The membrane process had been applied for the advanced treatment of pig farm biogas slurry. As studied, this physical pretreatment, with low cost and high efficiency of the suspended solids removal and nutrient retention, is required to control membrane fouling. The combined process of paper filtration and ultrafiltration in a pilot scale was applied in the present study. The main objective was to explore and identify the feasibility of the new process for the pretreatment of the separation liquid of pig farm biogas slurry. A precision identification experiment of paper filtration and the multi-batch repetitive experiments of the combined process were designed. The results showed, at the identified paper filtration precision of 50μm and an operating pressure of 0.3 bar of the ultrafiltration process, that the flux rate at the stable stage of the multi-batch operation was around 295.00 L/h. The combined process achieved an overall processing rate of 345.41 ± 18.81 L/h and a volume permeation proportion of 82.45% ± 0.85%. The TSS was removed by 95.71%, but total nitrogen (TN) and ammonia nitrogen (NH₃-N) were retained by 76.29% ± 2.04% and 73.74% ± 2.10%, respectively. Comprehensively, the requirement for the pretreatment was obtained.

Nanofiltration (NF) or reverse osmosis (RO) process has been widely applied for concentrating nutrient in biogas digestate. However, efficient pretreatment is key to the sustainable operation of NF or RO. In this study, the combination of NF and RO for concentrating biogas digestate was compared using different pretreatments of hollow fiber ultrafiltration membrane (HFUFM) and ceramic membrane (CUFM). Pilot-scale batch tests were conducted (500 L). CUFM showed a higher membrane flux than HFUFM (100 ~ 180 L·(m²·h)⁻¹ vs. 17 ~ 35 L·(m²·h)⁻¹), but they showed little impact on the NF + RO process. Membrane fluxes of NF and RO were 20 ~ 48 L·(m²·h)⁻¹ and 16 ~ 40 L·(m²·h)⁻¹, respectively. In the RO permeates, the removal rates of total suspended solids (TSS), total solids (TS), chemical oxygen demand (COD), total nitrogen (TN), NH₄⁺-N, and Cl⁻ were above 91%. In the concentrates, TN and total potassium (TK) were concentrated by 1.60 and 2.00 folds in the NF stage, and by 2.10 and 2.30 folds in the RO stage. Further attention should be paid to the antibiotics risks in the concentrates before they are utilized as plant fertilizers.

In-depth exploration of filtration behavior and fouling characteristics of polymeric ultrafiltration (UF) membranes can provide guidance for the selection of materials and the control of membrane fouling during the purification of digestate. In this study, four types of polymeric membranes, (polyethersulfone (PES), polysulfone (PS), polyvinylidene fluoride (PVDF), and polyacrylonitrile (PAN)), were employed to filter digestate from swine manure. The results showed that the viscosity of the digestate dropped from 45.0 ± 11.3 mPa·s to 18.0 ± 9.8 mPa·s, with an increase in temperature from 30.0 °C to 45.0 °C. The four membrane fluxes all increased by more than 30%, with the cross flow velocity increasing from 1.0 m s⁻¹ to 2.0 m s⁻¹. During the batch experiments, the flux maintenance abilities of the membranes were in the order: PAN > PS > PVDF > PES. There were no significant differences in the effects of membrane materials on the removal of COD, TN, and TP (p < 0.05). For UV₂₅₄ removal efficiency, PS showed the highest efficiency (68.6%), while PVDF showed the lowest efficiency (63.4%). The major fouling type was irreversible hydraulic fouling, and the main elements of scaling were C, O, S, and Ca. Pseudomonadales were the dominant bacteria in the PS (26.2%) and in the PVDF (51.4%) fouling layers, while Bacteroidales were the dominant bacteria in the PES (26.8%) and in the PAN (14.7%) fouling layers. The flux recovery rates (FRRs) of the cleaning methods can be arranged as follows: NaClO > NaOH > Citric acid ≈ Tap water. After NaClO cleaning, the PVDF membrance showed the highest FRR (73.1%), and the PAN membrane showed the lowest FRR (30.1%).

To address the severe membrane fouling caused by swine manure digestate, this study provides a systematic evaluation of how pore size governs purification efficiency and flux recovery. SEM-EDS and ATR-FTIR were employed to characterize the foulants across five different pore sizes. The results revealed significant differences in flux retention and permeate quality, with smaller-pore membranes exhibiting superior antifouling performance. At a volume reduction factor of 5, the flux decline rates of the 50, 200, 500, 800, and 1200 nm membranes were 34.7%, 8.8%, 33.6%, 40.5%, and 72.2%, respectively. Notably, the 200 nm membrane achieved the highest flux maintenance and recovery after chemical cleaning, whereas the 1200 nm membrane suffered the greatest flux decline and poorest effluent quality. The 800 nm membrane experienced severe fouling likely driven by particulate deposition and cake layer formation. Based on the combined characterization results, polysaccharides and protein-like substances are suggested to be the primary organic foulants. The proposed fouling mechanisms likely involve cake layer formation on the membrane surface, along with concentration polarization and pore blocking. Furthermore, the internal adsorption of soluble organics likely contributes to irreversible fouling. Overall, this study not only elucidates the pore size-dependent fouling mechanisms but also establishes the 200 nm ceramic membrane as a highly promising pre-treatment candidate for digestate concentration processes.