Protecting Your Groundwater Through Farmstead Assessment:
Assessing Your Household Wastewater Treatment Practices

AE-1975, July 1998

John Nowatzki, Water Quality Associate
Bruce Seelig, Water Quality Specialist
Thomas Scherer, Agricultural Engineer

Household wastewater treatment systems are used to treat and dispose of wastewater from the home. A household wastewater treatment system that is properly constructed and maintained will function for many years and minimize the potential for groundwater contamination.

Basic septic system design and siting requirements for North Dakota are outlined in Chapter 16 of the State Plumbing Code. Local health districts may have more stringent septic system requirements.

An individual household wastewater treatment system, sometimes called a septic system, typically consists of a septic tank and drain field (see Figure 1). Wastewater from bathrooms, kitchen, and laundry room is routed to the septic tank where liquids and solids are separated. Solids such as grease and soap float to the top and form a scum layer. Other solids settle to the bottom where they can be partially decomposed by bacteria. Liquid from the septic tank is discharged into the drain field where harmful, disease-causing microorganisms, organics, and nutrients are removed by adsorption or biological degradation. Consider both design and location when planning construction of a household wastewater treatment system to assess the potential for groundwater contamination.

Figure 1. Typical septic tank and drain field.

Design of your household wastewater treatment system

When designing a septic system, you need to consider both the septic tank and the drain field. Septic tanks must be watertight and built of a corrosion and decay resistant material such as concrete. The capacity of a septic system should match the size of your household. Table 1 shows the recommended septic tank liquid capacities for households of various sizes. The liquid capacity of a septic tank is the capacity below the tank outlet.

Table 1. Recommended septic tank 
liquid capacities.
-----------------------------------
Number of	Minimum liquid
bedrooms	capacity, gallons*
-----------------------------------
3 or less	     1,000              
4	             1,250              
5	             1,500
-----------------------------------
* Liquid capacity is the tank volume 
below the outlet. An additional 
internal volume equal to 20 percent 
of the liquid capacity is needed 
for floating scum storage.

If you know the liquid capacity of your septic tank in cubic feet, multiply that figure by 7.5 to find its capacity in gallons. Compare the liquid capacity of your tank with the guidelines given above. If your septic tank is smaller than recommended, you will probably need to pump your septic tank more frequently than otherwise recommended.

If you do not know the capacity of your septic tank, a reasonable estimate would be to assume that the capacity is equal to the size recommended for the number of bedrooms that were in the house when the septic tank was constructed.

Some soils do not allow adequate filtration of septic tank effluent and create potential contamination problems. Extremely coarse soil, such as sand and gravel, do not filter out fine solids and bacteria from sewage tank effluent. Extremely fine soil, such as clay, is an excellent filter, but it may be too tight to allow much sewage to pass through it. Instead, effluent moves upward to the ground surface. Also, clay soils tend to have high seasonal water tables that do not allow proper flow through the drainage field, causing inadequate treatment of sewage backup.

This circular contains a brief discussion of each question on the Farmstead Assessment checklist, and a section discussing what you can do and who to call if you answer "Yes" to any of the questions.

1. Is your septic tank or drain field less than 50 feet from your well?

A primary concern related to the location of your septic system is safe distance from your well. In North Dakota, private septic systems are required to be at least 50 feet from a well that is at least 100 feet deep and 100 feet from a well less than 100 feet deep.

2. Do you fail to pump out your septic tank on a regular basis?

Management of your septic system can affect the risk of your groundwater becoming contaminated. Maintenance is one of the most important considerations in making sure a septic system will function properly over a long period of time.

Most properly sized septic tanks need the solids pumped out every two to three years. If a garbage disposal is used, a septic tank should be pumped every one to two years. Remember, these are just estimations and the actual interval between septic tank pumping will depend on the quantity of solids entering your tank.

To determine when to have the septic tank pumped, the scum and sludge layer may be measured. To measure the scum layer, attach a hinged flap to the bottom of a 2x2 as shown in Figure 2. Measure the distance from the top of the scum layer to the bottom. If the scum layer is thicker than 12 inches, have the septic tank pumped. Also measure the sludge layer in the bottom of the tank. Wrap 3 feet of white terry cloth or toweling around a 2x2 and push to the bottom of the tank. Turn slowly two or three revolutions, let sit a minute, then slowly and carefully withdraw the 2x2. 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, have the tank pumped.

It is important that you know where the inspection hole or manhole of your septic tank is located. Scum and sludge levels should be monitored on a yearly basis. If you need to pump the septic tank, have a qualified service person do it and make sure it is pumped from the manhole and not through the 4-inch inspection hole.

Figure 2. Checking sludge and scum level.

3. Do you commonly dump grease, oils, or leftover household chemicals down the drain?

You should always avoid dumping grease and oils down the drain. They may plug sewer pipes or build up in the septic tank and plug the inlet. Keep a separate container for waste grease and oil and throw them out with the trash.

Household chemicals that are poured down the drain can cause damage to the septic system. Bacteria present in the septic system breakdown the sewage. When household chemicals are added to the system, they may destroy the bacteria, reducing the effectiveness of the sewage treatment process.

Assessing Household Wastewater Treatment Practices

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If you answered 
"Yes" to the 
following 
questions    What you should do	 Who to call		Other references
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Question 1.  Know the location 	 County health unit	Individual Home Sewage 
	     of all components	 or county or		Treatment Systems. 
	     of your septic	 regional sanitarian.	NDSU Extension Service
	     system.					Circular AE-829.

	     Test water for bacteria
	     and nitrates. 
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Question 2.  Monitor septic tank Local septic tank	Midwest Plan Service.
	     annually and pump 	 pumping service.	Onsite Domestic Sewage 	
	     scum and sludge 				Disposal Handbook. 
	     when needed.				MWPS-24.
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Question 3.  Do not allow 	Local sanitation 
	     grease, oil or 	department to		Chapter 16, North
	     other household	determine where these	Dakota State Plumbing
	     chemicals to be	products can be		Code (Revised 1996),
	     put down the 	disposed.               Published by the State
	     drain.				        Plumbing Board, Bismarck,
                                                        North Dakota.
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AE-1075, July 1998