Pig slurry contains valuable nutrients and organic matter, although its high water content makes its management and use as a fertilizer more expensive. It is also an interesting bioenergy resource for biogas production. We propose a treatment that consists of solid–liquid separation followed by the anaerobic digestion of liquid fraction of slurry (LFS) while a microfiltration membrane module concentrates the solids in the digester and removes a liquid fraction of the digestate (LFD). The aims of the work were to evaluate the fertilizer value of the LFS, digestate and concentrated digestate and the possibility of reusing the LFD in agriculture. The LFS contained 72% less dry matter than the slurry. The solid–liquid separation mainly removed N and P. Thanks to microfiltration, the remaining solids were partially recirculated to the digester, concentrating there. To do this, the membrane module continuously removed the LFD, which was made up of 99% water with dissolved elements, mainly C, N, K and Na. The concentrated digestate contained less K, similar amounts of N and P, and more dry matter and C than the initial LFS. The entire slurry treatment affected the mineralization dynamics by increasing recalcitrant C and decreasing labile C without modifying N release. The proposed process allowed taking advantage of the nutrients and stabilized organic matter contained in the LFS, producing a concentrated digestate. LFD did not meet the reclaimed water requirements. However, it could be useful as a fertirrigation solution and a post treatment could be enough to comply with the regulations.
Graphical Abstract
In this work, 124 samples of slurry from 32 commercial farms of three animal categories (lactating sows, nursery piglets, and growing pigs) were studied. The samples were collected in summer and winter over two consecutive years and analyzed for physicochemical properties, macronutrient and micronutrient, heavy metals, and major microbiological indicators. The results were found to be influenced by farm type and to deviate especially markedly in nursery piglets, probably as a consequence of differences in pig age, diet, and management. The main potential hazards of the slurries can be expected to arise from their high contents in heavy metals (Cu and Zn), especially in the nursery piglet group, and from the high proportion of samples testing positive for Salmonella spp. (66%). Linear and nonlinear predictive equations were developed for each animal category and the three as a whole. Dry matter, which was highly correlated with N, CaO, and MgO contents, proved the best predictor of fertilizer value. Using an additional predictor failed to improve the results but nonlinear and farm‐specific equations did. Rapid on‐site measurements can improve the accuracy of fertilizer value estimates and help optimize the use of swine slurry as a result.
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