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Water Treatment Plant (WTP) Optimization Techniques

Technical Document

Introduction

Given the seemingly ubiquitous, yet episodic nature of pathogen presence in source waters, monitoring and optimizing the performance of water treatment plants is an increasingly important aspect in many suppliers’ plans for maintaining water quality¹.

Filtration Plant Optimization (FPO) - The Filtration Plant Optimization (FPO) analysis was developed by the EPA to replace the modified Microscopic Particulate Analysis (MPA) procedure that had been used to assess filter plant performance². These methods are similar in that both involve concentration of large volume water samples using ten-inch (10") cartridge filters from the treatment plant’s raw and finished water. These samples are analyzed and the data compared to assess the efficiency of removal of biological particulates due to treatment. Laboratory analysis involves elution of the particulates from the filter, concentration of the eluant by centrifugation and microscopic examination. The FPO procedure includes separating a portion of the filter extract and processing it according to a modified Information Collection Rule (ICR) method for Giardia and Cryptosporidium.

Bacteria and viruses are not detected by the FPO analysis. However, results from the FPO analysis do indicate the number and type of other microorganisms (i.e., protozoa, algae) present in several size classes. This allows WTP managers an opportunity to review the performance of a particular filter, coagulation and flocculation scheme, etc. and a means of measuring performance improvement that result from optimization efforts.

Analytical Services, Inc. (ASI) has offered FPPE/FPO analyses since 1989. We supply sampling equipment and complete, detailed sample collection instructions and prompt turnaround time on the analyses. After you receive your report, our senior technical staff is available to review the tabulated results and log removal (by size class) data with you. We can also help you interpret the significance of these data in terms of WTP performance and the removal of Giardia and Cryptosporidium.

Laser Particle Counters (LPC) - Laser particle counters (LPCs) count, size and categorize discrete particles and are a substantial improvement over nephlometers (turbidity meters) which simply give an indication of total particulate load based on light scatter³. LPCs operate either on light blockage or light scatter principles, but both techniques generate quantitative data on the number and size of particles in the sample. LPCs can be installed on-line in a WTP, with sensors on each filter or the total influent and total effluent can be monitored. Other types of LPCs are stand-alone units and are capable of analyzing grab samples, typically using a syringe-type sampling device. The primary advantage of LPCs is their ability to develop real time, quantitative data regarding the number and size of particulates in the water and the removal due to the filtration process⁴. The disadvantage of LPCs is that they are unable to identify the particulates being counted. In addition, LPCs, which are calibrated using latex spheres, have been shown to size biological and non-biological particulates differently. This is because biological particulates, being composed of mostly water, have a refractive index very different than a solid particle. ASI offers LPC on discrete bulk water samples and can supply prepared bottles, coolers and sample collection instructions.

Bacillus & Other Aerobic spores - The use of total aerobic spores or Bacillus spore removal efficiency as a measure of overall filtration plant performance is a relatively new technique. The WTP performance assessment technique depends upon the assumption that sufficient quantities of Bacillus spores will be present in the untreated water to allow demonstration of the plant’s full removal capability. Influent and effluent samples are collected, analyzed and the resulting data are compared. For Bacillus spore counts, sample collection is relatively simple; two (2) 500-mL grab samples are collected in disinfected plastic bottles. Laboratory processing involves heating the sample to kill vegetative bacteria, which would otherwise interfere with the colony counts, and to induce the spores to germinate. The samples are then plated on nutrient agar (with soluble starch and blue dye), incubated and the colonies enumerated. The total number of colonies indicates the number of aerobic spores present in the sample; colonies with cleared zones around them, indicating the presence of amylase (an enzyme which degrades starch), are presumptive indicators of Bacillus spores. ASI offers aerobic spores/Bacillus spore counts for our water supply clients located throughout the United States.

Zeta Potential (ZP) - Zeta potential, unlike the above methods, is not a means of assessing WTP performance. ZP is a measure of the ions absorbed on colloidal particles in the water. In most natural waters, the net ionic charge (ZP) on a colloid is negative. Because of their like electrical charges, these particles do not readily form conglomerates or flocculate into larger particles which would settle out or be easily removed by filtration. However, with the addition of cationic coagulants or polymers, the net ionic charge or zeta potential can be reduced to near zero, where the maximum agglomeration will occur. Analyzing the ZP of samples collected during different coagulant dosing schemes allows for optimization of coagulation and filtration. ZP samples should be collected in sterile, 250-mL plastic containers and shipped refrigerated by overnight delivery. ASI routinely performs ZP analysis for drinking water clients throughout the US.

Zeta potential is measured using a Zeta-Meter, which passes an electrical current through a water sample contained within a cell. Colloids in the sample with a net negative charge will gravitate towards the anode (positive pole); conversely, colloids with net positive charges will move towards the cathode (negative pole). The rate of movement is an indication of the strength of the charge on the colloids. The particles are viewed with a microscope to determine the relative speed of migration and the ZP is calculated from the rate of movement.

References

1. Barsotti, M.G., et al. 1997. Using a Filtration Plant Performance Evaluation to Assess Treatment Effec-tiveness. NEWWA, 61:201-211.

2. USEPA. 1996. Microscopic Particulate Analysis (MPA) for Filtration Plant Optimiza-tion. EPA-910-R-96-001.

3. O’Shaughnessy, P.T., et al. 1997. Evaluating Particle Counters. JAWWA, 89:60-70.

4. Hargesheimer, E.E., et al. 1992. Evaluation of Particle Counting as a Measure of Treatment Plant Performance. Denver, CO: AWWA Meeting.

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