Design of arsenic removal plant by coagulation, sedimentation
Design of arsenic removal plant by coagulation, sedimentation
Design of arsenic removal plant by coagulation, sedimentation
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CeTAmb<br />
Università degli Studi di Brescia<br />
Facoltà di Ingegneria<br />
EXERCISE<br />
DESIGN OF ARSENIC REMOVAL<br />
PLANT BY COAGULATION,<br />
SEDIMENTATION AND<br />
FILTRATION<br />
FAUSTA PRANDINI<br />
International Summer School - VI Refresher Course<br />
“Appropriate Technologies for Environmental Management in Developing Countries”<br />
20th – 24th June 2011<br />
Faculty <strong>of</strong> Engeneering – University <strong>of</strong> Brescia
ARSENIC WATER CONTAMINATION<br />
WHO guideline: 10 μg/L<br />
Some countries retain the older WHO<br />
guideline <strong>of</strong> 50 μg/L<br />
Arsenic concentrations in<br />
some cases as high as<br />
2,500 μg/L<br />
B. Petrusevski, S.Sharma -IRC, 2007
ARSENIC REMOVAL PROCESSES<br />
• Arsenic oxidation states in natural<br />
waters: inorganic form as<br />
oxyanions <strong>of</strong> trivalent arsenite (As<br />
(III)) or pentavalent arsenate (As<br />
(V))<br />
• Redox potential (Eh) and pH are<br />
the most important chemical<br />
factors controlling the <strong>arsenic</strong><br />
speciation<br />
• Removal efficiency for As (V) is<br />
usually better than <strong>removal</strong> for As<br />
(III)<br />
• So, reduced inorganic As (III)<br />
should be oxidized to As (V) to<br />
improve its <strong>removal</strong> Eh-pH diagram for aqueous As species in<br />
the system As–O 2–H 2O at 25 C and 1 bar<br />
Oxidation step<br />
As(III)<br />
uncharged<br />
total pressure (Smedley and Kinniburg,<br />
2002)
ARSENIC REMOVAL PROCESSES<br />
Technologies<br />
Technologies for removing <strong>arsenic</strong> from<br />
drinking water include:<br />
• Precipitation processes: alum or<br />
ferric salts, …<br />
• Adsorptive processes: activated<br />
alumina, iron/manganese oxide, …<br />
• Ion exchange processes: specifically<br />
anion exchange<br />
• Membrane filtration: nano-filtration,<br />
reverse osmosis and electrodialysis<br />
reversal, …<br />
• Biological <strong>arsenic</strong> <strong>removal</strong><br />
– Community scale<br />
treatment <strong>plant</strong>s<br />
– Household treatment<br />
units
• 10 villages<br />
• 20,000 inhabitants<br />
• 20 km 2 <strong>of</strong> Area<br />
CONTEXT<br />
Rural community in the South <strong>of</strong> Calcutta (India)<br />
5
�School: 600 students<br />
�Ambulatory<br />
�Bank<br />
Hand pumps<br />
Analysed hand pumps<br />
Analysed<br />
surface water
WATER USES<br />
• Human uses: drinking and food preparation � groundwater<br />
• Human uses: hygiene � groundwater and/or superficial water<br />
• Animal uses � superficial water<br />
CONTEXT<br />
Rural community in the South <strong>of</strong> Calcutta (India)
