Investigating biological filtration for manganese control in Lorne water supply system
Creators: name: Wang, Jiteng
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400411
subject: Water treatment processes

description: In recent years, significant fluctuations of manganese (Mn) concentration have been observed in the raw water at Lorne Water Treatment Plant (WTP) in Victoria. The high Mn concentration of plant influent (e.g., above 1 mg/L) could lead to treatment difficulties, and possible dirty water events and customer complaints. It has been established in previous studies on the causes of the Mn fluctuations that biological treatment could potentially be a cost-effective and environmentally friendly option for Lorne WTP to control Mn in the water supply system. An investigation of the biological filtration for manganese control in Lorne Water supply system was therefore conducted. The biological filtration Mn control method was analysed with a triple bottom line approach to understand its technological and economic feasibility, and environmental impact. <br><br>It was found that manganese oxidising bacteria (MOB) present in Lorne raw water were successfully inoculated into the two lab biofilters packed with sand and GAC, respectively. The bio-oxidation process led to high removal efficiency of dissolved manganese. The residual dissolved manganese concentration was below Australian Drinking Water Guideline (ADWG) value (0.05 mg/L) for the influent with the various initial dissolved Mn concentrations (up to 0.6 mg/L). The sediments in the high total Mn concentration raw water (&gt; 1 mg/L total Mn) could accumulate on the top of the filter media, resulting in the improved total Mn removal efficiency. Doubling the empty-bed contact time (EBCT) from 20 min to 40 min further improved the removal efficiency for the high Mn concentration raw water, and the effluent Mn concentration was close or below the guideline value. By testing various operating conditions, it was found that the biofilters performed better at the temperature around 25 °C, pH at 7 and 7.5, and dissolved oxygen level around 9 mg/L. <br><br>The investigation of the observed Mn `bleed' phenomenon where the Mn concentration of the effluent could exceed that of the influent showed that backwash and sudden increase of Mn concentration in the feedwater would only affect the Mn removal efficiency for few days. However, a sudden decrease of Mn concentration in the feedwater would affect the biofilter over the long term due to the potential dormancy and death of the MOB. The sand-based biofilter was more robust and stable in operation than the GAC based one, which could be a better option for the biofiltration system.<br><br> The 16s metagenomic analysis identified the top Mn-oxidising and Mn-reducing bacterial species. Pseudomonas spp. was the most dominant Mn-oxidising species for both biofilters due to its greater tolerance in extreme conditions.  Geobacter spp. was the most populous Mn-reducing species for both biofilters because of its ability to survive at the presence of oxygen.<br><br> Preliminary cost analysis suggested a new 10 ML/day biological Mn removal plant would cost US$ 7.1 million (AU$ 9.6 million) which was approximately 45% cheaper than a conventional chemical oxidation plant at the same capacity. The life cycle assessment suggested that biological Mn process could contribute 20-40% less environmental impacts compared to the conventional chemical oxidation process.<br><br> For developing the more reliable and efficient operation of the biofilters, some recommendations were made for future studies. For example, higher Mn concentrations and more extreme operation conditions could be tested to verify the robustness of the process. Inoculation methods and nutrients could be studied for the most efficient way of inoculation as well as for the recovery from Mn `bleed' state. The 16s metagenomic analysis of the filter media could be carried out at different stages of the operation to observe the change of MOB communities and further understand the performance and operation of the biofilters. With more detailed design data or references, more accurate cost analysis and life cycle assessment can be conducted in the future. Further pilot scale trials may be necessary to obtain more engineering data for the possible full-scale applications on the site.