Got Cryptosporidium? Why You Should Consider Ozone Treatment

Got Cryptosporidium? Why You Should Consider Ozone Treatment

May 2, 2024
Timothy Glessner, PE, DBIA
Three metal drums with pipes attached for use in water ozonation.

We all know drinking directly from surface waters, such as lakes and rivers, is unsafe because microorganisms and bacteria living in the untreated water may make us sick. Traditional treatment methods help water suppliers provide their customers safe and healthy drinking water, but what about treating water contaminated with hard-to-kill organisms like Cryptosporidium?

What is Cryptosporidium?

Cryptosporidium is a cyst-forming parasite commonly found throughout the U.S. and worldwide. Cryptosporidium cysts live in the intestines of infected animals and humans and can be released by the hundreds of millions in a single bowel movement. Cryptosporidium can be spread by swallowing as few as 10 Cryptosporidium cysts from contaminated recreational water like swimming pools or lakes, touching your mouth with contaminated hands, or drinking un- or under-treated water from a contaminated source. Consuming Cryptosporidium can cause cryptosporidiosis, a gastrointestinal illness that may be severe and sometimes fatal for people with weakened immune systems.

Why Does Cryptosporidium Need to be Treated?

Cryptosporidium is a significant concern in drinking water because it can contaminate surface water used as drinking water sources and has caused waterborne disease outbreaks. It is protected by a tough outer shell, making it tolerant to chlorine and other disinfectants, which are the primary treatment methods used by many water systems. It is also very small, making it difficult to remove by filtration.

Improving Cryptosporidium Control: LT2 Regulations

The U.S. Environmental Protection Agency’s Long Term 2 Enhanced Surface Water Treatment Rule (LT2) amended the Safe Drinking Water Act to target additional Cryptosporidium treatment requirements for higher-risk water systems and strengthen protection against pathogenic organisms. LT2 will improve Cryptosporidium control and reduce the incidence of cryptosporidiosis by up to 1.5 million cases annually.

LT2 applies to all water systems that use surface water or groundwater under the direct influence of surface water, which can be sources of Cryptosporidium contamination. Water from these sources must complete sampling for Cryptosporidium and then be sorted into one of four categories, or bins, corresponding to the concentration of Cryptosporidium oocysts found. The level of treatment required increases from Bin 1 to Bin 4.

Sources in Bin 1 can meet the treatment requirements using properly operated conventional filtration. Sources in Bins 2 through 4 require a higher level of treatment. The additional treatment is measured by log treatment, a logarithmic scale measuring the oocyst percent removal or inactivation. Conventional filtration systems must provide an additional 1-log treatment if they fall into Bin 2, 2-log treatment in Bin 3, and 2.5-log treatment in Bin 4.

The second round of source water sampling required under LT2 has moved many sources in Bin 1 after the initial round of sampling into Bin 2. Therefore, many systems need to implement treatment for Cryptosporidium in addition to conventional filtration.

Further treatment can be provided by:

  • Enhanced operation of conventional filters.
  • Additional chemical disinfection.
  • Ultraviolet light (UV).
  • Membrane filtration.
  • Ozone.

Traditional Disinfection Methods

Four common chemical disinfectants include chloramine, free chlorine, chlorine dioxide, and ozone. Chemical disinfection level is measured by concentration x time (CT). Units are generally mg/L-min. Increasing disinfectant concentration or contact time increases CT. The CT value required for disinfection will vary based on the log treatment required, disinfectant used, water temperature, and potential of hydrogen (pH) of the water. Higher CTs are required at lower water temperatures and, in the case of free chlorine, at higher pH.

Chloramine and free chlorine do not easily penetrate the tough outer shell of Cryptosporidium cysts, and as a result, their required CTs are too high to be practical for drinking water use. The CTs required for disinfection with chlorine dioxide are much lower but are still significantly higher than ozone, making all three less effective for Cryptosporidium treatment than ozone.

Ozone for Treatment

Ozone is a powerful disinfectant capable of quickly neutralizing bacteria, viruses, and parasites. It is produced by converting oxygen into ozone through a high-voltage process, which is then dissolved in the water to achieve disinfection. Ozone’s high reactivity allows it to penetrate Cryptosporidium cysts effectively and inactivate them at lower CT values than other disinfectants, making it a superior choice for combating this resilient parasite.

Calculating CT Credit With Ozone

CT can be visualized using a graph with time on the X-axis and disinfectant residual concentration on the Y-axis. Plotting the trend of disinfectant residual with time, CT would be the area under the curve. For chemical disinfectants with a stable residual such as chlorine, the trend line is flat, and the area under the curve can be calculated as chlorine residual concentration (measured at the downstream end) times the detention time between the point of chlorine application and the point of residual measurement.

Ozone’s instability requires careful calculation of CT credit, as its concentration decreases quickly over time due to its reactiveness. The decay rate of ozone is typically exponential, making it crucial to use decay constants in calculations to determine the disinfectant’s effectiveness accurately. Simply using an average of the initial and final ozone residuals times detention time does not provide a correct result.

For example, the ozone residual in a basin immediately after application is 1.2 mg/L, and after a detention time of 22 minutes, it has decayed to 0.32 mg/L.

The ozone decay curve can be defined by the formula:

The rate of ozone decay will vary with water temperature. If you know the ozone residuals immediately after application and at the end of the detention time, the decay formula can be solved for C:

The decay constant is used to calculate the area under the residual curve to determine CT. Using the decay constant, CT can be calculated using the formula:

The example below approximates the ozone decay curve using the beginning and ending residuals. If residual measurements are made at intermediate points in the contact basin, those points can be used to calculate the decay curve more accurately.

Implementation of Ozone Treatment

Ozone will react very quickly with other constituents in the water. Therefore, ozone is most effective for disinfection after sedimentation or filtration because most of the material ozone will react with has already been removed by this point in the treatment process. The initial ozone residual after application will likely be less than the applied ozone dose due to part of the ozone being consumed by constituents in the water. This needs to be accounted for when determining ozone generation requirements.

Detention times for calculating CT (for ozone or any other chemical disinfectant) must be effective detention or contact time. Effective detention times account for short-circuiting through contact basins and can be determined using a tracer test.

Ozone doses required for other water treatment purposes are generally lower than the doses needed for Cryptosporidium disinfection. If your water treatment facility already uses ozone, you may need to increase ozone production capability to meet the higher required disinfection doses.

For all its benefits, it should be noted that ozone is a toxic gas. It is fed in enclosed contact basins to prevent its release into the atmosphere. Residual ozone must be destroyed before water leaves the contact basin to prevent its release into the atmosphere. A positive residual at the detention’s downstream end is essential to obtain CT credit. Therefore, when using ozone for disinfection, water systems must quench remaining ozone residuals before the water leaves the contact basin. Ozone gas detectors are used to monitor for ozone in the air as a safety precaution.

Despite the higher initial costs associated with ozone treatment systems, they effectively achieve the required Cryptosporidium disinfection levels, especially in water treatment facilities already using ozone or where ozone could be used to provide additional treatment capabilities.

Questions About Ozone?

Our water treatment team includes some of the nation’s leading ozone treatment experts. To continue the conversation, reach out to us.

A male engineer wearing a dark suit jacket and maroon patterned tie smiles for a headshot.
Timothy Glessner, PE, DBIA
Chief Engineer, Water

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