Tertiary filtration of wastewaters

Tertiary filtration is used widely to remove suspended solids remaining in effluents from secondary biological wastewater treatment processes. A variety of filter configurations are available and the design engineer needs to be familiar with the parameters affecting filter design and operation. Typical tertiary filtration system options include single- or multiple-media filter beds, shallow or deep beds, and pressure or gravity operating modes. Basic design parameters include media size and type, filtration rate, and characteristics of the influent wastewater solids. Treatment efficiency is, to a large extent, affected by the characteristics of the suspended solids, while operating requirements are related closely to the design of the backwashing system. A number of operating problems - poor efficiency, short run lengths, loss of media, and mudballs in the filter bed - occur frequently because of improper design or inadequate operating flexibility. Costs of filtration systems are related to filtration rate, pumping requirements, chemical use, and the need to retreat dirty backwash water.