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Using membrane distillation crystallisation for the treatment of hypersaline mining and industrial wastewater
Expanded Title:Membrane Distillation Crystallisation (MDC) offers a sustainable wastewater treatment process, particularly when using excess heat from peripheral processes to produce pure water as well as salt(s) products, thereby converting a waste material into a product of value that can be reused, recycled or sold to offset water treatment costs. MDC is also attractive because it requires lower operating temperatures (40-60°C) than evaporative crystallisation, enabling the efficient use of waste heat or renewable energy. Further, MDC requires lower hydrostatic operating pressures than reverse osmosis (RO) and so can be constructed from less expensive materials. Other advantages of MDC include: less membrane fouling than other pressure driven processes, improved rejection factors for feed streams containing non-volatile solutes, and the ability to treat high total dissolved solids (TDS) feed solutions that are close to their saturation limit. Furthermore, when applied to mine water treatment, MDC is able to operate in very acidic or basic streams, so such streams do not need to be pre-treated. This project investigated the applicability of MDC for the treatment of industrial wastewater with a specific focus on the treatment of mine water, and the impact that the presence of antiscalants had on the MDC process. Aqueous thermodynamic modelling using two different brines showed that the propensity for crystallisation increases with an increase with water recovery in the MDC process; this is directly linked to the increase in the supersaturation and consequently the driving force for crystallisation as a result of the removal of permeate water from the brine. Except for species that exhibit an inverse solubility, a lower operating temperature results in a higher crystal product yield due to the lower solubility of scaling species at lower temperatures - but for scaling species exhibiting inverse solubility characteristics, the inverse is true. The purity of the resultant crystallisation product depends on the extent of water recovery as the number of additional crystallising species (contaminants/impurities) increases with an increase in the water recovery. Different brine compositions produce diverse salts and salt yields, sometimes at the same operating conditions. Consequently, each brine composition should be assessed independently during the process design of an MDC process. Experimental investigation into MDC with PTFE membranes to validate the model demonstrated that increased feed temperatures and increased temperature difference led to the highest flux rates, and that changes in feed water concentrations (TDSs of 8000-16000 mg/L) did not have any significant effect on the flux rates. Increased flow rates (from 0.15m/s to 0.49m/s) led to improved fluxes. Phosphonic based antiscalants did not have an effect on flux, whilst organic based antiscalant resulted in a slight reduction in flux. The product waters had low conductivity (<10μS/cm) and only started to increase once scaling on the membrane surface and pore wetting caused a decrease the flux rate. Importantly, the high flux rates obtained from these experiments are significantly higher than those reported in literature and indicates a possibility that MDC could be used to increase water recovery from RO brines whilst generating usable salts as a by product.
Date Published:01/01/2017
Document Type:Research Report
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2223/1/16
ISBN No:978-1-4312-0867-8
Authors:Nathoo J; Eggers L; Randall D
Project No:K5/2223
Organizations:NuWater, Cape Town
Document Size:10 085 KB
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