Struvite and Vivianite Screw Presses and Heat Exchanger Removal Case Study
A regional wastewater treatment plant in Sonoma County was experiencing problems with struvite on their screw presses and vivianite on their heat exchangers. The design plant flow rate is 7.9 mg/day. They were using 7 mg/L to 10 mg/L (dry basis) of ferric chloride to control hydrogen sulfide and improve solids removal in the primary clarifier. The plant maintenance staff was forced to shut down the dewatering process every three (3) weeks to clean because of struvite deposits on the screen of the screw press. They also notice that they were having a difficult time maintaining the temperature in the anaerobic digester. Upon inspecting all the heat exchangers, the maintenance staff noticed a purple metallic deposit that was over 0.75 inches thick.
A sample of the deposit was analyzed and it was determined to be vivianite scale deposited on the surface of several heat exchangers. The maintenance supervisor obtained several cleaning compounds in an attempt to clean the heat exchanger and screw press. The plant supervisor wanted a product that was environmentally safe for disposal and could quickly remove vivianite and struvite - Struvite Remover was selected for use.
The plant staff obtained an 8 oz sample of Struvite Remover and had the lab supervisor run a test to see how fast it would dissolve the deposit. The lab supervisor took a 5-gram sample of vivianite and struvite and placed them in separate beakers with 100 ml of Struvite Remover. After 25 minutes, the samples of vivianite and struvite were dissolved in the beakers. The lab supervisor then ordered a 5-gallon pail to run a larger test. This time they soaked a six inch (6”) diameter valve that encased in struvite. After 24 hours, the valve looked like it was brand new and was free of any traces of struvite.