Modelling particle degradation and intermediate dynamics in a dispersed activated sludge microcosm

Municipal wastewater consists of a large fraction of particulate organic matter. During biological wastewater treatment these particles undergo extracellular depolymerisation before products are taken up by bacteria (MW < 0.6 kDa). Particle degradation and intermediate formation dynamics is important in process analysis of wastewater treatment as the transport regime differ. This work aims to develop a model for particle degradation that includes intermediate dynamics as observed in experimental work. A model for particle degradation including intermediate dynamics, bacterial growth and endogenous respiration is proposed. Particle hydrolysis was modelled using the particle breakup model. Depolymerisation products were separated into five different size groups: colloids; high, medium and low molecular weight (HMW, MMW and LMW) polymers; and one fraction for oligomers and monomers (SB). Depolymerisation of colloids, HMW and MMW polymers was modelled using first order kinetics. LMW polymer degradation was modelled using Michaelis-Menten kinetics, while growth was based on traditional Monod kinetics and endogenous respiration followed ASM3. The proposed model was implemented in AQUASIM for a batch reactor system, and parameter estimation by LSE fitting to experimental data on particulate starch degradation over 117 days in a dispersed biomass microcosm was performed. Validation of the model against experimental data gave a very good fit to the PBM. The intermediate dynamics seen in the experimental data was also qualitatively demonstrated by the model, with accumulation of HMW, MMW and LMW polymers in the bulk liquid. However, the accumulation of monomers and oligomers in the bulk liquid could not be reproduced in the suspended growth model proposed. Hence, a structured biomass model (biofilm) is suggested for future work.publishedVersio

Effect of temperatures on anaerobic granulated biofilm modelling

Anaerobic granulated biomass-based treatment is a sustainable alternative for municipal wastewater treatment. Each granule in the system is comprised of a complex community of anaerobic microorganisms embedded in a biofilm matrix. The aim of this work was to implement a biofilm model for simulation of biogas production and COD removal as observed in an experimental up-flow anaerobic sludge blanket (UASB) reactor system. Additionally, selected scenario simulations were carried out to assess the effect of temperatures (25, 16, and 12 °C) on granulated anaerobic reactor performance at different organic loading rates. The two main model components used are: Dynamic biochemical and physicochemical conversion processes (Anaerobic Digestion Model No. 1) and diffusive mass transfer within the granule (biofilm). The model was implemented in AQUASIM 2.1. Simulations gave insight into non-observables, especially intragranular biomass distribution and substrate profiles, which help our understanding of granule formation and evolution. Results reflected observed effluent COD concentrations and methane production rates at variable temperatures and reactor loadings. Simulations also confirmed observed steady-state reductions in COD removal efficiencies and methane fraction in biogas at increasing organic loading rate. Model simulations also showed intra-granular alkaline pH depth profiles with increasing organic loading rate which may explain calcium-based mineral core formation. The biomass composition and active regions in granules were not significantly affected by organic loading rate. At steady state, organic substrates especially monosaccharides and volatile fatty acids were predicted to degrade approximately within the outer 100 μm. In general, the model can be used as a tool to predict and simulate anaerobic granulated biofilm system performances in UASB reactor.publishedVersio

Starch degradation and intermediate dynamics in flocculated and dispersed microcosms

A large fraction of the organic substrate in municipal wastewater is particulate. Prior to uptake, particles have to be degraded through potentially a range of intermediates. However, research on intermediate dynamics during particle hydrolysis is limited. In this paper, batch experiments on flocculated and dispersed biomass microcosms using starch as particulate substrate are reported. Overall hydrolysis rate was not significantly different between the two systems. Particle colonization, and increased particle porosity in combination with particle breakup, led to increased substrate availability over time. Particle breakup was more important for flocculated biomass, while increased particle porosity and particle colonization played a larger role for dispersed biomass. During particle degradation intermediates were formed; however, all intermediate polymer sizes were not formed to the same extent. This can be explained by non-random enzymatic degradation, where some products are preferred over others. Intermediates' dynamics also depend on the biomass structure, and in a floc-based system, diffusion limitations allow glucose to accumulate in the system

Effect of low temperature and municipal wastewater organic loading on anaerobic granule reactor performance

Biogas production and municipal wastewater COD removal at low temperatures by granulated anaerobic biomass were investigated. Two anaerobic granule reactors were operated continuously for 1025 days by stepwise increase of organic loading from 1.3 to 15.2 g CODdissolved·l−1·d−1 at 25, 16, 12, 8.5, 5.5, and 2.5 °C. The sustained reactor performance was evaluated by COD removal efficiency, methane production, and microbial community analysis. Stable COD removal of 50–70% were achieved at 25–8.5 °C and up to 15 g CODdissolved·l−1·d−1, and no significant temperature effect was observed on specific methane production rate and yield. Below 8.5 °C, COD removal and methane yields reduced, but still significant methane formation was observed even at 2.5 °C. More than 90% of COD removed was converted to methane. Methanogenic archaea communities showed that temperature changes affected the major methane formation pathways, which explains temperature adaptability of the granules.publishedVersio

