Pilot trial of membrane distillation driven by low grade waste heat: Membrane fouling and energy assessment

• Direct contact membrane distillation (DCMD) was trialled for three months. • Trial pilot plant supplied with waste heat b 40 °C from power station • Water recovery 92.8% reached treating ion exchange regeneration effluent • Trial fluxes between 2 to 5 L/(m2·h), sensitive to heating or cooling temperatures • Membrane fouling observed, while total dissolved solid rejections always N99.8%.

This work explored the viability of DCMD for treatment of power station wastewater utilising waste heat. By conducting a site trial for three months, membrane fouling was explored and the ability to treat to practical water recoveries was shown. The waste heat from the power station was supplied to the MD module at 38 °C, and cooling was provided by the nearby saline river via the power station cooling system. Membrane flux was highly variable due to the variable nature of the power station operation (intermittently operated for peak loads) and seasonal variations, but the MD plant productivity was proportional to the available temperature. Normalising flux to the vapour pressure drop determined as a function of temperature compensated for the temperature variability and clearly showed membrane fouling effects. Surprisingly, the membrane performed consistently without significant flux lost even at very high concentration factors and water recoveries (up to 92.8%). This was attributed both to the collection of precipitating material by the 5 μm filter installed in the feed cycle, as well as the unique fouling mechanisms in MD where flux will only decline once the effect of vapour pressure reduction occurs in this characteristic fouling material. While dissolved solid rejections were on average 99.9% over the entire trial period, ammonia and carbon dioxide evolved into the MD permeate which would need consideration in exploring reuse opportunities, where the trace ammonia (10 mg/L) may be useful as a nutrient for irrigation applications. An assessment of the integration of the MD system at the 500 MW electric rated power station revealed that up to 8000 kL/day of desalinated water produced by MD is feasible, being sufficiently large to supply numerous industry, residential or agricultural sites. This is due to the abundance of very low grade waste heat (b 40 °C), where although more thermally efficient MD configurations cannot operate effectively, trade-off benefits include higher flux which translates to lower MD plant capital cost.