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 out of the effluent 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. Numerous anaerobic (water breathing) bacteria continue working to reduce some of the strength of the sewage, but not much treatment happens in the septic tank.

The human gut is an anaerobic (liquid) environment similar to that in the tank. The most hazardous materials in sewage are pathogenic (disease causing) bacteria and viruses from the human waste.

The separated liquid containing the dissolved sewage solids (called 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 containing anaerobic bacteria, organisms and other organic particles in the effluent.

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

FAQ's

Many new septic systems use the plastic vault technology to create the drainfield in areas where the vaults are allowed. Once excavators and contractors try vaults, they seldom go back to using drainrock due to ease of construction and favorable public acceptance. This is 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.

Almost all pipes used in septic systems are now plastic, although older systems used clay or concrete pipes.   Most metal components and even some concrete will dissolve over time in the corrosive environment of the septic system. Plastic is the best available material for construction of septic system pipes and components to resist corrosion. However, concrete septic tanks are much preferred over plastic and fiberglass tanks because of structural reasons.

In the standard gravel or drainrock type drainfield, the trenches are dug three feet wide at the required depth (usually three feet deep). The distribution pipe in each trench is 4 inch diameter plastic, PVC (polyvinyl chloride)  pipe with holes along the sides (perforations) and capped ends.  The perforated drainfield pipe is placed dead level in the trench by wiring it to a 1 x 6 plank staked to the center of the trench (the best way to ensure a level result) and then placing a foot of drainrock into the trench surrounding the pipe.

The drainfield can be built without the plank in the following way. A six inch deep layer of  the inch-and-a-half drainfield rock can be placed in the bottom of the trench and leveled. The pipe network is built on this base, and covered with a second six inch deep layer of drainrock, covering the pipe. The pipe may be checked along its length for level by pushing the drainrock aside in several places and checking with your instrument. The pipe may be raised a little at a time to level by working it on both sides with two pointed shovels.  Remember though, you may raise it a little this way but you may not lower it without a whole bunch of shoveling, and drainrock does not want to shovel.

The drainfield is supposed to work, (distribute the effluent evenly from all the holes), as ripples splash up and down inside the pipe.  It is proven that real working gravity drainfields do not distribute the effluent in this way, all the effluent comes out of a few of the holes.  However, gravity drainfields can still be allowed in some counties even though the 4 inch perforated pipe does not distribute very well. 

Common sense and numerous demonstrations have confirmed that it is impracticable to lay the pipe perfectly level. 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.)  This is why the center pipe has been eliminated from the gravity vault design.

In finer soils, fine 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.  Pressure distribution in the finer soils is of little additional value.  No better treatment is likely with pressure distribution, although the drainfield will probably last a little longer by being more evenly used.  

Coarse soils and gravel, on the other hand will drain too quickly if standard gravity drainfields are used.

Very coarse soils, or poor/shallow soils will require a pressure system in most counties nowadays.

FAQ's

In a pressure septic system, the drainfield vaults (seen here in the foreground) do have a center pipe, although a small one. Effluent is forced into the small (one inch diameter usually) pipe that is suspended from the center of the vaults with plastic ties, or tied to a row of blocks on the floor of the trench. This pipe is perforated with several dozen small holes (one eighth inch diameter). A pump sits in the bottom of the concrete dosing tank or pump chamber (the pump chamber is seen here with the white transport line coming out of it and connecting it to the drainfield). The 1000 gallon septic tank is behind the pump chamber and has a separate lid and riser (green). The sewer line from the house enters the septic tank from the far end.

Effluent from the pump chamber is pumped into the drainfield through the white pipe.  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 (sometimes known as a dosing tank) holds 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 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."

FAQ's

The Pressure System Squirt Test:  Upon completion, each system is field tested.  This is usually done by a local heath inspector, the designer, or sometimes both.  Among other tests such as checking the depth of 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 is probably the easiest way to accomplish this task. 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

Local health may require exposure of the entire piping network for the test with another inspection of the vaults 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.

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%.  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. One of the best ways to get even squirt is to use a center, rather than end manifold and to connect the transport line to the manifold in the center of the drainfield if possible, regardless of what the calculations show.

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.

FAQ's

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). Designs for all small septic systems are approved by local health inspectors based on drawings submitted by system designers, engineers, installers or homeowners. Who may submit site evaluation documents and drawings depends on state and local rules.

The site evaluation has replaced the traditional "perc test" in most states as a way to demonstrate the treatment qualities of your property to local health.  This is due to the superior results of the site evaluation over the perc test (where water is poured into a prepared hole and timed with a stop-watch as it soaks into the soil).  

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 designer in most cases will be on the site during the test hole digging to tell the excavator where the holes should be dug.  Usually the designer will know the excavators in an area.  Some subdivisions are well known by the excavator and designer so that no coordination is needed and the holes can be dug at the convenience of the parties. 

