ROBUST WATER RECYCLING PLANT FOR THE ANTARCTIC Stephen Gray 1, Jianhua Zhang 1, Mikel Duke 1, Adrian Knight 2, Michael Packer 3, Kathy Northcott 4, Peter Hillis 5, Dharma Dharmabalan 6, Peter Scales 2 1. Institute for Sustainability and Innovation, College of Engineering and Science, Victoria University, Melbourne, Victoria 2. Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, University of Melbourne, Melbourne, Victoria 3. Australian Antarctic Division, 203 Channel Hwy, Kingston, Tasmania 4. Veolia Australia, Bendigo Water Treatment Plant, Kangaroo Flat, Victoria 5. Peter Hillis, AECOM, Level 9, 8 Exhibition St., Melbourne, Australia 6. TasWater, Moonah, Hobart, Tasmania ABSTRACT This project is assessing the performance of an advanced water treatment plant (AWTP) for producing water suitable for drinking and for producing a waste brine stream of low environmental concern. The plant is being trialed in Tasmania before relocation to Davis Station, Antarctica. The remote location dictates that the energy and chemical volumes should be low and the requirement for skilled operators limited. Preliminary results to date have demonstrated that the final product water quality has been within the ADWG and the energy consumption is significantly lower than the current practice of desalination of a hypersaline tarn. The extent of operator involvment is still being assessed, as is chemical use. INTRODUCTION An advanced water recycling plant for Davis Station, Antarctic is being operated at Selfs Point, Tasmania to demonstrate the robustness of the process prior to shipping to the Antarctic. Davis Station s secondary treatment plant failed in 2005, and the station has since this time discharged macerated sewage in-line with the Madrid Protocol. An environmental impact assessment conducted by the Australian Antarctic Division (AAD) in 2010-11 identified the need for a secondary or tertiary treatment plant. The implementation of a secondary or tertiary treatment system provides the opportunity to further treat the effluent to drinking water quality for potential reuse within the station, and so avoid or reduce the need to source water for desalination from a hypersaline tarn. Energy production at Antarctic Stations is expensive and reducing the energy required for water production has considerable benefits for AAD. Recycling of water may also allow greater water use on stations, where it is currently limited. Therefore, a study is being conducted to demonstrate the reliability of a novel recycling process that is able to produce water of quality suitable for drinking with limited local operator involvement, low chemical volumes and low energy use. Davis Station has an average population of 120 in the summer and 25 over winter. A QMRA study 1,2 identified that for the small populations associated with Antarctic bases, the Log Reduction Values (LRV) required to maintain the tolerable annual disease burden of 10-6 DALY s per person per year during a disease outbreak were higher than for large municipal plants. LRVs of 12.4 for bacteria, 12.1 for virus and 10.4 for protozoa are required. Davis Station will install a secondary MBR plant to treat their wastewater, and an AWTP to produce drinking quality water and a brine stream of low toxicity. The AWTP demonstration plant consists of: ozonation, ceramic microfiltration, biologically activated carbon (BAC), reverse osmosis (RO), ultraviolet disinfection (UV), calcite contactor and chlorination (see Figure 1). The LRVs claimed and the LRVs achievable across each unit process is shown in Table 1. Performance results for the AWTP using Selfs Point biological nutrient removal effluent are reported. METHOD/ PROCESS The plant for Davis Station was constructed in 2 shipping containers (see Figure 2) and installed at Selfs Point, Hobart. The plant has been operating since July 2014 and the demonstration will continue until June 2015. This plant will be shipped to Davis Station during January 2016 for operation on MBR effluent, so it is the actual plant under consideration for delivery of drinking quality water. Self s Point final effluent prior to UV disinfection was sourced as feed to the AWTP. The feed was screened (2 mm) prior to entering the AWTP. The AWTP operates at a constant flow rate of 20 L/min. The dramatic variation in population at Davis Station requires the plant to operate in a near continuous manner during summer but for only 4 hours every second day during winter.
