Individual Home Sewage Treatment Systems (continued)

AE-892 (Revised), February 1997

Soil Absorption Systems (continued)

Absorption Beds

Absorption beds are wider than absorption trenches and utilize more than one distribution pipe (Figure 20). They cannot be used in locations having slopes greater than 6 percent. Absorption beds are not as effective as absorption trenches having the same bottom area because the absorption bed will have less sidewall area for percolation compared to a trench system. Therefore, about 25 percent more bottom area is required than for trenches. Absorption beds can be constructed on smaller house lots. Percolation tests are used to size absorption beds (Table 4).

Figure 20. Absorption bed construction and layout. (25KB b&w diagram)

Table 4. Recommended absorption bed area.

-----------------------------------------------------------
		               Treatment Area
			----------------------------
Percolation Rate	Loading Rate	per Bedroom*
  (min/in.)	        gal/ft2/day	Square Feet
-----------------------------------------------------------
   1 to 5		   1.0		   150
   6 to 15		   0.65		   230
  16 to 30		   0.50		   300
  31 to 45		   0.40		   375
  46 to 60		   0.35		   430
-----------------------------------------------------------
*Based on 150 GPD/bedroom loading rate

The bottom of the absorption bed must be level in all directions. Excavate the bed with a backhoe. Do not allow any equipment to be driven on the bottom of the bed.

Distribution pipe within beds is normally 4-inch diameter perforated pipe. Place the distribution pipes level, 4 to 6 feet apart and 1-1/2 to 3 feet from the edge of the bed. The pipes are normally joined at the ends to form a continuous loop, although they can be terminated as shown in Figure 20.

Place a minimum of 6 inches of rock under the distribution pipe and a minimum of 2 inches of rock over the distribution pipe. Use 3/4 to 2-1/2 inch diameter crushed rock that has been washed, then cover the rock with one of the following materials: A 4- to 6-inch layer of hay or straw, untreated building paper (called red rosin paper) or a geotextile specifically designed for drain fields. Cover the bed with 6 to 18 inches of topsoil and form a crown to account for any settling and also allow the bed to shed water.

Sewage Mounds

Sewage mounds are designed for locations having soils with very slow permeability and/or high water tables. A sewage mound is an elevated rock absorption bed with sand fill over the existing ground (Figure 21). The area where sand makes contact with the existing ground is called the basal area. A mound takes advantage of the higher permeability of surface soils compared to subsurface soils. On low permeability soils the total basal area must be large enough to allow percolation of effluent into the soil surface. In high water table conditions the elevated bed allows effluent treatment to take place before the effluent contacts the soil water table.

Figure 21. Construction features of a sewage mound. (16KB b&w diagram)

Sewage mounds can trace their origins to the first "Nodak" mounds designed in 1947 at North Dakota State University by J. Clayton Russell and Richard Witz. The design of mounds has evolved as more information on mound performance has become available. Present mound designs allow for higher water usage and different construction techniques than many earlier designs.

The septic tank effluent is sent to a mound system under pressure from a pumping chamber connected to the septic tank (Figure 22). The effluent is pumped to the mound through 1-1/2 inch diameter or larger plastic pipe. The pumping chamber should be large enough and the pump controls set so doses equal to about one-fourth of the daily sewage volume are discharged to the mound when the pump starts. A three-bedroom house has a design load of 450 GPD so the pump should discharge about 110 gallons per dose. This provides a rest period between doses and reduces the strain on the pump. Frequent starting and stopping of pumps will reduce their useful life considerably.

Figure 22. Septic tank, pumping chamber and sewage mound. (11KB b&w diagram)

Mounds should always be designed with a pressurized effluent distribution system in the rock absorption bed (Figure 23). A pressure distribution system evenly distributes the effluent through the entire absorption bed area. This helps prevent overloading in any one spot within the bed.

Figure 23. Pressure distribution system using two parallel pipes with a center feed. Effluent under pressure is supplied by the pump in the pump chamber. (5KB b&w diagram)

The pressure distribution system can have different configurations. For a 6-foot or narrower rock bed, a system of two parallel 1-1/4 or 1-1/2 inch diameter PVC pipes can be used. For a 7- to 10-foot wide rock bed, three parallel 1-1/4 or 1-1/2 inch diameter PVC pipes would be used. Pipe diameter is important because too large a diameter (such as 2 inches) could result in uneven distribution of the effluent in the rock bed.

Drill a � inch diameter hole every 40 inches in the bottom of the pipes. The cap should be glued to the ends of the pipe. Manifold the pipes together at the center of the rock bed. The effluent pipe coming from the pumping station will connect to the manifold. To protect from freezing conditions, the manifold pipe and effluent pipe must be designed to drain when the pump is off. This can be accomplished by using a 1/4-inch diameter weep hole in the effluent pipe in the pumping station.

