(University of Cape Town)

 

Cape

 

 

Research at the University of Cape Town by Marais’ research group in the seventies and eighties was the stimulus for the development of this process configuration as an attempt to resolve the problems of reduced P removal efficiencies due to nitrate return to the anaerobic zone. The innovation of this process was the direction of the return sludge to the primary anoxic zone, with a withdrawal of the denitrified mixed liquor from this zone and the return to the anaerobic zone.
This process is reported to operate with TKN/COD ratios of up to 0.11-0.14, which is above that seen in most domestic wastewaters.
The UCT process is usually operated at sludge ages up to 25 days. The ‘a’-recycle must be carefully controlled to just underload the primary anoxic basin with nitrate to avoid a nitrate discharge to the anaerobic basin. Under full- scale operation such careful control of ‘a’-recycle is difficult due to uncertainty in the TKN/COD ratio.
As the TKN/COD ratio increases, the ‘a’-recycle ratio needs to be decreased to avoid a nitrate discharge to the anaerobic basin, which in turn causes an increase in the actual anoxic retention time.  For influent COD > 500 mg/L and TKN/COD ratios > 0.11, the actual anoxic retention time exceeds 1 hr, which reportedly causes a decline of sludge settleability.
The ‘a’ recycle flow is tightly controlled to just underload the primary anoxic basin with nitrate, to avoid a nitrate discharge to the anaerobic basin.
In full-scale plants, control of the ‘a’ recycle is difficult due to variations in the TKN/COD ratio.  As the TKN/COD ratio increases, the ‘a’ recycle ratio needs to be decreased to avoid a nitrate discharge to the anaerobic basin, which in turn causes an increase in the actual anoxic retention time.
For influent COD’s > 500 mg/L and TKN/COD ratio’s > 0.11, the actual anoxic retention time exceeds 1 hr, causing the decline of sludge settleability.  Limit the anoxic retention time to 1 hr to improve sludge settleability.
Typical Influent TKN/COD: 0.11 to 0.14
Note: A variation by HRSD and CH2MHILL is also known as the Virginia Initiative Process (VIP), and typically operates with sludge ages of 4 to 7 days depending on temperature.