Agro-industrial by-products as alternative additives for the agronomic valorization of pig slurry through pH modification

Abstract

The use of animal slurry in modern agricultural systems is receiving considerable attention, mainly due to the latest increase in chemical fertilizer prices. However, the use of raw animal slurry to fertilize horticultural crops can, on the one hand, lead to significant ammonia emissions and, on the other hand, be an important source of zoonotic pathogens. Therefore, proper treatment is required. The present study explored the modification of the pH value of pig slurry using by-products from different agro-industrial activities as a strategy to control these two problems. Three approaches were used, namely acidification and biological acidification (bio-acidification) to pH 5 and alkalization to pH 9.5. The most promising of the 14 different additives and additive combinations initially considered should be able to reach the target pH at a dose of less than 20% (m/m). The presence of Salmonella and the number of Escherichia coli colony forming units (CFU) were evaluated in the slurry for each selected treatment. To ensure that treated slurry remains of interest as an organic fertilizer, the impact of treated slurry on plant germination and nitrogen (N) availability was also assessed. The by-products from the brewing and yeast industry failed to reduce the pH to 5. In fact, the bio-acidification of pig slurry was only possible with additives rich in carbon. Although slurry sanitization (<1000 CFU E.coli g−1 slurry) was not guaranteed by all treatments, a reduction in E.coli numbers was observed with all additives. The ammonium content of the treated slurry was significantly reduced (up to 20%) with treatments involving bio-acidification. N mineralization was evidenced for all additives used for slurry alkalinization but no N mineralization was observed when using acidification with H2SO4 or by-product spent acid. The phytotoxicity bioassay showed that most additives do not have a negative impact on germination. The results presented here suggest that agro-industrial by-products are capable of modifying the pH of pig slurry. Depending on the pH modification strategy and additive used, treated slurry may be safer and of higher fertilizing value than untreated (raw) slurry.