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Microscopic Particulate Analysis

Filter Plant Performance Evaluation: Interpreting the Microscopic Particulate Analysis (MPA)


The Microscopic Particulate Analysis (MPA) involves the identification, sizing and population estimates of microorganisms and organic or inorganic debris found in water. Samples for MPA are collected by passing water through a cartridge filter with a nominal pore size of one micrometer (µm). In the laboratory, particles trapped on the cartridge filter are washed from the filter, concentrated to a small volume, and observed at 100 to 1,000 magnification using light microscopy. Comparing the particles in a filtration plant's raw water and filtered water provides one tool in assessing the effectiveness of treatment and the ability to remove Giardia-sized and Cryptosporidium-sized material. The method is also used to detect surface water microorganisms in a suspect groundwater source or infiltration gallery.

In assessing performance of a surface water treatment plant, the MPA provides additional information to an evaluator who conducts an on-site survey of facilities (click here to access the full protocol on Filter Plant Performance Evaluations). The filter plant evaluator uses a multitude of information to establish a final rating, including:

  • Operational control of the plant, especially chemical pretreatment and filter operation;
  • Physical condition of the plant and its equipment;
  • Water quality data, including the plant's long-term turbidity performance and turbidity and particle count profiles of one of the filters; and,
  • Results of the MPA.

The laboratory analysis is, therefore, only one component of an evaluator's rating system and should not "override" the evaluator's final decision if plant operations and facilities are clearly in either excellent or poor condition. Sometimes, though, MPA of the filter effluent may identify problems that are not easily observable, and may reveal heavy particle breakthrough even in low turbidity water (less than 0.3 NTU). It should be made clear that MPA is only semi-quantitative; that is, the procedure cannot enumerate exact particle reductions, such as "log" removal.


The Bureau of Laboratories at the Department of Environmental Protection (DEP) employs experienced microbiologists who have analyzed numerous MPA samples collected from a variety of water quality conditions. The MPA assessments are not restricted by limited experience, and all samples are analyzed from a perspective with a consistent point of reference for scoring the categories.

To maintain consistency from one plant to another, laboratory staff remove a sub-sample representing 300 gallons from the filter effluent sample. As a result, the total number of gallons collected in samples from treatment plants does not require standardization. The microscopist always examines the standard 300-gallon sub-sample for particle identification and quantification. In addition, the water sample is then examined for Giardia cysts, Cryptosporidium oocysts and general characteristics.

Examination of the raw water sample is not as complex as the effluent analysis, involving a check for the presence of microorganisms (including cysts and oocysts) and organic or inorganic debris. These checks are used to ensure that the plant is actually removing a specific particle group, as opposed to total absence in the influent.

One focus of MPA is the identification and confirmation of Giardia cysts and Cryptosporidium oocysts. These organisms can cause a flu-like illness in humans and can be fatal for individuals with weakened immune systems. The organisms are recovered in the "cyst" form from high-volume water samples. The microscopist uses an immunofluorescent technique to assist in locating the cysts among the debris in the samples. The microscopist verifies identification with light microscopy to confirm at least internal structures and other unique characteristics of these organisms. The cysts and oocysts are considered viable unless they have been properly disinfected at the treatment plant.


Not all of the particles in an MPA sample are identified and sized. Specific categories of debris have been selected for these observations because they contribute more pertinent information about the efficiency of a surface water treatment plant. The significance of each category is described below:

Cryptosporidium-sized Debris (3 to 7 µm) - These organic and inorganic particles represent the size range of Cryptosporidium oocysts and, therefore, are critical indicators or plant performance.

Giardia-sized Debris (8 to 19 µm) - These organic and inorganic particles represent the size range of Giardia cysts and are critical indicators or plant performance.

Large Particulate Debris (20 to 100+ µm) - These particles of unknown nature are conglomerated masses of individual pieces. They are larger than cysts and oocysts and are useful tools for indicating plant performance.

Cellular Plant Debris - Pieces of vegetation that are characteristic of fecal material of some Giardia hosts, such as beaver and muskrats. The observations of large amounts of this material may indicate the presence of potential Giardia hosts in close proximity to the raw water intake. Cellular plant debris is variable in size, but generally larger than 20 µm in diameter.

Diatoms and Other Algae (less than 1 to 100+ µm) - These organisms are useful indicators of organic loading on a filtered system. Their concentration and size variability within a sample population provides valuable information on size-related filtration removals.

Protozoa - Common in raw water reservoirs, these unicellular organisms are often the same size or larger than cysts or oocysts.

Nematodes - Nematodes are sediment dwellers. They inhabit the bottom of reservoirs and streambeds so they and their eggs may indicate a low raw water intake level. These organisms are common to slow sand filters, infiltration galleries, and some filtration plants where their motility and body shape allow easy penetration into media beds.

