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i Technical Descripfion G. Comminution or screening and, where required, grit removal must precede the aeration/equalization chamber. H emergency overflow from the chamber may be provided. H. An emerge y Y TERTIARY FILTERS � Tertiary filters are normally designed to accept a suspended solids loading of 20-30 mg/L. Occasional shock loads can be handled; however, the filter will not function properly at continuous suspended solids loadings of 50 mg/L or greater. Therefore,design and operation of the secondary plant become an important aspect in the overall efficiency of any tertiary system. If the secondary treatment plant is operated to produce an effluent with a suspended solids concentration ofless than 30 mg/L,a properly maintained,designed,and efficiently operated filter can be expected to remove 95-98%, or produce a resultant final effluent suspended solids concentration of 2-5 mg/L.Bio-Pure tertiary systems have consistently produced final effluent with both BODS and suspended solids in the undetectable (ND) range. AquaClear recommends the use of DynaSand continuous-backwash filters wherever possible. Although their initial cost is higher than a standard pressure filter, cost of ownership over a period of time is considerably less, as the DynaSand has no moving parts, it is virtually maintenance-free, and produces a somewhat more consistent output. DynaSand filters are approved for California Title 22 Standards for Wastewater Reclamation. When using a single pressure filter, an important design consideration is the ability of the filter.to accumulate sufficient solids in the media during extended filter runs to eliminate the need for frequent backwashes.Frequent backwashes,which must be accomplished using a freshwater source,not only consume energy and plant operator's time, but impose additional hydraulic loading requirements on the secondary process if the backwash is directed to the aeration chamber. Because of this additional hydraulic loading,AquaClear recommends the use of dual filters which are designed to use the filtrate of one filter to backwash the other. Since the filtrate is already part of the system hydraulic loading, nothing is added. In order to obtain the desired solids holding capability, the media must allow r penetration into the depth of the bed rather than simply retaining solids on the media surface. L Conventional filter sands, as used in potable water filtration, provide only this surface removal and retention effect. This is satisfactory for the low solids concentrations involved in freshwater filtration. However, in wastewater filtration, solids concentrations are significantly higher and tend to rapidly seal the surface. During backwash of conventional filter sand media, another phenomenon occurs which contributes to this surface straining effect.When the bed is fluidized,the less dense,smaller grain-size media tends to accumulate as the top layer in the bed. This provides for efficient particle removal during filtration, but precludes any penetration beyond the surface layer. A more efficient media system incorporating varied media layers is recommended. 17