13. Drinking Water Treatment Flashcards
What is so bad about bottled water? (5)
- The air pollution created by its manifacture and the fossil fuels burned in its distribution
- 25% of all plastic bottled water is cleaned up city tap water just like you have at home
- Plastic bottles are an environmental nightmare, taking 450 years to disintegrate
- Bottled water on average costs $1.50 per bottle
Drinking water purification (3)
- Wastewater treated by secondary methods is not yet potable, or safe for human consumption
- It requires further treatment to remove pathogens, eliminate taste and odor, reduce chemicals (e.g., iron, manganese), and decrease turbidity
- A typical drinking water treatment installation purifies raw (untreated) water
Pathogens found in water (types) (3)
- Bacteria
- Viruses
- Protozoa and algae
Types of bacteria found in water (4)
- Salmonella sp.: typhoid fever, gastrointestinal (GI) problems
- Shigella sp.: dysentery
- Escherichia coli : most strains are harmless, some cause GI problems
- Vibrio cholerae: cholera
Characteristic of viruses found in water
Enteric viruses
What is an enteric virus?
Coming from your GI tract, feces contamination
Examples of viruses found in water (2)
- Hepatis
- Polio
Examples of protozoa and algae found in water (4)
- Entamoeba histolytica: dysentery
- Giardia sp.: diarrhea, (beaver fever)
- Cryptosporidium parvum: diarrhea
- Algal blooms: toxins (rashes, nausea, etc.).
○ Red tides: blooms of dinoflagellates
○ Pfiesteria piscicida: fish kills (estuaries) and human illnesses
Coliform test (def)
Detects fecal contamination in water, waste-water
What are coliforms?
Coliforms are enteric rod-shaped Gram-negative non-spore forming and motile or non-motile bacteria which can ferment lactose
→ Presence of coliforms indicates fecal contamination
What are the three stages of the coliform test?
- Presumptive test: gas production in lactose broth (35oC)
- Confirmed test: gas production in brilliant green lactose bile broth
- Completed test:
○ Coliform colonies (metallic sheen) on Levine’s EMB agar
○ Gas production
○ Gram negative non spore-forming rods
○ Takes ~ 4 days
Other common methods to enumerate bacteria in water samples (2)
- Filter plate method (petri dish)
- Direct count method (microscopy)
- Fecal streptococci
- E. coli
Can drinking water contain fecal coliforms, fecal streptococci or E. coli?
Drinking water cannot have any of these above organisms, and should not have coliforms
Guidelines for microbial parameters (2)
- Currently available detection methods do not allow for the routine analysis of all microorganisms that could be present in inadequately treated drinking water.
- Instead, microbiological quality is determined by testing drinking water for Escherichia coli, a bacterium that is always present in the intestines of humans and other animals and whose presence in drinking water would indicate fecal contamination of the water
Bacterial Guidelines
E. coli or total coliforms
- The Maximum Acceptable Concentration (MAC) of Escherichia coli or total coliform in public, semi-public, and private drinking water systems is none detectable per 100 mL.
- Testing for E. coli and total coliform should be carried out in all drinking water systems. The number, frequency, and location of samples for E. coli testing will vary according to the type and size of the system and jurisdictional requirements.
Bacterial Guidelines
Heterophobic plate count
No MAC is specified for heterotrophic plate count (HPC) bacteria in water supplied by public, semipublic, or private drinking water systems. Instead, increases in HPC concentrations above baseline levels are considered undesirable.
Bacterial Guidelines
Emerging pathogens
- No MAC for current or emerging bacterial waterborne pathogens has been established.
- Current bacterial waterborne pathogens include those that have been previously linked to gastrointestinal illness in human populations.
- Emerging bacterial waterborne pathogens include, but are not limited to, Legionella, Mycobacterium avium complex, Aeromonas hydrophila, and Helicobacter pylori.
