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Water Analysis
The risk of chemical contaminants to water
Introduction
The potential risks to human health, wildlife and domesticated animals caused by the presence of chemical contaminants and pathogens in water is of significant concern to the Water Industry. Faecal pollution of water, resulting in waterborne diseases such as cholera and typhoid, has been linked to the deaths of millions of people around the world, although technological advances in water treatment have virtually eliminated these risks in developed countries.
It is therefore, chemical contaminants that currently pose the highest risk to the safety of water supply, with surface waters and groundwater potentially containing thousands of these contaminants. There are many sources of chemical contamination including wastes from industrial and domestic wastewater treatment plants, run-off from agricultural land and leachates from landfills and storage lagoons. It is possible that some chemical contaminants may not be fully removed from raw water, by water treatment plants and therefore could also be present in drinking water.
Other substances may also be present in potable water including disinfection-by-products (DBP) which are formed when disinfectants used in water treatment plants react with bromide and with natural organic matter (e.g. decaying vegetation) present in the source water. Different disinfectants produce different types and amounts of DBP’s, including trihalomethanes (THMs), haloacetic acids (HAAs), bromate, and chlorite.
The analysis of wastewater, surface waters, groundwater and potable water is therefore essential in providing data that demonstrates the effectiveness of water treatment and ensures environmental and health regulatory requirements are met.
Wastewater analysis
Wastewater from industry or domestic use requires treatment to reduce the organic load so that the effluent can be discharged safely into receiving waters. Conventional wastewater treatment processes include:
- Pre-treatment/Screening – is used to remove sand, gravel, rocks and other solid materials
- Primary treatment/Sedimentation – to remove dissolved organic matter (as sludge) and fats, oil and grease
- Secondary treatment – is used to further remove suspended solids and soluble organic matter not removed by primary treatment. Removal is usually accomplished by biological processes which consume biodegradable, soluble, organic contaminants or produce a floc of non soluble contaminants
- Tertiary Treatment – is used as the final phase of treatment, for example when nitrate and phosphate levels must be reduced and where the intended receiving water is very vulnerable to the effects of pollution
The effectiveness of the primary treatment/ sedimentation process is monitored by the analytical parameter, Total Suspended Solids (TSS). Using this method, water samples from the process are filtered, the filter paper is washed and dried and then solids are determined gravimetrically. Results are expressed in mg/L if the sample was originally measured out by volume; or percent by weight, if the sample was originally weighed. An estimate of total suspended solids can be obtained by measuring the turbidity parameter.
Turbidity is an expression of the property by which suspended, or colloidal matter scatters light, thereby imparting opacity to the sample. Turbidity is measured by exposing a sample to light. The intensity of light scattered by the sample is compared with that measured for standard formazin suspensions and expressed as nephelometric turbidity units (NTU).
It is not just solids that are monitored in wastewater. The measurement of pollutants is essential for performance testing of wastewater treatment plants and the impact on receiving waters. Several parameters are routinely used for this analysis, including individual elements e.g. metals, nitrogen, phosphorous, parameters based on oxygen demand, total organic carbon and analysis that identifies specific compounds (target analysis).
Biological oxygen demand
One of the oldest known tests for determining wastewater quality is the Biological Oxygen Demand (BOD) parameter. The BOD result indicates the amount of oxygen in the wastewater, which is needed for biological decomposition of substances.
BOD is determined by adding microorganisms to the water sample. After a predefined time interval, usually 5 days, the oxygen consumed by bacteria during the decomposition of organic substances, is measured. The Royal Commission on Sewage Disposal originally developed the routine BOD test as a means of assessing the rate of biochemical oxidation that would occur in a stream when a polluting effluent was discharged to it.
However, predicting the effect of such a discharge on a stream requires consideration of many factors that are not covered in the determination of the BOD. A number of issues can influence the results of the BOD test including the activity of the micro organisms added to the sample, which may be inhibited by high or low pH, metals, free chlorine, phenols, pesticides and other substances toxic to micro-organisms. In addition, enhanced utilisation of oxygen may be caused by algae and nitrifying organisms.
Chemical Oxygen Demand
The depletion of oxygen in receiving waters is one of the most significant negative effects of water pollution. Preventing these substances from being discharged by wastewater treatment plants is essential to protecting the environment and aquatic life. However, the BOD (5 day test) test is far too slow to provide information useful for monitoring and controlling of wastewater treatment plants. Therefore a more rapid test is often used to estimate the BOD – the Chemical Oxygen Demand (COD) test.
The COD test is carried out by heating the sample with sulphuric acid and potassium dichromate. This process oxidises organic matter chemically and the remaining dichromate is measured. Alternatively, the amount of reduced chromium produced can be measured and translated into an oxygen demand value.
