Natural organic matter and trihalomethanes in drinking water
17 May 2016
Dr Connie O'Driscoll looks at the problem of trihalomethanes in Ireland's drinking water due to natural organic matter and outlines the problems facing water managers in meeting quality standards
On Thursday, 16 June (9am-6pm) and 17 June (9am – 1pm), a workshop is brought to members of the West region by NUI Galway & Trinity College Dublin which will provide a summary of international and national research from engineers and scientists in relation to natural organic matter and trihalomethanes in drinking water, specifically defining the problems water managers are faced with in relation to natural organic matter and looking at some solutions. For more details, please click here.
Successive reports from the Environmental Protection Agency (EPA) have shown that Ireland has an unacceptably high number of drinking-water supplies exceeding the parametric value of 100 μg L-1 for total trihalomethanes (THMs). In fact, Ireland has been reporting the highest non-compliance with respect to total THMs in drinking water across the 27 EU Member States (Figure 1).
Ireland, however, has a large number of public and private supplies (3,664) for a relatively small population, compared to other EU countries. For example, in comparison to Scotland’s 290 supplies, Ireland has 973 public water supplies for a similar population size.
Management of Ireland’s water supplies has been described by the EPA as complex owing to the variation in supply types, i.e. groundwater/surface water source, geographical location, size, treatment processes, management, consumers, ownership issues, distribution networks and a lack of investment in water infrastructure over many years.
THMs are the most well-known class of disinfectant by-products (DBPs) in treated water and the only one regulated by the European Union Drinking Water Regulations, though more than 600 different DBPs have been acknowledged. DBPs are formed when natural water (Figure 2) is treated to control microbial presence during the drinking-water treatment process.
THMs are used as an indicator for the presence of DBPs and do not give a true representation of the extent of DBP presence in drinking water. DBPs consist of halogenated and/or non-halogenated compounds, depending on the nature of disinfectants used and precursors present. THMs are typically formed when chlorine reacts with aromatic structures in natural organic matter (NOM). However, non-aromatic NOM can also form THMs.In a Joint Position Statement (2011), the Health Service Executive and the EPA reiterate the International Agency for Research on Cancer (IARC) review of chlorinated drinking water, which states that chlorinated drinking water was not classified as to its carcinogenicity to humans. Both chloroform and bromodichloromethane, two individual THMs, were classified as possibly carcinogenic to humans (Group 2B carcinogen – inadequate evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals).
To put the risk into perspective, processed meat is classified as a definite cause of cancer (Group 1 carcinogen – definite evidence of carcinogenicity) and belongs to the same group which includes smoking and alcohol. Red meat has been deemed a probable cause of cancer (Group 2a carcinogen – evidence of a link to cancer but no substantial proof).
WHO Guidelines for Drinking Water
The World Health Organisation (WHO) Guidelines for Drinking Water Quality clearly state: “The potential health consequences of microbial contamination are such that its control must always be of paramount importance and must never be compromised.” Microbial contamination can be life-threatening, as was observed in Walkerton, Canada, in May 2000. Cattle manure leaching into a shallow groundwater supply and a lack of chlorine residual caused by the increased chlorine demand of the contaminated water resulted in seven deaths and more than 2,300 illnesses. Water operators were sentenced on December 31, 2004.
In Ireland, water managers typically add enough chlorine to ensure a 0.1 mg L-1 chlorine residual at the end of the network. Statistical analysis of the national dataset on THMs (2006-2013) has shown that water supplies with higher THMs are likely to be caused by an absence of an adequate treatment to remove NOM and presence of treatment incapable of removing high NOM.
NOM refers to a wide range of carbon-based compounds originating from living and dead plants, animals and micro-organisms, and from the degradation products of these sources and can be correlated with highly coloured water. The number of carbon-based compounds is so vast that it is effectively impossible to provide a general chemical description of NOM.
NOM in raw drinking water varies with catchment characteristics. The soil organic carbon pool and percentage peat cover, precipitation and presence of forest plantations are all known to be strong positive determinants of NOM in water. Catchment slope has more recently been shown to be a negative determinant of NOM in raw drinking water, owing to reduced runoff rates and lengthy periods of contact time. Preliminary analyses of the national dataset on THMs has indicated that increased percentage pastureland upstream of a drinking water abstraction site coupled with higher percentage peatland can cause difficulty for treatment plants to remove NOM.
An upward trend in aquatic NOM concentrations has been observed at an international scale and will amplify the challenges for Irish drinking water in meeting the parametric value for THMs (100 μg L−1). Increases in NOM from peaty catchments were initially attributed to global warming and amplified by decreasing atmospheric sulphur. In a post-acidification era, land management, (e.g. peat harvesting and peatland forestry and agriculture) is the focus in altering aquatic NOM delivery and composition.
Measures to meet compliance
Short-term measures being looked at by water managers to meet compliance include ‘in-tank aeration’, the process of exposing finishing drinking water to air. THMs are very volatile and readily evaporate effectively ‘stripping’ water of THMs. In-tank aeration is being looked at as a cost effective, short-term measure by water managers to comply with drinking-water regulations.Medium-term measures involve upgrading infrastructure to meet the increasing amount of NOM that has been observed over the past 10–20 years in raw water supplies.
An increase in the amount of NOM will increase the coagulant dose, which will in turn result in an increase in sludge volumes. Sludge is formed as a by-product of the primary coagulating–flocculating process and contains mineral and humic material removed and precipitated from the raw water, together with the residues of any treatment chemicals used as coagulant (commonly aluminium or iron salts) and coagulant aids (mostly organic polymers). Management of sewage sludge is becoming an issue of growing importance from both an economic and environmental perspective. National sludge production statistics are generally limited; however, one figure released by Inniscarra WTP, Cork, estimates production of 72, 000 m3 day-1 of 16-18% sludge.
Compliance efforts have traditionally focused on engineering (within-plant) solutions and have not considered the underlying ecological mechanisms or anthropogenic influences that determine the production and fate of THM precursors. Protection of drinking water sources should be the ultimate goal and regulations should guide how source waters and their catchments can be managed to reduce THMs.http://www.engineersjournal.ie/2016/05/17/trihalomethanes-in-drinking-water/http://www.engineersjournal.ie/wp-content/uploads/2016/05/dirty-water.jpghttp://www.engineersjournal.ie/wp-content/uploads/2016/05/dirty-water-300x300.jpgChemenvironment,water