Bacterial Guidelines
Protozoa
- Giardia and Cryptosporidium
- It is not possible to establish MACs for these protozoa in drinking water at this time
- Routine methods available for the detection of cysts and oocysts suffer from low recovery rates and do not provide any information on their viability or human infectivity
Bacterial Guidelines
Viruses
- It is not possible to establish MACs for enteric viruses in drinking water at this time
- Where treatment is required, treatment technologies should achieve at least a 4-log reduction and/or inactivation of viruses
Lead contamination in drinking water guidelines
- Canadian: MAC < 5ppb
- Quebec: in 2019 changed from <10ppb to <5ppb
4 major steps of drinking water purification
- Sedimentation to remove particles
- Coagulation and flocculation form additional aggregates which settle out
- Filtration
- Disinfection: typically with chlorine, gas (ozone) or UV radiation
Two types of filtration processes
(step 3)
- Rapid filtration
- Slow sand filtration
Rapid filtration (3)
- Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand, traps organic C
- Fast filtration rates through media (sand or anthracite)
- Backwashing needed
Slow sand filtration (5)
- Are carefully constructed using graded layers of sand, with the coarsest sand, along with some gravel, at the bottom and finest sand at the top.
- Drains at the base convey treated water away for disinfection.
- Slow filtration rates through media (sand and gravel)
- Removal of biological layer needed
- Higher removal rates for all microorganisms
Why disinfect drinking water?
(Step 4)
Because 1.8 million people die from diarrheal diseases every year
What are the objectives of disinfecting drinking water? (2)
- Primary objective → kill, remove all pathogens
- Secondary objective → remove chemicals, contaminants, suspended solids and gases from drinking water
What are the 3 ways to disinfect drinking water?
- Chlorine
- Ozone
- UV irradiation
Chlorine
- Very strong oxidant with “residual activity” → meaning that when you add it it stays killing stuff for a long time in the water
- Sodium hypochlorite is commonly used to disinfect drinking water
- Inexpensive, relatively safe
- When dissolved in water it will slowly decompose, releasing chlorine, oxygen and sodium and hydroxide ions;
4 NaClO + 2 H2O → 4 Na+ + 4 OH− + 2 Cl2 + O2
What is the main advantage of using chlorine to disinfect water?
Organic molecules in drinking water become chlorinated forming: Trihalomethanes or THMs which are carcinogenic (DBPs)
What are some alternatives to chlorine? (3)
- Monochloroamine (NH2Cl)
- Dichloramine (NHCl2)
- Trichloramine (NCl3)
Pros (2) and cons (1) of these alternatives to chlorine
Much less THMs formed, very good residual activity, but less effective than chlorination against viruses, protozoa
Ozone as disinfection
Pros (1) and Cons (3)
- No THMs formed …
- But can produce bromate (carcinogen)
- But no residual activity … must be made onsite
How does ozonation work? (5)
- Ozonation helps disinfect the water
- The water flows from the dissolved air flotation tanks to the ozone chambers
- Liquid oxygen is pumped into an ozone generator to make ozone
- The ozone is added to the water to:
○ Improve the filter performance in the next treatment stage
○ Destroy most of the harmful bacteria
○ Improve the taste and odour of the water - At the end of the process, sodium bisulphite is added to the water to remove any leftover ozone
Ultraviolet light as disinfection
Pros (1) and cons (2)
- Effective
- No residual activity
- Optimization is difficult
How does ultraviolet disinfection work? (6)
- Purely physical, chemical-free process
- UV-C radiation (253 nm wavelength, damages DNA of bacteria / Giardia and Cryptosporidium cysts, which are extremely resistant to chemical disinfectants, are efficiently reduced
- Less effective against viruses
- UV also used to remove chlorine and chloramine species from water ; this process is called photolysis
- UV disinfection leaves no taint, chemicals or residues / byproducts in the treated water
- Difficult to optimize, UV light’s disinfection effectiveness decreases as turbidity increases
