Long-term performance evaluation of down-flow hanging sponge reactor regarding nitrification in a full-scale experiment in India

To date, the combination of upflow anaerobic sludge blanket (UASB) and subsequent post-treatment processes has been recognized as an attractive alternative for treatment of municipal sewage, especially in developing countries (Kassab et al., 2010, Khan et al., 2011). The post-treatment process has been extensively developed using numerous conventional technologies, such as activated sludge (AS), trickling filters (TFs), rotating biological contactors (RBCs), and waste stabilization ponds, and the number of review papers on these technologies has increased in recent years (Chan et al., 2009, Chong et al., 2012, Kassab et al., 2010, Khan et al., 2011). However, these review papers point out that most of the systems lack full-scale implementation, and further work is required to evaluate the full-scale performance.

To address the next challenge, a full-scale experiment using an aerobic post-treatment process called down-flow hanging sponge (DHS) has been conducted in India (Okubo et al., 2015). The DHS, a novel type of TF process using polyurethane sponge media as packing material, is recognized as a promising post-treatment alternative (Kassab et al., 2010). Unlike the gravel and plastic media, the sponge media provide a three-dimensional space for retention of sludge, resulting in much higher sludge concentration than in a conventional TF process (Onodera et al., 2015). The DHS reactor has advantages over other highly efficient processes, such as AS, including simple operation, easy maintenance, low operation and maintenance (O&M) costs and low excess sludge production (Tandukar et al., 2007). Although major disadvantages of the TF process are short resistance times, pumping costs, and biomass grazing by higher trophic organisms (Boller et al., 1994, Wik, 2003), the sponge media contribute to a longer residence time (Tandukar et al., 2006), compact reactor size provided by high sludge concentration (Onodera et al., 2013, Tandukar et al., 2007), and protection of the biomass against macrofauna overgrazing (Onodera et al., 2015). The bench-scale and pilot-scale DHS reactors have shown an excellent ability to polish the quality of UASB effluent in terms of suspended solids (SS), organic matter, ammonium, and pathogenic indicators (Machdar et al., 1997, Machdar et al., 2000, Onodera et al., 2014, Tandukar et al., 2007, Tawfik et al., 2006). Moreover, the pilot-scale experiment using the combined UASB and DHS system with a flow rate of 50 m³ day⁻¹ showed that the estimated energy requirement and excess sludge production of the system were approximately 75% and 85% lower than those of the conventional AS process, respectively (Tanaka et al., 2012). Moreover, a full-scale DHS reactor with a flow rate of 500 m³ day⁻¹ was constructed at a municipal sewage treatment plant in India, and has shown high removal efficiency in terms of organic substances and SS, less excess sludge production, and low energy consumption (Okubo et al., 2015).

Despite the anaerobic process having little effect on nitrogen compounds (Chong et al., 2012), this previous report lacks any description of the process performance. Since there is increasing concern about the water environment and human health in developing countries, particular attention should be placed on the removal efficiency with regards to nitrogen. Indeed, the nitrification performance is sensitive to the conditions of the DHS reactor, which can be attributed to its functional characteristics of high dissolved oxygen (DO) uptake without forced ventilation (Onodera et al., 2014), dense sludge and nitrifying bacteria retention in the sponge media (Kubota et al., 2014), and vigorous microbial activity of the retained sludge (Machdar et al., 2000). Therefore, the evaluation of nitrification is useful to determine if the DHS reactor can be scaled up successfully. Moreover, it is important to prove the applicability of the full-scale DHS reactor to developing countries that have limited budgets, lack of technically skilled workers, and less available land. To this end, the affordability and simplicity as well as efficiency of the post-treatment process need to be evaluated.

The aim of this study is to evaluate the process performance with regard to nitrification of the full-scale DHS reactor. This paper focuses on the distinctive characteristics of the DHS process, including high DO uptake, dense sludge retention, and high sludge activity. The applicability of the DHS reactor was also assessed based on an on-site experiment during a long-term monitoring period of 1800 days at a municipal sewage treatment plant in India.