Figure 1. Typical household wastewater treatment system with problems. Illustration by Andy Hopfensperger, University of Wisconsin-Madison Department of Agricultural Engineering.

The most common form of on-site wastewater treatment is a septic tank/soil absorption system. In this system, wastewater flows from the household sewage lines into an underground septic tank. The following then occurs:

• The waste components separate, with the heavier solids (sludge) settling to the bottom and the grease and fatty solids (scum) floating to the top.
• Bacteria partially decompose and liquefy the solids.
• Baffles in the tank provide maximum retention time of solids to prevent inlet and outlet plugging, and to prevent rapid flow of wastewater through the tank.
• The liquid portion (effluent) flows through an outlet on the septic tank to the soil absorption field.
• The absorption field is usually a series of parallel trenches, each containing a distribution pipe or tile line embedded in drain field gravel or rock.
• The effluent drains out through holes in the pipe or seams between tile sections, then through the drain field gravel or rock and into the soil.
• The soil filters remaining minute solids, some dissolved solids, and pathogens (disease-producing microorganisms). Water and dissolved substances slowly percolate outward into the soil and down toward ground water or restrictive layers. A portion of the water evaporates into the air, and plants growing over the drain field lines utilize some of the water.

Figure 1 shows a typical household system for wastewater generation, collection, treatment, and disposal. While such systems may be called by various names (such as septic tanks or subsurface treatment and disposal systems), they are similar. In this discussion the term septic tank is used. Note the list of options below each part of the diagram. You may wish to circle the parts found in your system. The ‘leakage’, ‘overflow’, ‘infiltration’ and ‘clearwater’ components represent possible problems with the system. Unfortunately, these problems are often difficult to recognize. 

Over-flow from systems may be noticed as wet spots, odors and changes in vegetation cover. Water entry (infiltration and clear water) will be more difficult to detect. To help detect any water entry, trace where the floors drain, the roof drains, the foundation drains, and where sumps are directing outside water 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 very difficult to assess. The flow chart at the bottom of the box follows the flow of wastewater and sludge through the treatment system.

Quantity of Wastewater

The best strategy is to minimize the volume of household wastewater entering the treatment system. This is important because less flow (volume) means better treatment, longer system life, and less chance of overflow. Less volume also lowers costs by reducing the number of times the holding tank must be emptied.

The quantity of water used depends upon the number of people using the dwelling, their water use rates, and the maintenance of the water supply system. Average water use in rural households is 40 to 50 gallons per person per day. With low-use fixtures and individual awareness and concern, a reduction to fewer than 25 gallons per person per day is possible. However, even conservative use by several people may exceed the capacity of the wastewater treatment system.

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

Consider the following ways to minimize water use.

• Eliminate non-functional uses, such as flushing toilets to dispose of tissues or other waste that should be handled as solid waste. Turn off water between uses, fix plumbing fixture leaks, and try to eliminate sources of clear water infiltration into the system. (For example, divert roof drains away from the soil absorption field.)
• Consider which actions use the most water. Toilet flushing usually ranks the highest. Low-flow models could decrease water use by more than half. Composting toilets allow even greater reductions, but they can present other waste disposal challenges.
• Bathing and washing clothes are next in order of water use. Consider installing low-flow or controlled-flow showerheads, which give good cleansing with less water; taking shorter showers; and taking ‘wet-down-soap-up-without-water-then-rinse’ showers.
• Wash full loads of clothes to save soap and water. Front loading washers use much less water. When running small loads, be sure to use the reduced water level setting.
• Use modern, efficient plumbing fixtures and appliances, including 0.5- to 1.5-gallon toilets, 0.5- to 2.0-gallons-per-minute (gpm) showerheads, faucets of 1.5 gpm or less, and front loading washing machines of 20 to 27 gallons per 10- to 12-pound dry load. These fixtures and appliances can reduce water use from 30 to 70 percent (see Fig. 2.).
• Use water softener devices with care, be-cause they may use significant amounts of water. Proper adjustment and timing of the softener’s regeneration mechanism can reduce excessive water use. The flush backwash should not go to your septic tank.

• Eliminate household water leaks. A leak in the home may be detected by opening the cleanout (usually located between the house stub-out and the septic tank) and checking for a continuous flow through the pipe to the septic tank. Since a small flow is sometimes difficult to see, you should drop a pinch of sand into the pipe and watch to see if the sand is carried away. If the sand is carried away, you have a continuous leak inside the home. To isolate the leak, check each fixture. If a leaking toilet is suspected, put a small amount of food coloring into the toilet and check to see if it appears in the flow through the cleanout.
• Prevent water from other sources from entering either the septic tank or the drain field. Divert water flowing from roof drains away from the septic tank and away from the drain field. Construct a berm or a swale upslope of the drain field if there is surface runoff flowing downslope over the drain field. However, avoid diverting the runoff toward your well.
• Do not allow water from basement floor drains or foundation drains to enter the septic tank through the house plumbing.

Keep in mind that your family’s awareness of your water use and how it can be reduced is as important as the use of water conservation devices.

Figure 2. Aeration tank of a household aerobic treatment system. Source: Onsite Domestic Sewage Disposal Handbook, MWPS-24, Midwest Plan Service, 1982.

Quality of Wastewater

Domestic wastewater usually contains relatively small amounts of contaminants – less than 0.2 percent (i.e., it is 99.80 percent ‘pure’ water), but even small amounts of contaminants can make a big difference in the usefulness of the water.

