Sewage Treatment

Though the biological treatment methods described here can be applied to any raw sewage, for the reasons described earlier, they are generally only used to process effluent that has already had the bulk of the solid material removed. As such, it is mostly water, though still containing unacceptable levels of pathogens and oxygen-consuming organisms.

An early method of biological treatment used natural soil. Sewage was allowed to percolate down through the soil where it was processed by aerobic organisms. Such treatment methods were not practical for the large volumes of sewage produced by towns of any appreciable size. If a significant amount of solids accumulated, the reaction would become anaerobic, with the attendant disadvantages of odor and slow decay.

Contact gravel beds are an improved form of the natural soil method. This type of processing is usually performed in batch mode. Gravel beds several feet deep enclosed in tanks are charged with effluent. Voids in the gravel guarantee aeration, and the aerobic decay process proceeds more rapidly than the natural soil method. After a batch is processed, the beds can be left empty so that the gravel can re-aerate.

A more efficient version of contact gravel beds are percolating or trickling filters, still in common use. Effluent is trickled over gravel beds continuously, and the voids between the gravel provide aeration. The beds rapidly become "charged" with a slime layer containing complex ecosystem made up of bacteria, viruses, protozoa, fungi, algae, nematodes, and insects. The various life forms in this biological mat maintain a balance, some feeding on the effluent, some feeding on one another, keeping the filter from becoming clogged. The new grown solid material can be flushed out with the purified water, then removed in settling tanks called humus tanks.

Scientists studying biological treatment methods at the turn of the century discovered that if sewage is left in a tank and aerated, with the liquid periodically removed and replaced with fresh sewage, the sludge that settles in the tank will develop into a potent "microorganism stew." This material, known as activated sludge, can oxidize organic sewage far more rapidly than the organisms in trickling filters or contact gravel beds.

In activated sludge processing systems, the effluent is introduced at one end of a large tank containing activated sludge and is processed as it travels down to the outflow pipe at the far end. The mixture is agitated to keep the sludge in suspension and ensure adequate aeration. Air can be bubbled through the tank to introduce additional oxygen if necessary. After outflow, the processed liquid is held in sedimentation tanks until the sludge settles out. The now purified water is then released to a river or other body of water and the settled sludge is removed and returned to the main processing tank. Over time, the activated sludge accumulates, and must be treated in the same way as the biosolids discussed earlier.

Algal ponds are a variation on the activated sludge method. Algae on the surface of a pond of effluent aerate the liquid by photosynthesis. The bulk of the processing is still performed by bacteria.

Some wastes contain too high a level of toxic materials to be processed using biological methods. Even small amounts of toxic chemicals can kill off activated sludge or other biological systems, causing the municipality to restart the culture while the sewage waits to be processed. If wastes are too wet to incinerate, wet air oxidation can be used in which oxygen and hot effluent are mixed in a reactor. Another process for dealing with toxic waste is vitrification, in which the material is essentially melted into glass by a pair of electrodes. The material is inert and immobilized, and can be buried with a higher degree of safety than in its previous state.