Variability of Waste Treatment Plant Performance

Time series techniques are used to analyze the variability in waste treatment plant performance of eight municipal plants. Statistical analyses of the data indicate a range in the coefficient of variation of secondary effluent BOD from 0.2 to 1.1. The coefficient of variation tends to increase as plant treatment levels increase. Power spectrum analyses show the presence of five-day-on and two-day-off oscillations for several plants. For one plant, the amplitude of this weekly oscillation is about 20% of the mean load. Cross-spectral analyses indicate a tendency for plug flow conditions across the primary treatment step. Coherencies are significant between raw influent and primary effluent but are insignificant between raw influent and secondary effluent indicating some statistical independence. Linear system analyses permit estimation of effects of this load variability on stream quality. Inputting the waste load discharge spectrum into a nondispersive stream indicates that DO variations of 1.0 mg per l appear possible in medium-size rivers (flow = 1,000 mgd).