Q: Our wastewater treatment system is more than 20 years old and showing its age. For as long as we can remember, the PVC pipe joints and valves on our caustic feed system for pH adjustment have been caked with crusty caustic solids, some looking like stalactites found in caves. About 6 months ago, we decided to replace this piping for safety reasons. However, just recently, our wastewater pretreatment operator has reported that several joints are beginning to seep caustic. Looks like our replacement project failed. Before we try again, do you have experience with this issue? J.W.
A: Many PVC glues contain fumed silica, very small glasslike particles used to thicken the glue. What is happening is that over time, the caustic is dissolving this fumed silica and, thus, degrading the glue.
If you decide to, again, replace this piping, we make these recommendations:
- Use a PVC glue that does not contain fumed silica and is specifically designed for bleach (bleach has caustic in it as a stabilizer) and caustic service; they are available, but you need to specifically request them.
- Have your pipe installers trained in accordance to ASTM D 2855-96, “Standard Practices for Making Solvent-Cemented Joint with Polyvinyl Chloride (PVC) Pipe and Fittings;” a rep from your PVC pipe supplier can usually provide training,
- While you did not mention if you also had threaded joints, if you do, be sure the threads are clean and sharp and use a Teflon tape that meets the MIL T-27730A specification.
- For any PVC valves, use Teflon or EPDM seals.
You did not mention whether or not you were utilizing gravity or pump to move the caustic from its storage tank to the pH adjustment tank. If you are using a pump, we strongly recommend that you fabricate a clear Plexiglas enclosure around the pump to prevent caustic from being sprayed due to pump failure.
Using the right methods and materials will provide you with a successful project, and, most importantly, a much safer operation for your fellow workers.
