Assessment of On-Line Particle Counters for Routine Control of Microbial Pathogens at Water Treatment Plants
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report No UWRAA 153
Purpose of the Project
It is the purpose of this project to develop the necessary procedures and protocols to enable the routine use of particle counting technology in maintaining optimal treatment plant operation and, hence, water quality within required guidelines. Of major concern is the possible transport of pathogen particles through the deep bed filters under various water treatment plant operating regimes. Attention in this study is thus focussed on i) examining the calibration and optimisation of particle counters for selected particles with particular emphasis given to Cryptosporidium, and ii) examining the potential for breakthrough of pathogenic particles as a result of operation of the contact water filtration water treatment process.
To this end, a range of literature, bench and pilot scale studies have been undertaken with the aim of understanding the operation and implementation of particle counters in water treatment and, more specifically, assessing the capacity of particle counters to detect the presence of particles in specific size ranges.
Summary of Key Findings
For routine on-line particle counting in a water treatment plant an obscuration sensor would appear to offer the most cost effective sensor type.
The issue of maintenance of floc integrity is critical to the set-up of any counter in a water treatment environment. Special care should be given to sample point placement and construction, sample tubing and joints, choice of a non-pumped flow controller, and proximity to electrical interference from high-load equipment such as backwash pumps.
Although the obscuration counter is the cost-effective option for on-line operation there exists tremendous scope in the laboratory and at the pilot scale for the application of more sophisticated equipment that provides a greater insight into the mechanisms encountered in water treatment processes.
The sensors of particle counting instruments will soon be routinely calibrated with industry standards for both size and counts. Following such calibration it is essential that the sensor be assessed for pathogen sizing using a water matrix that is representative of that which will routinely pass through the sensor in a water treatment plant.
The importance of coagulant dose to the rate of floc growth and the subsequent relationship to floc size and the ability of the floc to entrap 5m latex spheres are areas worthy of further investigation. Recent work also suggests that the fractal nature of flocs generated from coagulant addition has a dramatic effect on their ability to capture specifically sized individual particles.
Humic acid was found to have a significant detrimental effect on the coagulation-flocculation process and although this is not a new finding the results served to emphasise the complex nature of the effect of natural organic matter (NOM) on the coagulation process. No doubt this effect will vary from water source to water source.
Investigations into the filtration process have served to emphasise the knowledge gaps in this area, particularly with regard to the entrapment of flocs generated through the addition of iron salts.
A major finding of the study was the lack of association between particle passage through the filter and coagulant concentration in the finished water. The traditional view of particle breakthrough at the end of a filtration run is that floc material is the major constituent of such particulate material. However, if a filter is allowed to go through to filter breakthrough before backwashing and if iron is used as the dominant coagulant, it would appear that the particulate material that breaks through is discrete particles with little or no floc attached. If time or head loss triggers for backwashing govern, breakthrough is avoided.
Cryptosporidium oocyst seeding runs conducted using the Macarthur WFP pilot filter facility were effective in measuring the Cryptosporidium removal performance of the pilot filter under a limited range of operating conditions;
Cryptosporidium oocyst log reductions observed during the three seeding runs, (3.1 to 3.7-logs) were comparable to those reported previously for similar conditions;
The results of pilot filter performance evaluation for the removal of Cryptosporidium oocyst by filters of the specific water quality, coagulation, and filter design and operating characteristics of this study were entirely consistent with previously reported information on Cryptosporidium and filtration;
The data collected using the Macarthur WFP pilot filter facility during the three brief seeding runs suggest the potential existence of useful correlations between Cryptosporidium oocyst removal and removals measured in terms of one or more of the more easily measured conventional performance parameters, turbidity, particle concentration, and aerobic spore concentration.