Lecture 17: Measuring water quality Flashcards
(21 cards)
problems with drinking water treatment could occur if there was:
- pollution of source water
- overload filtering part of the water treatment - increased demand for treated water
- more water has to be put through which means purification could become inefficient, so need to put more chlorine in at the end to still meet the residual concentration at the end of the process - out of date pipes and plant
- prone to cracking, have water and sewage running side by side, so if they crack could combine - biofilms in pipes
- can occur naturally, common in inside of distribution pipes, not often associated with pathogens/microbes but could be - inefficient treatment programme
- contamination after treatment
- when does it become the householders responsibility to look after the water
what is an example of an inefficient treatment programme
A massive outbreak in Milwaukee of Cryptosporidium infection transmitted through the public water supply
what happened during the Milwaukee infection
- turbidity increased due to high rainfall, when turbidity rises the treatment plant becomes less effective
- at the time of the outbreak the USA regulation was the NTU was= <0.5 95% of the time
what is maximal turbidity measured by
nephelometric turbidity units
how does protozoa, Cryptosporidium parvum work
= a water borne pathogen (gets into water supply and results in gastro symptoms)
- animal reservoirs –> gastrointestinal tract of cattle and sheep, therefore widespread in the environment
- cryptosporidium enter human intestinal cells, causing frothy, watery diarrhoea and abdominal cramps
- after a phase of sexual reproduction, Cryptosporidium form oocysts –> a very thick walled form, makes them resistant to chlorine
- should be removed by sand filtration and UV radiation
why contamination occurred in Milwalkee ..
- pathogen was present in untreated source water
- sources = cattle farming, freezing works, human sewage (didn’t protect their source water)
- inadequate treatment processes, recycling backwash water
- spring rains and snow melt compounded problem
- mechanical faults
what was the difference between Havelock North and Milwaukee
Havelock north = contamination of untreated water
milwaukee = treated water
recommendations for preventing Havelock North and Milwaukee
- a high standard of care must be embraced
- protection of source water is of paramount importance
- maintain multiple barriers against contamination
–> recognises that having more than one barrier between consumers and possible sources of pollution reduces the likelihood of contaminated water being supplied, if one barrier fails, the others remain in place
important aspects when collecting a sample of water
- Sample must be representative of the whole body of water
- Must use a sterile container
- Try to be aseptic
- Microbes tend to be on the surface, so don’t sample from surface bc might get increased microbes which isnt representative, so take from middle of the middle
- Open facing down river, then turn it round 180 degrees once its under the surface (technique for in a moving river)
- Needs to be transported in a cool, dark (don’t want photosynthesis) container
- Sample must not be frozen, as it might cause injury or death to the microbes
- Need to sample at several different places
Biochemical oxygen demand (BOD)
measures demand for O2 by bacteria during degradation of organic matter, and indicates the concentration of organic compounds in water
- only interested in biodegradable part of the organic material
- microbes oxidise organic material, use up oxygen which is what we are measuring
- sample is put in the dark (no photosynthesis) and measured after 5 days
Advantages and disadvantages of biochemical oxygen demand (BOD)
advantages = only measures biodegradable portion
disadvantages = 5 days is a long time to wait for something as important as water quality
chemical oxidation demand (COD)
- potassium dichromate oxidises organic compounds CO2 and H20
advantages = quick
disadvantages = does not differentiate between biodegradable and non biodegradable portions
why don’t we look for pathogens when looking for bacterial indicators of faecal pollution
- many genera
- to many bacteria to look at individual tests for each one - special culture conditions required
- eg: different nutrients, different gaseous environment - cannot be cultured
- would miss these - present in low numbers
- could miss these as well - too late
- once the testing is done and you have come with a positive result, the water has probably been in circulation for many days and could be contaminated
how to test for faecal contamination
use an indicator organism
ideal indicator organism
- bacterial inhabitant of only large intestine
- because then we know these has been fecal contamination - non-pathogen
- normal microflora - easy to culture and identify
- need to process water sample rapidly - present in large numbers
- indicator, preferably in larger numbers than pathogen - survives longer in water than pathogens but does not reproduce in water
what indicator comes very close to the 5 requirements
–> Coliforms - Family Enterobacteriaceae
- aerobes and faculatative anaerobes
- gram negative
- nonspore forming, rod shaped
- ferment lactose, with gas
- within 48hrs at 35 degrees
eg: escherichia
–> Faecal coliforms
- coliforms which ferment lactose at 44.5 degrees
- usually E.coli
problems associated with measuring water quality
–> disadvantages of using indicator bacteria
- could fail to detect disease-causing viruses
- doesnt detect risk from toxic algae
- E.coli may have rapid die-off rates in certain waters
- viable but non culturable (VBNC) indicator bacteria could lead to erroneous conclusions
–> Can occur to some bacteria if they are subjected to stress, end up with false negative if bacteria isn’t culturable but in reality it is there
what is the impact of clustering of microorganisms
- bacteria like to naturally group together
- if there was a cluster inside our sample = much higher number of bacteria than your expecting
- outside of the sample = not representative of the whole sample, get a much lower number of bacteria than expected
tests for indicator bacteria, drinking water standards for NZ 2005 (revised 2018)
–> preferred method is the enzyme substrate coliform test (colilert) (ONPG-MUG)
ONPG = o-nitrophenyl-B-D-galactopyranoside
- coliforms produce B-galactosidase which acts on ONPG–> yellow
MUG
E.coli produces B-glucuronidase which acts on
MUG –> fluorescent blue
results;
clear =negative control
yellow = coliforms present
fluorescent blue (under UV light) = faecal coliforms (E.coli) present
what are acceptable lvls of drinking water
<1 E.coli/100ml
<1 (oo)cyst/100 L
interpretation of treated drinking water
–> coliforms present, faecal coliforms absent
- tells us the water is safe to drink, but there might have been some soil contamination, could be post treatment but still safe
–> faecal coliforms present
- not safe to drink, need to work out why faecal colifroms are present, did it occur within the water treatment, in the source treatment or post treatment
–> non-pathogenic bacteria in water
- councils will test for individually, if enough people complain they will investigate different types of bacteria which could be there, also carefully look at pH of the water
