The F/M ratio is dependent on the Solids Retention Time (SRT). As SRT increases (older sludge), the F/M decreases. SRT is calculated by taking the total mass of activated sludge inventory (lb or kg) and dividing by the mass of activated sludge wasted from the system each day. SRT is usually in the range of 4 to 10 days for conventional activated sludge systems and the F/M will be between 0.2 and 0.4 lb BOD5 per lb MLVSS per day. These systems are not designed to remove ammonia, but often do when water temperatures are warm and/or when the systems are lightly loaded. Systems that must remove ammonia need longer SRTs to give the nitrifying bacteria time to grow in the system. When water temperatures are at 10 oC (50 oF), the minimum aerobic SRT will be 12 to 16 days. The aerobic SRT calculation only includes the MLSS in the aerated zones of the activated sludge process. When water temperatures are at 20 oC (68 oF), the minimum aerobic SRT will be 4 to 6 days. F/M ratios in extended aeration systems tend to be lower -- in the range of 0.05 to 0.10 lb BOD5 per lb MLVSS per day. Remember though, F/M is determined by SRT and should not be used to control the process by itself.