Making a clean break
Frank Rogalla assesses the results of a study into the potential effects that changing disinfectant could have on water quality in the distribution system
To meet more stringent regulations for either microbial contaminants or disinfection by-products (DBPs), WTWs are changing disinfectants. While these changes will have beneficial impacts, it is not well known how they will affect distribution system water quality in the long term.
Consequently, the American Water Works Association Research Foundation (AwwaRF) sponsored a project ‘to document the long-term effects of disinfectant changes on distribution system water quality’. This involves collecting, analysing and summarising available data on changing disinfectants. The project focuses on the effects of altering a typical chlorine disinfection scheme to use either chloramines, ozone, chlorine dioxide, UV or booster chlorination. Data from utilities are presented along with potential applications of the project results for utility operations.
There are several major classes of impacts that a change in disinfection can have on a distribution system:
First, a literature search and background review was conducted, assembling all available information on changing disinfectants and the resultant effects on distribution system water quality. Second, issue papers were prepared on each impact listed above, to address key factors affecting water quality as well as their positive or adverse effect. The table on the next page summarises the impacts of chloramines, ozone, chlorine dioxide and UV irradiation, highlights gaps in the available information and suggests data that are needed to fill these gaps.
Evaluation of available data
Data regarding the impacts of changing disinfectants on water quality in the distribution system were obtained by a questionnaire covering WTW size, source of supply, processes, raw and finished water quality, and distribution system water quality. Utilities which had observed changes in water quality after changing disinfection strategy were requested to provide data for parameters that changed – one year of data before the change and two years of data after the change.
Completed questionnaires were received from 18 utilities representing 20 WTWs. Water quality information was received from 15 of the utilities. Several examples illustrating utilities’ experiences in switching disinfectants are presented below.
Conversion to chloramines
A utility in the north-eastern US utilises both surface and ground water. The treatment process includes coagulation, flocculation, sedimentation, filtration, and disinfection.
First, chlorine was used for both primary and secondary disinfection, and then secondary disinfection was switched to chloramines with the following impacts:
Samples were collected at about 80 locations for heterotrophic plate count (HPC) analysis. Before the conversion to chloramines, HPC levels typically were very low – below 15cfu/ml. A few sites had HPC levels between 15 and 150cfu/ml. After the conversion to chloramines, HPC generally remained low but varied with sample location. HPC counts at one location dropped from as high as 60 to less than five. At another site, HPC counts were zero when using chlorine, and increased to between three and ten after the conversion. At a third site, HPC levels went from less than ten to as high as 300.
Trihalomethane (THM) and halo-acetic acid (HAA) data were provided from 13 sites throughout the distribution system. The data shows the switch to chloramines resulted in an overall reduction in THMs from about 80-100ug/l to 40-60ug/l. For HAA, the data shows levels appear to have dropped at most locations. However, at some locations there was no change.
Customer complaint data were differentiated between discoloured water and taste-and-odour complaints. A record of discoloured water complaints shows the number appears to have dropped after the switch from chlorine to chloramines, possibly indicating less corrosive water with respect to iron.
Customer water quality complaints were differentiated between taste and odour as well as between types of taste and odour complaints (i.e. chlorinous, earthy, musty, fishy, etc). Figure 1 shows customer complaints regarding the odour of the water with an apparent reduction in odour complaints after the switch to chloramines, and also a change in the type of descriptors.
With the use of chlorine, the description of the odour was mostly chlorinous or musty. After switching to chloramines, the predominant descriptor has been earthy. It could be that these other types of odours were present with the use of chlorine, but were masked by the chlorine odour.
Conversion to ozone
A centrally-located US utility uses surface water as its supply with conventional treatment processes. Chlorine is used for both pre-oxidation and primary disinfection, as well as chloramines for secondary disinfection. Ozone was introduced for pre-oxidation, with primary and secondary disinfection remaining unchanged. Impacts of the switch were:
Extensive disinfection by-product research at each plant, as well as within the distribution system, showed conversion from chlorine to ozone as the pre-oxidant and primary disinfectant reduced the already low levels of THM and HAA in the distribution system. Since conversion to ozone, a 30-50% reduction in THM and HAA levels has been achieved. Observed simultaneously was a 50% reduction of total organic halides, a general measure of halogenated disinfection by-products. The ozone has also reduced UV-absorbing organics (measured at 254nm) in the raw water by 45-60%. Figure 2 presents TOX levels before and after the switch to ozone.
Customer calls regarding water quality complaints were reduced by 30% from 900 in 1998, when the switch to ozone was made. Since, a further reduction of more than 100 callers a year to less than 250 in 2002 has been observed.
Conversion to chlorine dioxide
Located in the south central US, a river supply is treated with coagulation, lime softening, filtration and disinfection. The disinfectant was changed from free chlorine for both primary and secondary disinfection to chlorine dioxide and to chloramines for several reasons:
The impacts from the changes in disinfection strategy are presented below.
Figure 3 compares average bacterial plate counts obtained at each monitoring location, starting with the implementation of chlorine dioxide. It is evident that initially, the use of chlorine dioxide increased standard plate counts above the desired maximum level of 500cfu/ml in distribution system samples. Hence chlorine was used as a post-disinfectant alongside chlorine dioxide; the improvement in plate count was almost immediate.
Additional data also clearly demonstrated that the population changes with disinfectant type, and that both number and organism type are affected. The microbial population found in the distribution system is therefore a function of the disinfectant. The distribution of bacterial species changed upon the onset of chlorine dioxide disinfection. Bacterial evaluation indicated that a shift from orange and yellow gram (-) rods to white gram (+) rods, similar to slime-forming Bacillus sp., was occurring in the treated water. As the disinfection strategy was refined, the shift in bacterial species distribution continued as the plate counts decreased, so that over 95% were either yellow gram (-) rods and/or white gram (+) rods.
Changes in disinfection strategies can have both beneficial and potentially adverse impacts on the water quality in the distribution system. Utilities should evaluate all the potential impacts before making any changes. The results of this research project are intended to assist utilities in evaluating a disinfection change and the potential impacts on water quality in the distribution system. Specifically, the results might be used in: evaluating current changes; making operational adjustments; evaluating future changes; designing future changes; diagnosing water quality problems.
This study was directed by John Dyksen of Black & Veatch, Oradell, NJ, under a project financed by the American Water Works Association Research Foundation (AwwaRF) under project manager Djanette Khiari.
This article first appeared in WWT magazine.