Damien J. Batstone

Pretreatment methods to improve sludge anaerobic degradability: a review.

This paper presents a review of the main sludge treatment techniques used as a pretreatment to anaerobic digestion. These processes include biological (largely thermal phased anaerobic), thermal hydrolysis, mechanical (such as ultrasound, high pressure and lysis), chemical with oxidation (mainly ozonation), and alkali treatments. The first three are the most widespread. Emphasis is put on their impact on the resulting sludge properties, on the potential biogas (renewable energy) production and on their application at industrial scale. Thermal biological provides a moderate performance increase over mesophilic digestion, with moderate energetic input. Mechanical treatment methods are comparable, and provide moderate performance improvements with moderate electrical input. Thermal hydrolysis provides substantial performance increases, with a substantial consumption of thermal energy. It is likely that low impact pretreatment methods such as mechanical and thermal phased improve speed of degradation, while high impact methods such as thermal hydrolysis or oxidation improve both speed and extent of degradation. While increased nutrient release can be a substantial cost in enhanced sludge destruction, it also offers opportunities to recover nutrients from a concentrated water stream as mineral fertiliser.

Microbial ecology meets electrochemistry: electricity-driven and driving communities

Bio-electrochemical systems (BESs) have recently emerged as an exciting technology. In a BES, bacteria interact with electrodes using electrons, which are either removed or supplied through an electrical circuit. The most-described type of BES is microbial fuel cells (MFCs), in which useful power is generated from electron donors as, for example, present in wastewater. This form of charge transport, known as extracellular electron transfer, was previously extensively described with respect to metals such as iron and manganese. The importance of these interactions in global biogeochemical cycles is essentially undisputed. A wide variety of bacteria can participate in extracellular electron transfer, and this phenomenon is far more widespread than previously thought. The use of BESs in diverse research projects is helping elucidate the mechanism by which bacteria shuttle electrons externally. New forms of interactions between bacteria have been discovered demonstrating how multiple populations within microbial communities can co-operate to achieve energy generation. New environmental processes that were difficult to observe or study previously can now be simulated and improved via BESs. Whereas pure culture studies make up the majority of the studies performed thus far, even greater contributions of BESs are expected to occur in natural environments and with mixed microbial communities. Owing to their versatility, unmatched level of control and capacity to sustain novel processes, BESs might well serve as the foundation of a new environmental biotechnology. While highlighting some of the major breakthroughs and addressing only recently obtained data, this review points out that despite rapid progress, many questions remain unanswered.

Influence of Environmental Conditions on Methanogenic Compositions in Anaerobic Biogas Reactors

ABSTRACT The influence of environmental parameters on the diversity of methanogenic communities in 15 full-scale biogas plants operating under different conditions with either manure or sludge as feedstock was studied. Fluorescence in situ hybridization was used to identify dominant methanogenic members of the Archaea in the reactor samples; enriched and pure cultures were used to support the in situ identification. Dominance could be identified by a positive response by more than 90% of the total members of the Archaea to a specific group- or order-level probe. There was a clear dichotomy between the manure digesters and the sludge digesters. The manure digesters contained high levels of ammonia and of volatile fatty acids (VFA) and were dominated by members of the Methanosarcinaceae, while the sludge digesters contained low levels of ammonia and of VFA and were dominated by members of the Methanosaetaceae. The methanogenic diversity was greater in reactors operating under mesophilic temperatures. The impact of the original inoculum used for the reactor start-up was also investigated by assessment of the present population in the reactor. The inoculum population appeared to have no influence on the eventual population.

Acetate Oxidation Is the Dominant Methanogenic Pathway from Acetate in the Absence of Methanosaetaceae

ABSTRACT The oxidation of acetate to hydrogen, and the subsequent conversion of hydrogen and carbon dioxide to methane, has been regarded largely as a niche mechanism occurring at high temperatures or under inhibitory conditions. In this study, 13 anaerobic reactors and sediment from a temperate anaerobic lake were surveyed for their dominant methanogenic population by using fluorescent in situ hybridization and for the degree of acetate oxidation relative to aceticlastic conversion by using radiolabeled [2-14C]acetate in batch incubations. When Methanosaetaceae were not present, acetate oxidation was the dominant methanogenic pathway. Aceticlastic conversion was observed only in the presence of Methanosaetaceae.

Linking microbial community structure, interactions and function in anaerobic digesters using new molecular techniques

Over the last decade there has been a rapid development in culture-independent techniques for exploring microbial communities, which have led to new insights into their structure and function in both natural environments and engineered systems. This review focuses on some of the most important recent advances and their applications to the diverse microbial communities associated with anaerobic digestion. The use of these approaches in combination with complementary imaging techniques, chemical isotope analyses and detailed reactor performance measurements provides a new opportunity to develop a fundamental understanding of how microbial community dynamics, interactions and functionality influence digester efficiency and stability.

