David C. Stuckey

A review of soluble microbial products (SMP) in wastewater treatment systems

Abstract A review concerning the characterisation, production, modelling, significance and implications of soluble microbial products (SMP) in wastewater treatment is presented. The precise definition of SMP is open to debate, but is currently regarded as “the pool of organic compounds that are released into solution from substrate metabolism (usually with biomass growth) and biomass decay”, although for anaerobic systems volatile fatty acids are not included as SMP. Some of the SMP have been identified as humic and fulvic acids, polysaccharides, proteins, nucleic acids, organic acids, amino acids, antibiotics, steroids, exocellular enzymes, siderophores, structural components of cells and products of energy metabolism. Crucially, SMP have been found to comprise the majority of soluble organic material in the effluents from biological treatment processes and their presence is, therefore, of particular interest in terms of achieving discharge consent levels for BOD and COD. SMP also exhibit several characteristics, such as toxicity and metal chelating properties, which effect the performance of the treatment system, and their presence has also been shown to adversely affect the kinetic activity and the flocculating and settling properties of sludge. This review outlines some important discoveries with regard to the MW distribution, biodegradability and treatability of SMP and also the effect of process parameters on their production. It also summarises the attempts at incorporating SMP formation into the modelling of wastewater treatment. However, it concludes that the knowledge regarding SMP is far from complete and that much work is still required to fully understand their contribution to the treatment process; some of these future research areas are then outlined.

The use of the anaerobic baffled reactor (ABR) for wastewater treatment: a review

Abstract A review concerning the development, applicability and possible future application of the anaerobic baffled reactor for wastewater treatment is presented. The reactor design has been developed since the early 1980s and has several advantages over well established systems such as the upflow anaerobic sludge blanket and the anaerobic filter. These include: better resilience to hydraulic and organic shock loadings, longer biomass retention times, lower sludge yields, and the ability to partially separate between the various phases of anaerobic catabolism. The latter causes a shift in bacterial populations allowing increased protection against toxic materials and higher resistance to changes in environmental parameters such as pH and temperature. The physical structure of the anaerobic baffled reactor enables important modifications to be made such as the insertion of an aerobic polishing stage, resulting in a reactor which is capable of treating difficult wastewaters which currently require several units, ultimately significantly reducing capital costs.

The effect of thermal pretreatment on the anaerobic biodegradability and toxicity of waste activated sludge

1982) A~traet--The management of sludges generated by biological treatment of wastewaters has become an increasingly severe problem in recent years. The objective of this study was to examine the effect of thermochemical pretreatment on the anaerobic biodegradability and toxicity of waste activated sludge (WAS). In order to accomplish this, the degradability and toxicity of pure nitrogenous organic compounds present in WAS, and mixtures of these compounds, were also evaluated. The anaerobic bioconvertibility and toxicity of the various organics were determined using batch bioassay techniques. It was found that WAS bioconvertibility increased with increasing pretreatment temperature up to a maximum at 175 °, and this resulted in an increase in methane production of 27~o over the control. With the compounds and cultures used, mesophilic bioconvertibility and toxicity were found to be significantly higher than the corresponding values under thermophilic conditions. Finally, it was found that most of the pure individual nitrogen compounds and simple mixtures tested were quite biodegradable, although at the concentrations evaluated (20 g 1 - m) most were toxic. It was also noted that small changes in structure could have a significant effect on both toxicity and bioconvertibility. In most cases thermocbemical pretreatment of these individual compounds resulted in decreased bioconvertibility and increased toxicity. In conclusion it can be stated that thermochemical pretreatment enhances WAS bioconvertibility, while under identical treatment conditions, resulted in a considerable reduction in the bioconvertibility of individual nitrogen compounds and mixtures. This effect appears to be due to the conversion of biodegradable organics to refractory ones. Further, the toxicity of WAS after thermochemical pre- treatment appears to be due to its solubitization, and conversion of these soluble products to toxic compounds under more extreme treatment conditions.

Recent developments in anaerobic membrane reactors.

