After the sheet-metal components are laser cut, punched, bent and welded, the parts move to the finishing department. Aluminum parts are subjected to a chrome conversion before some are powder coated. Cold rolled steel and galvanized parts are pretreated through a five-stage iron phosphate washer and powder coated.

This five-stage iron phosphate washer spray deserves a closer look.

• Stage 1 involves a 1,250-gallon alkaline cleaner solution with surfactants operating at a 3.5 to 4 percent concentration, with water pressure of 15-18 psi at the nozzles and temperatures of 135°F. The alkaline cleaner removes oil and grease from the parts, thereby preparing them to accept the iron phosphate or powder coating. The sodium hydroxide solution is purchased as a liquid, which is formulated with a silicate builder and nonionic and anionic surfactants. Stage 1 is followed by a spray rinse (Stage 2) using city water.
• Stage 3 centers on iron phosphating, which minimizes corrosion of the part. The first step is the dissolution of the metallic iron in a phosphoric acidic solution. As the acid attacks the metal surface, it is consumed, raising the pH of the liquid. The change in pH causes phosphate salts to precipitate and react with the metal surface, forming a crystalline coating that locks paints or powder onto the part and acts as a barrier to the flow of electrons. The third stage is followed by another spray wash (Stage 4) using city water.
• Stage 5 is the non-chrome seal, which enhances under-paint corrosion resistance on the surface of the iron phosphate-coated steel.

Keeping the rinse water at a sufficiently high quality level is critical to the efficiency of the finishing process. The rinse from Stage 2 is particularly sensitive to the amount of oil carried over from the Stage 1 cleaner, and the Stage 3 process cannot operate effectively if more than 100 ml of oil per liter of rinse is carried over from Stage 2. To ensure process efficiency, Metcam was forced to dump Stage 2 rinse water every other production day. In addition, as the cleaner in Stage 1 approached its six-month bath life, the quality of the solution carrying over to Stage 2 deteriorated. Maintaining the quality of its finishing line was of paramount importance to Metcam, but it was also expensive. Metcam estimated that it spent $35,515 every year just to generate Stage 1 and Stage 2 wastewaters, with evaporation costing approximately $0.31 per gallon of wastewater.

Managing the wastewater it generated added even more cost and headaches. Without access to a sewer connection, Metcam weighed two options: sending the waste offsite for treatment and disposal, or reducing the waste by evaporator onsite and sending the residual sludge offsite for final disposal. Evaporation represented the most cost-effective solution, but Metcam recognized that minimizing or even eliminating wastewaters at their source would be a much more effective solution. Explained Schwefel, "We were interested in finding an alternative to the two traditional options available to metalworking operations. Fortunately, the state of Georgia was just as interested in finding alternative methods to minimize waste. With the help of the state's P2 division (P2AD), we entered into a program to test a pollution prevention option."

Unlike a traditional filtration system that can foul within a short period of time, a cross-flow filtration system lets the membrane surface receive continuous "sweeping," thereby limiting the fiber cake buildup and extending processing times. In a crossflow UF system, contaminated cleaner solution is pumped across the membrane at relatively low pressures. Solution flow is parallel to the filter pores while a tangential pressure transports water, dissolved salts/metals and low-molecular weight cleaner compounds through the membrane pores to the cleaner bath. All suspended solids and soluble materials rejected by the membrane move away in a continuous, turbulent flow at the membrane surface.

The UF system that Metcam employed was designed to separate high molecular weight, colloidal or suspended solids from water-based cleaning solutions. Constructed of a semi-permeable, sintered titanium dioxide membrane attached to a tubular, stainless steel substructure, the membrane had a nominal pore size of 0.1 microns.