Assessmentof On-line Particle Counters for Routine Control of
MicrobialPathogens at Water Treatment Plants
ReportNo WSAA 153
October 1999
It is thepurpose of this project to develop the necessary procedures and protocols toenable the routine use of particle counting technology in maintaining optimaltreatment plant operation and, hence, water quality within required guidelines.Of major concern is the possible transport of pathogen particles through thedeep bed filters under various water treatment plant operating regimes.Attention in this study is thus focussed on i) examining the calibration andoptimisation of particle counters for selected particles with particularemphasis given to Cryptosporidium, andii) examining the potential for breakthrough of pathogenic particles as aresult of operation of the contact water filtration water treatment process.
To this end, arange of literature, bench and pilot scale studies have been undertaken withthe aim of understanding the operation and implementation of particle countersin water treatment and, more specifically, assessing the capacity of particlecounters to detect the presence of particles in specific size ranges.
For routineon-line particle counting in a water treatment plant an obscuration sensorwould appear to offer the most cost effective sensor type.
The issue ofmaintenance of floc integrity is critical to the set-up of any counter in awater treatment environment. Special care should be given to sample pointplacement and construction, sample tubing and joints, choice of a non-pumpedflow controller, and proximity to electrical interference from high-loadequipment such as backwash pumps.
Although theobscuration counter is the cost-effective option for on-line operation thereexists tremendous scope in the laboratory and at the pilot scale for theapplication of more sophisticated equipment that provides a greater insightinto the mechanisms encountered in water treatment processes.
The sensors ofparticle counting instruments will soon be routinely calibrated with industrystandards for both size and counts. Following such calibration it is essentialthat the sensor be assessed for pathogen sizing using a water matrix that isrepresentative of that which will routinely pass through the sensor in a watertreatment plant.
The importanceof coagulant dose to the rate of floc growth and the subsequent relationship tofloc 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 fromcoagulant addition has a dramatic effect on their ability to capture specificallysized individual particles.
Humic acid wasfound to have a significant detrimental effect on the coagulation-flocculationprocess and although this is not a new finding the results served to emphasisethe complex nature of the effect of natural organic matter (NOM) on thecoagulation process. No doubt this effect will vary from water source to watersource.
Investigationsinto the filtration process have served to emphasise the knowledge gaps in thisarea, particularly with regard to the entrapment of flocs generated through theaddition of iron salts.
A major findingof the study was the lack of association between particle passage through thefilter and coagulant concentration in the finished water. The traditional viewof particle breakthrough at the end of a filtration run is that floc materialis the major constituent of such particulate material. However, if a filter isallowed to go through to filter breakthrough before backwashing and if iron isused as the dominant coagulant, it would appear that the particulate materialthat breaks through is discrete particles with little or no floc attached. Iftime or headloss triggers for backwashing govern, breakthrough is avoided.
The results ofpilot 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 wereentirely consistent with previously reported information on Cryptosporidium and filtration;
The datacollected using the Macarthur WFP pilot filter facility during the three briefseeding runs suggest the potential existence of useful correlations between Cryptosporidium oocyst removal andremovals measured in terms of one or more of the more easily measuredconventional performance parameters, turbidity, particle concentration, andaerobic spore concentration.
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