Strategies for Rural Development in Areas with Limited Public Infrastructure: Alternative Septic Systems

Performance and Public Health

Onsite and clustered septic systems can have public health and ecological impacts on groundwater and surface waters. The traditional onsite septic system treats many of the constituents present in residential waste water.  Domestic sewage contains high concentrations of Total Suspended Solids (TSS), 5-Day Biochemical Oxygen Demand (BOD5), pathogens, ammonium nitrogen, total nitrogen, and total phosphorus, as well as varying amounts of heavy metals, organic compounds, pharmaceuticals, and other potentially hazardous materials. Domestic sewage characteristics are described in the Table linked here.

Treatment in the septic tank includes both physical and biochemical processes. The physical (or mechanical) processes include settling and flotation of solids, while the biochemical processes include digestion of the solid organic matter into soluble (dissolved) compounds.  Naturally occurring bacteria regard the organic matter as “food” within the anaerobic environment of the septic tank, and they will digest available organic matter and continue to reproduce as more organic material enters the tank, maintaining a healthy system balance. Undigestable solids and grease are held in the tank until they are pumped out by a septage hauler.

The effluent leaving a septic tank will have slightly reduced BOD5 levels .  If an effluent filter is added to the septic tank outlet, BOD5 and TSS reduction is enhanced.  The septic tank does not reduce phosphorus or nitrate concentrations, although human pathogens can be reduced somewhat since they may attach to solids or simply die in the tank.

Septage

Septage, the material that is pumped from septic and holding tanks, is a byproduct of septic systems that must be treated, managed, and dispersed. Wastewater treatment facilities and other facilities specifically designed for septage treatment are two options for treatment and management. Septage can also be treated and sprayed across undeveloped land under specific soils, site, and application conditions. A long-term plan for septage management should be developed for large projects that will use clustered wastewater systems, particularly in designated growth centers.

The disposal field in a septic system is designed to maintain unsaturated soil conditions immediately below the disposal field.  Both physical and biochemical treatment occur here as well. The physical processes include filtering and slowing the flow rate of the wastewater movement through the soil.  As the effluent moves through the soil, both solids and microbes adhere to the soil particles and are thus physically filtered out of the wastewater. The unsaturated soils between the leach field and groundwater, any impervious soil layers, or bedrock help to significantly reduce the potential for pathogen contamination of groundwater, but they do not always remove 100 percent of the nitrogen and phosphorus.

Most of the biochemical treatment of nitrogen and phosphorus in the disposal field occurs at the interface between the media (i.e., stone or a proprietary device) and the undisturbed soil, where a chemical and biological layer known as a biomat forms. The biomat works best when it is less permeable than the surrounding soils, and modern system design standards account for the formation period and expected long-term performance capability of this mat. Highly permeable soils with deeply placed disposal systems may never develop functional biomats, and thus must rely solely on the physical removal of organic nutrients from the effluent.  These systems may convey more nitrogen and phosphorus into surface waters than shallowly placed systems on finer-textured soils.

Nitrogen Conversion Process

Ammonium-nitrogen from the septic tank is converted to nitrite and then to nitrate through a series of aerobic processes, called nitrification.  For the nitrification to occur in soils, with oxygen diffusing from the atmosphere, the soil must be unsaturated and the wastewater must move through the soil under unsaturated conditions.

If the nitrified wastewater then encounters saturated, anaerobic conditions and there is enough remaining organic material, the nitrate can be converted to nitrogen gas and other gases by another process called denitrification. 

These two successive processes - nitrification followed by denitrification-can only proceed in nature when the conditions are exactly right to provide unsaturated, aerobic conditions for enough time for nitrification to occur, followed by saturated, anaerobic conditions for enough time for denitrification to occur. These conditions and timing are related to the rates of hydraulic loading of the disposal field, as well as to site conditions. Other important requirements include having a sufficient population of microorganisms to carry out the processes, a high enough temperature for the processes to occur and for the microbes to function, and the proper soil pH.

Time is a very important factor in these reactions.  If the system is located near a sensitive environmental feature, the travel time from the septic tank, through the leaching system, through the soil, and into the receiving environment must be long enough to allow all of these processes to occur. The travel time is also a function of the hydraulic loading rate, because the faster the water is loaded into the disposal field, the faster it will move down-gradient through the soil.  Therefore the travel time becomes shorter as the loading rate increases.  Pressurized systems help to control hydraulic loading and spread the effluent more evenly throughout the bed, leading to longer travel times and improved wastewater treatment.

Most Maine soils provide adequate treatment of effluent through the physical, chemical and biological processes. However, some of the nutrients (such as nitrate) are capable of moving through the soil with little or no treatment. Nitrate is a negatively charged compound, and therefore tends to move through the negatively charged soil without adhering to soil particles like other contaminants. In some cases, the site conditions and hydraulic loading rates will allow most or all of the nitrates to be transformed into harmless nitrogen gas, through the nitrification-denitrification process.  However, in cases where high organic loading is anticipated, particularly in areas near sensitive water bodies and other environmental features, pre-treatment using a system designed to remove nitrates and phosphorus is strongly recommended.

Other wastewater constituents

Other wastewater constituents that can cause problems in drinking water and surface waters include the following:

The Table - Domestic of Sewage Characteristics includes a listing of contaminants of special concern that are typical components of residential wastewater, and the capabilities of various septic system technologies to treat them.

Related Work Plan Components

Workgroup Contacts

In Aroostook County: Jay Kamm, Ken Murchison, Joella Theriault

In Washington County: Judy East