Reducing the Risk of Groundwater Contamination by

Improving Household Wastewater Treatment

Fact Sheet 6

Satisfactory treatment and disposal of residential wastewater can be accomplished by onsite systems. For these systems to function over a long period of time, they need to be properly designed, installed, and maintained. When all the site-specific criteria have been met, there will be minimal or no impact of the system on groundwater or surface water.

There are many types of wastewater treatment systems. A qualified designer must evaluate the site to determine the system that is best suited to your site and needs.

In Washington, minimum standards for onsite household wastewater systems are set by the Washington Department of Health (WDOH), but counties may mandate more stringent standards if they desire. These standards are detailed in Washington Administrative Code (WAC) 246-272. The codes are a minimum, so consider whether the minimum requirement is sufficient for your site.

The conventional septic system is the most common form of onsite wastewater treatment and, where soil conditions are suitable, it is the most desirable onsite system to use. Since the septic tank and laterals are completely covered with soil, the system is not visible and odor is nonexistent as long as wastewater does not surface.

Strategy: Minimize the volume of household wastewater. Collect all wastewater that needs treatment, but exclude from the system all water that doesn't need treatment.

Reducing the volume of wastewater entering the treatment system is important because less flow (volume) means better treatment, longer system life, and less chance of overflow. Excess flow is a principal reason for system failure (wastewater surfacing or backing up in house). All wastewater needing treatment should be collected, however, to avoid contamination of groundwater or surface water.

The quantity of wastewater is dependent upon the number of people using the dwelling, how water is used, and maintenance of the water distribution 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 a wastewater treatment system.

Less flow entering the system improves treatment by increasing the time waste spends in the septic tank, thus providing more time for solids separation, settling, and decomposition. Less flow also means improved aeration and increased soil contact, providing better treatment in a soil absorption field.

Consider the following ways to minimize water use:

• Eliminate wasteful uses, such as flushing toilets to dispose of tissues or other wastes that should be handled as solid waste. Turn off water between uses, and fix plumbing fixture leaks.
  
  
• Consider which actions use the most water. Toilet flushing usually ranks highest (40% of household use). Low-flow models could decrease water use by more than half. Washington (WAC 51-18) requires low-flow toilet fixtures in new construction and structures under repair. Composting toilets allow even greater reductions, but they can present other waste disposal challenges. (See "alternative treatment systems" section, page 7.)
  
  
• Bathing (30%) and clothes washing (15%) are usually next in order of water use. For bathing, consider such reduction options as using low-flow or controlled-flow showerheads which give good cleansing with less water, taking shorter showers, or turning the water off while soaping up.
  
  
• For clothes washing, use a suds saver, and run full loads. Front-loading washers use much less water, although finding one to buy may present a challenge. When running small loads, be sure to use the reduced water level setting. Space clothes washing activities over the week.
  
  
• Modern efficient plumbing fixtures, including 1.5–to 2.5–gallon toilets, 1.5–to 2.0–gallons per minute (gpm) showerheads, faucets of 1.5 gpm or less, and front–loading washing machines of 20 to 30 gallons per 10–to 12–pound dry load, offer the potential of substantial reduction in residential water use. These reductions have commonly amounted to between 30 and 70 percent of total in-house water use (see Table 1).
  
  
• Your awareness of your family's water use, and how each of you can reduce it, is as important as using water conservation devices.

Leaky piping or septic tanks ("leakage losses") can allow wastewater to enter the groundwater supply without adequate treatment and can cause contamination. Don't allow water that doesn't need treatment (foundation drains, infiltration of rain water, roof drainage) to add to your waste volume. Divert clear water, which doesn't require treatment, away from the house, well, and wastewater treatment system. For example, divert roof drains and surface runoff away from the soil absorption field.

In hard water areas, the water softener may be a significant user of water. Proper adjustment and timing of the softener's regeneration mechanism and using softened water for only essential uses can reduce excessive water use.

The quality of water refers to what is in the water, not to the water itself. Even wastewater is more than 99 percent 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:

• Bacteria and viruses, some of which can cause disease in humans. These microorganisms are usually removed by settling, or through filtration in the soil. Many will die from aging or the adverse conditions in the system.
  
  
• Suspended solids, particles which are more dense (sludge) or less dense (scum) than water. Most can be separated from liquid waste by allowing enough time in a relatively calm septic tank. Grease and fats are a part of the suspended solids. Soil absorption fields can be quickly clogged by wastewater high in suspended solids.
  
  
• Organic chemicals from cleaning solvents, pesticides, and fuels which usually are not degraded or removed through treatment and can pass along with the wastewater into the water supply.
  
