The Disposal of Sewage

THE DISPOSAL OF SEWAGE The disposal of the sewage of large cities is one of the most ficult problems with which the engineer is faced. Sewage contains, beside the waste products of human beings, the waste products of factories and the whole of the water used in the city in its farious daily operations. The bulk of sewage to be dealt with amounts to approximately thirty gallons per head of the tion. In small communities the matter is simple since the tively small amount of material can be run into streams without serious pollution or can be applied to the land, where the enous matter is decomposed and follows the cycle of changes described above. The land thus fertilized can be turned to good account in the cultivation of cabbages and other crops for the feeding of cattle. In tropical countries, where bacterial action is hastened by high temperatures, the sewage of even large towns may be similarly disposed of, but in the more temperate climates similar rapidity of bacterial action has to be induced by special engineering methods. In all these methods the soil organisms are brought into play. In the majority of town systems the sedimen tation tank, the septic tank and coke or sand filter beds figure in the process. The greater part of the solid matter settles in the first tank the turbid liquid passing over into the second tank from which a rather less turbid and less evil-smelling liquid passes on to the filter beds.

In both tanks the action of bacteria is for the most part anaero bic, under which the breakdown of the excretal matter is very rapid, but the destruction is only partial, resulting in simple solu ble nitrogenous substances possessed of the evil smells one asso ciates with the process of putrefaction. This putrefying matter is sprinkled on to beds of coke some twenty to fifty feet wide and four to five feet deep through which it rapidly percolates. In its passage it is so much exposed to air that the aerobic processes of ammonification and nitrification follow with extreme rapidity. In many systems the time of passage through the filters is as short as 20 or 25 minutes, after which the noxious substances have been converted almost entirely into nitrates and so forth, perfectly in nocuous matter. These form an effluent that can be run into the river without fear of causing pollution of the stream.

Activated Sludge Process.

The purification of sewage by the aerobic bacteria which it normally contains is so slow, requir ing many days for completion, that sewage disposal by this means alone has long been regarded as impracticable. A method of has tening the process was, however, discovered in 1913, and since 1916 the activated sludge process has actually been in successful operation. When sewage is well aerated the suspended matter gradually disappears, being acted upon by aerobic bacteria, and gives place to a granular brown mass which rapidly settles, leav ing a clear solution of the inorganic salts, such as chlorides and nitrates, with only quite small amounts of soluble organic matter. It was discovered that the brown sediment added to a fresh sup ply of sewage and aerated by a blast of very fine air bubbles con siderably hastened the oxidation process. On repetition each in crease in the amount of sediment in relation to the volume of sew age is accompanied by an increase in the rate of oxidation, so that, when the relative amount of sediment approaches 30% of the total volume, oxidation is complete in the space of a few hours. This brown sediment forms the so-called activated sludge and consists very largely of a mass of living organisms, bacteria and protozoa.

In practice two tanks are employed : (I) the aeration tank in which the sewage and activated sludge are blown with air delivered in tiny bubbles by being forced through porous porcelain ; (2) the settling tank in which the sludge is deposited and from which an effluent that requires no filtration is run away. Any excess of sludge over and above that required to maintain the necessary quantity of 25% to 3o% in the aeration tank is spread out to dry by evaporation and forms a valuable fertilizer.