Struvite Production at Commercial Dairies with Use of a Mobile System and Comparisons to Alternative Nutrient Recovery Systems

Abstract

HighlightsAnaerobically digested dairy manure is best for formation of struvitePhosphorus can be captured in forms reusable as fertilizerPhosphorus can be captured in a cost range of $0.003 to $0.02 per L of manure based on a partial budget analysis considering the variable cost of chemicalsEffective methods for capture of P from dairy manure are needed to promote recycling of PAbstract. Effective methods for capture of phosphorus (P) from dairy manure are needed to promote recycling of P. A mobile system consisting of a fluidized bed technology with a 3,200 L cone mounted on an 8-m trailer was used to evaluate the variability in capture of P from liquid dairy manure in the form of struvite. Batches of 13,000 L of manure were pre-treated with sulfuric acid or oxalic acid, and magnesium chloride prior to pumping through the cone at a flow rate of ~20 to 28 L min-1. Sodium hydroxide or aqueous ammonia were pumped into the base of the cone to raise the pH and form struvite. On many occasions the system did not work well and negative ortho-P removals were observed. When the struvite system was working well, ortho-P capture using dairy manure ranged from 1% to 76%, averaging 34% over the course of 7 to 9 h of operation. Ortho-P capture was highest at 84%, and total P capture of 62% using anaerobically digested manure with pH modifiers of oxalic and/or sulfuric acid and aqueous ammonia. Major factors and concentrations that promote formation of struvite were: total suspended solids (<10,000 mg L-1), Ca (<600 mg L-1), Fe (<25 mg L-1), and ortho-P (>50 mg L-1). Partial budget economic analysis indicated that number of lactating cows and cropland were critical factors for achieving whole farm P balance. A comparison to two other technologies for capturing P from manure indicated the three systems provide an initial end product for off-farm export in the range of $0.003 to $0.02 per L of manure based on a partial budget analysis considering the variable cost of chemicals. Keywords: Dairy manure, Phosphorus, Struvite.