University of Missouri Extension

EQ680, Revised June 2017

Reducing the Risk of Groundwater Contamination by Improving Household Wastewater Treatment

Properly installing and maintaining a system for treating and disposing of household wastewater minimizes the impact of that system on groundwater and surface water. State and local codes specify how wastewater systems must be designed, installed and maintained. For example, Missouri Code 10CSR 20-8.021 regulates private sewage systems. The state Department of Health and Senior Services' Onsite Wastewater Treatment Systems Owner's Manual provides information on servicing these systems. More information is available in the Resources section. Before proceeding, also always check with the county department of health for any county regulations and limitations.

At minimum, follow the codes, but also consider whether the minimum requirements are adequate for your site.

Septic tank/soil-absorption system: The most common system

The most common form of on-site wastewater treatment is a septic tank/soil-absorption system. In this system, wastewater flows from the household sewer into an underground septic tank. Once in the tank, the system works in the following way:

Quantity of wastewater

Minimize the volume of household wastewater.

Reducing the volume of wastewater entering the treatment system is important because less flow, or volume, means better treatment, longer system life and less chance of overflow. For holding tanks, less volume reduces costs by reducing the number of times the tank has to be emptied.

The quantity of water used depends on the number of people using the dwelling, how water is used and maintenance of the water supply system. Average water use in rural households is 75 gallons to 100 gallons per person every day. With low-use fixtures and individual awareness and concern, it is possible to reduce use to fewer than 50 gallons per person a day. However, even conservative use by several people may exceed the capacity of the wastewater treatment system.

Reducing the volume of water entering the system improves the treatment by increasing the time the waste spends in the system, thus providing more time for settling and aeration and more soil contact.

Consider the following ways to minimize water use:

Figure 1 shows a typical household system for wastewater generation, collection, treatment and disposal. Although systems for many farmsteads may be similar — groundwater supply, septic tank, subsurface treatment and disposal — note the lists of options below each part of the diagram. You may wish to circle the parts found in your system. The leakage, overflow, infiltration and clear water components represent possible problems with the system.

Unfortunately, these problems often are difficult to recognize. You may notice overflow from systems as wet spots, odors and in changes in vegetation cover. Water entry, such as infiltration and clear water, will be more difficult to detect, involving tracing where floor drains, roof drains, foundation drains and sumps are directing waters that do not need treatment into the treatment system. Leakage from the collection and treatment system — as well as infiltration of water into the system through unsealed joints, access ports and cracks — can be difficult to assess. The flow chart at the bottom of the box follows the flow of wastewater and sludge through the treatment system.

Table 1
Water use by conventional fixtures and water-saving fixtures and devices

Conventional fixture Gallons used Water-saving fixture/device1 Gallons used
Toilet 4 to 6 per flush Air-assisted toilet 0.5 per flush
Shower head 4 to 6 per minute Low-flow shower head 2.0 per minute
Faucets Faucet-flow control aerators
Bathroom and kitchen 4 to 6 per minute Bathroom
0.5 per minute
1.5 per minute
Top-loading clothes washer 40 to 55 per load Front-loading clothes washer 22 to 33 per load
1Installation of all these water-saving devises could reduce water use by about 35 percent.
Penn State Cooperative Extension Circular 302

Typical household wastewater treatment system with problems Figure 1
Typical household wastewater treatment system with problems. Illustration by Andy Hopfensperger, University of Wisconsin-Madison, Department of Agricultural Engineering.1 Overflow devices are not approved for installation in Missouri.

Quality of wastewater

Minimize the amount and complexity of contaminants in the wastewater.

The quality of water refers to what is in the water, not to the water itself. Even wastewater is more than 99.44 percent pure water. Wastewater usually contains relatively small amounts of contaminants, but they make a big difference in the usefulness of the water.

Contaminants found in wastewater include:

Consider the following ways to improve wastewater quality:

Collection of wastewater

Collect all wastes that need treatment. Minimize loss of untreated waste. Don't let water that doesn't need treatment or disposal infiltrate the treatment system.

Leaking piping or treatment tanks (leakage losses) allows wastewater to return to the local water supply without adequate treatment. Infiltration of clear water overloads the system and dilutes the wastes. Don't let water that doesn't need treatment — such as water from basement floor drain sumps, foundation drains, rainwater infiltration or roof drainage — add to your waste volume. Divert clear water, which doesn't require treatment, away from house, well and wastewater treatment system.

