Design of arsenic removal plant by coagulation, sedimentation

CeTAmb
Università degli Studi di Brescia
Facoltà di Ingegneria
EXERCISE
DESIGN OF ARSENIC REMOVAL
PLANT BY COAGULATION,
SEDIMENTATION AND
FILTRATION
FAUSTA PRANDINI
International Summer School - VI Refresher Course
“Appropriate Technologies for Environmental Management in Developing Countries”
20th – 24th June 2011
Faculty of Engeneering – University of Brescia

ARSENIC WATER CONTAMINATION
WHO guideline: 10 μg/L
Some countries retain the older WHO
guideline of 50 μg/L
Arsenic concentrations in
some cases as high as
2,500 μg/L
B. Petrusevski, S.Sharma -IRC, 2007

ARSENIC REMOVAL PROCESSES
• Arsenic oxidation states in natural
waters: inorganic form as
oxyanions of trivalent arsenite (As
(III)) or pentavalent arsenate (As
(V))
• Redox potential (Eh) and pH are
the most important chemical
factors controlling the arsenic
speciation
• Removal efficiency for As (V) is
usually better than removal for As
(III)
• So, reduced inorganic As (III)
should be oxidized to As (V) to
improve its removal Eh-pH diagram for aqueous As species in
the system As–O 2–H 2O at 25 C and 1 bar
Oxidation step
As(III)
uncharged
total pressure (Smedley and Kinniburg,
2002)

ARSENIC REMOVAL PROCESSES
Technologies
Technologies for removing arsenic from
drinking water include:
• Precipitation processes: alum or
ferric salts, …
• Adsorptive processes: activated
alumina, iron/manganese oxide, …
• Ion exchange processes: specifically
anion exchange
• Membrane filtration: nano-filtration,
reverse osmosis and electrodialysis
reversal, …
• Biological arsenic removal
– Community scale
treatment plants
– Household treatment
units

• 10 villages
• 20,000 inhabitants
• 20 km 2 of Area
CONTEXT
Rural community in the South of Calcutta (India)
5

�School: 600 students
�Ambulatory
�Bank
Hand pumps
Analysed hand pumps
Analysed
surface water

WATER USES
• Human uses: drinking and food preparation � groundwater
• Human uses: hygiene � groundwater and/or superficial water
• Animal uses � superficial water
CONTEXT
Rural community in the South of Calcutta (India)

Rural community in the South of Calcutta (India)
� GROUNWATER QUALITY
• Arsenic: 100 mg/L
• Turbidity: about 0 NTU
• Microbiological: good quality
� DATA
• 1200 inhabitants
• D = 20 L/inh/d
• Sodium hypochlorite and Alum (aluminum
sulfate, Al 2(SO 4) 3 • 14H 20) availability
CONTEXT
DESIGN OF ARSENIC REMOVAL PLANT BY OXIDATION,
COAGULATION, SEDIMENTATION AND FILTRATION
Q = 1200 inh x 20 L/inh/d = 24,000 L/d = 24 m 3 /d
� > 90% → As OUT = 10% * 100 mg/L = 10 mg/L
8

CONTEXT
Rural community in the South of Calcutta (India)
� CHEMICALS
• Oxidants: sodium hypochlorite, NaClO
• Coagulant: Alum (aluminum sulfate), Al 2(SO 4) 3 • 14H 20
9

DESIGN OF OXIDATION and
COAGULATION TANKS
OXIDANT DOSAGE: NaClO
• T ox= 1-2 min • V ox = Q x T ox = 24 m 3 /d x 2 min =
= 24 m 3 /(24 x 60 min) x 2 min = 0.033 m 3 =
• Oxidation test
Dosage = 1-2 mg/L
• 15% solution
= 33.3 L
= 0.3 kg/d
ALUM DOSAGE: Al2(SO4) 3 • 14H20 • Tc = 1-2 min
• Jar test
Dosage=10-50
mg/L
• 12% solution
• D NaClO = 2 mg/L = 2 g/m 3
• C NaClO = Q x D/0.15 = 24 m 3 /d x 2 mg/L/0.15 =
= 24 m 3 /d x 2 g/m 3 /0.15 = 320 g/d =
• Vc = Q x Tc = 24 m 3 /d x 2 min =
= 24 m 3 /(24 x 60 min) x 2 min = 0.033 m 3 =
= 33.3 L
• D Alum = 30 mg/L = 30 g/m 3
• C coag = Q x D/0.12 = 24 m 3 /d x 30 mg/L/0.12 =
= 24 m 3 /d x 30 g/m 3 /0.12 = 6,000 g/d =
= 6 kg/d

DESIGN OF FLOCCULATION TANK
FLOCCULATION
DESIGN OF OXIDATION and
COAGULATION TANKS
Total Volume of the mixing tank
• V = 33.3 L x 2 ~ 70 L
• T f = 15-20 min • V f = Q x T f = 24 m 3 /d x 15 min = 25 m 3 /(24
x 60 min) x 15 min = 0.25 m 3 = 250 L

DESIGN OF
SEDIMENTATION TANK
• T sed = 2-3 h • V sed = Q x T sed = 24 m 3 /d x 2 h =
24/24 m 3 /h x 2 h = 2.0 m 3
• Hydraulic loading
rate: R h = 0.8 –
1.5 m/h
• Ased = Q/R h = 24/24 m 3 /h / 0.8 m/h = 1.25 m 2
• fsed = (4 x Ased/∏) 0,5 = 1.3 m
• Hsed = Vsed/Ased = 1.6 m

• Hydraulic loading
rate: R h = 6 – 7 m/h
• H = 1 – 2 m • H sf = 1 m
FILTER BACKWASHING
• R h = 15 – 30 m 3 /(m 2 x h)
• Tc = 5 – 10 min
DESIGN OF
SAND FILTRATION
• A sf = Q/ R h = 24/24 m 3 /h / 7 m/h = 0.17 m 2
• f sf = (4 x A sf/∏) 0,5 = 0.46 m = 46 cm
• Q = R h x A sf = 20 m/h x 0,14 m 2 = 2.8 m 3 /h
• V = Q x Tc = 2.8 m 3 /h x 5 min = 2.8 m 3 /h x
5/60 h = 0.23 m 3
Backwashing frequency: from 1 time/d to 1 time/week (in relation with water
quality and quantity)

ALUM SLUDGE
• 1 mg alum produces
0.44 mg sludge
ALUM SLUDGE
• Sludge = Q x 0.44 kg sludge/kg alum x D Alum =
24 m 3 /d x 0.44 g sludge/g alum x 30 g/m 3 =
= 316.8 g/d = 0.32 kg/d

PLANT EXAMPLE