Changes in temperature affect all the biological activities that drive sewage treatment
Treatment of sewage relies heavily on biological activity. Many factors affect digestion, including temperature, retention time and pH. Microbial activity doubles every time the temperature increases by 18 degrees F. When warm wastewater causes microbial activity to double, the biodegradation of constituents increases. This means that oxygen uptake is more rapid at warmer temperatures, requiring air to be supplied at a higher rate. The waste degrades more quickly at warmer temperatures, so it need not be held in the treatment system as long when it is warm.
The converse is also true: In the winter, oxygen uptake is low and air need not be supplied as fast. However, the waste takes longer to degrade, and therefore needs to stay in the treatment system longer during cold months.
The practical implication of this is that aerobic treatment unit processes should be designed using summer temperatures, and detention tanks should be designed using potential winter temperatures.
The ideal range for aerobic microbes decomposing the waste is between 77 and 95 degrees F. For a commercial kitchen, the ideal temperature in a septic tank or grease interceptor to allow for FOG to solidify and bacterial activity to take place is below 80 degrees F. This is primarily a concern when the wastewater includes discharge from mechanical dishwashers that have a minimum required temperature of 160 degrees F.
Impacts and solutions
Having warm wastewater in treatment components is highly related to the wastewater influent temperatures and ambient air temperatures. With commercial kitchens using high-temperature dishwashers, removal of FOG can be challenging and the use of multiple tanks to allow for cooling can be beneficial.
As temperature decreases, so does microbial activity. The biological and chemical reduction of organic material proceeds very slowly under low temperatures. It has been found that microbes used in wastewater treatment become dormant from 35 to 39 degrees F. The good news is in cold climates, since tanks are buried, septic tank effluent on average is approximately 10 to 20 degrees F warmer than the ambient ground temperature.
Lower temperatures will reduce the biological activity by approximately one-half for each 50-degree F drop in temperature until almost all activity stops at about 35 degrees F, providing little more treatment than physical filtering and adsorption in the soil component and physical separation in the septic tank. Fortunately, most soil treatment areas, even in the winter, stay above 35 degrees F because of heat from the incoming sewage and heat from the surrounding soil.
Temperature also affects the flow and mixing characteristics in the septic tank. Very little research evaluating septic tank treatment at varying temperatures is available. One study of the anaerobic digestion of septic tank effluent and temperature effects (at 41, 50 and 68 degrees F) found organic removal efficiency impacts are minimal at higher hydraulic retention times. This is a positive outcome for cold climates with larger septic tank capacities. This study did find that starting up systems in the late fall or winter is not advisable. The reactor at 68 degrees F consistently achieved higher levels of performance compared to the other reactors at 41 and 50 degrees F. The reactor operating at 41 degrees F was the most affected by HRT changes (Viraraghavan and Dickenson, 1991).
Temperature has a significant effect on nitrification and is critical in the design process in cold climates. In general, colder temperatures require longer cell residence times in suspended-growth systems and lower hydraulic loading rates in attached-growth systems due to slower growth rates of nitrifying bacteria.
Pathogen removal in soil is dependent on vertical separation, loading rates appropriate for the soil conditions, distribution methods, clogging mat and soil temperature. In soil with aerobic conditions, bacteria use for food the organic material that produces the BOD (literally, they break down the BOD and solids, and incorporate them). Pathogens are trapped in the soil, either by being adsorbed onto soil particles (electrical and chemical interactions between the soil particles and the surfaces of the sewage microbes), or by becoming stuck to the microbial slimes laid down by soil bacteria. Once trapped, some pathogens die because of unfavorable temperature, lack of moisture and food, and other causes. Others are inhibited or killed by antibiotics given off naturally by soil fungi and other organisms. Still others are actually preyed upon by soil bacteria and are eaten. This process proceeds faster with warmer soil temperatures and slower with cooler temperatures. Cold temperatures will slow all biological reactions in the soil, reducing the rate of wastewater treatment. Both bacteria and viruses from sewage survive longer at low soil temperatures, because natural soil microbial activity is reduced.
Impacts and solutions
The temperature of the source water can lower the influent temperatures in the septic tank and downstream components. In cold climates, challenges exist when there are shallow installations, large septic tank capacity, and multiple tanks or components. These components are surrounded by frozen soils either partially or completely. In these applications, insulation is needed to maintain the internal temperatures necessary for active digestion when sufficient soil cover is not available or practical. It is also beneficial to insulate components coming to grade such as maintenance holes. Post-install insulating blankets can be used over components to help keep the components insulated.
Components can freeze in cold climates when there are shallow installations, large tanks and numerous components. In those cases, insulating blankets can be placed over a septic system to keep it safe from damage.
About the author: Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to firstname.lastname@example.org.