ENG421 (10abc) – Disinfection Micro-organisms Disinfection Disinfectant Concentration and Contact Time Disinfection – design considerations Disinfection – design examples

General Water Treatment Technologies (Week 4) Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

Micro-organisms (1 of 4) pathogenic micro-organisms any micro-organism capable of producing disease surrogate organisms (e.g. Escherichia coli, E.coli) used to indicate effectiveness of disinfection Week 1 revision : most common drinking water health risk contamination by human or animal excreta and the microorganisms contained in faeces recent contamination may cause communicable enteric diseases (diseases of the gut) drinking contaminated water or using it in food preparation may cause new cases of infection greatest risk of infection to : infants and young children people whose immune system is suppressed the sick the elderly Pathogenic (disease-causing) organisms of concern include : bacteria viruses protozoa diseases caused vary in severity mild gastroenteritis severe and sometimes fatal diarrhoea, dysentery, hepatitis, cholera or typhoid fever

Micro-organisms (2 of 4) - Week 1 revision (cont) : Giardia (protozoa) diarrhea, excess gas, stomach or abdominal cramps, and nausea, causing weight loss and/or dehydration Cryptosporidium (protozoan) upset stomach or acute, watery, and non-bloody diarrhoea nausea/vomiting and abdominal pain

Micro-organisms (3 of 4) Week 1 revision (cont) : Risk of disease from waterborne pathogens influenced by : • the concentration of pathogenic organisms in the water • the virulence of the strain • the per capita intake of contaminated water • the infectious dose of the particular pathogen • the susceptibility of individuals • the incidence of the infection in the community (which will determine the numbers of pathogens being excreted)

Micro-organisms (4 of 4) micro-organisms may be considered as suspended matter : Bacteria 0.5 – 5 μm Viruses 0.005 – 0.01 μm Protozoans 2 – 15 μm Most micro-organisms can attach to other suspended particles be enveloped by suspended particles Physicochemical treatment processes for suspended particle removal remove >98% of micro-organisms Direct filtration plants remove up to 99% of micro-organisms Conventional plants (properly optimised) remove up to 99.9% of micro-organisms Even at low concentrations micro-organisms may cause disease Filter “break-through” may occur micro-organism concentration (and suspended matter concentration) will rise

Disinfection (1 of 10) disinfection process removes, inactivates, destroys pathogenic micro-organisms usually final treatment in conventional treatment systems want to combat potential microbial contamination in distribution lines treated water leaves plant with residual disinfections in long transmission lines : add intermediate disinfectant injection physical means : ultra-violet light gamma radiation X-ray heat (boiling) chemical means chlorine (most widely used) chlorine dioxide chloramine hypochlorite bromine iodine ozone hydrogen peroxide (perozone) permanganate

Disinfection (2 of 10) Mechanism of disinfection depends on nature of disinfection and type of micro-organism four mechanisms :

Disinfection (3 of 10) Considerations in selecting disinfection process

Disinfection (4 of 10) many micro-organisms co-exist in water and have variable resistance to the disinfectants most resistant bacteria ↓ protozoans least resistant viruses within a group, resistance may vary e.g. protozoans Cryptosporidium oocysts very resistant to disinfectants (chlorine, chloramines, ozone) Giardia cysts much less resistant so, use surrogate or indicator organisms (E. coli, a bacteria) to measure effectiveness of disinfectant but, E. coli may die off before many pathogenic organisms to determine micro-organism levels in water, “plate counts” are performed regularly

Disinfection (5 of 10) Radiation ultraviolet and gamma waves may be used in small water treatment plants UV causes irreversible inactivation of DNA at wavelength 255 – 265 nm all micro-organisms must be exposed to UV otherwise non-UVed micro-organisms can replicate very effective for bacteria and viruses hindered by dissolved and suspended matter in water interferes with UV radiation reaching micro-organisms

Disinfection (6 of 10) Temperature the effect of temperature may be determined using :

Disinfection (7 of 10) Chemical Agents Chlorine chlorination is proven and reliable technique chlorine (Cl2) applied to water as aqueous solution or diffused directly in ammonium-free water hypochlorous acid (HOCl) and hypochlorite ion (OCl-) form composition (ratio) depends on pH and temperature hypochlorous acid (HOCl) is a better disinfectant than hypochlorite ion (OCl-) advantages cost effective residual chlorine (~ 0.5 mg/L) in water chlorine is efficient oxidant improves taste and odour control prevents algae and slime growth disadvantage chlorine reacts with organic compounds in water → formation of trihalomethanes (THMs, carcinogenic) avoid chlorine application in early treatment stages safer after suspended particle removal (fewer organics) best choice disinfection for organic free water

Disinfection (8 of 10) - Chemical Agents (cont) Chloramines chloramines are formed by adding chlorine and ammonia (NH3) to water three chloramines disinfect water NH2Cl, NHCl2, NCl3 less efficient than chlorine, but very effective good residual in water does not form significant quantities of THMs Chlorine Dioxide advantages more effective than chlorine and chloramines for bacteria and viruses effective at pH 8.5 – 9 minimises lead and other metal corrosion chlorine dioxide residual maintained in distribution system for some time disadvantages higher operating costs than chlorine formation of chlorate ion (ClO3-) and chlorite ion (ClO2-) possibly toxic

