The Green Way and the Gray Way

I love trade journals. I subscribe to about a dozen devoted to water and wastewater. I firmly believe that one can become an expert in the world of water just by spending an hour every day reading them. The most common type of article is the spotlight article, or case study, in which the author presents a problem that a water or wastewater facility faces and the design solution its engineers construct to meet that challenge.

One truism becomes very clear after reading several of these: Ten different facilities faced with the same problem will find ten different solutions, and all of them work. The August issue of WaterWorld exemplified this in a way it probably didn’t intend. It offered up two solutions to a very common problem. One was “green” and one was “gray.”

That problem is combined sewer overflows (CSO) in which rainwater and sewage combine in one set of distribution pipes that flows to the wastewater plant. (Excuse me, I meant to say Water Resource Recovery Facility.) The problem is that, when a big storm hits, the increased flow of rain water overwhelms the plant’s ability to process it and the combined flow has to be dumped into a river or ocean. Yes, in 2014, we still release raw sewage into the environment.

There are two ways to solve this problem: The first is to build a gigantic underground reservoir to temporarily hold the combined flow until the rain stops and release it at a rate that does not exceed the capacity of the plant. The other is to realize that our modern urban infrastructure prevents rain water what it’s supposed to do—go back into the ground. As Joni Mitchell once said, we paved Paradise and put up a parking lot. The second solution then is to wave our engineer’s wand and charm storm water to do what it did before civilization came along. We do that with miniature wetlands called bioswales, permeable pavement, tree trenches, rain gardens, tree plantings, green roofs, flow-through planters, rain barrels, and restoration of the natural habitat.

In this issue of <>I>WaterWorld one article described how Philadelphia contemplated both approaches. The green approach set forth the goal of removing the the first inch of rainfall from entering the city’s sewers—enough to prevent overflows in all but the biggest so-called 100 year storms. The cost is $2.5 million. The cost to implement the gray would have been $15 to $20 million. The green way won. Smart choice.

Another article in the same issue described the manner in which Indianapolis approached its CSO problem. This city—much smaller than the City of Brotherly Love—chose the gray way. The solution was a massive 18 foot diameter tunnel buried 250 feet below ground. The Deep Rock Tunnel Connector also required a shopping list of expensive infrastructure: new sewer connections, chamber vents, drop shafts, deaearation chamber vents, deaeration chambers, outlet boxes and and outlet box vents. The article’s main focus was on the use of computer modeling to design such a large scale project which no doubt saved a lot money in the design phase. The engineering firm was undoubtedly very proud of the complicated engineering it completed. The total cost of the project—$180 million.

Comparing the Philadelphia Story with the Indianapolis Tunnel Storage System is probably not fair. I was unable to compare “apples to apples” by locating numbers of volume of CSO intercepted by the gray solution and the green solution. In an article in Water Environment and Technology on the Philadelphia project the 25 year cost of installing green water infrastructure was estimated at $2.4 billion dollars. But the cost for the storage tunnels — $10 billion. Clearly going green saves a lot of green.

There are three take home lessons from this tale of two cities:
1. Nature’s a damn good engineer. She’s had 4 billion years of experience.
2. If all you have is a hammer, then everything looks like a nail.
2. Shelling out big bucks does not make a better mousetrap. It just makes a more expensive mousetrap.

Have you thanked your lawn lately?