The process diagram for a secondary treatment wastewater plant might look something like:
Suppose such a regulatory agency imposed new restrictions on effluent of this plant that mandated denitrification. What kind of unit operation would be added as tertiary treatment? First, recall that denitrification is an anaerobic process. The microbes that accomplish nitrification are mostly facultative. When placed in an anaerobic environment, they are able to use NO2 and NO3 instead of O2 as their terminal electron acceptor. In the presence of a carbon source, the net effect is denitrification.
A rapid sand filter mostly immersed in water provides an anaerobic environment and a media fot the denitrifying bacteria to grown on. Effluent from the secondary treatment process flows to the top of the filter, and as the fluid works its way through the filter the desired result are achieved. A carbon source (almost always methanol) is added to the system for microbial nutrition. To release any N2 that is trapped, the filter is periodically "bumped" by sparging oxygen to the bottom of the filter. Like any depth filter, the denitrifying sand bed requires backwashing on a regular basic. The backwash, which contains sloughed off microbial film, is recycled to the headworks of the plant where treatment begins.
The new nitrification-denitrification flow diagram might look something like this:
Note that denitrification precedes disinfection. This arrangement is not only required from a public health standpoint, it is also generally a boon to the efficiency of the disinfection process. Disinfection is inhibited by the formation of chloramines, which are compounds resulting from the interaction between nitrates and chlorine.
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