Phosphorus recovery from pig slurry through biological acidification and re-crystallization as struvite

Pig slurry is a phosphorus-rich waste stream that requires costly physical/chemical and-or biological treatment, particularly in areas vulnerable to eutrophication. Swine slurry commonly undergoes a solid-liquid separation: the liquid fraction is spread in the neighbouring fields while the solid phase has to be exported, often as compost, to phosphorus-depleted regions. There is currently a growing trend towards phosphorus recovery under a purified, marketable form such as struvite, a slow-release mineral fertilizer. Recycling phosphorus as struvite could reduce the treatment costs of pig slurry and provide a renewable phosphorus source for agriculture. Indeed, the phosphate rock used in the fertilizer industry is a non-renewable resource whose decreasing quality and progressive depletion could threaten the fertilizer-based agricultural model and harm global food supply. Phosphorus in swine slurry mostly exists as mineral solids. It is now possible to extract this phosphorus as ortho-phosphate by acidification and recover it as struvite by adding magnesia (MgO). Biological acidification of swine slurry using waste-type co-substrates would be more environment-friendly than chemical acidification and save the costs of chemical addition. Sucrose was used as a practical co-substrate in order to realize a preliminary study of the biological and chemical reactions taking place during the acidification of pig slurry. A continuous reactor and batch tests were used to ascertain the link between the amount of sucrose added, the type/amount of volatile fatty acids (VFA) produced, the resulting pH drop and the dissolved phosphorus obtained. The batch tests consisted in 12 flasks, each containing raw swine slurry and four different amounts of sucrose in triplicate (0, 15, 30 and 60g/L). At maximum initial sucrose concentration the pH dropped from 7.5 to 5.5 after 2 days, while dissolved phosphates reached 800 mgPO4-P/L from 50 mgPO4-P/L initially. Lactate was the main VFA all along (70-95%). At lower sucrose concentrations, the pH either increased or dropped slightly before re-increasing. At 30g-sucrose/L, 420 mgPO4-P/L were measured after 2 days when a pH of 6.2 was reached. However those phosphates re-precipitated when the pH increased to 6.5 after 60 hours. Lactate was the main VFA during the first 40 hours but disappeared afterwards, simultaneously with the pH re-increase. The pattern in term of VFA production at the highest sucrose concentration was comparable to what occurs in successful silage. The initial amount of water soluble carbohydrates (WSC) (i.e. sucrose in this case) was high enough to give a competitive advantage to Lactic Acid Bacteria (LAB). They were able to acidify the slurry down to a pH too low for other microorganisms to metabolize the lactate into less acidic VFAs or other end-products. This resulted in a stable, low pH and a constant VFA concentration and composition. On the other hand, at sucrose concentrations lower than 60 g/L, the pattern of VFA production was similar to what occurs in spoiled silage. At such WSC concentration, the initial lactic acid production was not sufficient and other microorganisms were able to consume the lactic acid, resulting in a pH increase and a decrease in VFA concentration (possibly due to the beginning of methane production). The continuous reactor was maintained at 36°C and fed with sucrose and the supernatant of digested, centrifuged pig slurry, at a fixed hydraulic retention time of 4 days, during 30 days. The reactor stabilized after 4 days at a pH of 4.8 while the sucrose concentration in the influent was decreased from 150 to 40g-sucrose/L in the feed. Sucrose was mostly converted to lactate (60-95% of removed sucrose) throughout the study, indicating that the biological process similar to ensiling could be maintained during 30 days in a continuous reactor. The results obtained indicate that the biological acidification of pig slurry with high sucrose concentration (40-60g/L), leads to the dissolution of most of the phosphorus (700-900g/L, 70-80% of TP). The critical pH range in which phosphorus got dissolved was between 5 and 6. The biological process taking place had many similarities with ensiling, with lactic acid being produced predominantly. This ensiling-like process could be maintained in a continuous reactor for 30 days. Based on these findings, the choice of real co-substrates for biological acidification of swine slurry should be oriented towards WSC-rich wastes, like fruits, vegetables, grass, or crop-residues.