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Flammable sand trap waste

A fellow MPCA staffer has asked me to post the following query.

The most environmentally benign method of dealing with flammable sand
trap wastes needs to be worked out.  Flammable sand trap waste are
typically generated when the waste water from vehicle engine washing
operations (ranging in size from mom and pop car washes to vehicle
fleets) flows to a floor drain where it is collected in a holding tank.
The material is then typically comprised of three-phases: a LNAPL phase,
dirty water, and a sand layer.

The LNAPL phase can be blotted off with absorbent pads and shipped off
by a waste oil hauler.  The sand can be dewatered, and in many instances
be taken to a permitted landfill (if no liquids).  It is however
difficult to deal with the remaining dirty water (containing metals and
NAPLs).  On-site pretreatment of the dirty water would appear to be
pretty expensive.  

Many smaller outstate (non-metro) waste water treatment facilities
(WWTFs) are reticent to take it because it may upset their plant, they
may be already close to their discharge limits, or they don't want to
take such liquids if not in their billing area.

Closed loop recirculation of the dirty water has been suggested as a
means of P2.  If you know of any methods of P2 for the dirty water or
dealing with flammable sand trap wastes as a whole please let me know.

Al Innes
Minnesota Pollution Control Agency
520 Lafayette Road North
St. Paul, MN  55155-4194