The COD value indicates the amount of oxygen needed to chemically oxidize organic compounds present in wastewater. In addition to organic compounds, other compounds, for example nitrites, bromides, iodides, metal ions and sulphur can also be oxidized using this procedure. The COD value can be correlated with BOD for specific types of samples and is generally higher than the BOD result (typically about 2.5 times). Total Organic Carbon (TOC) analysis is an alternative test to COD (it only produces a response to organic compounds present in water) and TOC can be correlated to both BOD and COD.
Alkalinity and pH
The pH of a sample is a measure of the acidity or alkalinity. Extreme pH in wastewater and in natural waters can:
- Be harmful to aquatic life
- Disrupt biological processes in wastewater treatment plants
- Produce hydrogen sulphide which is odorous and toxic if the pH is low
Most living organisms can only function at a pH close to neutral, with the exception of some microorganisms, which are tolerant to very acidic conditions.
Although an important measure within wastewater treatment plants, the pH of a sample only measures hydrogen or hydroxide ion concentrations. Therefore it does not measure the total acids or bases as pH because the total amount of a weak acid and base do not dissociate completely. To measure the total acidity or alkalinity of a water sample, the sample has to be titrated with a strong base or strong acid and the endpoint determined.
Water Framework Directive
Although the routine parameters discussed above provide useful information on the general quality of wastewater and environmental water samples, detailed analysis, to monitor specific compounds that may be harmful to the environment or living organisms, need to be measured individually. Detrimental effects on wildlife include acute and chronic toxicity as well as subtle effects, which include endocrine disruption i.e. the feminisation of fish due to the discharge of oestrogenic compounds into receiving waters.
Currently a range of legislation covers different aspects of water management. Over the next 5 years many of these regulations will be replaced by the recently introduced Water Framework Directive (WFD), which aims to introduce a simpler approach to environmental protection. For example, the following regulations will be replaced in 2013:
- Freshwater Fish Directive – 78/659/EEC
- Shellfish Waters Directive – 79/923/EEC
- Groundwater Directive – 80/68/EEC
- Dangerous Substances Directive – 76/464/EEC
The Water Framework Directive looks at the ecological health of surface water bodies as well as traditional chemical standards. The directive will help deal with diffuse pollution as well as point source discharges. Priority substances covered by the WFD are shown in Table 1.
Emerging pollutants
Over the past two decades there has been growing concern regarding the presence of biologically active contaminants in the aquatic environment, including endocrine disrupters, pharmaceutical personal care products and other substances. Some of these emerging compounds have become subject to regulation, particularly where there is significant ecotoxicological data to suggest they can cause adverse effects on wildlife or significant risk to human health.
For example, Pentabromobiphenylether, C10-C13 chloroalkanes, 4-nonylphenols and di(2-ethylhexyl)phthalate (DEHP) have been listed as priority hazardous substances and are controlled under EC Water Directive 2000/60/EC. Classes of emerging contaminants that need further research and are not currently covered by regulation are shown in Table 2.
Drinking water analysis
UK Drinking water analysis is carried out in accordance with the Water Supply (Water Quality) Regulations 2000 England and Wales – introduced in 2001 in Scotland. These are transposed from the EU Directive 98/83/EC from which, related regulations for private water supplies have also been produced (2006 in Scotland, due late 2009 in England, Wales and Northern Ireland). Bottled water is also tested according to similar regulations.
A range of analytical techniques are used for the analysis of drinking water quality.
General chemistry
General chemistry covers a variety of physical and inorganic parameters, ranging from pH, to nutrients such as nitrate and phosphate. Examples of the techniques used includes the following: Electrochemical –
- pH probes (semi permeable glass electrodes coupled with a reference electrode). Hydrogen ion concentration quoted as pH units
- Conductivity electrodes (measures the electrical conductance of a water sample) – useful in giving an approximate idea of water hardness and gross contamination such as in discoloured water incidents
- Ion-selective electrodes that specifically measure single parameters such as fluoride
Such basic determinations may be carried out by automated systems, requiring minimal operator input.
Metals analysis
A variety of techniques have been used over the years to determine metals in water samples and these still have their use today in smaller laboratories. However, in high throughput water testing laboratories, these have largely been replaced by one or two main techniques such as ICP-MS and ICP-OES.
ICP-MS (Inductively Coupled Plasma – Mass Spectrometry) – following acidification (nitric acid) of samples and heat treatment to ensure metals ions are in solution, multiple elements can be determined by ICP-MS. The Plasma is a high temperature, ionised gas (argon), contained within a torch. The metallic elements of interest are ionised (charged) and are detected by a mass spectrometer that is able to differentiate and detect them on the basis of their mass to charge ratio.
