Multifunctional role of anaerobic reactors in wastewater treatment

Abstract:Since 1860, anaerobic technology has undergone rapid development, particularly with the invention of the high-rate anaerobic reactors, and has been widely applied in treating different types of wastewater under extreme environmental conditions. However, the major functional role of anaerobic reactors is generally considered to be removing organic pollutants via methanogenesis. In fact, the actual reactions involved in anaerobic reactors are extremely complex, which also have positive effects on removing inorganic pollutants (N, P, S, etc.). The performance of the removal of inorganic pollutants could be further enhanced by ex-situ/in-situ methods. Recent studies also showed that calcium removal for water softening and in-situ biogas upgrading could be achieved through anaerobic reactors. Nevertheless, the research work of the above-mentioned field has appeared to be relatively independent and scattered so far. First, in this paper, the state of the art of anaerobic wastewater treatment and the traditional role of anaerobic reactors are summarized. Then, the expanded roles of anaerobic reactors in desulfurization, denitrification, phosphorus removal, softening treatment, and in-situ biogas upgrading were elaborately reviewed in terms of their feasibility and process description. A major problem for the anaerobic treatment of sulfate-contaminated wastewater is the production of hydrogen sulfide (H2S), which greatly inhibits the methanogenesis process. The introduction of the biological sulfide oxidation step could not only reduce sulfide toxicity, but could also recover sulfur in the form of the insoluble elemental sulfur. As for denitrification, the integration of methanogenesis with the traditional denitrification process or even the novel anaerobic ammonium oxidation (ANAMMOX) process has been proven to be able to remove organic pollutants and ammonia simultaneously. Phosphorus removal by physico-chemical and/or biological methods was also demonstrated in the anaerobic reactors; however, the involved mechanism and phosphorus transformation pathway need to be further investigated. High-strength of calcium ions was shown to have adverse impacts on the capacity and stability of both anaerobic reactors and post-treatment facilities. The combination of the stripping or crystallization devices with anaerobic reactors was effective at inducing precipitation of calcium carbonate to alleviate the inhibition of calcium ions. Problems due to the precipitation and accumulation of calcium carbonate in the anaerobic granules (hereafter referred to as the inorganic-nucleus based anaerobic granules) were also observed, which shows promise for the development of an innovative in-situ calcium removal strategy. Specifically, by enhancing the formation of the inorganic-nucleus based anaerobic granules and regularly peeling away the inorganic-nucleus, the calcium ions can be in-situ removed from anaerobic reactors. Nevertheless, its feasibility needs to be further studied. Anaerobic reactors also showed great potential for the in-situ biogas upgrading, which has significant advantages over other ex-situ and physico-chemical methods. Several in-situ strategies (such as side-stream CO2 desorption, operation at elevated auto-generated pressure) have also been developed that take advantage of the fact that carbon dioxide (CO2) is much more soluble than methane (CH4) in the liquid phase, especially under high-pressure conditions. However, they posed certain technical challenges such as CH4 loss and disposal of the retained CO2. In this case, biological processes such as the enhanced hydrogenotrophic methanogenesis, can be used to overcome some of these technical challenges. Conclusively, the multifunctional role of anaerobic reactors in wastewater treatment is gradually recognized. The overall performance of the multifunctional anaerobic reactors needs to be optimized in respect to the reactor structure, operational parameters, and the construction and regulation of the involved coupling models. The role of anaerobic reactors in the production of other bio-based energy (hydrogen, electricity, etc.) and chemicals (organic acids, alcohols, etc.) is another emerging research area for anaerobic technology.