Membrane technology can be expected to concentrate lots of nutrients in small volumes in biogas slurry, and then transport specific nutrients into other agricultural areas to serve as fertilizer, thereby partially solving the problem of surplus biogas slurry in high-density livestock areas. This article presents a critical review of the state-of-art research on the application of membrane concentration for volume reduction of biogas slurry. There are some common features for the water quality of biogas slurry that produced by anaerobic digestion, including complex composition, high concentrations of suspended solids, organic matter, ammonia nitrogen, and salt. First, a general review was made on the solid-liquid separation technologies that used for biogas slurry pretreatment, together with the physical and chemical processes related to separation. The separation efficiencies of common techniques were ranked in order for the removal of particles: centrifugation > sedimentation > non-pressurized filtration > pressurized filtration. Treatment with flocculants before separation can significantly improve efficiency. Aluminum sulfate (Al2(SO4)3), polymeric aluminum chloride (PAC), ferric chloride (FeCl3), and calcium hydroxide (Ca(OH)2) have been proved to be highly efficient for the remove suspended solids. In the organic-polymers test, cationic polyacrylamide (PAM) was found to be the most efficient flocculants. Second, the liquid products from solid-liquid separation can further be treated by microfiltration or ultrafiltration, which can be used to remove bacterium, colloid components, and macromolecular substance. Compared with organic membrane, ceramic membrane can act as more efficient solid-liquid separator due to its large membrane flux, strong anti-pollution ability, and high chemical stability. Third, the nanofiltration or reverse osmosis can be used to concentrate the nutrients, including dissolved organic compounds, nitrogen, phosphorus, and potassium, particularly that the biogas slurry concentration in 15%-50% of the initial volume has been reported with reverse osmosis systems. The reverse osmosis with proper pretreatment can be technically feasible for nutrient concentration and volume reduction of biogas slurry, but high costs and strong membrane fouling have posed a main obstacle on the application of the system. In further studies, a feasible and economical technology can include the following research fields: (1) Aiming at the removal of colloids and small size particles in biogas slurry, in-depth study can be expected to perform on the relationship between the physicochemical properties of different particles and their sedimentation performance, and thereby to develop a new solid-liquid separation technology. (2) In view of the complex characteristics of water quality in biogas slurry, such as high ammonia nitrogen and salinity, the cost-effective technology and equipment can be expected to reduce ammonia nitrogen and salt before membrane concentration. (3) Membrane modification can be developed by various approaches, such as blending, copolymerization, surface grafting, in order to improve its hydrophilicity, mechanical strength, permeation flux, and anti-pollution performance during filtration. (4) To achieve the maximum volume reduction and the best effluent water quality, it is necessary to optimize membrane filtration process and operating parameters. In reducing the energy consumption and operating cost of membrane concentration, solar energy-driven membrane separation technology may be under view in near future. (5) The concentrate can be used for organic liquid fertilizer, foliar fertilizer, plant nutrient solution, or mixed with other chemical materials to produce commercial fertilizer. The fertilizer value of concentrate still needs further evaluation and optimization based on field studies. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.
