Turbidity Monitoring: Making Water Quality Clear

Earth may be the “blue planet”—but clean water is shockingly rare. Every year, contaminated water causes over 485,000 deaths. Worldwide, over 2 billion people rely on an impure water source. A further 144 million only have access to surface water.

Wrongly, we don’t always think of these issues as affecting developed nations. We should. Water quality issues are present everywhere—think of the tragic case of Flint, Michigan, or the high levels of lead in Pittsburgh, Pennsylvania. Clearly, low water quality is a devastating, global problem, and one affecting us all—we drink, shower, cook, and play using water!

This problem has not been solved in part because many different factors make up the quality of water. As I discussed in my blog post on water quality monitoring, things like PH, temperature, flow, salinity, organic contamination, and more must all be within certain ranges to ensure water quality. Some of these factors are well-known, but others are less frequently considered. And that’s unfortunate, because you can never know too much about something so important.  

In this post, I delve into one of these lesser-known factors—turbidity. Without understanding this factor, it’s impossible to have a clear understanding of water quality.

What is Turbidity?

Image showing Lake Tahoe's clear water.
Lake Tahoe is famous in part for its extremely clear water (Source)

When you picture a lovely, scenic lake, you’re probably picturing one with little or no turbidity. That’s because turbidity is a direct measure of the clarity of water: clear water has low turbidity, and highly turbid water, as seen in this image, is more opaque. Turbidity is caused by a wide variety of both organic and inorganic compounds. Organic compounds contributing to turbidity include things like algae, plankton, microorganisms, and rotting organic material; inorganic ones include clay, silt, silica, and gravel. When determining whether water is safe to drink or play in, it’s obviously important to understand what sort of other materials are present in it—and turbidity helps us do just that.

One issue is merely aesthetic: no one enjoys drinking cloudy water. But there are more serious issues, too. 

Ecologically, turbidity is extremely harmful. Suspended particles trap heat from the sun, resulting in warmer water. Because of this, and because particles refract and absorb light, photosynthesis becomes far more difficult. Plants and algae are no longer able to grow, further decreasing the presence of oxygen. In turn, the aquatic animal life that relies on plants has a vital food source depleted. According to one study, “predator–prey interactions are often dramatically altered in response to changes in turbidity.” In this way, turbidity can cause a vicious ecological cycle:

A scientific diagram demonstrating the adverse ecological impacts of turbidity.
The cycle of turbidity and oxygen depletion, leading to ecological damage (Source)

Another key reason to monitor turbidity is the fact that suspended particles act as a “shield” for pollutants and bacteria. When solids float in water, contaminants have a surface on which to attach themselves and remain in the environment. Often, these bacteria (like E. coli) and pollutants (like lead) cause serious health impacts. So, when determining how best ensure water safety, treatment facilities look to turbidity as a representation of the level of pollutants and bacterial life.

Perhaps the most important application of turbidity monitoring is that it can be used to detect Combined Sewage Overflow (CSO) events—which you may have read about in one of our case studies. CSOs occur when heavy rain or snow events cause a sewer system to be overloaded with wastewater. After heavy rain or snowfall, many sewer systems are actually designed to “discharge excess wastewater directly to nearby streams, rivers, or other water bodies.”

Obviously, the discharge of raw sewage into the environment has adverse health consequences. Although data on the subject is surprisingly sparse, one estimate states that about “3,500–5,500 gastrointestinal illnesses each year are caused by CSO pollution of swimming waters.” That’s concerning, especially when “experts estimate reported illnesses are 10 to 1,000 times lower than the actual, unreported disease burden.” But, by making CSO events predictable and understandable, turbidity sensors can help minimize their adverse impacts. 

Measuring Murky Material

Turbidity sensors are not as intuitive as, say, a thermometer—so it’s useful to examine how they work. In order to measure turbidity, sensors send a beam of light into the tested liquid, usually water. By measuring how much the light is refracted, the sensors can determine the clarity, and therefore the level of suspended particles, in the water:

A schematic showing the workings of turbidity monitoris.
A simplified schematic depicting how turbidity monitors work (Source)

If this example is unclear, picture hundreds of bouncy balls rolling down a hill. If the hill is smooth, the bouncy balls’ trajectory will be straight. But if there are rocks, stumps, etc., they will end up bouncing off in all directions. By measuring where they end up, one can determine how many “interfering agents” are present. The principle is similar in turbidity monitoring.

Choosing a Turbidity Sensor

Carefully choosing a turbidity sensor for your environmental monitoring application is important. Turbidity sensors are fairly expensive—so it’s important to make sure you’re getting exactly what you need. 

First, you should consider how frequently you wish to monitor turbidity. If you are taking discrete samples you might not need continuous turbidity monitoring. This could save you money, as individual turbidity test kits can be far cheaper than continuous monitoring. An example application of discrete sampling would be something where the emphasis is on comparison, not on change over time—perhaps you are interested in seeing which regional lakes, after an exceptional rainfall, are the most turbid.

But more commonly, you would likely be interested in time series data, so as to see how the turbidity of a body of water changes over time. This requires the ability to conduct numerous measurements at regular intervals using a continual turbidity monitor.  

