Turbidity

October 2003

Help on accessing alternative formats, such as Portable Document Format (PDF), Microsoft Word and PowerPoint (PPT) files, can be obtained in the alternate format help section.

Table of Contents

• 1.0 Guideline
• 2.0 Executive Summary
  • 2.1 Health Considerations
  • 2.2 Analytical and Treatment Considerations
• 3.0 Application of the Guideline
  • 3.1 Monitoring Turbidity Levels
  • 3.2 Use of Alternative Filtration Technology by Waterworks Systems
  • 3.3 Criteria for Exclusion of Filtration in Waterworks Systems
  • 3.4 Considerations for Groundwater Systems
• 4.0 Identity and Sources in the Environment
  • 4.1 Description of Turbidity
  • 4.2 Sources
• 5.0 Relationship between Turbidity and Other Water Quality Parameters
  • 5.1 Microbiological Characteristics
    • 5.1.1 Relationship between Turbidity and the Presence of Pathogenic/Non-pathogenic Organisms
    • 5.1.2 Effect of Turbidity on Disinfection
    • 5.1.3 Effect of Turbidity on Microbial Enumeration
    • 5.1.4 Relationship between Turbidity Removal and Microbial Quality of Treated Water
  • 5.2 Chemical Characteristics
  • 5.3 Physical Characteristics
• 6.0 Analytical Methods
• 7.0 Treatment Technology
  • 7.1 Chemically Assisted Filtration
  • 7.2 Slow Sand Filtration
  • 7.3 Diatomaceous Earth Filtration
  • 7.4 Membrane Filtration
• 8.0 Health Considerations
  • 8.1 Microbial
  • 8.2 Chemical
• 9.0 Other Considerations
• 10.0 Rationale
• 11.0 References
• Appendix A: Determining if Groundwater Is Under the Direct Influence of Surface Water

1.0 Guideline

Waterworks systems that use a surface water source or a groundwater source under the direct influence of surface water should filter the source water to meet the following health-based turbidity limits, as defined for specific treatment technologies. Where possible, filtration systems should be designed and operated to reduce turbidity levels as low as possible, with a treated water turbidity target of less than 0.1 NTU at all times. Where this is not achievable, the treated water turbidity levels from individual filters:

1. For chemically assisted filtration, shall be less than or equal to 0.3 NTU in at least 95% of the measurements made, or at least 95% of the time each calendar month, and shall not exceed 1.0 NTU at any time.
   
   
2. For slow sand or diatomaceous earth filtration, shall be less than or equal to 1.0 NTU in at least 95% of the measurements made, or at least 95% of the time each calendar month, and shall not exceed 3.0 NTU at any time.
   
   
3. For membrane filtration, shall be less than or equal to 0.1 NTU in at least 99% of the measurements made, or at least 99% of the time each calendar month, and shall not exceed 0.3 NTU at any time. If membrane filtration is the sole treatment technology employed, some form of virus inactivation* should follow the filtration process.
   
   

It is not expected that all water supplies will be able to meet this revised turbidity guideline immediately. Therefore, supplementary treatment should be considered in the interim to ensure delivery of safe drinking water.

* Some form of virus inactivation is required for all technologies. The difference is that chemically assisted, slow sand and diatomaceous earth filters are credited with log virus reductions and membrane filters receive no credit.

2.0 Executive Summary

Particles of matter are naturally suspended in water. These particles can be clay, silt, finely divided organic and inorganic matter, plankton and other microscopic organisms. Turbidity is a measurement of how light scatters when it is aimed at water and bounces off the suspended particles. It is not a measurement of the particles themselves. In general terms, the cloudier the water, the more the light scatters and the higher the turbidity.

The best means of reducing turbidity and safeguarding a drinking water supply is to apply a multiple-barrier approach (i.e., source to tap) to protect drinking water. The focus of this approach is to look at the entire drinking water supply, identify potential and existing hazards and then develop strategies to deal with each of the hazards.

Treatment plants can reduce turbidity by filtering particles out of the water. All filtration systems should be designed and operated to reduce turbidity levels as low as possible. The treated water turbidity target is 0.1 NTU at all times. However, even though effective filtration can be accomplished using any one of a number of technologies, the actual levels of turbidity achieved will vary from technology to technology. For this reason, the turbidity guideline is broken down by type of technology.

The most important consideration when dealing with turbidity is to make sure the levels remain low and fairly constant over time. Concerns are most likely to result from a spike in the level of turbidity, due either to an increase in the amount of particulate matter in the source water (e.g., from heavy rains) or to a breakdown in the treatment process (e.g., inadequate coagulation, a ruptured filter). Because it might otherwise be difficult to notice a spike in turbidity when only one filter in a plant is not working properly, the guideline applies to each individual filter within a filtration system.

2.1 Health Considerations

It is important to control turbidity in public water supplies for both health and aesthetic reasons. Suspended matter can contain toxins such as heavy metals and biocides and can also harbour microorganisms, protecting them from disinfection. Recent research has correlated turbidity levels with treated water supplies being contaminated with Giardia and Cryptosporidium. These microorganisms can cause outbreaks of illness. As such, turbidity may be used as a health parameter to indicate the safety of water leaving a filtration system. Because turbidity can affect the microbiological quality of drinking water, this guideline should be read in conjunction with the bacteriological guidelines. Excessive turbidity may also be associated with unpleasant tastes and odours.

In addition, high turbidity can lead to an increase in the amount of disinfection by-products that form in treated water. Trihalomethanes (THMs), for instance, are a group of chemical compounds that form when chlorine reacts with organic material in water. By filtering out the organic matter to reduce turbidity, treatment plants also reduce the amount of THMs that may form in the water. For more information on these disinfection by-products, see the THMs guideline and supporting document.

The nature of turbidity and its health implications vary with the type of source water. Turbidity in surface water and groundwater that comes into contact with surface water (referred to as groundwater under the direct influence of surface water), however, is generally organic in nature and may contain toxins, harbour pathogens or lead to the formation of THMs. Turbidity in secure groundwater supplies (i.e., not under the influence of surface water) is generally non-organic and should pose no health threat. The health-based guideline and target for turbidity therefore apply only to surface water sources and groundwater under its influence.

2.2 Analytical and Treatment Considerations

Turbidity is easy and inexpensive to measure. In addition to being an indicator for determining the relative safety of drinking water, it is a useful tool for assessing the performance of water treatment processes.

Turbidity is measured in nephelometric turbidity units, or NTU, using a device called a turbidimeter. Modern turbidimeters can make measurements of 0.1 NTU or lower. Levels of turbidity in raw waters can range from 1.0 NTU to more than 1000.0 NTU. Levels vary at individual locations over time.

A number of studies indicate that properly designed and well-operated conventional, chemically assisted and direct filtration water treatment plants can readily achieve a safe finished water with turbidity levels lower than 0.2 NTU. Meeting the guideline level of 0.3 NTU for these systems should be straightforward. Slow sand and diatomaceous earth filtration plants can consistently achieve a safe finished water with turbidity levels of less than 1.0 NTU. Membrane filtration plants can consistently achieve finished water turbidity of less than 0.1 NTU. For all filtration technologies, these limits are achievable and expected in 95-99% of measurements, but a target of 0.1 NTU should be sought at all times.

3.0 Application of the Guideline

The health-based turbidity guideline applies to drinking water produced by systems that use either a surface water source or a groundwater source under the direct influence of surface water. The guideline is applied to individual filter turbidity. However, good operating practices suggest that both the individual filter turbidity and the combined filter turbidity should be continuously monitored. Drinking water taken from pristine sources may be exempt from the filtration requirements if it meets all of the criteria outlined below (see "Criteria for Exclusion of Filtration in Waterworks Systems").

