DISINFECTION:

APPROPRIATE TECHNOLOGIES FOR

WASTEWATER TREATMENT

KA SAMSON

Presented at the APPROPRIATE TECHNOLOGY CONFERENCEBMW CENTRE, CAPE TOWN, 3-4 SEPTEMBER 2007

Kevin.samson@capetown.gov.za

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)

• concentration and the type of disinfecting agent/facility

• wastewater demand (e.g. FSA)

• 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

CAPE FLATS WWTP

MATURATION PONDS (1.2km2)FOR 10 -12 DAY RETENTION

DISINFECTION WITH SUNLIGHT

CALCIUM HYPOCHLORITE CHIPS

Calcium hypochloriteAnother disinfection system with operational issues

Gas chlorination

ATHLONE WWTW

MATURATION PONDS (90 000 m2)FOR 1.25 DAY RETENTION

Major pump stations feeding the ATHLONE WWTPUV & COLOUR RESULTS

Grab samples taken during the day when a yeast factory does not discharge trade effluent (08h00)

DATE ColourUV Trans

(1cm)Abs (1cm) Colour

UV Trans (1cm)

Abs (1cm)

Hazen % Hazen %20-Jun-03 200 4.2 - 80 22.4 - 8h0024-Jun-03 100 16.1 - 70 30.1 - 8h0025-Jun-03 80 17.4 - 70 30.3 - 8h0028-Aug-03 125 14.7 - 65 34.4 10h0003-Sep-03 125 25.7 55 41.2 10h00

Samples w ere f iltered through 0.45 um filters prior to analysis (unless stated otherw ise)

RAAPENBERGLANGA

WWTP Colour UV transmissivityHazen %

Athlone 90 43.6Cape Flats 50 57.9Potsdam ASP 55 52.9Potsdam BIO 65 50.2Bellville <50 62.3Mitchells Plain 65 59.3

Final Effluent

Assessing final effluent characteristics for possible UV disinfection technology

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

Estimated cost of civil work

Chlorine contact tank 1,200,000 1,200,000 1,200,000 Buildings and tanks 4,000,000 1,800,000 800,000 600,000 2,200,000 2,000,000 Water supply and roads 800,000 300,000 200,000 400,000 400,000 400,000 Seawater pumping scheme (15l /s) 4,500,000 Additional storage tanks (2 x 750 m³) 2,200,000 Return seawater pumping schemes 3,500,000 Contingency 600,000 1,350,000 220,000 100,000 260,000 240,000

Total Civil Cost (incl. Contingency) 6,600,000R 14,850,000R 2,420,000R 1,100,000R 2,860,000R 2,640,000R

Total Capital Costs 18,000,000R 25,250,000R 4,370,000R 17,600,000R 35,860,000R 23,140,000R

Annual Costs

I & R @ 12% 2,515,000 3,470,000 595,000 2,563,000 5,209,000 3,346,000 Power Costs 1,150,000 1,200,000 50,000 1,273,000 1,350,000 870,000 Salt and Acid 3,150,000 Potable water 410,000 Replacement and Consumables 620,000 770,000 800,000 400,000 400,000 Pumping costs (seawater) 60,000 Chlorine gas 3,980,000 Liquid oxygen 2,825,000

Total annual costs 7,845,000R 5,500,000R 4,625,000R 4,636,000R 6,959,000R 7,441,000R

Costs c/kl (based on ADWF) 20.5 c/kl 14.4 c/kl 12.1 c/kl 12.1 c/kl 18.2 c/kl 19.4 c/kl

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THE END

Source: UV Technology for Municipal WWTPs in Canada (2003)

COMPONENTS OF THE DIFFERENT UV LAMP TYPES

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