3.3.1 Preventive measures and multiple barriers

Summary of actions

• Identify existing preventive measures from catchment to consumer for each significant hazard or hazardous event and estimate the residual risk.

• Evaluate alternative or additional preventive measures where improvement is required.

• Document the preventive measures and strategies into a plan addressing each significant risk.

Identifying and implementing preventive measures should always be undertaken within the context of a multiple barrier approach, so that failure of one barrier will be compensated by effective operation of the remaining barriers. This minimises the likelihood that contaminants will pass through the entire treatment system to be present in sufficient amounts to cause harm to consumers.

Traditional preventive measures are incorporated as or within a number of barriers, including:

• catchment management and source water protection

• detention in protected reservoirs or storages

• extraction management

• coagulation, flocculation, sedimentation and filtration

• disinfection

• protection and maintenance of the distribution system.

The types of barriers required and the range of preventive measures employed will be different for each water supply and will generally be influenced by characteristics of the source water and surrounding catchment (see Box 3.2). Selection of appropriate barriers and preventive measures will be informed by hazard identification and risk assessment.

Box 3.2

Examples of multiple barriers

Large parts of Melbourne are supplied with high-quality source water from a highly protected catchment. Melbourne Water focuses much of its attention and resources on maintaining prevention of contamination at the source. The series of barriers for the majority of the water supply system include:

• protected forested catchments for harvesting of water with no human or livestock access

• large catchment reservoirs with long detention times

• additional retention time in seasonal storage systems

• disinfection of water before it enters the distribution system

• closed distribution systems.

In contrast, Adelaide is supplied with surface water derived from multi-use catchments and the Murray River, where there is limited control over activities with potential impacts on water quality. As a result, the barriers applied are heavily weighted towards water treatment and downstream control to remove turbidity and microorganisms. Barriers include the use of multiple storage reservoirs, coagulation, flocculation, sedimentation, filtration and disinfection with long contact times before supply.

Provision of residual disinfectant through large parts of the distribution system is also an important barrier for both systems.

Catchment management and source water protection

Catchment management and source water protection provide the first barrier for the protection of water quality. Where catchment management is beyond the jurisdiction of drinking water suppliers, the planning and implementation of preventive measures will require a coordinated approach with relevant agencies such as planning authorities, catchment boards, environmental and water resources regulators, road authorities and emergency services.

Effective catchment management and source water protection include the following elements:

• developing and implementing a catchment management plan, which includes preventive measures to protect surface water and groundwater

• ensuring that planning regulations include the protection of water resources from potentially polluting activities, and are enforced

• promoting awareness in the community of the impact of human activity on water quality.

Whether water is drawn from surface catchments or underground sources, it is important that the characteristics of the local catchment or aquifer are understood, and the scenarios that could lead to water pollution are identified and managed. The extent to which catchment pollution can be controlled is often limited in practical terms by competition for water and pressure for increased development in the catchment.

Effective catchment management has additional benefits. By decreasing contamination of source water, the amount of treatment and quantity of chemicals needed is reduced. This may lead to health benefits through reducing the production of treatment by-products, and economic benefits through minimising operational costs.

In surface water catchments, preventive measures can include:

• selection of an appropriate source water (where alternatives exist)

• exclusion or limitations of uses (e.g. restrictions on human access and agriculture)

• protection of waterways (e.g. fencing out livestock, management of riparian zones)

• use of planning and environmental regulations to regulate potential water-polluting developments (e.g. urban, agricultural, industrial, mining and forestry)

• use of industry codes of practice and best practice management

• regulation of community and on-site wastewater treatment and disposal systems

• stormwater interception.

Groundwater from depth is generally microbiologically safe and chemically stable; however, shallow or unconfined aquifers can be subject to contamination from discharges or seepages associated with agricultural practices (pathogens, nitrates and pesticides), septic tank discharges (pathogens and nitrates) and industrial wastes. Preventive measures for groundwater supplies should include protecting the aquifer, the recharging surface water and the local area around the borehead from contamination and ensuring the physical integrity of the bore (surface sealed, casing intact etc).

