Failed or improperly functioning private residential septic systems threaten nearby water bodies. When a septic system is not working properly, untreated wastewater can enter the lake, directly or almost directly. Though a septic system is designed to eventually return “clean” wastewater to the groundwater, the waste must first go through the whole process of bacterial action and passage through appropriate materials that filter it.
Untreated Waste Can Be Harmful to Health
Waste from a home that has not passed through a functioning septic system can carry bacteria into the water body. The presence of E. coli bacteria demonstrates the presence of fecal material in the water. E. coli are seen as a marker; when they are in the water, other health hazards (bacteria or viruses) associated with human or animal waste may be present. (While not prevalent in the US, diseases such as cholera and typhus are water-borne.)
Nutrients Harmful to Lakes
Untreated residential wastewater is also heavy with nutrients that feed unwanted plant and algae growth in lakes and ponds. Algae blooms and thriving invasive plants can be supported by nutrients from septic systems that are not working well. Untreated septic waste can also contain chemicals that are harmful to plants, aquatic life, and humans, such as chlorines and various chemicals and hormones used in pharmaceutical and personal care products. Some of these are altered or removed as the waste passes through the microbial and filtration action of a septic system, but reach the lake water when the system is not working properly.
Dirty water leaving your house – from laundry, kitchen, or bathroom – ends up somewhere. But where? And how does it get treated before it ends up back in the water cycle? If you are connected to a sewer, municipal sewage treatment plants process the waste and the treated liquid is usually released back into rivers or larger streams. But if you have your own residential septic system, how does it work?
All wastewater leaving your house goes out an underground pipe and dumps into your septic tank, a large cement or plastic chamber. Here, the solids settle to the bottom, and a layer of scum made up of soaps, grease, and other lighter-than-water elements floats on the top. All effluent contains bacteria, and there are anaerobic bacteria (that do not need oxygen to work) in the sludge layer in the tank. These bacteria go to work decomposing the solid materials and reducing them to sludge that remains in the septic tank. The layer above the sludge and beneath the scum is a liquid layer with dissolved or suspended waste.
When wastewater enters the tank, a compensatory volume of the liquid layer in the septic tank passes on to the other end, past a baffle which holds back solids, into a pipe that empties in a distribution or diversion box (D box). The D box has multiple exits, and from it the water passes into a number of perforated pipes. From these pipes, it disperses into the material of the leach field or bed. Wastewater can flow by gravity or be pumped between the different parts of the system.
A typical leach field is made up of layers of sand and/or gravel that allow the wastewater to pass through at an appropriate rate into the soil below. Septic system design relies on permeable soils which will filter the wastewater or effluent as it passes through, so when a leach field is constructed, stone, permeable gravel, and other suitable materials are brought in to construct the bed, and regulations require an adequate depth of good permeable soil where the leach field is made.
Back to the Groundwater
The process of filtering or percolating through the soil cleanses the wastewater. Dissolved waste and bacteria cling to soil particles or are eaten by microorganisms that require oxygen for the process. Eventually the resulting “clean” water becomes part of the underground water systems, or ground water.