backup once it recovers. In the rare case where a pressurized drainfield requires replacement, preservation of the old failed drainfield is a good idea.

To fix a pressurized drainfield build the replacement drainfield in a fresh location and tee the transport line to split the flow to feed both drainfields. With a high pressure gate valve placed on both arms of the split, flow can now be directed to either drainfield old or new. Let the old drainfield rest for a year then bring it back on line and rest the new one. Each year you can switch drainfields on say July 4th, and each drainfield will get a years rest before being back in service. Just make sure nobody closes both valves at the same time which will be hard on the pipes. 

You should know that a septic system can be deliberately made to fail. Simply turn on any tap in the house and leave it running for a week or so. The excess volume of effluent will flood and fail the drainfield by killing the aerobic bacteria in the drainfield.  This is why it is always important to repair hissing (leaking) toilets and even dripping taps in a home on a septic system to keep excess water out of the system.

Poor distribution of the effluent in the drainfield causes local patches of failure, reducing the effective area of the drainfield.  This is why pressure systems are more reliable than gravity systems.  Pressure distribution is almost always required in poor or extremely gravelly soil conditions where water distribution over the whole drainfield area is helpful.

Looking at the failure will not tell you if it was caused by poor soil or over-load or some other reason. Lush plant growth around the failed drainfield like the picture to the right shows that the failure has persisted for several weeks or months.

The soil you have to work with on your site is determined by the Site Evaluation by a soil expert.

Generally the best soil in which to build your septic system is undisturbed medium sand.  Very fine sands, and darker organic (loamy) soils are second best.  Silt, a very fine soil with a smooth texture like talcum powder is even less absorptive.  There is simply less space between the particles to hold and pass water.

Clay, and extremely fine clay-like soils are even poorer for septic systems. Clay particles are very small and pack together in tight formation. The poorer the soil, generally, the larger the drainfield due to the slow rate of absorption.  Finer soils formed from ancient stream and lake beds can turn out to be compacted, and layered. A few fine bands of silt and clay can spoil otherwise good sandy soils.

Also, coarse gravelly soils can be too porous allowing the flow of effluent from the drainfield to penetrate deep into the soil too quickly. This places the effluent below the plant root portion of the soil. This upper soil zone is where treatment must happen because of the presence of oxygen and aerobic bacteria.

Even when soil is good, it may not be deep enough to allow a system to be built (usually a minimum of four to five feet deep).  Septic systems in shallow soil above a layer of solid rock, or above a shallow water table may not be approved by the local health department. Sometimes an alternative system such as a mound may be allowed if soil is too shallow.

If the soil is poor, or too shallow, why not bring in some good soil to replace it?   Once ordinary dirt from a site has been disturbed or moved, it will not support proper septic functioning.  The soil will fail in a few weeks.  This is just one of the mysteries of soil.  This applies to natural sand, dredging spoils, potting soil and all imported dirt.  Once soil is left undisturbed for a few thousand years, microorganisms and other large and small forces of nature create a condition called soil structure. This naturally changes the soil making it suitable for use as a treatment medium. A designer or soil expert is trained and experienced at determining soil structure by throwing a hand pick at the soil face, and by feeling (texturing) the soil and other tests. Most home-owners and do-it-yourselfers would not be able to recognize poor soil structure and should not determine system size in areas known to have difficult sites for septic.

ASTM C-33 Sand:  However, one special type of sand can sometimes be placed below the drainfield in excessively coarse or shallow soil to allow a system to work (as long as the soil is not too poor or less than a foot deep).   This graded and washed sand available from most gravel pits is called ASTM C-33 concrete sand.  It has many of the properties of natural undisturbed soil to support aerobic treatment of sewage effluent.  This special sand has the quality that once placed on a site, it will not fail as would normal dirt.  Therefore, this sand is used to enhance treatment in poor soils such as coarse gravel.  Systems using C-33 sand like mounds, intermittent sand filters and bottomless sand filters will cost more when compared with conventional pressurized septic systems due to increased excavation, media placement and media cost. 

This discussion should help to warn property owners, particularly those with small lots, that a site can be permanently ruined for the placement of any possible septic system by excavating the area without knowledge of the underlying soil conditions. I have seen property destroyed in an hour by an inexperienced excavator simply trying to "clean up" a piece of land by flattening a spot for the house.

Also placement of the water well should follow the septic assessment, not the other way around. The best soil on the site should belong to the septic system. The water well will sterilize a hundred foot circle for drainfield use. Only a qualified soil expert can determine the best place for the drainfield. Poor site planning at this stage can cost a lot more than you may think you are saving by sidestepping the need for an expert.

Local health regulations anticipate that people are often inclined to put drainfields in places where they may fail from poor soil or not enough soil. You will have to prove to local health that you have enough soil depth in the area of your proposed drainfield to allow treatment without failure (usually four to five feet of unrestricted soil depth at a minimum).  This proof is called a site evaluation.

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The regulations that specify the amount of unsaturated soil needed under a drainfield for proper treatment (vertical separation) are determined by each state. Vertical separation is needed to protect a water table or rock layer etc. from the presence of microorganisms. From various studies the vast majority of the hundred or so indicator viruses from the human gut were found to be unable to survive with a vertical separation of two to three feet. State and local regulations specify the vertical separation that the designer must provide.

