During 2009, District operations staff removed thousands of pounds of struvite scale from three anaerobic digesters and related pipes at the wastewater treatment plant.

Struvite scale is a rock-hard substance formed from ammonium, magnesium, and phosphate, which are all present in high concentrations in wastewater sludge. Like plaque in human arteries, struvite blocks critical pipes and valves, reducing efficiency and eventually leading to system failure.

The District has two small digesters, each with a capacity of approximately 0.6 million gallons, and one larger digester with a capacity of one million gallons. One at a time, each digester was shut down, drained, cleaned, and returned to service. The work on each digester took several weeks. Reducing treatment capacity to this extent required careful scheduling and management.

All three digesters are now operating at peak efficiency. District staff is altering the treatment process and devising a system to add iron chlorides to the sludge, which will reduce the amount of phosphate in the sludge and thereby reduce struvite formation.

The District saved an estimated $99,000 in fiscal year 2010 by using District personnel, rather than contractors, to harvest biosolids from one of its six facultative sludge lagoons. Annual savings may top $200,000 in future years, as the District recoups its investment in harvesting equipment.

Biosolids are the stable organic compounds left at the end of the sewage treatment process. The material resembles garden compost but is 95% water. After spending six years decomposing in a lagoon, the biosolids are dredged, pumped a short distance to the District’s dedicated land disposal site, and injected 18 inches into the soil. This is an economical and environmentally friendly way to dispose of biosolids.

While the District’s disposal site has at least 40 years of available capacity, the District is already working with a regional coalition of agencies to explore future alternatives that would generate electricity from biosolids.