Large-Scale Nutrient Harvesting: Making the Case for Resource Recovery in Atlanta

The City of Atlanta’s Department of Watershed Management (“the City”) has evaluated several resource recovery and energy management strategies as part of its 50-year wastewater master planning efforts. These strategies include evaluating technologies and proven processes that can save and/or recover resources and energy, while adhering to the concepts of the triple bottom line. The City conducted a pilot study and evaluation to determine the technical feasibility and business case for nutrient harvesting at the 122 MGD capacity R.M. Clayton Water Reclamation Center (RMC).In order to meet effluent total phosphorus limits for discharge to the Chattahoochee River, RMC utilizes enhanced biological phosphorous removal (EBPR) and, when required due to process conditions, chemical precipitation of phosphorus. Anaerobic digesters at RMC receive a combination of primary sludge and waste secondary sludge. After digestion, the sludge is dewatered using centrifuges and then incinerated. RMC’s influent phosphorous loading is estimated between 3,100 and 3,900 lbs per day based on an annual average daily influent flow. The dewatering recycle sidestream returned to the main treatment process can represent a significant component of the combined influent phosphorus and ammonia load. In addition, RMC experiences significant struvite (magnesium ammonium phosphate) scaling that impacts the operability of dewatering piping and equipment. Under this project, the City conducted a pilot study using the Ostara Pearl® nutrient recovery process to achieve controlled precipitation of struvite and yield a commercially valuable fertilizer product.The objectives of the project include the following:• Demonstrate the technical feasibility of nutrient harvesting at the RMC facility scale• Evaluate the potential reductions in phosphorus and ammonia recycle loads to the main treatment process• Evaluate reduced struvite formation potential associated with centrate return and anaerobic digestion at RMC• Assess the business case using triple bottom line decision criteria, potential for revenue generation at full scale nutrient harvesting, and field trial operational requirements for the system, and obtain feedback from RMC operations staffResults from the pilot study indicated up to 90% removal of phosphorus and up to 20% removal of ammonia from the dewatering recycle stream. Evaluation of projected design and operating conditions as well as capital investment requirements showed nutrient harvesting would have a payback period of approximately five years with net annual savings of approximately $1.7M on plant operations. While the revenue stream for the harvested nutrient product is significant, additional benefits and annual cost savings are realized in the following areas:• Reduced phosphorus and ammonia loading to the main treatment process• Reduced potential for nuisance struvite precipitation• Reduced reliance on chemical phosphorus removal• Reduced chemical purchases:○ ferric chloride for phosphorus removal○ lime for alkalinity recovery• Reduced operation and maintenance requirements and costs for chemical feed systems• Reduced energy consumption for mainstream treatment and solids handling processes due to reduced recycle load• Reduced production of chemical sludge and associated disposal requirementsThe pilot work was successful in demonstrating alignment with the triple bottom line criteria of social and environmental stewardship in conjunction with the potential for significant operational and financial benefits to RMC and the City. The City has decided to move forward with the installation for a full-scale process. The pilot study and future full-scale project represent one of the largest nutrient recovery systems in the US.