Contaminants found in wastewater include the following:

• Bacteria and viruses, some of which can cause disease in humans. These microorganisms are large enough to be removed by settling, or through filtration in sand beds or soil. Many will die as they pass through the system.
• Suspended solids are particles that are more dense (sludge) or less dense (scum) than water and that can be removed by filtration. Most can be separated from liquid waste by allowing enough time in a relatively calm tank. Grease and fats are a part of the suspended solids. Filtration beds and absorption systems can be clogged by wastewater high in suspended solids.
• High oxygen demand can lower water quality. The microorganisms that decompose organic wastes (such as blood, milk residues and garbage grindings) use lots of oxygen. The amount of oxygen required to ‘stabilize’ wastewater is typically measured as biochemical and chemical ‘oxygen demand.’ Aeration and digestion processes, in the presence of oxygen and organisms, produce stable, low-odor wastewater when given enough time. However, wastewater with excess oxygen demand can cause problems for soil absorption fields, ground water, streams and lakes by reducing levels of oxygen in the water.
• Organic solvents from cleaning agents and fuels may not be degraded or removed through treatment and can pass along with the wastewater back into the water supply.
• Nutrient contaminants are composed mainly of nitrogen from human wastes, phosphorus from dish washer detergents and some chemical water conditioners. Nitrate-nitrogen is a common ground water contaminant, and phosphorus is a common contaminant of surface water.

Here are some ways to improve wastewater quality:

• Minimize use of the garbage disposal unit. Garbage disposals deposit large amounts of suspended solids and organic matter into the septic system, as well as using additional water.
• Do not clog septic tanks by putting items such as fats, grease, coffee grounds, paper towels, sanitary napkins, tampons or disposable diapers down drains.
• Do not put toxic substances in drains. They might end up in the ground water. These include solvents, degreasers, acids, oils, paints, disinfectants and pesticides. However, this does not prohibit the use of bleach to disinfect laundry or the washing of clothing worn for pesticide applications.
• Do not use chemicals to clean your system. They may interfere with the biological action in the tank, clog the drainfield by flushing sludge and scum into the field, or add toxic chemicals to ground water.

Collection of Wastewater

Collect all waste that needs treatment and minimize the loss of untreated waste. Leaking pipes or treatment tanks can allow wastewater to return to the local water supply without adequate treatment. Exclude from the treatment system water that does not need treatment or disposal.

Infiltration of clear water overloads the system and dilutes the wastes. Do not allow rainwater or water from basement floor drain sumps, foundation drains or roof drainage to add to your waste water volume. Divert clean water away from the house, any wells, and any wastewater treatment system.

Treatment Systems

Make wastewater suitable for treatment and disposal in the soil by reducing the concentration of contaminants in the wastewater.

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

The design and construction of septic tanks influence their water tightness and effectiveness at retaining sludge and scum. Multiple tanks or chambers in series can improve sludge and scum removal. Gas deflectors and filter screens or inclined-plate settling units help to minimize solids entering the drain field. Tanks should be sized to accommodate at least 24 hours of wastewater flow while still allowing for sludge and scum retention. Pumping the tank before it is more than one-third filled with scum and sludge (generally every 3 to 5 years) improves the functioning of the system. When the tank is pumped, you should have the baffles checked and also check for tank leaks. Septic tanks should be made of reinforced concrete, polyethylene or prefabricated fiberglass.

Aerobic (oxygen using) biological systems, which are considered packaged systems, treat wastewater better than the typical anaerobic (no oxygen) septic units, thus improving solids separation, releasing volatile chemicals, and reducing sludge volume. These systems are, however, more expensive to operate and maintain and are more often subject to problems caused by changes in wastewater quality or environmental conditions. Aerobic systems are often used because they produce high quality effluent which may be disposed of through conventional trenches or drip irrigation, or sprayed on top of the ground.

If you have an aerobic system, you should have a maintenance person inspect the unit three times each year. The maintenance person should check the aeration mechanism, the disinfection unit (tablet chlorinator), and other critical aspects of the unit.

Between inspections you should check the tablet chlorinator for chlorine tablets. If there are no tablets, fill the chlorinator immediately. An empty chlorinator increases the chance that disease causing organisms will come into contact with people.

Additional treatment after wastewater has passed through the septic system can reduce the concentration and amount of contaminants in the wastewater and make it suitable for disposal.

Figure 3. Buried sand filter. Source: Onsite Domestic Sewage Handbook, MWPS-24, Midwest Plan Service, 1982.

Aerobic systems may be used for additional treatment of septic tank effluent, and these systems can yield a better quality effluent suitable for more disposal options.

Sand filters also improve the quality of wastewater after septic tank pretreatment. Effective treatment involves aerobic biochemical activity as well as physical filtration. Filters consist of 2 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.

Filters should be designed according to the pretreatment system used, the quality of the wastewater, the hydraulic loading rate, the dosing frequency, the temperature, and the distribution and collection systems used. Filter maintenance includes resting, occasional raking, removal of clogged or crusted surface media, filter media replacement, and attention to dosing equipment.

Wastewater treated in such systems is generally lower in bacteria, nitrogen, phosphorus, oxygen demand, suspended solids and organic matter. The amount of reduction depends on the design of the system.

A disinfection unit is a system that kills disease-causing microorganisms in wastewater, and is used when discharge on the surface is permitted. Chlorine, iodine, ozone and ultraviolet light systems are available to treat good quality effluent, such as that from properly functioning aerobic units and sand filters.