Surface water is defined as all waters open to the atmosphere and subject to surface runoff. Groundwater under the direct influence of surface water is defined as "any water beneath the surface of the ground with (i) significant occurrence of insects or other macro-organisms, algae, organic debris, or large-diameter pathogens such as Giardia lamblia, Cryptosporidium, or (ii) significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water conditions." ¹ Key issues that should be considered when determining whether groundwater is under the influence of surface water are given in Appendix A.

The health-based turbidity guideline does not apply to secure groundwater sources, i.e., those not under the direct influence of surface water. Turbidity in these cases is non-organic, should pose no health threat and should not hinder disinfection. However, for effective operation of the distribution system, it is good practice to ensure that water entering the distribution system has low turbidity levels of around 1.0 NTU.

3.1 Monitoring Turbidity Levels

For chemically assisted filtration (i.e., continuous feed of a coagulant with mixing ahead of filtration), source water turbidity levels should be measured at least once per calendar day directly in front of where the first treatment chemical is applied. Treated water turbidity levels from individual filters should be continuously measured (with an on-line turbidimeter) at intervals no longer than 5 minutes apart at a point upstream of the combined filter effluent line or the clear water tank.

For slow sand or diatomaceous earth filtration, water turbidity levels from individual filters should be continuously measured (with an on-line turbidimeter) at intervals no longer than 5 minutes apart at a point upstream of the combined filter effluent line or the clear water tank. However, the frequency of monitoring may be reduced to one grab sample per day if it can be demonstrated that this frequency gives a reliable measure of filter performance.

For membrane filtration, treated water turbidity levels from individual filters should be continuously measured (with an on-line turbidimeter) at intervals no longer than 5 minutes apart at a point upstream of the combined filter effluent line or the clear water tank. An individual membrane filter may be defined as a unit or group of membrane stacks or cartridges within a train that may be valved and isolated from the rest of the system for testing and maintenance. Process designs should include a minimum of two parallel trains, if practical. Consideration should be given to installing on-line turbidity meters to analyse the water unique to each individual filter.

3.2 Use of Alternative Filtration Technology by Waterworks Systems

A waterworks system can use a filtration technology other than the technologies stipulated if, in combination with disinfection, it reliably achieves at least a 3-log reduction of Giardia lamblia cysts and Cryptosporidium oocysts and a 4-log reduction of viruses. Pilot studies or equivalencies from other jurisdictions should demonstrate that the technology meets these criteria.

3.3 Criteria for Exclusion of Filtration in Waterworks Systems

Filtration of a surface water source or a groundwater source under the direct influence of surface water may not be necessary if all of the following conditions are met:

1. Overall inactivation is met using a minimum of two disinfectants:
   • ultraviolet irradiation or ozone to inactivate cysts/oocysts;
   • chlorine (free chlorine) to inactivate viruses; and
   • chlorine or chloramines to maintain a residual in the distribution system.
   Disinfection should reliably achieve at least a 99% (2-log) reduction of Cryptosporidium oocysts,* a 99.9% (3-log) reduction of Giardia lamblia cysts and a 99.99% (4-log) reduction of viruses. If mean source water cyst/oocyst levels are greater than 10/1000 L, more than 99% (2-log) reduction of Cryptosporidium oocysts and 99.9% (3-log) reduction of Giardia lamblia cysts should be achieved. Background levels for Giardia lamblia cysts and Cryptosporidium oocysts in the source water should be established by monitoring as described in the most recent "Protozoa" guideline document, or more frequently during periods of expected highest levels (e.g., during spring runoff or after heavy rainfall).
   
   
2. Prior to the point where the disinfectant is applied, the number of Escherichia coli bacteria in the source water does not exceed 20/100 mL (or, if E. coli data are not available, the number of total coliform bacteria does not exceed 100/100 mL) in at least 90% of the weekly samples from the previous 6 months.
   
   
3. Average daily source water turbidity levels measured at equal intervals (at least every 4 hours), immediately prior to where the disinfectant is applied, are around 1.0 NTU but do not exceed 5.0 NTU for more than 2 days in a 12-month period. Source water turbidity also does not show evidence of protecting microbiological contaminants.
   
   
4. A watershed control program (e.g., protected watershed, controlled discharges, etc.) is maintained that minimizes the potential for faecal contamination in the source water.

3.4 Considerations for Groundwater Systems

In keeping with the multi-barrier approach to drinking water quality management, systems using secure groundwater sources should:

1. Ensure that groundwater wells are properly constructed, are located in areas where there is minimum potential for contamination and have appropriate wellhead protection measures in place. These source protection measures protect public health by reducing the risk of the drinking water source becoming contaminated.
   
   
2. Ensure that treatment is sufficient to achieve 4-log reduction of viruses by disinfection. It is important to confirm that elevated turbidity levels will not compromise the disinfection process.
   
   
3. Maintain a chlorine residual throughout the distribution system and ensure that water quality is monitored and maintained. Well-designed and well-operated distribution systems are key to providing safe, clean drinking water to consumers.

* Studies on human volunteers have demonstrated that Cryptosporidium oocysts are less infectious than Giardia cysts by about one order of magnitude.

4.0 Identity and Sources in the Environment

The sources and nature of turbidity are varied and complex and are influenced by the physical, microbiological and chemical characteristics of water. In surface waters and groundwater under the direct influence of surface water, turbidity can vary significantly over time, which has important implications for drinking water treatment processes and the microbiological safety of the drinking water. Particulate matter is frequently a source of nutrients for microorganisms and can protect microorganisms from both chemical and ultraviolet light disinfection. Particles contributing to turbidity may also carry undesirable chemical contaminants such as heavy metals. Turbidity can seriously affect the safety and acceptability of drinking water to consumers.

4.1 Description of Turbidity

Turbidity is a "measure of the relative clarity of water."² Turbidity in water is caused by suspended and colloidal matter, such as clay, silt, finely divided organic and inorganic matter, and plankton and other microscopic organisms. However, turbidity is not a direct measure of suspended particles suspended in the water. It is, rather, a measure of the scattering effect that such particles have on light. A directed beam of light remains relatively undisturbed when transmitted through absolutely pure water, but even the molecules in a pure fluid will scatter light to a certain degree. Standard Methods for the Examination of Water and Wastewater defines turbidity as an "expression of the optical property that causes light to be scattered and absorbed rather than transmitted with no change in direction or flux level through the sample."³

In samples containing suspended solids, the manner in which water interferes with light transmittance is related to the size, shape and composition of the particles in the water and to the wavelength (colour) of the light that falls on the particles (incident light).⁴ A minute particle absorbs the incident light falling on it and then re-radiates the light in all directions.

The detection, measurement and visual perception of turbidity are influenced by a number of factors. Particle size has an impact on the direction in which light is scattered and on the intensity of scattered light of differing wavelengths (colours). The shape of the particle also influences light scattering, as do the refractive index of the water and the colour of the particles.

Light scattering intensifies as particle concentration increases. However, as scattered light strikes more and more particles, multiple scattering occurs, and absorption of light increases. When particulate concentration exceeds a certain point, detectable levels of both scattered and transmitted light drop rapidly, marking the upper limit of measurable turbidity. By decreasing the path length of light through the sample, the number of particles between the light source and light detector is reduced, extending the upper limit of turbidity measurement.

