An investigation of fluorescent organic matter in water during springtime at Borgunda water treatment plant

Abstract

An analysis of fluorescent organic matter was performed at Borgunda drinking water treatment plant. The plant draws water from the lake Vättern and provides about 100 000 consumers with drinking water. Previous spring seasons, the plant experienced problems with their sand filters being clogged and occasionally complaints have been made about an unpleasant taste and odour from the water. It is likely that these problems are caused by organic matter such as algae bloom. The objective of the thesis was to investigate how the composition of the water changed over time, how it changed over sand filters and if the sand filters gave different results. The aim of the thesis was to investigate the composition of dissolved organic matter (DOM) at the intake of the raw water, before and after sand filtration at Borgunda treatment plant, using fluorescence spectroscopy. In addition to this, three hypotheses were formulated such as; the fluorophore signal will increase with warmer temperature, with eastern wind direction and the presence of algae can be linked to operational problems in the sand filter. To investigate, a sampling campaign was performed at seven occasions during the spring season at the raw water, incoming water to the plant, outflow of each sand filter, after UV radiation and of the outgoing water. The samples were analysed in the laboratory by using fluorescence, absorbance, and dissolved organic matter measurements. The PARAFAC model was applied and showed that chlorophyll was found in the raw water, incoming and outgoing water. Two components above wavelengths of 550 nm were found that likely correspond to more than one algae species. During a visible pollution event on the surface in one of the sand filters, samples were also analysed in microscope and found diatoms and green algae. The removal performance of the sand filters was analysed, and results indicate that there are biofilms in the sand filters that during the sample period sometimes produce material that increases the fluorescence signal. Sand filer 2 most frequently had worse removal or was producing stronger signals when compared to the other sand filters. Sand filter 3 overall had the best removal performance over the sampling period. No clear connection could be found between the increasing temperature and increased signal in fluorescence components. The eastern wind direction could however be linked to increased signals and is a possible explanation for increased organic material, causing problems in sand filters. Future studies are recommended to investigate the impact of snowmelt and increased flow in runoff streams, that bring nutrient and enables phytoplankton growth in, on surface waters used as drinking water sources.