Rural community in the South <strong>of</strong> Calcutta (India)<br />
� GROUNWATER QUALITY<br />
• Arsenic: 100 mg/L<br />
• Turbidity: about 0 NTU<br />
• Microbiological: good quality<br />
� DATA<br />
• 1200 inhabitants<br />
• D = 20 L/inh/d<br />
• Sodium hypochlorite and Alum (aluminum<br />
sulfate, Al 2(SO 4) 3 • 14H 20) availability<br />
CONTEXT<br />
DESIGN OF ARSENIC REMOVAL PLANT BY OXIDATION,<br />
COAGULATION, SEDIMENTATION AND FILTRATION<br />
Q = 1200 inh x 20 L/inh/d = 24,000 L/d = 24 m 3 /d<br />
� > 90% → As OUT = 10% * 100 mg/L = 10 mg/L<br />
8
CONTEXT<br />
Rural community in the South <strong>of</strong> Calcutta (India)<br />
� CHEMICALS<br />
• Oxidants: sodium hypochlorite, NaClO<br />
• Coagulant: Alum (aluminum sulfate), Al 2(SO 4) 3 • 14H 20<br />
9
DESIGN OF OXIDATION and<br />
COAGULATION TANKS<br />
OXIDANT DOSAGE: NaClO<br />
• T ox= 1-2 min • V ox = Q x T ox = 24 m 3 /d x 2 min =<br />
= 24 m 3 /(24 x 60 min) x 2 min = 0.033 m 3 =<br />
• Oxidation test<br />
Dosage = 1-2 mg/L<br />
• 15% solution<br />
= 33.3 L<br />
= 0.3 kg/d<br />
ALUM DOSAGE: Al2(SO4) 3 • 14H20 • Tc = 1-2 min<br />
• Jar test<br />
Dosage=10-50<br />
mg/L<br />
• 12% solution<br />
• D NaClO = 2 mg/L = 2 g/m 3<br />
• C NaClO = Q x D/0.15 = 24 m 3 /d x 2 mg/L/0.15 =<br />
= 24 m 3 /d x 2 g/m 3 /0.15 = 320 g/d =<br />
• Vc = Q x Tc = 24 m 3 /d x 2 min =<br />
= 24 m 3 /(24 x 60 min) x 2 min = 0.033 m 3 =<br />
= 33.3 L<br />
• D Alum = 30 mg/L = 30 g/m 3<br />
• C coag = Q x D/0.12 = 24 m 3 /d x 30 mg/L/0.12 =<br />
= 24 m 3 /d x 30 g/m 3 /0.12 = 6,000 g/d =<br />
= 6 kg/d
DESIGN OF FLOCCULATION TANK<br />
FLOCCULATION<br />
DESIGN OF OXIDATION and<br />
COAGULATION TANKS<br />
Total Volume <strong>of</strong> the mixing tank<br />
• V = 33.3 L x 2 ~ 70 L<br />
• T f = 15-20 min • V f = Q x T f = 24 m 3 /d x 15 min = 25 m 3 /(24<br />
x 60 min) x 15 min = 0.25 m 3 = 250 L
DESIGN OF<br />
SEDIMENTATION TANK<br />
• T sed = 2-3 h • V sed = Q x T sed = 24 m 3 /d x 2 h =<br />
24/24 m 3 /h x 2 h = 2.0 m 3<br />
• Hydraulic loading<br />
rate: R h = 0.8 –<br />
1.5 m/h<br />
• Ased = Q/R h = 24/24 m 3 /h / 0.8 m/h = 1.25 m 2<br />
• fsed = (4 x Ased/∏) 0,5 = 1.3 m<br />
• Hsed = Vsed/Ased = 1.6 m
• Hydraulic loading<br />
rate: R h = 6 – 7 m/h<br />
• H = 1 – 2 m • H sf = 1 m<br />
FILTER BACKWASHING<br />
• R h = 15 – 30 m 3 /(m 2 x h)<br />
• Tc = 5 – 10 min<br />
DESIGN OF<br />
SAND FILTRATION<br />
• A sf = Q/ R h = 24/24 m 3 /h / 7 m/h = 0.17 m 2<br />
• f sf = (4 x A sf/∏) 0,5 = 0.46 m = 46 cm<br />
• Q = R h x A sf = 20 m/h x 0,14 m 2 = 2.8 m 3 /h<br />
• V = Q x Tc = 2.8 m 3 /h x 5 min = 2.8 m 3 /h x<br />
5/60 h = 0.23 m 3<br />
Backwashing frequency: from 1 time/d to 1 time/week (in relation with water<br />
quality and quantity)
ALUM SLUDGE<br />
• 1 mg alum produces<br />
0.44 mg sludge<br />
ALUM SLUDGE<br />
• Sludge = Q x 0.44 kg sludge/kg alum x D Alum =<br />
24 m 3 /d x 0.44 g sludge/g alum x 30 g/m 3 =<br />
= 316.8 g/d = 0.32 kg/d
PLANT EXAMPLE