“Solids yield surprise” [June] presented results of calculated solids yields and biochemical oxygen demand (BOD) loadings from five trickling filter and activated sludge wastewater treatment systems with various solids retention times (SRTs). The relationship was counterintuitive because smaller BOD loadings seemed to produce higher solids yields.
The authors proposed three hypotheses for the phenomenon. Two of these related specifically to trickling filters (biomass sloughing from filters dampens the solids production, and anaerobic biogrowth on filter media lower solids production) and one related more generally to influent total suspended solids (TSS) loading (influent TSS/BOD ratio decreases when BOD loading increases). The authors found that biomass sloughing and TSS/BOD ratio hypotheses were not supported by data and need to be rejected. The anaerobic growth hypothesis did not have any supporting data.
It is my opinion that anaerobic growth hypothesis is unlikely the cause of the observed phenomenon because anaerobic conditions always exist in the deeper layers of the biofilm regardless of BOD loading. I venture to give it another explanation.
In a wastewater treatment system, observed BOD loading to the plant fluctuates because of influent BOD concentration changes or sampling and measurement errors.
The SRT of the wastewater treatment process may be kept constant in the midst of the daily BOD loading fluctuations. With the left side of the equation shown above being constant, the right side yield must increase when BOD loading decreases, given stable mixed liquor concentrations, and vice versa.
The mixed liquor TSS concentration stays stable because a large portion of it is inactive volatile suspended solids (VSS) and nonvolatile TSS. The VSS increase due to loading increase may not be significant given the large portion of mixed liquor suspended solids (MLSS) being inactive. The measurement error of mixed liquor TSS also could mask the small biomass growth because of BOD loading increases.
In addition, the biological system might not be in a steady state. As a result, increased BOD loading may not correspond to its steady state MLSS concentration, given specific SRT. This would lead to the condition in which BOD loading increases while MLSS concentration lags behind.
Another cause could be sampling and measurement errors of influent flows and BOD concentrations that result in calculated BOD loading errors. Normally, it is difficult to accurately attribute the significant influent BOD variation to portions because of real loading variations and sampling and measurement errors. It is almost certain that significant portions could come from the latter. This fictitious BOD loading change certainly would not affect the SRT or MLSS concentration, but it would affect the yield calculated, with higher BOD loading corresponding to lower yield, because more BOD was calculated as “removed” than is accurate.
If the above hypothesis is true, then the cause of the observed phenomenon — higher BOD loading that results in smaller solids yield — should not be restricted to wastewater treatment plants with trickling filter and activated sludge system combinations. It should apply to activated sludge treatment systems as well.
A series of graphs comparing calculated solids yields and BOD loadings for the San Jose/Santa Clara (Calif.) Water Pollution Control Plant shows inverse relationships such as those described in the article. These graphs support the argument that this phenomenon also can occur in an activated sludge treatment system.
We probably could conclude that the observed yield and BOD loading relationship might be an artifact rather than a fundamental biogrowth mechanism.
San Jose/Santa Clara (Calif.) Water Pollution Control Plant