THM monitoring in drinking water applications - Water and Wastewater - Drinking Water
The set of compounds that are formed in a water disinfection process, are collectively referred to as disinfection by-products (DBP). Trihalomethanes (THM) are byproducts of disinfection that are formed when chlorine (or a chlorine based product) is used as a disinfectant. The THMs commonly found in water for human consumption are: chloroform, bromodichloro-methane (BDCM), dibromochloromethane (DBCM) and bromoform , with chloroform usually being the main component. Many trihalomethanes are considered to be dangerous for health and suspected as carcinogens. The European Community Drinking Water Directive states that water used for human consumption should not exceed 100 µg/l of total THMs and US regulations state a maximum level of 80 µg/l of total THMs.
Chlorine is one of the most commonly used disinfectants in the world: historically it has been used for more than 100 years. It is easy to produce, store and the disinfected water is left with a level of chlorine which keeps it disinfected within the distribution network until it reaches the tap.
One of the main benefits of water chlorination is the drastic reduction of infection by bacterial, viral and parasitic diseases. The main disadvantage of chlorine disinfection is the potential for the formation of THMs. Other methods of disinfection like ozone or UV do not present this problem but are, however, are much more expensive to implement and run, so are less commonly used in larger operations.
THMs are generated during the disinfection of the water, through the reaction of chlorine (or chlorine dioxide/hypochlorite) with natural occurring organic matter (NOM) present in the water, such as humic acid and fulvic acid. The concentration of THMs in the water will depend on: the amount of chlorine present, the contact time of the chlorine with organic matter, the amount of organic matter, the concentration of bromides in the water, the pH and temperature.
“THM Precursors” are dissolved organic matter, naturally occurring in water, comprising a heterogeneous mixture from breakdown of organic materials (plants, bacteria etc.). Generally it has been found that THM precursors are those substances containing aromatic structures, but the picture is complicated in terms of predicting THMs by additional factors such as:
- The distinctive chemical characteristics associated with origins
Chloroform is the most abundant THM and is usually the main by-product of the disinfection found in chlorinated water. This is mainly due to the fact that the majority of water sources do not contain an abundance of brominated species.
- Seasonally dependencies connected to the life cycles of flora
- Residency time dependence.
For example, during the “first rain”, a lot of organics which are accumulated in the ground are flushed into the river. This leads to higher levels of THMs. Also the longer the chlorine has to interact with the organics, resulting in demand patterns influencing the levels of THM.
- Hydrological and biogeochemical processes
Studies concerning the formation of Trihalomethanes as a consequence of adding chlorine to water were first carried out in the United States in the 1970s. For this purpose procedures based on gas chromatography and mass spectrometry were used. Epidemiological studies associate exposure to disinfection by-products, mostly THMs, with health effects such as bladder cancer and certain birth defects in newborns of exposed mothers.
Studies into bladder cancer found an increased risk with long exposures to THM (more than 30 years). The US EPA classifies Chloroform and Bromoform as probable carcinogens and Dibromochloromethane as a possible carcinogen for humans under certain exposure conditions. This means, although there are indications of carcinogenicity in experimental animals, the evidence is limited for humans.
According to the World Health Organization (WHO), the potential risks posed by THMs are greatly outweighted by the dangers of not chlorinating water supplies. The risk posed by THMs is considered long-term, since it would require the consumption of water over a lifetime, as is the case of most carcinogenic products, to create problems. According to the WHO, exposure to these substances poses a risk of cancer of 10-5, that is, a case of cancer per 100,000 people who consume water in a minimum period of 70 years. In the case of the European Union, the risk is considered 10-6.
As an alternative, some developed countries are using sodium chlorite as a replacement for sodium hypochlorite thus avoiding the production of THM in the purification of water.
THMs can be incorporated into the human body through several routes: ingestion of tap water, inhalation of evaporated THMs and dermal absorption. Many daily activities such as water consumption, bathing or showering and swimming in the pool can also contribute to the total exposure to THM.
The WHO provides recommended guide values for individual maximum concentrations of each of the THM in water for human consumption. The guide values represent the limiting concentration of a compound that does not result any significant risk to health through lifelong consumption. These are:
- Chloroform: 300 μg/l
- Bromodichloromethane (BDCM): 60 μg/l
- Dibromochloromethane (DBCM): 100 μg/l
- Bromoform: 100 μg/l
According to the WHO, calculation of the combined toxicity of all THMs can be achieved using the following equation:
The level of awareness of control and monitoring technology for THMs in drinking water varies significantly around the world, despite strengthening regulations globally. In some places THMs are considered not an important (or even known) issue.
Many other water treatment plants rely on a bi-annual or monthly laboratory analysis of the THM levels. This usually consists of sampling the water, transporting it to a laboratory, analysing it using Gas Chromotography / Mass Spectroscopy (GCMS) techniques. The elapsed time for this approach can be up to two weeks.
When THM levels are consistently and historically low (i.e. < 10 ppb of Total THMs) this approach is generally thought to be sufficient to comply with current regulations. Having said that, this approach means that the operators are relying on a very limited sample to determine the overall levels. A spot sample every month or six months is not likely to be representative.
