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Biogas slurry is the liquid part of anaerobic fermentation of excrements from breeding industries. It is rich in organic matters, nitrogen, phosphorus, potassium, and other nutrients. However, biogas slurry is an environmental pollution source if it is discharged without proper treatment. To recover the nutrients and remove Suspended Solids (SS) and trace heavy metals in the biogas slurry, a membrane filtration system was designed in this work. Herein, Ceramic Membrane (CM) was applied for pretreatment because of its large membrane flux, strong anti-pollution ability and high chemical stability features. The key parameters that affect the performance of the membrane filtration system, such as membrane pore size, biogas slurry temperature, membrane surface velocity and Volume Reduction Factor (VRF), were investigated. The effects of pretreatment were studied via analyzing the membrane flux, Chemical Oxygen Demand (COD), turbidity, ammonia nitrogen, and the content of heavy metals under different conditions. The results showed that the membrane flux could reach 250 L/ (m² ·h) for 50 nm CM, which was 75 L/ (m² ·h) higher than that of 20 nm CM. However, in order to achieve high permeate water quality and heavy metal removal efficiency, ultrafiltration by 20 nm pore size CM was selected for pretreatment. The membrane flux increased from 165 to 275 L/ (m² ·h) upon increasing liquid temperature from 15 to 40 °C. With the increase of membrane surface velocity, the membrane flux increased gradually, but the increasing rate decreased. The membrane flux could reach 175 L/ (m² ·h) under the membrane surface velocity of 3.0 m/s.When the concentration rate increased, the membrane fluxes showed a downward trend of first fast and then slow. The maximum VRF of 20 nm pore CM could reach above 10, and the corresponding membrane flux was 62.6 L/ (m2 ·h). The pretreatment of biogas slurry by CM filtration had the optimal economy, when the frequency converter operated in the range of 40-45 Hz. The removal efficiency of turbidity by 20, 100 and 1 000 nm pore CMs was 99.77%, 83.91% and 81.87%, respectively. And there was no significant difference on the COD removal between 20 nm and 100 nm, which was 15.97% and 13.12%, respectively. There was almost no retention of ammonia nitrogen for all CMs. The removal rate of Ca, Mg, K, P by 20 nm CM was 14.7%, 66.3%, 3.9% and 32.9% respectively. The retention rate of Cu, Zn, Pb, Fe and Mn in biogas slurry by 20 nm CM was 96.0%, 95.5%, 100%, 86.5% and 76.0%, respectively. However the removal efficiency of Cr was quite low owing to its anionic state in biogas slurry. To draw a conclusion, pretreatment of biogas slurry by ultrafiltration using 20 nm CM could effectively remove SS and heavy metals, and retain majority of the dissolved organic and inorganic nutrients in biogas slurry. After pretreatment by ultrafiltration, nanofiltration(NF) or reverse osmosis(RO) could be used to concentrate the dissolved nutrients including organic matters, nitrogen, phosphorus, and potassium. This study provides a useful reference for the harmless treatment and resource utilization of swine wastewater and biogas slurry. It is anticipated that membrane concentrate system could render versatile applications for biogas slurry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]