P8.4.2.1 Fouling of GAC by bulk organic matter, or other contaminants, in the water. Ensure treatment processes removing organic matter are in place, and functioning properly, prior to the GAC bed. TOC concentration. Treatment achieves little or no reduction in TOC. Implement, or optimise, upstream processes for removing natural organic matter. P8.4.2.2 Too little GAC in the bed to continue to provide adsorption until planned GAC replacement. Ensure any GAC lost from the contactor/ filter bed is replaced. Ensure initial GAC bed depth is sufficient for the planned number of bed-volumes of water to be run through it, using calculations and pilot trials. TOC concentration. GAC bed Treatment achieves little or no reduction in TOC. Add GAC to the bed. Carry out calculations and trials, if this has not already been done. Determine the maximum backwash rate that can be tolerated without significant loss of GAC and limit rates to this value. Reduce backwash rate. Event: GERMS GROW IN THE GRANULAR ACTIVATED CARBON BED Possible hazards: Germs4 Level of risk: Moderate–high5 P8.4.3.1 Poor quality water passing through the GAC. Provide treatment to improve the microbiological quality of the water, and its organic matter content, prior to GAC treatment. quality of water before and after treatment. Heterotrophic count higher following the GAC bed than before. Ensure there is a disinfection stage following the GAC. 1 Too little PAC may result in less natural organic matter being removed from the water before the disinfectant is added. This is likely to lead to increased concentrations of disinfection by products. 2 The risk associated with this is probably low except in situations in which reliance is placed on the activated carbon to remove algal toxins. In these instances the level of risk should be regarded as high. 4 The germs may be free in the water or carried on fine carbon particles. 5 The consequences of the event, and therefore the level of risk, will be influenced by the efficacy of subsequent disinfection processes. Contingency Plans If an happens despite preventive and corrective actions you have taken, you may need to consult with the Medical Officer of Health to assess how serious a problem is.
What to measure or observe: Coliforms [by first stage in E. coli monitoring by enzyme substrate method]; or Heterotrophic Plate Count (HPC). organic carbon (TOC). Disinfection by-products. Complaints of taste and odour from consumers. How often: When the first stage [coliform] in monitoring E. coli in water leaving the treatment plant using the enzyme substrate method indicates the presence of significant numbers of coliforms. TOC monitoring is for process control, and can be carried out at a frequency considered appropriate by the supplier (at monthly). What to do with the results: Results need to be recorded to meet legislative requirements or to allow water safety plan performance assessment. The WINZ database is good for this. The collected data need to be periodically reviewed to see whether problems with this supply element are developing. This should be done as frequently as the manager responsible considers necessary to minimise risk to public health arising from this supply element. Should this review show any unusual incidents, indicate that proper procedures are not being carried out, highlight poor laboratory results or indicate that poor water quality is reaching customers, then review the procedures for managing the trace organics removal process. Evaluate the monitoring results to see if the water safety plan needs modification – eg, preventive measures are up to date; and changes to the trace organics removal process are recognised in the plan. Responsibility: Manager designated responsible for the water supply.