Operating Experience with Ostara Struvite Harvesting Process

The Madison Metropolitan Sewerage District operates a 158 ML/d (42 MGD) advanced secondary wastewater treatment plant with biological phosphorus and ammonia removal. The 11th addition to the wastewater treatment plant included upgrades to the solids handling and anaerobic digestion facilities including: 1) Change from DAF thickening to GBT thickening of waste activated sludge (WAS); 2) Addition of acid phase digestion prior to methane phase; 3) Heating of the thickened WAS with steam injection heating prior to acid phase digestion; 4) Release of phosphorus from the WAS prior to thickening by combining and holding a recirculation stream of acid phase sludge with the WAS; 5) Struvite recovery from the combined filtrate streams generated from digested sludge thickening and WAS thickening; 6) The capability for time/temperature batching of a portion (~20%) of the digested sludge to produce a Class A biosolids. A process diagram is attached.The Ostara process for struvite harvesting has been operational since November, 2013. The District operates and owns the process, with control software, operational parameters, and constant input and advice provided by Ostara operational personnel. Bagged product is sold to Ostara and picked up about every two to three weeks. The soluble phosphorus in the feed is produced mainly by WAS thickening. The acid phase digestion process has total VFA concentrations in the 5,000 mg/l range for recycling and releasing P from the WAS. The acid phase digestion process also solubilizes phosphorus and magnesium to concentrations of approximately 600 mg/l and 180 mg/l respectively. These concentrations in the recirculation stream also add to concentrations released from the WAS due to the VFA additions. Anticipated benefits of struvite harvesting have been met in some areas, but operational experience with the system and other process needs are requiring modifications and research for improving process performance. The following are some process details and notes on changes either implemented or being investigated:The soluble P removal in struvite harvesting has met the projected 80 to 85% removal projections.Concentrations in the combined feed to the Pearl reactors are approximately 160 mg/l orthophosphate, 80 mg/l magnesium, and 220 mg/l ammonia. These concentrations were increased from design concentrations by pre-thickening approximately 30% of the WAS in the retired DAF thickening process.Soluble P concentrations in the digesters have decreased from ~200 mg/l to ~100 mg/l. Evidence supports that this will likely result in significantly less struvite precipitation in the digesters and in digested sludge piping, translating to decreased O&M costs in these areas.A comparable decrease in the water extractable phosphorus in the biosolids has positively impacted land re-application limitations for phosphorus.The total phosphorus removal by the struvite harvesting process has been lower than expected due to loss of struvite fines in the reactor effluent. This recycle is increasing the phosphorus loading to the secondary process and could negatively impact biological phosphorus removal.Elimination of the ferric chloride addition was planned with inclusion of struvite harvesting facilities. Management of H2S levels in the plant's permit for air emissions from engines is necessitating a low dose of ferric chloride to the digester feed.A low ferric chloride dose is also added to the sludge feed to thickening and dewatering processes. The pH elevates in the filtrate and centrate during digested sludge thickening and dewatering. This results in the formation of struvite in the filtrate and centrate in spite of much lower soluble P concentrations in the methane phase digesters. A low ferric chloride dose into the digested sludge feed suppresses the nuisance struvite formation as well as improved thickening and dewatering.The District and Ostara are continuing to work together to optimize the process performance. Several process modifications have been made and others are currently being investigated.Integral in these process investigations is the new paradigm that various unit operations in the treatment plant, such as return sludge, waste sludge, and thickening/dewatering processes and struvite harvesting , are now quite intertwined interconnected, resulting in a need for more careful analysis of upstream and downstream impacts within the process with successful operation of struvite harvesting. And an efficiently operating struvite harvesting process is now an integral requirement of the system necessary to protect the plant biological phosphorus removal process. Extensive laboratory work and process monitoring are on-going to find keys to achieving the expected performance from the 11th addition modifications.