Because several factors affect the intensity of light scattering, it is not possible to relate scattered light measurements directly to the number or weight of suspended solids in a given volume of water with any accuracy. Direct correlations can be made only if such factors as the size, distribution, shape, refractive index and adsorptive capacity of the suspended solids causing the turbidity remain constant; this can be achieved only in a laboratory and is therefore impractical and unnecessary in most cases.⁴

4.2 Sources

Levels of turbidity in raw water can range from less than 1.0 NTU to more than 1000.0 NTU. The particles that cause turbidity in water range in size from colloidal dimensions (approximately 10 nm) to diameters of the order of 0.1 mm and can be divided into three general classes: clays, organic particles resulting from decomposition of plant and animal debris, and fibrous particles from asbestos minerals.⁵ Clay particles generally have an upper diameter limit of about 0.002 mm, but can be as large as 0.02 mm. Biological organisms may also cause turbidity.

Particulate material in natural waters is mostly made up of eroded soil particles from the surrounding area. Coarser sand and silt fragments are at least partially coated with organic material. Clay particles are composed of clay minerals, usually phyllosilicates, as well as non-clay material, such as iron and aluminum oxides and hydroxides, quartz, amorphous silica, carbonates and feldspar.⁵ Clays and organic particles are often found together as a "clay organic complex."⁵ To a certain extent, it is artificial to treat the organic (humic) component in isolation from the inorganic component when considering the behaviour of suspended matter. However, humic substances have a much higher adsorptive capacity than clays (870 meq/100 g and 80 100 meq/100 g, respectively⁶); the effect of humic components likely predominates in many instances.

Other particles in raw water and drinking water supplies include the group of naturally occurring hydrated silicate minerals with fibrous structures known as asbestos; inorganic precipitates, such as metal (iron or manganese) oxides and hydroxides; and biological organisms, such as algae, cyanobacteria, zooplankton, and filamentous or macrobacterial growths.⁷,⁸ Due to the numerous types of source particles and their implications in the treatment process, raw water quality monitoring for turbidity should be done at least daily, and preferably more often.

5.0 Relationship Between Turbidity and Other Water Quality Parameters

Table 1 summarizes some of the relationships between the source of turbidity and water quality/treatment implications.

Table 1: Turbidity and implications for water quality and water treatment

Source of turbidity	Possible water quality/chemistry implications	Treatment implications
Inorganic particles (silt, clay, natural precipitants, e.g., CaCO3, MnO2, Fe2O3, etc.)	- raise/lower pH and alkalinity - source of micronutrients - affect zeta potential - source of metals and metal oxides - cloudy/turbid appearance - affect taste	- major influence on coagulation, flocculation and sedimentation design - harbour/protect microorganisms
Organic particles (decomposed plant and animal debris, humic substances)	- source of energy and nutrients for microorganisms - cause colour - impart taste and odour - serve as precursors for the formation of chlorinated or ozonated compounds - possess ion exchange and complexing properties that include association with toxic elements and micropollutants - affect pH - affect zeta potential	- high disinfectant demand - potential to form chlorinated organics - potential to form ozonation by- products - high coagulant dose - reduce clarifier overflow rates - increase flocculation/sedimentation times - harbour/protect microorganisms - reduce filter runs - can compete with pollutant compounds for adsorption sites in activated carbon adsorption - can precipitate in the distribution systems
Biological organisms (algae, cyanobacteria, zooplankton, filamentous or macrobacterial growth)	- impart taste and odour - potential source of toxin (microcystin-LR) - disease transmission - corrode tanks, pipes, etc. - stain fixtures - cause aesthetic problems due to sloughing of growths from tanks, filters, reservoirs and distribution system	- plug filters - high disinfectant demand - need multiple barriers to ensure effective microbial inactivation - flotation may be more effective than sedimentation - microbial inactivation required

5.1 Microbiological Characteristics

5.1.1 Relationship between Turbidity and the Presence of Pathogenic/Non-pathogenic Organisms

The microbiological quality of drinking water can be significantly affected by turbidity. Microbial growth in water is most extensive on the surfaces of particles and inside loose flocs, both naturally occurring and those formed during treatment (see section "Treatment Technology"). This growth occurs because nutrients adsorb to surfaces, allowing bacteria to grow more efficiently than when in free suspension.⁹,¹⁰ Similarly, river silt has been shown to readily adsorb viruses.¹¹

Studies of distribution systems have shown conflicting findings with respect to turbidity and microorganisms. Haas et al.¹² noted that increased values of pH, temperature and turbidity were associated with increased concentrations of microorganisms. Heterotrophic plate count (HPC, formerly known as standard plate count) increases that parallel increases in turbidity have been found at turbidity levels lower than 2.0 NTU.¹³ Similarly, work by Goshko et al.¹⁴ found positive correlations between HPCs and turbidities in the 0.83-8.89 NTU range. On the other hand, a study reported by Reilly and Kippin¹⁵ suggested that turbidity around 1.0 NTU does not affect the frequency with which either coliforms or HPC organisms occur in the analysis.

In water with turbidities ranging from 3.8 to 84.0 NTU, Sanderson and Kelly¹⁶ found coliform organisms even after the water was treated with chlorine (free chlorine residuals between 0.1 and 0.5 mg/L after a minimum contact time of 30 minutes).

Huck et al.,¹⁷ in their investigation of Cryptosporidium removal by granular media filtration, noted that an increase in turbidity associated with suboptimal coagulation and breakthrough at the end of filtration runs resulted in deterioration in oocyst reduction, even at turbidity levels less than 0.3 NTU. Utilities should therefore carefully consider the effects of reducing coagulant dosage. To avoid breakthrough, plants should specify a maximum head loss and filter run times and should consider using particle counters to monitor for early breakthrough.

5.1.2 Effect of Turbidity on Disinfection

Particulate matter (e.g., organic, inorganic, higher microorganisms) can protect bacteria and viruses from the effects of disinfection. LeChevallier et al.,¹⁸ studying the efficiency of chlorination in killing coliforms in unfiltered surface water supplies, found a negative correlation with turbidity. A derived model predicted that an increase in turbidity from 1.0 to 10.0 NTU would result in an eight-fold decrease in the disinfection efficiency at a fixed chlorine dose. A study by Hoff¹⁹, which examined the efficiency of disinfection at turbidities of 1.0 and 5.0 NTU on poliovirus and sewage effluent coliforms, found that viruses and coliforms that adsorbed to organic matter were more resistant to disinfection than those that adsorbed to inorganic material such as clay and aluminum phosphate. For organic particulates, a reduction of turbidity from 5.0 to 1.0 NTU reduced the concentrations of disinfectant-resistant organisms approximately five-fold.

Hoff and Geldreich²⁰ reiterated that particulate characteristics have a significant impact on protection effects. Studies with ozone by Sproul et al.²¹ confirmed that alum and bentonite afforded little protection to a variety of test organisms at 1.0 and 5.0 NTU, whereas faecal material and, in particular, human epithelial carcinoma cells did provide protection. Chlorine dioxide studies by Scarpino et al.²² suggested that temperature and turbidity affected the rate of inactivation of bentonite-adsorbed poliovirus. At 25°C, turbidities in excess of 2.29 NTU reduced inactivation rates.

Free-living nematodes are relatively common in North American municipal water supplies. Nematodes of the Rhabditae family are known to ingest pathogenic bacteria and viruses and hence are able to protect these pathogens from chlorine disinfection.²³ Studies indicate that more nematodes are found in higher-turbidity raw and treated waters.²⁴,²⁵ In a study of the San Francisco water supply, coliform organisms were detected at chlorine levels of 0.35 mg/L or greater. Crustaceans apparently harboured the coliforms; on passing through a spigot, the crustaceans ruptured, and viable coliforms were released.²⁶ In laboratory tests, various clays and humic acid were shown to protect Klebsiella aerogenes from ultraviolet light disinfection.²⁷

Chlorine (as hypochlorous acid) reacts readily with organic matter containing unsaturated bonds, phenolic groups and nitrogen groups, giving rise to taste- and odour-producing compounds²⁸ and trihalomethanes (THMs).²⁹ Hence, waters with high turbidity from organic sources may give rise to a substantial chlorine demand. This could result in reductions in the free chlorine residual in distribution systems as protection against possible recontamination. For Ottawa River plants, Otson et al.³⁰ noted that increased pre-chlorination dosage requirements were strongly correlated with increases in turbidity. In Oregon surface waters, chlorine demand had a positive correlation with both turbidity and total organic carbon levels.¹⁸ The resultant model suggested a 180% increase in chlorine demand for a turbidity increase from 1.0 to 5.0 NTU.

