There are a number of reasons why refineries treat their wastewater right on site instead of sending it off to a municipal treatment plant, but for the most part, it has to do with the contaminants present in the samples. Wastewater from processes such as desalting, distillation, thermal and catalytic cracking, and coking possibly contain benzene (toxic aromatic compound), H2S, NH3, heteroatoms, and acids. Requirements for wastewater in municipal treatment plants vary from state to state so refineries are just playing it safe by treating their water in-house. The refineries have the equipment and knowledge to detect the exact amount of contaminant species, therefore they should be the ones who treat it. On top of all this, you can’t combine the four different types of wastewater (cooling water, process water and steam, storm water, and sanitary sewage water) so transporting it to the municipal plant would require four different vehicles, a bunch of unneeded money spending, and communication of specifics. Even though industrial wastewater treatment plants are capable of removing chemicals and acids, it is not safe to use the facilities for water that has came in contact with petroleum fractions. The contaminants found in this water is basically human poison and is best left to the people who know where the all the water has came from.
Sources:
Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 290-293.
http://en.wikipedia.org/wiki/Industrial_wastewater_treatment#Oils_and_grease_removal
http://10statesstandards.com/wastewaterstandards.html#52