Assuming that you want to monitor turbidity continuously, you’ll have to base your approach on the expected level of turbidity, too. Areas of low turbidity are more suited to nephelometry, whereas areas of high turbidity are more suited for turbidimetry. These are different methods of measuring the presence of suspended particles in water. Depending on your application, you’ll want to know whether candidate meters are EPA or ISO compliant—this will ensure that your instruments are in-line with industry standards. 

Another important consideration, particularly for highly turbid areas, is biofouling:

The underside of a ship encrusted with algae and barnacles.
Biofouling can often be seen on the hulls of ships (Source)

Biofouling refers to the growth of organic material, like algae or barnacles, on underwater surfaces. Non-biological fouling can also occur due to silt and sediment build-up. Turbidity meters are often subject to both biological and non-biological fouling. This can confound the results of turbidity monitoring, because fouling means that the sensor cannot accurately send light through the water sample. For instance, water could be relatively clear, but a fouled sensor would report it as being highly turbid. 

If your application is likely to result in fouling, you’ll have to consider whether you’ll be able to clean the sensor frequently. In more turbid environments, you would likely have to manually clean your sensor, anywhere from once a week to once every day. However, there are self cleaning options available. Often, these use a pair of miniature wipers to de-foul the sensor. These wipers are either built into the sensor, or added by affixing an additional wiping mechanism. Another method—often used in combination with wiper systems—is the use of chemical agents, like chlorination dispensers. These make the aqueous environment of the sensor inhospitable to biological growth, without introducing particles large enough to affect turbidity measurement. 

At that point, it’s key to consider how many feet of cable you’ll need. A sensor which would be located only a few feet under the water would, obviously, require a far shorter cable than one designed to measure turbidity farther below the surface. It’ll also be important to make sure that your sensors are capable of measuring turbidity to the required specifications, and given other aspects of the operating environment. Turbidity sensors have different operating temperature ranges, levels of ruggedization, and measurement capabilities. 

Keep in mind, too, that many turbidity sensors need to be calibrated before use. When considering a sensor, you’ll also have to purchase compatible calibration solution.

A Sampling of Sensors

While there are fewer turbidity sensors on the market than, say, air quality sensors, there are still enough to cause analysis paralysis. To that end, I’ve made a list of four high-quality options. Note that, although some sensors are designed for specific non-water applications (such as monitoring dairy or brewery production), these sensors are water focused:

SensorMeasurement CapabilitiesSelf Cleaning?Operating ParametersForm FactorPrice
Global Water WQ 730Range: 0-50 NTU and 0-1000 NTU options available
Accuracy: + 1% of full scale
Resolution: 12 bit
NoOperating Temperature: 14 to 122°F
Maximum Pressure: 30 psi
Power Requirement: 10-36 VDC
Output: 4-20 mA
Weight: 1 lb
Dimensions: 1.5 (diameter) x 8.5 inches (length)
Campbell Scientific OBS-501Range: 0 to 4000 NTU
Accuracy: ±2% of reading or 0.5 NTU (which ever is greater)
Yes—uses both wiper and biocideOperating Temperature: 0 to 104°F
Maximum Pressure: 30 psi
Power Requirement: 9.6-18 VDC
Output: 0-5 VDC
Weight: 1.3 lbs
Dimensions: 1.8 (diameter) x 10.3 inches (length)
Greenspan Turbidity Sensor TS-1000ARange: 0 to 100, 400, and 1000 NTU available
Accuracy: ±1% for 100/400 NTU, ±3% for 100 NTU
Resolution: 100 NTU (0.1NTU), 400 NTU (0.2 NTU), 1,000 NTU (0.3 NTU)
Yes—uses wiper bladeOperating Temperature: 0 to 122°F
Maximum Pressure: N/A
Power Requirement: 8-30 VDC
Output: 4-20 mA or 0-2.5V DC
Weight: 1.1 lbs
Dimensions: 1.85 (diameter) x 16.4 inches (length)
Susix Turbidity and TSS TransmitterRange: 1 to 10,000 NTU
Accuracy: ±3% of full scale
Optional—wiper system availableOperating Temperature: -4 to 140°F
Maximum Pressure: N/A
Power Requirement: 24 VAC
Output: 4-20 mA
Weight: 2.42 lbs
Dimensions: ~1 (diameter) by ~10 inches (length)

Making Waves

Measuring turbidity is a vital aspect of assessing water quality. Increasingly, scientific studies are focusing on turbidity for monitoring various environmental events, like Combined Sewage Overflows (CSOs). 

One notable paper focused on the Pau region of France—which is known for CSO events—and concluded that “using turbidity (and conductivity) measurements to estimate continuous concentrations of pollutants in the sewage system has been validated and is working for several months… it is possible to obtain an instantaneous and continuous estimation of the pollutant concentration, which is very useful for the sanitation manager…” The Pau study demonstrates, as do others, the significant benefits of turbidity monitoring.

The bank of the Pau river in France, looking scenic.
The Gave de Pau river in Pau, France, suffers the consequences of CSOs (Source)

Turbidity monitoring need not be confusing, and, hopefully, this post served as a refresher on some key aspects. Without understanding turbidity monitoring, our picture of water quality will always be murky.

If turbidity monitoring (or capturing other kinds of environmental sensor data) sounds like something you’re interested in, please feel free to reach out! We’ve designed Kosmos, our environmental engagement platform, to meet all sorts of environmental monitoring needs—we’d love to discuss yours.