The amount of coagulant which can be safely used is dependent upon the alkalinity of the raw water. When sulphate of alumina is added to water it is decomposed, as explained above, with the formation of alumina, which is alone useful in the work of purification, and sulphuric acid, which combines with the calcium carbonate or lime present in the water. There should always be an excess of alkalinity or lime in the raw water. If for any reason there is not, there is nothing to combine with the liberated sulphuric acid, and the decomposition of the coagulant is not complete, and a portion of it goes undecomposed into the effluent. The effluent then has an acid reaction, and is unfit for domestic supply. When distributed through iron pipes, it attacks the iron, rusting the pipes, and giving rise to all the disagreeable consequences of an iron containing water.
The amount of lime in a water available to combine with the sulphuric acid can be determined by a very simple chemical operation, namely, by titration with standard acid with a suitable indicator. The amount of coagulant corresponding to a given quantity of lime can be readily and accurately calculated, but it is not regarded safe to use as much sulphate of alumina as corresponds to the lime. The quantity of coagulant used is not susceptible to exact control, but fluctuates somewhat, and if the exact theoretical quantity should be employed during 24 hours, there would surely be an excess during some portion of that time from which bad results would be experienced. It is therefore considered only prudent to use three quarters as much sulphate of alumina as corresponds to the lime in the water. With sulphate of alumina containing 17 per cent of soluble aluminum oxide and the corresponding amount of sulphuric acid, the amount which can be applied to a water in grains per gallon is slightly less than the alkalinity expressed in terms of parts in 100,000 of calcium carbonate.
Many waters contain sufficient lime to combine with the acid of all the coagulant which is necessary for their coagulation. Others will not, and it thus becomes an important matter to determine whether a given water is capable of decomposing sufficient coagulant for its treatment. It is usually the flood-flows of rivers which control in this respect. The water at such times requires much larger quantities of coagulant for its clarification, and it also usually contains much less lime than the low-water flows. The reason for this is obviously that the water of the flood-flows is largely rain-water which has come over the surface without coming into very intimate contact with the soil, and consequently without having taken from it much lime, while the low-water flows contain a considerable proportion of water which has percolated through the soil and has thus become charged with lime.
In some parts of the country, as, for instance, in New England, the soil and underlying rock are almost entirely free from lime, and rivers from such watersheds are capable of receiving only very small quantities of coagulant without injurious results.
The deficiency of alkalinity in raw water can be corrected by the addition to it of lime or of soda-ash. Lime has been used for this purpose in many cases. When used only in moderate amounts it hardens the water, and is thus seriously objectionable. The use of so large a quantity as would precipitate out, as in Clark’s process, has not been employed in practice. If it should be attempted, the amount of lime would require to be very accurately controlled, and the effluent would have to be treated with carbonic acid to make it suitable for supply.
Waters so hard as to require the use of the Clark process almost always have sufficient alkalinity, and do not require to be treated with lime in connection with the use of sulphate of alumina.
The use of soda-ash is free from the objections to the use of lime, but is more expensive, and would require to be used with caution. Its use has often been suggested, but I do not know that it has ever been employed in practice. In small works the use of a filtering material containing marble-dust, or other calcareous matter, would seem to have some advantages in case of deficiency of alkalinity, although it would harden the water so treated.
The alkalinities of a number of waters computed as parts in 100,000 of calcium carbonate (approximately equal to the safe doses of sulphate to alumina in grains per gallon) are as follows:
| Maximum. | Minimum. | Average. | |
|---|---|---|---|
| Boston water, 1898 | 2.87 | 0.33 | 1.08 |
| Conestoga Creek, Lancaster, Penn. | 12.20 | 3.70 | 6.80 |
| Allegheny River, Pittsburg | 8.00 | 1.02 | 2.90 |
| Mahoning River and tributaries, 1897 | 20.00 | 2.20 | 10.00 |
| Scioto River and tributaries, 1897 | 35.00 | 10.00 | 20.00 |
| Ohio River, Cincinnati, 1898 | 7.00 | 2.00 | 4.50 |
| Ohio River, Louisville | 10.87 | 2.12 | 6.70 |
| Lake Erie, Lorain, Ohio | 9.50 | ||
| Lake Michigan, Chicago | 11.50 |