July 14, 2008
The environmental community is rallying behind efforts to reduce combined sewer overflow (CSO). CSO occurs in New York City when rainfall overwhelms sewage treatment plants resulting in raw sewage being dumped into the adjacent estuary. The city’s solution has been to build huge retention tanks to store the rainfall until the sewage plants can process the load. Unfortunately, retention tanks are extremely costly and the city can not afford to complete construction of the proposed tanks anytime soon.
In response to NYC’s procrastination, the environmental movement has proposed a myriad of best management practices including point of use retention and detention systems. The most popular solution is green roofs. Green roofs are simply roof top gardens that retain rain water to irrigate planting on the roof, hence retaining rainwater on the site. They have the added benefit of eliminating heat island effect, reducing CO2 and literally making the city a greener place. Green roofs are a very elegant solution when intelligently designed and maintained. The drawback is that in cash strapped properties green roofs would be the first system to be neglected. Since environmental realists do not want their solution dependent on building management, they have proposed blue roofs. The concept is quite simple; store rainfall on the roof and slowly allow it to trickle out of the building. In this manner you mitigate the effects of excessive rainfall on the sewage treatment plant.
Recently, many groups have been proposing mandating green and blue roofs. Proponents recommend that all buildings with flat roofs be required to install either a green or blue roof. I wholeheartedly encourage the proliferation of green and blue roofs. However, I oppose any efforts to mandate green or blue roofs. They are just one tool in a plumbing engineer’s toolbox to combat CSO but the toolbox also includes retention tanks, water reuse, irrigation, gray water and a myriad of other design choices. Do we really want to prohibit the installation of rooftop playgrounds and swimming pools? Do we want to find architects specifying sloped roofs solely to exempt a building from green roof requirements? I propose that we require any building applying for sewer permits (SD1&2) to demonstrate that they will retain enough water on site so as to eliminate local CSO occurrences. The DEP should specify retention/detention parameters to accommodate the local infrastructures. Some neighborhoods may not require any retention while other locations with poor sewer infrastructure may have to retain a large percentage of their rainfall. This customized solution might appear to be cumbersome but in reality it is similar to the current procedure used by the DEP to minimize storm water flooding. All the DEP has to do is enhance their requirements for storm water retention. With these requirements in hand, the plumbing engineer can devise an ingenious solution to meet the DEP’s parameters. After careful consideration the engineer might decide on a green roof, blue roof or devise a new solution and term it a yellow roof. The field of study is too new for the city to mandate a specific solution that applies across the board. The DEP should mandate specific requirements and allow the plumbing engineers to do their job.