ABSTRACT
Recent scientific advances have made headway in addressing pertinient issues in climate change and the sustainability of our natural environment. This study makes use of a novel approach to desalination that is environment friendly, naturally sustainable and energy efficient, meaning that it is also cost efficient. Evaporation is a key phenomenon in the natural environment and used in many industrial applications including desalination. For a liquid droplet, the vapor pressure changes due to the curved liquid-vapor interface at the droplet surface. The vapor pressure at a convex surface in a pore is, therefore, higher than that at a flat surface due to the capillary effect, and this effect is enhanced as the pore radius decreases. This concept inspired us to design a novel biporous anisotropic membrane for membrane distillation (MD), which enables to desalinate water at ambient temperature and pressure by applying only a small transmembrane temperature gradient. The novel membrane is described as a super-hydrophobic nano-porous/micro-porous composite membrane. A laboratory-made membrane with specifications determined by the theoretical model was prepared for model validation and tested for desalination at different feed inlet temperatures by direct contact MD. A water vapor flux as high as 39.94 ± 8.3 L m-2 h-1 was achieved by the novel membrane at low feed temperature (25 °C, permeate temperature = 20 °C), while the commercial PTFE membrane, which is widely used in MD research, had zero flux under the same operating conditions. As well, the fluxes of the fabricated membrane were much higher than the commercial membrane at various inlet feed temperatures.
ABSTRACT
Desalination and wastewater treatment technologies require an effective solution for brine management to ensure environmental sustainability, which is closely linked with efficient process operations, reduction of chemical dosages, and valorization of brines. Within the scope of desalination brine reclamation, a circular system consisting of seawater reverse osmosis (SWRO), membrane distillation (MD), and forward osmosis (FO) three-process hybrid is investigated. The proposed design increases water recovery from SWRO brine (by MD) and dilutes concentrated brine to seawater level (by FO) for SWRO feed. It ultimately reduces SWRO process brine disposal and improves crystallization efficiency for a zero-liquid discharge application. The operating range of the hybrid system is indicated by a seawater volumetric concentration factor (VCF) ranging from 1.0 to 2.2, which covers practical and sustainable operation in full-scale applications. Within the proposed VCF range, different operating conditions of the MD and FO processes were evaluated in series with concentrated seawater as well as real SWRO brine from a full-scale desalination plant. Water quality and membrane surface were analyzed before and after experiments to assess the impact of the SWRO brine. Despite their low concentration (0.13 mg/L as phosphorous), antiscalants present in SWRO brine alleviated the flux decline in MD operations by 68.3% compared to operations using seawater concentrate, while no significant influence was observed on the FO process. A full spectrum of water quality analysis of real SWRO brine and Red Sea water is made available for future SWRO brine reclamation studies. The operating conditions and experimental results have shown the potential of the SWRO-MD-FO hybrid system for a circular brine reclamation.
Subject(s)
Distillation , Water Purification , Membranes, Artificial , Osmosis , Salts , SeawaterABSTRACT
Forward osmosis-membrane distillation (FO-MD) hybrids were recently found suitable for produced water treatment. Exclusion of synthetic chemical draw solutions, typically used for FO, can reduce FO-MD operational costs and ease its onsite application. This study experimentally validates a novel concept for the simultaneous treatment of different produced water streams available at the same industrial site using an FO-MD hybrid system. The water oil separator outlet (WO) stream was selected as FO draw solution and it generated average fluxes ranging between 8.30 LMH and 26.78 LMH with four different feed streams. FO fluxes were found to be governed by the complex composition of the feed streams. On the other hand, with WO stream as MD feed, an average flux of 14.41 LMH was achieved. Calcium ions were found as a main reason for MD flux decline in the form of CaSO4 scaling and stimulating the interaction between the membrane and humic acid molecules to form scale layer causing reduction in heat transfer and decline in MD flux (6%). Emulsified oil solution was responsible for partial pore clogging resulting in further 2% flux decline. Ethylenediaminetetraaceticacid (EDTA) was able to mask a portion of calcium ions and resulted in a complete recovery of the original MD flux. Under hybrid FO-MD experiments MD fluxes between 5.62 LMH and 11.12 LMH were achieved. Therefore, the novel concept is validated to produce fairly stable FO and MD fluxes, with few streams, without severe fouling and producing excellent product water quality.
