Study of High‐Rate Filtration at the Dalecarlia Plant, Washington

FILTRATION of water at high rates has received continued attention over the past decade, with increasing interest being manifested in the last few years. A brief review of recent literature on the subject indicates that there is much to support a raise in design rates from the normally accepted value of 2 gpm per square foot. Baylis (1) stated in 1949 that, assuming there is no sacrifice of water quality, a reduction in filter cost can be gained by operating at elevated rates, as the filter size can be reduced. He has substantial data (2) to show that excellent water quality can be maintained at high rates through the use of floc-strengthening aids and close control of water treatment. Economy may be gained through reduction in the size of filters but not necessarily in the effluent piping and the rate controller, where considerable head is lost at high rates. These appurtenances should be carefully designed if high rates are to be used. Hazen (5), in a 1953 discussion of the elements of filter design, also placed emphasis on the necessity for good control of pretreatment and adequate supervision at high rates. He pointed out that the chances of providing such supervision in smaller plants, or in plants where sudden changes in rawwater quality are likely, were not good, in general, and that the safety factor involved was much lower than elsewhere. The fact that heavy dosages of carbon, which are necessary in some plants for taste and odor control, may pass to the clear well is a deterrent to operation at high rates. In such circumstances, it might even be found preferable to design more filters at the conventional rate of 2 gpm per square foot. In a review of the data on several recently designed filters, Hazen found that higher rates are being used in some plants, particularly the larger ones, while most plants are still holding fairly close to 2 gpm as a normal rating. Cox (4), in a 1953 article on highrate filtration, also stressed pretreatment. Great care should be exercised