This study focuses on the performance of an integrated forward osmosis (FO) and membrane distillation (MD) process for wastewater reuse. FO worked as a pretreatment barrier to remove most contaminants in the feed water and MD was used to recover the draw solutes from FO effluent and simultaneously produce high-quality reusable water. A unique three-channel FO-MD membrane module was designed and built to test water flux and contaminant removal in bench-scale experiments. It was found that the integrated FO-MD system possessed inherent flux balancing mechanism that enabled a stable and equal water flux for both FO and MD membranes for effective recovery of draw solution over long-term experiments. The FO-MD system was able to achieve more than 3 logs (> 99.9%) removal of ammonium, COD, arsenic, and combined solutes in both synthetic and real wastewaters. Such a synergistic integration of FO and MD membrane processes offers three major advantages. First, the upstream FO process removes most contaminants and foulants from the feed solution, thus potentially diminishing the fouling and wetting problem for the downstream MD process. Second, the downstream MD process successfully recovers the draw solution for the FO process, enabling a constant water flux for FO. Third, the synergistic removal capability of FO and MD enabled the production of extremely high-quality product water. Additional benefits of the integrated process also include ambient pressure operation and potential use of renewable low-grade heat as the energy source.
An integrated forward osmosis (FO) and membrane distillation (MD) system has great potential for sustainable wastewater reuse. However, the fouling and long-term durability of the system remains largely unknown. This study investigates the fouling behaviour and efficiency of cleaning procedures of FO and MD membranes used for treating domestic wastewater. Results showed that a significant decline in flux of both FO and MD membranes were observed during treatment of wastewater with organic foulants. However, shear force generated by the increased cross-flow physically removed the loosely attached foulants from the FO membrane surface and resulted in 86-88% recovery of flux by cleaning with tap water. For the MD membrane, almost no flux recovery was achieved due to adsorption of organic foulants on the hydrophobic membrane surface, thus indicating significant irreversible fouling/wetting, which may not be effectively cleaned even with chemical reagents. Long-term (10 d) tests showed consistent performance of the FO membrane by rejecting the contaminants. However, organic foulants reduced the hydrophobicity of the MD membrane, caused wetting problems and allowed contaminants to pass through. The results demonstrate that combination of the FO and MD processes can effectively reduce irreversible membrane fouling and solve the wetting problem of the MD membrane.
The objective of this study was to evaluate the performance of anaerobic digestion (AD) within the intermediate zone, specifically at 45˚C. Single-stage batch anaerobic digestion system was developed in the lab and performance was monitored for more than 2 years. The AD system was able to achieve high biogas production with about 62%-67% methane content. The digester exhibited high acetate accumulation, but sufficient buffering capacity was observed as the pH, alkalinity and volatile fatty acids-to-alkalinity ratio were within recommended values. The system achieved 36.5% reduction of total solids (TS) and 47.8% reduction of volatile solids (VS), which exceeded the required VS destruction efficiency for Class A biosolids. The pathogen counts were less than 1000 MPN/g total solids in the effluent, which also satisfied Class A biosolids requirements. The accumulation of acetate was presumably due to the high temperature which contributed to high hydrolysis rate. Consequently, it produced large amount of toxic salts that combined with the acetate, making them not readily available to be consumed by methanogens. The slower degradation of acetate was observed by the kinetic parameters. Accumulation of acetate contributed to 52% to 71% reduction in acetate degradation process, but was not completely inhibitory. The methanogens existing in the system were mostly thermo-tolerant acetate-utilizing methanogens, and specifically from Methanosarcinaceae species.
Separate treatment of high-nutrient sidestream is an efficient and cost effective way to decrease the loading on the main plant, resulting in lower effluent nutrient concentration. This study investigated the use of a combined forward osmosis-membrane distillation (FO-MD) system for the removal of nitrogen present in high concentration in sidestream from anaerobic digestion process. The combined system was able to achieve almost 100% rejection of solids and acetic acid, and more than 98% rejection of NH3-N from the sidestream. The high rejection of NH3-N was mainly achieved by the FO process. The solids in the feed solution contributed to fouling problem in both FO and MD, resulting in significant decline in flux. However, 76% or higher flux recovery was achieved for FO membrane by cleaning with tap water. We observed that flux recovery was due to removal of solids from the membrane surface by the cleaning process. FO membrane also demonstrated excellent performance for continuous operation when cleaned for 15 min in every 24 h interval. Overall, the combined FO-MD system was found to be an effective solution for treatment of nutrient rich sidestream.
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