So the typical stormwater system has catch basins and it may have a water quality and water quantity system. Let’s keep water quantity for the future and discuss water quality first.
As I alluded to in my last blog post, both water quantity systems and water quality systems can really be considered under the heading of water quality. Water quantity systems restrict the flow off of a constructed site to approximately the rate of flow that occurred from that site before construction. The intent of this is to keep downstream scouring of river beds, etc. to a minimum. That means that less sediment gets mingled into the flowing water downstream, thus improving water quality.
Water quality systems, though, are the systems that are designed and installed on a site to clean the stormwater that is generated from a site of the pollutants that it picks up while on that site. For instance, if oils are present on a site, a stormwater quality system that can remove oils will be installed on the site. While there are many other pollutants that are encountered on a site, let’s start our series on water quality systems by looking at some systems for removing oils from stormwater. Admittedly, this will be a fairly simplistic look at oil/water separators. If you have questions about what kind of system you might have, feel free to call our office.
Typically, oils are treated by one of three different stormwater quality systems, referred to as a group as Oil/Water Separators. The first, still in heavy use and seen frequently, is the down-turned elbow inside of a catch basin or manhole. This is a 90 degree downturned pipe on the end of the outflow pipe (the pipe leaving the CB or manhole). The theory here is that, since oil floats on water, the down-turned elbow will discharge the cleaner water below the floating oil, which will keep the oil from discharging downstream in the stormwater system.
A second type of oil-water separation system is the vault oil/water separator, sometimes called an API separator (API standing for American Petroleum Institute). This type of separator is essentially a large concrete box with baffles and some down-turned elbows in it. This system is used when there is a significant amount of oil expected and the oil is expected to be laden with sediments. The API separator uses a series of dead pools, baffles, and down-turned elbows to separate out not only the oil, but the sediments as well.
The last type of system in use for oil-water separation is the coalescing plate separator. This is a bit more technical than the API system, but not that much. It works by the same methods as the API separator, more or less. The coalescing plate separator is comprised of a vault that contains within it a large metal box of various plates. These are, you guessed it, the coalescing plates. The plates are wavy steel plates that create small domes about the size of a golf ball. These domes trap a small volume of water and oil and allow the quick removal of the oil from the oil/water mixture by floatation on a smaller basis. Because the domes hold only a small volume of oil/water mixture, the oil rises to the top fairly quickly. Once larger amounts of oil are trapped, it begins to travel upward through other plates in the coalescing plate structure until it can be seen at the top of the overall plate assembly. While this is happening, the sediments are also being directed downward.
So this is a brief description of some of the systems typically in use for the separation of oil and water. However, we often see that there is oil removal from a site but none of these types of oil/water separation structures are present on a site. This may be because there is a filter style stormwater quality system on the site that removes a spectrum of different pollutants at the same time. Those filter-style systems will be the subject of our next blog post.
– See more at: http://www.catchallenvironmental.com/blog/water-quality-systems-the-oil-water-separator#sthash.CWYONly6.dpuf