CE 428 Water and Wastewater DesignGrit Chambers Dr. S.K. Ong

Grit Removal

Characteristics of grit - sand, gravel, cinders, eggshells, bone chips, seeds, coffee grounds and other heavy materials- predominantly inert, composition variable- moisture content 13 - 65%, volatile organic content - 1 - 56%- specific gravity - clean grit particles - 2.65, for material with substantial organic material attached to inerts - approx. 1.3- bulk density in the range of 1600 kg/m3- most grits are retained on a No. 100 mesh sieve (0.15 mm or larger)- typical settling velocity for 100 mesh grit is 1.3 cm/s or 2.6 ft/min

Purpose of Grit removal- to protect mechanical equipment from abrasion and abnormal wear- to reduce conduit clogging caused by deposition of grit particles in pipes and channels- to prevent loading the treatment plant with inert matter that might interfere with the operation of treatment units such as siltation of anaerobic digester and aeration tanks

Type of Grit Removal - horizontal flow (square or rectangular configuration)- aerated (rectangular)- vortex-type

Horizontal flow type- open channels with sufficient detention time to allow particles to settle and to maintain constant velocity to scour organics- the velocity of flow controlled by

(i) dimensions of unit(ii) special influent distribution gates(iii) special weir sections at the effluent end (proportional weir)

- designed to remove grit > 0.21 mm dia. to as low as 0.15 mm dia.- grit removal is accomplished by a conveyor with a scraper, buckets or plows- may require grit washing equipment to remove organics

Typical Design Information for Horizontal Grit Removal Parameters Range Typical Detention time (s) 45 - 90 60Horizontal Velocity (ft/s) 0.8 - 1.3 1.0 - 1.25Headloss in a control sectionas percent of depth in channel 30 - 40% 36%Allowance for inlet and outletturbulence 2Dm - 0.5 L (at least 50% increase in theoretical length is

recommended, Dm - max. depth, L - length)Grit quantities 0.004 - 0.037 m3/1000 m3

Aerated Grit Chambers- popularity of aerated grit chamber

- less wear on grit-handling equipment- in many cases, no need for separate grit washing equipment

- normally designed to remove particles 65 mesh (0.2 mm) or larger- velocity of roll or agitation governs the size of particles of a given specific gravity to be removed

- quantity of air is adjusted to provide the roll and washing of the grit to remove organic matter

- grit removed by using grab buckets on monorails centered over the grit collection and storage trough or by a flushing through a drain

Typical Design information for Aerated Grit Chambers Range Typical Detention time (mins) at max. flow 2- 5 3Dimensions Depth (ft) 7 -16

Length (ft) 25 - 65Width (ft) 8 - 23

Width -depth ratio 1:1 - 5:1 1.5 : 1Length-width ratio 3:1 - 5:1 4:1

Air supply (ft3/min/ft of length) 2 - 5Grit quantities (ft3/mgal) 0.5 - 27 2.0 Vortex-type Grit Chambers

- vortex created- grit move to the outside of the unit and collected- air scour to remove organics- grit removed by air lift pump or by bottom drain- typically designed to remove 95% of 100 micron (150 mesh) grit- head required to achieve this removal is a few meters, for example, 95% removal of 25 micron - head requirement - 5 - 7 m- typical units can handle peak flows up to 0.31 m3/s (7 mgd)

Typical Design Information for Vortex Grit Chambers Range Typical Detention time (s) 30Dimensions Diameter (ft) 4 - 24

Upper Lower 3 - 6

height (ft) 9 - 16Removal Rates %

50 mesh (0.3 mm) 95+70 mesh (0.24 mm) 85+100 mesh (0.15 mm) 65+

Design Example for horizontal aerated grit chamber

Design an aerated grit chamber with an average flow of 11.4 mgd (0.5 m3/s) with a peaking factor of 2.75

1. Peak hourly flow rate for design = 11.4 x 2.75 = 31.35 mgd

2. Grit chamber volumeAssume 2.0 minutes for peak hourly flowAssume two chambers in use all the time

=

4. Dimensions

Use width to depth ratio of 1.1:1 and a depth of 8 ft

Width = 1.1 x 8 = 8.8 ftLength = 2,910 ft2/(8 x 8.8) = 41.3 ft Use 41 ft.

5. Check horizontal velocity

(on the low side, but acceptable)

6. Air supply use 5 ft3/m. ft of length air required = 41 ft x 5 ft3/m. ft of length = 205 ft3/min

7. Average grit removal assume 7 ft3/mgdvolume of grit = 11.4 mgd x 7 ft3/mgd

= 79.8 ft3/day

Primary Sedimentation

purpose – to remove settable organic solids and to reduce the solids load on the biological treatment unit primary sedimentation or clarification is achieved in large basins under relatively quiescent conditions

Two types of design available: horizontal flow and circular

Design Criteria overflow rate (gpm/ft2 or m3/m2/d) weir overflow rate (gpm/ft or m3/m/d) detention time (hours) solids loading rate (lbs/ft2/d or kg/m2/d) (more important for secondary sedimentation tanks)

Design Data Range Typical Overflow rates

Average flow (gal/ft2/d) 800 – 1200(m3/m2/d) 32 – 48

Peak hourly flow (gal/ft2/d) 2,000 – 3,000 2,500(m3/m2/d) 80 – 120 100

Weir overflow ratePeak hourly flow (gal/ft2/d) 10,000 – 40,000 20,000

(m3/m2/d) 125 – 500 250

Detention time (hrs) (average flow) 1.5 - 2.5 2.0

Typical dimensionsRectangular

Depth (ft) 10 – 15 12Length (ft) 50 – 300 80 – 130Width (ft) 10 – 80 16 – 32Bottom slope 1 – 2%Speed of scraper (ft/min) 2 – 4 3

CircularDepth (ft) 10 – 15 12Diameter (ft) 10 – 200 40 – 150Bottom slope (in/ft) ¾ - 2Speed of scraper (rev/min) 0.02 = 0.05 0.03

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Expected BOD and suspended solids removal between 30 – 40% and 50 – 70%, respectively (see Figure 12- 4, may be different for different wastewater).

Design example:

Design a sedimentation tank for a municipal wastewater with an average flow of 5.000 m3/day and a peak hourly flow = 12,500 m3/dayUse Figure, assume 60% SS removal, overflow rate = 35 m3/m2/day

1. Required surface area = 5,000/35 = 143 m2

2. Circular tank diameter d = (4x 143/3.142)1/2 = 13.5 m(have to select size appropriate for circular scraper)

3. Assume 15 m diameter (to fit a 15 m diameter scraper)surface area needed = 3.142 (15)2/4 = 176.7 m2

4. Assume side wall depth = 3 m Volume of tank = 176.7 x 3 = 530.1 m3

5. Detention time = 530.1/5000 = 0.106 days = 2.54 hrs (ok)

6. For peak hourly flow requirements, find overflow rate = 12,500/176.7 = 70.7 m3/m2/d (ok)

7. Check weir overflow rate= 12,500/(3.142 x 15) = 265 m3/m2/day (ok)

8. SS removal at peak flow rate 40%