Anaerobic digestion and microfiltration of the liquid fraction of pig slurry: N Mineralization, C-CO2 emissions and agricultural value of the products

Abstract

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 regulation