Transcript CITY OF CAPE TOWN WASTEWATER DEPARTMENT

DISINFECTION:
APPROPRIATE TECHNOLOGIES
FOR
WASTEWATER TREATMENT
KA SAMSON
Presented at the APPROPRIATE TECHNOLOGY CONFERENCE
BMW CENTRE, CAPE TOWN, 3-4 SEPTEMBER 2007
[email protected]
INTRODUCTION
WASTEWATER TREATMENT IS ONE OF THE FEW INDUSTRIES
WHICH HAVE LIMITED INFLUENCE OVER THE QUALITY AND
QUANTITY OF THE INCOMING RAW MATERIAL
THE PRIMARY PURPOSE OF WASTEWATER TREATMENT IS TO
ENSURE THE SAFE DISCHARGE OF THE TREATED EFFLUENT
INTO THE ENVIRONMENT IN ORDER TO PROTECT PUBLIC
HEALTH AND THE ENVIRONMENT
IMPORTANCE OF DISINFECTION: it is the last barrier protecting
the receiving water bodies and/or users from pathogenic
organisms
Disinfection can be achieved through the application of heat,
light, oxidizing chemicals. Some technologies are:
•
•
•
•
Sunlight (natural disinfectant)
Oxidizing chemicals: chlorine, ozone, chlorine dioxide,
hypochlorites (calcium & sodium), hydrogen peroxide
UV (Ultra-violet light) using mercury vapour lamps
Membranes
Effective disinfection of wastewater is influenced by:
•
•
contact time (dependent of type of disinfectant)
•
•
wastewater demand (e.g. FSA)
•
concentration and the type of disinfecting agent/facility
flow rate (average vs peak)
concentration of the interfering substances (TSS,COD,FSA)
e.g. typical Cl2 dosage for ww is 2-10mg/l vs 0.2 to 2 mg/l for
potable
water
To determine the appropriateness of the disinfection
technology for a specific WWTP, the following needs
to be considered:
•
•
•
•
•
•
•
•
influent wastewater characteristics
effectiveness of the treatment process
impact on the receiving environment
risks associated with the technology
costs (O&M and CAPEX)
simplicity of operation
reliability and effectiveness
maintenance & the degree of skills required
DISINFECTION WITH SUNLIGHT
CAPE FLATS WWTP
MATURATION PONDS (1.2km2)
FOR 10 -12 DAY RETENTION
CALCIUM HYPOCHLORITE CHIPS
Another disinfection system
with operational issues
Calcium hypochlorite
Gas chlorination
ATHLONE WWTW
MATURATION PONDS (90 000 m2)
FOR 1.25 DAY RETENTION
Assessing final effluent characteristics for possible UV disinfection technology
WWTP
Athlone
Cape Flats
Potsdam ASP
Potsdam BIO
Bellville
Mitchells Plain
Final Effluent
Colour
UV transmissivity
Hazen
%
90
43.6
50
57.9
55
52.9
65
50.2
<50
62.3
65
59.3
Major pump stations feeding the ATHLONE WWTP
UV & COLOUR RESULTS
Grab samples taken during the day when a yeast factory does not discharge trade effluent (08h00)
DATE
Colour
20-Jun-03
24-Jun-03
25-Jun-03
28-Aug-03
03-Sep-03
Hazen
200
100
80
125
125
LANGA
UV Trans
(1cm)
%
4.2
16.1
17.4
14.7
25.7
Abs (1cm)
-
RAAPENBERG
UV Trans
Colour
Abs (1cm)
(1cm)
Hazen
80
70
70
65
55
Samples w ere filtered through 0.45 um filters prior to analysis (unless stated otherw ise)
%
22.4
30.1
30.3
34.4
41.2
-
8h00
8h00
8h00
10h00
10h00
ESTIMATED COST COMPARISON OF VARIOUS DISINFECTION SYSTEMS
(PRESENT DAY PRICES MAY 2007)
Description
OSEC (Salt)
OSEC (Seawater) Chlorine Gas
U-V(35%)
Ozone (PSA) Ozone (LOX)
Disinfection Plant
Nett Cost (incl. Contingency)
11,400,000
10,400,000
1,950,000
16,500,000
33,000,000
20,500,000
1,200,000
4,000,000
800,000
1,200,000
1,800,000
300,000
4,500,000
2,200,000
3,500,000
1,350,000
1,200,000
800,000
200,000
600,000
400,000
2,200,000
400,000
2,000,000
400,000
220,000
100,000
260,000
240,000
Estimated cost of civil work
Chlorine contact tank
Buildings and tanks
Water supply and roads
Seawater pumping scheme (15l /s)
Additional storage tanks (2 x 750 m³)
Return seawater pumping schemes
Contingency
600,000
Total Civil Cost (incl. Contingency)
R
6,600,000 R
14,850,000 R
2,420,000 R
1,100,000 R 2,860,000 R
2,640,000
Total Capital Costs
R 18,000,000 R
25,250,000 R
4,370,000 R 17,600,000 R 35,860,000 R 23,140,000
Annual Costs
I & R @ 12%
Power Costs
Salt and Acid
Potable water
Replacement and Consumables
Pumping costs (seawater)
Chlorine gas
Liquid oxygen
Total annual costs
Costs c/kl (based on ADWF)
2,515,000
1,150,000
3,150,000
410,000
620,000
3,470,000
1,200,000
595,000
50,000
770,000
60,000
2,563,000
1,273,000
5,209,000
1,350,000
3,346,000
870,000
800,000
400,000
400,000
3,980,000
2,825,000
R
7,845,000 R
20.5 c/kl
5,500,000 R
14.4 c/kl
4,625,000 R
12.1 c/kl
4,636,000 R 6,959,000 R
12.1 c/kl
18.2 c/kl
7,441,000
19.4 c/kl
Membrane Filtration
Solids
Virus
Air
Bacteria
Treated effluent
Membrane
0.04-0.4um
MEMBRANE TECHNOLOGY
S ource: TechnicalMem oradum T M-1WQ subm itt ed by CT E/ AE COM
DIS INF ECTI ON S TUDY
ME TROP OLI TAN WAT ER RE CLAM AT ION DI ST RICT OF GRE ATE R CHI CA GO
MA S TER PLA N: NORTH S IDE WA TE R RECLA MA TI ON P LANT, 2005
The reason why disinfection is so important!
THE END
COMPONENTS OF THE DIFFERENT UV LAMP TYPES
Source: UV Technology for Municipal WWTPs in Canada (2003)
UV DISINFECTION (… in general)
• One installation in Cape Town - at the Potsdam WWTP
• Principal parameter controlling the effectiveness is the UV dose
• UV Dose (mW.s/cm2 or mJ/cm2)= Intensity (mW/cm2) * contact time (s)
• Recommended dose = > 30 mJ/cm2 or UVT > 50%
• Usually installed in the effluent channel eliminating the need for a
contact tank - short contact time required  2 sec to 15 sec
• Lamp technologies: low pressure/low intensity (LP/LI), LP/HI, MP/HI
• Power consumption: 87.5W per lamp
• Lamp operating life: 12000 hrs