Mound Sizing

The main sizing criteria for mound construction is the basal area or contact area of the fill sand with the existing soil. For slowly permeable clay and clay loam soils, a loading rate of from 0.20 to 0.25 gallons per day per square foot is usually satisfactory.

Example: for a three-bedroom house producing 450 GPD of sewage effluent, the contact area between the fill sand and the existing ground should be between 450 GPD � 0.20 GPD/ft2 = 2,250 ft2 and 450 GPD � 0.25 GPD/ft2 = 1,800 ft2. Figure 24 shows a mound for a three-bedroom home. The fill sand-original soil contact area for this mound located on level ground is 2,150 square feet. The amount of clean, washed sand needed is about 72 cubic yards.

Figure 24. Sewage mound on flat terrain. The dimensions were calculated based on the effluent from a three-bedroom house with the mound built above a clay-loam to clay soil. (14KB b&w diagram)

The rock absorption bed should be sized on the basis of the intake rate of medium sand. Use a loading rate of 1.2 gallons per day per square foot to calculate the bottom area of the absorption bed. Using the three-bedroom home example, the absorption bed area is 450 gal/day � 1.2 gal/day/ft2 = 375 ft2. For soils with low permeability, keep the rock absorption bed width at 4 to 6 feet. For our three-bedroom home example, the rock absorption bed will then be 375 ft2 � 6 = 63 feet long with a 6-foot-wide bed that is 12 inches thick. A rock bed of this size will require about 14 cubic yards of washed rock.

Mounds should be located on flat areas or the crests of slopes if at all possible, however, they can be built on sloping terrain. (Figure 25). For design and location purposes, as the slope increases, the desired percolation rate of the soil should become faster. For example, if the percolation rate is as slow as 120 MPI, then the ground slope should not exceed 3 percent (3 foot vertical drop in 100 feet). If the percolation rate is between 60 and 120 MPI then the ground slope should not exceed 6 percent and if the percolation rate is 30 MPI or less then mounds can be constructed on slopes up to 12 percent.

Figure 25. Sewage mound on sloping terrain. For most soils the maximum slope should be no greater than 6 percent (6 foot vertical drop in 100 feet). The dimensions were calculated for the amount of affluent from a three-bedroom house with the mound built above a clay-loam to clay soil. (13KB b&w diagram)

When a mound is constructed on a slope only the sand-soil contact area under and down-slope from the rock absorption bed can be considered. Under sloping soil conditions, the ends and up-slope portion of the mound receive very little effluent and thus does not contribute to the area of infiltration.

Mound Construction

Mounds require very diligent and careful construction practices. Mounds have been known to fail due to two main causes; improper sizing and poor construction practices. To ensure a mound works as planned, construction practices must be adhered to very closely.

The first step in mound construction is to mow the grass or vegetative cover to a maximum 2-inch height and remove the cuttings. Then dig in the effluent line from the pumping station. The line must be installed below frost level or sloped uniformly back to the pumping chamber so it drains after the pump shuts off. The excavated trench must be backfilled and the soil firmly compacted to prevent effluent from flowing along the pipe.

Ground preparation comes next. The ground must be ripped or scarified by a chisel plow or the teeth from the backhoe bucket. Once the surface is prepared, no wheel traffic can be allowed in the area. Wheel traffic will seal the soil. Do not work in wet soil conditions. Working in wet soil will compact, smear and seal the soil.

Place the fill sand. The fill sand should be a medium texture sand. The sand should be washed and checked to be sure it contains no more than 10 percent fines. To test, put 2-1/2 inches of sand in a quart jar and add water until about three-fourths full. Cover and shake to mix the sand and water. Let the mixture stand for an hour and measure the silt and clay accumulation on top of the sand. If the depth is 1/4 inch or less, the sand is clean enough for use in the mound.

Pit run sand varies widely, even from the same area of a pit. If in doubt, select a coarser fill.

Shape the sand with a front end loader or blade. Small crawlers work much better than wheel tractors as wheel tractors compact the fill and are difficult to maneuver in the fill. Do not allow the tracks to run directly on the earth. Keep at least 6 inches of fill sand under the tracks while building the mound. After forming with the tractor blade, level and do the final shaping by hand. Keep the absorption bed floor level the total length of the bed.

Place 6 inches of 3/4 inch to 2-1/2 inch diameter rock in the bottom of the absorption bed and level. Place the pressure distribution system on the rock. Connect the distribution system at the center of the mound with a manifold and tee the manifold into the effluent pipe coming from the pumping station. Cap the ends of the pipe. Make sure the manifold and effluent pipe will drain. Place 2 inches of rock over the distribution pipe.