Anaerobic membrane reactors (AnMBRs) have recently evolved from aerobic MBRs, with the membrane either external or submerged within the reactor, and can achieve high COD removals (~98%) at hydraulic retention times (HRTs) as low as 3 h. Since membranes stop biomass being washed out, they can enhance performance with inhibitory substrates, at psychrophilic/thermophilic temperatures, and enable nitrogen removal via Anammox. Fouling is important, but addition of activated carbon or resins/precipitants can remove soluble microbial products (SMPs)/colloids and enhance flux. Due to their low energy use and solids production, and solids free effluent, they can enhance nutrient and water recycling. Nevertheless, more work is needed to: compare fouling between aerobic and anaerobic systems; determine how reactor operation influences fouling; evaluate the effect of different additives on membrane fouling; determine whether nitrogen removal can be incorporated into AnMBRs; recover methane solubility from low temperatures effluents; and, establish sound mass and energy balances.

Toxicants inhibiting anaerobic digestion: a review.

Anaerobic digestion is increasingly being used to treat wastes from many sources because of its manifold advantages over aerobic treatment, e.g. low sludge production and low energy requirements. However, anaerobic digestion is sensitive to toxicants, and a wide range of compounds can inhibit the process and cause upset or failure. Substantial research has been carried out over the years to identify specific inhibitors/toxicants, and their mechanism of toxicity in anaerobic digestion. In this review we present a detailed and critical summary of research on the inhibition of anaerobic processes by specific organic toxicants (e.g., chlorophenols, halogenated aliphatics and long chain fatty acids), inorganic toxicants (e.g., ammonia, sulfide and heavy metals) and in particular, nanomaterials, focusing on the mechanism of their inhibition/toxicity. A better understanding of the fundamental mechanisms behind inhibition/toxicity will enhance the wider application of anaerobic digestion.

Hydrodynamic characteristics of the anaerobic baffled reactor

Abstract The anaerobic baffled reactor (ABR) contains a mixed anaerobic culture segregated into compartments. This paper presents residence time distribution studies done on both clean and working reactors, to investigate the mixing patterns and dead spaces in this novel type of reactor. The results show that the mixing is characterized by a number of theoretical perfectly-mixed compartments which correlates closely with the actual number of compartments in the reactor, especially at low hydraulic residence times (HRT). The fraction of dead space in the ABR is low compared to other designs of anaerobic digesters. The dead space can be shown to be made up partly of biological dead space (due to presence of biomass) which decreases with HRT, and partly of hydraulic dead space (due to the flow patterns) which increases with HRT. Hence there is no direct correlation between dead space and HRT.

Bioaugmentation and its application in wastewater treatment: A review.

Bioaugmentation (the process of adding selected strains/mixed cultures to wastewater reactors to improve the catabolism of specific compounds, e.g. refractory organics, or overall COD) is a promising technique to solve practical problems in wastewater treatment plants, and enhance removal efficiency. The potential of this option can now be enhanced in order to take advantage of important advances in the fields of microbial ecology, molecular biology, immobilization techniques and advanced bioreactor design. Reports on bioaugmentation in WWT show the difficulties in evaluating the potential parameters involved, leading frequently to inconclusive outcomes. Many studies have been carried out on the basis of trial-and-error approaches, and it has been reported that reactors bioaugmented with pure cultures often fail to perform as well as the pure cultures under laboratory conditions. As an interesting technical challenge, the feasibility of bioaugmentation should ultimately be assessed by data from field implementation, and this review highlights several promising areas to explore in the future.

The effect of shock loads on the performance of an anaerobic baffled reactor (ABR). 1. Step changes in feed concentration at constant retention time

