(a) Diatomite-type filters shall be designed for operation under either pressure or vacuum. The design capacity shall not exceed three gpm per square foot of effective filter area for pressure filters and shall not exceed two and one-fourth gpm per square foot of effective filter area for vacuum filters.
(b) The determination of the filter area shall be made on a basis of a true and effective supported septum surface. In the case of fabric septums, the area computation will be made on the basis of measurements of the septum support in a reasonably constant plane. Area allowance shall not be granted for folds in the septum fabric or deviations in the septum surface which would easily bridge.
(c) The tank containing the filter elements shall be constructed of steel, plastic or other suitable material, which will satisfactorily provide resistance to corrosion, with or without coating. Pressure filters shall be designed for a working pressure equal to the shut-off head of the pump, with a factor safety of four. Vacuum filters shall be designed to withstand the pressure developed by the weight of the water contained therein and closed vacuum filters shall, in addition, be designed to withstand the crushing pressure developed under a vacuum of twenty-five inches of mercury, both with a factor safety of 3.5.
In either type filter where the tank is fabricated from material which is corroded by water, a suitable corrosion resistant coating shall be applied to retard such attack.
(d) The filter elements shall be fabricated of corrosion resistant materials throughout. They shall be designed to be adequately resistant to a differential pressure between influent and effluent of not less than the maximum pressure which can be developed by the circulating pump and shall be of adequate strength to resist any additional stresses developed during the cleaning operation. The filter septum, on which the filter-aid cake is deposited, shall be provided with openings, the minimum dimensions of which shall be not greater than 0.005 inches.
(e) Provision shall be made to introduce filter aid into the filter in such a way as to evenly precoat the filter septum before the filter is placed in operation. The amount of filter aid shall be selected to provide at least the same protection to the filter septum as would be provided by the use of 0.1 of a pound of diatomite filter aid per square foot of filter area. Use of additional equipment to provide body feed to the filter influent is optional. Where provided, such equipment shall have the capacity to continuously and uniformly feed at a rate of not less than 0.1 of a pound of filter aid per square foot of filter area per twenty-four hours.
(f) The filter piping shall be so designed that, during the precoating operation, the effluent from the filter shall be recirculated or run to waste and shall not be fed into the pool until free of filter aid. An exception to this requirement may be made if the filter septum is of such construction that no perceptible quantity of filter aid is introduced into the pool when the effluent discharges therein.
(g) Where dissimilar metals are used in the construction of the filter, which may set up galvanic currents, then suitable provision shall be made to resist electrolytic corrosion.
(h) Filters shall be so designed and installed that they can be readily disassembled and the filter elements removed.
(Ord. B-751. Passed 7-9-62.)