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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.4 ION EXCHANGE
4.4.8 Residuals
The quantity and nature of the residuals generated from ion exchange treatment of chromium baths appears to be the most significant concern of platers using this technology (PS 198, PS 234, PS 244). The process generates a concentrated acidic waste during regeneration that contains mostly trivalent chromium and tramp cations such as iron and aluminum and dilute streams from backwashing and cleansing of the bed. Only two shops provided quantity data for this residual. PS 045 indicated that they generate 700 gallons per month from the maintenance of a 650 gallon chromic acid anodizing tank with a production rate of 363,670 amp-hrs/year. PS 244 indicated that they generated 7.7 gallons of mixed water and acid for each gallon of chromic acid anodizing solution treated.
Nearly all of the shops that reported residuals data indicated that regenerant is treated in-house and that the resultant sludge is sent off-site for disposal or recovery. One shop indicated that the regenerant is sent off-site for deepwell injection (PS 198).
It appears that a batch treatment system is the best method for processing the regenerant on-site. Shops that attempted to treat the regenerant in their continuous treatment systems experienced overloading problems. For example, PS 244 purchased the technology in 1983 and discontinued its use eight months later (currently recycle chromic acid as "another product"). This shop, which has a total wastewater flow of approximately 26,000 gpd, was unable to assimilate the regenerant into their general wastewater flow. PS 244 provided the following input: "Adds a large load to the pollution control unit. To reduce labor it needs it's own automatic pH adjust and chrome reduction unit for it's effluent [sic]."
The need for a separate treatment system is due to the non-continuous nature of the waste stream and its high concentration of acid and metals. The required batch treatment process consists of chromium reduction with a sulfur compound (to reduce any hexavalent chromium present), pH adjustment (8.0 to 9.5), flocculation (polymer addition), settling/clarification, and solids dewatering. Eco-Tec recommends the use of magnesium hydroxide for pH adjustment. Alternatively, sodium hydroxide or lime could be used.
PS 049 generates 700 gal/mth of regenerant plus 2,300 gal/mth of other chromium wastewater. These wastes are batch treated in a 750 gal treatment tank. The wet sludge generation rate from this batch treatment process is high, 1,400 gal of sludge from the treatment of only 3,000 gal of wastewater. The high sludge production rate is due to the concentrated nature of the regenerant waste. The wet sludge is dewatered using a filter press (4,800 lbs/mth of filter cake) and the resultant filter cake is dehydrated with a sludge dryer (1,200 lbs/mth of dry sludge). The 1,200 lbs/mth of dry sludge is sent off-site for disposal. PS 049 is able to directly sewer their supernate from the batch treatment tank and the filtrate from the filter press that dewaters the sludge. Most shops will find it necessary to return the filtrate to the batch treatment process because it will contain residual concentrations of pollutants. Since much of the water from the original feed stream is contained in the filtrate, the quantity of this solution should be closely evaluated and considered when sizing the batch treatment tank.
Another residual generated by this process is spent cartridge filters. Cartridge filtration is used to prevent suspended solids from entering and fouling the ion exchange bed. No quantity data were provided by the respondents for the spent cartridge filter waste.