EFFICIENCY OF MECHANICAL FILTERS.
The efficiency of mechanical filters depends entirely upon the use of coagulants. Without coagulants they can only be used to remove very large particles. The efficiency of the filtration depends much more upon the kind, and amount, and method of application of coagulant than upon the arrangement of the filter. In fact, the arrangements of the filter are more directed to the convenience and economy of operation and washing than towards the efficiency of the results.
The conditions which control the efficiency of mechanical filters have been discussed in connection with coagulation. With sufficient coagulant the removal of turbidity or mud is complete. Color also can be removed with these filters. The bacterial efficiencies secured with them have been discussed at length in connection with the Pittsburg experiments.
With careful coagulation and manipulation it is possible to get 98 per cent bacterial efficiency without difficulty. The results are somewhat irregular, for reasons not as yet fully understood. On some occasions higher bacterial efficiencies are secured with smaller quantities of coagulant, while at other times the efficiencies are less without apparent reason. There seems to be a limit to the bacterial efficiency which can be secured with any amount of sulphate of alumina and rapid filtration, and it is doubtful if a plant could be operated to regularly secure as high a bacterial efficiency as 99 per cent with any amount of sulphate of alumina.