In the old days, the excavator was the designer.  Newer regulations and the expansion of information in the septic design field make it difficult for an excavator to perform all roles.  Excavators after all make money keeping the equipment working, not by handling paper.  Large excavation companies will either have a designer on the payroll or work with an independent and reliable favorite.  Small excavators who are designers (where this is still allowed) may retain the qualification to lure customers with statements like; "I'll do the design for $100", or "I'll throw the design in for free".  Customers who bite on this apparent money saving offer will have trouble getting bids for their septic system.  The excavator may pick up the permit from the health department and appear on the site ready to start without the homeowner having a chance to get another excavator to estimate the job.   Be sure to ask the excavator what he would charge for the design if another excavator was to perform the work.  You may find that the design now costs a few hundred more, and the system costs more than a few hundred less.

Sometimes, subdivision regulations require a site evaluation when a new lot is created by platting.  

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.

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.

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 are required.  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 special plow blade before the sand is placed to provide flow into the ground.  The sand placement is with a lot of hand work.  Construction equipment or trucks are kept off the mound area, so construction must happen from the side.  The pressure drainfield 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.  The slope is 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.

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 a bottomless sand filter. 

On poorer soils or protected areas, Lined sand filters are designed with a thick, 30 mil PVC or vinyl liner under the whole thing.  Clean coarse gravel with a long drain along the bottom channels the treated sewage effluent out of the filter downhill to a gravity drainfield through to a special "boot" that is factory built at one end at the bottom of the liner.  Or, the filter may have another pump in a vault inside the sand that sends the treated effluent over the top of the liner into a small disposal drainfield nearby.  Some newer sand filters use a coil of tubing in the sand at the bottom of the filter and a small electric air pump to blow air into the system and help the aerobic action of the the bacteria.   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.

A STEP System is a community system. The sewage collection system in alternative communities is usually a Septic Tank Effluent Pumping or STEP system.  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 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 many states including California, Oregon, Washington and Alaska.

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. 

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 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.  This is a promising design if replacement area has been lost to building or ruined by improper excavation. 

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.

FAQ's

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 require alternative systems in sensitive areas, or when soil conditions are inadequate to ensure treatment.

In extremely difficult conditions, the more expensive and complicated systems may be required. Local health using state and federal guidelines has laws and rules that govern what they will require a homeowner or developer to do.  If the local health authority is being even handed in their decisions, you will probably be required to build the system type that the regulations specify.  Research has proven that the more expensive pressure distribution provides more reliable systems that discharge fewer disease causing micro-organisms.

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 require more of a system than is really required.  Neighbors who have shouldered the expense of alternative designs will call their county commissioners if a newcomer is required to provide only a simple system.  However, side-by-side homes can require vastly different systems.   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 septic system costing over twice what is really needed.

Read the section on site evaluation to find out how to maximize your property for the septic system.

FAQ's

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 $1,800 to $4,000 complete, to a county licensed excavator plus state or local taxes. If plastic vaults are used, the cost will be on the higher side of this range. Vault systems will usually be smaller than gravel designs, but the cost of the vaults is more than the relatively inexpensive drainrock.

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 for a 1000 gallon tank delivered within 50 miles of the yard.  A 1250 gallon tank  for an extra $90 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.

The number of excavators licensed in the area and how busy they are will have a huge affect on 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.  $3500 to $5000 is an average range for a small, simple 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.

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 $5,000 - $12,000 depending on the site, with sand filters sometimes called intermittent sand filters to distinguish them from recirculating sand filters costing about the same as a mound or a few thousand more due to the need for a disposal drainfield and the probable custom welded vinyl liner. 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 this usually special system to be almost comparable to a standard pressure system in another area.

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 $13,000 zone if these designs are relatively rare in your area. If everyone has one, the cost could be as low as $6,000 to $7,000, but not much less.

A Wisconsin At-Grade-System should run about $6,000 to $7,000.

Evapotranspiration Systems should run about $25,000. The cautions involving pressure systems above, 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.  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.

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.

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 house system permit. Alternative designs may require additional inspections during construction, and a higher permit fee than the standard system.

In some places, local health inspectors even design the system as part of the permit fee.   It is a way, after all, for public health to be served in areas where resources are strained.  Some counties and states allow excavators to design 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, 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 special designers who create plans for systems.  In some places on the other hand only professional engineers or private health inspectors or soil scientists are allowed to submit designs.

Professionally licensed designers charge from $250 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, the design cost should be about $450 to $550 for a house with costs increasing up to several hundred dollars more for a site with difficult soil.

Complex and alternative designs involve more work, and design fees could be more by a hundred to three hundred dollars or more over a standard pressure design.

FAQ's

If the soil surrounding and below the drainfield remains moist and not wet, the aerobic bacteria will dominate the drainfield and things are working fine.  If the soil becomes saturated with sewage effluent or water, anaerobic (water breathing) bacteria from the effluent 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."

A completely failed system generally must be replaced in a fresh location.  If the drainfield is abandoned for a year or more, it probably will recover.  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 as a backup. 

A septic system can be deliberately made to fail simply by turning on any tap in the house and leaving 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.

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 complex systems and poor soil conditions.

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 poorer for septic systems.  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 be compacted, and layered.   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 poor soil.

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 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 can cost more when compared with conventional 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.

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 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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