Air compressor blowdown.  Older, smaller air compressors of the piston-type typically blow some lube oil by the piston rings and this enters air that’s being compressed in the cylinders.  Frequently, a moisture scavenger is attached to the compressor output line to remove water. The oil gets scavenged out along with the water, resulting in an oily water discharge.  In some cases a small oil/water separator is inserted between this discharge and the sanitary sewer line to remove the oil, but in many cases it’s just a straight shot to the sewer.  The concentrations of oil are not very high (several hundred ppm) and the lube oil typically is one that can be easily broken down by sewage treatment plant processes.

Building Maintenance Wastewaters.  Many industrial operating floors and surfaces (as well as those at commercial establishments and institutions) require periodic cleaning and upkeep.  To discharge the wastewaters, you need to verify that the cleaners being used don’t contain any toxic chemicals, that the surfaces being cleaned don’t introduce any toxic chemicals, and that the discharges are otherwise compatible with the processes at the municipal sewage treatment plant.  Things to watch out for are low or high pH, heavy metals, and a lot of grease or oil.  The MSDS sheet for the cleaner should be checked carefully for any constituents that are highly toxic.

Contact Cooling and Heating Water.  A number of industrial processes use water that is in contact with the material in a manufacturing process, typically to cool it down, heat it up, or keep it equilibrated at the appropriate temperature for a reaction to occur.  This is common in calendering and other textile operations.  So long as the raw material doesn’t partition into the water stream (or if it partitions but is innocuous), such wastewaters typically cause no problems at sewage treatment plants.  (However, the general permit excludes certain specific manufacturing processes subject to pretreatment standards.)

Fire Protection System Testing Discharges.  Most manufacturing facilities which have processes susceptible to fire, or which store raw materials that are flammable or combustible, have an insurer requirement to test their fire protection system annually (and sometimes more often).  If it’s a water sprinkler system (rather than CO2 or Halon), this can produce a fair amount of water when tested.  Typically, the water contains some scale and iron deposits.  That’s usually the extent of the water contaminants, unless the system is laid up with a corrosion inhibitor.

Non-Destructive Testing Wastewaters. Rotating equipment parts, engine blocks, cylinder heads and sometime structural steel components are tested with penetrating dyes to inspect for small cracks and defects not apparent to the naked eye.  The dye solutions typically are water-soluble and are rinsed off after the testing is completed.  Most commercial dyes are not used in sufficient quantities to be a problem for sewage treatment plant operations, although several of them are listed as toxic substances. 

Undesignated Miscellaneous Sewer-Compatible Wastewaters.  This category covers a wide range of possibilities.  One example would be bearing cooling water or bearing seal water on big pumps, in which there is the possibility of lube oil carryover into the water.  (But note that this situation could also be non-contact cooling water, which is a different general permit.)