Wastewater characterisation by combining size fractionation, chemical composition and biodegradability

The potential for resource recovery from wastewater can be evaluated based on a detailed characterisation of wastewater. In this paper, results from fractionation and characterisation of two distinct wastewaters are reported. Using tangential flow filtration, the wastewater was fractionated into 10 size fractions ranging from 1 kDa to 1 mm, wherein the chemical composition and biodegradability were determined. Carbohydrates were dominant in particulate size fractions larger than 100 μm, indicating a potential of cellulose recovery from these fractions. While the particulate size fractions between 0.65 and 100 μm show a potential as a source for biofuel production due to an abundance of saturated C16 and C18 lipids. Both wastewaters were dominated by particulate (>0.65 μm), and oligo- and monomeric (<1 kDa) COD. Polymeric (1–1000 kDa) and colloidal (1000 kDa-0.65 μm) fractions had a low COD content, expected due to degradation in the sewer system upstream of the wastewater treatment plant. Biodegradation rates of particulate fractions increase with decreasing size. However, this was not seen in polymeric fractions where degradation rate was governed by chemical composition. Analytical validation of molecular weight and particle size distribution showed below filter cut-off retention of particles and polymers close to nominal cut-off, shifting the actual size distribution

A review on occurrence and spread of antibiotic resistance in wastewaters and in wastewater treatment plants: Mechanisms and perspectives

This paper reviews current knowledge on sources, spread and removal mechanisms of antibiotic resistance genes (ARGs) in microbial communities of wastewaters, treatment plants and downstream recipients. Antibiotic is the most important tool to cure bacterial infections in humans and animals. The over- and misuse of antibiotics have played a major role in the development, spread, and prevalence of antibiotic resistance (AR) in the microbiomes of humans and animals, and microbial ecosystems worldwide. AR can be transferred and spread amongst bacteria via intra- and interspecies horizontal gene transfer (HGT). Wastewater treatment plants (WWTPs) receive wastewater containing an enormous variety of pollutants, including antibiotics, and chemicals from different sources. They contain large and diverse communities of microorganisms and provide a favorable environment for the spread and reproduction of AR. Existing WWTPs are not designed to remove micropollutants, antibiotic resistant bacteria (ARB) and ARGs, which therefore remain present in the effluent. Studies have shown that raw and treated wastewaters carry a higher amount of ARB in comparison to surface water, and such reports have led to further studies on more advanced treatment processes. This review summarizes what is known about AR removal efficiencies of different wastewater treatment methods, and it shows the variations among different methods. Results vary, but the trend is that conventional activated sludge treatment, with aerobic and/or anaerobic reactors alone or in series, followed by advanced post treatment methods like UV, ozonation, and oxidation removes considerably more ARGs and ARB than activated sludge treatment alone. In addition to AR levels in treated wastewater, it examines AR levels in biosolids, settled by-product from wastewater treatment, and discusses AR removal efficiency of different biosolids treatment procedures. Finally, it puts forward key-points and suggestions for dealing with and preventing further increase of AR in WWTPs and other aquatic environments, together with a discussion on the use of mathematical models to quantify and simulate the spread of ARGs in WWTPs. Mathematical models already play a role in the analysis and development of WWTPs, but they do not consider AR and challenges remain before models can be used to reliably study the dynamics and reduction of AR in such systems.publishedVersio

