Water Recycling Technology Through Ion exchange

Introduction

The cost and maintenance associated with recycling metal bearing waste streams has decreased in recent years. This case history will press upon the technical and financial aspects of recycling metal bearing rinse waters. It is LT Technologies' belief, that by understanding the "theory" and environmental impact of water recycling, more companies will follow this path into the new millenium.

Ion Exchange Treatment

The water recycling system LT Technologies designed recycles 100 % of all rinses associated with the etching process. All copper and ferric chloride rinses are double counter flow with a retainer to assist in the reduction of etching solution drag out. The inorganic (copper and ferric chloride rinses) constituents cascade to the sump tank where it is then pumped from the etchers directly to an ion exchange feed tank. In an etching process the use of non-volatile organics are required. These organic components coat the etched substrate and give it a protective layer. As with all organics present in the use of ion exchange, pretreating with carbon is imperative. This organic rinse water from the stripper/developer and benzotriazole are fed to two 4.0 ft3 FRP columns of organo clay and carbon at 8 gallons per minute. At this flow rate the organic absorption unit will run at 2 gallons per minute, per cubic foot. This is a sufficient amount of media and volumetric flow rate to efficiently absorb all organics present. The organic treatment phase can be backwashed with city water. Having the ability to backwash insures that the columns will not saturate prematurely.  The previously organic rinse water is then fed to the ion exchange feed tank.

The etching rinse water is now ready for treatment. The water will be pumped using a dual set of centrifugal pumps with a double face corrosion resistant seal. A pressure transducer controls the feed tanks. This pressure transducer is wired back to the control system and converts pressure into feet (each psi = 2.31 feet). This will in turn control the function of the feed pumps. If the feed tank should drop to a low-level point an automatic actuated ball valve opens to allow the passage of make up water. The city water is treated through a standard deionization (SDI) make up system.

The water is fed through a 5 micron pre-filter for sedimentation removal. The ion exchange unit is designed with two dual bed exchangers. Each cation exchanger houses 14 ft3 of a polystyrene cross-linked strong acid cation resin. The basic function of the strong acid cation resin is to exchange hydrogen for all cations present in the feed stream.  Each of the anion exchangers contains 21 ft3 of a macroporous weekly basic anion resin. As with most metal finishers the need for 5 to 8 meg water is not required for recycling. The weak basic anion resin effectively removes acids such as chlorides, sulfates, and nitrates, but will not exchange hydroxide for all present anions. The capacity is a tremendous benefit of the week basic anion. Standard strong basic anions can hold 18,000 kilograins per ft3. The weekly basic anion resin has a capacity of up to 35,000 kilograins per ft3. With almost twice the holding capacity, the resin we selected cut down the frequency and volume associated with a regeneration without sacrificing quality. This system was designed for 20 gpm but currently is only running at 15 gpm. There is 14 ft3 of strongly acidic cation resin in each of the cation vessels.  At this flow rate the exchange process of hydrogen for positively charged cations(+) equals (1.07) gallons per minute, per cubic foot. As with most acidic waste solutions there is a greater amount of negatively charged anions(-) than positively charged cations(+). For this reason, a greater volume of weekly basic anion resin is present in each anion vessel. The anion resin will exchange OH(-) for negatively charged anions(-) present in the waste stream at (0.71) gallons per minute, per cubic foot. At a feed TDS (total dissolved solids) of approximately 300 mg/l (600 Mhos) this efficiently exchanges all dissolved solids to an average of 25 mg/l (50 Mhos).

The "treated" water is fed through a post macroporous filter for possible resin leakage. The now clean water is continuously recirculated through an ultraviolet disinfection unit for bacteria control. Stagnant water can promote the growth of bacteria very quickly; continuously recirculating the water through the UV lamp decreases the growth probability. The treated water is pumped back to the etching process through a pressurized pumping system, 15 gpm, at 60 psi. A pressure transducer (mentioned above) monitors the transfer tank and pump from high and low level alarms.

Ion Exchange Chemical Regeneration

Ion exchange resins are widely used due to their ability to be chemically regenerated. Cation exchange resins are regenerated with a positively charged acid (H(+)). The two most commonly used acids are sulfuric (H2SO4), and hydrochloric (HCl).  HCl was selected over H2SO4 for its ability to regenerate the resin without causing sulfate precipitates on the cation resin bed. Both the cation and anion resin beds are regenerated with deionized water to insure proper chemical to contaminant exchange. Before regeneration it is important that the column being regenerated is backwashed. This backwash will "fluff" the resin bed and discard any colloidal matter present. A sample port is used to observe the backwash water. All of the water generated from the backwash is recycled to a holding tank where it can be used for make up water or additional water for the decationized dual bed rinse explained later. The backwash water is passed through a macroporous filter to remove any sediment present. When the water becomes relatively clear the backwash is complete. The cation regeneration is ready to be initiated, and volumes are as follows:

14 ft3 of Cation resin with Hydrochloric Acid treatment: 10% maximum @ 10 lbs. ft3 = 45 gallons with 32% Hydrochloric Acid. 4.0 GPM of water to 1.5 gallons of 32% HCL=5.5 GPM total for 30 minutes.