Abstract A 10-litre anaerobic baffled reactor (ABR), with eight compartments, was used to examine the effect of organic shock loads, in the form of a step change in the feed chemical oxygen demand (COD) at constant hydraulic retention time (HRT), on reactor performance in terms of COD removal, and to obtain a greater insight into microbial responses and interactions during these shocks. In order to minimise feed variations, and have a totally biodegradable substrate, a synthetic carbohydrate (sucrose)-protein (meat extract) substrate was used. The reactor was operated at 20-h HRT, 4 g/litre COD (4.8 kg-COD/m 3 d), and 35°C for 1 month as a base-line condition, and this resulted in 98% COD removal. It was found that a step change in the feed to 8 g/litre COD (9.6 kg-COD/m 3 d) at 20-h HRT for 20 days did not affect the substrate removal efficiency at all; however, when the concentration was increased to 15 g/litre COD (18 kg-COD/m 3 d) from 4 g/litre for 20 days, the removal efficiency decreased to 90%. It was found that the compartmentalised ABR consisted of three general zones (acidification, methanation, and a buffer zone where little acidification and methanogenesis occurs), and the function of high solids concentrations in the reactor was to enhance stability rather than improve COD removal. Hence, the structure of the ABR prevents most of the biomass being exposed to low pHs during shock loads, and enhances reactor stability. In addition, due to low pHs and high substrate concentrations in the first compartment, a microbial population seems to be selected which produces primarily acetate and butyrate rather than formate and propionate, and this also enhances the stability of the reactor during shock loads. In contrast to past results, formate did not seem to be an important interspecies electron carrier except under high mixing conditions and shock loads, and this was postulated to be due to differences in the structure of the microbial flocs. Based on these observations, the ABR holds some promise as a reactor design for anaerobic industrial wastewater treatment.

Microbial Populations Associated with Treatment of an Industrial Dye Effluent in an Anaerobic Baffled Reactor

ABSTRACT Fluorescent in situ hybridization (FISH) using 16S and 23S rRNA-targeted probes together with construction of an archaeal 16S ribosomal DNA (rDNA) clone library was used to characterize the microbial populations of an anaerobic baffled reactor successfully treating industrial dye waste. Wastewater produced during the manufacture of food dyes containing several different azo and other dye compounds was decolorized and degraded under sulfidogenic and methanogenic conditions. Use of molecular methods to describe microbial populations showed that a diverse group of Bacteria andArchaea was involved in this treatment process. FISH enumeration showed that members of the gamma subclass of the classProteobacteria and bacteria in theCytophaga-Flexibacter-Bacteroides phylum, together with sulfate-reducing bacteria, were prominent members of a mixed bacterial population. A combination of FISH probing and analysis of 98 archaeal 16S rDNA clone inserts revealed that together with the bacterial population, a methanogenic population dominated byMethanosaeta species and containing species ofMethanobacterium and Methanospirillum and a relatively unstudied methanogen, Methanomethylovorans hollandica, contributed to successful anaerobic treatment of the industrial waste. We suggest that sulfate reducers, or more accurately sulfidogenic bacteria, together with M. hollandicacontribute considerably to the treatment process through metabolism of dye-associated sulfonate groups and subsequent conversion of sulfur compounds to carbon dioxide and methane.

Analytical methods for soluble microbial products (SMP) and extracellular polymers (ECP) in wastewater treatment systems: A review

Effluents from biological processes contain a wide range of complex organic compounds, including soluble microbial products (SMP) and extracellular polymers (ECP), released during bacteria metabolism in mixed culture in bioreactors. It is important to clearly identify the primary components of SMPs and ECPs in order to understand the fundamental mechanisms of biological activity that create these compounds, and how to reduce these compounds in the effluent. In addition, these compounds constitute the main foulants in membrane bioreactors which are being used more widely around the world. A review on the extraction of ECP, characterization, and identification of SMPs and ECPs is presented, and we summarize up-to-date pretreatments and analytical methods for SMPs. Most researchers have focused more on the overall properties of SMPs and ECPs such as their concentrations, molecular weight distribution, aromaticity, hydrophobic and hydrophilic properties, biodegradability, and toxicity characteristics. Many studies on the identification of effluent SMPs show that most of these compounds were not present in the influent, such as humic acids, polysaccharides, proteins, nucleic acids, organic acids, amino acids, exocellular enzymes, structural components of cells and products of energy metabolism. A few groups of researchers have been working on the identification of compounds in SMPs using advanced analytical techniques such as GC-MS, LC-IT-TOF-MS and MALDI-TOF-MS. However, there is still considerably more work needed to be done analytically to fully understand the chemical characteristics of SMPs and ECPs.