Finally there are plants/regions where, due to the characteristics of the source water, THMs are a significant issue and need to be closely monitored. In these applications it is very important to strike a balance between the accuracy of the system, the cost of the purchase, the running costs and the ease of operation. Usually this happens when Total THMs are on average around 30 ppb: there’s always a risk of TMHs exceeding the limit of 80 - 100 ppb and it is in the interest of the WTP to keep levels as low as possible.
Following international trends, regulation of THM concentrations is likely to become stricter in the future, so whilst 100 ppb is on the limit of acceptability today (2019), in many parts of the world (at least from a regulatory point of view), it is very likely that this won’t be the case in a few years’ time when legislators and consumers will expect much higher standards of water quality.
A number of different technologies are available on the market for THM analysis. Each system has strengths and weaknesses and particular interest should be paid to the following criteria before purchasing an instrument:
- Cost of ownership
- Ease of use
- Accuracy and reliability of the measurement
- Measurement time
This is an on-line system that can give a reading every 4 hours or more.
In this method the instrument extracts THMs from the water sample by P/T on an adsorbent material and the concentrated THMs are subsequently desorbed into a proprietary reagent mixture that generates a colored product.
Because of the cost of reagents the system is very expensive to run (tens of thousands of dollars per year for the reagents) and usually it is not feasible to have a measurement every hour.
The method also means that the instrument has an inherent complexity and many different components, which can lead to expensive repair and ongoing issues.
Purge/Trap – Gas Chromatography – Surface Acoustic Wave Detector
This is a laboratory instrument which has been adapted for on-site application.
The monitor is an integrated purge-and-trap system connected to a compact gas chromatograph column and a surface acoustic wave (SAW) detector. The system requires Helium gas, an external manual calibration and needs to be operated by skilled technicians.
Also, because of the nature of the technology it is impossible to use it as an on-line system: the operator will have to go on site, take a sample, wait 30 minutes and get the result.
Membrane Permeation – Gas Chromatography – Electron Capture Detector
This is an experimental method, not yet widely accepted in industry
In this kind of system, THMs are extracted from a water sample into a carrier gas. This is achieved by forcing the water through a permeable membrane, then adsorbing the THMs onto a trap, followed by separations onto a GC column. Finally the Trihalomethanes are detected by an Electron Capture detector. Using a gas chromatographer for online monitoring generates stability problems as retention times, calibration and system validation.
Extraction through a semi-permeable membrane has a huge impact on sample matrix including ionic strength, pH and temperature. Fouling is a problem that could arise in this kind of systems. The use of the radioactive element 63Ni in the detector is not desirable and such a system would require a carrier gas and be quite expensive.
Fluorescence Detector with Membrane Permeation and Chemical Reaction:
This is an experimental method, not widely accepted in the industry.
In this method, the Trihalomethanes move from the water into the chemical reaction mixture across a semi-permeable membrane in a continuous flow of reagents, followed by measurement of the fluorescence generated from the reaction of the THMs with alkaline nicotinamide. This method requires a lot of reagents. Extraction through a semi-permeable membrane has a huge impact on the sample matrix including ionic strength, pH and temperature. Also fouling is a problem that could arise in this kind of systems.
Process gas chromatograph (GC) with TID (Thermal Ionization Detector) and PID (Photo Ionization Detector)
This system is a traditional process gas chromatograph with TID and PID technology. This technology is quite accurate and allows for speciation. Given the complexity of the instrument it demands a high purchasing cost and frequent recalibration (once per month). Also, in order to work, a source of compressed air is needed not to mention a skilled system’s technician.
Electronic nose technology uses sampling of headspace gases in a sample tank and passing these gases over sensitive sensors to measure the THM level. An example is the Multisensor Systems MS2000.
The advantage of electronic nose technology is that it lends itself to on-line operation, is robust in a variety of environments and it does not use any reagents. The sample presentation is also simple reducing the possibility of fouling.
Being a non-contact technique cleaning of sensors is not necessary.
The principle of operation of the MS2000 THM Monitor is the measurement of headspace gases from a sample tank containing the water to be measured.
Henry’s Law dictates that the continuous flow of the concentration of gases in the headspace is proportional to the concentration of the analyte in the water.
Calibration of the instrument is done by presenting a known concentration to the sensors and generating calibration coefficients from the responses obtained.
The MS2000 works by passing a continuous flow of water through the sample tank as shown in Figure 2. The volatile components in the water will pass into the headspace above the water until an equilibrium is reached.
A sample of the headspace gases is then passed across sensors in the MS2000 THM Monitor sensor head, which respond to the THMs in the headspace. This response is then analysed by the instrument and a concentration value is generated based upon the relationship between the concentration present in the headspace and that in the water.
The monitoring of THMs in drinking water is becoming more important globally as drinking water standards increase. Monitoring of THMs cannot be left as a regulatory “chore” and is vital in maintaining public confidence in water supply companies.
There are many considerations in deciding on the THM analysis technique to use. There are varying approaches with differing outcomes in terms of price, running costs, performance and reliability.
Electronic nose technology has established itself as a cost effective method of monitoring THMs when measured against alternative techniques and is gaining growing acceptance in the market.