Disposal of Sewage

Sun (Auckland), Volume IV, Issue 986, 31 May 1930, Page 8

Disposal of Sewage

TWO POPULAR FALLACIES Problems for Engineers (Written /or THE SUN by BACILLUS.) FOLLOWING an investigation by Mr. H. 11. Watkins, engineer and secretary of tlie Anckland and Suburban Drainage Board, into the sewage disposal systems in use in various countries of the world; a report will shortly be presented to the board dealing with possible solutions of Auckland’s sewage problems, a subject which, no doubt, will attract keen public interest.

There are two popular fallacies which confront engineers and others responsible for the design and maintenance of sewage disposal works, and occupy an appreciable part of their time in discussion and argument.

The first consists in a belief in the infallibility of the septic tank as a solution of all sewage disposal troubles. This belief is easier to shake now than it was 30 years ago, mainly because so few sewerage schemes rely to any extent on this instrument of treatment. The original idea was so attractive that its popularity is easy to understand. The septic tank was one of the results of Pasteur’s discovery of the action of bacteria or bacilli, and was designed as a closed box or vessel whose inlet and outlet for the passage of sewage were trapped to prevent access of air. The sewage was supposed to enter the tank, give up its solid content as food for the bacteria living in the tank, and flow out as clear water.

Exaggerated claims by fraudulent and ignorant people have misled public opinion, while the real though limited value of the septic tank has suffered from the activities of patentees. The basis of truth on which the septic tank ■was founded lies in the existence of bacteria which live only in the absence, of air (hence called anaerobic bacterial, and which break up some of the solid substance of sewage into inert mineral residue and soluble compounds. But the further the action goes, the more septic the effluent becomes, and the more difficult it is to obtain final purification. On the other hand, “aerobic” bacteria, which work only in the presence of air, and “facultative” bacteria, which work either with or without air, have much the same liquifying effect on the sewage; and as the final stage of purification of sewage always consists of oxidation induced by bacterial activity, it is usually desirable to maintain as far as possible the supply of oxygen dissolved in the water. The old-fashioned septic tank works satisfactorily an isolated house on porous soil, for the air in the soil can be replenished quickly enough to maintain the oxygen supply. But where the effluent from a septic tank enters river or sea water, or where it undergoes further treatment on filter beds before reaching its outfall, it is desirable to leave the sewage open to the air as much as possible. Septic or liquifying tanks in England are commonly open to the atmosphere, but in New Zealand they are usually roofed over. The root principle that purification of sewage depends finally on oxidation is recognised in the “activated sludge” method of treatment. This method, the most promising development in sewage treatment of recent years, consists essentially of aerating by pumping finely divided air dubbles through the contents of the liquifying tank. It owes its name to the fact that the sludge which remains in the tank, and is exposed to days and weeks of aeration, becames activated so that newly arriving sewage is attacked by the bacteria at about ten times the rate obtaining in the ordinary liquifying tank. MAKING WORKS PAY A late prominent Auckland en-

gineer used to say that the septic tank has retarded the science of sewage treatment by 50 years, and when the fundamental difference . between it and the activated sludge tank is remembered, it must be admitted that there is no great exaggeration in the criticism.

The second great popular fallacy regarding sewage treatment is that the effluent from a main sewer should be valuable for mammal purposes. Although this is correct, the “watercarriage” system is desirable, even in comparatively small villages and townships, while it is an essential to the health of every city; and the separation of the sewage from the water costs more than it is worth. The belief dies hard that the use of sewage as manure should make sewage works pay, and there is justification for hoping that some day a discovery by some bio-chemist experimenting with colloids or emulsions will enable sewage to be economically separated from water. At the present time it can’t be done. The scientists, medical officers of health, engineers, and works manahave not solved the problem. It is true that sewage farms have existed in the past, and still exist in various parts of the world, and some return is obtained from the products of the farms. But in nearly every case this method would not be adopted if there were a hole in the ground at the out-*-11 to which the sewage could go. For the price of the produce practically never equals the cost of ploughing in the sewage, draining off the water, and harvesting the crop. As the ocean is the nearest approach to a large hole in the ground, it follows that every port discharges its sewage into the sea, with or without preliminary treatment. There are also examples of sewage treatment works from which manure is being sold. But whereas the sewage in these cases passes through three or four or more stages of treatment before the effluent is ready for discharge, the addition of another stage for converting the sludge into manure, and the last stage of marketing it, may bring in more by the sales than has been added to the cost of disposing of the sludge by burying or dumping at sea. Settlement of sewage in tanks has some effect upon separation, but quite thick-looking sludge may contain US to 99 per cent, of water, and sludge which consists of 99 per cent, water must be reduced to half its bulk before it comes down to 98 per cent., and then there are 98 tons of water to handle for every two tons of solid substance.

To complicate the problem, every cheap method that has been devised so far of improving the clarity of the effluent has resulted in an increase in the volume of the sludge. Thus the addition of lime to sewage in settling or liquefying tanks usually produces a clearer effluent, hut it also produces a greater bulk of sludge, and the sludge is even less willing to separate from the water. Sewage disposal is as much an art or craft as a science, and while scientific knowledge may some day point out a “short cut” -which will save monev, its success on the economic side has so far been conspicuously lacking.