Water Quality and Treatment Flashcards
Why is drinking water quality important
- To protect the consumers health
- To be sure that the water is acceptable
- To protect the distribution system
Water quality characteristics
- Physical- may affect acceptability, treatment or indicate possible contamination
- Chemical- may affect health, acceptability, aquatic life, treatment or distribution
- Biological- usually applies to flora or fauna in natural water and used for assessing pollution
- Microbiological- organism too small to be seen by the naked eye, many will be harmless but some may be pathogens
Quality issues which may need treatment
- Physical- suspended solids in surface water
- Chemical- arsenic in groundwater
- Microbiological- removal of pathogens
- Aesthetic- unacceptable taste, odour, colour
What do many countries base their water quality standards off
WHO drinking water guidelines- most recent 4th edition in 2011. They are only guidelines but many countries base their standards off these
What do the WHO guidelines provide detail on
- Microbial aspects
- Chemical Aspects
- Radiological Aspects
- Acceptability aspects
Transmission categories for water related diseases
- Water Borne
o Diseases where pathogens are ingested in water
o E.g., Cholera and Typhoid
o Transmission relates to the quality of water drunk - Water-Washed/Water-Scarce- a number of possible pathogen routes
o Ingested because of poor hygiene- faecal-oral route
o Will affect the skin, e.g. scabies or the eyes e.g. trachoma because people can’t wash themselves regularly
o Infections carried by lice, e.g. typhus - Water-Based
o Pathogen spends part of its life in an aquatic animal
o All are caused by parasitic worms
o E.g. Bilharzia and Guinea Worm - Insect-Vector Route
o Diseases carried by insects that breed in or bite near to water
o E.g. Malaria, River blindness
What does the F diagram show
Faecal Oral transmission routes
Primary chemical water quality concerns
- Arsenic
- Fluoride
- Nitrate/Nitrite
- Mercury, Lead, Selenium, Uranium (Heavy Metals)
- Pesticides
- Salinity
- pH and Hardness
- Iron and Manganese
Diseases caused by chemicals in drinking water
- Geogenic (naturally occurring)
o Arsenic- Arsenicosis
o Fluoride- Fluorosis - Anthropogenic
o Nitrate (main source agriculture and wastewater)- methemoglobinemia
o Lead-plumbism (lead poisoning)
o Pesticides – poisoning
How should water samples be stored and for how long before testing
- Within 2 hours if samples are kept in a cool dark place without refrigeration
- Within 6 hours if samples are chilled rapidly to 4 degrees
Cold temperatures prevent reproduction, and dark places prevent bacteria being killed by UV light. Any sample which has been kept for more than 24 hours after collection should be discarded.
Why do we use indicator organisms rather than testing for specific pathogens
When testing water we don’t test for specific pathogens, as there are so many different types of pathogens which could be present in water, and doing so would be lengthy and expensive. Also testing for specific pathogens would fail to show the presence of other pathogens. Instead we use indicator organisms.
Characteristics of an indicator organism
- Indicate that faecal contamination has occurred
- Relatively easy to culture and detect
- Present in faeces in large numbers
- Not particularly harmful, but indicate that far more dangerous organisms may be present
- Principal bacteria used as indicators of faecal pollution: the coliform family
- Total coliforms can be cultured at 37 C but not all are of faecal origin
- Some coliforms can grow at 44 C and ferment lactose to produce acid, these are thermotolerant coliforms
What is turbidity
- Most visible aspect of contamination
- Optical property
- Suspended solids and colloids
- Also includes pathogens and algae
How do you test turbidity
- Testing for turbidity involves filling a tube with the sample until you can no longer see that pattern at the bottom of the tube
Issues with turbidity
Pathogens can adhere to solid particles, which inhibits disinfection
Limitations of water quality testing
- Analysis only gives a snapshot of the current situation
- Small discrete samples- what about the wider area
- Limited information on the causes of contamination
- Also need to consider transport and storage- need to consider behaviour not just hardware
Critical parameters for water treatment
- Thermotolerant coliforms- because it is an indicator for faecal contamination
- Turbidity of Water- many bacteria are attached to suspended soils and these can inhibit the disinfection process
- Disinfectant Residuals- usually chlorine, the chlorine left over once it has reacted with the pathogens. If there was no residual left would assume not all pathogen had been reacted with, and there is no ongoing disinfection throughout the pipe network
- pH- affects the disinfection effectiveness
Stages of Water Treatment
Sedimentation
Filtration
Disinfection
How to remove larger particles
Screening/Sedimentation/Filtration
How to remove colloids
Coagulation with Chemical (aluminium sulphate), creates small flocks in the water which are then large enough to settle out as solids
How to remove ions/molecules
Reverse Osmosis/Distillation
Which is easier to improve bacteriological quality or chemical quality
- Improving the bacteriological quality is relatively easy
- Improving the chemical quality is more difficult and expensive
Continuous vs batch treatment
- Continuous= flow in and out is steady
- Batch= treat a certain volume, then treat the next volume
Where it is continuous, parallel treatment facilities required to maintain flows during cleaning, maintenance or breakdown
How do sedimentation tanks work
Particles tend downwards due to gravity and settle at the bottom. Valve at the base to drain sludge and settled particles. Outlet weir allows a constant flow of water
How do air flotation plants remove sediment
uses micro air bubbles to attach and float suspended solids to the water surface for removal. The floated sludge is then removed periodically