Through automated controls in our Allen Bradley PLC, we are able to recover a large percentage of the regenerant for reuse. During the coarse of the cation regeneration and slow rinse a volume of 125 gallons is recovered from the 325 generated. The regeneration is followed by a deionized slow rinse. This slow rinse allows for further exchange to take place and also rinses the free acid off of the resin bead. After the anion regeneration phase a dual bed fast rinse is performed. A fast rinse will not have to be completed during the cation regeneration.

Anion resins are regenerated with a negatively charged hydroxide chemical (OH(-)). With the use of sodium hydroxide (NaOH) we are able to exchange negatively charged anions off the resin for (OH(-)). As with the cation regeneration, the anion resin bed must be back washed prior to sodium hydroxide regeneration. All of the water generated from the backwash is recycled to a holding tank where it can be used for make up water or additional water for the decationized dual bed rinse explained later. The backwash water is passed through a macroporous filter remove any sediment present. The anion regeneration volumes are as follows:

21 ft3 of Anion resin with sodium hydroxide treatment: 6% @ 10 lbs. per cu ft. = 35 gallons with 50% sodium hydroxide. 7.3 GPM of water to .67 of 50% NaOH=7.97 GPM total for 30 minutes.

Through automated controls in our Allen Bradley PLC, we are able to recover a large percentage of the regenerant for reuse. During the coarse of the anion regeneration and slow rinse a volume of 150 gallons is recovered from the 450 generated. The regeneration is followed by a deionized slow rinse. This slow rinse allows for further exchange to take place and also rinses the free hydroxide off of the resin bead.

After the dual bed deionizier is regenerated a decationized fast rinse is performed. The water that was recovered from the cation and anion regeneration will be used in the decationized fast rinse. The water is fed through a centrifugal pump to the dual bed exchanger where it is recirculated for 45 minutes. A TDS meter inline monitors the incoming total dissolved solids. If the feed TDS becomes greater than 1500 mg/l an alarm will sound and the decationized rinse water is diverted to the regeneration waste holding tank. Deionized water is fed to the recirculation tank if a low-level point is reached. A level pressure transducer controls the function of the make up water valve. When complete the dual bed exchanger regenerated is ready for operation.

Regeneration wastewater

During the regeneration, a total of 800 gallons was generated, with 35 % recycled for the decationized rinse. This recycling process will reduce the historical volumes of regeneration with a "final fast rinse" by 500 gallons. The now contaminated 800 gallons from regeneration must be treated. There is much free HCl in the regenerant wastewater. The pH of this waste solution is approximately (1) to (2) pH units. The pH must be adjusted to between (7) and (9), in order to successfully treat the solution. The wastewater will be treated through a closed atmospheric evaporator. The closed system will concentrate the solution and the distillate will be condensed through a heat exchanger. This distillate can be used for future make up water. There are level controls internally that adjust the solution being fed to the unit. All components are controlled by an Allen Bradley PLC with modem capabilities. The closed evaporator reduces the volume of regenerant wastewater by 90%. Furthermore, there was no air discharge permitting required the customer is RCRA (Resource Conservation and Recovery Act) exempt. Due to this exemption, the customer forgoes the environmental and financial obligation of filing for an air discharge permit.

Ion Exchange System Controls

The ion exchange dual bed deionizer is controlled with an Allen Bradley SLC 500 controller. A OIT digital display is present for visual inspection of the level percentage in the primary system tanks. Level requirements can be changed with the touch pad of the OIT display. During the regeneration the DTAM display can be used to inspect stages and make alterations if needed. An influent/effluent total dissolved solids meter monitors the feed and discharge water quality. The meter is looped to a dual pen chart recorder, where a permanent record is kept of the influent/effluent TDS. A dual electrode pH meter is also present in the influent and effluent of the ion exchange unit. As mentioned previously, a percent chemical cell is part of the regeneration sequences. This meter monitors %HCl and %NaOH during the regeneration process and allows the operator to better observe actual chemical dosage without using a hydrometer or checking the chemical feed reservoirs.  The newly designed control system was a key component to the operator friendly environment the customer required.

Conclusion

LT Technologies closed the loop of a company that lacked the ability to discharge to a sanitary POTW sewer. In addition they understood the importance of recycling their wastewater. The initial capital cost was the only liability. They saw the financial and environmental payback of water recycling. More companies upper management must become more enlightened, and not blindfolded only by the capital involved in water recycling. There is more to look at than financial pay back. We all share a responsibility in achieving environmental stability.