Improving nutrient fixation and dry matter content of an ammonium-rich anaerobic digestion effluent by struvite formation and clay adsorption

The anaerobic digestion (AD) of organic wastes that contain nitrogen leads to its mineralization, yielding a digestate rich in ammonium (NH₄⁺), an important fertilizing nutrient. The applicability of AD digestate as fertilizer can be improved by fixating the nutrients and increasing its dry matter content. Methods for the fixation and recovery of the digestate's NH₄⁺ and possible also PO₄³⁻ include struvite precipitation and adsorption in clay materials such as bentonite. These techniques were tested in batch experiments employing the liquid fraction of a digestate originating from the AD of a substrate mix containing lignocellulose, cattle manure and fish industrial waste. The concentration of NH₄⁺-N in this digestate was 2,300 mg L⁻¹. Struvite precipitation conditions at a molar ratio of 1.2:1:1 (Mg²⁺:NH₄⁺:PO₄³⁻) and pH 9.5 were best in terms of simultaneous removal of NH₄⁺-N (88%), PO₄³⁻ (60%) and soluble chemical oxygen demand (44%). Bentonite adsorption gave comparably high removal levels for NH₄⁺-N (82%) and PO₄³⁻ (52%). Analysis of the precipitates' morphology and elemental composition confirmed their struvite and bentonite nature. Dry matter content was increased from 5.8% in the AD digestate to 27% and 22% in the struvite and bentonite sludges, respectively.