Here are two Rules of Thumb for Drainfield Depth that should probably be universal:

1. Vertical Separation bottom of drainfield rock or plastic vaults to water table or restrictive rock layer etc = 2 foot minimum.

2. Maximum drainfield depth from finish grade to bottom of drainfield vaults or gravel = 3 foot maximum.

The septic designer must take the anticipated flow of sewage from the house (based on the number of bedrooms), and the soil type that the drainfield sits in, and determine the overall square footage of drainfield required.  The finer soils require larger drainfields because the finer soils absorb water more slowly. Depending on the soil type in the area of the drainfield, a single family three-bedroom house can need a drainfield of from three hundred square feet in coarse sand to over eighteen hundred square feet in fine clay.

A geometric arrangement of trenches is prepared with scale drawings, usually with a computer CAD program, making sure that nowhere in the drainfield does the vertical separation fall below the required minimum.  On a sloping site, with a variety of soils, this can lead to a somewhat more difficult exercise.  Surface contours and soil depth data from test holes must be shown on the submitted plans. 

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Nitrate: Beginning in the septic tank, organic nitrogen compounds are broken down (mineralized) and inorganic ammonium (NH₄+) is released.

Ammonium is soluble in water but is weakly retained in soil by attraction to negatively charged soil surfaces. Under aerobic conditions inorganic ammonium is rapidly oxidized to nitrate (NO₃-) through a microbial process called nitrification.

Nitrate as an ion is very soluble in soil solution, and is often leached into the ground water.

Nitrate poisoning of infants caused the establishment of drinking water standards for this substance. The increased use of breast feeding and liquid infant formula concentrates have almost eliminated reported cases of Methemoglobinemia in the United States. However, nitrate will continue to be an important indicator of subsurface pollution because it is associated with many other harmful substances that can pollute drinking water.

According to a 1995 US Department of the Interior study in central Washington State, 30% of wells have nitrate concentrations exceeding the US Government MCL (maximum contaminant level ) of 10 PPM (parts per million). Central Washington contains both vast areas of dryland wheat farms and one of the largest irrigation systems in the world spread out below the famous Grand Coulee Dam.

One map from this study shows the application of nitrogen in pounds per acre. Average application is around 120 to 150 pounds per acre per year on farmland, or up to 50 tons of nitrogen per year per square mile from agriculture.

A report by the State of Washington is available to assist designers and operators of septic systems and sewage treatment plants in understanding and estimating the mass loading of nitrogen from residential dwellings (Methodology to Predict Nitrogen Loading from Conventional Gravity On-Site Wastewater Treatment Systems 1995). From this report, the nitrogen from people is about 22 grams per person per day entering the septic system with reduced amounts entering the ground water through the waste stream. This amounts to about 16 pounds of nitrogen per year produced by an average household.

The report then documents the nitrogen removal performance of several recently built test facilities in the US including the Anderson facility in Florida. Not theoretical models, these studies document the performance of actual septic systems.

Septic systems are simply not a significant source of nitrogen.

Few municipal systems use any method of nitrogen removal. Municipal sewage treatment unlike on-site septic systems, concentrates nitrate at the treatment plant. Typically nitrate from municipal sewage treatment is discharged underground in huge drainfields or expelled to surface water. Everything goes somewhere.

Phosphate: Although phosphate is not a toxic substance, excess levels in lake waters can promote eutrophication, the excessive growth of aquatic plants and eventual depletion of oxygen.

The major source of phosphate in surface water is from fertilizer. Application practices can cause soil adsorbed particles to run off into surface water.

Over the years, the amount of phosphate used in households is declining. Very few laundry or kitchen products use phosphates anymore. Certain powder dish washing soaps still contain this substance due to its superior spot resistance.

Phosphate is a minor by-product of organic decomposition of sewage, and small amounts of phosphorus are present in sewage. However, Anderson, and Bomblat in their discussions of 1994 do not include phosphorous or phosphate as materials of interest in their detailed analysis. It is almost impossible to link phosphate in the environment with septic systems because the amounts produced are so small when compared with natural sources and surface application of phosphate on farms and lawns. This has not prevented speculation by individuals who continue to point the finger at septic systems as a source of phosphate pollution, again without direct proof.

It is well known that phosphate is absorbed into and strongly attaches to soil particles close to the drainfield. Phosphate travels only a few inches in a hundred years

Organic compounds: Organic matter comprises the bulk of the solids in wastewater. Chemical and biological oxygen demand (COD and BOD), total organic carbon, and suspended solids are water quality analyses commonly used to indicate the amount of organic matter present in wastewater. Nearly all organic matter in household wastes is biodegradable, and it does degrade readily in soil.

Toxic Synthetic Organic Compounds: It is a popular myth that domestic household sewage contains significant levels of synthetic organic compounds referred to as household chemicals.  Priority pollutant scans by the state department of health of municipal wastewater have discovered that these compounds do not enter the waste water from the building sewer.

The only known products of this type used in houses and likely to enter the sewage stream are shampoos used to treat head lice. The compounds are malathion, carbaryl, and phenothrin.

The only other sources for synthetic compounds are found in the garage and the storage shed. These compounds are used around the house not in it. They are used for ornamental plants and to control pests such as termites.

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