Water and ammonium recovery from the digestate of short-fiber residues in paper recycling mills using membrane filtration and natural zeolite

ABSTRACT Flowchart of a model paper recycling mill water circuit, highlighting recycling, recovery, and discharge pathways. Icons indicate each unit (from http://www.flaticon.com). This study investigates nanofiltration (NF) and reverse osmosis (RO) treatment of the liquid fraction from digestate of short-fiber residues from paper recycling mills, focusing on water reuse and ammonium (NH4-N) recovery. NF removed up to 86% chemical oxygen demand (COD) and fully retained phosphorus (Ptot), but separated only 40–55% of total nitrogen (Ntot) and NH4-N. RO performed better, removing 96% COD, 86% NH4-N and Ntot, while fully retaining Ptot. RO permeate met the freshwater quality for paper recycling, with small NH4-N remaining. To improve NH4-N recovery, natural zeolite was tested with RO in three scenarios: treating the liquid fraction of digestate, RO retentate, and RO permeate. The highest NH4-N adsorption (79%, 7.8 mg/g) occurred with RO retentate, and the highest desorption (53%, 2.9 mg/g) with RO permeate. Adsorption performance varied with NH4-N concentration and pretreatment, with higher NH4-N levels leading to greater adsorption. COD was also reduced by up to 70% during adsorption. RO treatment of digestate in a model paper mill (0.7 Mt/year production capacity) could recover about 38,000 m3 of water annually, saving 71,000 €. Combined savings from water reuse and avoided digestate disposal could alone reach 0.2 million € annually.