  
• Inorganic chemicals which may seriously compromise your onsite treatment system. Household onsite systems are generally designed to degrade only biological contaminants. Inorganic chemicals introduced into your onsite system may even harm the microorganisms which break down household wastes.
  
  
• Nutrients. Nitrogen from human wastes and phosphorus from detergents and some chemical water conditioners are the most notable. Nitrate-nitrogen is a common groundwater contaminant. In addition, phosphorus can contaminate surface water.

Oxygen demand is used as an indicator of wastewater strength. The microorganisms that decompose organic contaminants in wastewater use oxygen. The amount of oxygen required to break down wastewater is measured as biochemical and chemical oxygen demand, commonly known as BOD and COD, respectively. Organic wastes or contaminants such as blood, milk residues, and garbage grindings have high oxygen demand. Aerobic processes (in the presence of oxygen) produce stable, low-odor effluent when given enough time. Wastewater with excess oxygen demand can cause problems for soil absorption fields, groundwater, streams, and lakes by reducing levels of oxygen.

Consider the following ways to improve wastewater quality:

• Do not use a garbage disposal unit unless the system was designed and sized for one. Garbage disposals contribute a large load of suspended solids and organic matter with their higher oxygen demand to wastewater and use additional water.
  
  
• Do not put items that may clog your disposal system—such as fats, grease, coffee grounds, paper towels, sanitary napkins, tampons, or disposable diapers—down drains. Dispose of these as solid waste.
  
  
• Toxic substances—such as solvents, degreasers, acids, oils, paints, disinfectants, and pesticides—should not be put down drains since they may end up in groundwater. This does not include bleach used to disinfect laundry or to wash clothing worn for pesticide applications.
  
  
• Do not use additives to clean or "sweeten" a septic system. They may interfere with the biological action in the tank, cause the drainfield to be clogged by sludge and scum to be carried into the field, or add toxic chemicals to groundwater. Introducing additives may be prohibited by your county health department; check with them for further information.

Strategy: Make wastewater more suitable for further treatment, disperse wastes, take advantage of the additional treatment afforded by contact with soils, and minimize the opportunity for wastewater to contaminate water supplies.

In areas where a municipal sewage system is not available, an onsite system for treatment and disposal of household wastewater is needed. The systems available for use in Washington include: septic tank-soil absorption systems, aerobic treatment devices, holding tanks, and other alternative treatment systems. Direct discharge of household wastewater to the soil surface or surface water is against Washington regulations.

A sketch of the existing wastewater disposal system should always be retained by the owner. Any underground components should be shown on the sketch with reference points and distances to at least two permanent objects at cross angles to each other. This allows location of buried components with minimal problems.

In the septic tank-soil absorption system, wastewater flows from the household sewer into an underground septic tank and is then introduced to the soil through a piped distribution system (Figure 1). In the septic tank the waste components separate—the heavier solids (sludge) settling to the bottom, and the grease and fatty solids (scum) floating to the top (Figure 2). Up to 50 percent of the solids retained in the tank is decomposed by bacteria in the anaerobic digestion process. The partially treated water moves on to additional treatment and disposal in the soil absorption system.

Septic tanks and other chambers must be designed and constructed to be watertight. Among the most important components of a septic tank are the baffles. Baffles are placed in the tank to provide maximum retention of solids, prevent inlet and outlet plugging, and prevent short-circuiting of wastewater through the tank.

Two-compartment tanks, or tanks in series improve sludge and scum removal, and effluent quality. Such tanks are required in Washington. Effluent filters also help minimize solids carryover.

Septic tanks remove solids by holding wastewater in the tank. This allows the solids to settle and the scum to rise to the top. Tanks should be sized to hold at least 3 days' worth of wastewater flow, while still allowing for sludge and scum retention. Properly selected tanks have enough space for sludge to accumulate for an average of three to five years without needing solids removal.

Subsurface treatment and disposal using soil absorption—such as trenches and beds—is the common practice for household wastewater after treatment in a septic tank. The liquid portion (effluent) flows through the septic tank outlet to the soil absorption field, which is usually a series of trenches (laterals), each containing a distribution pipe embedded in drainfield gravel or rock. The effluent flows out through holes in the pipe, then down through the drainfield gravel or rock and into the soil. The soil filters out remaining solids and pathogens (disease-producing microorganisms), and dissolved substances degrade, as the wastewater slowly percolates through the soil to groundwater.

Absorption fields must be maintained properly to operate at peak efficiency and minimize potential health hazards:

• Do not drive over an absorption field. Compaction from vehicles or equipment will cause settling, shifting, or breakage of lateral lines. This can lead to wastewater surfacing, and the creation of a health hazard.
  
  
• Never plant a vegetable garden over an absorption field. Microbes from the effluent may travel through the soil and contaminate the crop, especially root crops.
  
