Redundancy and diversity - when is enough enough?

The redundancy and diversity which is built into Water Treatment Plant (WTP) design varies considerably between provincial and large urban water suppliers. The main drivers are:

• The acceptable level of risk associated with maintaining adequate water supply to the community,
• Meeting the quality required by the Drinking Water Standards of New Zealand and
• Cost

Failure of a critical element of a water treatment plant may compromise either water quality or the provision of an uninterrupted supply. Smaller communities may accept a higher level of risk due to a limit in available funding. Arguably, the consequence of any hazard increases when serving a large population, and decreases for smaller populations thereby changing the level of risk associated with that hazard. Therefore, depending on the way in which consequences are assessed, the risks for a small supplier are lower than for a large supplier.

Large water providers may opt for sophisticated, automated systems with redundancy and diversity which ensure that the water supplied meets quality standards and meets the demand at all times.

A small supplier may opt for a simpler, proven process which has less potential for failure. The system may have significantly less diversity and / or redundancy and the control system may automatically shut down on equipment failure / non-compliance. Although all mechanical and electrical equipment has a risk of failure, the more complex equipment requires much more rigorous testing, both by the manufacturer and during on site commissioning thereby increasing cost. When including redundancy in a design, consideration must be given to a number of factors including equipment failures, how the process will continue to operate while equipment is being maintained, and cost.

In the past, most WTPs used the conventional three stage treatment process comprising coagulation, sedimentation, and filtration. However, there are now a variety of proven technologies which has diversified the equipment available for water suppliers to use. Water suppliers must now consider the diverse treatment options available as well as equipment redundancy and the ever important costs of these when designing a new or upgrading an existing process.

This paper discusses the benefits of building redundancy and diversity into a process in the context of achieving the requirements of the current Drinking Water Standards of New Zealand, including a discussion of how the risks may vary depending on the size and type of population supplied. The design of a medium sized water treatment process which compares conventional three stage treatment and membrane treatment is presented in this paper as a case study.