The application of membrane contactors for ammonia recovery from raw and digested manure

Anaerobic digestion and solid-liquid separation of animal wastes are a viable technology for waste management to reduce transport cost and environmental hazards in areas with increased livestock production. However, post processing of manure is necessary due to significant amount of nitrogen and phosphorus, which even though they are essential nutrients also pose potential risk of over fertilizing fields leading to contamination of water streams and air. Liquid-liquid membrane contactors (MC) present a possible technology for ammonia recovery. In general one of the main obstacles though impeding the implementation of membrane contactors is membrane fouling. This must also be expected to be the case for the recovery and concentration of ammonia from swine manure. The presence of organic matter and multivalent ions in pig slurry may greatly influence the membrane fouling due to surface adsorption and pore plugging. The aim of this work is to add knowledge on how the overall mass transfer coefficient of ammonia (km) is affected by different solid-liquid separation techniques of pig slurry at different temperatures (30ºC, and 50ºC) and feed flow velocity (0.9 m/s and 1.8 m/s). Further a comparison on how digestion of manure influences membrane fouling at 40ºC and a feed velocity of 0.9 m/s is conducted. Ammonia stripping of the liquid fraction of both digested and undigested manure was performed using tubular polypropylene (PP) membranes. Intensity, morphology and composition of fouling layers have been determined using Scanning Electron Microscopy and Fourier Transform Infrared Spectrometry (ATR-FTIR). Based on the experimental results, it can be concluded that mass transfer of ammonia from the liquid fraction is neither influenced by solid-liquid separation nor by anaerobic digestion. For digested manure the overall mass transfer coefficient km was equal to 19±1∙10-3 m/h and for undigested km was found to be 18.9±0.9∙10-3 m/h at 40ºC, and flow velocity of 0.9 m/s. Further, investigations show that increasing the temperature from 30ºC to 50ºC doubled the overall mass transfer coefficient of ammonia, while increasing feed flow velocity had negligible effect on the overall mass transfer coefficient of ammonia. No significant difference was found between undigested manure effluents with different dry matter content but similar particle size distribution, meaning that pre-processing of manure has little influence on the mass transfer coefficient of ammonia as long as the particle size is unchanged. Membrane fouling in case of digested and undigested pig slurry was a combination of organic fouling, biofouling and colloidal fouling. SEM observations revealed presence of bacteria suggesting biofouling, while the obtained ATR-FTIR spectra are characteristic for proteins and carbohydrates deposit.