Facts About Cryptosporidium and Giardia

Cryptosporidium and Giardia are protozoan pathogens that most surface water systems receive frequent questions about. This article is the first of a number of articles that Clarksville Gas and Water (CGW) plans to post on its website for transparency and information sharing to its customers.

CGW has never had an incident where Cryptosporidium or Giardia has been found in its finished drinking water. CGW continually spends much effort and cost to protect its customers from contaminants including protozoan pathogens. 

As stated below, CGW has been operating microfiltration technology in its water treatment plant since 2013. Microfiltration provides a direct barrier to many protozoan, bacterial, and viral pathogens. Utilizing conventional pre-treatment, microfiltration, and chlorination ensures that Clarksville Water System customers receive the best quality drinking water possible while meeting or exceeding all regulations associated with drinking water quality. 

If you have any questions regarding drinking water quality, please call the Clarksville Water Treatment Plant at (931) 553-2440.

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What is Cryptosporidium and Giardia lamblia?


Cryptosporidium is a microscopic parasite (approximate size ranges 4-6 µm) that causes the diarrheal disease cryptosporidiosis. Both the parasite and the disease are commonly known as “Crypto.”

There are many species of Cryptosporidium that infect animals, some of which also infect humans. The parasite is protected by an outer shell that allows it to survive outside the body for long periods of time and makes it very tolerant to chlorine disinfection.

While this parasite can be spread in several different ways, water (drinking water and recreational water) is the most common way to spread the parasite. Cryptosporidium is a leading cause of waterborne disease among humans in the United States. (CDC, https://www.cdc.gov/parasites/crypto/index.html)

Giardia is a microscopic parasite (approximate size ranges 8-14 µm) that causes the diarrheal illness known as giardiasis. Giardia (also known as Giardia intestinalis, Giardia lamblia, or Giardia duodenalis) is found on surfaces or in soil, food, or water that has been contaminated with feces (poop) from infected humans or animals.

Giardia is protected by an outer shell that allows it to survive outside the body for long periods of time and makes it tolerant to chlorine disinfection. While the parasite can be spread in different ways, water (drinking water and recreational water) is the most common mode of transmission. (CDC, https://www.cdc.gov/parasites/giardia/)

Water Source for the City of Clarksville

The City of Clarksville has been utilizing the Cumberland River for its drinking water since it started up its first water treatment facility in 1912. The current water treatment facility has been in operation and providing Clarksville with drinking water since 1951.

The Cumberland River, like all surface water in Tennessee, is susceptible to contamination risks, whether those risks are natural or man-made. Through Clarksville’s required Source Water Assessment (updated annually), it has been determined that the Cumberland River (within five-miles upstream of Clarksville’s water intake structure) is a “Moderate” risk for contamination, primarily from urban and agricultural sources. The risks of a “raw” water source take into account contamination of Cryptosporidium and Giardia lamblia along with many other contaminants.

Cumberland River
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The City of Clarksville has been utilizing the Cumberland River for its drinking water since it started up its first water treatment facility in 1912. The current water treatment facility has been in operation and providing Clarksville with drinking water since 1951.

Drinking Water Regulations pertaining to Cryptosporidium and Giardia

Clarksville Water System (CWS) is regulated by the State of Tennessee Department of Environment and Conservation (TDEC) Division of Water Resources (TCA Chapter 0400-45-01, https://publications.tnsosfiles.com/rules/0400/0400-45/0400-45-01.20190217.pdf), which, in turn, follows EPA regulations. EPA regulations directly pertaining to the removal/inactivation of Cryptosporidium and Giardia oocysts can be found on the EPA website (https://www.epa.gov/dwreginfo/surface-water-treatment-rules). The following information has been borrowed from that source.

The purpose of the Surface Water Treatment Rules (SWTRs) is to reduce illnesses caused by pathogens in drinking water. The disease-causing pathogens include Legionella, Giardia lamblia, and Cryptosporidium.

The SWTRs require water systems to filter and disinfect surface water sources. Some water systems are allowed to use disinfection only for surface water sources that meet criteria for water quality and watershed protection.

The following is a brief overview of the major components of each rule. This combination of rules is designed to provide protection from microbial pathogens. Simultaneously, the rules minimize health risks to the population from disinfection byproducts.

