Water Journal : Water Journal December 2011
water treatment refereed paper technical features 66 DECEMBER 2011 water and LT425 (Ciba Specialty Chemicals, Australia), were also dosed periodically during coagulation as required. Stream 3 -- MIEX/Conventional Coagulation/GAC The third treatment stream was comprised of the product water from Stream 2 with the addition of two parallel granular activated carbon (GAC) filters utilising Filtrasorb 400 GAC (Calgon Carbon Corporation, USA). F400 is a bituminous coal-based GAC with effective granule size 0.55- 0.75mm, which is commonly applied in water and wastewater applications for organic contaminant removal. Filtration was achieved using packed bed columns with gravity-fed empty bed contact times (EBCT) of approximately 14 minutes at 125L/hr per column. The filtrate from both columns was blended into a common storage prior to analysis. Stream 4 -- Microfiltration/ Nanofiltration Dual membrane filtration consisted of microfiltration (MF) pre-treatment for particulate removal using a single submerged hollow fibre module (Memcor CMF-S system, US), followed by a single FILMTEC NF 270-4040 spiral-wound nanofiltration (NF) membrane (DOW Chemical Company, US). The microfiltration system was fed with 1000L/ hr and operated at 75% permeate recovery. The nanofiltration system operated in cross-flow configuration at 50% recovery, producing 325L/hr. Nominal pore size for the microfiltration is reported as 0.2μm, with the molecular weight cut-off for the NF being 270 Daltons, achieving high levels of DOC removal with significant reduction in hardness. Analyses Grab samples for dissolved organic carbon (DOC) and UV absorbance at 254nm (UV254) analyses were filtered through 0.45 μm pre-rinsed membranes. Colour was measured after further filtration through 0.22 μm pre-rinsed membranes to negate the light-scattering effect of ultrafine silt present in this source water, which artificially elevates colour readings. Analysis was by the method of Bennet and Drikas (1993). UV254 was measured through a 1cm quartz cell and colour through a 5cm cell using an Evolution 60 Spectrophotometer (Thermo Scientific, USA). DOC was measured using a Sievers 900 Total Organic Carbon Analyser (GE Analytical Instruments, USA). Turbidity measurements were conducted on a 2100AN Laboratory Turbidimeter (Hach, USA), with results expressed in nephelometric turbidity units (NTU). Total trihalomethanes (THMs), the sum of chloroform, dichloro- bromomethane, chloro-dibromomethane and bromoform, was analysed by headspace gas chromatography with electron capture detection. Results and Discussion This paper is based on two years of water quality data for four treatment processes treating the same source water between July 2009 and June 2011. Within that time, significant water quality changes were experienced, largely as a result of two major events. Following a period of extended drought where river inflows were minimal and source water quality was relatively stable, turbidity spikes in the influent signified the beginnings of two subsequent and different water quality periods, the Mannum-Adelaide Pipeline (MAP) flush and the arrival of floodwaters from Eastern Australia denoted as 'MAP Flush' and 'Flood Inflows' respectively. Data for turbidity and colour is presented in Figure 1, with assigned water quality periods indicated. A clear disparity in the trends of the two parameters is evident between the water quality periods, with the MAP flush producing high turbidity but relatively low colour water. However, with the arrival of the flood waters, the impact of colour was markedly increased. Following the initial spike in the flood inflow period the turbidity diminished rapidly, several months before the decline of raw water colour. Although these are the two most commonly analysed quantitative (turbidity) and qualitative (colour) parameters in drinking water treatment, their relationship to the quality of the water in terms of treatability and disinfection efficiency is limited. The colour reduction for the four treatment streams is shown in Figure 2. Despite variation in raw water colour between 6HU in the stable period to peaks of 107HU within the flood inflows, colour reduction was consistently high, especially for the advanced multi-stage processes (MIEX/Coag/GAC and MF/ NF), which averaged greater than 98% reduction over the two years. As a consequence of the consistently high removals, monitoring of treated water colour yielded limited information on the performance of these processes. For the traditional coagulation treatment stream, the colour data retained sufficient resolution to describe the treatment performance variation. Some difficulty was encountered in maintaining optimum coagulation conditions throughout the changing water quality periods, especially when rapid changes occurred. This was in part due to the reactive nature of coagulation control where decline of treated water quality dictated the operational changes. Within several challenge periods, additional chemicals were dosed to maintain target pH and floc settleability for acceptable filter run times, but only after water quality showed deterioration, leading to the largest span between maximum and minimum reduction percentages of all the treatments. 0 25 50 75 100 125 150 0 50 100 150 200 250 300 1-Jul-09 1-Oct-09 1-Jan -10 3-Apr-10 4-Jul-10 4-Oct-10 4-Jan -11 6-Apr-11 7-Jul-11 Colour (HU) Turbidity (NTU) Mt.Pleasant WTP Source Water Turbidity & Colour Turbidity Colour Figure 1. Raw water turbidity (NTU) and colour (HU) at Mt Pleasant WTP, July 2009 to June 2011. MAP = Mannum-Adelaide pipeline. 0% 25% 50% 75% 100% Conv MIEX/Coag MIEX/Coag/GAC MF/NF % reduced Colour reduction - July 2009 to June 2011 Average Figure 2. Average, maximum and minimum colour reduced by each treatment technology. Conv = conventional coagulation; MIEX/Coag = MIEX adsorption and coagulation; MIEX/Coag/GAC = MIEX, coagulation and GAC adsorption; MF/NF = microfiltration and nanofiltration.
Water Journal April 2012
Water Journal November 2011