Operation at Self s Point mimicked these operational modes. The plant operates in a semi-batch mode. Flow is maintained at 20 L/min through the plant but the final chlorination stage consists of two contact tanks. Operation of the plant has one contact tank filling while the other is providing the contact time for chlorination. Prior to discharging a batch of water from the final contact tanks to storage, each critical control point is checked to confirm that the specific batch of water has had suitable treatment. A key objective of the trials was to demonstrate attainment of reliable water quality, as well as to ascertain the level of operator attention and maintenance required for plant operation. Therefore, demonstration of the process involved substantially more water quality testing than would be anticipated at Davis Station, as well as frequent verification of on-line instruments to identify calibration requirements. The operation of the plant was automated and the role of the operators at Selfs Point was to assess the performance of the automated operation. Once installed at Davis Station the plant will have the ability to be remotely operated, with expert advice provided to local operators to maintain plant performance. Water quality analyses were performed by a mixture of on-line instruments, TasWater s laboratory at Selfs Point, Victoria University, University of Melbourne and a range of specialist analytical laboratories. On-line sensors measured turbidity, ozone residual, flowrate, conductivity, UV dose, and chlorine residual. TasWater s laboratory performed microbiological tests, colour measurements, alkalinity, ammonia, phosphorus, TSS and nitrogen analyses. Victoria University analysed samples for DOC, total nitrogen, metals, and fluoride. The University of Melbourne measured micro-contaminant concentrations and removal through the process as well as performing recombinant receptor reporter gene bioassays. Only limited data is reported in this paper, with micro-contaminants results being report by Allinson et al 3. RESULTS/ OUTCOMES Commissioning of the purified water plant has been completed and the plant has been operating for 6 months of a planned 12 month trial. The design and performance of each unit process is critiqued to provide background prior to a discussion of the overall plant performance. Feedwater Selfs Point feedwater quality is generally good, with turbidities between 1-2 NTU, ammonia concentrations of approximately 0.3 mg/l and DOC concentrations of 8-10 mg/l. However, there have been maintenance and wet weather events that have resulted in effluent quality of 3-5 NTU, ammonia concentrations of 6-12 mg/l and no increase in DOC. The feedwater quality at Davis station is anticipated to be of higher quality than that of the Selfs Point effluent, as a membrane bioreactor (MBR) will be used to treat the Davis Station wastewater. Therefore, the trials at Selfs Point represent a poorer feedwater quality than that anticipated in future service. Ozone The ozone system used was a Wedeco, OCS- GSO 10 packaged unit containing facilities for both ozone production and ozone contact. At 20 L/min the average HRT was 24 min. and the HRT 10 as determined by tracer studies was 5.0 min. The HRT 10 was used in determining the CT values for ozone contact as is required by the US EPA guideline 4. Initial commissioning tests identified the need to match the maximum output of the ozone generator with the specific characteristics of the ozone cells, as several plant breakdowns occurred prior to tuning of the system. However, once the maximum output was limited, the system performed reliably with no further mechanical issues. Residual ozone concentrations of over 1 mg/l could be achieved when the feed water quality was good with turbidity of 1-2 NTU. The ozone dose was measured to be 13.5 mg/l, giving an ozone:toc dose of approximately 1.4-1.7 mg O 3 /mg TOC (measured DOC was 8.5 mg/l). The required ozone residual set point for achieving 0.5 LRV protozoa (>4 LRV virus) is 0.4 mg/l (2.0 mg.min/l) based on the US EPA guidelines 4 and particle free water. Therefore, under conditions of low turbidity feedwater the ozone capacity was sufficient to meet the design objectives for the plant. However, during periods of poor feed water quality, when the feedwater turbidity varied from 2-5 NTU and ammonia levels rose, maintaining ozone residual became difficult. Nevertheless, LRV across the ozone process based on Ecoli was always >2.5, as shown in Table 2. Inactivation of Ecoli and virus by ozone has been shown to be similar 5, so comparison of Ecoli inactivation can be compared to the CT values required for virus. The required CT for 4 LRV virus in particle free water is 0.5 mg.min/l corresponding to an ozone residual of 0.1 mg/l in the current system, while a 2 LRV virus can be achieved with a CT of 0.25 mg.min/l (0.05 mg/l) in particle free water 4. The LRV E.coli results in Table 2 show that LRV > 2.5 was always achieved, although the residual ozone concentration did not always correlate with the LRV obtained. Hence, a LRV of 2 is claimed across the ozone system and further monitoring is being conducted to confirm if this is appropriate.
