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AquaClear recommends the use of DynaSand continuous-backwash NOTES 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, is virtually maintenance-free, and produces somewhat higher quality output. DynaSand filters are approved for California Title 22 Standards for Wastewater Reclamation. In systems where pressure filters are used,the following applies: An important design consideration in a tertiary suspended-solids removal filter 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 not only consume en- ergy and plant operator's time, but impose additional hydraulic load- ing requirements on the secondary process. In order to obtain the desired solids holding capability, 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 concentrations involved in freshwater filtration. How- ever, in wastewater filtration, solids concentrations are significantly higher and tend to rapidly seal the surface of conventional filter sands. During backwash of conventional filter sand media, another phe- nomenon 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 pene- tration beyond the surface layer. A more efficient media system incorporating varied chemical mate- rial (media) layers is recommended. The different materials are conventional filter sands with different specific gravities plus anthracite coal. The denser filter sands are placed below the coarser but less dense coal, thus allowing deeper penetration in the direction of flow and maintaining a coarse surface layer even after backwash.This filter media design concept is termed multimedia. Fifty to seventy percent of the secondary effluent BODS concentra- tion can be removed in the tertiary filter.The BOD that is entrapped in the suspended solids is removed. Filtration has no measurable effect on the removal of soluble BOD (10 microns or less). The most important consideration in the sizing of tertiary filters, and one requiring considerable engineering judgment, is selection of the application rate, which is expressed as influent gallons per minute applied per square foot of filter bed area. 19