Water. desalination + reuse

TECHNOLOGY Product water quality has consistently met Australian Drinking Water Guidelines (ADWG) requirements. Analysis has indicated that total dissolved salts (TDS) performance is amongst those parameters likely to be the first water quality trigger for membrane replacement while also providing an indication of rejection performance for a range of parameters of interest. Figure 5 indicates the TDS performance of the plant relative to ADWG guideline value of 600 mg/L, showing that after 4 years of operation, membrane performance is still highly satisfactory. No significant increase in normalised differential pressure indicative of biofouling has been observed during Hot Standby. Chemical cleans have to date only occurred as part of short-term membrane preservation events related to maintenance. Regular permeate flushing of membranes and sound pretreatment performance have contributed to the positive performance of the plant under Hot Standby mode. SDI15 achieved recently during Hot Standby have averaged 2.6 relative to a 90th percentile plant limit of 4. In line with the above described water quality performance, membrane replacement has not yet occurred with the plant now in its fifth year of operation. Process Stability Due to the short operating time and the permeate flushing practice, it is challenging for the operational team to maintain stable reliable performance from online instruments. Figure 4: Membrane bed-in One example is that pH instruments have been changed to a different design which better accommodates both seawater operation and permeate flushing, as permeate is not able to dilute the KCl solution utilised by the instrument. A similar issue exists with ORP instruments but to date has not been solved, which means that keeping these instruments in calibration between Hot Standby runs still requires intervention. Because control loops take time to stabilise, parameters such as pH and chlorine are outside of desirable limits for approximately half an hour on start-up as shown by figure 6. This period of operation outside of limits is approximately 4% of the typical Hot Standby run duration of 15 hours. In continuous operation the proportion would be negligible but in a brief period of operation the effects on product water quality become greater. Also, commencement of chemical dosing is based on achieving significant flow rates of permeate into the remineralisation part of the process. So there is a period of lower flow permeate production at startup during which dosing does not occur. Again this would normally comprise a very low proportion of operating time in a continuous operation scenario but the issue becomes more significant in Hot Standby operation. Other risks are increased due to the low turnover of product including an increased tendency for lime system blockages and sodium hypochlorite degradation with the associated chlorate formation. AssET MANAGEMENT Cyclic Effects Intuitively, plants perform better when processes and parameters (primarily pressures) are stable due to the reduced dynamic stresses on the system. However, there has been no evidence to support this theory in operation of the plant under Hot Standby to date. The risk of unforeseen outcomes related to the stop/start cycle needs to be monitored into the future: by definition the likelihood of an undesirable outcome will increase with time and any failure root cause analysis will need to consider whethermore stop/starts could be a contributing factor. Figure 5: TDS performance | 34 | Desalination & Water Reuse | August-September 2013 Corrosion Operation of the GCDP intermittently has created a situation in which filtered seawater is much more likely to be left

• Cover