Performance of a full-scale processing cascade that separates agricultural digestate and its nutrients for agronomic reuse

Abstract

The application of animal manure on agricultural land in the Netherlands is bound by legal limits to prevent the leaching of nitrogen (N) and phosphorus (P) to ground and surface waters. The surplus of animal manure is transported abroad at high costs. In this study, a full-scale cascaded membrane filtration system (GENIUS) comprising two decanter centrifuges, microfiltration (MF) reverse osmosis (RO) and an ion exchanger was monitored. The system processed agricultural digestate from anaerobically co-digested animal manure into two solid fractions (SFs), RO concentrate, MF concentrate and purified water. The goal was to separate P and ammoniacal nitrogen (NH4-N) and remove water from the digestate. From the initial digestate, 66% of P was recovered in the first SF, which constituted 15% of the total mass, without the addition of iron or aluminium salts or polymer flocculants. Another 29% of P was recovered in the MF concentrate and used as a liquid organic fertiliser. Of the P in the initial digestate, 98% was removed before RO. For N, 34% ended up in the RO concentrate and this product can be regarded as an alternative for synthetic N fertiliser as it contains N solely in mineral form. Overall, around 18% of the total mass of initial digestate was discharged as purified water and 31% was locally applied in the form of RO concentrate. We found that aqua regia digestion before chemical analysis can decrease the measured S content of processed digestate. Compared to the transport of raw (unprocessed) digestate, the implementation of the GENIUS system led to a 53% reduction in the mass-weighted average transport distance.