Abstract
In this study, the pretreatment of concentrated blackwater using ultrafiltration (UF) was shown to improve the permeability, selectivity and robustness of membrane distillation (MD) for application to wastewater treatment. Concentrated blackwater comprises urine and faeces, with minimal flushwater added. The faecal contribution increased the soluble organic fraction and introduced coarse and colloidal particles into the urine, which increased resistance to filtration during dead-end UF. Ultrafiltration removed the particulate and colloidal fractions (MW > 500 kDa) from the blackwater, which permitted similar permeability and robustness for MD to that observed with urine (29.9 vs 25.9 kg m−2h−1), which comprises a lower colloidal organic concentration. Without UF pretreatment, a higher density organic layer formed on the MD surface (197 vs 70 gCODm−2) which reduced mass transfer, and transformed the contact angle from hydrophobic to hydrophilic (144.9°to49.8°), leading to pore wetting and a dissipation in product water quality due to breakthrough. In comparison, with UF pretreatment, MD delivered permeate water quality to standards satisfactory for discharge or reuse. This is particularly timely as the ISO standard for non-sewered sanitation has been adopted by several countries at a national level, and to date there are relatively few technologies to achieve the treatment standard. Membrane distillation provides a robust means for concentrated blackwater treatment, and since the energy required for separation is primarily heat, this advanced treatment can be delivered into areas with more fragile power networks.
Highlights
Membrane distillation (MD) provides a high rejection barrier to organics, including low molecular weight non-volatile solutes [13], inorganics and pathogens, similar to that of reverse osmosis [11,7]
The greater permeability loss in the concentrated blackwater extended the time to filtration, terminating in a pseudo steady-state flux approaching around 0.21 kg m−2h−1
Evaluation of the resistance to filtration during dead-end ultrafiltration evidenced the significant contribution of the particle fraction that is incorporated through faecal contamination, which introduces both coarse and colloidal particles
Summary
Membrane distillation (MD) provides a high rejection barrier to organics, including low molecular weight non-volatile solutes [13], inorganics and pathogens, similar to that of reverse osmosis [11,7]. The hydrophobic membrane used in MD is characterised by a contact angle exceeding 90° and when combined with a small pore diameter, confers a high breakthrough pressure (ΔPinterface) sufficient to repel water [10]: cosθ. The repulsion of water creates a vapour liquid interface at the air filled pores, through which the driving force for water vapour mass transport is provided by the vapour pressure difference initiated between the feed and permeate sides of the membrane [21]. Liquids with low surface tension (γL) or alterations to the solid-liquid contact angle caused by adsorption or other surface fouling phenomenon (cosθ), can reduce the breakthrough pressure (Eq (1)), leading to an increased probability for pore wetting. Several authors have proposed that fouling of MD membranes by humic acid (molecular weight 227 g mol−1) is less significant than other
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