The operators at the Somers STP, on the eastern outskirts of Melbourne, were having maintenance issues with their pumps, caused by struvite build-up in them. It would take struvite as little as 30 days to build up enough to trigger maintenance events.
Struvite is magnesium ammonium phosphate [MgNH4PO4. 6H2O]. It is white, yellowish white or brownish in colour and is very insoluble in water. Anaerobic sludge digestion releases ammonium, magnesium and phosphate, which can form struvite in digesters and downstream dewatering facilities. Struvite formation occurs when the conditions are such that the concentration product exceeds the struvite conditional solubility product and can result in scaling in pipelines and on the walls of process equipment [such as pumps].
Struvite forms in the supernatant liquid and adheres to exposed metals within pumps and pipes, which was the problem being experienced at Somers. Submersible pumps used to transfer the supernatant needed constant and continuous attention to keep them running. Lale Rogeon, Operation & Maintenance Coordinator, said that the plant had tried coating pump internals, which extended pump life, but after a month, they still needed to be removed from service and cleaned. See photos showing the amount of struvite needing to be cleaned to get pumps back into operation.
Lale said that the continuous need to clean the pumps was costing the plant between $3,000 and $4,000 per month. Having to comply with the OH&S regulations associated with working in confined spaces (on the submersible pumps), was adding to costs because of the additional personnel needed to complete the operation. The plant operators thought there should be a better way and were ready to investigate an alternative.
After discussions with Hydro Innovations (authorised Gorman-Rupp pump distributors in Australia), self priming pumps were believed to hold the key to at least part of the solution. Whereas submersible pumps needed a minimum of 3 persons to execute a pump removal, Gorman-Rupp self primers would not need to be removed, and only one operator would be needed to clean pumps of struvite build-up. So if struvite growth could not be slowed, at least labour costs would be reduced by two thirds.
SE Water decided to go with Gorman-Rupp Super T Series self priming pumps with an internal coating to attempt to slow the struvite growth. These pumps can be installed above the liquid (high and dry) to enable operators to easily access and monitor at ground level. They have a large removable cover-plate to enable inspection and repair in minutes. Since installation, operators have not had to perform any de-scaling operations. The “experiment” was a complete success, delivering enormous savings as well as improving OH&Sconditions associated with keeping these pumps operational.
This story was featured on the Pump Industry in October 2014.
