Flooding, lake sewage releases: What we've learned

There's an ick factor to the storms that swept across Minnesota this weekend. Torrential rain dropped as much as five and a half inches in parts of the state by Monday morning. That caused sewer overflows into Lake Minnetonka that have already closed beaches. But experts say that's supposed to happen. And it did, in 20 different places across the state. Here's what you need to know:

What happened?

Huge amounts of rain fell in a short time over the weekend and sewer systems weren't big enough for deluge. There are actually two types of sewers. One carries storm water -- either rain or snow runoff from storms. The other is the sanitary sewer that carries the stuff we flush down toilets and drains.

Storm water happens in big amounts, very occasionally, and gets sent into holding ponds and rivers. Wastewater is constant, and typically low enough in volume that it can be chemically treated. But that's expensive and it takes a lot of time. With really big storms, the two systems tend to mix, and that fills up the wastewater system until it overflows.

That's what happened this weekend, basically from the Iron Range to the Iowa border.

Why not just build more treatment plants?

That's one option. The city of Mound, for instance, is getting a lot of attention because a release there ran into Lake Minnetonka. Officials in Mound say that the Met Council isn't providing them with a big enough pipe to the council's waste water treatment plant to handle big rain events.

But the question is, what's big enough? Officials might plan, for instance, to handle a 25-year rain event -- the four biggest storms in a century. They could factor in even bigger storms.

What's the other option? Flush it into Lake Minnetonka?

One alternative is to try and limit what runs into the waste water system, to keep the rain out of what we flush down the drain. Originally, the storm and waste water systems were often built together -- rain actually helped dilute sewage before modern treatment.

Now, we're splitting these two systems, but that doesn't always work. The storm system can leak into the waste stream. And some of it is deliberate: people illegally run their sump pumps into their drains. This mixing is called inflow and infiltration.

And here's where cost comes in. Extra treatment is the gold-plated solution, according to Tim O'Donnell, spokesman for the Metropolitan Council's Environmental Services, the regional sewer authority in the Twin Cities area.

"To add that type of capacity, to handle the extreme rainfall that mixes in with the waste water, would be upwards of a billion dollars regionwide. We think an approach that targets inflow and infiltration reduction efforts at the local level, targeted at where the water is getting into the sewers is a much more cost effective alternative, that we figure could be done for about $150 million regionwide," he said.

So that's what the Met Council is proposing.

And what if that doesn't work?

Well, we make choices. You could have huge treatment plants that you mostly don't use. You could shut off the system when it gets full and let sewage back up into people's basements. Or a few times a century, you could pump it away and contaminate a lake or river. In Minnesota, we choose the latter.

"When we have large rain events, we can have bypasses from our waste water collection systems or from a waste water facility. Obviously this is something we don't like to have happen, but what we want to make sure is that we don't have waste water backing up into people's homes because we view that as an immediate public health threat," said Wendy Turri, the municipal wastewater manager for the state's pollution control agency. "And so the better of two evils is for them to bypass into a river or lake."

Basically, we choose to dump it out, monitor the affected water and keep people away until the contamination breaks down naturally. And that's what's going on now in Lake Minnetonka. They're testing the water and closing beaches where bacteria is a health threat.