31 Mar 97
AFFF solution should be metered discharge to "flowing sewers"
because discharge to an intermittently flowing sewer could cause
waste to collect and to be flushed to aeration basins at higher
than recommended concentrations. Uncontrolled sewer discharge
rates could also result in foam backing out of sewer drains.
When too much fire fighting foam containing fluorochemical
surfactants is discharged to a wastewater treatment system at one
time, severe foaming can occur, even at low concentrations. This
results in aesthetic concerns in rivers and streams as well as
operational problems in sewers and wastewater treatment systems.
Therefore, the rate of discharge must be controlled.
Rate Of Discharge
It is generally recognized that the concentration of foam
solution in the influent reaching a wastewater treatment plant
needs to be no greater than 1,700 parts per million (ppm). This
degree of dilution is considered sufficient to prevent "shock
loading" and foaming which can upset treatment plant operation.
As an example, if a discharge is to be made to a 6-million-
gallon-per-day treatment plant, the solution could be discharged
at a rate of 7 gallons per minute (gpm). Since such a low rate
of discharge is apt to be difficult to control, dilution of the
foam solution by say 10 to 1, would permit a discharge rate of 70
gpm. In any case, it could take several days or even weeks to
dispose of the solution, depending upon the amount of the foam
solution release. Since this level of dilution may not apply to
all wastewater treatment plants, operators of affected plants
should be consulted in advance. Discharge levels of AFFF must be
determined well in the early stages of design. In some
instances, treatment plant modifications may be necessary, new
environmental permits may be needed, or existing permits updated.
Solar Evaporation Pond
Disposition of AFFF solution through solar evaporation is
feasible under certain circumstances. Feasibility of this
disposal method is related to the rate of evaporation which
depends upon the holding area surface area, the difference in
saturation pressures at the air dewpoint and the surface water
temperature, wind velocity and the latent heat required to change
water to vapor. High humidity present in many locations during
the summer has the effect of slowing the rate of evaporation.
The ideal location for utilizing solar evaporation as a means for