A common question that arises when studying the clarity or quality of water is what the difference between turbidity and total suspended solids (TSS). They both offer measurements on clarity but what does each test for? And is there any correlation between the two?
What is Turbidity?
When we refer to Turbidity, we are looking at how clear or translucent the liquid is by looking at the water’s light scattering properties. Testing turbidity can reveal some suspended solids, algae, organic material, and any other minuscule particles that cause the liquid to become cloudy or murky resulting in a higher Nephelometric Turbidity Unit (NTU) reading. NTU’s are units that are used to describe turbidity. A low NTU reading indicates clearer liquid and higher readings indicate low water clarity. NTU readings generally range from 1 to 4000 where 1 would indicate pristine clarity and 4000 would have the transparency similar to that of milk. An NTU reading of less than 1 is generally considered quite good for tap water.
What are (TSS) Total Suspended Solids?
Total Suspended Solids (TSS) refers to any particles that are suspended in the water column. These particles can include silt, algae, sediment, and other solids floating in the water (both organic and inorganic). These particles are defined as being large enough to not pass through the filter (through the filtration process) used to separate them from the water. Suspended solids absorb heat from sunlight and as a result, the water temperature increases resulting in a deprivation of dissolved oxygen in the water which can be disastrous to aquatic life if levels are too high. TSS can be measured in ppm, mg/L, g/L and %. To determine TSS, you need to run sample liquid through a filtering process where the sample is filtered, dried, and weighed. Results can be ran through the below formula to determine the TSS in mg/L.
Total Suspended Solids (mg/L) = (weight before filtering – weight after filtering) / sample volume in litres
There are portable instruments available that do measure TSS but they can get quite expensive. The best meter we have found measures both TSS and turbidity is made by Hach. It is a portable hand-held meter complete in a carry case which measures turbidity, suspended solids, and sludge blanket level . More information on this product can be found here.
When looking at TSS readings, it is generally considered that a reading of less than 20mg/L is clear. Readings between 40 and 80mg/L will begin to appear cloudy and readings over 150mg/L will appear quite dirty. These numbers can vary depending of the type of particles present and are provided as a guide only.
What is the difference between turbidity and TSS?
Turbidity and TSS are similar in the sense that they are both measuring clarity of liquid but they aren’t actually measuring the same thing. It is worth noting that measuring turbidity from a sample allows you to get an instantaneous reading of NTUs meaning you can take the reading directly from out in the field. Measuring TSS on the other hand, is a manual and drawn out process requiring a precise technique and measurements that often have to be conducted back in a laboratory. While portable meters are available as mentioned earlier, they are expensive and depending on the application, may or may not be worth the investment. It is worth considering the regularity of testing required and whether testing needs to be done on-site or can be taken back to a laboratory to go through the filtering process.
Put simply, turbidity looks at how well a light passes through liquid and TSS is a quantitative expression of suspended particles. Even though turbidity and TSS compliment each other, they are both influenced differently. For example, TSS can calculate sedimentation rates, while turbidity can’t. Turbidity and TSS do overlap in the measurement of some particles as shown in the illustration below but as mentioned, they do actually differ making it extremely difficult to form any kind of correlation between the two.