Executive Summary This report presents the motivation, outcomes and industry recommendations for the total 10 week trial of two technologies, struvite precipitation and vacuum membrane distillation (VMD), for the recovery of nutrients (specifically P and N) as a possible alternative to their removal via lime dosing system. The location of the trial was at Ballarat South Waste Water Treatment Plant (BSWWTP), which receives primarily residential wastewater and is therefore a suitable site to trial the technologies due to its similarity in function to wastewater treatment plants operated by Victorian water utilities. Thus the trial has wider relevance to the industry. The project received funding from the Intelligent Water Networks (IWN) with Central Highlands Water (CHW) as the industry champion. The investigation was a collaboration between CHW and Victoria University (VU). Background knowledge for this study was from research previously conducted by VU. Applying this knowledge to the side stream belt press filtrate where the current lime dosing is applied, was identified to be the most suitable location for the trial, as this area of operation has high concentrations of P and N, with phosphate at 45 mg-P/L and ammonia at 78 mg-N/L respectively. N and P in wastewater are key components of nutrients and their removal/reduction form a considerable level of effort and energy utilized in the process. It is envisaged that the recovered nutrients can be added to the biosolids produced from the belt press operation before being sent off site for drying and aging. The trial setup involved arrangements of the two technologies fed with anaerobic digester effluent after belt press filtration to remove suspended solids. Currently at BSWWTP, the filtrate from the belt press is treated with lime to remove P. This treated supernatant still carrying high ammonia is sent to the head of the plant. Two separate pilot units were set up featuring the struvite crystalliser (primarily to precipitate P as struvite) and VMD unit for the reduction of N. The agreed deliverables on the project were:
Perform small scale site trial and critically examine nutrient removal performance; Carry out cost assessment; Produce conceptual design for full scale solution; and Develop recommendation for N and P recovery at BSWWTP and wider industry.
Setup and key technical outcomes During the trial’s first stage (Stage 1) lasting 6 weeks, pre-lime dosing filtrate was fed to the crystalliser to capture P and N as struvite. In a parallel setup, VMD received wastewater from post lime treatment, recovering N as ammonium sulphate solution. After three weeks, the configuration was changed to feed the struvite crystalliser effluent to the VMD (referred to as the in series trial). A second stage of testing (Stage 2) lasting 4 weeks featured only the crystalliser to precipitate struvite, thus remove P and recover some proportion of N and examined sources of input materials. The key technical outcomes of the trial were:
Struvite process for P and N recovery: o Stage 1: The optimum conditions were obtained at HRT of 6 hours (266 L/day) and Mg 2+ dosing rate at 12.2 mg/min (Mg2+:PO43- molar ratio is 2:1). The removal percentages of phosphorus and nitrogen were recorded at 93% and 27% respectively for analytical grade anhydrous MgCl2. Dosing rate of Mg2+ could be reduced from Mg2+:PO43- molar ratio 5:1 to 2:1 without impacting the P removal efficiency. Residual Mg2+ in final discharge was reduced significantly due to decreasing Mg2+ dosing rate; o Stage 2: Removal of phosphorus and nitrogen were recorded at 92% and 30% for commercial grade hydrous MgCl2 (MgCl.6H2O). At this operating condition, no significant difference was found in the performance of nutrient removal from anaerobically digested sludge filtered water via struvite precipitation. Using commercial grade Mg2+, P removal efficiency was not
significantly affected (around 90%). Mg2+:PO43- molar ratio 2:1 would be the ideal lowest dosing rate for future designs and estimates on the full-scale plant. ď‚ˇ VMD process for N recovery: o Lime dosing effluent feed: VMD achieved up to 93% reduction in ammonia (as N). Increasing the VMD plant throughput by 3-fold decreased ammonia reduction to 70% highlighting tradeoff of increasing plant handling at the expense of lower ammonia reductions; o Struvite effluent feed from Stage 1: VMD achieved up to 85% reduction in ammonia (as N). Further reduction may be possible but was not explored due to time constraints. Increasing the VMD plant throughput by 5-fold lowered ammonia reduction to 41% again highlighting trade-off for increasing plant handling at the expense of lower ammonia reductions. ď‚ˇ Combined struvite and VMD process: o By implementing struvite and VMD in series, the combined process reduced both P and N in the belt press filtrate by 89%. Cost and Scope 2 emissions Struvite operation cost was estimated at $0.17 per kL of water to be treated, which was incurred mostly due to the use of chemicals to maintain pH and provide Mg. VMD was found to require mostly energy, which led to the estimated operating cost of $0.02 per kL of water to be treated. Sulphuric acid, the capturing chemical that was found to be convenient for the trial was not included in the VMD cost assessment as low chemical alternatives could be used. Therefore in total the operating cost for struvite and VMD would come to $0.19 per kL water to be treated. This does not include other costs such as maintenance and labour. In terms of energy, VMD required 46% less energy than the BSWWTP aeration blowers on a per kL water to be treated basis. Compared to the entire site energy, this was 82% less energy. This shows VMD energy saving potential compared to current aerobic treatment which in turn corresponded to the same reduction in Scope 2 emissions as a result of the reduced electricity consumption. The thermal energy requirement for VMD can be provided by the combustion of biogas from anaerobic digesters if the process can be designed with 90% internal heat recovery, which is expected to be achieved by integrating the heat exchangers. Recommendations Based on the technical and cost assessment of the struvite and VMD trial, recommendations were made to the Victorian Water Utilities. BSWWTP could serve as a case for trial to implementation. The additional electrical energy and related Scope 2 emissions from struvite precipitation was broadly considered negligible if the current lime dosing system was retrofitted, since both use similar mechanical equipment and devices. However the alkaline (NaOH) use for pH adjustment is a major cost, while the Mg2+ salt was less significant. Although Mg(OH)2 could avoid the NaOH cost it was not successfully shown with the product used in this trial. It was therefore recommended to investigate other Mg(OH)2 products before designing the full-scale plant. VMD technical performance was viable for nitrogen recovery and removal where electrical energy savings were estimated when an appropriate condensation system is utilised. However the trial found the full-scale membrane requirements tied to equipment needs to be large and further work would be needed to optimise performance at large scale. Less membrane requirements are expected on anaerobic digester effluents with higher concentrations than those seen in this trial, where for example values in the order of 1000 mg/L have been reported in literature. Both struvite and VMD for P and N recovery and removal as an alternative to current nutrient removal are therefore technically feasible, but further tests, improvements and engineering designs targeted for scaled up use are recommended.