Heavy Metal (Cu, Ni, Zn) Removal with pH Neutralization & VOC Reduction
Scope : The system depicted above is a skid mounted waste treatment system designed to handle plating rinse waste with flows up to 20,000 GPD. This system is configured to remove most heavy metals to less than 0.5 ppm. Incoming concentrations as high as 1,000 ppm can be handled. Above that maintenance becomes a bit high and alternate sludge handling options need to be considered. This system was specifically designed to handle Ni, Cu and Zn. Most heavy metals can be handled with little or no configuration change. Hex. Chrome would require reduction stage ahead of the system.
Reuse / Closed Loop : The effluent from this system is quite clean and can be considered for reuse. Typically a nanofiltration or RO system followed by a mixed bed ion exchange column will provide excellent quality water for reuse
Operation : The primary mode of treatment is metal hydroxide precipitation followed by an ion exchange polishing step. Effluent enters the first stage equalization tank from a transfer station ahead of the system. The pH of this tank is maintained at 9.0 using a combination of magnesium hydroxide ( Mg[OH]2 ) and caustic ( NaOH). Caustic alone could be used however a heavier sludge that is easier to de-water is formed with this combination. When the second stage is ready to accept a batch a volume of 250 gallons is transferred for processing. In this stage the pH is elevated further to 10.2 (to ensure total precipitation of Ni). After the pH setpoint is attained and held for some time agitation stops. This tank is equipped with a sloped bottom for sludge collection. The precipitants formed within the tank are allowed to settle. The actual settling time is a function of the first stage level and volume processed in the last 60 minutes. Essentially the longest possible time for settling is used. As demand increases settling time decreases. A minimum of 5 minutes is allowed, a settling time of 20 minutes or more is targeted. After the software determined settling time has expired a transfer pump is started at a very slow rate to transfer the sludge off the bottom of the tank. A sludge blanket detector is used to determine when all of the sludge has been pumped. At this time system automatically transfer back to the recirculation / treat mode and agitation is restarted. The pH is lowered to less than 8.5 and allowed to stabilize. After a brief wait the effluent is now pumped through an activated carbon bed (for VOC reduction and coarse filtration) followed by a 1 micron bag filter (for removal of any TSS that may have not settled) followed by a pair of ion exchange bottles for final effluent polishing.
Performance : When properly maintained this system typically achieves a total metals count of less than 0.5 ppm at the discharge, likewise TSS is normally less than 0.5 ppm as well. This system is designed to be operated totally automatically with no operator intervention. Periodic maintenance is required and is a function of solids loading. Higher volumes translate into higher maintenance.
Sludge Handling : There are several options for handling of the solids that are precipitated from the system. For very low flows (i.e. < 1,000 GPD) a duplex bag filter is sufficient and very economical. For large flows a sludge thickening tank and a small filter press may need to be considered.
Cost of Operation : The cost of operation is quite cheap and is directly related to volumes and influent metals loading. we can provide estimates based upon influent flows and volumes.
Application : This system is best applied when the flow requirements are relatively low (i.e. < 15 gpm) and the metal loading is normally low (i.e. influent metal loading < 300ppm, peaks to 1,000 ppm). For larger applications we offer a variety of custom built options.
For more information Email : info@digital-analysis.com