To secure the best results the amount of coagulant used must be proportioned to the condition of the water; the amount varies from one-quarter grain to two grains per gallon, the exact amount for any water being determined by experiment. If sufficient coagulant is not fed to the raw water, it will result in an inferior filtrate, and if too much coagulant is used, it will not only increase the cost of operation, but coagulant will pass through the filter bed to the delivery mains. Some waters are so soft that insufficient base is present for coagulant to react upon. When such is the case a base, usually of lime, is also added to the raw water.

Fig. 110

To feed coagulant to the raw water, some form of pump or apparatus is required that will be automatic in operation and feed a measured quantity of coagulant proportional to the quantity of water. A meter pump for this purpose is shown in Fig. 110. A coagulant solution of the required strength is mixed in wooden coagulant tank, a, which is connected by feed pipes to a meter-actuated duplex pump, b.

Table XL - Efficiency Of Filters

Remarks

Filtered water in perfect physical condition, being free of turbidity and color and very brilliant and sparkling. Average rate, 110,010,000 gallons per acre.

As will readily be seen, this has been a week of bad water. The large number of bacteria in the applied water, in connection with large sewage contamination, have made obligatory a larger use of chemicals than is necessary under normal conditions. Turbidity and chlorine contents have also been high.

With sulphate of iron at $9.00 per ton and caustic lime at $5.00 per ton, the chemical cost for the week would average $1.77 for iron and 21 cents for lime; total, $1.98 per 1,000,000 gallons filtered.

It has been shown by rather extensive experiments conducted at this station that to accomplish the same work with alum there is required fully as much alum as iron sulphate, and with high turbidities a trifle freer use of alum obtains. It is probable, therefore, that an average use of 3 grains of alum per gallon would have been necessary to get the same percentage reduction. With alum at $20 per ton, the cost would have been $4.25 per million filtered, or a saving of $2.30 per 1,000,000 gallons filtered in favor of the iron process.

(Signed) C. Arthur Brown, Superintendent. March 20, 1904.

The meter measures the quantity of raw water passing through; the raw water operates the pumps which discharge a proportional quantity of solution into the raw water. All working parts of a pump or other coagulant apparatus should be made of bronze to withstand the corroding effects of sulphate of alumina or sulphate of iron, which energetically attacks and destroys iron.