4stages

 

This process was developed in an attempt to completely denitrify the process effluent by the inclusion of a secondary anoxic zone after the aerobic basin to supplement the denitrification capacity of the primary anoxic zone.  The secondary anoxic zone is able to remove small amounts of nirate due to the slow rate of denitrification available (energy for denitrification is provided by small amounts of organic carbon produced endogenously from death and lysis of bacteria).
Operationally TKN/COD ratios of less than 0.10 are required to achieve low levels of effluent nitrogen. The small aeration basin (flash aerator) at the end of the process is necessary to remove small amounts of ammonia produced in the secondary anoxic zone and to return the sludge to the aerobic condition prior to clarification, which minimizes the risk of sludge blanket denitrification and disruption of the settlement process.
Anoxic retention times vary depending upon a number of parameters but while primary anoxic retention times are usually reported at 1-3 hrs, secondary anoxic retention times between 2-4 hrs are common.
The process should be operated to provide the lowest amount of nitrate in the return sludge possible, unless there is a large amount of RBCOD in the incoming wastewater. Operationally TKN/COD ratios of less than 0.10 are required to achieve low levels of effluent nitrogen.
The small aeration basin (flash aerator) at the end of the process is necessary to clean up ammonia and condition (make aerobic) the mixed liquor by raising its redox potential prior to secondary clarification.  Another function of flash aeration is to remove any bubbles of nitrogen associated with the floc Рan issue often caused by the rapid denitrification arising with methanol dosing.
Anoxic retention times vary depending upon a number of parameters but while primary anoxic retention times are usually reported at 1-3 hrs, secondary anoxic retention times between 2-4 hrs are common.