Sewage Disposal

Sludge Disposal.

This is still the greatest problem con nected with sewage treatment, the difficulties of which have been rather increased than diminished by the modern aeration processes, owing to the greater diffusion of the sludge particles by agitation. Formerly it was the custom to either spread the sludge on the land or dig it in, but the great area of land necessary for successful operation has forced this method out of use for large towns. The chief difficulty arises from the fact that the sludge is very retentive of the water, which forms by far the greatest proportion of the mixture, the proportion being greater with the modern aeration processes.

These remarks do not apply to the sludge from detritus tanks, which can be easily handled and hence removed, and which only comprises about one-twelfth of the whole. A further amount will be removed in the sedimentation tanks. Chemical precipitation is successful for clarification pur poses but greatly increases the amount of sludge, and has been largely abandoned for that rea son, but is once more being tried on improved lines. Pressing the sludge into cake is still in use but is costly and often unsatis factory. The cake is not appreciated as manure and still contains much moisture. It is usually impracticable to mix the sludge with dry house refuse or street sweepings, as the supply of these is insufficient. Drying by heat is usually too expensive, and removal of the water by vacuum filters has not passed the experimental stage. Centrifugal machines have so far not been successful, and chemical flotation methods do not appear to have come into common use. The most popular method is still the spreading of the sludge on cinder beds termed lagoons, the water passing away into underdrains and being conveyed back by pipes to the sedimentation tanks. Such lagoons may be merely de pressions in the ground, the excavated material forming the banks.

The sludge should not be more than 9 inches deep and about io gallons per square yard can be treated in this manner. The amount is, however, very variable owing to the varying humidity of the atmosphere, and the sludge may take anything from one day to several months before it is dry enough to be lifted by a spade and carted away.

Large towns like Manchester, Glasgow and London pump their excess sludge into specially designed steamers of about i,000 tons capacity and carry it out to sea, where it is dumped.

Attempts have also been successfully made to separate out the sludge by "digestive" methods. Of these, those processes in use on the Ruhr in Germany and at Birmingham are the most noteworthy. At Essen Dr. Imhoff has developed a tank in which the liquid travels continuously through an upper tank at a velocity of from 3o to i,000 ft. an hour, and the solids fall through slots into a lower quiescent tank where digestion takes place. This sludge is afterwards forced up by the static pressure and spread over cinder drying beds. The water is said to evaporate rapidly and the sludge left to be quite innocuous. The methan gas evolved is now collected and forms a valuable by-product for light and power purposes. Similar gas collection is now in operation at Birmingham. At this latter town the sludge is pumped to separate shallow tanks, the digestion being accom plished in two stages to encourage fomentation. The pumping main is inoculated with digested sludge to encourage bacterial 'action and heat has also been utilized, and the effluent is returned to the percolating filters. The sludge, originally 18 in. thick, dries down to 6 in. on ash beds, and is then removed by wagons to sheds, spread in thin sheets, air-dried, ground and finally mixed with chemicals to form a fertilizer. The process eliminates grease, preserves the nitrogen and is effective.

Storm Water Treatment.

Storm water to an extent ranging from 3 to 6 times the average dry weather flow, must be treated in special plant after being screened. The total capacity of such plant is usually taken at one quarter the dry weather flow, and there should be at least two units provided for, such units usually consisting of tanks or roughing filters. In dry periods they should be empty, but when full they should act as continuous flow tanks. They are often merely extra sedimentation tanks in which the sludge is dealt with in the usual manner. The roughing filters may be used where much sediment is expected, but cleaning is thus made more difficult. If suitable land is available the screened liquid may be passed on to it and then discharged direct to the stream, after passing through separate detritus tanks.