Turbidity: Definition, Causes, and History as a Water Quality Parameter
Turbidity is a principal physical characteristic of water and is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample. It is caused by suspended matter
or impurities that interfere with the clarity of the water. These impurities may include clay, silt, finely divided inorganic and organic matter, soluble colored organic compounds, and plankton and other microscopic organisms. Typical sources of turbidity in drinking water include the following:
Simply stated, turbidity is the measure of relative clarity of a liquid. Clarity is important when producing drinking water for human consumption and in many manufacturing uses. Once considered as a mostly aesthetic characteristic of drinking water, significant evidence exists that controlling turbidity is a competent safeguard against pathogens in drinking water.
The first practical attempts to quantify turbidity date to 1900 when Whipple and Jackson developed a standard suspension fluid using 1,000 parts per million (ppm) of diatomaceous earth in distilled water. Dilution of this reference suspension resulted in a series of standard suspensions, which were then used to derive a ppm-silica scale for calibrating turbidimeters.
The standard method for determination of turbidity is based on the Jackson candle
turbidimeter, an application of Whipple and Jackson's ppm-silica scale. The Jackson candle turbidimeter consists of a special candle and a flat-bottomed glass tube, and was calibrated by Jackson in graduations equivalent to ppm of suspended silica turbidity. A water sample is poured into the tube until the visual image of the candle flame, as viewed from the top of the tube, is diffused to a uniform glow. When the intensity of the scattered light equals that of the transmitted light, the image disappears; the depth of the sample in the tube is read against the ppm-silica scale, and turbidity was measured in Jackson turbidity units (JTU). Standards were prepared from materials found in nature, such as Fuller's earth, kaolin, and bed sediment, making consistency in formulation difficult to achieve.
In 1926, Kingsbury and Clark discovered formazin, which is formulated completely of traceable raw materials and drastically improved the consistency in standards formulation. Formazin is a suitable suspension for turbidity standards when prepared accurately by weighing and dissolving 5.00 grams of hydrazine sulfate and 50.0 grams of hexamethylenetetramine in one liter of distilled water. The solution develops a white hue after standing at 25EC for 48 hours. A new unit of turbidity measurement was adopted called formazin turbidity units (FTU).
Even though the consistency of formazin improved the accuracy of the Jackson Candle Turbidimeter, it was still limited in its ability to measure extremely high or low turbidity. More precise measurements of very low turbidity were needed to define turbidity in samples containing fine solids. The Jackson Candle Turbidimeter is impractical for this because the lowest turbidity value on this instrument is 25 JTU. The method is also cumbersome and too dependent on human judgement to determine the exact extinction point.
Indirect secondary methods were developed to estimate turbidity. Several visual extinction turbidimeters were developed with improved light sources and comparison techniques, but all were still dependent of human judgement. Photoelectric detectors became popular since they are sensitive to very small changes in light intensity. These methods provided much better precision under certain conditions, but were still limited in ability to measure extremely high or low turbidities.
Finally, turbidity measurement standards changed in the 1970's when the nephelometric turbidimeter, or nephelometer, was developed which determines turbidity by the light scattered at an angle of 90E from the incident beam. A 90E detection angle is considered to be the least sensitive to variations in particle size. Nephelometry has been adopted by Standard Methods as the preferred means for measuring turbidity because of the method's sensitivity, precision, and applicability over a wide range of particle size and concentration. The nephelometric method is calibrated using suspensions of formazin polymer such that a value of 40 nephelometric units (NTU) is approximately equal to 40 JTU. The preferred expression of turbidity is NTU.
Turbidity's Significance to Human Health
Excessive turbidity, or cloudiness, in drinking water is aesthetically unappealing, and may also represent a health concern. Turbidity can provide food and shelter for pathogens. If not removed, turbidity can promote regrowth of pathogens in the distribution system, leading to waterborne disease outbreaks, which have caused significant cases of gastroenteritis throughout the United States and the world. Although turbidity is not a direct indicator of health risk, numerous studies show a strong relationship between removal of turbidity and removal of protozoa.