In the United States, well-operated slow sand filtration plants may be allowed to have higher turbidity in filter effluents if there is no interference with disinfection and the turbidity level never exceeds 5.0 NTU.³¹ Non-interference with disinfection may be assumed if the finished water meets the coliform maximum contaminant level and if there are fewer than 10 HPC bacteria per millilitre during times of highest turbidity.³¹

5.1.3 Effect of Turbidity on Microbial Enumeration

The presence of turbidity may interfere with the quantification of bacteria and viruses. Bacteria are enumerated by counting the number of visible colonies that form on nutritive media when bacterial cells are incubated on the media for a fixed period of time. This process assumes that each colony represents one cell; however, a single colony could emanate from a particle containing many bacterial cells adsorbed on its surface. Fewer cells than were actually present would then be recorded. This phenomenon would also lead to an underestimation of bacterial numbers with the most probable number technique.

Geldreich et al.³² noted that turbidity in a potable water sample may make membrane filtration impractical because of the volume of water the filter can pass, the character of the suspended material and the thickness of the deposit on the surface of the membrane. Although crystalline or siliceous materials may not be a problem, other substances may clog filter pores or cause a confluent growth to develop during incubation, hampering microbial enumeration. Coliform masking has been observed with membrane filters, with false-negative results occurring 17, 45 and more than 80% of the time for turbidities of less than or equal to 1.0, 5.0 and more than 10.0 NTU, respectively.¹⁸,³³ Additional studies suggested that levels of turbidity per se (up to approximately 10.0 NTU) did not greatly affect coliform discovery, although associated non-coliform bacteria seriously inhibited detections.³⁴

Viruses can also be adsorbed on or within particulate matter and may be very difficult to remove; 1% recovery is not unusual.³⁵ A review of virus detection methods concluded that no simple and accurate system for enumerating viruses in highly turbid waters was available.³⁶

5.1.4 Relationship between Turbidity Removal and Microbial Quality of Treated Water

Historically, filtration has been shown to substantially block disease-causing organisms from entering into the drinking water supply.³⁴ During coagulation, protozoa, bacteria and viruses, along with other sources of turbidity, become trapped in the floc and are removed by the filter.³⁷,³⁸ However, sometimes floc breaks through filter beds; such breakthroughs have been shown to be accompanied by an increase in virus penetration, even though the turbidity of the finished water remained below 0.5 NTU.³⁵

Studies have shown a correlation between decreased filtrate turbidity (down to 0.1 NTU) and reduced bacterial and algal counts.³⁹ Increases in the turbidity of filter effluent can signal the potential for increasing passage of unwanted organisms, even if the turbidity in the effluent is less than 1.0 NTU. For example, increasing concentrations of Giardia cysts can occur with turbidity increases of only 0.2-0.3 NTU.⁴⁰,⁴¹

The Pennsylvania Department of Environmental Protection, in its 1996 Regulatory Basics Initiative Program report,⁴² gave its view of the relationship between turbidity and pathogen occurrence in finished filtered water. It stated that a relationship exists between turbidity spikes and Giardia breakthrough such that a stable filter with low turbidities that experiences a 0.1 NTU increase in turbidity can experience a 10- to 50-fold increase in cyst breakthrough from disturbance of the media.

In evaluating plant performance using endospores, researchers found that the log reduction of spores was similar in magnitude to the individual reduction of turbidity, total particles and particles in the Cryptosporidium oocyst size range. More important, spore removal closely paralleled particle and turbidity removal in response to coagulant dosage under all the water quality conditions examined.⁴³

In examining relationships between turbidity and parasites, it was found that for every log removal of turbidity, a 0.89-log removal was achieved for Giardia and Cryptosporidium.⁴⁴ Conversely, increases in filtrate turbidity paralleled increases in the risk of Cryptosporidium breaking through the filter due to floc material breaking through that contained, or was associated with, oocysts. These increases occurred even with efficient chemical coagulation. It is therefore reasonable to assume that during the filter "ripening" period at the beginning of a run, when turbidity is often greater than normal for the filter, the risk of Cryptosporidium breakthrough is higher.⁴⁵

Table 2 shows average potential removal credits estimated for Giardia lamblia, Cryptosporidium and viruses, when treated water meets the prescribed turbidity limits. The log reduction credits outlined in Table 2 are based on the mean or median removal established by the U.S. Environmental Protection Agency (EPA) as part of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR).⁴⁶ Facilities that do not meet the requirements, or facilities that believe they can achieve a higher log credit than is automatically given, can be granted a log reduction credit based on a demonstration of performance by the appropriate regulatory agency.

Table 2: Giardia lamblia, Cryptosporidium and virus potential reduction credits for various technologies

Technology	Cyst/oocyst creditc	Virus credit
Conventional filtrationa	3.0 log	2.0 log
Direct filtrationa	2.5 log	1.0 log
Slow sand or diatomaceous earth filtrationa	3.0 log	2.0 log
Micro- and ultrafiltration, nanofiltration and reverse osmosisb	Removal efficiency demonstrated through challenge testing and verified by direct integrity testing	No credit for micro- and ultrafiltration; for nanofiltration and reverse osmosis, removal efficiency demonstrated through challenge testing and verified by direct integrity testing

^a Conventional/direct/slow sand/diatomaceous earth filtration should be followed by free chlorination to obtain additional virus credit.

^b Micro- and ultrafiltration should be followed by free chlorination for the inactivation of viruses.

^c Depending on cyst/oocyst levels in source water, additional treatment is required using ultraviolet light, ozone, chlorine or chlorine dioxide (refer to protozoa/virological quality supporting documents for the level of inactivation required and the CT/IT tables for various disinfectants).

5.2 Chemical Characteristics

Because of their adsorption capacity, suspended particulates have the ability to entrap undesirable organic and inorganic compounds; as such, an indirect relationship exists between turbidity and the chemical characteristics of these compounds in water. Most important in this respect is the organic or humic component of turbidity.

Humic substances are able to bind substantial amounts of metals and hydrous oxides together, forming complexes. An excellent review of metal-humate complexes, the mechanism of their formation and their properties is provided by Schnitzer and Kahn.⁴⁷ The ability of a number of natural waters in Ontario to complex copper has been demonstrated, with complexing capacities of up to 2.35 μmol Cu/L (0.149 mg/L) being reported.⁴⁸ A wide variety of heavy metal ions were found to be complexed in sediments of the Ottawa and Rideau rivers. A positive correlation between the unit surface area of the sediment and the concentration of adsorbed metal ions was observed.⁴⁹ In a study of mercury sorption and desorption characteristics of Ottawa River sediments, it was found that sorption rates were higher for organic-rich sands. Desorption of mercury was difficult, with less than 1% of the mercury being leached during a 7-hour contact period.⁵⁰ The strength of some metal-humate complexes may lead to negative errors in the analytical measurement of trace metals in natural water samples if turbidity exists.⁵¹

One method that is used to remove undesirable metal ions during water treatment is adsorption with activated carbon. This process is aided by the presence of organic matter.⁵² Organic molecules are also adsorbed by natural organic matter. DDT, for example, is solubilized in 0.5% sodium humate solution by a factor of at least 20 over its solubility in pure water.⁵³ Herbicides such as 2,4-D, paraquat and diquat can be adsorbed onto clay/humic acid particulates, the adsorption being greatly influenced by metal cations present in the humic material.⁵⁴ The presence of turbidity, therefore, might also interfere with the detection of biocides in water samples.

