Processing tanks are a special use of an anaerobic treatment tank to increase nitrogen removal in an onsite wastewater treatment system. Processing tanks are used in conjunction with some advanced pretreatment components, such as recirculating media filters.
The processing tank is a combination septic tank, surge storage/equalization tank, pump tank, and recirculating tank. In order for this technology to be successful, the aerobic effluent from the advanced treatment component is recirculated or returned to the processing tank where anaerobic conditions exist.
The ammonia (NH4+) from the processing tank is pumped to the advanced treatment component and converted to nitrate (NO3-) in the aerobic advanced treatment component. This aerobic effluent is recirculated back to the processing tank in order for the anaerobic bacteria to convert it to nitrogen gas (N2) through a process called denitrification.
Nitrogen gas forms 78% of the air we breathe so the vented N2 gas is harmless. In the anaerobic conditions in the processing tank, bacteria that break down the nitrate can thrive and significant denitrification can take place. If there is too much recirculation, the processing tank can turn aerobic, thus inhibiting the conversion of nitrate to nitrogen gas.
If not enough recirculation takes place, the amount of ammonia that is converted to nitrate might not be sufficient and the effluent treatment in terms of BOD5 and TSS removal might be compromised. The O&M for this component is similar to the other anaerobic treatment tanks with the additional step of setting and adjusting the recirculation ratio.