Water Journal : Water Journal July 2012
water JULY 2012 91 disinfection Victor Harbor is located on the peninsula approximately 80km south of Adelaide. It has a base population of approximately 11,500; however, during peak holiday periods and long weekends this number can be as high as 30,000 (www.victor. sa.gov.au; accessed 8/7/2010). The Victor Harbor WWTP was commissioned in 2005 under a 20-year Build Own Operate Transfer (BOOT) Agreement between TRILITY Victor Harbor (UUVH) and SA Water. The plant is a membrane bioreactor (MBR), consisting of an activated sludge process for biological nutrient removal, followed by immersed flat sheet Kubota ultrafiltration membranes and ultraviolet disinfection (non-validated nominal dose of 36 mJ/cm2). In 2009, the client embarked on a process to implement wider scale reuse across the region of Victor Harbor. In order to achieve this, water quality requirements were specified by the South Australian Department of Health (SA DoH), and upgrades to the plant were required to achieve these targets. Within the Australian Guidelines for Water Recycling (DoH, 2006), Log Reduction Credits (LRV) for virus, protozoa and bacteria are required for specific reuse applications ("fit for purpose"). The expansion of reuse across Victor Harbour will incorporate unrestricted outdoor irrigation. As such, the requirements for disinfection are the same as for dual reticulation systems. The SA DoH has assigned LRV credits for the activated sludge, membrane filtration and UV disinfection processes at VHWWTP, and specified the LRVs required for the expanded reuse program. The existing process steps exceed the LRV credits required for protozoa and bacteria; however, additional treatment is required to satisfy LRV 6 for virus. Table 1 shows the LRV credits assigned for VHWWTP, and the additional credits required for residential garden watering. The options available for attainment of the additional LRV 3.0 for virus are shown in Table 2. Due to the time pressures to produce compliant water, an interim chlorine disinfection system was installed and commissioned at Victor Harbor WWTP in 2009, with a design CT of 30 mg.min/L. The commissioning period identified periods when CT dropped rapidly. This was attributed to peak loads of ammonia coming through the plant, an issue particularly evident during holiday periods. Installation of an ammonia analyser confirmed this theory. At times, the peak loads of ammonia were so high that the chlorine dose required for breakpoint chlorination exceeded the dosing limit of the chlorination system. The bioreactor operates with intermittent aeration, alternating between aerobic and anoxic periods to enhance Simultaneous Nitrification and Denitrification. However, if the plant receives a severe peak load from Pump Station 1 (PS-1), there is insufficient aeration capacity for full nitrification -- this means that ammonia spikes can break through into the treated effluent. The existing in-basin balancing volume is also of little benefit in load peak attenuation. This has not historically been a problem, as both EPA and client quality requirements are based on 21-day composites, allowing for some fluctuation in treated wastewater quality over a given period. However, with the implementation of a chlorination system, plant compliance essentially changed from a 21-day composite to an instantaneous requirement as CT instantaneously reacts to the increased chlorine demand imposed by the presence of ammonia. The problem of CT drop was overcome during regular periods of operation by programming the PLC to increase chlorine dose during periods of expected ammonia increase. However, additional complications were given by the highly variable flow rates through the plant, ranging from 4L/s up to 114L/s. Thus, the control system was required to flow pace dosing of chlorine to a Call 1300 735 123 to find out more about our Data Supply Rental Service options Stormwater, Sewer & Trade Waste / Wastewater & Re-Use / Rivers, Reservoirs & Organics / Com::pass feed forward coagulation control Know what's happening in your water every minute of every day DCM Process Control specialises in "real-time" water quality parameter measurement utilising the s::can range of UV/Vis spectro::lyser's in both the water and wastewater industries. Our unique in-situ water characterisation capabilities are ideal for optimisation, event detection, design, water security and plant control processes. The multi-parameter s::can spectro::lyser has NO moving parts, NO reagents or consumables and is fully submersible. Eliminate the guesswork. Table 1. LRVs attributed to Victor Harbor WWTP. LRV Credits Process Viruses Protozoa Bacteria Activated Sludge Process 1.0 0.5 1.0 UF Kubota Membranes 2.0 3.0 3.0 UV Disinfection (UVTA 'Terminator' Unit) 0.0 1.5 2.0 TOTAL (without Chlorination) 3.0 5.0 6.0 Requirement for Unrestricted Outdoor Irrigation (AGWR Phase 1, 2006) 6.0 4.5 5.0 Table 2. Options for additional LRV 3 for virus. Treatment Step CT/Dose Comment Chlorination 30 mg.min/L Contact time must be based on USEPA "LT1ESWTR Disinfection Profiling and Benchmarking Technical Guidance Manual", incorporating baffling factors. Chloramination 712 mg.min/L at 15°C 534 mg.min/L at 20°C UV Disregarded due to cost and time constraints.
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