How do you remove struvite?
The photographs above demonstrates what can happen if you don’t control or remove struvite. This is a 14” diameter line used to transfer sludge from an anaerobic digester to a sludge holding pond. Every year this agency would mechanically clean this transfer line. However, every year plant operation notice that it took longer to transfer equivalent amounts of liquid from the digester. Finally, the maintenance department had to replace a valve and this is what they saw!!
So do you clean it chemically or mechanically? What is the cost to dig up and replace the line? Well if the line is buried and it 14,000 feet long, your replacement cost could run as high as $200 to $250 per linear foot. You also have to consider the cost of downtime.
Mechanical cleaning will use either a hydro-jetting system or a mechanical grinder. The hydro-jetting system will remove the soft loose struvite buildup but will not dislodge the hard layered build up of struvite. Using a mechanical grinder to remove struvite is very labor intensive and time consuming project. Mechanical cleaning requires taking equipment out of service for weeks to months.
Chemical cleaning is another alternative. However, you need to understand the type of chemicals used to remove hard layered struvite deposits. Conventional acids like HCL, H2SO4 and H3 (PO4) will not dissolve the struvite but will dissolve calcium carbonate if present in the struvite crystal. Struvite is a magnesium ammonium phosphate (MgNH4PO4-6H20) compound which is also called MAP. In most system where MAP is a problem, you need to have a specific ratio of magnesium to ammonia to phosphate to precipitate struvite. In wastewater treatment plant, the major controlling factors are the magnesium level, CO2 release and pH. So to control the precipitation of struvite you need to control magnesium. For years, to control scale (calcium and magnesium), we have been using crystal modifiers. Recently, we have found specific crystal modifiers have the ability to resolubilize magnesium from the MAP crystal. Once you have removed the magnesium from the MAP crystal, the phosphate and ammonia goes back into solution and is easily removed. The rate of solubilizing magnesium from the MAP crystal is a function of concentration and time.
Running a study, with a product developed by Jenfitch, LLC called JS 9325, we were able to quickly clean a pump and valve assembly with an environmental safe product. We found that it took 2 lbs of JS 9325 to remove 1 lb of struvite. In our first test, we recirculated a 30% ( by weight) solution through a closed loop system that had a 2” build up in the pipeline and 1” build up on the pump inlet and outlet. After recirculating for 6 hours, we inspected the lines and the pump. The lines were clean with trace amounts of grayish film (struvite deposit less than 1/64”). We tested the spent solution. The pH was 3.5. We found soluble magnesium (chelated magnesium), ammonium phosphate, and carboxylic acid.
In another application, the plant manager had notice a struvite build up in the overflow line from the digester and in the elbows going to the sludge holding ponds. He said they were pumping 24 hours per day and could not maintain levels in the digester. He had a maintenance person check the sludge pumps and they discovered the pumps were operating at a high back pressure. The plant manager purchased 2 drums of JS 9325 and make up a 30% (by weight) solution. He fed it into the digester overflow box –filled the pipeline and let it sit overnight. The next morning they started up the transfer pumps and reported that they were able to get the levels in the digester down after 18 hours instead of 24 hours. We checked back with the plant manager several months later and the pumps were controlling the sludge level in the digester with only 18 hours of operation.