Two tanks installed more than 30 years ago to catch overflows from the city’s “combined sanitary-and-storm sewer system are working well to minimize harm caused by effluent discharged to area streams, according to preliminary data collected by the Altoona Water Authority.

Prior to installation of the tanks at the overflow sites near Tuckahoe Park and Bellwood Avenue, dissolved oxygen levels plummeted downstream after heavy rains to the detriment of aquatic life, but now oxygen levels hardly budge following storms, according to the findings two years into a five-year study ordered by the Department of Environmental Protection.

The authority diverts runoff into the 1-million-gallon-plus tanks after it rains hard enough to fill the “outfall” lines beyond the tanks connecting with the Westerly and Easterly sewer treatment plants, according to Dave Pedersen, engineer with Gwin Dobson & Foreman, who spoke to the authority board Thursday.

The tanks are sized to take 30% of the storm flow that would otherwise go into the streams at those points in the system — a portion that contains 60% of the sanitary effluent, because it is the heavy first flush that includes sewage that dropped out in the lines during dry periods, officials said.

Prior to installation of the tanks, all the overflow would have gone into either Mill Run near Tuckahoe or the Little Juniata River near Bellwood Avenue.

“It was pretty bad,” said Mark Glenn of Gwin Dobson.

It was bad enough to be obvious to observers downstream that the waterways contained sewer pollution, according to Glenn and Pedersen.

Tests conducted in 1983 showed that in the Little Juniata, dissolved oxygen went as low as 1 milligram per liter, even as upstream water contained 7 milligrams, Pedersen said.

The benchmark minimum for a healthy stream is about 5 milligrams, he said.

By contrast, last year, with 11,000 samples taken, there were no readings lower than 5 milligrams in the Little Juniata, Pedersen said.

There were readings as low as 4.8 milligrams in Mill Run, but they were related to low flow, not to the combined sewer overflow discharges, Pedersen said.

One of the indications of success is the reclassification by the Pennsylvania Fish & Boat Commission of the Little Juniata as a high quality stream starting in Pinecroft, Glenn said.

“The tanks are doing what they are supposed to,” Pedersen said.

There is one issue with the data: when plotted on a graph, the dissolved oxygen levels show irregularities during storm events, Pedersen said.

But when averaged out, those don’t amount to a decrease in the oxygen levels, he said.

It’s a puzzle he’s trying to solve, he said.

There are still questions that need to be explored by the study about whether the CSOs add to levels of E coli and fecal coliform bacteria in the streams, according to Pedersen and Glenn.

The study ultimately may enable the authority to identify potential capital projects that could lead to elimination of more — or even all — the untreated effluent discharged to the streams, according to Pedersen.

To help accomplish that, Gwin will identify “problem” areas where water is entering the system especially fast during storms or where it’s traveling through the system especially fast, Pedersen said.

If the authority can slow down the onrush sufficiently, the sewer plants, coupled with the tanks, may be able to handle the flow without the system diverting contaminated water to the streams, he said.

That might be accomplished by larger pipes, pipes with gentler slopes or some other means of detention in strategic locations, Pedersen said.

Or it could be accomplished by separating the sanitary flows from the storm flows in certain areas, he said.

Alternatively — or in addition — it may be possible that the authority could add capacity in the lines downstream from the CSOs, provided the plants have sufficient treatment capacity, according to Pedersen.

“It could go in a lot of different directions,” he said of the potential capital projects.

Mirror Staff Writer William Kibler is at 814-949-7038.