The ion-exchanger–loop-stripping process: ammonium recovery from sludge liquor using NaCl-treated clinoptilolite and simultaneous air stripping
The technical feasibility of an ammonium recovery process (‘ion-exchanger–loop-stripping’) for sludge liquor from municipal wastewater treatment plants is examined. The proposed process combines ion-exchange on natural zeolites with simultaneous air stripping of ammonia to produce an industrial NOx-removal agent. Column experiments with continuously recycled NH4Cl-solutions and a real sludge liquor sample were conducted to determine basic ion-exchange kinetics of the applied clinoptilolite. Mass balances of consecutive loading/regeneration cycles show the positive influence of NaCl-pretreatment as well as simultaneous air stripping on the NH4+-exchange capacities. Removal rates for NH4+ between 61.5 and 84.6% were achieved at NH4+-concentrations typical for sludge liquor (900 to 2,300 mg L−1). Zeolite loadings ranged from 5 to 8 mg NH4+ g−1 after 90 min of loading. Regeneration rates were between 42.9 and 49.7%, but increased to 64.8% with simultaneous air stripping. A minimal decrease in the ammonium removal rate was observed as a result of matrix effects in sludge liquor (e.g. flocculants, competing ions). Liquid analyses showed a considerable phosphate-reduction in the sludge liquor sample after ion-exchange due to potential struvite or apatite precipitation. The obtained results enable a detailed design, scale-up and further optimization of the ion-exchanger–loop-stripping process in future.