Stability enhancement of thermophilic anaerobic digesters treating municipal sewage sludge

Abstract

Stability of anaeobic digesters is important for efficient and effective treatment of sludge generated in wastewater treatment plants. One of the factors governing the process stability of anaerobic digesters is the balance between the microbial groups involved in the digestion process, particularly between acid producers (bacteria) and acid consumers (archaea). It has been frequently observed that changes in the environmental conditions or feed composition disturb the balance between acid producing bacteria and acid consuming methanogenic archaea causing accumulation of intermediate products which leads to digester failure. Maintaining the balance between the microbial communities is even more challenging at thermophilic temperatures possibly due to the lower microbial diversity reported at higher temperatures. Studies on many macro-organisms in large scale ecosystems and a few on microbial ecosystems suggest that highly diverse ecosystems are more resilient towards disturbances and hence are functionally more stable. In the current study, it was hypothesized that increasing the microbial diversity in thermophilic anaerobic digesters (TADs) will make the digesters more resilient towards sudden changes in the feed composition and hence increases the process stability of the digesters. Two different approaches were adopted to increase the microbial diversity in the TADs. In the first approach diversity was increased at the micro-organism level i.e. increasing the species diversity of the methanogens. Three different methods were used to increase the diversity of methanogens such as using inocula from range of environments, changing the method of mixing and using support media in the digesters. In the second approach microbial diversity was increased at the functional group level by introducing a completely new functional group of micro-organisms i.e. the anodophiles (electrogenic bacteria), which are not found in anaerobic digesters naturally but which are in many ways capable of replacing the function of the methanogens in the anaerobic microbial community. No increase in methanogen diversity was observed when inocula from range of environments was used. Based on 16S rRNA gene analysis, the methanogens dominating the sewage sludge were found to be dominant in the tested and control samples. The species diversity of methanogens was successfully increased by changing the method of mixing from mechanical stirrer to sludge re-circulation. The dominant methanogens in the digesters using sludge re-circulation for mixing were closely related to thermophilic archaea such as Methanothermobacter thermoautotrophicus, Methanobacterium thermoformicicum, Methanosarcina thermophila, whereas the dominant methanogens in the mechanically stirred digesters were closely related to Methanosaeta concilli, uncultured Methanosaeta sp and Methanosarcina barkeri. The sludge re-circulation mixing method promoted proliferation of true thermophilic micro-organisms and may also have helped to maintain close syntrophic associations. An inert carbon fibre cloth (CFC) was also tried in the digesters to increase the diversity of methanogens. The digesters with CFCs recovered at faster rates than the digesters without CFCs. The 16S rRNA gene analysis on the adhering fraction of CFCs and suspended fraction from the digester showed higher diversity of methanogens on CFCs. The use of CFCs also increased the average residence time of micro-organisms in the digesters and facilitate the maintenance of close syntrophic associations of syntrophic micro-organisms. To add an entire functional group of micro-organisms to the digesters, microbial fuel cells were combined with the TADs. The anaerobic digester/ microbial fuel cell hybrid system was tested for increased stability by giving acetic acid shock to the system. The hybrid system recovered at a faster rate after the acid shock compared with either of the components of the hybrid system when operated individually. The results from this study suggested that diversity-stability ecological theories can be applied to engineer new digester designs to promote digester stability and performance in the event of environmental and/or operational disturbances. It must be emphasized that the techniques used to measure the microbial diversity of TADs in the current study were limited to measuring the diversity of only dominant micro-organisms. In this work implications for operating a thermophilic anaerobic digester with lipid rich waste are outlined.