The London sewage for some years has been treated by adding 2·5 grains of sulphate of iron and 3·7 grains of lime to every gallon of sewage; a reduction of 15 to 20 per cent. in the amount of dissolved organic matter being secured. Polarite or magnetic spongy carbon is used as a filter in certain places, the solid and some of the dissolved sewage being first precipitated by magnetic ferrous carbon (ferrozone). The Amines process consists in applying a mixture of herring brine and lime to the sewage. The sewage is stated to be sterilized by this means. Electrolysis has also been applied to the purification of sewage, as in the Hermite process. In this process sea-water is electrolysed, oxygen-yielding compounds and chlorine being produced.
With regard to all the chemical processes hitherto introduced, the following general statement appears to hold good: they are expensive and not thoroughly efficient.
Sewage sludge is deposited in the tanks in chemical processes and needs separate disposal. At Birmingham the amount of sludge produced daily from the sewage of a thousand persons is nearly a ton. This sludge has been run into rough filter beds and left to dry or carted away for manure, but in its crude state its manurial value is very slight. At Ealing it is mixed with house-refuse and burnt in a destructor. The more modern method is to pass it through a filter-press, thus compressing it into solid cakes which can be sold for manure.
Filtration of the sewage matter has been accomplished in various ways.
Intermittent downward filtration through a considerable depth of soil was stated by the Rivers Pollution Commission to be attended with good results. A porous soil is chosen, and the purified water is received in drains under it. A large part of the organic matter is removed by bacterial agency. Frankland’s experiments shewed that upward filtration through the same media did not purify.
Filtration through artificial media has not been successful with crude sewage. Precipitation by ferrozone followed by filtration through polarite is said to be satisfactory.
Broad Irrigation purifies the sewage efficiently under favourable conditions, the possible exceptions being during rainstorms and during frosty weather. The effluent into the river cannot, however, be regarded as certainly innocuous, though it is better than the effluent from most other processes. Sewage farms are not a commercial success. In such a farm liquid sewage is allowed to flow at intervals over the land, different fields being irrigated in rotation. Immense crops of grass are obtained, but the grass is coarse and rank.
The soil to be irrigated should have a gentle slope, and the effluent be conveyed by subsoil drains about 5 or 6 feet deep into the nearest water-course. The sewage should be delivered in as fresh condition as possible, and should be freed from its coarser portions by settlement or precipitation. The amount of land required is about 1 acre for the sewage of 100 persons. The irrigation must be on the intermittent plan, in order that the soil may undergo aeration; as it is only in this way that the best purifying results can be obtained. The sewage farm should be well drained by deep-laid agricultural drains. The chief purification of the sewage occurs in the superficial layers of the soil. Nitrification ceases at a depth of about 18 inches. The great point, therefore, is to keep the superficial soil in good condition. A similar nitrification occurs in earth-closets (page [195]). No nuisance need arise in connection with a sewage-farm, and the supposition that milk and other products from such a farm are less wholesome than the same products from other farms has proved to be unbased.
Bacterial Methods of Treating Sewage.—Chemical precipitation of sewage is likely to be completely superseded by biological or bacterial methods of sewage disposal. When sewage is treated by filtering through land or by broad irrigation the process is bacterial, bacteria or microbes in the soil converting injurious organic matter into innocuous mineral products. The typical process is one of nitrification. The novelty of recent methods is in utilising bacteria for the whole process of purification, and not only for its final stages. The object is, in fact, not as in chemical processes to arrest, but by confining the sewage in tanks to aid and hasten decomposition or putrefaction. Two kinds of microbes serve in this process; those living in air, known as aerobic, and those living in other gases than air, called anaerobic.
Three biological methods of preliminary treatment of sewage are employed. (1) Mr. Scott Moncrieff passes the sewage slowly upwards through a filter 14 inches thick, consisting of successive layers of flint, coke, and gravel. This is called a “cultivation tank.” The solid sewage becomes liquefied in passing through this medium, the microbes in the filter dissolving the sewage. (2) In the “septic tank method,” introduced by Mr. Cameron at Exeter, a tank is employed which is covered in to exclude light, and to a large extent air. The tank is large enough to hold 24 hours’ flow of sewage. The microbes in the sewage under these conditions multiply rapidly, attack, and liquefy the sewage. As in the first process little or no sludge is left. The ultimate products of the decomposition are water, ammonia, and carbonic acid, and other gases. The effluent from the tank is comparatively clear and inoffensive. (3) Aerobic biological filters are employed, as in Mr. Dibdin’s installation at Sutton, where the filtering material is coke. The sewage slowly passing through the filtering medium becomes liquefied, the solid matter being peptonised. This action is in part at least due to anaerobic microbes. The filtering beds are used intermittently to allow of aeration, and the liquefaction of solid organic particles entangled in the filter probably chiefly occurs at this stage. It is desirable to have small subsidence tanks, for the removal of large suspended matters and of road debris, etc., before the sewage is spread over the filtering beds. The material used in the filter varies. Most commonly coke-breeze has been employed, but coal slack and other material have also been utilised.