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Water Filtration Plant Performance Evaluations

During the past 30 years Giardiasis and Cryptosproridiosis have been recognized as two of the most frequently occurring waterborne diseases in the United States.

The occurrence and detection of the Giardia and Cryptosporidium parasites causing these diseases and drinking water source identification and protection has become a matter of urgent concern to those responsible for water utility operation in areas endemic with these parasites. Because of these concerns, the Surface Water Filtration Rule was established and protocols were developed for determining if a source was ch

As part of the Surface Water Filtration Rule, all community water supplies identified as surface water sources require a minimum of filtration rather than just disinfection prior to consumption. The Surface Water Filtration Rule has led to the development of a protocol to evaluate the performance of filtration plants. The protocol is similar to the method used for the Groundwater Under the Influence Investigations, except the risk rankings and interpretations are based on a combination of the system configuration, source water quality, degree of particle reduction, and distribution of particles.

When I was the Director for the Center for Environmental Quality at Wilkes University, I (Brian Oram) conducted research in pilot filtration plant performance and optimization; the laboratory was also involved in the evaluation of a number of bench-scale, pilot, and full-scale filtration plants in Pennsylvania, New York, and New Jersey. When I was in graduate school, I worked on projects using diatomaceous earth filtration, slow sand-filtration, direct filtration, submicron filters, cartridge filters, and up-flow filtration systems. In addition, we have conducted bench scale and pilot scale experiments to assist in optimization of pretreatment processes and filtration of water supplies and have developed protocols and conducted testing to assist in the development of solid-flux loadings for secondary clarifiers for wastewater applications and living filtration systems for on-site wastewater disposal systems.

Microscopic Evaluation Technique for Filtration Plant Performance

The purpose of this procedure is to evaluate the performance of public water supply filtration systems. A minimum of 300 gallons of both raw and finished water should be collected. The evaluation is based on the filtration plant’s ability to remove Giardia-sized and larger particles. In addition to the MET evaluation, the evaluator should also consider the operational conditions of the plant, physical condition of the plant/ raw water, and effluent quality of the finished water.

Each analysis culminates in a filtration performance rating which reflects the overall effluent quality. The rating is always accompanied by an explanation which details the justification of the assessment.

Filtration plants with excellent and good ratings are reported as "Acceptable Filtration Performance.” Filtration plants which are rated “questionable” or “poor” are reported as "Unacceptable Filtration Performance".

The amounts of each of several specific groups of particulate matter and microorganisms are recorded. These groups include: small particulate debris, large particulate debris, cellular plant debris, diatoms, algae, protozoa, insects and crustaceans, nematodes, and rotifers. Also, pollen grains,

Giardia, and other parasitic protozoa should be noted.

Number of Specific Particles in a Category Avg. Field at 100x
  Quantity Quality  
+ Rare Excellent 0 - 49
++ Few Good 50 - 99
+++ Some Moderate 100 - 199
++++ Many Poor > 200


These systems remove essentially all of the Giardia-sized debris along with much smaller particles. There is no evidence of turbidity breakthrough to indicate any risk for Giardia contamination of the effluent and 300 gallons of effluent can be observed in a single sample without any Giardia-sized particles reaching the rare level (+).


300 gallons of effluent can be observed in a single subsample without significant amounts of Giardia-sized particles present, i.e., the levels of diatoms, algae, and protozoa remain at the few (++) level.

Moderate (Questionable)

Giardia-sized particles are at the moderate (+++) level in samples which represent 300 gallons of effluent. These systems have filtration which does not provide an obvious assurance for Giardia removal as in the above two ratings. It is not possible to comfortably predict the potential for Giardia breakthrough with this result. Facility evaluations rely heavily on information obtained in the operations survey.


These systems are unable to remove Giardia-sized particles and are therefore vulnerable for Giardia passage through the facility. Subsamples represent 300 gallons of effluent, and Giardia-sized debris is in the moderate (+++) to many (++++) range. Operational changes are required to improve the effluent quality. The facility must be reevaluated to determine whether correction of the problem was achieved. Corrections must be performed within a reasonable time period and the public notified if the potential for Giardia contamination remains.

It was my experience that the water treatment systems that relied on surface water sources (lakes, streams, etc.) when we conducted this assessment, would either fail because of excessive coagulant carry over, poor settling-pin-floc, and over-dosing of chemical coagulant. If the sample passed but you then checked the sample using the Groundwater Under the Direct Influence assessment, it would fail big time. It is my professional opinion that we need a multi-barrier approach that should start with watershed monitoring and development control, in-line and real-time particle size/turbidity monitoring, in-line chlorine and ORP monitoring, and end with point-of-use filtration, i.e., ultrafiltration.

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