Prior to 2007, Concord, Massachusetts wastewater treatment plant operated under a NPDES permit that allowed a seasonal phosphorus limit of 0.7 mg/L. But with the issuance of a new five-year permit with a phosphorus limit of 0.2 mg/L and the probability that future limits could reach as low as 0.05 mg/L, Concord needed a treatment solution that would be reliable and cost-effective today and over the long term.