The particles of turbidity provide “shelter” for microbes by reducing their exposure to attack by disinfectants. Microbial attachment to particulate material or inert substances in water systems has been documented by several investigators and has been considered to aid in microbe survival. Fortunately, traditional water treatment processes have the ability to effectively remove turbidity when operated properly.
Waterborne Disease Outbreaks
Notwithstanding the advances made in water treatment technology, waterborne pathogens have caused significant disease outbreaks in the United States and continue to pose a significant problem. Even in developed countries, protozoa have been identified as the cause of half of the recognized waterborne outbreaks. The most frequently reported waterborne disease in the United States is acute gastrointestinal illness, or gastroenteritis. The symptoms for this disease include fever, headache, gastrointestinal discomfort, vomiting, and diarrhea. Gastroenteritis is usually self-limiting, with symptoms lasting one to two weeks in most cases. However, if the immune system is suppressed, as with the young, elderly and those suffering from HIV or AIDS, the condition can be very serious and even life threatening. The causes are usually difficult to identify but can be traced to various viruses, bacteria, or protozoa.
Giardia and Cryptosporidium are the two most studied organisms known to cause waterborne illnesses. These two protozoa are believed to be ubiquitous in source water, are known to occur in drinking water systems, have been responsible for the majority of waterborne outbreaks, and treatments to remove and/or inactivate them are known to be effective for a wide range of waterborne parasites. Giardia and Cryptosporidium have caused over 400,000 persons in the United States to become ill since 1991, mostly due to a 1993 outbreak in Milwaukee, Wisconsin. Giardia and viruses are addressed under the 1989 SWTR. Systems using surface water must provide adequate treatment to remove and/or inactivate at least 3-log (99.9%) of the Giardia lamblia cysts and at least 4-log (99.99%) of the enteric viruses. However, Cryptosporidium was not addressed in the SWTR due to lack of occurrence and health effects data. In the mid-1980's, the United States experienced its first recognized waterborne disease outbreak of cryptosporidiosis. It was soon discovered that the presence of Cryptosporidium in drinking water, even in very low concentrations, could be a significant health hazard. In 1993, a major outbreak of cryptosporidiosis occurred even though the system was in full compliance with the SWTR. Several outbreaks caused by this pathogen have been reported.
The ESWTR's primary focus is to establish treatment requirements to further address public health risks from pathogen occurrence, and in particular, Cryptosporidium.displays several instances of past outbreaks of cryptosporidiosis in systems using surface water as a source, along with general information about the plant and turbidity monitoring. In three out of four of the cases displayed in the table, turbidity over 1.0 NTU was occurring in finished water during the outbreaks.
The Relationship Between Turbidity Removal and Pathogen Removal
Low filtered water turbidity can be correlated with low bacterial counts and low incidences of viral disease. Positive correlations between removal (the difference between raw and plant effluent water samples) of pathogens and turbidity have also been observed in several studies. In fact, in every study to date where pathogens and turbidity occur in the source water, pathogen removal coincides with turbidity/particle removal.
One of the water treatment operator’s primary jobs is controlling turbidity. Turbidity control is usually associated with surface water systems and groundwater systems under the direct influence of surface water. This brieflyexamines turbidity control through the entire water treatment process from the raw water source to the clear well.
What is Turbidity?
Turbidity is caused by particulates in the water and is synonymous with cloudiness. Measured in NTUs [nephelometric turbidity units] or occasionally in JTUs [Jackson turbidity units], it is significant because excessive turbidity can allow pathogens to “hide” and, hence, be resistant to disinfection.
