A case study to determine the efficacy of ozonation in purification processes

A case study to determine the efficacy of ozonation in purification processes S Morrison*, A Venter and S Barnard School of Environmental Sciences and Development: Botany, North-West University, Potchefstroom 2520, South Africa Abstract The aim of this study was to determine the efficacy of ozone in water purification processes at the Midvaal Water Company, which uses the hypertrophic Middle Vaal River for source water. It was found that pre- and intermediate ozonation had no significant effect on pH, conductivity, dissolved organic carbon (DOC) and total organic carbon (TOC). Chlorophyll-a, total chlorophyll, spectral absorbance coefficient (SAC 254) and total algal cells were not influenced by pre-ozonation (as desired) but were greatly reduced after intermediate ozonation. The dissolved air flotation step which occurs after preozonation and prior to intermediate ozonation contributed to an average total chlorophyll removal of 74%. The effect of ozonation on the removal of manganese, iron and aluminium could not be determined during this study since these elements were present in relatively low concentrations in the source water. Intermediate ozonation had variable effects on the removal of Cyanophyceae, Dinophyceae, Euglenophyceae and Chlorophyceae, but Chrysophyceae, Bacillariophyceae and Cryptophyceae were greatly reduced after this stage. Keywords: water treatment, pre-ozonation, intermediate ozonation, SAC254, chlorophyll, algal cells Introduction Water is a scarce resource in South Africa and this situation may very well intensify over time, forcing purification companies to utilise every available water source, even a river, reservoir or wetland with water that looks like pea soup, or one clogged from bank to bank with aquatic plants (Davies and Day, 1998). However, due to the efficiency of water treatment processes, water for potable supply need not be of the highest quality (Mason, 1991); if a water source of better quality is not available, advanced treatment methods have to be implemented and developed. Potable water with tastes and odours can often be linked to a polluted or eutrophic natural resource and conventional water treatment methods may not effectively remove it. Therefore, it may be necessary to use more advanced water treatment processes in order to produce potable water of an acceptable quality (Pryor and Freeze, 2000). Midvaal Water Company (Fig. 1) is situated on the banks of the Middle Vaal River (26º 48’ 1.4” East and 26º 56’ 4.5” South) in South Africa and abstracts between 95 and 180 Mℓ of water from the Vaal River per day. It has a capacity to treat 320 Mℓ of water daily. The Middle Vaal Water Management Area is mostly rural, where agriculture, mine de-watering, and the subsequent discharge into the river system, impact on the water quality (Anon, 2008). Tributaries in the catchment of the Vaal River also contribute to the deteriorating water quality of Figure 1 Schematic diagram of the different water treatment processes at Midvaal Water Company (midvaalwater.co.za) 1 = Water abstraction (not shown on diagram) 2 = Pre-ozonation and flocculation channels 3 = Dissolved air flotation 4 = Intermediate ozonation 5 = Control room 6 = Sedimentation 7 = Filtration 8 = Disinfection with chlorine gas 9 = Water recovery 10 = Storage and distribution 11 = Office (not shown on diagram) 12 = Laboratory * To whom all correspondence should be addressed.  +27 18 299 2517; fax: +27 18 299 2370; e-mail: Received 28 June 2010; accepted in revised form 14 December 2011. the Middle Vaal system (Anon., 2008) by introducing various pollutants into the system at times. The production of drinking water from natural water necessitates the removal of numerous compounds present, mainly inorganic species, humic substances and pollutants (Camel and Bermond, 1998). Ozone, a potent germicide, is also used as an oxidising agent for the destruction of organic compounds producing taste and odours in drinking water, for the destruction of organic colouring matter and for the oxidation of reduced iron or manganese salts to insoluble oxides (Eaton et al., 1995). http://dx.doi.org/10.4314/wsa.v38i1.7 Available on website http://www.wrc.org.za ISSN 0378-4738 (Print) = Water SA Vol. 38 No. 1 January 2012 ISSN 1816-7950 (On-line) = Water SA Vol. 38 No. 1 January 2012 49 0.5 0.4 60 0.3 40 0.2 20 0.1 Final water total chlorophyll (ug/l) Source water total chlorophyll (ug/ℓ) 0.6 80 0 0 2009 2008 2007 2006 2005 2004 Source water 2003 2002 2001 2000 1999 1998 1997 1996 Final water 140 0.8 120 0.7 0.6 100 0.5 80 0.4 60 0.3 40 0.2 20 0.1 0 0 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 Source water Final water turbidity (NTU) Source water turbidity (NTU) Figure 2 The average annual total chlorophyll (µgℓ-1) of the source and final water from 1994 to 2009 1995 50 0.7 100 1994 Water samples were collected weekly for a 1-year period (October 2007 to September 2008), from the source water and after the water had been treated with an average of 2.4 mg∙ℓ-1 ozone for 4 min (intermediate ozonation). The pre-ozonation step was implemented only during the latter part of the study and water samples were collected weekly for the last 4 months of the study after the source water had been pre-ozonated with 1.3 mg∙ℓ-1 ozone for 2 min. Standard methods, some of which were accredited by the South African National Accreditation System (SANAS), were used to determine the pH, conductivity; turbidity (NTU); chlorophyll-a; total chlorophyll (chlorophyll-a and phaeophytin-a); dissolved organic carbon (DOC); total organic carbon (TOC); manganese (Mn), iron (Fe) and aluminium (Al), as well as Spectral Absorbance Coefficient (SAC 254) at Midvaal Water Company Scientific Services. Methylisoborneol (MIB) and geosmin analyses were done at Rand Water using the Purge and Trap system coupled to GC-MS. Phytoplankton identification and enumeration were done according to the sedimentation technique using gravity as described by Swanepoel et al. (2008). Due to the financial implications it was not expedient for the company to obtain multiple replications of a specific variable at each time interval for this case study. In some instances, problems with the infrastructure resulted in missing values at certain time intervals. Due to this fact, and the time-dependent nature of the data, statistical inference and other analyses such as principle component analyses were not an option. Therefore, effect sizes (Ellis and Steyn, 2003), instead of the usual p-values, were used to determine the importance of a specific 0.8 1995 Materials and methods 120 1994 Due to its high oxidation potential ozone has been widely applied to water treatment. Midvaal Water Company has two U-tube reactors for intermediate ozonation (Fig. 1, No. 4) and also has a radial diffuser in the pre-ozonation reactor (Fig. 1. No. 2) in order to transfer (...truncated)