Chlorination of water containing organic matter such as humic acids can produce THMs, a group of chemical compounds that includes chloroform, bromodichloromethane, chlorodibromomethane and bromoform. The Canadian drinking water guideline for THMs is based on the known health effects associated with chloroform. Morris and Johnson⁵⁵ observed a relationship between raw water turbidity and THM concentration in finished Iowa City water. In laboratory tests, Stevens and co-workers⁵⁶ found that THM production was reduced if the water was filtered prior to chlorination. Harms and Looyenga⁵⁷ also reported that raw water turbidity was positively correlated with chloroform concentration in a South Dakota water supply. Strategies for addressing turbidity have implications related to controlling the potential formation of THMs, including the removal of organic matter, the use of alternative disinfectants, disinfectant application points and dosages, and the use of activated carbon.⁵⁸

For plants using aluminum salts as coagulants, highest particulate aluminum concentrations (>200 μg/L) were measured when the turbidity was greater than 0.5 NTU.⁵⁹ The results of the study suggest that filtered water turbidity should be less than 0.1 NTU to minimize particulate aluminum concentrations that contribute to residual aluminum.⁵⁹

At the asbestos levels commonly found in drinking water (of the order of 10⁴-10⁶ fibres/L),⁶⁰ very little, if any, correlation has been observed between turbidity and asbestos concentration.²,⁶¹,⁶² However, a general but non-linear relationship has been reported at high levels of asbestos (10⁹-10¹¹ fibres/L).⁶³ Further studies on treatment efficiencies for asbestos removal have resulted in a recommendation by Logsdon and co-workers⁶⁴,⁶⁵ that plants designed for asbestos removal should produce filtered waters with turbidities of 0.1 NTU or lower. McGuire et al.⁶⁶ suggested that this objective would help but not necessarily guarantee low asbestos counts (<10⁶ fibres/L). Boatman⁶⁷ reported that turbidity could impede asbestos analyses because of restricted filter volumes. Asbestos-cement pipes are used in some localities to transport drinking water, and it has been demonstrated that water with an aggressivity index of less than 10 can cause the release of asbestos fibres into the drinking water.⁶⁸

5.3 Physical Characteristics

A considerable body of evidence suggests that a large part of colour in water arises from colloidal particles. These tiny particles have physical and chemical properties that allow them to stay suspended in the water, rather than settling down or dissolving. Black and co-workers⁶⁹,⁷⁰ used electrophoretic studies to demonstrate the predominantly colloidal nature of colour in water; it has been claimed that about 50% of colour is due to a "colloidal fraction" of humic substances.⁷¹ True colour is therefore defined as the colour of water from which turbidity has been removed.²

The relationship between high turbidity, in both raw and filtered water, and taste and odour has long been recognized.⁷² Algal growths, actinomycetes and their debris also contribute to taste and odour problems.⁷ At 5.0 NTU and above, there is an increasing visual detection, which many consumers find unacceptable.

6.0 Analytical Methods

Turbidity is measured using the nephelometric method. Nephelometry determines turbidity using the intensity of scattered light. Table 3 lists four nephelometric methods that meet the criteria of the American Water Works Association/American Public Health Association (AWWA/APHA) or the U.S. EPA and one International Organization for Standardization (ISO) criterion for determining turbidity in drinking water.

Table 3: Recognized analytical methods for measuring turbidity in drinking water^a

Method	Citation	Description
Nephelometric	AWWA/APHA 2130B3	Tungsten lamp@2200-3000°K and one or more perpendicular detectors (& filters) with spectral response peak of 400-600 nm; light path ≤10 cm
Nephelometric	U.S. EPA 180.173	Tungsten lamp@2200-3000°K and one or more perpendicular detectors (& filters) with spectral response peak of 400-600 nm; light path ≤10 cm
Optical	ISO 702774	Tungsten lamp (& filters), diode or laser as radiation source at 860 nm (or 550 nm if sample is colourless) with a perpendicular detector and aperture angle 20-30°
Great Lakes Instruments (GLI)	U.S. EPA GLI 275	Two perpendicular 860-nm light sources alternately pulse each 0.5 seconds, & two perpendicular detectors alternately measure "reference" and "active" signals
Hach Filter Trak	U.S. EPA 1013376	Laser diode @660 nm at 90° to detector/receiver (light path ≤10 cm), which may use photo-multiplier tube and fibre-optic cable; range is 0-1000 mNTUs

^a Additional methods may be approved before this guideline is revised/updated in the future.

Nephelometric turbidity instrumentation varies in design, range, accuracy and application. The design of nephelometric instruments should take into account the physics of scattered light. As noted in a previous section ("Identity and Sources in the Environment"), the size, shape and concentration of the particles affect the intensity pattern and distribution of the scattered light. Small particles less than one-tenth of the light wavelength will scatter light uniformly in both forward and backward directions. As the particle size approaches and exceeds the wavelength of the incident light, more light is transmitted in the forward direction. Because of this intensity pattern, the angle at which the light is measured is a critical factor; the current international standards have determined the most appropriate angle to be 90 degrees.⁷⁴ As noted above, as the concentration of particles increases, more particles reflect the incident light, which increases the intensity of the scattered light. As the concentration exceeds a certain point determined by the specific optical characteristics of the process, the particles themselves begin to block the transmission of the scattered light. The result is a decrease in the intensity of the scattered light. The intensity at which various wavelengths of light are reflected or absorbed is also determined by the colour of the liquid and the reflecting surface. Industry standards require nephelometers to operate in the visible or infrared ranges: 400-600 and 800-900 nm, respectively.⁷⁷

All these factors, along with the optical geometry of a particular instrument, cause measured values between instruments to vary widely; thus, criteria for instrument design have been developed to minimize these variables. Manufacture of turbidimeters is guided by recommendations provided by the U.S. EPA⁷⁸ and the ISO (ISO 7027).⁷⁴

Using special experimentation methods with a quartz iodine light source, the nephelometric response of exhaustively filtered de-ionized water has been shown to be 0.022-0.003 NTU.⁷⁹ Air bubbles and dirty sample tubes can cause false high readings for turbidity; very turbid samples or samples with colour due to dissolved substances will give low readings.³,⁸⁰

For a finished water turbidity goal of 0.1 NTU, rigorous standard operating procedures and a high level of quality control are required; a small numeric change may result in a large percent change.⁸¹,⁸² However, according to the U.S. EPA's Analytical Methods for Turbidity Measurement (180.1 and GLI 2) and Standard Methods for the Examination of Water and Wastewater (2130B), the sensitivity of nephelometers is such that turbidity differences of 0.02 NTU or less can be detected in waters having a turbidity of less than 1.0 NTU.³,⁷³,⁷⁵ All three methods cite reporting to the nearest 0.05 NTU where the turbidity range is 0-1.0 NTU. Thus, the practical lower limit of the standard nephelometric method can be considered to be 0.1 NTU.

