Chapter 1: Disinfection Calculations

The US Environmental Protection Agency (EPA) has developed interrelated regulations to control microbial pathogens, disinfectants, and disinfection byproducts (DBPs) in drinking water. These rules, collectively known as the microbial/disinfection byproducts (M/DBP) rules, primarily address two key public health concerns: acute threats from microbial contamination and chronic threats from disinfectant residuals and byproducts of disinfection.  The protozoa Giardia is the most difficult to inactivate. Viruses are the most difficult to remove. Remember, inactivation refers to killing the Giardia or destroying its ability to reproduce.

Factors Influencing Disinfection 

Let’s review several physical characteristics of a microorganism and chemical factors that can influence the disinfection of water.

pH 

Worldwide, many agencies have historically limited the range of pH values of distributed water between 6.5 and 8.5. Although this range is not a regulatory limit, many jurisdictions have used it as one. The antimicrobial activity of chlorine is optimal at a neutral pH of 7.0. But an increase in pH will reduce its efficacy (pH over 8.0).

Temperature 

The activity of most disinfectants will increase as the temperature increases.  Chlorine is very effective at temperatures above 65 ℉. Temperature measurements for calculating treatment contact times is important.

Turbidity 

Turbidity is the measure of the relative clarity of a liquid. Cloudy (turbid) water would be due

to the presence of clay, silt, tiny inorganic and organic matter and other microorganisms. This would definitely interfere with the disinfection process and is why a multi-barrier treatment approach with filtration precedes disinfection.

Organic Matter 

A disinfectant like chlorine or bromine will react with natural organic residue found in water. This reduces the amount of disinfectant available. But more importantly the formation of chemical disinfection by-products called trihalomethanes (THMs) also occurs. These compounds are regulated by the EPA. Consequently, most water agencies in California now use chloramines instead of chlorine as a primary disinfectant.

Inorganic Matter 

Inorganic compounds found in residual silts or a compound such as ammonia (NH3) in water will reduce the amount of disinfectant.

Reducing Agents 

A reducing agent loses electrons and is oxidized in a chemical reaction. Examples of reducing agents include earth metals, formic acid, and sulfite compounds. Chlorine combines with reducing agents, thereby affecting the amount of chlorine available to disinfect. Examples of reducing agents in water include hydrogen sulfide (H2S), ferrous ion (Fe+2), manganous ion (Mn+2), ammonia (NH3), and nitrite (NO2-).

Microorganisms 

The physical characteristics of a microorganism can influence the disinfection process. For example, spores are resistant to disinfectants because the spore coat and cortex act as a barrier. However, most microorganisms can be filtered or settled out prior to disinfection.

CT Concept 

In order to better provide for simultaneous compliance with EPA’s M/DBP rules a “CT” method was developed to evaluate the inactivation of pathogens using a base 10 logarithmic scale and is referred to as “log-activation”. It represents the order of magnitude in which inactivation occurs. A 2-log inactivation corresponds to a 99 percent inactivation. 3-log is 99.9 percent inactivation. A CT value is a measure of disinfection effectiveness for the time that microorganisms in the water are in contact with a disinfectant CT is defined by EPA as disinfectant residual concentration (C) multiplied by contact time (T). The units of CT are always expressed as min-mg/L. The table below shows CT values for the inactivation of Giardia using chloramine disinfection.

Figure 1.1 CT Values for Inactivation of Giardia Cysts by Chloramine, pH 6.0-9.0.8

Notice the CT values (in min-mg/L) depend on the pH and temperature of the water.

Tables showing CT values to achieve inactivation of Giardia or viruses for other chemical disinfectants including free chlorine, chlorine dioxide, and ozone are also available.

CT Credits for Filtration

The EPA also provides log “credits” if water has been filtered and meets specified turbidity goals. For this discussion we will assume conventional filtration technology (i.e., Coagulation & Flocculation, Sedimentation, Filtration) prior to disinfection. We will go through an example later.

Other alternative technologies must be approved on a case-by-case basis.