Removal of DOC across the ozone unit could not be detected, and DOC concentrations in the feed and post-ozone were between 8-10 mg/l. Bromate concentrations have not yet been determined, and at Davis Station the bromide concentrations are such that complete conversion of all bromide to bromate would not represent a health issue for the product water. However, bromate and iodate concentrations will be determined during later trials, and dosing of bromide and iodine in the feed is planned to assess performance over a wide range of feed concentrations as may be encountered at other locations. Ceramic MF The ceramic microfiltration (MF) membrane system used was from Metawater. It operates at 50 L.m -2.hr -1, with one element in operation while a second is in standby mode. Metawater membranes have been validated by the Californian Department of Health for 4 LRV protozoa and 1 LRV virus removal based on pressure decay tests (PDT). A PDT is performed for each batch of treated water (batched in the chlorination stage) as well as a backwash. Chemically enhanced backwashing is performed every 15 backwashes. Fibre breakage is not an issue with ceramic membranes of this type and membrane failure has not been observed in 15 years of operation in Japan. This reliable integrity performance was one of the key reasons for choosing these membranes. Each PDT has been within 0.25-0.45 kpa/min, and has been significantly lower than the 1.3 kpa/min required for verification according to the Department of Health, California. Backwashing of the elements has removed all fouling, with no build-up of irreversible fouling observed over the 5-6 months of operation (see Figure 3). The high ozone dose up-stream of the ceramic MF was designed to assist in keeping the ceramic MF clean, as was the low operating flux and chemically enhanced backwashing. Turbidity values of 0.2-0.3 NTU are generally obtained from the ceramic MF. Biologically Activated Carbon (BAC) The purpose of the BAC is for removal of microcontaminants, and there is no CCP for the BAC. Good micro-contaminant removal has been observed 3. DOC removal across the BAC was initially 50% but has decreased to 40% after 3-4 months operation, suggesting that the adsorption capacity of the BAC is reducing (See Figure 4). Iron and manganese are also reduced by 50% across the BAC suggesting that iron and manganese oxidising bacteria are present in the BAC. Readily biodegradable organic carbon (RBOC) increased from 2.7 mg/l in the plant feed to 4.4 mg/l following ozonation. It then decreased to 3.8 mg/l following ceramic MF and to 1.9 mg/l post-bac. While the reduction across the BAC is significant, lower RBOC values (0.5 mg/l) are required to produce biologically stable water. Turbidity of water post-bac experiences peaks when operation re-commences after shut down, but stabilises at 0.2-0.3 NTU after approximately 10 minutes operation. Reverse Osmosis (RO) The RO process consists of 5 elements contained in 5 single element housings in series. A recycle loop is used to increase the process recovery and the process recovery is set at 70% to avoid scaling. No anti-scalant is used to minimise chemical consumption and also to reduce compounds discharged into pristine Antarctic waters. The RO feed pressure is limited to 14 bar by the RO feed pump. The batch processing nature of the feedwater requires that the RO elements be rinsed with permeate at the end of each campaign. The design of the system is also such that when in standby mode, permeate osmotically backwashes the RO membranes. Should the time between batches be longer than 2 days, sodium meta-bisulphite is used to prevent fouling and membrane degradation. RO is used to further remove micro-contaminants and as a barrier for pathogens. Conductivity is used to determine the LRV on-line, and this is able to reliably achieve an LRV >1.5. However, additional LRV for protozoa was desired and a PDT was implemented following laboratory trials that indicated an LRV of 2.5 could be achieved for protozoa (TMP=45 kpa). Laboratory tests on a 4 element also repeated the PDT >300 times to demonstrate that no detrimental impact to the RO element arose from the PDT. Thus far, the RO system has always achieved an LRV >1.5 from conductivity measurements and an LRV of 2.5 from the PDT. Fouling of the RO membranes was observed after 5 months operation, with the pressure at the maximum 14 bar and a reduction in recovery. A CIP was conducted by caustic cleaning followed be an acid clean that returned the RO system to good performance, with operation at 10 bar and increased salt rejection (99%). An autopsy is being conducted with results soon available. Annual replacement of membranes during AAD s summer maintenance period is an option that they are considering to reduce requirements for CIP. Ultra-violet disinfection (UV) The UV system is comprised of two Wedeco Specktron 6 UV systems in series. The UV lamps are designed to achieve 4 LRV virus for a single