Disinfection (9 of 10) - Chemical Agents (cont) Ozone very powerful disinfectant depends on pH and temperature most widely used after chlorine advantages significant reduction in THMs formation low dosage requirement may produce ozone on-site few by-products disadvantages expensive (high capital cost) low solubility of ozone in water → bubble ozone through water unstable, decomposes in water → does not provide residual protection → use chlorine or chloramine as secondary disinfection (residuals) aldehydes (by-product) may effect human health contact time 3 – 10 minutes low dosage 1 – 2 mg/L

Disinfection (10 of 10) - Chemical Agents (cont) Ozone

Disinfectant Concentration and Contact Time (1 of 7) Disinfectant Concentration and Contact Time are prime considerations in disinfection as concentration goes up, micro-organism kill goes up as contact time goes up, micro-organism kill goes up low concentration and long contact produces similar results to high concentration and short contact time C.t (min.mg/L) : the product of concentration (C, mg/L) and contact time (t, min) identifies inactivation (kill) for each type of micro-organism contact time is not the same as hydraulic retention time contact time determined using a tracer study t10 : time taken for 10% of tracer to pass at effluent end is used

Disinfectant Concentration and Contact Time (2 of 7) in general, the longer the contact time the greater the kill (Chick’s Law)

Disinfectant Concentration and Contact Time (3 of 7) Contact Time (cont) modify Chick’s Law to include disinfectant concentration (Chick-Watson Model)

Disinfectant Concentration and Contact Time (4 of 7) C.t and “log-removal” disinfection efficiencies are compared in terms of log-removal log-removal is the absolute value of the logarithmic of the micro-organism fraction remaining after disinfection 99% inactivation = 2 log-removal 99.9% inactivation = 3 log-removal 99.99% inactivation = 4 log-removal 99.68% inactivation = 2.5 log-removal

Disinfectant Concentration and Contact Time (5 of 7) C.t and “log-removal” (cont) Performed chlorine is shock addition of sodium hypochlorite, Giardia is a protozoan

Disinfectant Concentration and Contact Time (6 of 7) C.t and “log-removal” (cont)

Disinfectant Concentration and Contact Time (7 of 7) C.t and “log-removal” (cont)

Disinfectant Mixing (1 of 3) disinfectant in liquid form is mixed with water in a rapid mixing arrangement disinfection takes place in subsequent contact chamber (tank) disinfectant mixing chambers :

Disinfectant Mixing (2 of 3) disinfectant mixing chambers (cont)

Disinfectant Mixing (3 of 3) disinfectant contact chamber (tank) must avoid dead spaces, short circuiting and dispersion → 1. high length to flow width ratio 70 – 80 : 1 200 : 1 would be ideal (space and cost considerations) use of baffles helps to reduce tank size 2. water height to flow width ratio < 1

Disinfection – design considerations (1 of 2) factors to consider : disinfectant dose mixing arrangements contact time contact chamber feeding systems ozone generator dechlorination system off gas destruction unit Most systems are very specific to source water characteristics and flow rate Hydraulic retention time dictated by C.t value C.t value affected by temperature, pH, micro-organisms As a result of above considerations, specific design criteria are not followed in disinfection system design

Disinfection – design considerations (2 of 2) As a result of above considerations, specific design criteria are not followed in disinfection system design Typical values :

Disinfection – design example 1 (1 of 2) Problem 1 : Determine the required chlorine dose for a relatively clean groundwater source with pH = 8. A 3 log-removal is required. It is 3 km from source to service area and peak flow is 1 m/s in the supply pipe. Solution : assume separate contact chamber not required as groundwater is clean only virus inactivation is required assume design safety factor of 0.7 (reduces residence time) residence time in supply pipe = 3000 m x 0.7 = 2100 sec = 35 min 1 m/s

Disinfection – design example 1 (2 of 2) Solution (cont): for 3 log-removal of viruses, at pH = 8 temp = 5oC C.t = 6 min.mg/L C = (6 min.mg/L)/(35 min) = 0.17 mg/L Use 0.2 mg/L dosage at entry point to supply line

Disinfection – design example 2 (1 of 4) Problem 2 :

Disinfection – design example 2 (2 of 4) Solution :

Disinfection – design example 2 (3 of 4) Solution (cont) :

Disinfection – design example 2 (4 of 4) Solution (cont) :

References Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D76 1997), pages 513 – 544 Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H46 2006) , pages 997 – 1042 Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K38 2000), pages 292 – 312 MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W375 2005), pages 1035 – 1161 Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E58 2009), pages 163 - 169 Parsons, S.A. and Jefferson, B., 2006, Introduction to Potable Water Treatment Systems, Blackwell, Oxford (TD430 .P37 2006), pages 128 – 139 Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages 424 - 447