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l 1 [ 1 11-3 SINGLE-PASS (INTERMITTENT) PACKED-BED FILTERS .733 The head on orifice n is equal to 12_ i hn — 1(2.45CD2)22g gn = kqn = k(mg,)Z = rnzkgi -- mZh, I where k = constant q, = discharge from first orifice,gal/min qn = discharge from the nth orifice, gal/min h, = head on first orifice, ft m = constant with decimal value less than 1.0 The head lost between orifices 1 and n, which corresponds to the headloss in the distribution pipe between orifices 1 and n, computed from Eq. (11-8), is Ah(,-,) = hfdp = ht — hn (11-9) The difference in discharge between orifices 1 and n, for a given distribution pipe aand orifice size, can now be determined by using Eqs. (11-6) through (11-9). If the computed value of m is too low(e-g., less than 0.95 or some other acceptable value, typically 0.95 to 0.90),the size of the distribution pipe can be increased.Use of these equations is illustrated in Example 11-5. Additional details on pressure dosing may be found in Ball (1996). It should also be noted that it is important that the.orifices be drilled with a drill press. Often,contractors will use a hand-held drill to drill the a orifices in the field,which usually will result in orifices with large enough differences in diameters to cause poor distribution (Ball, 1996). i Construction Considerations. j Because contractors often lack adequate training in the'proper installation of ISFs; the engineer or designer should provide inspection to ensure that proper materials and construction techniques are used. Important construction considerations include i sand cleanliness, the use of proper liner material, proper placement and assembly of the individual components, and proper construction and testing of the distribution i manifold. The sand in an ISF is usually retained by a PVC liner (see Fig. 11-15). A liner thickness of 30 mil (0.76 mm) is recommended as being tough enough to withstand handling during construction, and is easily repaired if a puncture occurs. Heavier liners are difficult to work with and do not offer improved performance. PVC liners can be patched, much like an inner tube, with cement available from liner suppliers (Ball, 1991). The easiest approach to the construction of ISFs is to purchase a kit containing all the necessary components for a predetermined size of filter from a supplier. k t Operation and Maintenance f Properly sized intermittent sand filter systems require some limited operation and maintenance_ Routine monitoring involves the periodic checking of the various I counters and elapsed-time meters. Noting the elapsed time the effluent pump has been on is extremely helpful in identifying the presence of leaky fixtures (e.g., the J