The chemical composition is also practically unaltered. When bleach is used there is a proportionate increase in the hardness but the amount is usually trifling and is without significance. During 1916 when the Ottawa supply was entirely treated with bleach at the rate of 2.7 parts per million (0.92 p.p.m. of available chlorine) the average increase in the total hardness as determined by the soap method was 2.5 parts per million.
When chlorine is added to prefiltered water, as an adjunct to filtration, an increase in the number of gallons filtered per run has been noted at some plants. This increase is not so great with rapid as with slow sand filters but in some instances it has led to appreciable economies.
Walden and Powell[1] of Baltimore, found that the addition of a quantity of bleach equal to approximately 0.50 p.p.m. of available chlorine enabled the alum to be reduced from 0.87 to 0.58 grain per gallon. The percentage of water used in washing the filters was also reduced, from 4.1 per cent to 2.9 per cent, whilst the filter runs were increased on the average by one hour and ten minutes. The net saving in coagulant alone amounted to 30 cents per million gallons.
Clark and De Gage[2] found that the use of smaller amounts of coagulant during the period of combined disinfection and coagulation resulted in an increase of nearly 25 per cent in the quantity of water passed through the filter between washings, and also in a material reduction of the cost of chemicals, which averaged $2.62 per million gallons for combined disinfection and coagulation as against $4.86 for coagulation alone. The water used in these experiments was obtained from the Merrimac River at Lawrence.
The effect of hypochlorite on the reduction of algæ growths on slow sand filters was first noticed by Houston during the treatment of the Lincoln supply in 1905. Two open service reservoirs were fed with treated water and were themselves dosed from time to time. “Previous to 1905 they developed seasonally most abundant growths, but during the hypochlorite treatment it was noticed that they remained bright, clear, and remarkably free from growths” (Houston[3]).
Ellms,[4] of Cincinnati, has also noted the effect of hypochlorite on algæ. When the bleach was added to the coagulated water the destruction of the plankton was not as satisfactory as had been anticipated and it was found that large doses destroyed the coating of the sand particles and rendered the filters less efficient. The use of bleach in the filtered water basin was more successful and cleared it of troublesome growths.
In 1916, during the treatment of the London Supply with bleach (dosage 0.5 p.p.m. of available chlorine), Houston made further observations on this point. The Thames water, taken at Staines, had previously been stored for considerable periods in reservoirs, but this necessitated lifting the water by pumps which consumed large quantities of coal that were urgently needed for national purposes. As a war measure, the storage was eliminated and the water treated with hypochlorite at Staines and allowed to flow by gravitation to the various works where the slow sand filters are situated. The treatment resulted in a marked reduction in the growths of algæ, the reduction in the area of filters cleaned in 1916 (June to September) as compared with 1915 being as follows:
| Filter Works. | Percentage Reduction (Approximate). |
| Grand Junction (Hampton) | 6 |
| Grand Junction (Kew) | 43 |
| East London (Sunbury) | 30 |
| Kempton Park | 33 |
| West Middlesex | 56 |
A portion of this reduction can probably be attributed to the elimination of storage.
Chlorination, by decreasing the load on filter beds, has enabled the rate of filtration to be increased in some cases. This increased capacity, which would otherwise have necessitated additional filter units, has been obtained without any further capital outlay. At Pittsburg (Johnson[5]) the rate of filtration, after cleaning, was increased 250,000 gallons each hour until the normal rate was reached; restored beds were maintained at a 250,000 gallon rate for one week. After the introduction of chlorination it was found possible to increase the rates more rapidly without adversely affecting the purity of the mixed filter affluents.