Pretreatment system

Make wastewater more suitable for further treatment or disposal.

Septic tanks retain most of the suspended and settled solids — the sludge and scum — from wastewater. In the tank, bacteria digest and compact the sludge. The partially treated water moves on to additional treatment or disposal, for example, in a soil-absorption field.

Septic-tank design and construction influence their water tightness and effectiveness in retaining sludge and scum. Multiple tanks or chambers in a series can improve sludge and scum removal. Gas deflectors and filter screens or inclined-plate settling units help minimize solids carry-over. Tanks should be sized to accommodate at least 24 hours of wastewater flow or the minimum size, as stated earlier in this publication, while still allowing for sludge and scum retention. Pumping the tank before it is more than one-third filled with scum and sludge improves system function. When the tank is pumped, you also should check the baffles and look for tank leaks.

Aerobic (oxygen-using) biological systems (packaged systems) provide more extensive treatment of wastewater than the typical anaerobic (no oxygen) septic units, improving solids separation, releasing volatile chemicals and reducing sludge volume. However, these systems are more expensive to operate and maintain and are more subject to problems caused by changes in wastewater quality or environmental conditions.

Aerobic lagoons contain oxygen-breathing bacteria that decompose waste in a storage pond three feet to five feet deep. You can apply overflow from the system to lawns or allow it to flow through a grassed waterway. This system requires low maintenance and is inexpensive to construct.

Holding tanks collect and hold the entire wastewater flow. Disposal is generally done by a licensed contractor who spreads the waste on an approved site or hauls it to a municipal waste-treatment facility. Tank size should allow for ample capacity to accommodate pumpage and disposal at convenient and appropriate times, especially for land-spreading. When pumped, check the tank for leaks.

Alternate treatment systems

Reduce concentration and amount of contaminants in the wastewater to expand options for appropriate disposal.

Low-pressure pipe systems use a small diameter pipe in shallow, narrow trenches in direct contact with the soil. The system is dosed under pressure using a pumping chamber after the septic tank. Typical installation uses 1.5-inch polyvinyl chloride, or PVC, with holes every 5 feet of pipe length. Costs for absorption field components and construction are reduced, but the expense of the dosing chamber and pumping systems have been added.

Drip irrigation systems are soil absorption systems that have been modified to account for sites that may have slowly permeable soils, shallow soils or steep slopes. They consist of the dosing tank, pump and controller, filter system and the drip field. The drip irrigation system uses a small diameter pipe that is buried in a shallow trench just below the surface. The dosing tank and controller can provide uniform distribution of wastewater through dosing and resting cycles. Dosing helps maintain aerobic conditions in the soil, which improves treatment of the wastewater and maintains soil permeability.

Aerobic systems, described in the previous section, may be used for additional treatment of septic-tank effluent, yielding a better quality effluent suitable for more disposal options.

Sand filters improve the quality of wastewater after septic-tank pretreatment (Figure 2). Effective treatment involves aerobic biochemical activity, as well as physical filtration. Filters consist of 2 feet to 5 feet of sand or other media in a bed equipped with a distribution and collection system. Wastewater is applied by dosing, and it may be recirculated to improve treatment.

Wetlands, or vegetated submerged beds, are designed to have one or two cells or zones of shallow excavated earthen ponds filled with up to 18 inches of rock or other granular media and planted with aquatic vegetation. Wastewater from the septic tank enters the front end of the wetlands and is spread out across the width of the bed through a perforated pipe. The wastewater is treated by bacteria and other microorganisms found in the plants and granular media in the wetlands. As wastewater is treated, it flows from the first cell to a second cell to a soil treatment system or lagoon for further treatment. Treatment of the wastewater is based on retention time in the wetland cell and granular media. During winter months when the plants use less water, the retention time should be increased.

Wastewater treated in such systems generally is lower in bacteria, nitrogen, phosphorus, oxygen demand, suspended solids and organic matter. The amount of reduction depends on design of the system. These systems must be engineered and can be costly to construct. They also must be maintained properly to operate effectively.

