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Membrane Distillation and Applications for Water

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Water treatment is an important auxiliary process in all thermal cogeneration plants. In this context membrane distillation (MD) is a novel technology that is potentially advantageous to technologies like reverse osmosis in the following ways: ability to utilize low-grade heat, reduced sensitivity to fluctuations in pH or salt concentrations, and lower capital and O&M costs. This research is a continuation of a Värmeforsk prestudy and encompasses field trials conducted with a test rig deployed at Idbäcken Cogeneration Facility (Nyköping) with a five-module MD unit capable of producing 1-2 m3/day purified water (see Figure 1). A long-term performance evaluation including thorough chemical testing of product water quality is presented. District heating supply and return lines were employed for heating and cooling, respectively, feed stocks include municipal water and flue gas condensate. Target groups for this study include environmental engineers with particular interest in emerging water purification technologies.

The field trials can be divided into three phases: (1) parametric study of yield, (2) long term operation, and (3) evaluation of flue gas condensate as a feed stock. Testing commenced in the beginning of April 2006.

The performance of MD concerning production rate is highly dependent on the feed stock temperature, flow rate and temperature difference across the membrane. Initial results for municipal water feedstock showed that product water fluxes were in line with previous experiments, thus confirming the assumptions made in the prestudy regarding specific thermal energy requirements (i.e. 4.0-5.0 kWh/m3). Figure 2 illustrates the dependency of pure water production on flow rate supplied to five MD modules. Connecting two MD modules in series was found to be a viable system alternative. Specific electricity consumption has been revised to 0.9 – 2.4 kWh/m3 due to new data on module pressure drop, these values are nearly 50% lower than originally reported in the prestudy.

Regarding the long-time performance, the test period lasted for 13 days on a continuous operation basis before the first flux deterioration was encountered, which was later attributed to scale formation on the membrane cassettes feed inlets, flow distributors inside each cassette and the membranes’ upper half.

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