  
• Do not allow trees to grow over the system. Roots from the trees can cause damage to lines, as well as plug them.
  
  
• Keep a grass cover over the absorption field. This will help use some of the nutrients available and aid in evapotranspiration.

Soil absorption systems are not suitable on some sites because of slow soil permeability, shallow depth to restrictive soil layer or bedrock, shallow water table depth, or other factors. Deep, well-drained, well-developed, medium-textured soils (such as silt loam and loam) are most desirable for soil absorption systems. Coarse, sandy soils allow effluent to flow too quickly downward to groundwater and do not provide adequate time for solids and pathogens from the liquid to filter.

At least three feet of suitable, aerated soil beneath the bottom of a soil absorption system is needed to renovate wastewater before it reaches a limiting layer. A limiting layer may be bedrock, impervious soil (claypan, hardpan, or fragipan), or seasonal high water table. Unsaturated soils allow movement of air, which helps keep the soil profile aerobic.

Disposal sites that are more distant and downslope from your well increase the isolation of your water supply from contaminated wastewater. A soil absorption system is required to be at least 100 feet from any water supply and 10 feet from the foundation of the house and 5 feet from property lines. However, separation distances of greater than 200 feet to water supplies are highly recommended because they provide greater protection to your drinking water supply.

Pumping the tank before it is more than one-third filled with scum and sludge improves functioning of the system. When the tank is filled beyond this point, sewage has less time to settle and solids can pass through to the absorption field, causing premature failure. The tank is pumped through the access manhole. Consider installation of an outlet screen or filter, with service access to ground surface.

When the tank is pumped, have the baffles checked, check for tank leaks, and make any needed repairs. All other components of the septic system should be checked at this time. Keep a record book on the system and record all maintenance procedures in it. Septic system maintenance should be performed by a licensed professional.

The frequency of pumping depends on the capacity of the storage tank, the flow of wastewater (related to number of people in the household and water-use habits), and the volume of solids in the wastewater (more solids if garbage disposal is used).

The importance of safety around septic tanks should not be overlooked. The space within a septic tank contains gases which are toxic when inhaled. Because of this, never go into or lean into a septic tank. Fatalities have occurred from unsafe acts during septic tank maintenance and repair.

The Washington State Department of Health (WDOH) defines alternative treatment systems as any system other than a conventional septic tank and drainfield. Although use of alternative treatment systems is encouraged, design and maintenance of such systems should be consistent with state technical review committee guidelines. Systems approved for use in Washington are briefly described below.

Aerobic (oxygen-using) treatment devices (packaged systems) are treatment units that provide biodegradation or decomposition of wastewater by bringing the wastewater in contact with air by some mechanical means. These units come in different configurations and sizes and incorporate a variety of mechanical (and non-mechanical) means to enhance aerobic degradation of wastewater. Air pumps, air injectors, and biological-contact surfaces (such as pipes, fabric, grids, gravels, and rotating disks) are included.

Holding tanks collect and hold the entire wastewater flow. Disposal should be done by a licensed contractor. Tank size should allow ample capacity for pumping and disposal at convenient and appropriate times. When the tank is pumped, it should also be checked for leaks. Use of holding tanks is not recommended as a standard practice for a year-round household. Installation of a holding tank as the sole method of wastewater disposal in new construction is not permitted in Washington.

Sand filters are enhanced wastewater treatment systems that are characterized by a large container with means for distributing septic system effluent atop a layer, or layers, of graded sand (or gravel). As the wastewater moves downward, it undergoes biochemical degradation. There are many different designs of sand filters, but they can generally be divided into two types: single-pass filters and multiple-pass filters.

Nitrogen removal can be achieved through denitrification (conversion of nitrate to nitrogen gas) or ion exchange. These processes are not used extensively at this time because they are quite expensive to install, operate, and maintain.

Further information on guidelines for these and other wastewater treatment systems is available from WDOH, (206) 586-1249.

If you suspect your household wastewater treatment system is backing up or your distribution system is clogged, contact your plumber or treatment system installer. They may have suggestions for extending the life of your system. If your septic system is discharging to the soil surface or otherwise failing, contact your county health department for permits to repair or replace your wastewater treatment system.

If you have a septic tank-soil absorption system, do not wait for the system to fail before pumping the septic tank. Once a system fails, it is too late to pump the tank and salvage the absorption field. Also avoid using septic tank additives. They can contaminate groundwater. If wastewater is surfacing near or above your soil absorption field, don't cover it with more soil. This does not fix the system, and it will soon surface again.

If your wastewater treatment system is leaking or showing signs of failure, seek help to correct the problem. Do not pipe the sewage to the road ditch, storm sewer, stream, or farm drain tile—this pollutes the water, creates a health hazard, and is illegal. Also do not run the sewage into a sinkhole or drainage well because it will pollute groundwater.

