Approval testing of membrane filtration systems
The increased interest in the use of membrane filtration systems for the treatment of public drinking water supplies requires that these products are approved by the regulators who are concerned by the potential adverse impact of materials on water quality. Bernard Hegarty, from WRc, examines the issues and reports on a recent initiative aimed at developing harmonised test protocols for testing membrane filtration systems.
- increasing concern over health effects of disinfection by-products;
- the ability of membrane filters to remove pathogenic micro-organisms, such as cryptosporidium oocysts, from the raw water; and
- the need for alleviation of potential drought situations caused by climate
change effects, which has forced water suppliers to consider water resources
previously considered untreatable economically, such as brackish or sea
However, these technologies present a challenge to regulatory and approval organisations, who must decide whether the materials of construction of these (and other) products adversely affect the quality of water through the release of harmful substances. In many cases, testing methods used by the testing bodies are based on those used for simpler products such as plastic pipes. These test methods may require modification or re-interpretation when applied to membrane filtration systems.
One of the most important requirements for any product (including membrane filtration systems) used in contact with drinking water is that it should not contaminate the water. Contamination can arise from the leaching or migration of residual traces of chemicals from the materials into the water.
Suppliers of membrane systems usually consider them as technologies to remove contaminants from water, rather than as potential sources of contaminants. The contact time between membrane elements and the water being treated is usually much shorter than the contact time between water and a reservoir or pipe coating, so the potential for contamination may be limited. Nevertheless, most regulators consider that these products need to be assessed and tested using the same principles applied to other products.
The substances of concern to regulators include contaminants, such as residual monomers or additives, that are not monitored routinely in water supplies. In the case of membrane systems, for example, the type of compound that might be released includes the substances used to preserve the membrane, which are mostly flushed out to waste during commissioning.
It is difficult to measure such substances directly under field conditions with enough sensitivity to satisfy toxicological concerns, so laboratory tests are applied that exaggerate leaching. These test concentrations are used to estimate concentrations that would occur at customers' taps. The estimation requires information on the surface area of the material, the volume of water in contact with that area and the contact time, for both the test conditions and the normal conditions of use of the products.
Most approval schemes require detailed compositional information on all components and raw materials of the products, so that the potential for contamination can be assessed. Compliance with positive lists (lists of permitted ingredients) may need to be verified. This can pose problems for filtration systems that contain many small components, such as ŒO' rings and seals, where the suppliers of the components and their ingredients may be reluctant to provide the required information. Confidentiality issues frequently arise.
The major European countries have, independently, developed schemes for assessing and testing products for use with drinking water and approving suitable products.
The use of products used in public water supplies in the UK is subject to Regulation 25 of the Water Supply (Water Quality) Regulations 1989. The Drinking Water Inspectorate (DWI) is the government agency responsible for checking that water companies comply with the Regulations. Approvals under Regulation 25 are granted on the basis of testing and scrutiny and a toxicological assessment by the Government-appointed expert Committee on Chemicals and Materials (CCM). The focus of the assessment is the potential for a product to release harmful organic substances into the public water supplies. The CCM specifies its requirements, usually for leaching and analytical test work, individually for each product. For membrane systems, the CCM asks for extensive information on the product, including:
- its intended use, construction and operational flows;
- the complete composition of each component that contacts the feed and treated water;and
- British Standard BS 6920: 1996 tests on each component (tests for taste of water; turbidity and colour, the release of microbial nutrients, toxic metals; and cytotoxic substances).
From this data the CCM identifies those chemicals that might be released from the membrane system, and specifies leaching tests to monitor the extent to which they are released. The tests involve flushing the intact membrane element in accordance with manufacturer¹s instructions and then leaving fresh test water to stand in the membrane element for three sequential periods of 24, 48 and 72 hours. The water is analysed after each leaching period for leaching substances.
Then the CCM considers whether the leaching observed is acceptable on a toxicological basis. As part of that assessment, CCM converts the concentrations of substances detected in the static leaching tests (which are often measured in mg/l) into concentrations that would be found in practice with a dynamic product.
In the case of membrane filtration systems, the test protocols used in other countries can be significantly different:
In the Netherlands, the Netherlands Waterworks' Testing and Research Institute (Kiwa N.V.) is responsible for the testing and certification of materials used in the production and distribution of drinking water. Leaching tests on membrane elements are undertaken by Kiwa.
The complete element is flushed in accordance with the manufacturer's instructions and then disassembled into its constituent components. The components are then placed in contact with fresh test water at 23oC for three sequential 72-hour stagnation periods. Leach-ates are analysed for leaching substances. Estimated exposures to contaminants (tap concentrations) are generated with a conversion factor that relates the surface area of the part tested to the area of the membrane.
In France, the test elements are installed in a test rig and rinsed according to manufacturers¹ instructions. Test water is then re-circulated through the element for 24 hours, passing through the membrane 50 times during that time, and then analysed for leaching substances. After disinfection, under the same conditions as recommended by the manufacturer, a second 24-hour re-circulation test is carried out with fresh test water and this water is also analysed for contaminants.
The different national approval schemes constitute a significant barrier to trade between countries, and progress in developing and agreeing harmonised standards for materials in contact with water is slow. At present, there is little, if any, mutual acceptance in each country of foreign approvals and manufacturers seeking to have their products approved for use abroad must submit their products to each national approval body for assessment. Costs to applicants of providing the required information for the application can be significant, especially if manufacturers seek to market the products in many countries.
While this problem applies to all materials, including pipes and pipe linings, there is an extra difficulty associated with membrane filtration systems. The complex designs and use of novel materials has led to a corresponding complexity in the test protocols and conversion factors used in each country, so that it is difficult to predict how products tested under one regime will fare in another. WRc has undertaken research, commissioned by the DWI, into alternative leaching test procedures for membrane systems. For example, results using a static test protocol (the UK protocol) were significantly different (leaching rates up to 40 times lower) to those using a dynamic protocol (based on the French re-circulation test) on the same type of membrane element.
The DWI, in collaboration with NSF International and Kiwa N.V, organised an international workshop on this issue. The aim of the workshop was to provide a forum for the development of an international consensus for the regulatory assessment and testing of membrane filtration systems used in the treatment of drinking water. The workshop reached a consensus on the preferred use of dynamic test protocols (rather than static methods) for testing products that are used in a dynamic way. However, more research was required to address the issues of whether re-circulation was likely to underestimate leaching of some contaminants. An alternative would be to use single pass, low-flow test conditions. A comparative study is currently underway at WRc, Kiwa and NSF International. It is intended that the results will become part of the programme within CEN to develop standards for membrane filtration systems.
Suppliers of membrane technology to the water industry have to develop and
market their products with an understanding of their customers' needs. This
may require significant investment in their products, to gain the relevant
approvals. However the current work should lead to a soundly-based test
protocol that will be accepted by international approval bodies and
regulators as a common test protocol.