Catchment Influences on Dissolved Organic Carbon Concentration and Character; Implications for Potable Water Treatment and Trihalomethane Control

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  • Rachel Gough

    Research areas

  • PhD, School of Biological Sciences

Abstract

The presence of dissolved organic carbon (DOC) in raw water can cause a
number of issues in potable water treatment due to its effect on aesthetic, chemical and biological water quality parameters. Of particular concern is the role of DOC as a precursor to potentially harmful disinfection by-products (DBPs), most notably haloacetic acids (HAAs) and trihalomethanes (THMs),
which are formed during the chlorination of natural waters. Rising concentrations of DOC in surface waters draining upland catchments have been reported in the past few decades. This is a major concern for drinking water companies in these areas given the regulatory requirement to maintain
THM concentrations below maximum permissible levels. Coagulation-flocculation is recognised as the most effective means of removing DOC from raw water during potable water treatment and controlling the formation of THMs in finished water.
The present study is focussed on upland drinking water reservoir catchments in the North Wales area. In this area many catchments comprise extensive areas of forest and peatland coverage, both of which have been associated with high DOC loading of surface waters. In this study the concentration and character of leachable DOC was compared in soils derived from different habitats in order to assess the effect of habitat type (beech, spruce, larch and pine forests and blanket peat) on DOC treatability and THM formation potential (THMFP). Significant differences were identified between different habitats, with the beech habitat identified as most appropriate for drinking water catchments due to
it having relatively low leachate DOC concentration, THMFP and colour.
In the 20th century many peatland areas were drained for agriculture, game and forestry, resulting in the destabilisation of their vast carbon stores, and an increase in the export of DOC to surface waters. In this study the effect of peatland restoration via ditch blocking on pore water DOC treatability and THMFP was simulated using a water table manipulation experiment. Overall there was little evidence of differences in THMFP or DOC treatability between different water table regimes. There was also no evidence of variation in microbial activity, based on CO2 flux and enzyme activity measurements,
between different treatments. The results suggest that in the short term at
least, ditch blocking may have little or no effect on DOC export from peatlands, though this could be the result of a delay in the microbial response to a change in water table regime.
A number of catchment characteristic s, including geological, pedological and ecological features are known to exert an influence on surface water quality. In this study GIS mapping was used to display and quantify the spatial extent of ecological and pedological features in two upland drinking water catchments and the subcatchments of their main input streams. Contrasting surface water quality in the two reservoirs could be explained by differences in catchment characteristics, with greater coniferous forest coverage, deeper soils and
shallower mean catchment slope explaining higher DOC loading in Reservoir A. UV absorbance measurements (A253:A203, and to a lesser extent , SUVA) were
also shown to provide a proxy for standardised THMFP (STHMFP), suggesting they may be useful as a means of monitoring THMFP.
Seasonal variations in DOC concentration and character were observed in fluvial and reservoir samples, including THMFP and THM formation rate, indicating temporal variations in THM yields. However, mixing of waters in large reservoirs was found to temper this variability, resulting in a more consistent supply to water treatment works (WTWs). Seasonal variations in DOC character
were found to be partly the result of a seasonal increase in algogenic DOC inputs. A laboratory based experiment was set up to examine the effect of an algal bloom on the treatability and STHMFP of DOC in a water source formerly dominated by terrigenous DOC. Though the algal bloom was associated with a shift towards lower STHMFP and a slower rate of THM formation, it was also
associated with an increase in brominated THMs (BrTHMs; thought to be more carcinogenic) and a decrease in treatability due to it being dominated by hydrophilic, aliphatic DOC. Characterisation of algogenic DOC using high pressure size exclusion chromatography (HPSEC) also showed that DOC
produced during different growth phases of an algal bloom can be identified by peaks in specific regions of the HPSEC chromatogram.
The contribution of individual treatment processes to DOC and THM precursor removal was assessed over a 12 month period at the WTW located at Reservoir A. DOC concentration and STHMFP decreased significantly following coagulation
-flocculation. However, selectivity in the removal of DOC also resulted in an increase in the proportion of BrTHMs. The precipitation-filtration process
designed to remove manganese also resulted in a shift in DOC character.
A jar testing experiment demonstrated the benefits, in terms of DOC and THM precursor removal, of switching the chemical coagulant used at WTWs. However, with all three coagulants included in the study, low molecular weight, hydrophilic, aliphatic DOC was found to be recalcitrant to removal. Enhanced removal of this DOC fraction will be important for THM amelioration if DOC concentrations in surface waters continue to rise.

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Original languageEnglish
Awarding Institution
Supervisors/Advisors
Thesis sponsors
  • Knowledge Economy Skills Scholarship (KESS)
Award date21 Apr 2014