Enhancing nutrient and water recovery from liquid digestate: A comparative study of selective electrodialysis and conventional treatment methods

Abstract

Ultrafiltration and reverse osmosis proved effective in treating the liquid fraction of anaerobically digested sewage sludge, both with and without an intermediate selective electrodialysis treatment step. The incorporation of selective electrodialysis significantly increased clean water recovery rates (from 38% to 88%) while concurrently reducing its dissolved solids load by almost two orders of magnitude, from 869.0 to 10.3 mg/L. Selective electrodialysis efficiently separated ions into stable fractions that could be post-processed to produce tailored solid fertilizers, such as struvite and ammonium sulfate, on demand. Throughout the system, heavy metals and pharmaceuticals were monitored for potential toxicity and harm to ecosystems. Heavy metals in the effluents of the treatment process both with and without the selective electrodialysis inclusion were below regulatory limits. Traces of ibuprofen, ciprofloxacin, oxytetracycline, and tetracycline were found in ultrafiltration and reverse osmosis concentrates. Currently, there are no established regulations for monitoring pharmaceuticals in digestate, but their presence could potentially complicate the direct use of these concentrates in agricultural applications.