Primary sewage treatment separates solid material (sludge) from the liquid by settling. Few of us are concerned with how this is accomplished in a municipal system, but those of us with septic tanks do need to understand what takes place. A septic tank is isolated from the atmosphere. Microbes quickly consume oxygen gas that enters the tank through the pipes. A variety of microbes prosper under these “anoxic” conditions. Provided with abundant "food" in the form of organic material, they begin the process of decomposing the organic material. The products of decomposition include dissolved organic and inorganic compounds and gasses like methane, carbon dioxide, ammonia, nitrogen and hydrogen sulfide. These gasses leak out the vents through our roofs. Some gasses may leak out of the top of the septic tank where they are oxidized in the soil and fertilize the ground cover (the grass grows greener over the septic tank, especially if it leaks). The liquid floating above the sludge, containing microbes, dissolved organic compounds and dissolved gasses (but not oxygen), passes on to the next treatment stage. The sludge that settles out is usually a black sticky gunk which contains organisms such as fecal coliform bacteria that pose risks to human health. It stinks because of the presence of ammonia, hydrogen sulfide and volatile organic compounds.
Because the reactions in septic tanks are microbial, decomposition occurs slowly. There are many things we can do to ensure that our septic tanks continue to function properly. The more solids like dirt, food scraps and toilet paper we flush down the drain, the faster the sludge will accumulate and the harder the microbes must work. Garbage disposals are a "no-no" because they produce too much solid material. Compost your kitchen wastes instead and profit from the free fertilizer and mulch. Harsh chemicals (bleach, acids, organic solvents, etc.) should never be poured down the drain or we risk killing the microbes that are doing the chemical "work" for us. A properly maintained septic system does not require any of the various kinds of "additives" that are marketed. Grease, which is decomposed very slowly, should never be poured down the drain or a thick scum may form on the surface of the water that will not only retard the loss of gasses, but might plug up the pipes which carry the liquid out of the tank.
Water conservation is also very important. The less water you use, the longer the water resides in the septic tank, the more gasses are released out of the roof vents, and the more time the microbes have to break down the solids. Water conservation is additionally important if you use a deep (artesian) well because water levels are falling at least 1.5 feet each year as we deplete our deep groundwater reserves. And finally, tanks should be inspected at least every five years, and pumped if necessary (it is the law!). Unpleasant as it may sound, the top of the tank should be opened every year and the amount of solids in the tank measured with a "dipstick". If the tank is more than about one quarter full of solids it should be pumped. Why? Consider a freshly pumped tank filled with liquid. Water entering the tank will eventually find its way to the effluent pipe, but will remain in the tank for days, allowing time for complete settling and for the microbes to “do their job". If the tank fills with sludge, then the water entering the tank has a more nearly direct path to the effluent pipe, and there is insufficient opportunity for solids to settle out before the water exits the tank. If solids pass out the effluent pipe into the absorption field they will clog it. The only solution to this problem is expensive - replace the absorption field. Septic systems are like cars and boats in the sense that you either spend a little time and common sense in maintenance, or you spend a lot of money in repairs.
Scientists and engineers are trying to improve simple tanks so as to increase the efficiency of decomposition of organic matter. Partitions in the tank can reduce the concentration of particles that exit the tank. Coarse effluent filters not only retain particles in the tank, but provide a substrate with a large surface area that microbes can colonize so that they can attack both dissolved and particulate organic material with greater efficiency. None of these schemes are easy to retrofit on existing systems, and only experience will dictate which ideas produce significant, inexpensive low-maintenance results over the long term. A good source of information about conventional septic systems is the National Small Flows Clearinghouse at (800) 624-8301 or www.nsfc.wvu.edu.