Manure nutrient recovery, removal, and reuse on California dairies

California is the leading US dairy state, with over 1.7 million cows. Surplus nutrients in manure, especially nitrogen (N), present significant environmental challenges to local air and water quality. Novel nutrient recovery technologies can reduce adverse manure nitrogen impacts, including losses to groundwater, while increasing the availability of recycled nitrogen, phosphorus, and organic amendments for nondairy farms, creating a circular nutrient economy. We estimated and characterized surplus N on dairy farms in high-priority N management zones in California's San Joaquin Valley by location and farm size (537 farms). Stocking rates and correlated nutrient surpluses were not a function of overall farm size in terms of either on-site land available for manure application or mature cow numbers. Using geospatial, techno-economic, and life cycle assessment modeling, we compared environmental performance (impacts and benefits) of three prominent systems for managing dairy N surplus: vermifiltration, chemical flocculation, and ammonia stripping and recovery. Based on modeled greenhouse gas and nutrient flows within and from dairy manure management systems, tradeoffs between climate impacts and nutrient surplus reduction were observed among these technologies. The largest combined climate and surplus N management benefits occurred when nutrient recovery was combined with renewable natural gas recovery from anaerobic digesters. The collocation of dairies in some subregions affects potential solutions to managing surplus manure N and other nutrients.