In the early 1990s, the Jackson Pike Wastewater Treatment Plant in Columbus, Ohio, upgraded its single-stage anaerobic digestion system.
That change, aided by the industry’s first large installation of multi-port sliding valve mixers, plus improved heating and continuous sludge feeding, would enable six digesters to do work that required 16 digesters under the earlier two-stage process.
But boosting the new system to peak efficiency depended on finding the right recipe to feed it. After six years of experimenting, with the help of automatic sampling, Jackson Pike found the key to fine-tuning its new system, and a new way to cut costs and nearly eliminate the plant’s most time-consuming and least pleasant task: collecting sludge samples for analysis.
The automatic samplers gave the plant an accurate way to assess the solids content of primary and waste activated sludges fed to the digesters. That in turn helped the staff determine the optimum mix of sludges for efficient digestion.
Finding the blend
The Jackson Pike plant, built in 1937, has an average flow of 80 mgd. Sludges harvested from primary and secondary clarifiers are processed by anaerobic digestion. About 15 percent of the resulting digested biosolids is dewatered by centrifuges to 10 percent solids and hauled away in tank trucks for land application. The remainder is centrifuged to 20 percent solids and incinerated, producing ash that can be recycled into various commercial uses, such as cover material.
Bringing the new digesters up to peak performance was complicated because primary and waste activated sludge (WAS) are digested together. “Having committed to single-stage digestion, our big challenge was to maximize WAS digestion using the plant as-is, with no substantial equipment changes,” says assistant plant manager Doug Wise.
Primary sludge and WAS initially were piped to a mixing vessel, and whatever blend occurred there was fed to the digesters. “Sometimes it worked well and sometimes it didn’t,” he says. “We didn’t know when it would or wouldn’t, or why.
“We get highest efficiency when the solids ratios in both sludges are correctly balanced, so digestion of both components will be completed at the same time. If there’s too much WAS, it gets only partially digested and foams up into the digester’s fabric canopy. We refer to that as the digester getting sick.
“Our first step toward learning how this works was to re-route the two sludges into separate holding tanks so we could vary the mixture and study how different solids ratios affected digestion. Then we experimented with many different blends of WAS and primary sludge, also trying them on different types of anaerobic digestion to see what technology would work best with our plant equipment. For that, we needed to draw and test a lot of samples.
“The key to pushing the system to its limit without overloading it is knowing the solids ratios of both sludges going in, and controlling the solids ratio of the blend by varying the proportions of both sludges.”
