settling tank with a mechanical collector. The equipment shall also
collect scum from the top of the sludge-settling zone and pump it to the
[aeration] [reaeration] zone. Pumping shall be accomplished by air lift
pumps.
2.7.1
Piping and Valves
the [aeration] [reaeration] zone or the sludge holding zone or both
scum to the [aeration] [reaeration] zone. Piping shall be galvanized steel
pipe or PVC pipe with solvent weld or push-on joints. Valves shall be gate
valves.
2.7.2
Air Lift Pumps
If the sludge settling zone is the hopper bottom type, at least one air
lift pump shall be provided in each hopper. If the sludge settling zone
contains a mechanical sludge collector, an air lift pump shall be provided
at the collector discharge. Each air lift pump shall consist of a foot
piece located at the bottom of a vertical eductor tube and air supply
piping and control valves. The foot piece shall provide maximum
practicable lift through the formation of small air bubbles. Eductor tubes
and foot pieces shall be constructed of PVC plastic, galvanized steel, or
stainless steel. Air control valves shall be globe valves. Sludge
recirculation air lifts shall be sized based on pumping a sludge volume of
[100] [_____] percent of the treatment plant design flow.
2.7.3
Mechanical Sludge Collector
If the sludge settling zone does not have a hopper bottom, a mechanical
sludge collector shall be provided to scrape the entire tank bottom.
Mechanical sludge collectors shall be either the rotating arm or conveyor
type. The collector shall be driven by an electric motor through an
appropriate gear or chain drive. Motors and drives shall be positioned
above the plant high water level or shall be suitable for submerged
service. Motors shall be totally enclosed conforming to NEMA MG 1. Each
motor shall be furnished with magnetic starter with thermal overload
protection and control devices conforming to NEMA ICS 1. Enclosure shall
be NEMA 250, Type 3R. The tip speed of rotating arm collectors shall not
exceed 1500 mm per minute 5 feet per minute. Linear speed of conveyor type
collectors shall not exceed 300 mm per minute l foot per minute. Collector
speed shall be fast enough to avoid sludge-residence time exceeding 3
hours. Scraper blades shall be replaceable. Collector drive shall have
overload protection.
2.8
FROTH CONTROL SYSTEM
**************************************************************************
NOTE: Retain "aeration zone" for extended aeration
type treatment plants. Retain "contact zone" and
"reaeration zone" for contact stabilization type
treatment plants.
**************************************************************************
A system shall be provided to control froth in the [aeration] [contact and
reaeration] zones. The system shall consist of a pump, piping, valves, and
spray nozzles.
SECTION 11390
Page 20