trapezoidal section constant velocity horizontal flow grit channel is discharging unacceptable amounts of grit into the primary facultative pond of a stabilization pond. In addition the channel overtops regularly with peak wet weather flow during normal rain storms. An initial investigation confirmed levels of grit present in the particular wastewater to be within normal loading levels. Furthermore, the provision for grit storage capacity in the channel is adequate for the operational cleaning-out cycle employed. According to the authorities, the likely cause of the grit discharge problem is that the channel length is inadequate for settling out the nominal particle size required. Assume 1. Average dry weather flow (ADWF) = 5000m3 /day 2. Peak dry weather flow (PDWF) =13000 m3 /day 3. Peak wet weather flow (PWWF) = 15600 m3 /day 4. For settling out particles ≥ 0.20mm with a constant horizontal flow velocity of 0.3 m/s, the required channel length (L) is given by L=15d 5. The existing grit channel has a overall depth of 600 mm and length of 6 m 6. A parshall flume is situated directly downstream from the grit channel and the flow rate/head relationship for the flume is given by: Q = 0.3812H1.58 Where; Q = Flowrate through flume in m3 /s H = Upstream head to flume in m Determine: a. The overall grit channel depth required with a freeboard of 300 mm. b. Under the PDWF condition the channel length required for setting out all particles of nominal size ≥0.20 mm. c. The importance of grit chambers in relation to the secondary treatments.
trapezoidal section constant velocity horizontal flow grit channel is discharging unacceptable amounts
of grit into the primary facultative pond of a stabilization pond. In addition the channel overtops regularly
with peak wet weather flow during normal rain storms. An initial investigation confirmed levels of grit
present in the particular wastewater to be within normal loading levels. Furthermore, the provision for grit
storage capacity in the channel is adequate for the operational cleaning-out cycle employed.
According to the authorities, the likely cause of the grit discharge problem is that the channel length is
inadequate for settling out the nominal particle size required.
Assume
1. Average dry weather flow (ADWF) = 5000m3
/day
2. Peak dry weather flow (PDWF) =13000 m3
/day
3. Peak wet weather flow (PWWF) = 15600 m3
/day
4. For settling out particles ≥ 0.20mm with a constant horizontal flow velocity of 0.3 m/s, the
required channel length (L) is given by L=15d
5. The existing grit channel has a overall depth of 600 mm and length of 6 m
6. A parshall flume is situated directly downstream from the grit channel and the flow rate/head
relationship for the flume is given by:
Q = 0.3812H1.58
Where; Q = Flowrate through flume in m3
/s
H = Upstream head to flume in m
Determine:
a. The overall grit channel depth required with a freeboard of 300 mm.
b. Under the PDWF condition the channel length required for setting out all particles of nominal size
≥0.20 mm.
c. The importance of grit chambers in relation to the secondary treatments.
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