CORRECTING ACTIVATED SLUDGE INHIBITION BY ADDITION OF TRACE AMOUNTS OF COPPER

A large brewery wastewater treatment plant was upgraded/expanded by installation of a high rate anaerobic pretreatment process ahead of the existing activated sludge process. All of the brewery wastewater is now pretreated anaerobically before final secondary treatment in the activated sludge system. The activated sludge system operated and performed well for many years prior to start-up of the anaerobic pretreatment system. However, since the start-up of the anaerobic pretreatment system, the activated sludge system has experienced long term (several years) chronic inhibition symptoms. The primary symptom of the inhibition was recurring episodes of increased effluent TSS coinciding with proliferation of flagellated protozoa. The inhibitory condition was treated by increasing the target MCRT which resulted in MLSS increasing from around 6,000 to over 17,000 mg/l over a seven (7) year period (1992–1999). Flagellate blooms increased with lower wastewater temperatures and decreased with warmer temperatures and the corresponding MCRT adjustments resulted in a sinusoid trend in the MLSS which varied seasonally. However, each year the required MLSS levels were greater than during the preceding year and were reaching the point where concerns arose for clarifier performance and operation. An investigative program was initiated to determine the cause of the inhibition and implement corrective action. The investigation initially searched for chemicals in the process wastewater stream which have known inhibitory characteristics, but none were found. All inorganic elements and nutrients required for bacterial growth were determined to be present except for copper. Copper deficiency was suspected and confirmed based on the symptomatic nature of the operational trend. On an experimental basis full scale addition of copper sulfate at 14 ppb was begun in April 1999. The trend reversed and over six (6) months, the MLSS was lowered to 1994 levels. Copper addition has continued, and the activated sludge treatment system remains stable with good clarifier operation and performance.