Cover the rock with one of the following: A 4- to 6-inch layer of hay or straw, untreated building paper (red rosin paper) or a geotextile material designed for septic system drain fields. Cap the mound with a loam or loamy sand soil (Figure 21). Make the cap 12 inches high at the center of the bed and 6 inches high at the end of the bed. Taper the cap down the sides of the mound.

Last, place 6 inches of good topsoil over the entire mound. Plant grass over the entire mound. Water tolerant shrubs may be planted around the base and up the sideslope of the mound if desired.

Construct mounds to follow the contour of the existing ground. Never place a mound in a low area where water will accumulate. If the mound is on sloping ground, use a berm on the uphill side to divert runoff water around the mound. Mounds can be constructed to complement your landscaping design. Shrubs at the base of the mound will use water and help trap snow.

Lagoons

Small lagoons (Figure 26) have been used for final disposal on some farms. Construct a lagoon only in high clay content soils which will seal the lagoon bottom.

Figure 26. Cross section of a small farm lagoon sized to handle the sewage effluent from a typical three-bedroom home. (13KB b&w diagram)

The lagoon surface area should be sized at about 500 square feet per person. A lagoon serving a four-person household would then have a surface area of about 2,000 square feet. The lagoon should have a depth of 3 feet with a minimum freeboard of 2 feet. Shape the sides of the lagoon to a 3:1 slope. A 2,000 square foot lagoon with 3 feet working depth and 3:1 side slopes would have a 50-foot diameter at its working depth and a 62-foot diameter at the top of the dike. The lagoon may also be square or rectangular. A 2,000 square foot lagoon would be 45 feet square at its working depth and 57 feet square at the top of the dike.

A lagoon must be fenced to exclude children and animals. The lagoon must also be maintained to keep animals from burrowing in the sides of the lagoon.

Care and Maintenance of a Septic System

A septic system will perform as designed for many years if four very important principles are followed. One, don't put any excess water into the sewer system; two, know what not to flush; three, pump the solids from the septic tank on a regular basis, and four, do not disturb the drainfield by driving on it or planting shrubs on top of it.

The major function of a soil absorption system is to infiltrate the water portion of the effluent from the septic tank. Using water in the house wisely will aid the operation of the soil absorption system. Any device or procedure that reduces water use in the house will be beneficial. Low flush toilets, low flow shower heads, fixing leaking faucets, using the dishwasher and clothes washer only with full loads, spacing out clothes washing to two or three times a week instead of one day a week, and avoiding long showers are some of the most common ways to reduce water use in the house.

Another source of water that can end up in the septic system is the water from the footing drain tiles in a basement. The output from the sump pump should be directed onto a slope that carries it away from the house and the soil absorption area. Also, water from roof gutters should be directed away from the soil absorption area. Never allow any heavy vehicle to drive on the soil absorption area. This area is always moist and the heavy vehicle will sink into the drain field and damage its components.

What you flush into the house plumbing system will end up in the septic tank. The septic tank is a vessel for decomposing biological materials. Hazardous chemicals such as paints, varnishes, thinners, waste oils, pesticides and other harmful chemicals will destroy the ability of the bacteria in the septic tank to do their job. Dispose of these types of substances some other way. Judicious use of household cleaners will also benefit the septic tank. Other items that should not be flushed are coffee grounds, disposable diapers, kitty litter, tampons, cigarette butts, condoms and any other items that can easily be put in the trash.

Powdered laundry and dishwasher detergents have been shown to plug the inlet baffles in septic tanks. In addition, when combined with oil, fat or grease they will form a solid cake instead of a scum layer which floats on top of the liquid in the septic tank. Use of liquid detergents is highly encouraged. Garbage disposals add greatly to the amount of organic matter in a septic tank. Use of a garbage disposal usually results in increased cleaning frequency of the septic tank.

Soil absorption fields require very little maintenance, but they can be damaged by driving on them with heavy equipment. A common riding lawn mower will not hurt the absorption field, but a full sized tractor, car or truck will cause damage. Planting shrubs or trees on top of the absorption field can cause problems due to root intrusion. However, planting trees and shrubs on the periphery of the absorption field will help it do its job.

If an absorption field is becoming soggy or some effluent is surfacing, resting it can be beneficial. When allowed to rest for six months to a year, the field will dry out and the soil pore spaces reopen. An absorption field will regain much of its original absorption capacity after resting. If the absorption field uses trenches, individual trenches can be rested by blocking off the entrance to the trench within a drop box or distribution box.

Septic Tank Cleaning

Even the best designed and constructed system will fail unless proper maintenance is performed. The major maintenance is the periodic removal of solids from the septic tank. If solids are not removed they will continue to build up and be conveyed to the absorption system. Sewage solids in the absorption system will eventually clog the soil and cause system failure. A soil absorption system clogged with solids must be abandoned and a new field constructed.