3. Calculate Disinfectant Contact Time (T). Water does not flow through all treatment processes in a perfectly mixed condition. In some treatment units there can be substantial short-circuiting. The disinfectant contact time (T), also referred to as T10 in the Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water (USEPA, March 1991), is an estimate of the detention time within a basin or treatment unit during which 10 percent of the water has passed at the effluent end of the contact basin or treatment unit. T can be determined experimentally through a tracer study, or it can be estimated based on a theoretical detention time and a baffling factor (BF) to account for short-circuiting. Let’s discuss these.

A) Theoretical Detention Time. Determine the peak hourly flow for each disinfection segment you are working on and compute the theoretical detention time. From water treatment math you learned that detention or contact time is

We need to think of this as an ideal or “theoretical” detention or reaction time because actual flows in a treatment plant (which you determine by performing tracer studies) are often less.

B) Determine Baffling Factor. The T in each basin, pipe, or unit process is a function of the physical configuration and baffling.  Baffles help steer the flow to help ensure that all of the water passing through a contact basin has the same contact time. The flow through a pipe is very different than the flow through an unbaffled basin (see Figure 2.3). The longest path a particle can take through a pipeline does not vary substantially from the shortest path (we call this “plug flow”). In the case of an unbaffled basin, however, some percentage of the flow may follow a path that goes directly from the inlet to the outlet. As a result, short-circuiting occurs and microorganisms in this path will only be in contact with the disinfectant for a relatively short time.

Figure 1.3: Baffling Factors account for short-circuiting¹⁰

Top: This pipe demonstrates a plug flow condition in which all of the material sent through the pipe discharges at the theoretical detention time (T) of the pipe.

Bottom: This unbaffled basin demonstrates short-circuiting in which some of the material entering the basin would come out almost immediately, while other material that enters at the same time will be detained for a longer period of time. Short-circuiting occurs in basins with poor baffling.

Baffling factors (BFs) help estimate the contact time of a basin, pipe, or unit process based on the volume of and flow rate through the basin, pipe, or unit process. Baffling factors recommended by EPA and shown below were developed based on tracer studies of basins with varying sizes and configurations.

Figure 1.4: Baffling Factors.

C) Determine Contact time T10

The contact time for a particular disinfection segment with a single basin is: [latex]T_10=T x BF[/latex]

Where T is the theoretical detention time and BF is the baffling factor, we can arrange this equation to solve for the baffling factor as well: [latex]BF= T_10/T[/latex]

This shows the baffling factor is simply the ratio of the actual contact time to the theoretical contact time. Remember, if you have tracer test data, T10 will already be computed so you won’t need to look up a baffling factor.

4. Calculate actual CT.

Determine the ratio of actual CT to the CT needed.

[latex]〖CT〗_(calculated )=C x T_10[/latex]

Where C is the residual disinfectant concentration measured during peak hourly flow in mg/L.

Check to see if the ratio of CT calculated to CTneeded is greater than 1.0 You want

[latex]〖CT〗_calculated/〖CT〗_needed >1[/latex]

If the ratio is less than 1.0 you are not in compliance.

Breakpoint Chlorination Zones

The breakpoint chlorination curve shown in Figure 2.5 is the visual representation of chlorine’s ability to react with a variety of compounds to form a combined chlorine residual or to completely react with compounds to form a free chlorine residual.

If you were to add chlorine to an untreated natural surface water source and then measure the chlorine residual, you would likely find that it would be zero. This is because chlorine is reacting with natural organic and inorganic compounds in the water including dissolved organic matter, particulate organic matter iron, nitrite, sulfide, and ammonia.

Let’s focus on ammonia. It exists in water as “free ammonia” (NH3) and “ammonium ion” (NH4+). Total ammonia is the sum of free ammonia and ammonium ion. This distinction will be important when we study chloramination in more detail.

Total Ammonia = Free Ammonia + Ammonium Ion

NH3          +         NH4+

Chlorine reacts with all of these organic and inorganic compounds in natural water by taking away an electron from them. If you continue to dose more chlorine, you will eventually reach a point where a residual will just start to be detected. We call this dose, the chlorine demand.   It is shown in the breakpoint chlorination curve below as Zone I. Higher dosing above this point is what creates a chlorine residual. We can write the following equation:

Total Chlorine Dose = Chlorine Demand + Chlorine Residual

From this relationship we can also write:

Chlorine Residual = Total Chlorine Dose – Chlorine Demand.