Source Water and Watershed Protection
Turbidity control can and should start with the source water and the area around that source. Owning or having control of the land in the watershed area of your source water can make a tremendous difference in the source water’s quality. Controlling land use for the purposes of lessening contaminants, and especially soil erosion and sedimentation control, is becoming more important as populations increase. Water systems should work with local watershed groups, farmers, developers, Natural Resource Conservation Service offices, local extension offices, county commissions, and state environmental departments on water issues. The message they need to convey is better the source water, the easier and less expensive treatment will be. Turbidity reduction or control at the source is a cost effective and efficient way to eliminate multiple levels of treatment.
Raw Water Intake
The key to turbidity control is having a good awareness of your intake and the quality of the source water. There are measures that an operator can adopt to help minimize the intake of turbid or dirty water. Most intakes have a screen or a structure of some kind to hold back debris. These screens or areas can become silted in with dirt and must be cleaned. If your system does not have a screen on the intake, install one as soon as possible.
Some raw water pumps can be reversed to flow the water back out to flush the immediate area around the intake. Never do this at the beginning of the shift or the beginning of the production day. The flushing should be done at the end of the day before shutting down if your plant does not produce water 24 hours a day. If the plant operates continuously, then the flushing should be at the time of lowest demand so that the can water clear up.
The time it takes for the water to clear following the flush depends on the velocity of the water that carries the silt downstream. The faster the water flows in the river, the less time it should take to clear up. If a reservoir is your source water, it may take several hours. If the pumps do not have the capability to reverse or there is not enough water to push back through the intake, the cleaning must be done manually using an excavator and a vacuum truck (and possibly even a diver). The cleaning intervals vary from once a year to once a decade, depending on how clean the source water is. Visual inspections may need to be done with a camera or diver. Don’t forget to turn off the raw water pumps when inspecting.
The operator should record the raw water turbidity every day that he or she produces water, even if it is not a state requirement. Sampling the raw water should be done upstream from the intake. Even though the intake might be a long distance away from the plant, it is useful to see the difference between source water turbidity and the turbidity prior to treatment. Record data related to different operating scenarios, weather conditions, and other incidences that increase or decrease turbidity in the water.
If the plant does not operate continuously and can shut down temporarily, a good practice is to produce water before any rain storms when the water is the cleanest. Some plants can’t handle extremely turbid water. Watching or listening to the weather forecast can be helpful in controlling turbidity.
Another good practice for operators is to keep in daily contact with other water system or sewer system personnel up and downstream from your plant. Things like flash floods or problems with industries or sewage plants are easier to deal with if you know about the problem before it gets to your intake. The operator can make as much water as possible before shutting down and letting the problem pass. Don’t forget to relay the information downstream to other water systems.
Coagulation, Chemical Feeds, Flash Mix
Turbidity reduction is best achieved when the water is run through a series of chemical and physical treatment methods before reaching the filter. The terms coagulation, flocculation, and flash mix are often discussed together. Basically, coagulation is the process of getting particulates to stick together, flocculation is when this process becomes visible, and the flash mix is the fast mixing that makes it happen.
Coagulants include alum or polyelectrolytes such as polyaluminum chloride. Some water will react better with one chemical than the other. The correct dosage is determined with jar testing and feed pump calibration. The coagulant is usually injected into the line before the flash mix, sometimes using a static mixer (a short piece of pipe with internal spiral fins). If your plant is not equipped with an in-line static mixer, it would be a fairly inexpensive investment and an improvement to the treatment process.
Proper mixing is important to coagulation, as is the proper dosage of the coagulant. Coagulation can start as soon as the chemical is added, but the flash mix kicks the process into high gear. The flash mix is usually aided with a motorized paddle or the water is allowed to fall (splash) into a chamber making the water turbulent. Although it isn’t required, the operator can take a daily turbidity reading at the end of the flash mix.
