Christchurch City Council
August 2009
Quality of Drinking Water
A good quality water supply has long been an
important feature for communities.
Dating as far back as the Roman era, Vitruvius, a Roman architect
and engineer in the first century, considered such factors as how
the water reacted when boiled, how vegetables reacted when
cooked in it and the health effects of the people who drank it to
determine the quality of the water.
Today, the Drinking Water Standards of New Zealand 2008 set the
requirements for the communities quality of water, and provides
a number of criteria to ensure that safe clean water is delivered to
consumers.
The Health (Drinking Water)
Amendment Act
In 2007 the Health Amendment Act made it mandatory for all
Drinking Water Supplies to comply with the Drinking Water
Standards. A period of grace was provided to drinking water
suppliers to allow time for the water supplies to be brought up to
the required standard. Supplies serving larger populations have
less time to comply with the standards than supplies serving
smaller populations.
It is a top priority of Christchurch City Council to provide
clean and safe drinking water that meets the Drinking
Water Standards to all homes and businesses in
Christchurch and Banks Peninsula.
Bacterial monitoring is regularly undertaken, in
accordance with the Drinking Water Standards, on all
Council operated water supplies, at the treatment plant
and in the distribution system.
Protozoal contamination
A proven method for removing or deactivating Protozoa, such as
Giardia and Cryptosporidium, is required by the Drinking Water
Standards. This is done at the water treatment plant and includes
treatment methods:
Membrane filtration, Ozonation, UV disinfection, Diatomaceous
earth filtration, Cartridge filtration, and others.
The treatment system selected depends on the risk of
contamination in the supply catchment and the quality of water to
be treated.
Chemical contamination
Chemical contamination of a water supply can be divided into
three main groups:
1. Inorganic Chemicals
“Maximum Acceptable Values” are provided in the Drinking
Water Standard for a long list of inorganic chemicals including
those that come from ground waters (boron, copper), from
geothermal flows (arsenic, mercury), from water treatment
processes (aluminium, chlorine) and from water corroding the
pipe material (nickel, antimony, zinc, lead, chromium).
This criteria is primarily achieved through ensuring that E-coli
(an indicator of bacterial contamination) is not in the drinking
water when it leaves the water treatment plant or in the pipework
transferring the water to homes and businesses (known as the
distribution network).
2. Organic Chemicals
These consist of pesticides and disinfection by-products. If
waters high in organics are chlorinated prior to filtration, or
if the filtration is insufficient to remove the organics, then
disinfection by-products can be formed. These disinfection
by-products, such as trihalomethanes, may be carcinogenic
at higher levels. For this reason the Drinking Water Standard
provides strict limits on the level of disinfection by-products.
The Council regularly tests for these parameters. Bore water
supplies generally have low levels of organics and are therefore
less susceptible to disinfection by-product formation.
To make sure the water leaving the treatment plant meets
the Drinking Water Standards, a selection from the following
treatments are used:
Chlorine, Chlorine dioxide, Ozone, UV, Or a higher frequency of
E-coli monitoring is required.
3. Aesthetic Determinands (taste and odour)
These are parameters that may cause a nuisance to the
consumer but are not a health risk. They include taste and
odour issues (sulphate, sodium), staining of laundry (iron and
manganese) or scale formation (hardness).
To make sure the water in the pipework itself meets the Drinking
Water Standards, the following is used:
Chlorine, or a higher frequency of E-coli monitoring is required.
Chemical tests are carried out annually by the Council, and
chemical determinands that are found to be higher than 50%
of the Maximum Acceptable Value are tested more frequently.
Drinking Water Standards Criteria
Bacterial contamination
Water Gradings
Chlorination
To provide an indication on the level of contamination risk to
a drinking water supply the Ministry of Health have derived a
grading system.
According to the World Health Organization “disinfection by
chlorine is still the best guarantee of microbiologically safe water.”
The purpose of grading the water supply is to “provide a public
statement of the extent to which the community drinking water
supply achieves and can ensure a consistently safe wholesome
product.”
Water Supplies that comply with the Drinking Water Standards
and provide adequate barriers to contamination and other risks are
scored higher than those that don’t.
The Grade is made up of two parts.
• Grading for the Source and Treatment (A1-E)
• Grading for the distribution system (a-e). An ‘A1’ or ‘a’ grade indicates a “negligible level of risk” while an ‘E’
or ‘e’ grade indicates a “very high level of risk”.
To achieve an A grade a disinfection residual (e.g. chlorine) must
be provided in the water supply.
Bore Water Supplies
Bore water supplies (i.e. water drawn from a ground water source)
can be given a ‘Secure’ status under the Drinking Water Standards.
Water from a ‘secure’ well does not need to satisfy the Bacterial
or Protozoal criteria of the Drinking Water Standards. Generally
to be ‘secure’, the water must be drawn from confined aquifers or
from unconfined aquifers greater than 30m deep. The well must be
water age tested, have a secure well head and have a record of no
contamination.
Storage Tanks
Many homes have storage tanks on-site with up to two days
water storage provided. Storage tanks are common sources for
contamination through overflow vents and access lids. Assuming
there is some turn-over of the water in the tank, the chlorine will
help to remove contaminants in the storage tanks to provide safer
water to the consumers tap.
Chlorination has been used since the 12th Century and is a proven
method for reducing contamination in the pipe work system. It
continues to remove the bacterial contaminants from the treatment
plant through to the consumers tap.
The Drinking Water Standards provides an aesthetic
guideline for chlorine of 0.6-1mg/L for the taste and odour
threshold. Christchurch City Council targets a residual of
0.2mg/L for the whole water supply system.
With this level of chlorine residual bacterial contamination is
unlikely to occur. The chlorine residual continues to decay with
the water age. Chlorine residuals have been well studied and found
to be safe to humans at this concentration. To remove chlorine
residual (if preferred because of a noticeable residual after the
storage tank) an under-sink carbon filter can be used.
The Council’s chosen method of chlorination is gas chlorination.
Using this method reduces operator input into the chlorination
system and increases the operator safety. All gas chlorination
systems will be fitted with chlorine sniffers and auto-shutoff valves.
According to the Water Quality and Health Council:
“Chlorine-based disinfectants are the only
disinfectants that provide lasting residual protection
to protect the water from waterborne disease
throughout the distribution system from treatment
plant to the consumer’s tap. Alternatives to
chlorination for primary disinfection such as ozone
or ultraviolet light can not provide this residual
protection. All chemical disinfectants produce
byproducts. Chlorination byproducts have been well
studied compared to the byproducts of alternatives
which are just beginning to be studied.”