Ex. Assume that you are working as an operator at a water treatment plant. Your chief operator would like to maintain a residual of 2.0 mg/L of chlorine residual in the distribution system. The demand is 1.5 mg/L. What is the dose you must add to achieve a residual of 2.0 mg/L?

Dose=Demand + Residual=2.0mg/l+1.5mg/l=3.5mg/l

Therefore, you would need to maintain an average of a 3.5 mg/L dose of Chlorine to achieve the residual requested by your chief operator.

As you begin to add free chlorine above the chlorine demand, it will quickly react with free ammonia to form chloramines, which are also on the list of approved activators as disinfectants. So just beyond the chlorine demand, as you add more free chlorine, you will be forming chloramines, primarily monochloramine. Chloramines which form when water containing free ammonia is chlorinated are referred to as combined chlorine.

Figure 1.5 Breakpoint Chlorination Curve. Total chlorine residual is the sum of free chlorine and combined chlorine residuals.

Refer to Zone II in Figure 2.5. We see chlorine residual increasing. In this zone, it is primarily monochloramine NH₂CL that is forming according to the reaction.

This is also the preferred chloramine for treatment because it presents the least taste and odor problems. The unit weight of Chlorine is 70 and the unit weight of Nitrogen (in the ammonia) is 14. When chloramine treatment is employed, the ratio of chlorine to nitrogen in the ammonia will always be 5:1 because 5 mg/L of chlorine will always combine with 1.0 mg/L of ammonia (70 ÷ 14 = 5). At the top of the pyramid in Figure 2.5, there is no free ammonia left (you still have ammonium ion however). You will always target a 5:1 ratio to avoid excess free chlorine.

Now refer to Zone III in Figure 2.5. As you continue to apply free chlorine to the water beyond the 5:1 ratio, you will actually reduce the amount of total chlorine residual. This is because monochloramine will react with hypochlorous acid (HOCl) to form more dichloramines and trichloramines. These are unstable forms, and much less effective than monochloramine.  They also cause taste and odor problems.

When you get to a chlorine to nitrogen ratio of 7.6:1 you will reach a breakpoint. After that, the addition of free chlorine adds directly to the total chlorine residual. This is designated as Zone IV in Figure 2.5. In this zone, there is no ammonia left. The total ammonia is zero.

Other Disinfection Processes

In addition to or instead of adding chlorine, chloramine, or chlorine dioxide, water treatment plants can also disinfect water using UV light and ozone. They work well to disinfect water in the treatment plant, but these disinfection methods do not continue killing germs as water travels through the pipes between the treatment plant and your tap (no disinfection residual).

Ultraviolet Rays

Advances in UV disinfection have been made in recent years. An Ultraviolet (UV) disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism's genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell's ability to reproduce. UV radiation, generated by an electrical discharge through mercury vapor, penetrates the genetic material of microorganisms and retards their ability to reproduce. Unfortunately, this method leaves no disinfectant residual and is expensive.

Heat

Primarily used in emergencies, heating water to a boil for 5 minutes will kill all microorganisms that may have breached the distribution system. From a practical perspective, it is an expensive and inefficient way to disinfect water for a public water system.

Chemical Disinfection other than Chlorine

Iodine

Iodine is a good disinfectant and has been utilized for potable water treatment since the early 1900’s in concentrations between 2.5 – 7 ppm. The US Army continues to issue iodine-based tablets to field soldiers and other personnel to disinfect water in emergencies.  It is expensive and is also not recommended for use as a primary disinfectant due to lack of knowledge on long-term toxic effects.

Bromine

Bromine is primarily used as an alternative disinfectant for swimming pools, spas and cooling tower water, but not for municipal drinking-water, partly due to cost and partly to concerns about the formation of brominated DBPs.

Ozone

Ozone is used to disinfect water along with reducing taste and odors. Drawbacks of ozone for disinfection include a high cost, big physical footprint, lack of residual, difficulty in storing, and maintenance requirements.

Ozone has its advantages and disadvantages. Ozone treatment has the ability to achieve higher levels of disinfection than chlorine or UV, however, the capital costs as well as maintenance expenditures are not competitive with available alternatives.