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Due to the nature of backwash operations, SFBW is generated sporadically. Consequently, even without SFBW treatment, flow equalization prior to recycle to the main process is desirable in order to avoid contributing flow surges which could disrupt the main filtration process. Flow equalization becomes essential if SFBW treatment is involved prior to recycle. This was investigated in Cornwell et al. (2001). Therefore, while flow equalization is not “treatment” per se, application of proper flow equalization is a recommended, if not essential, preparatory process prior to recycle to the main treatment system or prior to SFBW treatment preceding recycle. In particular, in the latter case when SFBW treatment is used, equalization provides the additional benefit of minimizing the size of SFBW treatment facilities needed, thereby minimizing cost of these treatment processes as well.

Furthermore, in addition to equalization of flow, equalization of particulates and contaminants is also important prior to recycle. Without mixing in equalization basins, particulate material can settle out in the basin. If this particulate material builds up then each succeeding backwash event can stir up this settled material and it can create sporadic spikes of particulate content in the equalized SFBW flow leaving the basin. Consequently, a SFBW equalization basin not only needs to function to reduce hydraulic spikes, it also needs to reduce or eliminate fluctuations of particulates and other contaminants. This latter function can best be achieved by keeping equalization basins well mixed. The case studies described in the report include instances where temporary mixing was provided to existing basins that do not have permanent mixing. Some of these temporary mixing facilities did not function sufficiently well to normalize contaminant spikes. However, permanent mixing facilities can be designed and operated to function more satisfactorily.

For example evaluations of required EQ capacity, see the following sections in the Preliminary design report in “Case Studies” for “Ohio”:

  • “SFBW Equalization Design Assumptions”
  • “SFBW Treatment Design Assumptions”, and
  • “Option A: Additional Equalization Capacity”