Flush Tanks for Sewers.

Flush Tanks for Sewers.

The advantage of the use of flush tanks for sewers has become so well recognized of late that it is hardly necessary to enlarge upon that branch of the subject. Although their general use dates only from about 1885, the benefits derived are so obvious that few sewer systems are now planned without some form of flushing appliance. The sewers of the average town, if complete, will usually be divided into mains, submains and laterals, the latter having the length of one block. If the system is properly constructed the mains will receive sufficient water at all times to keep them clean. As for the laterals, supposing them to be eight incites in diameter and laid on a gra le of one fool to the hundred, the quantity of water per minute necessary to keep them clean is about twenty-five gallons. If the block has a length of 300 feet and contains twenty-four families, the amount of water furnished bv them at the time of maximum discharge will not exceed eight gallons per minute. The sewer will thus accumulate matter that will have to be removed by expensive process of hand cleaning, or by the occasional use of a hose. The same reasoning applies to combined systems, as at certain seasons, notably summer and winter, rains are too infrequent to be depended upon for flushing purposes.

Sewer flushing may be divided into two general heads, first, flushing with sewage, and second, flushing with clean water. In the former case gates are usually placed in man-holes and the sewage allowed to accumulate behind them until it reaches a certain height, when the valves are opened, either by hand or automatically. There are but few in use in this country and those mainly on large conduits. The second, or clean water flushing, is secured either by hand or from an automatically discharging reservoir. The objections to the hose method are that it is expensive, as in even a small town one or two men would have to be employed, and that it is constantly dangerous when not impracticable in the winter from the liability of damage to hydrants by freezing.