..------,-----.----,-------,------.----,

The first thing to note in Figure 7.6 is that the MLSS concentration has little tank-to-tank variability. The same is true for the heterotrophic and autotrophic biomass concentrations, although they are not shown. This lack of change is because the SRT is large relative to the HRT, causing the biomass recycle to make the mixed liquor homogeneous, as discussed previously. A decline in the MLSS concentration as the solids move down the chain would only occur when the HRT approaches the SRT.