Membrane Diffuser Acid Cleaning and Bumping
There are a few exercises you can try to lower diffuser pressure without draining the tank. It is good practice first to check moisture purge lines and ensure that they are either dry or the color of the discharge is clear.
1) Bumping – Isolate one grid (close the air off to most of the other grids so you have more air available for that one grid. Then pulse the air on and off over 15 minutes trying to stretch out the membranes and knock off/out some of the deposits on the surface or in the slits). Our past experience with bumping is mediocre, but it may bring a temporary result.
2) Acid cleaning – It is possible to dose formic acid, or an acid replacement product like Enviro-TerraTM into a grid, and blow it through the diffusers. If you have bacteria or calcareous deposits, it will kill the biofilm around the slits and clean some of the deposits. If you have an old diffuser sitting around, you can test it by pouring a bit of acid on it and see if it foams white and smells like sulfur. If it does, there is a good chance that acid cleaning may bring some results. We like to use 25 g (25 ml) of acid per diffuser and the Enviro-TerraTM product is what we prefer to work with in our lab because it is relatively safe. http://www.athea.com/products/135/enviro-terra-concentrate That said formic acid will have a more favorable vapor pressure and may get the job done with less acid and in a shorter period of time. Ascetic acid (vinegar) has also been used but it is very mild and must be used frequently to be effective.
3) Acid cleaning with Bumping – Probably the best results come from a combination of acid dosing + bumping. Clean with acid, bump the diffusers, then repeat the cycle.
It’s good to watch pressure before and after and you may not see results until you clean all of the diffusers because the blower pressure is determined by the highest pressure demand, not the average. I’m assuming from Mike’s explanation that all the blowers are piped together to a common air main.
Normally we use a cart and pump to dose acid but you could also take the moisture purge apart at the top and use a funnel to try to fill the grid with acid, put the purge back together, then blow it out. That would be to test it and see if it brings results. If it does, then we can guide you how to weld a tap to the drop pipe and show you the acid cart that has been used in the past.
It’s not a given that if there is foulant on the diffusers that it’s acid soluble or that it’s bacterial. It could be grains of sand or grit settling inside of the slits, particularly in the case of on/off cycles such as in an SBR/CASS process. You may need to go through a process of elimination to get to the root cause.