lamp, and 2 are used in series should 1 lamp fail. The small size of the treatment plant and the limited range of packaged UV systems means that each UV lamp is able to achieve a dose of 500 mj/cm 2, far in excess of the 186 mj/cm 2 required. The system uses low pressure high efficiency lamps, and has an on-line UV dose measurement system. Thus far there has been no issue with the UV system and the UV dose has always exceeded the CCP. A consistent drop in DOC across the UV system is suggested by DOC measurements, although the change in DOC is minor. Further fluorescence characterisation and more sensitive organic concentration tests are following up this possibility. Calcite Filter The calcite filter is designed to re-stabilise the RO permeate before chlorination. The water passes through a calcite bed and the final ph and alkalinity is consistently between ph 6.5-7.5 with alkalinity of 50 90 mg/l. The degree of alkalinity varies depending upon how frequently the filter is re-filled. Refilling is occurring every 3 months or more. Chlorination Chlorination is achieved by dosing of hypochlorite with a contact time of 50 minutes in the batch contact tank. The design is such that water is pumped through a side stream to maintain mixing in the contact tank, and the residual chlorine can be measured on-line in the feed and discharge from the tank. RO permeate has very low chlorine decay with little to no change in the residual chlorine with time. DISCUSSION Product water quality has always met the Australian Drinking Water Guideline values, although analyses for disinfection by-products are yet to be conducted (THM s, HAA, bromate, Iodate). Given the very low organic carbon concentrations in the permeate, organic based disinfection by products are expected to be low and tests to confirm this will be undertaken. The 5 barriers for pathogen control provides reliable protection, as does the batch based processing mode where all CCPs are confirmed before a batch of water is sent for storage. On-line CCPs can be measured for all barriers, assuming an ozone residual can be achieved. For Davis Station, particle free MBR effluent will be fed to the plant, and maintaining an ozone residual should be possible. For Selfs Point, the higher feed water turbidity has meant that an ozone residual is not always detected. Nonetheless, monitoring of E.Coli LRV across the ozone system has demonstrated >2.5 LRV on all occasions, even when a low ozone residual was present. The relatively high turbidity of the Selfs Point feed water means that an ozone residual cannot be reliably obtained leading to reliance on ozone dose or microbial testing to verify the LRV across the ozone process. Further work is required before ozone dose can be presented to regulators as a CCP. Micro-contaminants are effectively removed by ozonation and BAC 3, and provides a brine stream more suitable to discharge in the Antarctic Ocean than secondary wastewater. These water quality data suggest that the demonstration plant is technically able to achieve the desired water qualities. Further operation and water quality testing until May, 2015 is intended to confirm its reliability for producing suitable quality water. Furthermore, current water production at Davis Station is achieved by RO treatment of a hypersaline tarn. The water is heated from 4 C to 20 C before RO treatment, and the resultant energy consumption is estimated to be >50 kwh/m 3. A preliminary estimate of the energy consumption for the AWTP is 3 kwh/m 3, representing a significant saving in energy by use of recycled water. It is also a significant cost saving given all electrical energy is obtained by diesel generators on-site. Further analysis of the energy consumption is required to provide a more accurate energy consumption estimate as well as to provide comparisons against other alterative water sources (e.g. brackish water) that may be available at other remote sites. Imported chemicals need to be limited because transport to the Antarctic is costly and available space on the few ships that travel to site is highly competitive. Therefore, production of ozone onsite is attractive, and the only imported chemicals required for operation are those associated with chemically enhanced backwashing of the ceramic MF (hypochlorite, sulphuric acid), clean-in-place cleaning of RO elements (sodium hydroxide, hydrochloric acid), sodium meta bisulphite, calcite and hypochlorite for chlorine disinfection. The volume of chemicals required for each CIP is <1 L in total, sodium meta bisulphite is made on-site from powdered form, and sulphuric acid cleaning of ceramic membranes is only every 5 CEB. In comparison, the volumes of sodium hypochlorite (200ml/day for disinfection during summer @ Davis Station) and the volume of calcite used (estimated at 175 L/year at Davis Station) are significantly larger than the other chemicals. The volumes of chemicals used are still being determined, with aging of sodium hypochlorite and the effect of this on dosing volumes, as well as the intermittent nature of filling the calcite filter currently limiting the current accuracy of estimates.