The advantage of ICP-MS is the ability to analyse many elements simultaneously, at very low levels and without the chemical interferences that older technologies may be subject to. The use of a non-flammable gas in the torch, as opposed to an air/acetylene flame also allows unattended operation. Routinely up to 30 metallic and semi-metallic elements can be analysed in each determination.
ICP-OES – (Inductively Coupled Plasma Optical Emission Spectrophotometry) related to ICP-MS but ICP-OES determines the elements found in the plasma by emission of light radiation. These instruments are unable to analyse the same range of elements to very low detection limits as the ICP-MS. However, many samples do not require analysis of the full range of elements and therefore this technique is suitable for analysis of iron, aluminium, manganese, calcium and magnesium, required upon a significant proportion of water samples.
Mercury can be analysed by ICP-MS although Atomic Fluorescence Spectrophotometry is highly sensitive and is a preferable method for potable water analysis. As mercury is a volatile metal, addition of acidic tin chloride volatilises the mercury to its elemental form, which is purged and detected by measuring fluorescence. The addition of bromine to samples breaks down organomercury compounds so that they can be detected as free inorganic mercury.
Organics analysis
There are a wide range of organic compounds that can have a detrimental effect on drinking water including volatile disinfection by products such as trihalomethanes (THMs), to polyaromatic hydrocarbons (PAHs) and many different pesticides. These compounds have different properties so no one single analytical method is capable of analysing all of them simultaneously. To be able to analyse the compounds using different instruments and detect to low levels a process of sample preparation is usually required. Many of the methods used are capable of detecting organic compounds at low ng/l levels (parts per trillion).
Volatile organics are usually determined by purging with an inert gas, trapping onto an absorbent before being heated (thermally desorbed) onto the detection system. Volatiles such as THMs, benzene, and dichloroethane can be analysed by sampling the “headspace” in a sealed vial. Less volatile compounds may be extracted from water samples using an organic solvent such as hexane (liquid/liquid extraction) or by using cartridges containing a specific absorbent (solid phase extraction). In many cases, the latter technique can extract a greater range of compounds and uses less solvent, so is preferable from a health and safety and environmental point of view.
Organic compounds that are reasonably volatile, can be introduced into Gas Chromatography (GC) systems where they are separated from each other on a specifically designed narrow-bore column. A gas e.g. helium moves the compounds along the length of the column at elevated temperatures and they emerge at the other end at consistent “retention times”. A variety of detectors can be used to measure the compounds – mass spectrometry being one of the more popular because of its specificity and sensitivity.
LC (or HPLC) is used for compounds that are less volatile, unstable to heating, high molecular weight etc. The principles of separation are the same as for GC, but a liquid is used rather than a gas, to separate compounds upon a column that is designed to operate at high pressures (up to 600 bar). Detection may take place using ultra-violet absorbance, fluorescence (for compounds such as PAHs) or mass spectrometry (MS).
Many pesticides are now analysed by tandem mass spectrometry in which MSMS measurements take place, significantly reducing background noise and increasing signal to noise ratios. The main advantage of this technique is that samples can be introduced into the system without the need for sample preparation – saving a great deal of labour. Also, a wide range of compounds can be analysed simultaneously at very low detection limits.
Summary
In recent years there have been significant advancements in technology that have enabled Analytical Chemists to identify an increasing range of emerging contaminants in the aquatic environment and in potable water. Changing legislation and the introduction of new regulations continues to drive Analytical Chemistry Laboratories to invest in new technology and continue to develop advanced analytical methods. As a result of these advances, Analytical Chemists are able to monitor compounds at levels that were impossible or difficult to achieve just a decade ago.
Emerging contaminants have been detected in the environment and in drinking water around the world. Further development in analytical protocols and application of analytical technology will allow researchers to identify emerging compounds producing biological or chemical effects and help evaluate the environmental impact of these contaminants on wildlife and human health. In the EU, the Water Framework Directive requires priority substances to be reviewed regularly and this research will enable the most appropriate compounds to be included in the list of regulated substances.
Severn Trent Laboratories are a leading environmental testing organisation. They provide water and wastewater analysis to a wide range of customers including Water Utilities, Waste Management Companies, Industry and Manufacturing and Local Authorities. Severn Trent Laboratories can be contacted by email info@stl ltd.com or telephone 024 7642 1213. The authors of this article can be contacted directly by email [email protected] and [email protected].
Published: 01st Sep 2009 in AWE International
