Question

a 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) =13000m3
/day
3. Peak wet weather flow (PWWF) = 15600m3
/day
4. For settling out particles greater than or equal to mm with a constant horizontal flow velocity of 0.3m/s, the required
channel length (L) is given by L=15d
5. The existing grit channel has a overall depth of 600mm and length of 6m
6. A parshall flume is situated directly downstream from the grit channel and the flowrate/head )
b. Under the PDWF condition the channel length required for setting out all particles of nominal size
greater than or equal to 0.20 mm.
c. The importance of grit chambers in relation to the secondary treatments.
d. The difference between Biomagnification and Eutrophication.
e. The stages which results in eutrophication.

Answer 1