The following paper was presented at the 2015 Anodizing Conference, San Diego, CA and prepared by Loren McCune, McCune Enterprises, LLC USA
George Patrick of ARCADIS
Loren McCune (Environmental Manager, Retired, Bonnell Aluminum)
Abstract: Bonnell Aluminum owns and operates an aluminum extrusion and anodizing operation in Newnan, GA. Prior to 2005, the wastewater from the anodizing operations was treated by conventional chemical precipitation consisting of neutralization, flocculation and clarification. Since the effluent was discharged to a zero flow stream, toxicity tests indicated that the salts in the effluent could be harmful to water organisms in the stream. Bonnell evaluated many techniques to reduce the toxicity of the effluent. Ultimately, Bonnell decided to further treat and reuse the effluent. This paper presents the challenges faced with implementing one of the first zero liquid discharge (ZLD) facilities for aluminum anodizers and the results of 10 years of successful operation of a ZLD facility. Lessons learned that may assist others are presented. Key results were:
Reasons for Changes and Challenges
Around the year 2000, biotesting of Bonnell’s NPDES outfall consistently indicated toxicity to water organisms in the zero flow stream that received Bonnell’s treated wastewater. The high dissolved solids level in the wastewater (primarily sodium sulfate) was the suspected primary toxin. Bonnell’s NPDES permit was due for renewal and something had to be done about the toxicity of the treated wastewater.
Some of the challenges Bonnell faced included an extremely high flow of wastewater (just under 0.5 MGD), high levels of dissolved solids in treated water, widely varying compositions of wastewater into treatment, high production demands in anodizing, and the need for a cost-effective solution due to certain business conditions. The strategy selected was one that dealt with each of the challenges.
Reduction in Water Use
The premise at the beginning of the project was that the water conservation at the Newnan plant was possible and that water conservation would be more cost effective because it would reduce the size of the wastewater treatment system. Bonnell compared the water usage at Newnan to the usage at the Bonnell plant in Carthage and found that there would be feasible to reduce usage at Newnan.
For conventional wastewater treatment facilities, the cost of equipment is proportional to the logarithm of flow rate. However, for advanced treatment where water is recovered and reused, the equipment cost is proportional to flow rate. Thus, it was imperative for Bonnell to reduce the water usage.
Plant management in Newnan was committed to pollution prevention and on-board with addressing the water use issue. The plant manager appointed the anodizing manager to head up a team inside the plant to work with a design team being organized by the division environmental manager. A project engineer was also assign to the team and perhaps the most important fact was that all of these team members were committed to the project effort. The team included consultants and eventually equipment suppliers. An EPA waste minimization paper was useful as Bonnell focused on water conservation technologies and to bench-mark Bonnell’s progress.
Activities taken to reduce water use in anodizing were:
As water use was being reduced, various options for treatment, recovery and reuse were considered.
Bonnell evaluated city sewage facility treatment, reverse osmosis, and several end-of-pipe treatment schemes. Bonnell had worked with the local publicly owned treatment works (POTW) and even with reduced wastewater, this option offered little hope. Bonnell piloted a reverse osmosis unit which was operated out of a trailer on site. Useful information was obtained. End-of-pipe treatments were pursued. A small test plot wetlands was installed. Bench work with peat moss and other media was pursued. These varied in effectiveness with none showing significant results.
The design team selected DMP Corporation from Rock Hill, SC for the recovery/reuse project. They joined the team and detailed design began.
Chemical precipitation was implemented as the first treatment process. It was clear the cost of reuse and recovery equipment would be reduced if the dissolved solids were reduced. The dissolved solids in the treated wastewater were the result of neutralizing sulfuric acid waste streams with sodium hydroxide either from alkaline waste streams or from storage tanks. Sodium sulfate is very soluble, however, a few alkalies produce salts that will precipitate above certain concentrations. Lime (calcium hydroxide) an inexpensive alkali reacts with sulfuric acid to produce calcium sulfate or gypsum. This compound precipitates above 2500 ppm (roughly), thus removing some of the sulfates from the waste stream.
To affect this, the waste streams were segregated into acidic waste streams and non-acidic waste streams. The more acidic waste streams were collected into what is called the segregated acid streams (SAS) and the nonacid streams (NAS). The SAS stream contained sulfuric acid content from 1.5% to 2%. Dilute acids and alkaline streams were directed to the NAS system. The SAS wastewater was neutralized with lime to a pH of around 5. The precipitate calcium sulfate was flocculated and removed in a clarifier and thickener. The resulting pH 5 stream then joins the NAS wastewater but with a reduced dissolved solids content.
The treatment system involves the SAS as described above and further treatment before reverse osmosis. The NAS waters are treated to remove metals and hardness by soda ash softening and precipitation at two pH levels, pH 10 and pH 7. This removes calcium, magnesium, nickel, and other metals that precipitate and pH 10 and the aluminum is removed at pH 7. Each stage of treatment includes two-stage neutralization, flocculation, and clarification.
Prior to reverse osmosis, the treated waters go through carbon adsorption, bag filtration and ion exchange as a polishing step to remove metals, calcium and magnesium.
The membrane system involved three trains, each with two stages of membranes.
The evaporator is a double effect, steam heated vacuum operated unit. The stainless steel evaporator operates at 80% recovery and the only real moving part is a centrifugal pump. Air pressure transfers distillate and brine.
The system has been operating for over ten years. Permeate and distillate are reused and the reuse water meets the design goal unless there is a failure with the RO membranes. Life on the membranes is usually one to 2 years. The short life of the RO membranes is the biggest problem that Bonnell has experienced. The brine, containing sodium sulfate salts, is shipped off site for treatment. Water use has been reduced by 85% since 2000.
Many changes have occurred at the Newnan plant in the last 10 years:
Reducing Water Use in Anodizing
Bonnell’s experience is that minimal water use in anodizing can be achieved without sacrificing quality. Leadership from plant management, tracking conductivities in anodizing, improving rinsing, and installing crane drainage systems are some of the techniques shown to be effective. The water savings should justify the changes needed in anodizing lines.
Reusing Water in Anodizing
Membrane systems can be effective, but may or may not be cost justified. For Bonnell, the justification was environmental and necessary for survival. City water may be needed in one or two rinses, but reusing water is feasible.