Laser turbidimeters have recently entered the market. The manufacturers claim that these instruments are far more sensitive than the standard turbidimeters, purporting to accurately measure in the mNTU range. The U.S. EPA has approved a laser turbidity method, "Method 10133, Determination of Turbidity by Laser Nephelometry."⁷⁶ Currently, laser turbidimeters are not widely used in the industry.⁸³

Turbidity, as defined by the above methods, is a non-scientific measure of particle concentration. Electronic particle counters are now available that are capable of accurately counting and recording the number of particles as a function of size (often in the 1-150 μm range). Although there is a general relationship between particle counts and turbidity (below 1.0 NTU), a firm correlation does not exist.^84-86

A simple conversion factor relating particle counting and turbidity measurements is not possible because the two techniques differ fundamentally in terms of discernment. Particle counting measures two characteristics of particulates: particle number and particle size. Samples with identical clarity can be distinguished on the basis of these two features; one sample may contain many small particles, whereas another may contain few large particles. Turbidity, on the other hand, cannot distinguish between two samples of identical clarity and different particulate composition.⁸⁷

Particle counters are an excellent tool for optimizing treatment processes and for detecting the onset of filter breakthrough. Particle counters are restricted to performance verification only, and no limit is set as a maximum acceptable concentration for the number of particles in the treated water.

7.0 Treatment Technology

Turbidity is reduced by removing particles from the water through filtration. Adequate filtration can be achieved by a variety of technologies: chemically assisted filtration, slow sand filtration, diatomaceous earth filtration, membrane filtration or an alternative proven filtration technology.

7.1 Chemically Assisted Filtration

The chemically assisted filtration process generally includes chemical mixing, coagulation, flocculation, sedimentation (or dissolved air flotation) and rapid gravity filtration. Aluminum and ferric salts are used as primary coagulants. Cationic and anionic polymers are most commonly used as flocculation aids, and both, along with non-ionic polymers, have been used as filter aids. The coagulants and polymers are used to destabilize the generally negatively charged colloidal particles, which allows aggregation to occur via chemical and van der Waals interactions.⁸⁸,⁸⁹ The resulting (much larger) particles are filtered out when the water passes through sand beds or other single-, dual- or mixed-media granular filters. In systems where the combined water from all filters is monitored continuously, this treatment process is capable of producing water with a turbidity of less than 0.3 NTU; turbidities of less than 0.2 NTU have been demonstrated to be achievable on an ongoing basis. Filter loading rates generally range from 3.0 to 12.0 m/h.⁹⁰,⁹¹

Changes in alkalinity, colour, turbidity and orthophosphate concentrations affect coagulation reactions and the properties and rate of settling of resulting floc particles. Temperature affects efficiency by influencing the rate of chemical reactions and the viscosity of water, thereby affecting the particle settling velocity and the filter backwash rate. The lower the temperature of the water, the more difficult it is to treat the water.

All filtration plants should provide for continuous monitoring of the effluent turbidity from each individual filter, as well as for continuous monitoring of the combined filtered water turbidity from all filters. Continuous monitoring is required because short-term turbidity spikes represent a process failure and potential health risk. Peak turbidity levels in the filtered water are a particular concern immediately after filter backwashing; therefore, all filters should be designed so that the filtered water immediately after filter backwashing is directed into a waste stream ("filter-to-waste" provision). When operating the filters, every effort should be made to minimize the magnitude and duration of turbidity spikes.⁹²

Discharge of filter backwash water into a raw water reservoir should not be permitted unless the filter backwash water receives off-line treatment or is returned to a location upstream of the coagulant dosage point, so that all processes of a conventional or direct filtration plant are employed. The off-line treatment may be acceptable depending on the method used to treat the backwash water.

Following filtration, turbidity in a waterworks may increase if any of the following occur:

• coagulants escape into the filtered water;
• dissolved metals oxidize;
• bacteria and other microflora grow;
• chemicals are added for stability or corrosion control;
• deposited materials (especially in low-flow parts of the system) are resuspended; or
• pipes corrode or lines break.⁹³,⁹⁴

Uncovered distribution system reservoirs may also lead to increased turbidities, mainly by encouraging biological production.⁹⁵,⁹⁶

In 1989, the American Water Works Association Research Foundation sponsored a study that identified design provisions and operational practices at high-rate filtration plants. For the study, researchers chose 21 plants that were successful in producing finished water with turbidity of less than 0.2 NTU.⁹¹ In choosing the participating plants, consideration was given to geographic coverage as well as diversity of raw water types and treatment processes. In a different study, the Pennsylvania Department of Environmental Protection undertook performance evaluations of 150 surface water treatment plants that used filtration from 1988 to 1990 and found that a goal of 0.2 NTU was achievable for most plants.⁹⁷ An internal report, prepared in 1995 for the same department, also found that filtration plants can readily achieve finished water with turbidities of less than 0.5 NTU and that most plants can achieve less than 0.2 NTU.⁹⁰ In pilot tests involving treatment of Boston's low-turbidity surface water supply with dissolved air flotation, the turbidity goal of 0.1 NTU was met in more than 90% of the runs.⁹⁸ Operational studies at specific plants have indicated that low turbidities in plant effluent are readily achievable when competent operations are in place.⁹⁹,¹⁰⁰ In another study, it was demonstrated that well-operated conventional treatment plants or direct filtration plants that produce water with low turbidity (less than 0.5 NTU) can achieve up to a 3-log reduction of Giardia cysts and up to a 2-log reduction of viruses.¹⁰¹ The same study demonstrated that source waters with low raw water turbidity require filter effluent turbidities to be substantially lower than 0.5 NTU in order to effectively remove Giardia cysts and viruses.

The U.S. EPA's 1997 Notice of Data Availability for Interim Enhanced Surface Water Treatment Rule (IESWTR) shows that systems serving more than 10 000 people are able to meet low turbidity limits. The same study indicated that chemically assisted filtration is able to achieve a 2-log reduction of Cryptosporidium through filtration.¹⁰²,¹⁰³ The U.S. EPA has now concluded that conventional treatment plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3-log reduction of Cryptosporidium. Direct filtration plants achieve an average of 2.5-log reduction of Cryptosporidium.⁴⁶

7.2 Slow Sand Filtration

In slow sand filtration, filter effectiveness depends on the formation of schmutzdecke, which is a layer of bacteria, algae and other microorganisms on the surface of the sand, and the formation of a biopopulation within the sand bed. Raw water passes through the sand bed, where physical, chemical and biological mechanisms remove contaminants. The most important removal mechanism has been attributed to the biological process. No chemicals are added, nor is there a need to backwash.

Researchers have observed variation in the ability of slow sand filters to reduce turbidity. Fox et al.¹⁰⁴ found that when water was filtered at 0.12 m/h, after an initial ripening period had allowed the biopopulation to become established on new sand, the treated water turbidity was consistently less than 1.0 NTU. Raw water turbidity ranged from 0.2 to 10.0 NTU. Cleasby et al.¹⁰⁵ reported that typical effluent turbidity was 0.1 NTU for raw water, with turbidity ranging from lower than 1.0 to 30.0 NTU, except during the first 2 days after scraping of the schmutzdecke. Pyper¹⁰⁶ observed slow sand filtered water with turbidity of 0.1 NTU or lower 50% of the time and 1.0 NTU or lower 99% of the time; raw water turbidity in this study ranged from 0.4 to 4.6 NTU. Slezak and Sims¹⁰⁷ reported that nearly half of the 27 slow sand filtration plants they surveyed produced filtered water turbidity of 0.4 NTU or lower; at the same time, 15% of the plants produced water with an average turbidity of 1.0 NTU or higher. Consistent 3-log reductions of particles sized from 2 to 4 μm upwards were also observed in this study. The size range of 7-12 µm is considered to be representative of the size of Giardia cysts. Bellamy et al.¹⁰⁸ studied the water treatment efficiency of slow sand filtration to ascertain removal of Giardia cysts, total coliform bacteria, HPC bacteria, particles and turbidity. Results showed that slow sand filtration is an effective water treatment technology. Using a biologically mature filter, Giardia cyst removal was virtually 100%; total and faecal coliform removal was approximately 99%; particle removal averaged 98%; HPC bacteria removal ranged from negative to 99%, depending on the influent concentration; and turbidity removal ranged from 0 to 40%.¹⁰³ The U.S. EPA has now concluded that slow sand filtration plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3-log reduction of Cryptosporidium.⁴⁶

Slow sand filtration is appropriate for use when raw water turbidities are relatively low (e.g., <10.0 NTU).

