Abstract
Ultrafiltration (UF) can effectively remove large particles, suspended solids, and colloidal substances from anaerobic digestate. However, membrane fouling is a technical challenge in the purification of the digestate by UF. In this study, polyethersulfone (PES) membranes with four pore sizes (50.0, 20.0, 10.0 and 5.0 kDa) were employed to filter anaerobic digestate from swine manure. The effects of temperature, transmembrane pressure (TMP), and cross-flow velocity (CFV) on flux were investigated. The purification effects and fouling characteristics of the four membranes were analyzed. The results revealed that the increase of temperature and CFV can effectively promote UF separation efficiency, but as the TMP exceeded 3.0 bar, the flux increase rates of the four membranes were almost zero. The larger membrane pore size caused the faster flux increase with the increase in pressure. During the batch experiment, the 20.0 kDa membrane showed the lowest flux maintenance ability, while the 5.0 kDa showed the highest ability due to the smaller pore size. All four membranes can effectively remove tetracyclines residues. Elements C, O, and S were the major membrane foulant elements. The dominant bacteria orders of membrane fouling were Pseudomonadales, Xanthomonadales and Burkholderiales. Compared with tap water and citric acid, the membrane cleaning by NaOH and NaClO showed higher flux recovery rates. The 50.0 kDa membrane achieved the best cleaning effects under all cleaning methods.
Highlights
Anaerobic digestion (AD) can convert organic matters of livestock manure into renewable energy and realize the energy utilization of manure [1,2]
AD digestate was collected from a swine manure treatment plant located in the Changping district, Beijing, China
After solid–liquid separation, the solid fraction was used for organic fertilizer production, and the liquid fraction was pumped into an anaerobic digester for the biogas production
Summary
Anaerobic digestion (AD) can convert organic matters of livestock manure into renewable energy (biomethane) and realize the energy utilization of manure [1,2]. The digestate of AD contains high nutrients, suspended solids, and water content, making effective treatment and utilization of digestate difficult to achieve [3]. The solid–liquid separation process is usually used to divide the digestate into the solid and liquid fraction. The liquid fraction contains high nitrogen and potassium contents, and can be used for nutrient recovery by membrane concentration or struvite production [4]. The conventional solid–liquid separation methods, such as gravity sedimentation, plate filtration and screw extrusion, can only remove large particles [5,6,7], and the liquid fraction still contains large amount of colloids (size of 0.001–1.0 μm) and fine particles [5,6]
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