Bearspaw Water Treatment Plant Actiflo Discharge (Calgary, AB)

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The Importance of the Multiple Barrier Approach for Managing the Extremes

The increased human, industrial, and agricultural development along our surface water sources has contributed to increased risk of sediment loading, chemical, and biological contaminants present in our water supplies. Even our socalled pristine water sources are increasingly susceptible to upsets caused by variable and extreme weather events resulting from climate change. When these occur, a single barrier treatment approach is not sufficient enough to protect human health from water-borne pathogens or contaminants.

What are multiple barriers

Safeguarding against microbial presence in drinking water supplies is based on the use of multiple barriers from catchment to consumer, and reliably reducing contamination to safe public health levels. These “source-to-tap” barriers include protecting water resources, properly selecting and operating a series of treatment steps, managing distribution systems, and monitoring. All of these measures help to maintain and protect treated water quality.

Source-to-tap protection starts with controlling contaminants from entering the watershed and water source and reducing the loading and reliance on downstream barriers. As an example, for sources on the Eastern Slopes in Alberta, a collaborative approach is required with ranchers and farmers with respect to livestock controls, waste management, and chemical usage, as well as agreements with those involved with resource, land, and industry development within our watersheds.

Following source water protection, the next steps are physical barriers or processes within water treatment plants, such as physical removal and disinfection of contaminants. Treatment processes (or barriers) may include chemical conditioning, coagulation, flocculation, sedimentation, filtration, and disinfection. It is important that processes are optimized and monitored to achieve consistent and reliable performance. Chemical coagulation is the most important step in determining the removal efficiency of coagulation, flocculation, and clarification processes. It also directly affects the removal efficiency of granular media filtration units and has indirect impacts on the efficiency of the disinfection process. With proper design and operation, either granular media or membrane filtration can be effective barriers for pathogenic micro-organisms.

Application of an adequate concentration of disinfectant is the most essential barrier for treatment systems to achieve the necessary level of microbial risk reduction. Measures to minimize unwanted disinfection by-product formation should also be taken into consideration. The most commonly used disinfection process is chlorination. However, ultraviolet irradiation, chloramination, ozonation, and application of chlorine dioxide are also used.

Applied in the right combinations or with other processes, these disinfection methods are very effective in killing bacteria, inactivating viruses, and inactivating protozoa, including Giardia and Cryptosporidium. Storing water after disinfection and before supplying it to consumers improves the effectiveness of disinfection by increasing disinfectant contact times. This can be particularly important for more resistant micro-organisms, such as Giardia and some viruses.

Water entering the distribution system must be microbially safe and biologically stable. Maintaining a disinfectant residual throughout the distribution system is required to provide an additional barrier against recontamination and microbial growth within the distribution system.

Why multiple barriers are so important

The greatest microbial risks are associated with ingesting water that is contaminated with faeces from humans or animals (including birds), which are a source of pathogenic bacteria, viruses, and protozoa. Microbial water quality is often inconsistent and varies rapidly and over a wide range. Of particular concern are storm events which result in increased surface runoff into our water supplies. Short-term peaks in pathogen concentration may considerably increase disease risks and may trigger outbreaks of water-borne disease. Thus, while monitoring is useful to establish baseline conditions, it is important to always have effective treatment barriers in place.

High turbidity events caused by intense rainfall can stress or overwhelm individual treatment processes and mask microorganisms from the effects of disinfection. The additional risks associated with infrastructure failures or human error during extreme events emphasizes the importance of multiple barriers to mitigate the risks of pathogen or contaminant breaking through, and prevent pathogens from entering into treated water and the distribution system.

Other concerns include chemical contaminants in our water sources such as pesticides, arsenic, lead, nitrate, selenium, and uranium. There is also increasing concern over various trace chemical contaminants that are not fully removed by most current treatment processes. Contaminants such as lead or microbial pathogens can be introduced into the water during storage and distribution of the water from the treatment plant to the consumer. Conditioning the water appropriately at the treatment plant helps to protect the water against these risks.

The strength of the multiple barrier approach is that when one barrier is compromised, effective operation of the remaining barriers minimizes the likelihood of contaminants passing through the entire system and being present in sufficient amounts to cause harm to consumers. This approach has successfully been applied at many water treatment plants. The recent addition of ballasted flocculation at the Bearspaw and Glenmore Water Treatment Plants helped the City of Calgary maintain production of safe drinking water during the 2013 extreme flooding events.

About the author:

Doug Olson, P.Eng. is Senior Vice President, Water for the Associated Engineering group of companies. He is a Senior Process Engineer with 25 years of experience and specializes in multiple barrier water treatment approaches.