The Pearl 2000 is capable of producing 2,000 kg/d of finished product, known as Crystal Green, which will then be sold back to the technology developer for resale as a fertilizer ingredient. Courtesy of Clean Water Services
The advanced wastewater treatment facility removes phosphorus and nitrogen from the liquid stream, helping the plant comply with permits. The nutrients are then converted into a revenue-generating fertilizer ingredient.
June 12, 2012—In early May, Clean Water Services, of Hillsboro, Oregon, formally unveiled the world’s largest nutrient-recovery facility employing the Pearl process developed by Ostara Nutrient Recovery Technologies, Inc., of Vancouver, Canada. Located at the Rock Creek Advanced Wastewater Treatment Facility, the nutrient-recovery plant removes phosphorus and nitrogen from the liquid stream generated during the solids-handling process and converts the nutrients into a valuable fertilizer product. In this way, the new installation is expected to help the Rock Creek facility comply with its strict discharge limit for phosphorus while simultaneously providing a revenue source for Clean Water Services, a water resources management utility outside of Portland.
A leader in the field of nutrient recovery, Clean Water Services added the Pearl process to its 26 mgd Durham Advanced Wastewater Treatment Facility in 2009, marking the first full-scale installation of the technology at a municipal wastewater treatment facility. After two years of successful operations of the Pearl process at the Durham facility, Clean Water Services opted to incorporate the process as part of an overhaul of the secondary treatment system at its Rock Creek facility, says Nathan Cullen, P.E., M.ASCE, the engineering division manager for Clean Water Services.
During summer months, the 39 mgd Rock Creek facility is required to limit concentrations of total phosphorus in its effluent to less than 0.1 mg/L. Until earlier this year, the facility had used chemical treatment to remove phosphorus. But in early 2012, Clean Water Services completed efforts to convert the Rock Creek plant to the use of biological methods for removing phosphorus. Adding the Pearl process greatly helped to facilitate the conversion of the Rock Creek facility to the biological approach to removing phosphorus, Cullen says. By treating the liquid that is removed from solids during the dewatering process, the Pearl process reduces by 85 to 90 percent the phosphorus load that otherwise would be returned to the head of the plant for further treatment. In this way, the Pearl process “all but eliminates” the phosphorus that must be recycled within the plant, Cullen says, reducing the burden on the secondary treatment system. “It makes meeting the permit [limit for phosphorus] easier and more reliable,” he says.
For the installation at the Rock Creek facility, the Pearl process consists of two 26 ft tall fluidized bed reactors in which magnesium is added to the liquid stream to precipitate the formation of magnesium ammonium phosphate, commonly known as struvite. Within the fluidized bed reactors, the struvite takes the form of small crystal-like pellets that remain in suspension and continue to grow until they reach the desired size of 1.5 to 3 mm in diameter, says Phillip Abrary, the president and chief executive officer of Ostara. The reactors include different sections that are designed to generate varying fluid velocities that facilitate the downward movement of pellets through the reactor as they grow. In this way, properly sized pellets eventually are harvested at the bottom of the reactors. Known as the Pearl 2000, the two reactors installed at the Rock Creek plant are each capable of producing 2,000 kg/d of finished product, which is known as Crystal Green. “It’s more than double the size of anything that we’ve done before,” Abrary says.
Along with removing nutrients from the wastewater, the Pearl process helps decrease the extent to which struvite accumulates in piping at the plant, a common nuisance at wastewater facilities. What is more, the process is expected to lower the costs associated with chemical usage and biosolids disposal, Cullen says. At the Durham plant, the Pearl process enabled Clean Water Services to reduce by 40 percent the amount of alum used for final polishing of the effluent. The reduced chemical usage also led to an 18 to 20 percent decrease in the amount of biosolids generated at the plant, lowering the related disposal costs. Similar results are expected at the Rock Creek facility, Cullen says.
Under the terms of an agreement with Clean Water Services, Ostara will buy all of the Crystal Green produced at the Rock Creek facility. In turn, Ostara will sell the Crystal Green to entities that use it as part of a blended fertilizer product that is sold to the turf, nursery, and specialty agriculture industries. With the combination of the anticipated revenues from sales of Crystal Green and the operational cost savings, Clean Water Services expects that the $4.475-million nutrient recovery facility will pay for itself within 6 years. “After that, it will be a source of revenue,” Cullen says.