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434 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
of the sand. The maximum drainage area that can be treated by a sand filter has been reported
to be about 5 acres (2 ha). Sand filters generally are used to treat impervious areas, such as park-
ing lots, so that smaller sediment particles typical of pervious areas will not clog the sand filter.
Maintenance Requirements. Sand fitters require minimal maintenance, consisting of pe-
riodically removing accumulated sediment and the top layer of sand from the filtration
chamber and removing accumulated sediment and floatables from the catch basin. Regular
inspections of the filter system can indicate when this maintenance is required.

Limitations on Use. Because of their small size, sand filters are designed to be used for
pretreatment in large watersheds or full treatment in small watersheds. They cannot pro-
vide sufficient treatment for large watersheds.

12.5.7 Water Quality Inlets
Water quality inlets, also known as oil and grit separators, are similar to septic tanks used
for removing floatable wastes in on-site wastewater disposal systems. These inlets provide
removal of floatable wastes and suspended solids through the use of a series of settling
chambers and separation baffles as shown in Fig. 12.20. Given the limited pollutant removal
expected from water quality inlets, they are usually used in conjunction with other BMPs.
Fairly effective at removing coarse sediments and floating wastes, water quality inlets can
be used to pretreat runoff before it is discharged to infiltration systems or detention facilities.
In this way, some of the routine maintenance the other BMPs require (e.g., sediment removal
and unclogging of outlet structures) can be reduced. Water quality inlets also can serve to
capture petroleum spills that could enter other treatment structures or surface waters.

Pollutant Removal. The primary pollutant removal mechanisms of water quality inlets are
separation and settling. The use of three chambers in these inlets serves to increase the deten-
tion time of the runoff in the tank, allowing settling to occur. In this way, suspended solids, and
the attached pollutants, are removed from the runoff. In addition, the use of baffles and in-
verted elbows helps to remove floating litter and petroleum products from the stormwater. The
level of removal of these pollutants depends on the volume of water permanently detained in
the tank, the velocity of flow through the tank, and the depth of the baffles and inverted elbows
in the tank. By increasing detention time and decreasing flow velocity, the level of sediment
and floatables expected to be removed from water quality inlets can be improved.
Design Considerations. Water quality inlets design depends on the size of the watershed
being treated and the detention time required. Because suggested detention times are usu-
ally measured in terms of minutes rather than days, water quality inlets generally do not re-
move pollutants from stormwater runoff as effectively as some of the more intensive deten-
tion facilities. Water quality inlets have the advantage of being relatively small so they can
be placed throughout a drainage system rather than just at the downstream end of the sys-
tem. The flow and velocity of the entering runoff can be hydraulically restricted by limit-
ing the size of the inlet pipe.

Maintenance Requirements. Water quality inlets require periodic maintenance to remove
accumulated pollutants; in general, these inlets should be cleaned about twice a year.
Cleaning can be performed with a vacuum truck similar to those used to clean catch basins.
Periodic inspections between scheduled maintenance are also required to determine the
level of accumulated pollutants.