city speak xii - water we drink: bevis mak of aecom

City Speak - The Water We Drink:

Water Treatment Technology and Demand Management

Bevis MakExecutive Director, Water, AECOM

15 May 2010

From Source to Tap – How Does It Work?

• A brief history of water treatment

• What is in our raw water?

• Treatment philosophy and method

• Sustainable water resources planning– Demand management– New water sources: Seawater– New water sources: Reuse

• Conclusion

A Brief History of Water Treatment

Historical Development of Water Treatment (1)

• 1676: Microorganisms found under microscope

• 1746: First patent for a filter design. Commercial product in 1750 consisting sponge, charcoal and wool

• 1804: First treatment plant in Scotland using filters

• 1854: first report case tracing a terrible epidemic of cholera due to contamination of water by a cholera victim recently returned from India

• 1931: Virus identified by electron microscope

Historical Development of Water Treatment (2)

• 1930s: With filtration and chlorination, no more waterborne disease outbreak in US

• 1950-80: Major improvements in cost effective treatment design:- clarification + filtration

• 1974: Chlorine + organics in raw water = trihalomethanes + other carcinogens (DBP disinfection by-products)

• 1993: Outbreak in Milwaukee identified a new organism: cryptosporidium and giardia (C&G) [54 deaths, 700+ ill; False alarm in Sydney in1998]

• 1990s: Advanced treatment technology - membranes, ozone and UV

Giardia (4-14 micron)

Cryptosporidium (4 - 6 microns)

What is in our raw water?

Our Water

• Source water from rivers, lakes, groundwater reservoirs

• Many inorganic, organics and biological matters in raw water

• Inorganics : calcium, magnesium, sodium, potassium, carbonate, sulphate, chloride, nitrogen,

phosphorus, iron, manganese etc

• Organics: mostly natural decomposition of plant and animal material, + synthetic (industrial and agricultural)

• Biological: bacteria, viruses, algae, other micro-organisms (protozoans such as C&G etc, helminths etc)

Particles in Water – How dirty does it look?

• Finely divided solids not generally distinguishable by the unaided eye

• Mainly from soil weathering processes and biological activities

• Potential absorption sink for toxic substances such as heavy metal and DBP

• Can cause strong scattering of incident light and leads to degradation of visibility

• Clear water < 1NTU; runoff with sediment 40+ NTU

Treatment Philosophy and Method

World Health Organization (WHO) “Guidelines for Drinking-water Quality” 2006 (WHO 2006)

• … Securing the microbial safety of drinking-water supplies is based on the use of multiple barriers, from catchment to consumer, to prevent the contamination of drinking-water or to reduce contamination to levels not injurious to health. Safety is increased if multiple barriers are in place…

• Impounding reservoirs - self cleaning and dilution

• Treatment processes – minimum 2 processes, and each process enhances efficiency of next process

Our Water Sources

• 30% local catchment water and 70%+ Dongjiang water

• Reservoirs:– Local catchment with excellent water quality: High Island, Shek Pik,

Tai Tam, Kowloon and Aberdeen Groups of Reservoirs– Local catchment with partial mixing by Dongjiang water: Tai Lam

Chung, Shing Mun Reservoirs– Main storage for Dongjiang water: Plover Cove Reservoir

• Most major treatment plants treats Dongjiang water direct, eg, Sha Tin Water Treatment Works handles 40% of supply daily

• Quality of Dongjiang water affects treatment options

What Are the Key Concerns in Our Water?

• 5 most “difficult” parameters:– Avoid DBP formation (health) – oxidant / dosing point– Ammonia (health) – oxidation, biological– Manganese (aesthetics, brown water) - oxidation– Turbidity (health and aesthetics) – clarification + filtration– 4 log (99.99%) removal/reduction of C&G (health) – advanced

treatment needed

• What’s in Dongjiang water:

Parameter Acceptable Historic High Max. Av.Ammonia 0.05 4.69 2.14 0.05

Manganese 0.05 1.25 0.72 0.05

Post 2003 at STWTW (in mg/L; ppm)

Conventional Treatment Approach

• Soluble organics / inorganics: chemical reaction to make them insoluble [Mn2+ → Mn4+ (MnO2)]

– Ozone if available (chlorination no long practiced)

• Insoluble particles – make them stick and bigger, carry with them the micro-organism and other toxic substance, and settle out faster – Addition of alum at pH 6.0-6.5 (polymer optional)

• After most organics are removed in above process, add chlorine for oxidation if needed to avoid formation of DBP

• Finer particles are removed by filtration

• Chlorine added for final disinfection

The Process Diagram

Before and After Clarification

Solids formed after coagulation and flocculation

Traditional Clarifiers

Filtration

• Dual media

• Biofilters

Site Utilization – When Land Becomes An Issue

• Traditional clarifiers robust and stable

• Gravitation, with minimum energy

• But they are slow with large land intake

• Can they be more efficient?

