What is Dirty Water?

Wastewater can be one of the biggest environmental challenges involved in quarry planning & management. Wastewater can be grouped into three categories: contaminated, clean and dirty.  Contaminated water may contain metals, organics or salt that requires treatment to remove these contaminants prior to discharge.  Clean water may be reused or discharged without treatment.  Dirty water contains sediment and requires settling or flocculation prior to discharge or reuse on-site.  It’s this category which presents the greatest problem for quarry managers. 

Turbidity 

When it comes to the criteria for discharging dirty water regulatory authorities may apply limits based on either turbidity or total suspended solids (TSS).  Turbidity is defined as an expression of the optical properties of impurities in the water which cause light to scatter and be absorbed rather than transmitted in straight lines.  It is measured by passing a beam of light through the water sample, then comparing how much light passes through to a standard solution.  This is known as the Nephelometric method, and the results are expressed as NTU or Nephelometric Turbidity Units.   

High turbidity is caused by small particles such as clay or silt, micro-organisms or organic matter (including tannins).  Material that remains in suspension can cause light to scatter and be absorbed, particularly if the particles are themselves a darker colour.  Fine air bubbles can also cause a high interference.  The true colour of the water, that is the water colour resulting from dissolved substances in the water can cause turbidity to be slightly lower than expected. 

The measurement of Turbidity is important in waters where the transmission of light may affect the use of the discharge.  Ecosystems reliant on sunlight reaching deeper waters are particularly vulnerable to the discharge of water high in turbidity. 

Total Suspended Solids 

Total Suspended Solids (TSS) is a measure of the suspended insoluble solids in the water.  An aliquot of the sample is passed through a standard filter paper (usually 1 micron) and the remaining solids dried and weighed.  Results are expressed as milligrams per litre (mg/L).  Another common name for this test is Non-filterable Residue (NFR).  The test is a direct measurement of the amount of solids in the water greater than the filter pore size.  Micro-organisms and organic matter such as algae are less dense materials and therefore weigh less causing a lower TSS, while silt and clay particles are much denser, meaning that a small amount of sediment can cause a much higher TSS.  This is not an interference in the method but a definition of the result. 

The measurement of TSS is important for waters reused or recycled where solids present could interfere in a process or clog up machinery.  TSS is also important in water discharged into sensitive ecosystems where the increased solids may stop the flow of shallow creeks, increase sediment load in lakes or leave a choking layer over bottom dwelling organisms and accumulate in the digestive systems of aquatic fauna. 

Water may appear visually cloudy however the particles present may be too small to be caught on the standard filter.  This results in a water with high turbidity and having a low TSS.  Conversely a visually clear sample with a low turbidity may have a few heavy sediment particles that could be caught on a filter resulting in a high TSS. 

Sampling and Testing 

The sampling of the water is very important for both tests.  Oils and vegetable matter floating on the surface of the water should be excluded.  Stirring up the sediment from the bottom of a water body should also be avoided particularly where chemical flocculants have been used.  It is critical to allow enough time for complete sedimentation prior to sampling.  The best sample is taken from 5 to 10 cm below the surface of the water and sampled in a single aliquot, if possible, to avoid disturbing the bottom sediments.  The collection container should be free from dirt (obviously) but should also be clean: no Coke bottles.  Acids, for instance in a bottle previously containing soft drink or juice can change the solubility of some substances and therefore negatively interfere with the results. 

Turbidity is easy to measure in-line and can be incorporated into automatic water treatment systems.  Errors may creep in if instruments are not properly and regularly calibrated.  Turbidity standards have a short shelf life, and formazin is made using carcinogenic constituents, so expensive store-bought references are not usually replaced as often as they should be. 

TSS requires discrete aliquots and cannot be incorporated into in-line systems.  The actual test only takes between 30 minutes and 2 hours but does require a consistent technique and equipment that is not usually available on standard work sites. 

To answer the question what is dirty water?  We must decide whether the criteria is how much light can penetrate the water, that is the measure of turbidity or how much sediment is in the water that is measure TSS.  The answer depends on where the water is going, who is going to use it, and what the regulatory authorities’ request.  We need to keep in mind that turbidity does not equal TSS, and make sure we request the correct test from the laboratory.