How does the slow sand filter work
- Continuous filtration process
- Filtered through layers of sand and gravel
- Schmutzdecke- complex biological film that grows naturally on the surface of the sand
- Reduces flow through filter, so need periodic maintainance to disturb filter, but then reduce treatment efficacy
- The sand itself doesn’t do any filtration function but simply acts as a substrate
- Water rises in collection chamber
- Very slow process
- Multiple methods of pathogens being deactivated
How do nano filters work
- Some dissolved chemicals can be converted to insoluble forms by oxidation
- Fluorides can be passed through ion exchange resins or specific filter media to remove the dangerous chemicals
- Many chemicals require expensive and energy intensive treatment methods, such as reverse osmosis and multi-stage distillation
Methods of dissinfection
- Chlorination
o Most widely used as it also provides residual protection against future contamination
o Not effective for Giardia and Cryptosporidium - UV
o UV inactivates pathogens including Giardia and Cryptosporidium
o Requires energy, no residual protection - Ozone
o More effective oxidant than chlorine, short reaction time
o Effective for Giardia and Cryptosporidium
o No residual protection
Why do we need household water treatment
- Deals with post-collection contamination of water taken from a water source that may be of good quality
- Reduce the health risk of consuming water known to be of poor quality
- Only water used for drinking and washing uncooked food needs to be treated- no wastage of resources
Advantages of household water treatment
- Improve microbiological water quality
- Significantly reduce disease
- Cost effective
- Quickly implemented
- Close, natural sources of raw water can be treated to provide drinking water instead of households having to rely on distant, communal potable water supplies
Considerations of what type of household water treatment to undertake
- Household Decisions
- Short/Long Term Context
- Sustainability
Common stages of household water treatment
- Sedimentation (if necessary)
- Filtration (if necessary)
- Disinfection (if pathogens are not removed by filtration)
Methods of pathogen inactivation (x7)
- UV
- Predation (other bacteria eating pathogens)
- Time (if they are deprived of nutrients or natural cycle)
- Desiccation (lack of water)
- Temperature (boiling inactivates pathogens)
- pH/Chlorine
- Starvation
Not all pathogens are inactivated by all methods
Methods of household sedimentation
- Storage allows sedimentation. Pathogens, particularly those that are attached to suspended particles will settle out
- The storage period can be a few hours to a few days. As well as pathogens settling out, some will die as the conditions are not right for their survival
- Three pot System- each day transfer water to next pot after 24 hours cleaning out sediment as you go. Partial treatment, but can’t guarantee pathogen free
- Jerry Cans
- Animal Skin- allows some evaporation so water kept cool
- Clay Pot
household flocculation
- Chemical assisted sedimentation by adding a coagulant to the water and stirring slowly, suspended particles clump together as flocs which settle faster
- Requires subsequent straining through a cloth
- There are both commercial products and natural coagulants
Household filtration
- Best used after reducing the turbidity to some extent- straining or sedimentation
- Flow through filter usually by gravity, but sometimes pressurised by a hand pump
- Can be combined with:
o Natural biological layer that removes pathogen
o Chemicals that inactivate pathogens - Slow rates of filtration usually lead to the best result
- Filters need appropriate cleaning as performance reduces overtime
How do bio filters work
- Uses principles of slow sand filtration, scaled for intermittent household use
- Processes
o Mechanical trapping between the sand particles
o Adsorption on the surface of the particles
o Predation by natural bacteria
o Natural death to kill of pathogens - Requires correct operation and maintainance to be effective
- Tends to be more sustainable in areas where there is plentiful supply of water
- May Still need disinfection
How do household ceramic filters work
- The size of the pores created in the porous ceramic is key for their effectiveness
- Some ceramic filters are impregnated with colloidal silver that acts as a biocide or include activated carbon
- Pot filters
o A ceramic pot is filled with water that seeps through the walls of the pot into a receptacle below
o Candle filters- Hollow ceramic cylinders sealed at one end with an outlet at the other
How do membrane filters work
- The process is classified by the size of the pores in the membrane. For water treatment the category is usually ‘ultrafiltration’ which uses pore size of around 0.01 micron or less
- Most membranes operate by gravity which means filtrations takes place at a slow rate so requires storage of the raw water and treated water. A few methods use a small hand-operated pump to increase the rate at which the water can be treated
Methods of household disinfection
Household Disinfection
* Boiling
o Bring to a rolling boil for 1 minute should kill or inactivate most pathogens
o Expensive in relation to the fuel costs
* Solar Disinfection
o Uses the UV component of sunlight and rise in temperature
o Leaving clear bottles in sunlight for 6 hours
* Chlorination
o Can be added in the form of tablets, powders and solutions
o Challenge is applying the right dose for varying qualities of water
o Some pathogens are resistant
* UV Disinfection, using electrically powered lamps
Limitations of household water treatment
- No magic bullet- depends on raw water quality
- Prevention better than cure
- Applied without knowing whether or not it is really working
- After disinfection faecal coliforms not valid as indicators of pathogen removal
- Depends on high level of awareness and personal responsibility
