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Biological Filtration

Figure 1:.  Ozone as a pre-oxidant with biological filtration.
Figure 1:. Ozone as a pre-oxidant with biological filtration.

Biological filters remove contaminants by three main mechanisms: biodegradation, adsorption of micropollutants, and filtration of suspended solids. The microbial growth attached to the filter media (biofilm) consumes the organic matter that would otherwise flow through the treatment plant and ultimately into the distribution system. The end products are carbon dioxide, water, biomass, and simpler organic molecules. Particle filtration takes place on the bare filter media as well as the biofilm. In biofilters used for biological denitrification, nitrate is converted to nitrogen. In this case, microorganisms are fed a form of carbon, and they use nitrate as an electron acceptor in place of oxygen. Granular activated carbon (GAC) is often used to provide the necessary surface to promote the development of the biofilm.

The conventional treatment overview discusses coagulation, flocculation and clarification preceding biofiltration.

Biofiltration is often used by systems that use ozone as adding a strong oxidant converts some of the total organic carbon (TOC) to biodegradable dissolved organic carbon (BDOC). To promote biological activity ozone is added upstream to the filter beds. Ozone may be applied prior to rapid mix, as shown in Figure 1. It may also be applied to prior to the biofilter.

Biologically active carbon filters are typically used in place of conventional filtration, as a biologically active layer in dual media filters, or downstream of conventional (or membrane) filters. In the first configuration, conventional filter media is replaced with activated carbon which performs biodegredation and particle removal. In the second configuration, the top layer (activated carbon) of a dual media filter provides biodegradation and some particle removal and additional particle removal is achieved in the sand layer. In the third configuration, particle removal is achieved by the conventional (or membrane) filter and the biologically active GAC column is used as polishing step.

In all cases, filter sloughing can be a problem leading to possible taste and odor problems and release of bacteria in the finished water. This problem is of greater concern for smaller systems that use biofilters for denitrification, and may compromise proper secondary disinfection.

Key factors in controlling biofilter performance include temperature, contact time, backwash operations, and water quality parameters like pH, alkalinity, turbidity and BDOC. The water quality parameters impacting performance are the pH and BDOC and dissolved oxygen concentrations. In particular, temperature controls biogrowth kinetic. Biological activity is high during the summer when temperature is above 20 °C and decreases during the cooler months with relatively insignificant activity during the winter (10 °C and below). Filter loading rates are similar to those used in rapid sand filtration (2 to 4 gpm/sf) .

Pretreating the water to prevent fouling of the GAC and controlling microbial activity are other important aspects of biological filtration. High concentrations of calcium carbonate can lead to the blockage of GAC pores. Iron and manganese precipitation may also adversely affect adsorption. If the ammonia concentration is high, pretreatment should be used to reduce it. Lastly, the system must be kept aerobic to avoid taste and odor problems, and proper controls should be implemented to ensure that the oxygen supply is sufficient at all times.

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