Clarification Process Foot Print

Conventional100%

Solids Contact - 40 %Superpulsator - 17%DAF - 14%DensaDeg - 10%Adsorption Clarifier - 6%Actiflo - 3%

• Proprietary systems are smaller, but are more energy and/or chemical intensive

• Long term O&M cost

C&G Treatment

• Mainly from human and animal waste pollution

• Can cause serious gastrointestinal problems to immuno-compromised population: 54 death and 700+ ill in Milwaukee 1993

• 4-log removal of C&G required for new WTW built after 2003

• Conventional treatment (clarification + filtration) inadequate: either by membrane (replacing clarification+filtration) or addition of ozone/UV disinfection

• Not a major problem for Hong Kong

How’s Our Treated Water Like?

• Treated Water Quality Standards:– Primarily WHO Guidelines 2006– USEPA standards– EC standards

• WSD samples regularly:– 4 Microbiological parameters– 92 Chemicals of health significance– 15 other parameters

• Full compliance based on the annual average of monitoring data in accordance with international practice

Sustainable Water Resource Planning

Sustainable Water Resources Planning Portfolio

Conservation Reuse

Desalination

Groundwater Surface Water

Brackish WaterSea Water

GeneralIndirect PotableDirect Potable

http://www.pbase.com/monkeylove/photo_a_day


Demand Management

Water Demand Management – Conserve What We Have

• Education on water conservation: good practices on use of water and water savings

• Promotion on use of water saving devices

• Leakage control – pressure management and replacement of aging pipes

• Extension on use of sea water for toilet flushing


New Water Source –Seawater Desalination

Seawater Desalination – Thermal Process

• Not quality or salinity dependent

• Multi-stage systems with heat recovery

• Much more efficient than the 1973 system but still significantly more expensive than membrane system; viable only co-locate with power plant

Photo from: WSD “Water for a Barren Rock”

Completed 1976

Demolished 1992

Seawater Desalination – Thermal Process

Al Hidd MED Plant, Bahrain (273 ML/d)

Seawater Desalination – Reverse Osmosis Membrane Process

• A membrane is like a paper filter, a physical barrier, but holes are much smaller, and is designed to take much higher pressure

• Latest material: Polyvinylidene Difluoride (PVDF)

• Pore size/ Pressure– Micro/ultra filtration (MF/UF)– Nanofiltration (NF)– Reverse osmosis (RO)

4 to 6 micron0.1 micron pore size

0.0001 0.001 0.01 0.1 1 10 100mm

hairCrypto-sporidium

smallest micro-

organism

polio virus

Suspended solids

Parasites

Bacteria

Org. macro. molecules

Viruses

ColloidsDissolved salts

Sand filtration

Microfiltration

Ultrafiltration

Nanofiltration

Reverse Osmosis

Membrane Pore Size Smaller than Most Microorganisms

MF/UF: -0.2 to 2.0 bar (1 bar = 10m depth of water)NF: 5 to 9 barRO: 10 to 100 bar

Seawater Desalination

• All membrane units are VERY expensive; so is energy cost

• Many contaminants in seawater will cause fouling:

– Some contaminants can precipitate on the membrane surface

– Microrganisms can grow on the membrane surface– Some chemicals can chemically degrade the membranes

• Must have good pre-treatment and screening: Normally MF/UF + RO

• RO cannot completely eliminate boron; second pass RO needed

Process Diagram

Pilot Testing of Desalination in Hong Kong

• WSD conducted pilot testing of sea water desalination at Tuen Mun and Al Lei Chau

• At Tuen Mun – dilution from Pearl River but seasonal variations and organics increase bio-fouling

• At Ap Lei Chau – higher salinity but more consistent seawater quality


New Water Source –Reclamation / Reuse

Water Reclamation / Reuse

• Treated secondary effluent is a major water source

• Technologically, easier to treat than seawater – more consistent quality

• Australia, California, Singapore – indirect potable or industrial use using MF/UF + RO, similar to seawater desalination

• Public acceptance is a major obstacle

Water Reclamation / Reuse – Hong Kong Applications

• Ngong Ping: Sand filtration + UV after secondary treatment

• Shek Wu Hui STW Pilot Scheme: MF/UF + chlorine disinfection after secondary treatment

• Water used for irrigation, toilet flushing and water features

Summary and Conclusion

Conclusion

• Technology has an important role to play in – providing a safe drinking water, and– Tapping into new water sources

• There is a price tag to new technology:– Higher capital cost of more expensive equipment– Higher energy and chemical cost because of higher level of

treatment– Higher maintenance cost because of expensive spares and

replacements– Higher carbon footprint

To put the numbers into perspective: O&M Cost(Process +power / maintenance / staffing)

* high cost due to high membrane replacement cost against small production/demand]

There is a price to be paid!

O&M Cost Power Only

Conventional $0.16/m3 $0.02/m3

Desalination (MF+RO) $4.0/m3 $1.8/m3

Reuse (MF+Cl2)* $5.2/m3 $0.8/m3

But …we do not wish to see this again!

So, conserve now!

Thank You