Low-temperature anaerobic digestion for wastewater treatment

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Methanogenesis is an important biogeochemical process for the degradation of organic matter within cold environments, and is associated with the release of the potent greenhouse gas, methane. Cold methanogenesis has been harnessed, in engineered systems, as low-temperature anaerobic digestion (LTAD) for wastewater treatment and bioenergy generation. LTAD represents a nascent wastewater treatment biotechnology, which offers an attractive alternative to conventional aerobic and anaerobic processes. Successful, high-rate, LTAD of sewage and industrial wastewaters (e.g. from the brewery, food-processing and pharmaceutical sectors), with concomitant biogas generation, has been demonstrated at laboratory-scale and pilot-scale. A holistic, polyphasic approach, which integrates bioprocess, physiological and molecular biological datasets has been critical to the development of the LTAD concept.

Highlights

Anaerobic digestion (AD) wastewater treatment is environmentally sustainable. ► Conventional, heated AD is not suitable for cold, low-strength wastewaters. ► New information on biology has underpinned low-temperature LTAD process development. ► LTAD now successfully implemented at laboratory and pilot-scale. ► Potential for integration with other technologies to maximize resource recovery.

Section snippets

Introduction to anaerobic digestion and wastewater treatment

Fresh water is a diminishing resource. Two billion people lack adequate sanitation, or the means to afford it, and the development of a sustainable water infrastructure is a global challenge. For example, although The Millennium Development Goals (MDGs) call for halving ‘by 2015, the proportion of people without access to safe water and basic sanitation’, the UN MDGs Report [1] concluded that ‘with half the population of developing regions without sanitation, the 2015 target appears to be out

Methanogenesis under low-temperature conditions

Methanogenesis has been reported in diverse cold habitats, including Arctic and sub-Arctic peatlands, freshwater sediments and high-altitude rice-paddy soils [10, 11], although the microbial interactions and biochemical pathways involved are not well understood. The initial, hydrolytic, stage is viewed as the rate-limiting step at low/ambient temperatures. Reduced temperature may limit the energy gain from syntrophic volatile fatty acid (VFA) degradation and the critical importance of

Microbial acclimation to low temperature

Microorganisms have evolved sophisticated adaptation strategies, which enable them to function under sub-optimal conditions. Cultivating microbial biomass under sub-optimal conditions may result in a lag phase of reduced metabolic activity during acclimation. With respect to the biotechnological application of LTAD for wastewater treatment, the use of pre-acclimated inoculum sludge could reduce start-up times, increase toxicant tolerance and enhance treatment efficiencies, thereby increasing

Low-temperature anaerobic digestion of industrial and domestic wastewaters

Several challenges must be overcome for successful application of LTAD. With decreasing temperature: first, the rate of substrate utilisation may be reduced; second, reduced maximum microbial specific growth may be observed; and third, methane may become more soluble in the reactor liquor [8]. It is now clear, however that enhanced methanogenic activity can develop at low temperatures, despite the inoculum being sourced from a mesophilic reactor. Newly grown organisms are efficiently retained

Knowledge gaps and opportunities for LTAD

During the past decade, molecular and micro-analytical approaches have provided important new information on the nature of anaerobic biofilm communities and on the limitations and potential of the methanogenic consortia involved in LTAD, which have underpinned process developments [16, 22•, 31, 49] (Figure 3). Furthermore, molecular fingerprinting may predict potential operational instabilities before they occur [23, 50]. A holistic, polyphasic approach to bioprocess monitoring has begun to

Conclusions

By combining LTAD with post-treatment processes designed to remove recalcitrant and mineralised compounds; and residual organic residues, essentially all the criteria for sustainable environmental protection can be satisfied [1, 2]. In particular, huge economic, social and environmental benefits are to be gained from the implementation of LTAD as a low-cost, low-technology strategy for the decentralised treatment of domestic wastewaters in developing, and developed, countries. A comprehensive,

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

The financial support of Science Foundation Ireland (Charles Parsons Energy Research Award (06/CP/E006), The Irish Environmental Protection Agency (STRIVE 2005-ET-MS-29-M3) and the Irish Research Council for Science Engineering and Technology (IRCSET; Embark Initiative) is gratefully acknowledged. The authors wish to thank Florence Abram, Joseph O’Reilly, Padhraig Madden, Changsoo Lee, Fabio Chinalia, Anne-Marie Enright and Carol Morris for valuable scientific discussions.

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