Page 200 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Grit Chambers 155

baffles were used, air header placement, grit removal equip- Shape and size: Aerated grit chambers have been constructed
ment, and whether grit washing was used. with a variety of shapes as seen in Table 7.12. To promote the
The conclusion was that the RMC plant (see Table 7.12) spiral flow, a rectangular cross section is recommended with
was an ‘‘optimal’’ design in terms of operation and perform- corner fillets in the corners to reduce the hydraulic dead zones.
ance. The dimensions were length, 27.2 m; width, 3.7 m; and Londong (1989) recommends a width to depth ratio of 0.8,
depth, 4.0 m. The plant had no inlet or outlet baffles, which that is, w=D ¼ 0.8.
were deemed not essential due to its long-narrow geometry. Air diffuser placement: The header placement of the air
The aeration header was a perforated pipe (i.e., resulting in diffusers line is almost always parallel to the direction of the
coarse bubbles and nonclogging operation) with a parallel flow and near one of the sidewalls, the deviations of
longitudinal baffle that aided the hydraulic roll. Regarding Table 7.12 notwithstanding. Londong (1989) recommended
performance, the grit removal was 0.95 fraction for particles that the air header be placed at a depth, 0.7 D, below the
0.2 mm, the volatile solids fraction of the grit removed was water surface.
0.08 fraction, and the solids fraction was 0.79 (i.e., only 0.21
Types of air diffusers: Most diffusers are coarse-bubble,
fraction water). The tracer test for the RMC plant showed an
which are less likely to clog. Fine bubbles, however, exert a
inverted ‘‘V-shape,’’ that is, close to the plug-flow ideal. The
higher amount of drag on the water mass. Fine bubble
theoretical detention time was 3.7 min with actual measured
diffusers require a higher pressure drop than a coarse bubble
(based on the tracer curve) at 4.6 min. The bottom roll vel-
diffuser for a given airflow. Whatever the diffuser type, the
ocities were 0.03 v T 0.24 m=s.
airflow versus pressure drop (across the diffuser) should
While some three of the other grit chambers, that is, IC, SR,
be determined, that is, Q a vs. DP(diffuser) so that the com-
FR (see Table 7.12), had high grit removal fractions, the
pressor can be sized accurately.
organic fractions were high. Operational differences for these
plants were considered as contributing to the higher levels of Airflow control and measurement: The airflow is controlled
grit removal, for example, lower overflow velocities. In some by a valve to the header pipe located just after the compressor.
cases, grit had deposited on the bottom (i.e., not in the collec- The airflow, Q(air), should be measured so that performance
tion zone). Also, tracer tests showed accentuated short-circuit- can be related to airflow. An orifice meter or a Venturi meter
ing with long tails (the latter indicating dead zones). Detention with differential manometer are instruments common for flow
times varied 3.6–7.8 min and bottom velocities were generally measurement.
0.2–0.4 m=s. Required airflow: The required airflow, Q(air), may be deter-
Two general deficiencies of the grit-chamber designs mined: (1) by a pilot plant study, (2) by an experimental use of
included: (1) airflows could not be measured, that is, no a full-scale grit chamber, or (3) by data from practice (i.e.,
flow meters; (2) the airflows could be adjusted manually, operating plants). The airflow ‘‘capacity’’ should be in excess
but with difficulty. The characteristics of a well-designed of the maximum airflow needed. In a pilot study, the top and
grit chamber included not only effective performance but a bottom velocities of the roll should be measured for each
unit that is relatively trouble-free and had operating flexi- airflow. The compressor power required is based on an equa-
bility. Grit that has low organic matter was deemed import- tion for an ‘‘adiabatic’’ compression, with inputs, Q(air) and
ant so that grit handling and land disposal may be done p 2 (where p 2 is the pressure at the discharge side of the
easily, that is, with minimum nuisance and in accordance compressor).
with regulations. In summary, the RMC grit chamber had Equations for experimental system: Equations for airflow for
the closer-to-ideal dimensions (based on theory) and the coarse bubble and fine bubble diffusers were obtained by
overall best performance. Londong (1989) for a particular experimental system and are
7.3.3.2 Summary of Guidelines given here as Equations 7.24 and 7.23, respectively. The
equations give airflows that result in tangential velocities of
The following paragraphs summarize some of the important
0.2 m=s, that is, v T ¼ 0.2 m=s and give airflow as a function of
design criteria for aerated grit chambers. As noted previously,
the depth of the air header:
most are empirical.
Grit quantity: Table 7.12 shows a range of grit collected
3
3
ranging 7–39 mL grit=m water (0.9–5ft =mg), as measured Q(air, coarse bubble, v T ¼ 0:2 m=s)
‘‘wet.’’ For comparison, Tchobanoglous and Burton (1991, ¼ [0:07 þ 0:76 ln d(air header)] 1:33 (7:24)
3
3
p. 462) give 4–200 mL grit=m water (0.5–27 ft =mg) for
aerated grit chambers. Q(air, fine bubble, v T ¼ 0:2 m=s)
Detention time: About 3–5 min for detention time seems to be 0:62
¼ [0:63 þ 0:52 ln d(air header)] (7:25)
entrenched in the lore of American practice. However, Table
7.12 shows a range 3.6–7.8 min, and for comparison, Tcho-
banoglous and Burton (1991, p. 462) give 2–5 min for peak where
flow. By contrast, Londong (1989) recommended, based on a Q is the air flow necessary to maintain a circulation vel-
3
modeling study, u > 10 min for storm water flow and u > 20 min ocity of 0.2 m=s, that is, v T ¼ 0.2 m=s(m =s)
for dry weather flow. d is the depth of submergence of air diffusers (m)