An activated sludge system treating domestic wastewater is operated at a solids retention time of 10 d with a mixed-liquor temperature of l8C. For many weeks nitrification has occurred, with an effluent NH4-N concentration reported at less than 1.0 mg/L. After some time, the nitrification performance declines with effluent NH4-N concentrations exceeding 10 mg/L. As the city engineer you are requested to investigate the cause of the decline in performance and to make recommendations for actions that will get the discharge quality back in compliance. Describe possible causes for the decline in nitrification efficiency and how you would evaluate the problem.