Insects/Crustacea/Rotifers (generally 50+ µm) - These organisms can become numerous in surface water supplies. When this group is abundant, examination for Giardia cysts and Cryptosporidium oocysts is difficult.


Each of these groups are scored based on the following system:

CategoryNumber of Specific Particles in an Average Field at 100x 0 = None 0 1+ = Rare 1 to 50 2+ = Few 51 to 100 3+ = Moderate 101 to 200 4+ = Many Over 200

Filtration Performance Rating

Each MPA culminates in a filtration performance rating that reflects the overall filter effluent quality. The analytical results and ratings are recorded on an MPA sheet and are always accompanied by an explanation detailing the justification for the particular assessment. The general performance ratings are described as follows:

Acceptable - Approximately 300 gallons of filter effluent can be observed in a single sub-sample with no significant amounts of Giardia-sized or Cryptosporidium-sized particles present. That is, cyst- and oocyst-sized debris, diatoms and other algae, and protozoa are not present or remain at the Rare (1+) or Few (2+) range to indicate minimal contamination of the filter effluent. There is no apparent risk for Giardia or Cryptosporidium breakthrough in these water treatment plants.

Unacceptable - Sub-samples represent approximately 300 gallons of filter effluent with Cryptosporidium-sized debris, Giardia-sized debris, and/or larger particle categories in the Moderate (3+) or Many (4+) range. These filter plants are experiencing performance problems and may not provide the obvious assurance for Giardia and Cryptosporidium removal. Operational changes may be required to improve the effluent quality, and the facility should be re-evaluated to determine whether correction of the problem was achieved.

A Note About Floc Samples…

If a substantial amount of floc (pretreatment chemicals) is prevalent in a sample, a detailed particle quantification may not be possible. In this case, the word "floc" is entered into the particulate debris categories and no other particle categories are quantified. Sometimes, the floc only consists of chemicals and is relatively free of debris, which signifies acceptable particle removal. At other times, however, laboratory staff assess the overall amount of organic and inorganic debris that may be bound to the floc, and thus determine that the water treatment plant is passing too much Cryptosporidium-sized debris, Giardia-sized and/or larger particles into the finished water. Floc samples usually occur when the coagulant doses or pre-pH levels are improper, when rapid mixing is insufficient, or if floc and solids loading onto the filters is excessive. 

Photos and Descriptions Of Some Organisms Fournd In MPA Samples

Cryptosporidium Oocysts and Oocyst-sized Debris

Cryptosporidium is a protozoa with a complex life cycle. In the water, it exists in an oocyst form and is typically 3 to 5 microns in size. When even a few oocysts are consumed, they enter the small intestine, where they excyst, or "hatch," and can multiply into millions of protozoa. Various animals can deposit oocysts into the water, including humans. Normal chlorine concentrations that are maintained at public water systems are ineffective against Cryptosporidium.

Giardia Cysts and Giardia Cyst-sized Debris

Giardia cysts and Giardia-sized particles are typically 6 to 19 microns in size. Part of a two-stage life cycle, the cyst form is found only in surface water sources and can remain viable for up to 60 days in colder water. However, cysts do not multiply or move about. Several warm-blooded animals (not just beavers and muskrats) can deposit cysts into the water, including humans. At least two of the three internal organs of a cyst must be microscopically identified to qualify as a verifiable Giardia cyst.

Cellular Plant Debris

Vegetation that is characteristic of fecal material of some rodents, such as beaver or muskrats among others, possibly indicates a higher susceptibility to Giardia contamination. The plant material resembles interlocking or individual pieces of a jigsaw puzzle. Care must be taken to differentiate this material from fungi, roots in shallow groundwater, or decomposing woody material.

Diatoms and Other Algae

These are categorized as photosynthesizing organisms with size variability from less than 1 to over 100 microns in diameter. This size range provides a good spectrum for evaluating a water treatment plant's ability to remove large and small particles.


Nematodes are microscopic worms that may dwell in terrestrial or aquatic habitats. They are often found in slow sand filters and infiltration galleries. Their motility and slender body shape contribute to their passage through rapid sand filter beds. They are 3 to 4 microns in diameter and may be up to several hundred microns long.

Insects and Crustacea


This large category of organisms consists of a diverse biological group. Certain insect larvae and crustacea are normal inhabitants of slow sand filters. They are generally greater than 50 microns long. They should not be numerous in rapid-rate filter effluent, but their slender shape and motility contribute to the passage of some of smaller sizes through these media beds.



Common to surface water sources, rotifers may be 70 microns to greater than 500 microns in length. They involve shapes from microscopic & worm-like to spherical organisms.


When post-flocculation is identified in a sample, it indicates excessive breakthrough of pretreatment chemicals. Concerns are raised when the chemicals contain too much large-diameter debris. The photo on the left shows floc (pretreatment chemicals) with "acceptable" amounts of organic debris. Conversely, the photo on right contains floc with "unacceptable" amounts of debris.