The conventional method for removal of heavy metals by industrial wastewater treatment plants (IWTPs) uses hydroxide or sulfide precipitation, which produces a hazardous metal sludge. Molecular recognition technology (MRT) has a unique capability to remove, separate and make metal ions available for recovery when applied to Department of Defense (DoD) IWTP waste streams, without forming a hazardous metal sludge. A demonstration was conducted at the Puget Sound Naval Shipyard (PSNSY) in Bremerton, Washington for six months on a high-volume acid/alkali and chromium influent waste stream to show MRT's applicability to this wastestream.
The MRT system uses the high affinity and selectivity of macrocyclic chelating ligands, bonded to stable, solid polymer supports, to target the toxic metals found in DoD industrial influents. These specialized, polymer-supported ligands may be used in a packed bed or a membrane configuration at the high flow rates required by typical DoD IWTPs.
The MRT technology removed a wide range of heavy metals to below the monthly, regulated discharge levels at PSNSY and to much lower levels than those achievable by the conventional precipitation process. Annual cost savings over the conventional process were estimated to be $73 thousand. The payback period for a system to replace the IWTP at PSNSY was 9 years. However, for an add-on installation to remove chelated metals, which periodically arise in DoD waste streams and require pretreatment before conventional precipitation, the payback period would be less than 2 years.
Future Metal Products and Machinery (MP&M) regulations may mandate levels for treated effluent achieved by the MRT process. To achieve these low metal concentrations, the operating costs for the conventional process would further increase because more precise pH control would be necessary to optimize the precipitation process. Furthermore, MRT can remove metals separately and sequentially from aqueous industrial waste streams for recovery, unlike ion exchange, which does not discriminate between metal ions. MRT's capability to remove and regenerate metals in purified form eliminates the formation of hazardous metal sludge, the disposal of which in the future will be increasingly costly. The MRT system also has a small footprint.
The MRT's unique recovery capability could qualify an IWTP for exemption from Resource Conservation and Recovery Act (RCRA) permit requirements when designed as a closed-loop recycling process that produces products for use, reuse, or reclamation. The expected savings from the MRT system designed for future waste streams would exceed $30 million per year for the Navy. (Project Completed - 2003)