REMOVAL OF ORGANIC CONTAMINANTS FROM DRINKING WATER
Report No FR0252
To review the occurrence and identity of organic contaminants present in raw water and drinking water in the U.K. To assess published information on the removal of organic contaminants from water, review the performance of processes that have been inves tigated and make recommendations based on the findings of the review.
Raw water contains a wide range of organic contaminants, the identity and concentration of which depend on the catchment. Some of these contaminants are subject to the Water Quality regulations and therefore if present in the raw water must be removed. Other, background organic compounds, often measured as colour, can react with chlorine to produce undesirable by-products (CBPs); some of these are regulated. Considerable effort has been spent on examining ways of removing organic contaminants. A review of this work will aid in the evaluation of the capabilities of installed treatment plant, selection of new treatment plant, and identification of areas requiring further research.
There is a general lack of detailed information on the occurrence of many specific organic contaminants, including some that are regulated.
Pre-chlorination is responsible for the production of a large proportion of chlorination by-products (CBPs) found in drinking water. CBPs can not easily be removed by any existing treatment process. Reduction of their concentrations can only be achieved by preventing their formation by the removal of precursors prior to addition of chlorine.
Conventional coagulation is capable of removing background organics such as colour but neither it nor biological treatment is able to remove the majority of organic contaminants.
Activated carbon can be effectively used in a powdered or granular form and is identified as the process most capable of removing organic contaminants. Much of the information available on the performance of carbons has been derived under conditions that do not relate directly to treatment conditions; extrapolation of such information for selection and design purposes is difficult.
Ozonation is less versatile than activated carbon and can produce by-products that are less desirable than the parent molecule; some of the by-products are biodegradable and these should be removed by biological processes such as slow sand filtration or biological activated carbon. The latter should not be considered as a long term adsorption process. Ozonation is an effective process for reducing the production of CBPs.
Advanced oxidation processes (AOP) such as ozone/peroxide and ozone/UV are capable or mineralising organic compounds and of oxidation at much higher rates than ozone alone. They are still in the development stage but are proving themselves for relatively simple tasks such as organics removal from ground water. The same is true for membrane processes, in particular nanofiltration.
The formation of CBPs should be reduced by confining chlorination to a point in the treatment process where organic carbon is at a minimum. Coagulation should be operated to maximise organics removal by the application and control of the correct coagulation chemistry. Ozone should be used if further reductions in CBPs are required.
Activated carbon should be installed at surface water treatment works when there is a need to remove contaminants that can not be removed by coagulation. This should be done in conjunction with the identification and implementation of the correct coagulation chemistry to remove background organics.
Contaminated ground water supplies should be treated by either air stripping, GAC, or a continuation of both. The use of an AOP such as ozone/UV or ozone/peroxide can be considered in some cases.
The selection and design of an organics removal process should be based on data that relates as closely to the water to be treated as possible. Where such data is not available it should be obtained by means of laboratory and/or pilot scale tests.
Further work should be undertaken into the assessment and development of AOP and membrane processes.
A fully resourced survey of organic contaminants present in U.K. raw and drinking waters should be undertaken. This will permit better targeting of research effort into the removal of specific compounds.
V RESUME OF CONTENTS
Published information on the removal of organic contaminants has been reviewed in the context of a list of compounds based on current and possible future water quality regulations. The performance of coagulation, activated carbon, ozonation, advanced oxidation, membranes, biological processes, and air stripping is reviewed for a number of specific compounds or groups of compounds. A separate section on chlorination by-products is provided. Cost functions for unit processes are also given. A bibliography of 278 references is appended.
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