As is the case with chemically assisted filtration, a "filter-to-waste" feature should be provided so that the filtered water immediately after filter cleaning is directed into a waste stream.

7.3 Diatomaceous Earth Filtration

Diatomaceous earth filters operate by passing water through a thin layer of diatomaceous earth about 3 mm thick supported on a septum or filter element. To prevent turbid water from clogging the filter, a small amount of diatomaceous earth is continually added as body feed to maintain a permeable filter cake. Once the head loss across the filter cake becomes too great or the filter cake begins to slough, the filter is removed from service and the filter cake is washed and reused. New precoat is applied, and the cycle starts again.

Diatomaceous earth filtration has been shown to attain excellent removal of Giardia cysts over a broad range of operating conditions. Cyst removals exceeding 99%, and often 99.9%, were reported by Lange et al.¹⁰⁹ for filtration rates of 2.4-9.6 m/h and for temperatures from 3.5 to 15°C. Logsdon et al.¹¹⁰ reported that when sufficient diatomaceous earth and body feed were used, removal of 9-μm radioactive beads was nearly always 99.9% or higher. The same study reported that 11 filter runs were made with Giardia muris cysts at filtration rates of 2.2-3.5 m/h. Cyst removal exceeded 99% in all runs and exceeded 99.9% in five of the runs. The U.S. EPA has now concluded that diatomaceous earth filtration plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3-log reduction of Cryptosporidium.⁴⁶

Diatomaceous earth filtration is appropriate and effective in treating waters with low turbidity. Logsdon et al.¹¹⁰ reported that turbidity reductions of 56-78% were attained with diatomaceous earth when raw water turbidity ranged from 0.95 to 2.5 NTU. Pyper¹⁰⁶ reported an average turbidity reduction of 75% with an effluent quality of 0.5 NTU.

As is the case with chemically assisted filtration, a "filter-to-waste" feature should be provided so that the filtered water immediately after filter backwashing is directed into a waste stream.

7.4 Membrane Filtration

Four membrane treatment processes are currently used in the water industry, and all involve pressure-driven semi-permeable membranes. The most appropriate type of membrane depends on a number of factors, including targeted materials to be removed, source water quality characteristics, treated water quality requirements, membrane pore size, molecular weight cut-off, membrane materials and system/treatment configuration.¹¹¹ The four processes are:

1. Reverse osmosis: a high-pressure membrane treatment process originally developed to remove salts from brackish water;
   
   
2. Nanofiltration: a low-pressure reverse osmosis process for the removal of larger cations (e.g., calcium and magnesium ions) and/or organic molecules;
   
   
3. Ultrafiltration: a lower-pressure membrane process characterized by a wide band of molecular weight cut-off and pore sizes for the removal of dissolved organics and particulates; and
   
   
4. Microfiltration: a low operating pressure membrane process used to remove particulates, including pathogenic cysts.¹¹¹,¹¹²

Reverse osmosis and nanofiltration are very effective for absolute removal of cysts, bacteria and viruses.¹¹² Ultrafiltration (pore size 0.01 μm) and microfiltration (pore size 0.1 µm) are effective for absolute removal of Giardia cysts and partial removal of bacteria and viruses.¹¹³ Filtrate turbidity can be achieved consistently at or below 0.1 NTU.¹¹⁴,¹¹⁵

Prefiltration and/or the addition of a scale-inhibiting chemical may be required to protect membranes from plugging effects, fouling and/or scaling.

If membrane filtration is the sole treatment technology in use, then a form of virus inactivation should be incorporated into the treatment train after the filtering process.

A "filter-to-waste" feature should be provided for initial start-up and commissioning of the membrane system and for emergency diversion in the event of a membrane integrity breach.

8.0 Health Considerations

8.1 Microbial

The most important health-related effect of turbidity is probably its ability to protect microorganisms from disinfection. Turbidity, which has been shown to be correlated with the contamination of water supplies by Giardia and Cryptosporidium,¹¹⁶ serves as a surrogate measure for indicating the risk of contamination by these pathogens. A dramatic increase in turbidity levels at one of the Milwaukee water treatment plants (levels many times higher than those of the preceding 14 months) was associated with the outbreak of cryptosporidiosis in April 1993, when more than 400 000 people developed symptomatic gastrointestinal infections as a consequence of exposure to contaminated drinking water.¹¹⁷ An outbreak of giardiasis in Rome, New York, where an unfiltered but chlorinated water supply was used, has been cited as illustrating the problem of particulates possibly protecting pathogens and interfering with marginal disinfection.⁶² In another incident, high turbidities (>4.0 NTU), resulting from poor plant operation coupled with a malfunctioning chlorinator, were considered as causal factors in an outbreak of giardiasis.³⁹

In most water treatment plants, Giardia removal is a physical process involving coagulation, flocculation and filtration; chlorine contact times alone are insufficient to result in complete destruction or removal.¹¹⁸ Monitoring turbidity can therefore be a useful indicator of plant performance, including cyst removal. Studies have shown that small increases in turbidity (about 0.2 NTU) can result in significant passage of Giardia cysts.⁴⁰ It has been suggested that 0.1 NTU should be set as a goal or objective for treated water.³⁹,⁴⁰,¹¹⁸,¹¹⁹ Giardiasis problems have, however, occurred where turbidity limits have been met, and it should not be assumed that achieving a turbidity limit will by itself prevent waterborne disease.⁴¹,¹²⁰

A study in Philadelphia by Schwartz et al.,¹¹⁵ which asserted a correlation between levels of turbidity and hospital admissions of elderly residents with gastrointestinal illnesses, highlights the fact that meeting the turbidity limits does not necessarily mean that disease can be prevented. The authors found that an increase in the weighted average turbidity of approximately 25% (0.035 NTU) was associated with a 9% increase in hospital admissions of elderly residents with gastrointestinal illness 9-11 days after exposure, even though the recorded average turbidities were well below the regulated limits. A similar study by Aramini et al.¹²¹ has demonstrated a relationship between reported gastrointestinal illness and turbidities in excess of 1.0 NTU, the previous Canadian health-based drinking water guideline. Using a generalized additive model, the authors demonstrated that excess turbidities during the period 1992-1998 could explain 2.1%, 0.8% and 0.9% of emergency-associated, gastroenteritis-related physician visits by persons residing within the three water distribution areas, respectively. In addition, 1.3%, 0.2% and 0.3% of gastroenteritis-related hospitalizations of persons residing in the same three areas were explained by variations in turbidity. It is evident from these studies that change in turbidity levels in drinking water is a potential indicator for breakthrough of pathogenic organisms and increased consumer risk.