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

Household wastewater treatment and regulations

Your local health department, or WDOH Community Environmental Health Programs, Onsite Wastewater Treatment, (206) 753-3764.

Publications are available from sources listed at the end of the reference section. Refer to number in parentheses after each publication.

Groundwater contamination, protection and testing

Washington Groundwater: A Vital Resource EB1622. (1)

Protect Your Groundwater: Survey Your Homestead Environment EB1631. (1)

How Fertilizers and Plant Nutrients Affect Groundwater Quality EB1722. (1)

Role of Soil in Groundwater Protection EB1633. (1)

Septic System Waste Treatment in Soil EB1475. (1)

Drinking Water: Bacteriological Safety and Treatment EB0995. (1)

Water Quality Improvements for Farmstead and Rural Home Water Systems F2274. (1)

Water Quality for Domestic Use: Resource Handbook, 1988. (1) A wide-ranging collection of information including a list of certified water testing laboratories, bulletins on various water quality contaminants, and

Onsite Domestic Sewage Disposal Handbook, MWPS-24 (see below).

Design, installation, use, and maintenance of onsite sewage systems

Care and Feeding of Septic Tanks EB0707 (1)

Properly Managing Your Sand Filter System EB1670. (1)

Properly Managing Your Septic Tank System EB1671. (1)

Properly Managing Your Mound System EB1672. (1)

Properly Managing Your Pressure Distribution System EB1673. (1)

Home Water Softening EB1583. (1)

Water Saving Guideline 1 (3)

Basic Principles of Onsite Sewage, 1991. A fact-filled, illustrated booklet describing the fundamentals of onsite sewage treatment. (3)

Understanding and Caring for Your Septic Tank System DOH 334-009. (3)

Understanding and Caring for Your Pressure Distribution System DOH 334-012. (3)

Understanding and Caring for Your Mound System DOH 334-013. (3)

Onsite Sewage Disposal Systems, 1989. Rules and regulations (WAC 246-272). Pub. DOH 334-006A. (3)

The Washington Department of Health publishes a series of guideline documents, each one detailing standards for a specific type of onsite treatment system. (3)

Onsite Domestic Sewage Disposal Handbook. First Edition. 1982. MWPS-24. (2) Includes information on septic tanks, soil absorption systems, site selection, distribution systems, and other systems such as aerobic treatment and holding tanks.

Design Manual: Onsite Wastewater Treatment and Disposal Systems. 1980. U.S. Environmental Protection Agency. EPA Technology Transfer 625/1-80-012. (4) Contains information on site evaluation procedures, wastewater characteristics, onsite treatment and disposal methods, and management of onsite systems.

Water-saving toilets and showerheads

Home Water-Saving Methods EB0732. (1)

"How To Save Water," Consumer Reports, July 1990, pages 465-473.

Publications available from...

1. Your county Cooperative Extension office. There may be charges for publications, postage, and sales tax.
2. Your county Cooperative Extension office or directly from the Midwest Plan Service, Iowa State University, Ames, Iowa, 50011, (515) 294-4337. There may be charges for publications, postage, and sales tax.
3. Washington Department of Health, Community Environmental Health Programs, Building 3, Airdustrial Center, PO Box 47826, Olympia, Washington, 98504-7826, (206) 586-5529.
4. U.S. Environmental Protection Agency, 401 M Street S.W., Washington, D.C. 20460.

The Homestead Assessment System is a cooperative project of Washington State University Cooperative Extension, Washington Department of Ecology, and the U.S. Environmental Protection Agency Region X

Home·A·Syst team members: Christopher F. Feise and Edward B. Adams, WSU Cooperative Extension Water Quality Coordinators; James D. LaSpina, Homestead Assessment System Project Associate.

Household Wastewater Treatment Technical Reviewers: Enid Cox, WSU Cooperative Extension; Jerry Stonebridge, Sr., Washington Onsite Association; Karen Paugh, Yakima Health District; Cheryl Freeman, Inland Empire Public Lands Council; Mark Soltman, Washington Department of Health.

Adapted for Washington from material developed by the Wisconsin, Minnesota, and Kansas Extension Services and Farm·A·Syst Programs. Washington Home·A·Syst development was supported by the National Farmstead Assessment Program.

Information derived from Home·A·Syst worksheets is intended only to provide general information and recommendations to rural dwellers regarding their own homestead practices. It is not the intent of this educational program to keep records of individual results.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8, and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, religion, national origin, age, disability, and sexual orientation. Evidence of noncompliance may be reported to your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Published September 1993. Subject Code 376. A. EB1746-F6