1. Reuse water where it will enhance the quality of cleaning. Reuse deionized (DI) water first to maximize the benefit of its purity and because of its cost (see Rochester Powder Coating below). DI effluent from a single stage rinse will generally be cleaner (less dissolved solids) than fresh city water.
2. Cascade the rinses where the overflow volumes are greatest (i.e., greater than five gpm). Cascading the phosphatizing rinse back to the cleaning rinse can cut the rinse flow in half. And, the acidic phosphate chemical that is returned with the rinse neutralizes the alkaline cleaning carryover that would otherwise degrade the phosphate chemistry. No new contaminants are introduced.
3. Use overflow from the subsequent stage as make up for heated tanks because heated tanks loose the most volume to evaporation.
4. Add a prerinse before the cleaner stage to loosen and remove soils. This keeps a significant amount of soils out of the system, making the cleaning stage more effective. Reuse water by cascading overflow from the cleaning rinse to the prerinse.

Water and Chemical Use Reduction Tips
Phosphorus alternatives. Research on substitutes for phosphatizing compounds is ongoing. A few options are currently available. A number of phosphatizing chemistry vendors have developed sophisticated seal rinses (microprimers) that eliminate the iron phosphatizing step while providing equal corrosion protection and adhesion. Phosphoric acid in the phosphatizing step also maintains a specific pH. This function can be accomplished by other acids to lower the phosphorus concentration of this stage. Talk to your supplier about these possibilities. If improved paint adhesion to aluminum substrates is the goal, Sol-Gel processes1 and abrasive blasting procedures2 are available.

Also, a one-stage, no-rinse process for small to medium scale operations is possible. It incorporates residual surface oils into the coating so excess solution can be captured and reused creating zero-effluent from the process3.

Bath concentrations. Maintain phosphate bath concentrations and chemical metering of wand applicators within the correct operating range, using the chemical supplier’s recommendation. If a range is given, try operating at the low end. This may require greater care by the operators. Lower operating concentrations reduce loading to the rinses, which can lead to reduced flow rates and phosphate losses to effluent.

Reduce carryover. Keep chemistries in their tanks by reducing carryover. Design drain holes in parts where possible and avoid blind holes and recesses. Rack parts for good drainage. Angle them so solutions drain off one point—not an edge­—of the part back into the bath.

Design the system with adequate drip time. For dip tank operations, hold parts above the tank to allow the solution to drain back into the tank. Holding parts above the tank for 15 to 30 seconds returns 40 to 50 percent of the dragout into the tank.

Modify drain boards between stages to drain back to the previous stage. Multistage spray systems should have drain zones between stages that provide for similar drain times—15 to 30 seconds minimum. Consider a fine, low-volume mist arc or spray rinse between stages to remove additional carryover.

Use clean water. Consider using DI or reverse osmosis (RO) water for making up chemical baths and possibly for rinses. DI water greatly decreases the dissolved solids present. This lengthens bath life and reduces the volume of chemicals used and discharged. It also cuts scaling in heated tanks and the volume of sludge generated by treatment of wastewaters.

Automated systems. Ensure all process controls are properly set (i.e., speed, chemical additions) and that they are periodically calibrated and maintained. Conductivity controls are particularly sensitive—consider using inductive conductivity sensors to reduce maintenance requirements. Quality parts are not an indicator of good system control. Poorly maintained control systems can create quality products by overusing water and chemicals. Frequency of bath turnover may be a better indicator.

Water flow. Measure and control water flow. Flow meters give a quick indicator of water overuse and malfunctions that can lead to overuse. Metering valves can be used with flow meters to control flow rates. In the absence of flow meters, use flow restrictors to control flow. Avoid using ball valves in water lines unless a wide open flow is desired. Small changes in ball valve position can result in large changes in the water flow rate. Although cheap, they are only appropriate as on/off valves.

Filter baths. Filter baths remove solids that could build up in the tank or clog nozzles. Skim oil off the alkaline cleaning tank to lengthen the bath life.

Spray nozzles. Clean spray nozzles. Plugged nozzles can cause areas of the parts to be poorly cleaned or coated. A common response to quality failures is to increase the flow and frequency of bath changes when merely cleaning the nozzle could ensure that the solution cleans the parts. Properly position nozzles for an ideal spray pattern to ensure the solution cleans the parts.

Procedures. Train employees on proper operation of the phosphatizing system. Conduct daily inspections to look for tank leaks, valve leaks, evidence of controller malfunctions and plugged nozzles.

Rochester Powder Coating
Reducing Phosphorus Discharge
Rochester Powder Coating (RPC) in Rochester is a job shop that paints sheet metal parts using powder coating. Prior to painting, the sheet metal goes through a phosphatizing line. By embracing pollution prevention practices, RPC reduced its phosphorus discharge by 98 percent over two years.

In October 1995, discharge from the RPC phosphatizing system was 410 milligrams per liter (mg/L) going into the City of Rochester’s sewer system. With impending phosphorus limits, RPC and the City of Rochester began to look for ways to reduce this phosphorus discharge. RPC implemented an aggressive approach to maximize the use of phosphate instead of discharging it. First, RPC began to monitor solution content every two hours. This ensured that the concentration was within the proper operating range. They also added more-efficient spray nozzles to the phosphating risers to improve solution contact with parts.

In 1995, RPC installed a five-stage cleaning/phosphatizing system. The system had a partial rinse return to the phosphating bath to prevent loss of phosphorus. Along with installing this system, RPC instituted rigorous monitoring, maintenance and worker training. In 1997, a sixth stage, DI rinse was added to enhance corrosion resistance and recycle phosphate rinse solution for reuse in the mist rinse just prior to phosphatizing. This provided cleaner parts going into phosphatizing. Although RPC increased its production and increased discharge by 30 percent, phosphorus discharge concentrations were lowered to eight mg/L.

Federal-Mogul Corporation
Identifying Phosphorus Sources
MnTAP funded a student intern at Federal Mogul, Lake City, a manufacturer of diesel and compressor pistons and cylinder sleeves, to identify the sources of phosphorous in its manufacturing plant and to determine a strategy for reducing the quantity ending up in the wastewater. The two main sources of phosphorus were a phosphate coating process (96 percent) and plant maintenance cleaning chemicals (four percent).

During the student’s time at the company, all of the cleaning chemicals were switched over to non-phosphorus containing materials. The substitutes performed as well as or better than the conventional phosphorous containing materials.