  |
|
|
Two (2) silver recovery canisters in a series are required. Businesses choosing to use silver recovery canisters at their office for on-site silver recovery need to use 2 in a series, unless routine testing shows that their sewer outflow consistently meets hazardous waste and local sewer silver discharge limits. Two canisters in a series can reduce silver concentrations to about 1 part per million (ppm). One canister, even of high quality, will show diminishing returns after being used a few times and will eventually stop working.
Consider Cost. Setting up 2 silver recovery canisters in a series can range from $200 to $1,700. Operating, changeover and testing costs can add an additional $150-$300 annually. Most dental offices only generate ½ to 1 gallon of used fixer per month, making proper on-site silver recovery not very cost-effective.
Monitor the flow of used solutions into the canisters. If the flow is too fast, the proper reaction won’t happen inside the canister and you won’t meet silver discharge limits for sewer. If it is too slow, the canister may deteriorate too soon. Use a metered pump system or a restricted gravity feed system and keep flow rates at manufacturers recommendations, usually between 1-3 gallons per hour.
Have a sample valve installed between your two silver recovery canisters.
Use this valve to take samples of the effluent from the first canister. Using silver test papers, check the sample to see when the first canister is spent. Silver test paper can detect silver at levels between 200 and 500 parts per million (ppm). When your first canister reaches this level, it is time to rotate it out, putting your second canister first in line and adding a new, second canister. In addition, if your tubing between canisters is clear plastic, you can visually inspect the solution flowing through — if it is brown or has debris in it, this is a good sign that the working ability of the first canister is spent.
Test your outflow. Take periodic samples of recovered silver-bearing waste over the life span of a canister and have the waste analyzed for silver to see if it meets hazardous waste and local sewer silver discharge limits. Keep a file with all test data in it — you’ll have a starting point from which to make refinements to your on-site process. See Testing for information on how to and testing services available.
Perform regular maintenance as recommended in the manufacturer’s instruction manual and keep a maintenance/changeover log. Work closely with your supplier for help in developing a changeover schedule based on your volumes of silver-bearing solutions. Ask your supplier if they provide a full service waste management arrangement.
Fill CRCs with water before initially putting them into service. This will extend the life of canisters by preventing the steel wool from dissolving as they fill with fixer.
Strict Local Sewer Discharge Limits Some sewer districts in the state have set their own local silver discharge limits for businesses. This is in order to help the sewage treatment plant meet its own discharge levels for silver into waters of the state. In many locations, sewer discharge levels are so low that businesses using on-site silver recovery technologies such as electrolytic recovery and CRCs will have difficulty meeting these levels. Similar low limits are continuing to be developed in other areas of the state. Businesses located in areas with strict current or future local sewer limits may have no choice but to explore off-site options.
| Municipality | Silver | Limit (ppm) |
| Aberdeen | 0.2 | No |
| Chehalis | 0.2 | No |
| Clark County | 0.1 | No |
| Everett | 0.69 | Yes |
| Federal Way | 0.5 | No |
| Kalama | 0.1 | No |
| Lynnwood | 0.5 | Yes |
| Olympia (LOTT) | 0.2 | Yes |
| Pierce County | 0.2 | Yes |
| Richland | 0.2 | Yes |
| King County (Metro) | 3.0 | Yes |
| Spokane | 0.43 | Yes |
| Tacoma | 0.2 | Yes |
| Vancouver | 0.1 | Yes |
All dental offices using silver recovery canisters to recover silver from X-ray fixer should contact their local sewer utility for more information about local limits.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.