Buried sand filter Figure 2
Buried sand filter. Source: Onsite Domestic Sewage Handbook, MWPS24, Midwest Plan Service, 1982.

Additional treatment

Important considerations in designing filters include pretreatment and quality of wastewater, hydraulic loading rate, depth and type of filter media, dosing frequency, temperature and distribution, and collection systems. Maintenance includes resting, occasional raking, removing clogged and crusted surface media, filter media replacement and attention to dosing equipment.

Remove nitrogen through denitrification, the conversion of nitrate to nitrogen gas, or ion exchange. Denitrification requires anaerobic conditions in the presence of decomposable organic matter for bacteria to reduce nitrate to nitrogen gas for removal from wastewater. Denitrification and ion exchange processes are not used extensively at this time because they are expensive to install, operate and maintain.

Disinfection systems kill disease-causing microorganisms in wastewater and are used where discharge to surface water is permitted. Chlorine, iodine, ozone and ultraviolet light systems are available for treatment of good-quality effluents, such as those from properly functioning aerobic units and sand filters.

Disinfection of holding-tank waste prior to land-spreading has been studied, but it is not in common use. Disinfection with lime is feasible.

Disposal of wastewater and pumpage

Strategy: Dissolve wastes, take advantage of additional treatment afforded by contact with soils and minimize opportunities for waste to contaminate water supplies.

Off-site disposal of wastewater, by connection to a municipal sewage system, hauling to a municipal treatment facility or land-spreading can help protect the local farmstead water supply. Discharging treated wastes to surface water from private systems is not permitted in Missouri. Improper waste management off the farm can endanger the health of others in your community.

Application of wastewater to the soil surface, though not recommended in Missouri, can provide an opportunity to recycle nutrients and to reduce the contaminant content of wastewater safely. Choose the application time so there will be no runoff, maximum use of nutrients by plants and additional reduction in microorganisms. Consider these characteristics when selecting a site: soil, land use, depth to groundwater, weather, climate and hydrogeology. Check with your local health department before considering this option.

Subsurface treatment and disposal using soil absorption (trenches, beds, at-grade and gravelless) is the common practice for household wastewater after pretreatment in a septic tank or aerobic system. There are, however, sites where soil-absorption systems are not acceptable because of high or low soil permeability, depth to bedrock or the saturated zone, or other factors. Deep, well-drained, well-developed, medium-textured soils such as silt loam and loam are desirable soil-absorption sites.

Soils and separation from the water supply are important factors. Unsaturated soils allow movement of air, keeping the wastewater aerobic. A minimum of 3 feet of unsaturated soils is recommended for bacteria removal. Finer-textured soils (clay loams and clay) retain water better, allowing plant roots to take up wastewater and nutrients and encouraging increased die-off of microorganisms. Coarse, sandy soils cause effluent to flow too quickly downward to groundwater, not providing adequate time for filtering solids and pathogens from the liquid. Disposal sites that are more distant and downslope from the well increase the isolation of your water supply from contaminated wastewater.

Disposal of pumpage from septic tanks and other treatment systems on-site should follow similar rules as for wastewater. Sludges are more concentrated than treated wastewater, so lower application rates are recommended.

Only land-apply wastewater and sludge where permitted by the Department of Natural Resources (DNR). Approved sites for land application must meet requirements found in Missouri Code of State Regulations (10CSR20.800), including requirements for soil, depth to groundwater or bedrock, slope and distance from well and residences.

Assistance with failing systems or new designs

If you suspect your household wastewater treatment system is backing up or your distribution system is clogged, first contact your plumber or treatment-system installer, who may have suggestions for extending the life of your system.

A properly designed, constructed and maintained septic system can effectively treat wastewater for many years. For more information on septic systems, contact your local MU Extension specialist or local health department.


Farm•A•Syst: Farmstead Assessment System Fact Sheet: This guide, previously named MU publication WQ680 Reducing the Risk of Groundwater Contamination by Improving Household Wastewater Treatment, was originally produced as part of the Missouri Farmstead Assessment System — a cooperative project of MU Extension; MU College of Agriculture, Food and Natural Resources; and the Natural Resources Conservation Service — and was adapted from Wisconsin and Minnesota prototype versions of Farm•A•Syst.
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