When it comes to separating the solids from the liquid in a wastewater treatment plant, delivering thick dewatered sludge to the anaerobic digester is the order of the day. And in this regard, Miami-Dade’s gravity concentrators were not performing as well as they should have been. Thin-settled sludge solids from the gravity concentrators to the anaerobic digesters resulted in shorter digester retention times with increased solids to dewatering and more sludge to dispose of – and of course more money spent on disposal. Also, biogas production was not optimized.
A second, connected, challenge facing the plants were the high hydrogen sulfide levels, which were causing corrosion in the cogeneration engines powered by the gas.
Kemira has had a long and close relationship with both plants and knew of these concerns. In stepped Richard Waterous, Senior Specialist in AWT Sales and Ricardo Colon, Associate in Technical Sales and Service, both specialists in Advanced Water Treatment. They knew the effect that ferric sulfide has on concentrator performance and digester gas quality and realized that it was probably the answer to both of Miami-Dade’s challenges, too.
Contributing to circular economy with biogas
“Biogas is becoming an increasingly valuable byproduct of the wastewater treatment process,” explains Ricardo. “Using a byproduct to power the process creates a circular economy and is also a source of additional revenue for the plant. To be most useful and valuable the biogas should have as low a hydrogen sulfide value and as high a methane value as possible,” he continues. “A low hydrogen sulfide value means less corrosion and associated maintenance at the cogeneration plant, while a high methane content means that the gas is a better source of energy and therefore has increased commercial value.”
Trialing a solution to the hydrogen sulfide challenge
Previously the hydrogen sulfide contamination had been treated using a gas cleaning process consisting of an iron sponge system and wet scrubber. After cleaning, the digester gas continued to a cogeneration process. However, high levels of digester gas (prior to cleaning) hydrogen sulfide (5,000 ppm+ at the South plant and 10,000+ ppm at the Central plant) led to high maintenance and operating costs for the cogeneration plant. “Considering that the manufacturers of the cogeneration engines specified that a value of 400 ppm was needed in order to reduce the challenges posed by corrosion, these levels were clearly not even close to being good enough,” explains Richard.
Based on Kemira’s recommendation, the South plant initiated a trial using inline ferric sulfate injection. When applied to a strategic location in the process, ferric sulfate is a highly effective coagulant for hydrogen sulfide control, pulling the dissolved sulfides out of solution and solidifying it before it gets to the digester and becomes a harmful gas. The precipitate is then removed with the bio-solids during the digestion process, leading to thicker solids, longer digester retention times, dramatic reductions in gas-phase hydrogen sulfide levels, and increased levels of methane. The trial was such a success that it was soon replicated at the Central plant.
Bypassing iron sponges and sodium hypochlorite used as wet scrubber saves more than $550,000 per year in operating and maintenance costs.
Top-class improvements all round
The most significant implication of ferric sulfate technology is the digester biogas enhancement, especially the reduction in hydrogen sulfide levels. At the Central and South plants, the levels nosedived from 10,000 and 5,000 ppm respectively to as low as 200 ppm. Bypassing iron sponges and sodium hypochlorite used as wet scrubber saves more than $550,000 per year in operating and maintenance costs.
The risk of corrosion in the cogeneration engines has been significantly reduced, which in turn lowers costs by reducing the frequency of engine maintenance and oil changes. At the same time methane levels increased from 57 to 63% at the Central plant and from 62 to 67% at the South plant, meaning more kilowatts per cubic foot of biogas.
Additionally, the performance of the gravity thickeners at both plants improved, and concentrator throughput rates have increased significantly, from 300 gallons per minute to up to 500 gallons per minute. Underflow sludge solids have increased from 1.8–2% to 3.5–4% at the South plant. “I’m very enthusiastic about the future of this application,” says Richard. “It is common in some European countries and more well known in California. It has massive potential in Florida. We’re looking forward to helping other customers improve their concentrator performance and biogas quality in the future,” he concludes.