Mechanical failures in potable reuse plants: component and system reliability considerations

Abstract

The aim of this thesis was to evaluate the reliability of potable reuse applications from a component and system perspective. Mechanical properties of polyethersulfone (PES) and polyvinylidenefluoride (PVDF) membranes are often used as a surrogate for component reliability. Accelerated ageing with sodium hypochlorite (up to 240,000ppmhr) resulted in a 6% decrease in tensile strength (2.44 to 2.30MPa) for PVDF compared to a 11% decrease (2.47 to 2.19MPa) for PES. Changes in mechanical properties may be interpreted as evidence of potential for failure, however, fibre strain measured in-situ was 60% below loads typically used in tensile tests. Strain, measured by microstrain gauges, was inversely proportional to fibre stiffness and increased at higher aeration intensities and modules’ fibre looseness. Lateral strain ranged from a minimum of 1.3x10-6 (fibre base, no aeration, 2% looseness) to a maximum of 1.2x10-4 (top of fibre, 7.35m3/m2hr, 8% looseness). Measured strains were within the elastic region of PES and PVDF fibres indicating failure results from fatigue. PVDF fibres performance in high cycle fatigue analysis (100,000 cycles) was superior to PES fibres which failed between 94,000 and 42,000 cycles. The significance of component failure was investigated by studying epidemiology data from failures in drinking water systems, selected due to a lack of data on potable reuse plant performance. Of 83 confirmed outbreaks between 2002 and 2012 only 16% were attributed to mechanical failure, which usually occurred in conjunction with a suite of other failure events including operational and maintenance practices. The ability of an Advanced Water Treatment Plant (AWTP) to cope with comparable failure events was assessed using stochastic techniques over a 10 year period for a reference plant, built using mechanical maintenance data from 7 full-scale AWTPs (64 years cumulative operating history). Over a 10 year period, mechanical failure resulting in compromised capacity (downtime) was significantly more frequent than events compromising quality (7.6 vs 2.1 events per year). The frequency of capacity failure events could be reduced through improved maintenance and increased storage capacity compared to increasing critical component redundancy (67% versus 2% reduction in events/year). Consequently, component reliability, while important, is less effective than procedures that enhance system reliability.