In many cases, a below ground oil water separator is the best option due to space restrictions and layout. You can use gravity to drain into a below ground system and then into the sanitary sewer system so there is no need for pumps and controls. In other cases, a below ground system has been previously specified and it is not possible to change the specifications.
Some of the issues involved with a below ground system are that they are often forgotten after installation – one of those “out of sight, out of mind” matters. That is fine if there is only oily water going through the system. Troubles start when sediment and solids are introduced into the oil water separator. As the solids build up within the separator, the efficiency decreases. If the solids and oil are not removed from the separator, it will eventually fill up to the point that the separator will not separate the oil, but just pass the oil through with the water. Periodic maintenance on below ground separators, which includes pumping out the oil and sludge, is critical to their efficient long-term operation.
Finally, due to poor maintenance or bad soil conditions, if an underground system leaks, the potential clean-up costs can be very expensive! While most underground tanks are double walled, very few underground piping systems are, which is another source of potential leaks into the ground water. The longer the system remains underground, the more potential for leaks and the larger the potential liability.
Even if an above ground oil water separator is in a corner of the building, it is still “in sight” and accessible and therefore, has a much greater chance of being maintained. The maintenance is much less expensive than an underground system and usually involves just washing it out with a hose or a pressure washer.
For ease of maintenance, and minimizing liabilities, the additional costs of pumps and space issues of above ground oil water separators will almost always out weight the long-term liability of a potential groundwater clean up.