Upstream from St. Paul, the Minneapolis Water Works is engaged in an even more expensive capital investment. The utility, which also draws its water from the Mississippi, supplies water for Minneapolis and seven suburbs. It recently opened a new ultrafiltration plant in Columbia Heights capable of processing between 40 million and 75 million gallons a day, depending on the time of year (in winter, the water is colder, and therefore denser, slowing down filtration time). The utility also operates an older treatment plant in Fridley that can process about 125 million gallons per day.
Articles in our water quality series:
PFCs from 3M: an ongoing source of debate by Anna Pratt
Measuring water quality in Minneapolis, St. Paul lakes by Anna Pratt
St.Paul water: good to the last drop?
Drinking across the river in Minneapolis by Rich Broderick
Water as an economic resource by Rich Broderick
Savage water by Rich Broderick
|The Mississippi River, which is to say the Upper Mississippi watershed, is responsible for supplying St. Paul, Minneapolis and several surrounding suburbs with their daily ration of water.
Nutrients enter this water supply from the river itself, as well as from sediment that settles on the bottom of the lakes. Our mania for lawns that look like putting greens – and thus in need of massive doses of artificial fertilizers – has contributed some to the overload of phosphates and nitrogen, but the real culprits lie elsewhere.
“We sit in rich farmland and the problem is increased erosion, which is the result of urbanization,” according to Dave Schuler, chief engineer for the St. Paul water utility. Which is to say, the spread of soil compaction and impermeable surfaces that are part-and-parcel of urban sprawl. “Rain falls and storm water rushes into streams and the streams empty into the lakes,” Schuler explains matter-of-factly.
The difference is in the filtration systems employed at the two plants. At Fridley water goes through a sand filtration system, while at Columbia Heights it now passes through a special membrane that can screen out much smaller contaminants; the Fridley plant is scheduled to be replaced by a new ultrafiltration facility as well, with a total cost for both plants of about $160 million.
The project came out of recommendations from the Minneapolis Water Works Advisory Committee, convened in response to a couple of pressing developments. One was implementation of new EPA regulations that, when complete, will reduce the maximum amount of turbidity in drinking water by two-thirds. The other was growing concern about deadly waterborne illnesses, such as the 1992 cryptosporidium outbreak in Milwaukee, which sickened more than 400,000 residents and killed more than 100. All those who fell ill had been exposed to the pathogen from water processed by Milwaukee’s treatment plant – which used a sand filtration system.
Under normal condition, sand filtration would screen out the parasite responsible for the disease. But, explains Shahin Rezania, director of water treatment and distribution services for the Minneapolis Water Works, “There was probably a breakthrough of some kind in the sand filters in Milwaukee, so unfiltered water found a channel to pass through directly.” It didn’t help, he observes, that the treatment plant was located not far from where a sewage treatment plant dumped its effluent into the Lake Michigan – the source of Milwaukee’s drinking water.
“The Advisory Committee looked at several options for meeting our goals,” he says. “They included using chemical compounds like ozone to kill microbes. They’re effective, but they leave the dead microbes in the water, which effects turbidity. In the end, they decided that membrane filtration is the best way for us to go.”
|Talking about water |
aquifer – a layer of permeable, sand or gravel that contains large amounts of water
groundwater – water that is underground, as opposed to surface water in lakes and rivers
potable – fit for drinking
point source –specifically site of contamination, such as a wastewater outlet
non-point source – diffuse source of pollution, such as run-off from lawns
watershed – a region that drains into a particular river or body of water