Innovation in valorization of cow manure: Higher hydrolysis, methane production and increased phosphorus retention using UASB technology

Abstract

Cow manure has potential to serve as a sustainable secondary fuel and phosphorus resource and cut our reliance on finite primary resources. The efficient and sustainable valorization of cow manure faces compositional challenges because of a high solids content, the lack of soluble phosphorus, fine struvite particles being the main phosphorus species and high bicarbonate concentrations. Addition of calcium could result in higher methane production and conversion of struvite to calcium phosphate. To investigate this, cow manure was digested in two up-flow anaerobic sludge blanket reactors for 456 days, one with and one without CaCl2 addition. A positive effect of calcium addition was found for hydrolysis (29 % without calcium and 67 % with calcium), methane production (136 L-CH4 kgVS-1 without calcium and 301 L-CH4 kgVS-1 with calcium) and sludge bed development. Although calcium was added in a 3:1 ratio to phosphorus, it did not result in recrystallization of struvite to calcium phosphate. Instead, it precipitated as calcium carbonate, which was further induced by additional bicarbonate production through higher hydrolysis and methane production. Still, calcium addition caused better phosphorous removal (from 38 % to 61 %), which is attributed to the enhanced sludge bed capturing and accumulating both the calcium carbonate and struvite fines. Higher methane production and improved phosphorus retention enables better valorization of cow manure. The access to resources from cow manure through this technology can contribute to the circularity of agriculture and save on finite natural resources.