Further monitoring is required to determine the required level of operator involvement. Currently water sampling and verification of on-site meters is conducted frequently to demonstrate process reliability, but a reduced program is intended for operations at Davis Station. Target levels of operator involvement are weekly verification of online instruments and weekly water quality testing for biological pathogens, with instrument calibration upon drift of the instrument (>5% of the reading or a set absolute value). CONCLUSION Performance of a small scale purified water recycling plant for Davis Station is presented. The plant was designed for remote operation with minimal local operator requirement and low maintenance, chemical volumes and energy use. Current performance demonstrates that the operation of the process is capable of producing water quality that meets the ADWG. Ozonation of the unfiltered Selfs Point effluent demonstrates reliable 2 LRV E.coli and enhanced performance is expected on MBR effluent at Davis Station. The outcomes are expected to be applicable to other remote operations where purified water production is desirable. 2. S. Fiona Barker, Michael Packer, Peter J. Scales, Stephen Gray, Ian Snape, Andrew J. Hamilton, Pathogen reduction requirements for direct potable reuse in Antarctica: Evaluating human health risks in small communities. Science of the Total Environment, 461-462 (2013) 723-733 3. Mayumi Allinson, Graeme Allinson, Kiwao Kadokami, Daisuke Nakajima, Peter Scales, Adrian Knight, Jianhua Zhang, Michael Packer, Kathy Northcott, Stephen Gray, A simplified but extensive organic microcontaminant assessment method for recycled water. Ozwater15 4. US EPA Long term 2 Enhanced Surface Water Treatment Rule Toolbox Guidance Manual, April 2010 http://www.epa.gov/safewater/disinfection/lt2/ pdfs/guide_lt2_toolboxguidancemanual.pdf 5. Sigmon, C., Shin, G-A., Mieog, J. Linden, K.G. (2014) Establishing surrogate virus relationships for ozone disinfection of wastewater. Submitted to Water Research ACKNOWLEDGMENTS The authors acknowledge the financial support of the Australian Water Recycling Centre of Excellence, which is funded by the Australian Government through the Water for the Future initiative. The project is also supported through the Collaborative Research Network (CRN) program of the Australian Government, and by the Australian Antarctic Division. REFERENCES 1. SF Barker, M Packer, PJ Scales, S Gray, I Snape, AJ Hamilton, Do the Australian guidelines for water recycling protect small or remote communities? Water (AWA), 40(1), 2013, 85-87
Process Table 1: LRVs claimed for each unit process LRV Virus Bacteria Protozoa Helminths Claimed Credible Claimed Credible Claimed Credible Claimed Credible MBR 2 3 2 4 2 4 2 4 Ozone 2 4 2 4 0 0.5 0 0 Ceramic MF 1 4 1 4 4 4 4 4 Biologically Activated Carbon (BAC) 0 0 0 0 Reverse Osmosis 1.5 4 1.5 4 2.5 4 2.5 4 Ultra violet disinfection 4 4 4 4 4 4 0 1 Calcite Filter 0 0 0 0 Chlorination 4 4 4 4 0 0 0 0 Total 14.5 23 14.5 24 12.5 16.5 8.5 13 Required for Davis Station 12.1 12.3 10.4 8* Typically required for large municipal wastewater plant 9.5 8 8 8 Table 2 E.coli concentrations, ozone residual and LRV across the ozonation process Date Ecoli Residual Feed (MPN/ml) Post ozone (MPN/ml) LRV Ozone (mg/l) 17/9/14 1,986.3 3.1 2.81 0.47 24/9/14 1,553.1 4.1 2.58 0.28 1/10/14 1,732.9 5.2 2.52 0.42 8/10/14 1,553.1 4.1 2.58 0.67 15/10/14 >2,419.6 2.0 >3.08 0.21 22/10/14 >2,419.6 4.1 >2.77 0.42 29/10/14 >2,419.6 1.0 >3.38 0.92 5/11/14 >2,419.6 3.1 >2.89 0.36 12/11/14 >2,419.6 4.1 >2.77 0.37 19/11/14 >2,419.6 1.0 >3.38 0.04 26/11/14 13,500 1.0 4.13 0.06 2/12/14 24,300 2.0 4.08
Figure 1: Purified water process flowsheet Figure 2: Purified water plant at Self s Point, Hobart
Figure 3 Fouling trend for ceramic MF Figure 4 DOC reduction across the BAC