Most properly sized septic tanks need cleaning about every three years. However, the actual time will depend on the quantity of solids entering the tank. To determine when to have the septic tank cleaned, the scum and sludge layer can be measured.

To measure the scum layer, attach a hinged flap to the bottom of a 2 X 2 as shown in Figure 27. Push it through the scum into the liquid layer and pull up so the flap drops down. When it is slowly pulled through the scum, it will trap the scum on the flap. Measure the distance from the top of the scum layer to the flap. If the scum layer is greater than 12 inches, have the septic tank cleaned.

Figure 27. Equipment needed to measure the sludge and scum levels in a septic tank. (16KB b&w diagram)

When measuring the scum layer, also measure the sludge layer in the bottom of the tank. Wrap 3 feet of white terry cloth or toweling around a 2 X 2 and push to the bottom of the tank. Turn slowly two or three revolutions, let sit for a minute, then slowly and carefully withdraw. Sludge thickness can be determined by where the black particles cling to the rough cloth. If the sludge is thicker than one-third of the tank's liquid depth (about 12 inches), have the tank cleaned.

Cleaning a septic tank is more than just pumping out the liquids. The solids must also be removed. Cleaning a septic tank is best left to a professional with the correct equipment. Some of the liquid is normally pumped from the tank, then discharged back in to break up and mix the scum and sludge with the liquid. After thoroughly mixing the liquids and solids, the tank is pumped.

Common Septic System Problems

When a sewage system backs up, first check to see if the septic tank inlet or outlet pipe is plugged. If either one is, unplug the line. If the lines are open and sewage backs up, this can indicate one of two problems. Either the soil absorption system cannot accept the water or the pump in a pump chamber is not working. A high water table due to rainfall or flooding can cause temporary sewage backup, but for persistent backup, the most common problem is insufficient soil absorption area to absorb the liquid.

An absorption system which is too small must either have the system enlarged or water consumption reduced. If the absorption system is enlarged, do not abandon the old system. Make use of what capacity it has and add additional absorption area.

Problems with system overloading may also result from excessive water. Check these possible problems:

1. Leaky faucets and toilets. These may not look like much water, but running 24 hours per day can add considerably to the liquid load. Fix leaks.
2. Water from house footings drain tile. Drain tile water should empty to a separate sump, never to the septic system. Drain tile water can quickly overload the absorption system.
3. Leaky septic tanks or pumping chambers. In high water table soils, groundwater will seep into septic tanks and/or pumping chambers unless they are tightly sealed. This also can quickly overload the absorption field.

If the inlet or outlet lines to the septic tank plug, several things may be the cause. Potential problems include:

1. The inlet pipe is too flat or has a low spot. Either will cause solids to accumulate.
2. Solids such as diapers or sanitary napkins being flushed down toilets. Never dispose of these in a septic system. Plugging will result.
3. The septic tank has excess solids buildup. Solids are backing into either the inlet or outlet pipe.
4. Inlet or outlet baffles have been displaced. Inspect to determine if the baffles are in place and replace if they are missing.

Regulations from North Dakota State Health Department Governing Sewage Disposal Systems

SUBJECT: Sewage and Waste Disposal Facilities for Resort Homes, Cabins, Business Enterprises, Campgrounds, etc., in Shoreline Areas Adjacent to Recreational Reservoirs and Lakes.

FROM: North Dakota Department of Health, Division of Water Supply and Pollution Control.

The following policies are in effect in regard to sewage and waste disposal systems serving the above locations. These policies have been promulgated to protect the public health, abate nuisances and odor conditions, to control pollution and to abate the problem of nutrients from sewage and waste sources entering the water of recreational reservoirs and lakes.

1. All privies, cesspools, septic tanks, and drain fields and other waste disposal facilities must be located 100 feet or more back from the high water level of recreational reservoirs and lakes. Drain fields shall be adequate to handle all liquid wastes.
2. There shall be no pumping or discharge of liquid waste from the septic tank or the drainage field or other waste disposal facilities to the reservoir or lake waters.
3. There shall be no pumping or discharge of liquid waste from the septic tank or drainage field or other waste disposal facilities to ground surface in inhabited areas or to ground areas within 100 feet of the water shoreline or to any area from which such pumping drains into the lake or reservoir.
4. Sludge solids from septic tanks or solids from other waste facilities must be disposed of in remote areas away from habitation and in such a manner that no water pollution problems are created.
5. The recommended reservoir or lake shore lot size for individual homes or cabins which will be served by private water and sewage disposal facilities should be 40,000 square feet minimum. Water and sewage disposal facilities should be located and constructed so they will not endanger the facilities of adjacent residents.
6. All garbage refuse, rubbish and unwanted materials should be disposed of in an approved disposal facility. These materials shall not be disposed of in or adjacent to the waters of recreational reservoirs and lakes.

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AE-892 (Revised), February 1997