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Treatment plants achieve big phosphorus reductions

Use of naturally occurring bacteria has produced great results

A technology accidentally discovered decades ago in South Africa is reaping big benefits for clean water today in the Twin Cities metropolitan area.

The discovery was that certain kinds of naturally occurring bacteria like to consume phosphorus. Phosphorus is both an essential element for sustaining life and, in excess quantities, a pollutant in water.

Wastewater coming into the Metro Wastewater Treatment Plant in St. Paul typically has 6-8 milligrams of phosphorus per liter. Untreated, this phosphorus would — especially in years of low river flow — contribute to harmful algae blooms downstream on the Mississippi River at Lake Pepin.

Over the last decade, through the gradual introduction of the biological process known as “Bio-P,” phosphorus in effluent from the Metro Plant has been reduced to less than 1 mg/l on an annual average — the plant’s federal and state permit limit. Region-wide, the Metropolitan Council’s treatment plants are discharging half of the phosphorus they did two years ago. That’s on top of earlier reductions, all the results of a long-term phosphorus reduction strategy the Council adopted in 1997.

“The Council’s strategy has produced great results,” said Bill Moore, general manager of the Council’s Environmental Services division. “Since we instituted Bio-P, we have reduced the amount of phosphorus we are discharging from the Metro Plant by 1,000 tons a year, or more than 80 percent. Further attention to reducing phosphorus from urban and agricultural runoff will mean even better water quality for the river.”

1988 drought provided impetus

Phosphorus reduction efforts at the Metro Plant had their origins back in 1988. That’s when the state suffered its worst drought in years, and a massive algae explosion in Lake Pepin was blamed in large part on phosphorus discharged from the Metro Plant.

As a result of negotiations with the Minnesota Pollution Control Agency (MPCA) and the U.S. Environmental Protection Agency, the Council began a series of studies in 1990. Among the goals were to determine both the contribution that the Metro Plant makes to phosphorus loads in the Mississippi River and the most effective way to reduce phosphorus discharged from the plant.

The Council studied several options to remove phosphorus efficiently. One was a chemical process that adds ferric chloride or alum to the wastewater stream. Phosphorus binds to the chemical and drops out of the wastewater. But the process has disadvantages. The ongoing purchase of chemicals makes it more expensive, and it also creates more solids for disposal.

Microorganisms “eat” the phosphorus

Bio-P, on the other hand, was untested both in large facilities like the Metro Plant and in facilities in northern climates where regulation of the activity of the phosphorus-accumulating organisms might be problematic. The process had to be integrated with biological treatment for other pollutants. But a team of the plant’s environmental scientist, engineers and operators moved ahead with a pilot project from 1993-1998 that converted 25 percent of the plant’s treatment capacity.

In the complex Bio-P process, the wastewater stream is fed into treatment zones. The zones are configured with different levels of oxygen, depending on the time of year. The configuration creates favorable conditions for a variety of organisms to remove phosphorus, ammonia and biochemical oxygen demand in the same tank. Through trial and error, the staff learned how to regulate the process for the best results. And it turned out that Bio-P could be instituted without a costly expansion of secondary treatment facilities.

Regulatory flexibility contributed to success

In 1997 the Council adopted its phosphorus reduction strategy. In negotiations with the MPCA for the 1998 Metro Plant permit, the Council agreed to schedule conversion of the remaining 75 percent of the plant to Bio-P. But meeting a monthly average discharge of 1 mg/l of phosphorus would be a challenge, explained Rebecca Flood, environmental compliance manager for the Council’s Environmental Services division. In cold weather, the plant has to sacrifice some phosphorus reduction in order to achieve ammonia removal. In warmer weather, the organisms can more than make up for the loss.

Unlike some pollutants, phosphorus has a cumulative effect but not an immediate impact, Flood said. So the Council negotiated with the MPCA to accept a discharge limit of 1 mg/l on an annual — as opposed to monthly — average. Another way the MPCA showed flexibility, Flood said, was to allow the Council a staged process for meeting the new limit, in order to coordinate the conversion with the start-up of new solids incinerators at the plant. By May 2005, the plant achieved the standard, seven months ahead of schedule.

In addition to the Metro Plant improvements, the Council’s other wastewater treatment plants also have significantly reduced their phosphorus discharges. Many plants — like Blue Lake and Seneca — already meet the 1 mg/l limit, and will assure continued performance as their facilities are upgraded in the next few years. The Empire Plant is in the process of converting to Bio-P treatment, and the new plant proposed for Hastings will also use it.

Urban and agricultural runoff

Success at the Council’s treatment facilities is not the whole story. Phosphorus also reaches the region’s rivers through runoff from urban and agricultural areas. Common sources are leaves and grass clippings, animal waste and excess fertilizers from farmland. Controlling pollution from these diffuse sources — known as nonpoint source pollution — will continue to be a challenge in coming decades. Cooperative efforts of citizens, businesses, farmers, watersheds and other agencies will be essential.

The Council is currently developing a computer model that will help set appropriate pollutant limits for discharges from point sources and establish goals for reducing pollutant loads from watersheds that are tributary to the 40-mile, metro area reach of the Minnesota River, which joins the Mississippi near the Minneapolis-St. Paul International Airport. Especially during years of high river flow, the Minnesota is known to contribute the majority of phosphorus to the downstream flow.

State law requires the Council to develop target pollutant loads for each tributary watershed in the seven-county region. Each target is a goal set to limit the amount or load of a pollutant that is being discharged from a watershed via each stream. By reducing nonpoint source pollution throughout the region, further water quality improvements can be gained, and expensive and unnecessary investments in advanced wastewater treatment are avoided.