November 2013, Vol. 25, No.11

Problem Solvers

Innovative system bypassing digester tanks reaps rewards at Florida facility

Problem: Aging digester tanks cause odor problems and threaten disruption in service. Solution: Install a solids treatment system to enable bypassing the old tanks entirely.

Wear and tear is the unflagging enemy of the water resource recovery facility (WRRF). Operators can defend against it. But in the end, time will win, resulting in breakdowns and the occasional interruption in service. To cope with this, forward-thinking facilities develop extensive contingency plans.  

The Southwest Water Reclamation Facility in St. Petersburg, Fla., developed a contingency plan to deal with a pair of worn and aging digester tanks that were creating odor issues for the surrounding community. The plan involved bypassing the tanks entirely and processing biosolids through a Schwing Bioset Inc. (Somerset, Wis.) solids treatment process.  

The facility originally was built in 1955 to handle 15,140 m3/d (4 mgd) and was replaced in 1978 at the same location to handle 75,700 m3/d (20 mgd). Today, it is one of four facilities serving the greater St. Petersburg area.  

“Since the upgrade in 1978, we’ve all been running an anaerobic digestion process and creating a Class B product from the biosolids,” said Ken Wise, chief plant operator. “For us, that approach worked well until time caught up with us in the form of badly worn digester tanks, which were causing odor issues for an adjacent college and residential developments.” 

Considering that two of the tanks were built in 1955 with the original facility and the third was added with the expansion more than 35 years ago, the wear is not surprising, Wise said. Other WRRFs in the area also were experiencing failures in both the covers and their structures as a whole, he added.  

Initially, the city’s water resources department invested money to maintain the facility’s equipment, Wise said. “However, due to changing Florida regulations surrounding the land-application practices of the Class B biosolids they were producing at the time, the department started seriously looking into alternative biosolids treatment technologies, hoping to avoid repairing something that was not only at the end of its life but also might not be a fit for that new effort.”  

To find a solution, the department examined alternatives and conducted pilot projects with various technologies at other locations in the city. One of these projects involved installing the Bioset Process solids treatment system, which uses a combination of acidity and heat to stabilize biosolids and eliminates the need for digesters. The process proved to be quick to implement, low-maintenance, operator friendly, and successful overall at that location, Wise said. 

Ultimately, facility staff decided to install a different system from Schwing Bioset and had it on-line by August 2012, Wise said. The process had to be made to fit within the confines of an existing section of the facility, rather than at a new site. With assistance from the company, the facility was able to maximize its use of space while minimizing service disruption, Wise said. The system’s reactor is raised 3 m (10 ft) off the floor to fit an existing opening, he added. “Now, sludge comes off the belt presses, is mixed with quicklime and sulfamic acid, and is pumped up into the reactor, where it spends at least 40 minutes at 135°F [57 °C] and achieves a pH of 12.5 before being discharged to the trailers,” Wise said. 

The stabilized solids are kept in trailers onsite for 24 hours. Then, a sample is taken to ensure that pH is more than 11.5, as is required by federal regulations. Since the system went on-line, pH at the facility never has been less than 12.5, Wise said. 

In addition to eliminating almost all odor and complaints associated with it, the facility now produces a Class AA biosolids; its previously produced Class B material was only suitable for use on sod farms and pastures. “By contrast, the Class AA product we get off the Bioset Process can be applied on golf courses, pastures, food crops — pretty much anywhere,” Wise said.  

Currently, all of the Class AA material generated either is land-applied at a site located within an hour of the facility or sold as fertilizer to the local agricultural market. The Class B biosolids had to be hauled to sites more than 3 hours away. The higher-quality product also has enabled the facility to avoid operational difficulties and additional maintenance and expense associated with handling Class B biosolids. 

Even though the volume of biosolids produced at the facility has increased 10% because of the need to add lime, and there is a slight increase in the number of trailers shipped, the facility has achieved cost savings. Overall, costs dropped $100 each load, and even with the additional biosolids, the facility saves 40% more and produces a “much more usable product,” Wise said. The system also was installed at less cost than the tank-rebuild project would have cost.   

Installation of new technology enabled the facility to eliminate odor problems, improve the quality of biosolids — which also increases the flexibility for beneficial use — and achieve substantial net cost savings across the board, Wise said.