CDC Giardia Report

CDC Parasite Report

Surface Water Treatment Rule (SWTR) – June 1989:

  • Applies to all public water systems (PWSs) using surface water sources or ground water sources under the direct influence of surface water (GWUDI)
  • Requires most water systems to filter and disinfect water from surface water sources or GWUDI
  • Establishes maximum contaminant level goals (MCLGs) for viruses, bacteria and Giardia lamblia
  • Includes treatment technique (TT) requirements for filtered and unfiltered systems to protect against adverse health effects of exposure to pathogens

Interim Enhanced Surface Water Treatment Rule (IESWTR) – December 1998:

  • Applies to all public water systems using surface water, or GWUDI, that serve 10,000 or more persons
  • Sets a maximum contaminant level goal (MCLG) of zero for Cryptosporidium
  • Sets a 2-log Cryptosporidium removal requirement for systems that provide filtration  
  • Requires that watershed protection programs address Cryptosporidium for systems that are not required to provide filtration
  • Requires certain public water systems to meet strengthened filtration requirements
  • Establishes requirements for covers on new finished water reservoirs
  • Requires sanitary surveys, conducted by states, for all surface water systems regardless of size
  • Requires systems to calculate levels of microbial inactivation to address risk trade-offs with disinfection byproducts

Filter Backwash Recycling Rule (FBRR) – June 2001:

  • Applies to all public water systems using conventional or direct filtration to treat surface water, or GWUDI, regardless of size
  • Requires public water systems (PWSs) to review their backwash water recycling practices to ensure that they do not compromise microbial control
  • Requires recycled filter backwash water to go through all processes of a system’s conventional or direct filtration treatment. 

Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR) – January 2002:

  • Applies to all public water systems using surface water, or GWUDI, serving fewer than 10,000  persons
  • Sets a maximum contaminant level goal (MCLG) of zero for Cryptosporidium
  • Sets a 2-log Cryptosporidium removal requirements for systems that filter
  • Requires that watershed protection programs address Cryptosporidium for systems that are not required to provide filtration
  • Requires certain public water systems to meet strengthened filtration requirements
  • Requires systems to calculate levels of microbial inactivation to address risk trade-offs with disinfection byproducts

Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) – January 2006:

  • Applies to all PWSs that use surface water or GWUDI
  • Targets additional Cryptosporidium treatment requirements to higher risk systems
  • Requires provisions to reduce risks from uncovered finished water storage facilities
  • Provides provisions to ensure that systems maintain microbial protection as they take steps to reduce the formation of disinfection byproducts

What does the Clarksville Water System do to protect its customers from consuming Cryptosporidium and Giardia in its drinking water?

The South Clarksville Water Treatment Plant (SCWTP) employs a multi-faceted approach that goes beyond meeting the minimum regulatory requirements (see above) for removing/inactivating Cryptosporidium and Giardia oocysts. The water treatment plant’s original 1951 “conventional” design incorporated treatment processes including coagulation, flocculation, sedimentation, filtration (conventional gravity sand/anthracite), and chlorination (gas). The basic treatment processes from the original water treatment plant remain the same, but the plant has seen significant modifications to equipment and technology since then to enable it to produce a water quality that exceeds the regulations seen above. Below are treatment techniques and engineered processes that enable the Clarksville Water Treatment Plant to accomplish meeting those important regulations.

A. Enhanced coagulation – SCWTP utilizes coagulants that have become available in the last 20- years to provide the filters with a better quality settled water. In the past, aluminum sulfate solution (alum) was utilized for coagulation. Today, SCWTP uses polyaluminum hydroxychloride to coagulate the raw Cumberland River water. Polyaluminum hydroxychloride coagulates small colloidal particulates (even in the 3-14 µm size range), natural organic matter, and other particulates analyzed as “turbidity.” Generally, settled water quality is less than 1.0 Nephelometric Turbidity Unit (NTU).

B. Microfiltration – Until 2013, the SCWTP was equipped with conventional sand-anthracite filters, which adequately filtered settled water, but were limited in filtering particle sizes in the ranges that Cryptosporidium and Giardia are found. During engineering design for the 2010 SCWTP upgrade, it was determined that Clarksville would upgrade the filtration system to what is called “microfiltration.” A new membrane filter building was constructed during the 2010 project and the new membrane filters were placed in service in March 2013.

Membrane filters are considered a “direct barrier” to protozoan pathogens like Cryptosporidium and Giardia since the membrane fibers have a nominal pore size of 0.1 µm. The membrane filters not including other treatment processes can achieve up to a 6-log (99.9999%) removal of Cryptosporidium and Giardia.

C. Chlorination & Contact Time (CT) – Clarksville Water System disinfects filtered water utilizing chlorination (sodium hypochlorite) and must retain a minimum of 0.2 mg/L free chlorine residual throughout the water distribution system as per TDEC regulations. The Maximum Residual Disinfectant Level (MRDL) for free chlorine is 4.0 mg/L as per TDEC regulations.

The SCWTP is engineered to meet required chlorine “contact time” (CT). CT is a treatment technique that water systems use to ensure inactivation of pathogens including Cryptosporidium and Giardia. Since protozoan oocysts are resistant to chlorine, it is necessary to provide finished water with enough “contact time” to inactivate them. Utilizing mathematical models, SCWTP can determine the effectiveness of chlorination to pathogens based on chlorine dose, flow rate, water temperature, and engineered physical baffling in on-site water storage tanks.