Degradation of polymeric and particulate organic carbon in biofilms

Polymeric and particulate organic carbon (POM) are fundamental compounds in the global cycling of carbon, and constitute significant amounts of BOD in municipal wastewater. The main objective of this work is to study molecular size effects on degradation dynamics in biofilm systems. Specifically, the effect of substrate molecular weight on degradation kinetics and transport dynamics, location of depolymerisation enzyme activity and depolymerisation intermediate formation dynamics are assessed. A mathematical model for biofilm degradation dynamics is presented, and used for data interpretation and simulations. Dextran, an -1,6 Glucan, was used as model substrate during batch degradation in a Rototorque biofilm reactor, in addition to batch tests on biofilm sub samples retrieved from the Rototorque, and during pure endo- and exo-Dextranase studies. Oxygen utilisation rate (OUR) estimates and bulk phase TOC mass balances were used to evaluate the effect of variable initial molecular weight on the observed half order removal coefficient (Harremoës, 1978; Rittmann and McCarty, 1980). Size exclusion-HPLC analysis for determination of bulk phase depolymerisation intermediates, and specific enzyme assays were used to evaluate transport dynamics of polymers and location of enzyme activity in the enhanced mixed population biofilm system. Dextran removal rate decrease with increasing Dextran molecular weight. The observed areal half order removal rate coefficient, k1/2,A, demonstrate an approximate 10-fold decrease in the 1-500 kDa range, showing negative logarithmic correlation to the initial MW of Dextran. A less distinct correlation is observed above this transition limit (1-10 MDa). Evaluation of the Thiele moduli, from one step depolymerisation modelling, suggests that the logarithmic reduction in observed removal rate is caused by combined reaction rate and transport limitations. Transport limitations dominates as the polymeric substrate size increase and hinders biofilm matrix diffusion, and the removal rate becomes a surface limited process. Removal of Dextran is biomass dependent in what appears to be a non-linear dependency on biofilm thickness. Expressed as biomass areal density (g/m2), no depolymerisation is observed for thin biofilms (0.7 g/m2), slow for medium (3.7 g/m2) and high for thicker biofilms (5.2 g/m2). Depolymerisation intermediates accumulated in the bulk phase over the entire Dextran size range during pure Dexranase studies, with even size distributions. Final products were oligo-isomaltoses (DP 2-6). Dextran was not depolymerised by -Glucosidase nor Oligo-1,6 Glucosidase. During biofilm reactor and slide sub-sample tests, low MW Dextran intermediates (1-10 kDa) accumulated in the bulk during depolymerisation of 160 kDa Dextran at 250 and 200 mg/l initial concentrations, but were not detected during experiments with 100 mg/l initial concentrations. Intermediate range Dextran (10-100 kDa) did not accumulate in either case. At the same conditions, some assimilable range Dextran (0.2-0.9 kDa) accumulated in the bulk liquid during initial 250 and 200 mg/l batches, but was not detected during 100 mg/l initial Dextran concentrations. The extent of bulk phase accumulation seems to depend on the biofilm growth rate, where more bulk phase accumulation is observed during experiments with starved compared to more actively growing biofilms. More intermediates accumulate during low MW initial standards, compared to higher. These observations indicate that the extent of bulk phase intermediate accumulation is balanced by the rate of depolymerisation, and the substrate uptake rate (growth). Accumulation of intermediate hydrolysis products in biofilm systems is therefore dependent on the slowly biodegradable organic (SBCOD) loading rate. Dextranase was detected in the cellular fraction of the biofilms. The enzyme activity was not detected in any other biofilm sub compartments, implying that the exogenous enzyme remains attached to the cells while working on polymers. These findings support the conceptual model of Confer and Logan (1998), implying that bulk phase intermediate accumulation observed in this study and by others, is not a result of enzymatic activity in the bulk phase, but transport of intermediates from the biofilm matrix

Engineered methanotrophic syntrophy in photogranule communities removes dissolved methane

The anaerobic treatment of wastewater leads to the loss of dissolved methane in the effluent of the treatment plant, especially when operated at low temperatures. The emission of this greenhouse gas may reduce or even offset the environmental gain from energy recovery through anaerobic treatment. We demonstrate here the removal and elimination of these comparably small methane concentrations using an ecologically engineered methanotrophic community harbored in oxygenic photogranules. We constructed a syntrophy between methanotrophs enriched from activated sludge and cyanobacteria residing in photogranules and maintained it over a two-month period in a continuously operated reactor. The novel community removed dissolved methane during stable reactor operation by on average 84.8±7.4% (±standard deviation) with an average effluent concentration of dissolved methane of 4.9±3.7 mg CH4∙l−1. The average methane removal rate was 26 mg CH4∙l−1∙d−1, with an observed combined biomass yield of 2.4 g VSS∙g CH4−1. The overall COD balance closed at around 91%. Small photogranules removed methane more efficiently than larger photogranule, likely because of a more favorable surface to volume ratio of the biomass. MiSeq amplicon sequencing of 16S and 23S rRNA revealed a potential syntrophic chain between methanotrophs, non-methanotrophic methylotrophs and filamentous cyanobacteria. The community composition between individual photogranules varied considerably, suggesting cross-feeding between photogranules of different community composition. Methanotrophic photogranules may be a viable option for dissolved methane removal as anaerobic effluent post-treatment.publishedVersio

Granular sludge bed processes in anaerobic digestion of particle-rich substrates

Granular sludge bed (GSB) anaerobic digestion (AD) is a well-established method for efficient wastewater treatment, limited, however, by the wastewater particle content. This review is carried out to investigate how and to what extent feed particles influence GSB to evaluate the applicability of GSB to various types of slurries that are abundantly available. Sludge bed microorganisms evidently have mechanisms to retain feed particles for digestion. Disintegration and hydrolysis of such particulates are often the rate-limiting steps in AD. GSB running on particle-rich substrates and factors that affect these processes are stdied especially. Disintegration and hydrolysis models are therefore reviewed. How particles may influence other key processes within GSB is also discussed. Based on this, limitations and strategies for effective digestion of particle-rich substrates in high-rate AD reactors are evaluated.publishedVersio