31 Mar 97
underground tanks are undesirable or more costly. For open
systems, this system may require high volume pumps to pump the
discharge up to the above ground tank. These pump requires high
maintenance and increase long term facility maintenance costs.
6.2.1 For closed AFFF systems, containment systems receive AFFF
only during system testing. The AFFF discharge can be directed
to the containment system using hoses connected to a test header.
This eliminates the need for sump and pumps and makes aboveground
tanks cost effective for closed systems.
Earthen Retention Ponds
Earthen retention ponds may have an advantage where large
capacity containment systems are required. A disadvantage is
that a large amount of space is generally required. Ponds should
be designed to contain the greatest 24-hour rainfall in a 5-year
period. Ponds should be lined with an impermeable material in
potential problem. Liners should be protected from ultraviolet
(UV) radiation or be UV resistant. Gravity flow to the pond from
drainage piping system is preferred where the topography of the
site permits. If relative elevations preclude gravity flow, the
discharged solution would have to be pumped to the retention
pond. Disposal of the solution from the pond could be by
controlled flow to a wastewater treatment plant, solar
evaporation or a combination of the two. Valving and piping
should be provided to drain off rain water to the sewage
This method utilizes a lengthy containment trench with steel
safety and rain cover which would contain the foam solution until
it can be disposed.
Additional Capacity For Rainfall:
When open air AFFF retention ponds or tanks are used, or where
areas drained by AFFF drainage system are open to rain fall, the
capacity of the storage system shall be increased to accommodate
a 5 year - 24 hour maximum rain fall event, in addition to the
worst case foam discharge. Because of extremely unlikely event
of an AFFF discharge occurring simultaneously with a greater rain
event, capacity will not be designed for greater rain fall
events. Containment systems for only closed head fire protection
systems do not require rain fall allowance since the containment
system is only used for system testing.