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Water utilities have become more reliant on Advanced Water Treatment Plants (AWTP) to produce  climate-independent sources of high quality water for use in industry or augmentation of drinking water supplies.  The AWTPs use a range of treatment processes including membranes, ultraviolet light and advanced oxidation systems and thus, when compared with traditional treatment technologies, such as gravity driven media filters and disinfection chambers, the AWTP processes are more mechanically complex and usually attended by higher capital, operating and maintenance costs.

The additional mechanical and process complexity is justified on the basis that the role of the AWTP is to reduce exposure to pathogens and chemicals in the plant source water to levels acceptable for industrial and potable reuse applications while producing a reliable supply of water. To meet these dual objectives of quality and capacity, AWTP designers include provisions for process redundancies and use Hazard Analysis and Critical Control Point (HACCP) techniques to control production and monitor product water quality.  The inclusion of redundant process equipment and instruments will reduce the risk of failure to meet water quality and water production targets.

However, it is often difficult to assess a priori how the plant will perform with respect to these objectives over the long term. Without knowing if the AWTP is more likely to fail on product quality, production capacity or both, it is not clear if the redundant systems for quality and capacity are over designed or under-designed. This uncertainty, often results in added cost and complexity. Consequently, it would be prudent to take advantage of methods used in other industries to quantify failure in AWT systems and evaluate the merits of different capital and maintenance based strategies to improve performance.

Water reclaimation utilising alternative, renewable and sustainable sources of water can be used to supply high-quality water to electronics manufacturing industries as well as augment drinking water sources via Indirect Potable Reuse (IPR) and Direct Potable Reuse (DPR) schemes (Seah et al., 2003). For these reuse purposes, a high emphasis is placed on the resilience of AWTPs as it is critical to upholding reliability of the water supply.

Resilience is defined as a system’s ability to maintain routine function even under unexpected circumstances (Gunderson and Pritchard, 2002), therefore, it is an essential factor to ensure that an advanced water treatment plant has continuous process throughput whilst remaining compliant with strict water discharge guidelines. Although resilience modelling tools have been widely used in the Petrochemical, Oil and Gas, and Aviation industries to model process reliability and safety, there has been no standard resilience modelling method developed for the water treatment industry.

The aim of the project was to use OPTAGON to develop a mechanical resilience model that could be used to quantify and accurately predict equipment failure and determine the resilience of a typical large scale AWTP. Results were analysed to quantify the resilience of a dual membrane plants and differentiate between failure events resulting in a loss of production capacity versus events resulting in a loss of quality.

Further analyses of the results, based on sensitivity cases, were used to assess strategies that could be implemented to improve the resilience of the AWTPs.

In order to simulate and predict a plant’s resilience, this study utilised DNVGL’s Monte Carlo-based Reliability, Availability and Maintainability (RAM) simulation tool, OPTAGON, to accurately determine and quantify the resilience of a representative Reference AWTP. OPTAGON has been tried, tested and proven in the Oil and Gas industry over the last 15 years and is capable of assessing process equipment availability, criticality and the plant’s overall resilience. The RAM simulations provided insights as to how equipment failures impacted the Reference plant’s critical performance metrics; process equipment availability (Production capacity) and product water compliance (Quality).

The OPTAGON resilience model consisted of two separate models; a “Capacity” model that analysed the throughput of the plant and a “Quality” model that assessed the number and duration of the events experienced by the plant in a span of 10 operational years that have the potential to impact water quality.