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How Exactly Do Septic Tanks Work?
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By Scott Leahy
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A sewerage drain field and a septic tank make up a small scalesewage treatment system. A septic tank is basically arectangular shaped underground receptacle for wastewater. It iswatertight, made of concrete, and consists of tanks that aredivided into chambers. The first chamber has twice the volume asthe second and the majority of the organic solid matter,referred to as sludge, gets settled here. The second chamberfurther purifies the wastewater.
The septic tank collects wastewater from toilets, showers,sinks, etc. Due to the absence of a current the sludge quicklysettles down to the tank floor. Anaerobic bacterial digestiveactivity on the sludge leads to the generation of methane andcarbon dioxide; the sludge is stabilized and does not rotanymore. Particles of stabilized sludge stay settled, some mayfloat to the top as scum.
A septic tank should be constructed such that there is maximumcontact time with the microorganisms so as to enable swifterdigestion of the sludge. This is achieved by placing the inlet,overflow, and outlet diagonal to each other and by making thestreams flow in vertically placed pipes.
The semi-purified water moves to the second chamber via theoverflows that are constructed such that the upward motion ofwater prevents larger particles from flowing into the secondchamber. The feeding, digestion, and settlement of organicmatter continues in the second chamber. The retention time inthe second chamber is half that of the first chamber because ofits size and the fact that the amount of organic matter to treatis less. The outlet is constructed in the diagonal corner to theoverflow.
The treated wastewater is directed to a drain field, also knownas a seepage field, where any impurities that still exist getdecomposed naturally, the water is taken up by the plant rootsystem or it may even become a part of the groundwater. Thelarge sized drainage field is required if the soil is not veryporous. A septic system can be made to work entirely by usinggravitational forces; at some places, however, a pump may berequired to overcome topographical limitations.
The rate of sludge generation in septic tanks is around 0.05cubic yards every year for an individual. Septic tanks need tobe emptied every few years so that there is enough reactionspace for the microorganisms to attack the wastewater and forthe sludge to settle.
The septic tank may be emptied every 5-7 years depending uponthe amount of wastewater it processes. The tank is emptied usingan exhauster truck manned by trained personnel. The sludge thatis sucked into the exhauster truck is released into public wastewater treatment plants, sanitary landfills, or sludge dryingbeds.
Properly digested sludge can be used as a natural fertilizer;this is possible only if no fresh wastewater is added to thetank for at least one week.
The design of the septic tank should be such that the wastewateralways flows in its designated path and no short circuits in thewater flow occur. T-pipe-connectors with inflow and outflowopenings under the water level should be used for inlet,overflow, and outlet pipes. This prevents the floating scum fromclogging the pipes. A grease trap should be provided for thewastewater inflow pipe.
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