CE 428 Wastewater Grit Sedi (1)

8/13/2019 CE 428 Wastewater Grit Sedi (1)

1/4

CE 428 Water and Wastewater Design

Grit 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 - !", volatile organic content - 1 - !"

- specific gravity - clean grit particles - #.!, for material $ith substantial organic material

attached to inerts - appro%. 1.3

- bul& density in the range of 1'' &g(m3

- most grits are retained on a )o. 1'' mesh sieve *'.1! mm or larger+- typical settling velocity for 1'' mesh grit is 1.3 cm(s or #. ft(min

urpose of rit removal

- to protect mechanical euipment from abrasion and abnormal $ear

- to reduce conduit clogging caused by deposition of grit particles in pipes and channels

- to prevent loading the treatment plant $ith inert matter that might interfere $ith the operation of

treatment units such as siltation of anaerobic digester and aeration tan&s

/ype of rit 0emoval - horiontal flo$ *suare or rectangular configuration+

- aerated *rectangular+- vorte%-type

2oriontal flo$ type

- open channels $ith sufficient detention time to allo$ particles to settle and to maintain constant

velocity to scour organics

- the velocity of flo$ controlled by

*i+ dimensions of unit

*ii+ special influent distribution gates*iii+ special $eir sections at the effluent end *proportional $eir+

- designed to remove grit '.#1 mm dia. to as lo$ as '.1! mm dia.

- grit removal is accomplished by a conveyor $ith a scraper, buc&ets or plo$s- may reuire grit $ashing euipment to remove organics

/ypical Design 4nformation for 2oriontal rit 0emoval

arameters 0ange /ypical

Detention time *s+ 5! - 6' '2oriontal 7elocity *ft(s+ '.8 - 1.3 1.' - 1.#!

2eadloss in a control section

as percent of depth in channel 3' - 5'" 3"

9llo$ance for inlet and outlet

turbulence #Dm - '.! : *at least !'" increase in theoretical length is

recommended, Dm - ma%. depth, : - length+

rit uantities '.''5 - '.'3; m3(1''' m3

Aerated Grit Chambers

- popularity of aerated grit chamber

- less $ear on grit-handling euipment

- in many cases, no need for separate grit $ashing euipment

- normally designed to remove particles ! mesh *'.# mm+ or larger

- velocity of roll or agitation governs the sie of particles of a given specific gravity to be removed


2/4

- uantity of air is ad1 - !>1 1.! > 1

:ength-$idth ratio 3>1 - !>1 5>19ir supply *ft3(min(ft of length+ # - !

rit uantities *ft3(mgal+ '.! - #; #.'

Vortex-tye Grit Chambers

- vorte% 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 6!" of 1'' micron *1!' mesh+ grit- head reuired to achieve this removal is a fe$ meters, for e%ample, 6!" removal of #! micron -

head reuirement - ! - ; m

- typical units can handle pea& flo$s up to '.31 m3(s *; mgd+

/ypical Design 4nformation for 7orte% rit Chambers

0ange /ypical

Detention time *s+ 3'

Dimensions Diameter *ft+ 5 - #5?pper

:o$er 3 -

height *ft+ 6 - 10emoval 0ates "

!' mesh *'.3 mm+ 6!@

;' mesh *'.#5 mm+ 8!@

1'' mesh *'.1! mm+ !@

Design Examle !or hori"ontal aerated grit #hamber

Design an aerated grit chamber $ith an average flo$ of 11.5 mgd *'.! m3(s+ $ith a pea&ing factor of #.;!

1. ea& hourly flo$ rate for design A 11.5 % #.;! A 31.3! mgd

#. rit chamber volume

9ssume #.' minutes for pea& hourly flo$9ssume t$o chambers in use all the time


3/4

A3

3

ft61',#min#%

+hr

min'+*

day

hr #5+*

ft

gal58.;*

1'%+mgd#

3!.31*

=

5. Dimensions

?se $idth to depth ratio of 1.1>1 and a depth of 8 ft

=idth A 1.1 % 8 A 8.8 ft

:ength A #,61' ft#(*8 % 8.8+ A 51.3 ft ?se 51 ft.

!. Chec& horiontal velocity

s(ft35.'8.8%8

1'%+

+min

sec

'+*hr

min

'+*day

hr

#5+*ft

gal

58.;*

mgd#

3!.31

*

v

3

h ==

*on the lo$ side, but acceptable+

. 9ir supply use ! ft3(m. ft of length

air reuired A 51 ft % ! ft3(m. ft of length A #'! ft3(min

;. 9verage grit removal assume ; ft3(mgd

volume of grit A 11.5 mgd % ; ft3(mgd

A ;6.8 ft3(day

$rimary %edimentation

• purpose B 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 uiescent conditions

/$o types of design available> horiontal flo$ and circular

Design Criteria

• overflo$ rate *gpm(ft# or m3(m#(d+

• $eir overflo$ rate *gpm(ft or m3(m(d+

• detention time *hours+

• solids loading rate *lbs(ft#(d or &g(m#(d+ *more important for secondary sedimentation tan&s+

Design Data 0ange /ypicalOverflo$ rates

9verage flo$ *gal(ft#(d+ 8'' B 1#''

*m3(m#(d+ 3# B 58

ea& hourly flo$ *gal(ft#(d+ #,''' B 3,''' #,!''

*m3(m#(d+ 8' B 1#' 1''


4/4

=eir overflo$ rate

ea& hourly flo$ *gal(ft#(d+ 1',''' B 5',''' #','''

*m3(m#(d+ 1#! B !'' #!'

Detention time *hrs+ *average flo$+ 1.! - #.! #.'

/ypical dimensions

0ectangular

Depth *ft+ 1' B 1! 1#

:ength *ft+ !' B 3'' 8' B 13'=idth *ft+ 1' B 8' 1 B 3#

ottom slope 1 B #"

Speed of scraper *ft(min+ # B 5 3

Circular

Depth *ft+ 1' B 1! 1#

Diameter *ft+ 1' B #'' 5' B 1!'

ottom slope *in(ft+ - #

Speed of scraper *rev(min+ '.'# A '.'! '.'3 EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE

F%pected OD and suspended solids removal bet$een 3' B 5'" and !' B ;'", respectively *see Gigure

1#- 5, may be different for different $aste$ater+.

Design examle&

Design a sedimentation tan& for a municipal $aste$ater $ith an average flo$ of !.''' m3(day and a

pea& hourly flo$ A 1#,!'' m3(day?se Gigure, assume '" SS removal, overflo$ rate A 3! m3(m#(day

1. 0euired surface area A !,'''(3! A 153 m#

#. Circular tan& diameter d A *5% 153(3.15#+1(# A 13.! m

*have to select sie appropriate for circular scraper+

3. 9ssume 1! m diameter *to fit a 1! m diameter scraper+surface area needed A 3.15# *1!+#(5 A 1;.; m#

5. 9ssume side $all depth A 3 m 7olume of tan& A 1;.; % 3 A !3'.1 m3

!. Detention time A !3'.1(!''' A '.1' days A #.!5 hrs *o&+

. Gor pea& hourly flo$ reuirements, find overflo$ rate A 1#,!''(1;.; A ;'.; m3(m#(d *o&+

;. Chec& $eir overflo$ rate

A 1#,!''(*3.15# % 1!+ A #! m3(m#(day *o&+

8. SS removal at pea& flo$ rate ≈ 5'"

Documents

CE 428 Wastewater Grit Sedi (1)