Bio-acidification of animal slurry is a microbially mediated process that occurs under anaerobic conditions, where the slurry is acidified to organic acids produced by the microorganisms present in the slurry. This bio-acidification can be induced by adding easily degradable organic substrates, which are metabolized in a fermentation process by the microorganisms present in the slurry. Among the organic acids produced, lactic acid is one of the main ones, which is capable of decreasing the slurry pH. To favor lactic acid production over other weaker organic acids, desirable conditions for lactic acid bacteria should be established. The organic substrates to induce the fermentation would ideally be residues from agriculture with a high content of easily fermentable sugars.
To reach this main goal, it was necessary to understand first the main mechanisms happening during different acidification treatments. Manure composition is affected during acidification and there is no clear understanding of the chemical and microbial changes occurring when different substrates or acids are applied to different slurries. Therefore, several manures were characterized prior, during and after acidification and different treatments were performed. Treatments were carried out with sulphuric acid as the traditional inorganic acid used in farms, and with glucose as a carbohydrate source model and brown juice - a residue obtained during the extraction of protein from grass - as a carbohydrate source and acidifying agent to promote the bio-acidification treatment. The mechanisms happening during the storage of treatments were studied, by weekly following pH, glucose consumed and lactic acid produced, as well as by-products formation such as volatile fatty acids. This allowed us to determine the most efficient bio-acidification treatment where pH is maintained stable by lactic acid production and by-products avoidance.
Next experiment was carried out to find out the lowest substrate addition for the bio-acidification treatment able to maintain a long term stable pH, at which NH3 and GHGs emissions are minimized. Several treatments were performed to find out the best bio-acidification approach in terms of pH stability, reduction in substrate needs as well as effects on gaseous emissions reduction. The two best performing acidification and bio-acidification treatments maintaining a low and stable pH were selected to be applied to liquid fraction digestates from a biogas plant before the two pilot scale filtration units, ultrafiltration (UF) and forward osmosis (FO). We concluded that the bio-acidification treatment applied before UF increased the recoveries and acidification had a positive effect of the membrane permeability. Treatments promoted a higher N retention in the UF concentrate and increased P solubilisation. A higher fertilizer value could be obtained from treated digestates before filtration units, with the UF concentrate containing higher N and the FO permeate higher soluble P.
Finally, to study the effects of the treatments on main nutrients, plant fertilizer value and soil characteristics, pot experiments were carried out where we analysed the water extractability of P, N and P uptake and the plant biomass yield. We concluded that acidification increased the water-extractable P fraction of slurries and the plant P uptake.
