The flow rate and concentration of wastewater are variable during the course of the day, and also depend on the time of year. If the flow rate is too high, loss of microorganisms by washout may occur in secondary treatment processes. If the flow rate is too low, the lack of nutrients leads to a reduction of microorganism population. Wastewater entering a treatment plant usually flows first into a basin (a large agitated tank). Accordingly, the flow rate out of the basin is maintained at a constant level or between prescribed limits in order to keep subsequent processes within prescribed limits. The equalization tank also reduces the effect of toxic shocks on the biological processes within the main treatment plant.