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r.r Technical Description Criteria: 50,000 gpd flow t 31%= 0.065 MGD 0.065 x 250 mg/L x 8.34 x 1000 ft' = 94.11 CFM 1440 G. Comminution or screening and, where required, grit removal must precede the aeration/ equalization chamber. H. An emergency overflow from the aeration chamber to the buffer tank my be provided. TERTIARY FILTERS Tertiary filters are normally designed to accept a suspended solids loading of 20-30 mg/L. Occa- sional 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 secon- dary 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 con- centration of less than 30 mg/L, a properly maintained, designed, and efficiently operated filter can be expected to remove 95-98.5%, or produce a resultant final effluent suspended solids con- centration of 1.5 to 5 mg/L. Nitro-Raptor tertiary systems have consistently produced final efflu- ent with both BOD, and suspended solids in this range. 7H 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 fre- quent backwashes. Frequent backwashes, Which must be accomplished using a freshwater source, not only consume energy and plant operator's time but also impose additional hydraulic loading requirements on the secondary process if the backwash is directed to the aeration chamber. Because of this additional hydraulic loading, 7H recommends the use of dual filters which are desiened 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 ca- pability, the media must allow penetration into the depth of the bed rather than simply retaining solids on the media surface. Conventional filter sands, as used in potable water filtration, provide only this surface removal and retention effect. This is satisfactory for the low solids concentra- tions 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 contrib- utes to this surface straining effect. When the bed is fluidized, the less dense; smaller grain-size I media tends to accumulate as the top layer in the bed. This provides for efficient particle removal - 16- 7-H Technical Services Group, Inc.