Hudson,¹²² using 1953 data on infectious hepatitis and raw water turbidity for 12 U.S. cities, observed that infectious hepatitis incidence was greater with higher turbidity. A similar relationship appeared to exist between turbidity and cases of poliomyelitis, although this finding was based on a smaller sample.¹²² Shaffer et al.¹²³ reported detection of poliovirus in waters with chlorine concentrations greater than 1 mg/L and turbidities less than 1.0 NTU, which indicates that protection from disinfection occurs even at very low turbidity levels. Although a study of 16 U.S. cities in 1961 failed to reveal a clearly defined relationship between hepatitis incidence and finished water turbidity, the authors stated that, because of the many factors involved, it should not be inferred that there is none.¹²⁴ The infectious hepatitis epidemic in Delhi, India, occasioned by the massive contamination of the raw water source of a treatment plant by sewage, was also accompanied by a significant increase in raw water turbidity. Even though chlorination was practised, it was apparently insufficient to inactivate the infectious hepatitis virus.¹²⁵ The protection from disinfection offered by organic or cellular material in particular has been reported in other studies.¹⁹,²¹

8.2 Chemical

Particulate matter in water is not usually a potential chemical hazard in itself, but may have indirect effects.⁵ The concentrations of both heavy metal ions and biocides are usually much higher in suspended solids than in water. The possibility therefore exists that when such contaminated particles enter a different environment, such as the stomach, release of the pollutants could occur, with potentially deleterious effects.

The metal-ligand binding in humate complexes can be represented by the equation⁴⁷:

Mⁿ⁺ + H_mA = M_mA_n + mH

where:

• Mⁿ⁺ = the metal ion
• H_mA = humic acid
• M_mA_n = the metal complex.

If, for instance, the hydrogen ion concentration is increased by stomach acid, the equilibrium will be displaced in favour of the free ion and the undissociated humic acid.

Similarly, the absorption of some herbicides, in particular s-triazine compounds, by soil organic matter has been demonstrated to be pH dependent. Maximum absorption occurs at pH levels in the vicinity of the respective pK values of the herbicides (i.e., pH levels of about 4-6). Lowering or raising the pH decreases absorption and hence may lead to the release of free herbicides.⁴⁷

9.0 Other Considerations

Excessive turbidity has often been associated with unacceptable tastes and odours. Turbidity in excess of 5.0 NTU also becomes visually apparent and may be objected to by consumers. In some cases, if the level of turbidity is not lowered to reduce the organic loading in advance of applying certain chemicals, it may lead to other health concerns (e.g., the formation of THMs).

As noted above, turbidity measurement does not indicate the type, number or mass of particles. However, because of the ease of analysis and relative inexpensiveness of the equipment, it is a very useful tool to assess the performance of water treatment processes -- especially for conventional surface water systems. Moreover, turbidity can serve to signal potential contamination problems or difficulties within a distribution system. Drinking water should be aesthetically pleasing. Every effort should be made to keep the turbidity as low as possible by flushing and cleaning the pipelines. For aesthetic purposes, turbidity should not exceed 5.0 NTU within the distribution system, especially at the point of consumption.

10.0 Rationale

Turbidity is a characteristic of all water supplies. In surface waters and groundwaters under the influence of surface water, turbidity is a concern for both health and aesthetic reasons. In these waters, the particulate matter that creates turbidity can contain toxins, harbour microorganisms and interfere with disinfection. In addition, organic matter in the water can react with disinfectants such as chlorine to create by-products. These by-products may cause adverse health effects.

While turbidity may be measured in secure groundwater supplies (i.e., not under the direct influence of surface water), it is not a concern in treated water from these sources provided it does not hinder disinfection. It is good practice to ensure that water entering the distribution system from a secure groundwater supply has a low turbidity level around 1.0 NTU.

Turbidity is effectively reduced through filtration, using one of a number of common technologies. The most important consideration when dealing with turbidity is to reduce its level as low as possible and minimize fluctuation. For this reason, while the target is to reduce turbidity levels to below 0.1 NTU at all times, it is considered acceptable for treatment plants to achieve the following levels based on the type of technology used. The levels of turbidity in treated water:

1. For chemically assisted filtration, should be less than or equal to 0.3 NTU in at least 95% of the measurements made, or at least 95% of the time each calendar month, and should not exceed 1.0 NTU at any time.
   
   
2. For slow sand or diatomaceous earth filtration, should be less than or equal to 1.0 NTU in at least 95% of the measurements made, or at least 95% of the time each calendar month, and should not exceed 3.0 NTU at any time.
   
   
3. For membrane filtration, should be less than or equal to 0.1 NTU in at least 99% of the measurements made, or at least 99% of the time each calendar month, and should not exceed 0.3 NTU at any time. If membrane filtration is the sole treatment technology employed, some form of virus inactivation should follow the filter process.

Most problems associated with turbidity are caused when the level of turbidity in the treated water spikes. Spikes can occur when the natural levels of particulate matter increase in the source water, when the filtration rate increases or when an individual filter breaks down. In order to quickly figure out that a filter is malfunctioning and to identify which one it is, this guideline applies to individual filters within a system.

Turbidity measured to be less than 5.0 NTU is not discernible to the naked eye, but at higher levels the particulate matter in water may cause colour, taste and odour concerns for consumers. For this reason, utilities should try to maintain the level of turbidity in the distribution system to below 5.0 NTU. An aesthetic objective has not been set in order to avoid confusion with the health-related guideline.

11.0 References

Appendix A: Determining If Groundwater Is Under the Direct Influence of Surface Water¹²⁶

Determining whether groundwater is under the influence of surface water is a complex process. While there is considerable variation in the circumstances that may result in groundwater becoming contaminated with surface water, some key issues that should be considered include the following:

Geology

1. What are the characteristics of the aquifer deposit (i.e., lithology, stratigraphy and structure)?
   
   
2. How was the deposit created (i.e., geomorphology)?
   
   
3. What is the age of the deposit?
   
   
4. Is it a confined, semi-confined or unconfined aquifer?
   
   
5. If the aquifer was created during a recent geologic event (i.e., alluvial deposits/post-glaciation), is it likely to be connected to an existing surface water body? Is it susceptible to land surface use and influences? What are the consequences? What is the level of risk?
   
   
6. If the aquifer was created during an ancient geologic event (i.e., unconsolidated deposits or bedrock deposits/pre-glaciation), is it likely to be connected to an existing surface water body? Is it susceptible to land surface use and influences? What are the consequences? What is the level of risk?
   
   
7. What are the effects of local topography (e.g., mountains, foothills, plains)?
   
   
8. What are the effects of local geology and geologic events (i.e., facies changes, complex geology, glaciation)?
   
   

Hydrogeology

1. Do the aquifer characteristics (i.e., hydraulic head, intrinsic permeability, hydraulic conductivity, transmissivity, storitivity) imply direct surface water and/or land surface influence(s)?
   
   
2. Is there evidence of local surface water "discharge/recharge" to/from the aquifer?
   
   
3. What is the direction of groundwater flow (i.e., hydraulic gradient)?
   
   
4. What is the consequence of pumping groundwater from the aquifer (cone of influence)? Is there a change in gradient?
   

Hydrochemistry

1. Is there evidence of water quality similarities between the aquifer and surface water (i.e., chemistry, temperature, bacterial count)?
   
   
2. Is there evidence through environmental isotope analysis (³H, ¹⁴C, ¹⁸O, ²H)?
   
   
3. What is the chemical age of the groundwater (i.e., mineralization)? How does it compare with that of the local surface water?
   
   
4. Is there evidence of a hydrochemical facies change?
   
   
5. What is the level of bacterial contamination in the aquifer?
   
   

Land use issues

1. What types of local land use are there (e.g., agricultural, industrial, municipal, recreational)?
   
   

Soil horizon

1. What type of soil is there?
   
   

Tools for making the assessment

1. Cross-sections, isopach maps, topographic maps and geological maps
   
   
2. Aquifer pumping test, groundwater modelling and groundwater monitoring (observation wells, piezometers), soil percolation tests
   
   
3. Local water balance exercises (groundwater vs. surface water recharge/discharge)
   
   
4. Isotope analysis