compost station this is done to remove excess bio mass that has grown during the various phases fill mix and react phases so as to maintain the correct amount of the biomass in the tank 40 the waste phase can take place near the end of the decant phase or during the idle phase when the bio mass layer at the bottom of the tank has reached its highest concentration the idle phase is not needed as an operational phase but is rather than period between completion of the decant phase and the beginning of the next fill phase the idle phase only occurs when actual flows are substantially less than the design flows during an extended idle phase it may become necessary to periodically aerate the mixed liquor to prevent it from becoming anaerobic the various fill mix and react phases the settle phase and the waste phase are controlled on a time basis and the time spent in each phase can be readily adjusted to suit required operating conditions by reprogramming the microprocessor 21 typically an entire cycle will last several hours and 3 4 cycles can be performed per day the actual time of the decant phase is usually controlled by the liquid level in the reactor tank 40 following completion of the various fill mix and react phases the liquid level in the reactor can also be readily changed to suit required operating conditions one of the factors which must be taken under consideration in the process performed in the reactor 22 is the food to microorganism f m ratio this is the ratio of the weight of the organic pollutants available in the wastewater to the weight of the bio.