How does increasing Waste Activated Sludge (WAS) in the aeration tank affect the system?

Increasing Waste Activated Sludge (WAS) in the aeration tank primarily affects the Sludge Retention Time (SRT), Food to Microorganism ratio (F/M), Mixed Liquor Suspended Solids (MLSS), and Mixed Liquor Volatile Suspended Solids (MLVSS) in specific ways.

When WAS is increased, it typically means that more sludge is being removed from the system. Since SRT is defined as the average time that activated sludge remains in the system, removing more sludge implies that SRT decreases. This is because the amount of time solids are retained in the treatment system is reduced due to the increased volume of waste being removed.

The F/M ratio, which indicates the amount of food (organic matter) available per unit of microbial mass, tends to increase when more WAS is taken out. As the microbial population decreases due to higher wastage, the available biomass for the same amount of incoming organic load decreases, leading to an increase in the F/M ratio.

Regarding MLSS and MLVSS, increasing WAS generally reduces MLSS, as it directly corresponds to a decrease in the total suspended solids in the aeration tank. When more solids are withdrawn, the concentration of mixed liquor declines. MLV