Further information on integrated catchment management is provided in Appendix Section A1.6 Preventive Measures and Multiple Barriers (Box A1.1) and in the National Water Quality Management Strategy: Implementation Guidelines (ARMCANZ, ANZECC 1998).

Detention in reservoirs or storages

Detention of water in reservoirs can reduce the number of faecal microorganisms through settling and inactivation, including solar (ultraviolet) disinfection. Most pathogenic microorganisms of faecal origin (enteric pathogens) do not survive indefinitely in the environment. Substantial die-off of enteric bacteria will occur over three to four weeks. Enteric viruses and protozoa will survive for longer periods (weeks to months).

Detention also allows suspended material to settle, which makes subsequent disinfection more effective and reduces the formation of disinfection by-products.

Other preventive measures in reservoirs and storages include:

• reservoir mixing or destratification to reduce growths of cyanobacteria (taste, odour and toxin production)

• excluding or restricting human, domestic animal and livestock access

• diversion of local stormwater flows.

Extraction management

Where a number of water sources are available, there may be flexibility in the selection of water for treatment and supply. In such a situation it may be possible to avoid taking water from rivers and streams when water quality is poor (e.g. following heavy rainfall) in order to reduce risk and prevent problems in subsequent treatment processes.

Within a single water body, selective use of multiple extraction points can provide protection against localised contamination, either horizontally or vertically through the water column (e.g. cyanobacterial blooms).

Coagulation, flocculation, sedimentation and filtration

Coagulation, flocculation, sedimentation (or flotation) and filtration remove particles, including microorganisms (bacteria, viruses and protozoa). It is important that operations are optimised and controlled to achieve consistent and reliable performance.

As an alternative to conventional media-based processes, membrane filtration provides a direct physical barrier and generally achieves a greater removal of microorganisms.

Care should be taken in the selection and use of water treatment chemicals as they may contain undesirable contaminants. In addition, there can be variation in performance of the same chemical obtained from different sources.

Disinfection

The most commonly used disinfection processes are chlorination and chloramination, but ozone, ultraviolet irradiation and chlorine dioxide are also used. These methods are very effective in killing bacteria and can be reasonably effective in inactivating viruses (depending on type) and many protozoa, including Giardia. Cryptosporidium is not inactivated by the concentrations of chlorine and chloramines that can be safely used in drinking water, and the effectiveness of ozone and chlorine dioxide is limited with this organism. However, there is some evidence that ultraviolet light might be effective in inactivating Cryptosporidium, and that combinations of disinfectants can enhance inactivation.

Storage of water after disinfection and before supply to consumers can improve disinfection by increasing contact times. This can be particularly important for microorganisms, such as Giardia and viruses.

Providing a disinfectant residual throughout the distribution system can provide protection against contamination and limit regrowth problems; however, the issue of disinfection by-products needs to be considered. Chloramination has proved successful in controlling Naegleria fowleri in water and sediments in long pipelines.

Protection and maintenance of the distribution system

Water distribution systems should be fully enclosed and storages should be securely roofed with external drainage to prevent contamination. Backflow prevention policies should be applied and monitored. There should also be effective maintenance procedures to repair faults and burst mains in a way that will prevent contamination. Positive pressure should be maintained throughout the distribution system. Appropriate security needs to be put in place to prevent unauthorised access to, or interference with, water storages.

Corrosion of pipes, including those on customer premises, can result in leaching of metals, with implications for public health (e.g. copper, cadmium and lead) or aesthetic quality (e.g. copper, iron and zinc). This should be monitored.

Growth or persistence of biofilms should be minimised to reduce aesthetic problems, including off-tastes, odours and staining.

Adequate training of maintenance workers, including contractors, responsible for the distribution system is essential because of the potential for contamination during repairs and recommissioning.