Engineered addition of slag fines for the sequestration of phosphate and sulfide during mesophilic anaerobic digestion.

The potential for weathered steel slag fines (D50  < 1.6 mm) to sequester dissolved phosphorus and sulfur from fermenting biosolids was determined. Batch studies showed that sorption equilibrium between phosphates and sulfides was reached in less than 48 hr after adding basic oxygen furnace (BOF) slag grains to municipal digester sludge. When these same particles were added to pilot digesters (75 L) operating at a 30-day SRT, a classic dose-response was observed with respect to PO4 and H2 S sequestration, and an inhibitory performance threshold emerged above 20 g BOF/L. Only a fraction of the BOF slag solids contributed to the digester TS mass, because a substantial portion dissolved into the supernatant, some of which contributed to alkalinity. After 160 days of continuous operation, approximately 63% of the total phosphate was sequestered from the supernatant and 78% of the hydrogen sulfide from the biogas, when a steady-state BOF slag dose of 10 g BOF/L was applied. At or below this level, BOF slag fines had no significant effect on mesophilic digester performance as judged by conventional operational metrics. This study demonstrated that upcycling BOF slag directly into the anaerobic digestion process could offer wastewater resource recovery facilities a novel nutrient management tool. PRACTITIONER POINTS: Basic Oxygen Furnace (BOF) Slag fines can be engineered as a cost-effective alternative to conventional phosphorus and sulfur control strategies. A rapid phosphate and sulfide removal from anaerobic digestates occurs within hours after BOF slag addition. 63% of phosphate and 78% of hydrogen sulfide were removed when digester sludge was dosed with 10 g BOF/L, without impacting digester performance. Only a fraction of the dosed BOF slag contributed to digester TS, likely due to partial dissolution into the digestate. BOF slag can potentially increase the nutrient content of biosolids, facilitate biogas recovery, and reduce odors.