Septic Tank Systems for Large Flow Applications
(See endnote 7)
Large flow applications are modifications of the traditional septic tank system treatment technology for several homes and/or commercial discharges. This can be accomplished by having individual septic tanks for each of the structures with pipes leading to a single treatment system. For commercial establishments, the wastewater goes through a special oil and/or grease removal system prior to going to the septic tank which should contain the fluid for at least 24 hours to allow solids to settle. The biggest problem of this type of system is the carryover of solids, oil, and grease because of poor design and poor maintenance. (See the discussions on septic tank systems above and perforated pipe or tile field below.)
Septic Tank Effluents, Treatment, and Disposal Systems
Perforated Pipe or Tile Field (Leach Field or Drain Field)
(See endnote 8)
The distribution box empties its contents, in roughly equivalent amounts, into a series of underground drainage trenches which have had the sides and bottoms scored to allow proper movement of fluid. Gravel is placed in the bottom of the trenches, which have to be at least 3 feet above a nonpermeable layer of soil. Perforated pipe is placed on the gravel, and more gravel is backfilled over the tile or pipe. This is covered by untreated building paper or straw and finally soil is backfilled on top of it and grass is planted. The straw and untreated building paper disintegrate over time. In the absorption field, a biomat forms which helps renovate the wastewater as it percolates through the soil. This usually requires between 2 and 4 feet of unsaturated soil below the drain field. The effluent as it percolates down through the ground goes through physical, chemical, and biochemical processes which help clean the effluent.
The size of the drain field is determined by the nature of the land and soil percolation rate times, and the number of bedrooms which it is assumed will have at least two people in them. Numerous configurations of the perforated pipe or tile lines may be used as long as there is an equal distribution of the effluent in the drain field and one portion of it is not overburdened and another portion underused. A bed of gravel may be used in place of the drain field because of limited space. However, it must create the same level of cleaning of the effluent before it moves down to the groundwater supply. Another technique used is the dry well, which is a container without a bottom, with slit sides and surrounded by 3 feet of stone on all four sides resting on 3 feet of stone. There are usually two to three of these in tandem connected to the septic tank and utilized in areas where there are severe space limitations and extremely sandy soil. The distance above the seasonally high water table applies to this system as well as the others. (See Seepage Pits section below.)
Two to five feet of unsaturated, aerobic soil can do a good job in reducing the BOD and suspended solids. Phosphorus and metals are removed. The ability of the soil to continue doing this effectively varies with the soil mineralogy, organic content, pH, etc. Not much is known about viruses and toxic organic compounds going into the groundwater supply. Nitrates and chlorides leach readily down to the groundwater supply.
Older tile field systems and perforated pipe systems fail for a number of reasons. During installation heavy rains occurred and sealed part of the pores in the soil where the field tile or perforated pipe was being installed. Surface water crosses the system and creates conditions where there is more liquid to dispose of than the system can handle effectively. Too much water is used in the house. A garbage disposal unit is being used. All types of litter and inorganic materials are dumped down the toilets. Chemicals are dumped down the toilets. The septic tanks and/or distribution boxes are not level. The lines are installed uphill instead of downhill. Part of the system may have been crushed by heavy vehicles. The neighbors’ systems are overflowing and put more pressure on this property’s system. Flooding has occurred and inundated the ground. Trees and bushes are planted on the drain field or close to it and the roots invade the laterals. Poor maintenance of the septic tank leads to malfunctions of the system.
Best Practices for a Perforated Pipe or Tile Field Effluent Removal System
- • After excavating the trenches, score the bottom and the sides of each one to prevent sealing of the pores by the use of the equipment.
- • Recheck the header system or distribution box to make sure that it is level and that the inlet from the septic tank is above the outlets of each of the pipes going to the tile lines or perforated pipes.
- • Install, inspect, and properly maintain oil and grease traps where this may become a problem due to the use of the septic tank system by businesses.
- • Install the perforated pipe system according to the approved plans and make sure that the perforated pipes are gradually going downhill and not uphill.
- • Inspection of the system must be made before backfill is put on top of the straw or untreated building paper. The individual making the inspection must be wearing appropriate personal protective equipment, be highly trained, have an excellent understanding of the soil conditions of that particular site, and be a member of the appropriate health department. This individual must have the builder or contractor at random move aside the material to check the depth of the gravel underneath the pipe and above the pipe, the width and length of each of the trenches, and whether or not the septic tank and distribution box are level, and check the baffles and screens within the septic tank before approval. This should be completed prior to any heavy rains and the contractor should be ready to backfill as soon as approval is given. If the system is red tagged, then the contractor must make all necessary corrections and call the environmental health practitioner out for a re-inspection before closing the system.
- • All downspouts of the house must be redirected from the septic system and distribution field.
- • Surface water should not be allowed to cross the field from adjoining properties. If necessary, curtain drains should be placed around the entire property to prevent this or to lower high water tables. The water collected from the perimeter of the lot is discharged further down into a ditch, creek, or stream. This should not contain any contaminants from the septic tank system. (See endnote 6.)
- • Do not allow heavy equipment or trucks to be parked or cross drain fields since they may crush the lines.
- • Remove all bushes and trees from the drain field to prevent the roots from invading the lines.
- • Prevent flooding of systems by using proper flood control procedures on bodies of water and physical barriers. Large amounts of rain in short periods of time could still inundate the system. If so, it would be necessary for the inhabitants of the property to leave if possible for a period of time and allow the system to drain naturally.
- • If the system has failed because it is not well aerated due to high groundwater, heavy rains, or excess wastewater overloading the system, pump air into the septic tank and air into the leaching field which will allow aerobic bacteria to resume their role in cleaning the effluent and reducing biological clogs in the soil pores.
Evapotranspiration (See endnote 10)
In addition to the treatment and drainage of the effluent through the ground, evapotranspiration occurs during certain seasons of the year and allows the liquid to move to the surface and be used by grasses or transpiration by plants, or evaporate into the atmosphere from the soil. The problems of depending on evapotranspiration in climates other than those which are very dry includes the amount of precipitation, the wind speed, the humidity, solar radiation, and temperature. Also as the effluent evaporates salts are left behind which can end up clogging the surface of the unit. In wet and cold climates, there is very little or no evapotranspiration occurring.
Pressure System (Pumping and Dosing)
Where the existing drain field may not be operating properly, a pressure distribution system may be necessary. A pump pressurizes the effluent from the septic tank and sends it to the drain field lines through a small PVC line. This system distributes the effluent through the entire length of the trench when the pump is working and therefore a smaller amount of the effective soil for removal of the water is needed for processing the effluent. Typically, 2 feet of clearance is needed instead of 3 feet below the lines. Pumps fail because of poor maintenance and electrical failure.
Best Practices for Pressure Systems
- • Same as listed under perforated pipes or tile field systems.
- • All pressurized systems should have at least two drainage beds with a switch that can move the flow from one field to the other, giving each one an opportunity to rest.
- • The pump must be serviced and maintained properly in order for the system to work and not have sewage backup or pooling above the ground.
- • The pump must have a back-up electrical supply in the event of electrical failure.