Planting Waste Process Includes Microfiltration
A metal plating business in Massachusetts has installed a pollution control system that treats the company's wastewater to the point that it is cleaner than most household wastewater.
Absolute Metal Finishing is a metal plating firm that uses large quantities of the heavy metals nickel, chromium, gold, silver and copper, as well as hydrochloric, sulfuric and nitric acids. It is located in an area under the jurisdiction of the Massachusetts Water Resource Authority (MWRA), which has some of the strictest pollution control requirements in the nation. The installation of a microfiltration system has made it possible to meet, and by a large margin, exceed these requirements. As a result, the company is positioned to handle any foreseeable tightening of effluent discharge standards with no additional investment.
Absolute Metal Finishing, which has been in operation only for 19 months, provides a wide range of metal coating and finishing services, and is capable of plating virtually any metal. Most of the work, however, is carried out on steel and aluminum, and the business has grown rapidly in the short period since startup.
The new company's owners, who had 40 years of combined prior experience in metal finishing, had to make a key decision in determining where to locate their plant. Well aware of the fact that a number of plating companies had been assessed substantial fines for violating effluent limits, they looked at several towns in eastern Massachusetts. They settled on Norwood, a dozen or so miles southwest of Boston, largely because it comes under MWRA control. The Massachusetts Authority had developed regulations that were strict but were well enough defined to avoid misinterpretation. Requirements vary for each type of heavy metal, but generally fall in the area of one part per million (ppm).
A second step was to select an appropriate pollution control system to treat the plant's complex wastewater discharge. This called for careful definition of the objectives. A set of criteria was developed to establish the performance required of both the system purchased and its vendor. The system had to meet the existing and proposed limits of the environmental agencies, had to be easy to operate, service and maintain, had to have expansion capability, and, above all, had to be reliable.
The system's manufacturer would have to have demonstrated through experience in the industry, the ability to design, supply and support such a system, must have a clear understanding of the plating process and the accompanying wastewater considerations, and must understand the importance of furnishing a reliable, low maintenance and affordable wastewater treatment system.
Local Knowledge Aids Decision
With much local knowledge of industrial pollution control installations in the area, the company's owners reviewed a variety of systems from different manufacturers. A design incorporating microfiltration technology offered by Manchester Corporation of Harvard, Massachusetts, was selected, but not before another type of system available from the same supplier was considered.
The second unit was a conventional toxic metals precipitation system, while the equipment selected was based on the membrane filtration process. Advantages and disadvantages of each system were weighed, and treatment requirements, capital costs, operating and maintenance costs, energy requirements, facility needs and reliability were considered. The results of the study pointed to the membrane system as the best choice for the Absolute Metal Finishing project.
The company's coating apparatus is almost entirely automated. Toxic or hazardous materials, largely in the form of metal salts, are produced at nearly every stage of the coating process. Electroless nickel also is found in significant quantities. Three separate waste streams are generated and approximately 60 coating baths can be directed to any one of these streams. Each of them is segregated depending on its contents. In this way the most effective treatment may be applied depending on whether the liquid contains heavy metals or complexing agents like soaps and cleaners, or exhibits low pH, high pH, etc.
The liquid wastes from the plating and metal finishing operation flow by gravity to chrome and pH sump tanks. They are then pumped to the chrome reduction and pH adjustment tanks. In the chrome section, acid and bisulfate are added for chrome reduction. The reduced chrome wastes flow by gravity to the pH system where acid or alkali are added to precipitate metals as hydroxides. Adjustment of the pH minimizes solubility of the metals in the effluent. Cyanide rinse water from the copper tank is pumped to a cyanide destruction module, where sodium hypochlorite and pH adjustment bring about oxidation of the cyanides to carbon dioxide and nitrogen, and release the toxic metals as dissolved salts. The treated cyanide wastes flow by gravity to the pH adjustment sump for further treatment. Retention times are over two hours for the cyanide treatment, approximately one hour for the chrome reduction and 40 minutes for pH adjustment.
Adjustment of the pH causes formation of a precipitate which then is concentrated by the microfiltration system. Sludges containing heavy metal hydroxides are returned to the concentrator tank, which is pumped out automatically through a series of time-controlled valves to a holding tank for additional thickening. The "clean water" leaving the microfilter passes through a Parshall flume, where flow rate and final pH are measured and recorded prior to discharge of the effluent to the municipal sewer. Overflow from the sludge tank is directed back to the pH tank. Last, the thickened sludge is transferred to a filter press for dewatering. This step results in a dense filter cake, which is transported away as hazardous waste by a licensed hauler. The filtrate is returned to the pH system for reprocessing.
Once they understood its operation, the environmental authorities were suitably impressed with the performance of the system and provided full cooperation in approving the project. Only three months elapsed from the day of application to the day the required discharge permit was issued. Absolute Metal Finishing's staff began operations on the following day, and from startup the system's performance has been virtually trouble-free. It has demonstrated the ability to process the wastewater from the plating operations to a quality level which is better than the one ppm requirement.