Experimental Assessment of the Degradation of “Unbiodegradable” Organic Solids in Activated Sludge

Abstract

In current process models activated sludge consists of biodegradable and unbiodegradable organic fractions. Recent evidence suggests that this approach may not be accurate because some of this "unbiodegradable" material may indeed be degradable. To improve sludge production predictions, it is important to know to what extent the "unbiodegradable" organic fraction is degradable. Assuming that volatile suspended solids (VSS) is a measure of the sum of biodegradable and unbiodegradable organic solids and the integral of the oxygen uptake rate (OUR) is representative of the biodegradable organics, the combination of these measurements can be used to predict the change of unbiodegradable organic solids within an aerobic digestion batch experiment. This procedure was used to estimate degradation rates of "unbiodegradable" VSS between 0.006 to 0.029 d(-1). The advantage of the proposed method is that the degradation rate can be determined directly based on measurements and relies on a limited number of assumptions.

Development of Decay in Biofilms under Starvation Conditions—Rethinking of the Biomass Model

“…It became only evident here, due to the changed viewpoint of a new treatment challenge: intermittent runoff with high organic pollution. However, other investigations with activated sludge show similar findings [22], too. It should be worthwhile to revise the current approaches before extrapolating it to uninvestigated operational conditions.…”

“…The results showed that long starvation periods in the investigated biofilms are linked to a significant lower decay rate of just a tenth of the recommended decay rate used in the ASM [12]. Experiments with activated sludge support these findings [22,23]. Actually, both decay rate and particulate inert fraction are rather a function of nutritional state than a constant.…”

“…These are rather mechanistic assumptions and only valid for systems with rather low load variations. Current research in activated sludge systems proved that both the X E fraction [22] and the decay rate b H [23] are a function of sludge retention time SRT (i.e., function of substrate supply). Due to this observation at extreme conditions of low COD, the question arises if organisms can adapt their decay rate and the assumption of a constant decay rate has to be reconsidered or if the existing ASM has to be extended with an additional very slowly degradable COD fraction (consisting of "hard" external COD and/or cell-internal reserves) and according degradation process.…”

Physiological adaptation of growth kinetics in activated sludge

“…This work is based on an extension of a previous study (Friedrich et al, 2015). A brief description of the used materials and methods will be given here.…”

“…In particular, the model response for sludge production is strongly influenced by the decay rate parameter, where a small decay rate yields a high sludge production. To explain the good predictive capability of the existing models processes like the reduction of "unbiodegradable organics" (X U ) in activated sludge as described by Sp erandio et al, 2013, Ramdani et al (2010, Friedrich et al (2015) and others have to be taken into account.…”