Estimation of hydrolysis parameters in full‐scale anerobic digesters

In hydrolysis-limited anerobic systems, the key parameters describing degradation are degradability extent (f(d)), and the lumped apparent first order coefficient (k(hyd)). These are often measured in biological methane potential (BMP) tests. Using modern techniques, it should also be possible to estimate these parameters in full-scale systems, especially where inputs are dynamic. In this study, we evaluated f(d) and k(hyd) values and uncertainty based on nonlinear parameter estimation from (i) BMP tests and (ii) effluent gas and solids from two full-scale digesters fed with highly variable feed flows and concentrations (up to 6 kg COD m(-3) day(-1)). The substrate was thermally hydrolyzed activated sludge, and the inoculum for BMP tests was from the full-scale digesters. While identifiability of both parameters in the BMP tests was generally good, only f(d) could be well identified using continuous data. For k(hyd) using continuous data, normally only a lower limit could be found (upper was unbounded). In addition, parameters as estimated on different outputs (VS and gasflow) and two different digesters were consistent, with an f(d) value of 0.45-0.55, and a k(hyd) value of >5 day(-1). Gradual changes in f(d) over the 450 days could be related to upstream changes. f(d) values as estimated in BMP tests were consistent (if conservative) with continuous estimates, with a f(d) in BMP of 0.4-0.5. k(hyd) values were an order of magnitude lower (0.15-0.25 day(-1) vs. >5 day(-1)), and this translated to very poor model performance when BMP-estimated values were used in the continuous model. This means that while BMP testing may be used for project feasibility analysis, values obtained should not be used for dynamic modeling. The parameter confidence regions found were highly nonlinear, especially for continuous systems, indicating that iterative or sampling techniques are required for an estimate of real parameter uncertainty.

Phosphorus recovery from wastewater through microbial processes

Waste streams offer a compelling opportunity to recover phosphorus (P). 15-20% of world demand for phosphate rock could theoretically be satisfied by recovering phosphorus from domestic waste streams alone. For very dilute streams (<10 mg PL(-1)), including domestic wastewater, it is necessary to concentrate phosphorus in order to make recovery and reuse feasible. This review discusses enhanced biological phosphorus removal (EBPR) as a key technology to achieve this. EBPR relies on polyphosphate accumulating organisms (PAOs) to take up phosphorus from waste streams, so concentrating phosphorus in biomass. The P-rich biosolids can be either directly applied to land, or solubilized and phosphorus recovered as a mineral product. Direct application is effective, but the product is bulky and carries contaminant risks that need to be managed. Phosphorus release can be achieved using either thermochemical or biochemical methods, while recovery is generally by precipitation as struvite. We conclude that while EBPR technology is mature, the subsequent phosphorus release and recovery technologies need additional development.

Biomethanation and its potential

Biomethanation is a process by which organic material is microbiologically converted under anaerobic conditions to biogas. Three main physiological groups of microorganisms are involved: fermenting bacteria, organic acid oxidizing bacteria, and methanogenic archaea. Microorganisms degrade organic matter via cascades of biochemical conversions to methane and carbon dioxide. Syntrophic relationships between hydrogen producers (acetogens) and hydrogen scavengers (homoacetogens, hydrogenotrophic methanogens, etc.) are critical to the process. Determination of practical and theoretical methane potential is very important for design for optimal process design, configuration, and effective evaluation of economic feasibility. A wide variety of process applications for biomethanation of wastewaters, slurries, and solid waste have been developed. They utilize different reactor types (fully mixed, plug-flow, biofilm, UASB, etc.) and process conditions (retention times, loading rates, temperatures, etc.) in order to maximize the energy output from the waste and also to decrease retention time and enhance process stability. Biomethanation has strong potential for the production of energy from organic residues and wastes. It will help to reduce the use of fossil fuels and thus reduce CO(2) emission.

Variation of bulk properties of anaerobic granules with wastewater type.

Development of a granular sludge with high strength, high biological activity and a narrow settling distribution is necessary for optimal operation of high-rate upflow anaerobic treatment systems. Several studies have compared granules produced from different wastewaters but these have largely been from laboratory-fed reactors or compared granules from full-scale reactors fed similar wastewater types. Though two authors have commented on the inferiority of granules produced by a protein-based feed, the properties of these granules have not been characterised. In this paper, granules from full-scale reactors treating fruit and vegetable cannery effluent, two brewery effluents and a pig abattoir (slaughterhouse) were compared in terms of basic composition, size distribution, density, settling velocity, shear strength, and EPS content. The results supported previous qualitative observations by other researchers that indicate granule properties depend more on wastewater type rather than reactor design or operating conditions such as pre-acidification level. The cannery-fed granules had excellent shear strength, settling distribution and density. Granules from the two brewery-fed reactors had statistically the same bulk properties, which were still acceptable for upflow applications. The protein-grown granule had poor strength and settling velocity.

Decreasing activated sludge thermal hydrolysis temperature reduces product colour, without decreasing degradability

Activated sludges are becoming more difficult to degrade in anaerobic digesters, due to the implementation of stricter nitrogen limits, longer sludge ages, and removal of primary sedimentation units. Thermal hydrolysis is a popular method to enhance degradability of long-age activated sludge, and involves pressure and heat treatment of the process fluid (150-160 degrees C saturated steam). However, as documented in this study, in a full-scale system, the use of thermal hydrolysis produces coloured, recalcitrant compounds that can have downstream impacts (e.g., failure of UV disinfection, and increased effluent nitrogen). The coloured compound formed during thermal hydrolysis was found to be melanoidins. These are coloured recalcitrant compounds produced by polymerisation of low molecular weight intermediates, such as carbohydrates and amino compounds at elevated temperature (Maillard reaction). By decreasing the THP operating temperature from 165 degrees C to 140 degrees C, THP effluent colour decreased from 12,677 mg-PtCo L(-1) to 3837 mg-PtCo L(-1). The change in THP operating temperature from 165 degrees C to 140 degrees C was shown to have no significant impact on anaerobic biodegradability of the sludge. The rate and extent of COD biodegradation remained largely unaffected by the temperature change with an average first order hydrolysis rate of 0.19 d(-1) and conversion extent of 0.43 g-COD(CH4)g-COD(-1).