Modelling the degradation of endogenous residue and ‘unbiodegradable’ influent organic suspended solids to predict sludge production

Abstract

Activated sludge models have assumed that a portion of organic solids in municipal wastewater influent is unbiodegradable. Also, it is assumed that solids from biomass decay cannot be degraded further. The paper evaluates these assumptions based on data from systems operating at higher than typical sludge retention times (SRTs), including membrane bioreactor systems with total solids retention (no intentional sludge wastage). Data from over 30 references and with SRTs of up to 400 d were analysed. A modified model that considers the possible degradation of the two components is proposed. First order degradation rates of approximately 0.007 d(-1) for both components appear to improve sludge production estimates. Factors possibly influencing these degradation rates such as wastewater characteristics and bioavailability are discussed.

The effect of different aeration conditions in activated sludge – Side-stream system on sludge production, sludge degradation rates, active biomass and extracellular polymeric substances

“…Standard microbial growth yields were considered for heterotrophs (Y OHO = 0.67 244 mg COD mg COD -1 ) and autotrophs (Y ANO = 0.24 mg COD mg NO 3 -N -1 or mg COD mg N 2 -245 N -1 ) (Henze et al, 2000). A conversion factor f CV,X,Bio of 1.42 mg COD mg VSS -1 was 246 considered (Sperandio et al, 2013). …”

Net biomass production under complete solids retention in high organic load activated sludge process

“…Consequently, at PCMAS steady state operating conditions with high HRT and complete SRT, the fact that the observed relative constant MLVSS concentration after 180 days and that the net biomass growth tends to zero, lead to the conclusion that all organic matter, including cell debris and influent unbiodegradable compounds, is biodegradable (Xing et al, 2003;Sperandio et al, 2013). This conclusion does not contradict the prevailing view which considers cell debris non biodegradable (Metcalf and Eddy, 2003) for typical SRT values.…”

“…The first one (classic) takes into account the endogenous respiration concept and the second one (modified) adds an additional concept of X E and X U degradation, as a first order kinetic reaction. The values used for the simulation runs were from literature and ASMs default values presented in Sperandio et al (2013) work.…”

“…Studies conducted on membrane bioreactors operating at SRT up to complete retention of solids indicated that biomass (active and endogenous residue) production was equilibrated by biomass decay or that influent substrate was completely oxidized for cell maintenance (Sperandio et al, 2013). Other studies point out a need to introduce in ASMs a significant degradation of X U and X E fractions at a rate dependent on the influent composition and the WWTP operating conditions, demonstrating that zero net biomass growth can be achieved (Lubello et al, 2009;Fenu et al, 2010;Ramdani et al, 2012;Sperandio et al, 2013).…”

“…Other studies point out a need to introduce in ASMs a significant degradation of X U and X E fractions at a rate dependent on the influent composition and the WWTP operating conditions, demonstrating that zero net biomass growth can be achieved (Lubello et al, 2009;Fenu et al, 2010;Ramdani et al, 2012;Sperandio et al, 2013).…”