Flocculation usually consists of a two-or three-stage process, and begins when the particulates start sticking together more visibly. The process still uses the motorized paddles, but at a slower rate than during the flash mix. Stage one of flocculation is fastest, with the second and third stages working more slowly and sometimes with the paddles moving in the opposite direction. In some plants, it is possible to adjust the speed of these motors. Keep in mind you want the last stage to be at a slow, consistent speed so as not to break up the particles. This allows the particles to get heavier and helpd them settle to the bottom in the next part of the process, sedimentation. Again, it would not be a bad idea to take daily turbidity readings at the end of the flocculation process.
During sedimentation, the particles of dirt settle to the bottom of the basin. The sedimentation basin is the last step before the filters, so sedimentation must work effectively. The key to good sedimentation is having enough area and/or time for settlement and, subsequently, good sludge removal.
Sludge at the bottom of the tank is usually scraped with a slow-moving blade to a sump and drain. Some settlement basins have a cone-shaped bottom to direct the sludge to a drain. It still may be necessary to drain the sedimentation basin and clean the sludge every five to 10 years, depending on the water quality. Some sedimentation basins have tube settlers (slanted tubes that help with contact area and time for settling). Sometimes baffling is used or can be added to help the sedimentation time.
At the end of the sedimentation period in each basin, record settled water turbidity at four-hour time increments. This practice helps judge the performance of the sedimentation basin(s). It is important for the sedimentation basin(s) to operate optimally so the filters do not get overworked and to make sure the filters can handle the incoming turbidity with no bleed-through of dirty water. Individual sedimentation basin performance goals are shown below.
Summary of Optimization Monitoring and Performance Goals
Minimum Data Monitoring Requirements
Individual Sedimentation Basin Performance Goals
Individual Filter Performance Goals
Disinfection Performance Criteria
Filtration is the last stage in turbidity control before the clear well. Most filters can handle a wide range of turbidity, but don’t leave all the work up to them. The other processes—chemical mix, flash mix, coagulation, flocculation, and sedimentation— must work optimally for the life of the filters and to provide a safety factor or cushion for lower turbidities. The less turbidity going into the filters means longer filter runs and longer filter-media life, which saves money.
Individual filter performance goals are show in Table One.
The minimum data monitoring requirements for the filters are:
Continuous turbidity monitoring equipment needs to be cleaned and calibrated on a regular basis. Check with the manufacturer to get maintenance and calibration schedules for turbidity equipment. You may have to order supplies for cleaning and calibration procedure ahead of time. Be advised that some if not all of the calibration standards have a shelf life.
Inspect the filter media at least every quarter. A few things an operator can do to inspect the filter on a regular basis include:
The viability of the media is very important for efficient operation of the filtering process.
The clear well can, over a long period of time, accumulate sediment. The clear well should be inspected at the same time as the storage tanks are in the distribution system and be cleaned as necessary. A couple of things can be done to keep any sediment that may be in the bottom of the clear well from stirring up.
Sediment can accumulate in the distribution system over time and when a line break occurs or a fire hydrant is opened, it will get stirred up. Several things can be done to control this problem in the distribution system.
By following the procedures outlined, operators can lower turbidity in the water during all stages of the treatment process. Lower turbidity means lower treatment costs and better quality drinking water.
Accurate and repeatable turbidity measurements depend on good, consistent measurement techniques. Measurements are more accurate and repeatable if close attention is paid to proper measurement techniques. Four important considerations are:
Measure samples immediately to prevent changes in sample characteristics due to temperature shifts and settling. Avoid dilution whenever possible; particles suspended in the original sample may dissolve or otherwise change characteristics when the temperature changes or the sample is diluted. Thus, the measurement may not be representative of the original sample.
Cleaning Sample Cells
Cells must be meticulously clean and free from significant scratches. Glass imperfections and superficial scratches from manufacturing are effectively masked by the silicone oiling procedure outlined below. Clean the inside and outside of the cells by washing thoroughly with a nonabrasive laboratory detergent. Then continue cleaning with a 1:1 HCl bath followed by multiple rinses with distilled or deionized water. Air dry the cells. Handle sample cells by the top only to minimize dirt and fingerprints.