Recovery of Potassium-Rich Struvite in a Pig Farm Downstream a Nitrogen Removal Treatment Plant: Technological, Agricultural and Economic Assessment

Abstract

Simultaneous recovery of phosphorus (P), potassium (K), and magnesium (Mg) as precipitated phosphate salt (PPS) wasassessed (K-struvite and hazenite included) in a pig farm downstream a nitrification-denitrification unit. The precipitationprocess was addressed using denitrified effluent (DE) and waste sludge (WS) purged from the bioreactor. For DE, theinherent unbalanced Mg:K:P molar ratio (1.4:9.5:1.0; 1.6 g K/L) led to poor K recovery (≤ 15%) if considering P as thelimiting element. By considering K as limiting, the supply of external sources of Mg and P (final Mg:K:P as 1.0:0.5:1.0)allowed the recovery of most of the K available. For WS, a selective pretreatment with oxalic acid resulted in an avail-able Mg:K:P ratio of 0.8:1.2:1.0. Risk of interference by free calcium will exist if the operational parameters are notoptimized, thus constraining the recovery of K (≤ 51%). A fertilization field trial demonstrated the fertilizing value of therecovered PPS with respect to a mineral fertilizer, and the increased risk of soil salinization under a reiterative applicationof processed DE. Scenarios without extra addition of nutrients will introduce an enhanced handling in the current nutri-ent management planning of the pig farm. On the contrary, the trade of the PPS formed becomes essential when externalsources of Mg and P were added