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) was assessed (K-struvite and hazenite included) in a pig farm downstream a nitrification-denitrification unit. The precipitation process was addressed using denitrified effluent (DE) and waste sludge (WS) purged from the bioreactor. For DE, the inherent 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 the limiting 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 available Mg:K:P ratio of 0.8:1.2:1.0. Risk of interference by free calcium will exist if the operational parameters are not optimized, thus constraining the recovery of K (≤ 51%). A fertilization field trial demonstrated the fertilizing value of the recovered PPS with respect to a mineral fertilizer, and the increased risk of soil salinization under a reiterative application of processed DE. Scenarios without extra addition of nutrients will introduce an enhanced handling in the current nutrient management planning of the pig farm. On the contrary, the trade of the PPS formed becomes essential when external sources of Mg and P were added.