Carbon isotopic characterisation of dissolved organic matter during water treatment

John Bridgeman,Pauline Gulliver,Jessie Roe,Andy Baker

doi:10.1016/j.watres.2013.09.025

Abstract

Water treatment is a series of physio-chemical processes to aid organic matter (OM) removal, which helps to minimise the formation of potentially carcinogenic disinfection by-products and microbial regrowth. Changes in OM character through the treatment processes can provide insight into the treatment efficiency, but radiogenic isotopic characterisation techniques have yet to be applied. Here, we show for the first time that analysis of 13C and 14C of dissolved organic carbon (DOC) effectively characterises dissolved OM through a water treatment works. At the sites investigated: post-clarification, DOC becomes isotopically lighter, due to an increased proportion of relatively hydrophilic DOC. Filtration adds ‘old’ 14C-DOC from abrasion of the filter media, whilst the use of activated carbon adds ‘young’ 14C-DOC, most likely from the presence of biofilms. Overall, carbon isotopes provide clear evidence for the first time that new sources of organic carbon are added within the treatment processes, and that treated water is isotopically lighter and typically younger in 14C-DOC age than untreated water. We anticipate our findings will precipitate real-time monitoring of treatment performance using stable carbon isotopes, with associated improvements in energy and carbon footprint (e.g. isotopic analysis used as triggers for filter washing and activated carbon regeneration) and public health benefits resulting from improved carbon removal.

Highlights

Provision of a sufficient volume of appropriately treated water, free from potentially harmful chemical and microbiological contaminants is a fundamental requirement for human life
Whilst exact design and operational details will be determined by raw water quality characteristics, a surface water treatment works (WTW) will generally involve screening of gross solids, coagulant addition to precipitate and destabilise negatively charged colloidal matter, slow mixing to encourage collision and agglomeration of destabilised particles into flocs, settlement of flocs in a clarification stage, granular media filtration
Filter and granular activated carbon (GAC) media from the relevant treatment processes at each WTW were analysed for their carbon isotope properties

Summary

Introduction

Provision of a sufficient volume of appropriately treated water, free from potentially harmful chemical and microbiological contaminants is a fundamental requirement for human life. (UNESCO, 2009; EPRI, 2002; WaterUK, 2010); see Fig. 1 This is equivalent to approximately 3.6% of annual anthropogenic carbon emissions, and is 45% of the total carbon that is transported, mineralised and buried in inland waters (Battin et al, 2009). The use of chemicals in water treatment is widespread, being required for coagulation, as a flocculant aid, for pH correction and for disinfection (0.074 tonnes of chemical per megalitre (106 L) of drinking water produced (WaterUK, 2010)). This further compounds the urgent need for identification and elimination of process inefficiencies. Carbon accounting at the unit process level can help facilitate the development of new carbon-efficient technologies

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Conclusion