Schwing Bioset Application Report 17, St. Petersburg, Florida
Written by Larry Trojak, Trojak Communications
Version also published in WE&T Magazine, November 2013
Wear is the unflagging enemy of every wastewater treatment plant. Plant operators can defend against it to the best of their ability; but in the end, time will win out, resulting in breakdowns and the occasional interruption in service. To cope with such occurrences, forward-thinking plants will always have a solid contingency plan in place. For the Southwest Water Reclamation Facility (WRF) serving the Water Resources Department’s southwest sector (including St. Petersburg, FL), their contingency – designed to deal with a pair of worn, aging digester tanks – involved bypassing the tanks entirely and processing biosolids through a Bioset sludge treatment process. Doing so is not only helping them avoid an operational nightmare and additional maintenance and expense, it is allowing them to improve the by-product of that biosolids operation – all at a time when costs to land-apply their “standard” product have skyrocketed. Timing, it seems, really is everything.
Decades of Wear
Originally built in 1955 as a four million gallon per day (mgd) facility, the Southwest Water Reclamation Facility (WRF) was literally replaced at the same location with a 20 mgd plant in 1978. It is one of four which serve the greater St. Petersburg area: Plant #1, for the southeast section of the area which includes downtown St. Petersburg; Plant #2, to serve the northeast section of town; Plant #3, for the northwest section of the area and the beach communities; and Plant #4, for the southwest section of St. Petersburg, as well as the incorporated towns of Tierra Verde and Gulf Port. According to Ken Wise, chief plant operator for the Southwest WRF, volumes at each plant are pretty much equal.
“Plant #1 is called the Albert Whitted WRF and it’s a little smaller since there are fewer residents downtown than in other parts of the city,” he says. “But each of the other three plants are 20 mgd facilities and treat roughly the same amount of sewage. Since the upgrade in 1978 we’ve all been running an anaerobic digestion process and creating a Class B product from the biosolids. 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 in the area.”
Given that two of the tanks were built in 1955 with the original plant, and the third was added with the expansion more than 35 years ago, the wear factor is not surprising. Wise says other plants in the area were also seeing failures in both the covers and in their structures as a whole.
“We hadn’t had a failure yet, but the Water Resources Department was spending a good deal of money on
upkeep with us,” says Wise. “Under normal circumstances that would have probably sufficed and bought us a few more years. 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 that solution, the department looked at all possible alternatives, an initiative that included conducting pilot projects with various technologies at other locations in the city. One of those, at the Whitted plant, involved installing the Bioset Process sludge treatment system which uses a combination of pH and heat to stabilize the biosolids, thereby eliminating the need for digesters.
In addition to being extremely low maintenance and operator friendly, Wise says that it had proven quick to implement and very successful there. “Ultimately the decision was made to install another system from Schwing Bioset here at Southwest,” he says. “Installation took place in July of last year (2012) and we were online by August.”
The installation, he adds, went smoothly, despite the fact that the Bioset Process had to be made to fit within the confines of an existing section of the plant rather than in a totally new site.
“The Bioset crew really worked with us to maximize use of the space we had and minimize disruption,” he says. “As a result, we probably have one of the few Bioset systems in which the reactor is raised some ten feet off the floor to fit with an existing opening. 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 and achieves a pH of 12.5, before being discharged to the trailers.”
The newly-stabilized sludge is kept in the trailers on-site for 24 hours, at which point a sample is taken to ensure the pH is still in excess of 11.5 as required by Federal regulations. Since going online with the Bioset Process, Wise says the pH has never been less than 12.5.
In addition to the elimination of virtually any odor and the complaints associated with it, it is the end product of the Bioset process – now a Class AA biosolid (the Florida equivalent of Class A-EQ) – which is the real benefit for Wise and his operation.
“In the past, our Class B material was suitable for use on sod farms and pasture lands, but because of its designation would have to be set back from any kind of food crops. By contrast, the Class AA product we get off the Bioset Process can be applied on golf courses, pastures, food crops – pretty much anywhere. In addition, because of a recent change in regulations, the other three area plants still generating the Class B biosolid are now paying an extra $300 more per trailer, while our costs dropped $100 per load. Granted, by adding the lime, the volumes are up about 10%, so the number of trailers we are shipping has increased. But even with that added into the equation, we are still saving 40 percent when compared to the Class B and have a much more usable product,” says Wise.
All of the Class AA material generated at the plant is currently either land applied at a site within an hour of the plant or sold as fertilizer to the local agricultural market. The previous Class B, by comparison, was hauled to sites more than three-hours away where it often found limited use.
Tanks for the Memories
The St. Petersburg WWTP has proven to be something of a case study in how to best deal with a set of unfortunate, challenging circumstances. Faced with a pair of failing digesters that were going to require a significant investment to rebuild, and which were creating odor issues for nearby residents, businesses, and students – the plant was able to solve the problems by abandoning the existing tanks and by adopting new technology in their operation. That solution from Schwing Bioset was implemented for less money than the tank rebuild project would have cost, it eliminated the odor issue, and includes the added benefit of processing cake directly to a Class AA biosolid (and gain more flexibility in the beneficial reuse of the end product), resulting in substantial net savings across the board.
“Since bringing in the Bioset System things have definitely settled down around here,” said Wise. “It’s been a great solution for us.” And, it would seem, all the issues created by the failing tanks are just fading memories.
To download the entire #17 application report for St. Petersburg, Florida, click here.
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