Treatment of wastewater using an innovative biofilm reactor -- the air suction flow biofilm reactor
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The role of wastewater treatment infrastructure in protecting water resources and thus as a vital part of a country's infrastructure cannot be overemphasised. Failure to sufficiently treat wastewater threatens human health, ecosystems, biodiversity, food security and the sustainability of water resources. Population growth, increased economic activity, increasingly stringent discharge regulations and a need to reduce costs and emissions will increase pressure on the existing wastewater treatment infrastructure. Due to their size, large scale facilities may have the capacity to adapt to such challenges. However, small scale and decentralised wastewater treatment plants can pose particular problems including (i) lack of on-site maintenance staff (ii) inability to meet discharge limits and (iii) inefficient and costly operation regimes. Thus the development of innovative technologies that can meet the challenges of this sector is timely. In this research study, an innovative wastewater treatment technology, the Air Suction Flow-Biofilm Reactor (ASF-BR), developed by a research team in Civil Engineering at the National University of Ireland, Galway was investigated as an alternative to existing technologies; in particular for decentralised applications. The ASF-BR is a batch biofilm process that comprises two sealed reactors (R1 and R2), solenoid valves, a vacuum pump, and plastic media. Biofilm, which has accumulated on the media in each reactor, is passively aerated as wastewater is moved between the two reactors using a partial vacuum created by the vacuum pump. This novel technology potentially offers a number of advantages over conventional treatment process; (i) the passive aeration strategy can eliminate the need for mechanical blowers or stirrers, (ii) the sealed nature of the reactors allows control of gaseous emissions traditionally associated with wastewater treatment (iii) as a batch reactor the ASFBR can accommodate aerobic, anoxic and anaerobic phases during a treatment cycle and (iv) flexible operational regimes can accommodate periods of low flow which can be problematic in tourist areas. The research investigated the efficacy of the ASF-BR in the treatment of a high strength synthetic wastewater (Study 1), municipal wastewater (Study 2) and landfill leachate (Study 3). Furthermore the emission of nitrous oxide (N2O) from the ASF-BR during the nitrification and denitrification of municipal wastewater and landfill leachate was analysed (Study 4). Study 1 comprised two phases (Phase 1 - 166 days and Phase 2 - 264 days) with both configured to achieve the removal of organic matter, nitrification and denitrification from synthetic high strength wastewater. In Phase 2 the cycle configuration was adjusted resulting in a 66% reduction in the energy requirement of the treatment process while maintaining excellent performance. Maximum average removal efficiencies were 97% filtered chemical oxygen demand (CODf), 88% filtered total nitrogen (TNf) and 99% ammonium-nitrogen (NH4-N) Two phases (Phases 1 and 2) were conducted during Study 2, lasting 212 days and 117 days respectively. The objective of Phase 1 was to investigate the performance of the ASF-BR for the removal of organic carbon and ammonium-nitrogen from municipal wastewater. During the 8.5 h treatment cycle, removals averaged 68% CODf, 35% TNf and 89% NH4-N. During Phase 2 an anoxic period was added to facilitate denitrification of the wastewater. Removals averaged 89% CODf, 93% NH4-N and 75% TNf (augmenting the municipal wastewater with an external carbon source). Study 3 investigated the performance of the ASF-BR in treating landfill leachate over two phases (Phase 1 and Phase 2). Phase 1 sought to evaluate the performance of the ASF-BR in treating landfill leachate through nitrification and the removal of organic carbon. Removal efficiencies achieved averaged 63% CODf, 38% TNf and 86% NH4-N. During Phase 2, a 90 minute anoxic period was added to the beginning of the treatment cycle to facilitate the denitrification of the leachate and the cycle duration was also shortened for optimisation. Early results found that a lack of biodegradable carbon was likely limiting denitrification. Following the addition of an external biodegradable carbon source, removal efficiencies averaged 33% CODf, 73% NH4-N and 7% TNf. In Study 4, N2O emissions from the ASF-BR when treating municipal wastewater and landfill leachate were monitored. Maximum nitrogen removal was observed in Study 2 which resulted in 0.18 mg N2O produced/mg NO3-N removed. Nitrogen removal through denitrification was low in Study 3 and limited N2O emissions were observed. The results obtained from this study show the ASF-BR could offer a promising alternative to conventional wastewater treatment systems. The treatment performance observed was comparable to, or in excess of, many current technologies. The ASF-BR can also potentially offer benefits including (i) low energy requirements, (ii) operational flexibility, and (iii) an ability to capture the gases produced during the wastewater treatment process. While the ASF-BR could offer a reliable and robust alternative to existing processes, further studies, at a field scale, would be required to evaluate in detail the operation and maintenance of the unit in larger scale applications.