Professor Emeritus, Agricultural and Biosystems Engineering
Iowa State University
They are usually out of sight - so are easily forgotten - but septic systems do need maintenance, and failing to do it can lead to costly repairs.
Septic systems have two main components
A tank traps floating solids and sludge. A soil or sand filter (variously referred to as soil absorption trenches, leaching field, buried sand filter, filter mound or occasionally a mechanical treatment unit) reduces bacteria and soluble pollutants in septic tank effluent before the liquid is discharged into the soil (preferred) or (when soils are not suitable) into a ditch or stream.
The most important maintenance task on the tank component of the system is to have it checked and pumped out periodically. Though chemical and bacterial action inside the tank help to break down solids, the rate of decomposition is generally too slow to keep up with the amount coming in, and so a gradual buildup of solids is to be expected.
If the tank is allowed to fill with solids, then these damaging particles will be flushed into the filter/disposal component of the system, causing plugging - followed by development of wet areas in the disposal area - and ultimately requiring costly excavation and reconstruction.
The frequency of tank pumping varies with size of the tank, population of the household discharging into the tank, and whether or not a garbage disposal (which increases the rate of solids loading on the tank) is used. Guidelines published by Pennsylvania State University suggest that a 750 gallon tank serving a household of three people should be pumped approximately every 2.6 years, while a 1,000 gallon tank serving the same size household is predicted to need pumping every 3.7 years. If the household size increases to six persons, recommended pumping frequencies for the above tank sizes increase to 1.0 and 1.5 years respectively. If a garbage disposal discharges into the septic system, more frequent pumping will probably be necessary.
Be sure to ask the technician to check the condition of septic tank baffles each time the tank is pumped. Baffles help to retain solids inside the tank. The highly corrosive environment inside the tank can damage the baffles, resulting in a continuous and damaging discharge of solids.
The benefits of using special additives to maintain or restore septic tank performance continue to be debated by product vendors, researchers and environmental health officials (https://www.nesc.wvu.edu/files/d/39a22099-e56f-4213-99fe-08a9aed6210c/sfqw02.pdf). While some research has identified possible benefits, other work has found some additives to be detrimental. New products continue to be developed and marketed, and most experts agree further research is needed. According to information posted on the website of the USEPA – funded National Environmental Services Center at West Virginia University, “Claims made on the effectiveness of additives to either eliminate pumping of a septic tank or restore permeability of soil absorption systems are unsubstantiated. No product will allow a homeowner to escape a regular septic tank pumping and maintenance schedule.”
Several different types of soil/sand filters are used in Iowa to further treat household sewage before discharging it into the soil or a nearby water course. Maintenance on these so-called “secondary” treatment devices varies, but many depend on gravity drainage to deliver septic tank effluent to and throughout the secondary treatment area.
Gravity flow control devices, often called distribution boxes, splitter boxes, drop boxes, or dosing siphons, rely on perfectly level installation to split the sewage flow and evenly distribute it across the surface area of trenches or filter beds. If the splitter box becomes unlevel — due to poor installation, settling, frost heave or lawn maintenance equipment traffic — much of the flow may be directed into a small portion of the treatment area, causing local overloading, wet areas and plugging of soil or sand. To avoid this problem, the splitter box should be checked periodically to ensure that it is level and not obstructed by solids.
For further information on septic system design, installation and maintenance, contact your local county environmental health sanitarian, the Iowa Department of Natural Resources, or check out the excellent publications posted on the Septic Systems page of the National Environmental Services Center.