About one fifth of all the sewage handled by major wastewater treatment facilities in the Bay states is now being treated with biological nutrient removal — a practice that uses tiny microbes to remove most of the nutrients before the water is released into a river or stream.

Of the more than 2 billion gallons of wastewater handled each day by the watershed’s 273 largest treatment plants, 422 million gallons are treated with BNR technology, according to figures recently compiled by the Bay Program.

Based on information from the states, the Bay Program estimates that 976 million gallons of wastewater daily will be treated with BNR technology by the year 2000. More are anticipated after the turn of the century. Thirty-two plants in the watershed now use BNR, and that is expected to expand to 76 by 2000.

Implementation of BNR is considered critical because it is highly effective at removing nitrogen. Untreated wastewater typically has about 14 parts per million of nitrogen, but BNR — depending on the facility and the BNR design used — can reduce that to less than 8 ppm.

The Bay states have targeted nitrogen and phosphorus for 40 percent reductions by the turn of the century. While treatment plants for years have been incorporating phosphorus control technologies, nitrogen control has lagged.

In part, that was because nitrogen control was considered unimportant until the mid-1980s, and even then many considered it to be too expensive to be practical.

But that has not turned out to be the case, according to Bay Program figures. Years ago, some estimates had put the BNR costs as high as $30 for each pound of nitrogen removed. The cost per pound at the 22 Maryland facilities that have incorporated BNR is less than $4.

“It’s a lot cheaper than we thought,” said Allison Wiedeman, point source coordinator with the EPA’s Bay Program Office. In some cases, she added, BNR may prove to be less costly than other treatment options, especially when operating costs over a 20-year period are included.

That, she said, is because BNR, in some situations, can reduce energy costs at plants and may also reduce the amount of sludge that plants create and must dispose of.

Most of the costs of incorporating BNR are for construction. BNR moves the wastewater through a series of tanks, which have to be built at the plant. Each tank contains different microbes. The microbes in each tank convert the ammonia nitrogen in the wastewater coming out of the plant into a different form of nitrogen. Ultimately, it is turned into inert nitrogen gas, which is released harmlessly into the atmosphere.

The main reason for the initial high cost estimates was that engineers and consultants in charge of designing treatment plants were not familiar with BNR technology and erred on the high side when estimating costs, said Clifford Randall, a professor of environmental and civil engineering at the Virginia Polytechnic Institute and State University, who helped pioneer BNR technology.

“It’s really a matter of understanding or not understanding the technology,” he said. “Once people understand it, and see the logic behind it, then it is an easy sell.”

More recent estimates, Randall said, “are getting much closer” to actual costs. But many factors influence costs, he added. Certain treatment plant designs are not easily adapted to BNR, while others can easily make the transition.

If the plant was already scheduled to be upgraded to handle larger flows, he said, “usually you can work in the BNR with it for very small dollars.”

Also, as more plants are upgraded to include BNR, it encourages other plant operators and engineers to adopt the technology, Randall said. “It shows them how it can be done, reliably and economically, and encourages them to do the same thing. We’re making a lot of progress.”

Bay Program figures for BNR treatment show that:

  • In Pennsylvania, of the 133 largest plants in the watershed (with flows of more than 400,000 gallons a day), two use BNR technology. Those plants are unique, though, in that they use BNR to control phosphorus, not nitrogen. No new BNR facilities are anticipated by the year 2000.
  • In Maryland, of the 57 largest treatment plants (generally those with flows of more than 500,000 gallons a day) 22 have incorporated BNR technology. A total of 59 plants are expected to be using BNR by 2000.
  • In Virginia, of the 75 largest treatment plants in the watershed (generally those with flows of 500,000 gallons a day), six are using BNR. A total of eight are expected to be using BNR by 2000.
  • In the District of Columbia, the Blue Plains Wastewater Treatment Plant — the largest in the nation — began treating about 160 million gallons a day, or half of its average flow, with BNR last year. All of the plant’s wastewater is expected to be treated with BNR by 2000.
  • Of seven treatment plants at federal facilities in the watershed, one presently uses BNR. By 2000, six are expected to be using the technology.

After 2000, the number of plants using BNR is expected to increase.

The Virginia nutrient reduction strategy for the Potomac and Shenandoah rivers indicates that nitrogen reduction goals cannot be met without incorporating BNR at sewage treatment plants, and the General Assembly this year approved funding to help pay for some of those upgrades.

Pennsylvania may also upgrade plants to include BNR, pending the outcome of a review, overseen by Randall, that is examining the costs of incorporating the technology at 50 plants in the watershed, including 16 in Pennsylvania.

BNR is generally considered to be the most effective means of reducing the amount of nitrogen in sewage treatment plant discharges, which account for nearly 90 million of the roughly 370 million pounds of nitrogen entering the Bay each year.

Excess amounts of nitrogen and phosphorus spur large algae blooms in the Bay which block light to important underwater grasses that provide habitat for fish, blue crabs and other species. When the algae die and sink to the bottom, they deplete the water of oxygen needed by most Bay species. The Bay states are trying to reduce the amount of nitrogen and phosphorus reaching the Bay by 40 percent by the turn of the century.