Gravity: Analog Turbidity Sensor For Arduino

Total suspended solids

Total suspended solids (TSS) are particles that are larger than 2 microns found in the water column. Anything smaller than 2 microns (average filter size) is considered a dissolved solid. Most suspended solids are made up of inorganic materials, though bacteria and algae can also contribute to the total solids concentration .

These solids include anything drifting or floating in the water, from sediment, silt,and sand to plankton and algae . Organic particles from decomposing materials can also contribute to the TSS concentration. As algae, plants and animals decay, the decomposition process allows small organic particles to break away and enter the water column as suspended solids . Even chemical precipitates are considered a form of suspended solids . Total suspended solids are a significant factor in observing water clarity . The more solids present in the water, the less clear the water will be.

Some suspended solids can settle out into sediment at the bottom of a body of water over a period of time . Heavier particles, such as gravel and sand, often settle out when they enter an area of low or no water flow. Although this settling improves water clarity, the increased silt can smother benthic organisms and eggs . The remaining particles that do not settle out are called colloidal or nonsettleable solids .

These suspended solids are either too small or too light to settle to the bottom.

Settleable solids

Settleable solids are also known as bedded sediments, or bedload . These sediments can vary from larger sand and gravel to fine silt and clay, depending on the flow rate of water. Sometimes these sediments can move downstream even without rejoining the suspended solids concentration. When settleable  solids are moved along the bottom of a body of water by a strong flow, it is called bedload transport .

Turbidity is an optical determination of water clarity . Turbid water will appear cloudy, murky, or otherwise colored, affecting the physical look of the water. Suspended solids and dissolved colored material reduce water clarity by creating an opaque, hazy or muddy appearance. Turbidity measurements are often used as an indicator of water quality based on clarity and estimated total suspended solids in water.

Turbidity of Water

The turbidity of water is based on the amount of light scattered by particles in the water column . The more particles that are present, the more light that will be scattered. As such, turbidity and total suspended solids are related. However, turbidity is not a direct measurement of the total suspended materials in water. Instead, as a measure of relative clarity, turbidity is often used to indicate changes in the total suspended solids concentration in water without providing an exact measurement of solids .

CDOM

Turbidity can come from suspended sediment such as silt or clay, inorganic materials, or organic matter such as algae, plankton and decaying material. In addition to these suspended solids, turbidity can also include colored dissolved organic matter (CDOM), fluorescent dissolved organic matter (FDOM) and other dyes . CDOM is also known as humic stain. Humic stain refers to the tea color produced from decaying plants and leaves underwater due to the release of tannins and other molecules.

This discoloration is often found in bogs, wetlands or other water bodies with high amounts of decaying vegetation in the water. CDOM can cause water to appear red or brown, depending on the type of plants or leaves present. These dissolved substances may be too small to be counted in a suspended solids concentration, but they are still part of a turbidity measurement as they affect water clarity.

Turbidity and total suspended solids refer to particles present in the water column. Turbidity and water clarity are both visual properties of water based on light scattering and attenuation. All three parameters are related to particles in the water column, whether directly or indirectly.

Turbidity is determined by the amount of light scattered off of these particles . While this measurement can then be used to estimate the total dissolved solids concentration, it will not be exact. Turbidity does not include any settled solids or bedload (sediment that “rolls” along the riverbed) . In addition, turbidity measurements may be affected by colored dissolved organic matter . While this dissolved matter is not included in TSS measurements, it can cause artificially low turbidity readings as it absorbs light instead of scattering it .

Total suspended solids

Total suspended solids, on the other hand, are a total quantity measurement of solid material per volume of water . This means that TSS is a specific measurement of all suspended solids, organic and inorganic, by mass. TSS includes settleable solids, and is the direct measurement of the total solids present in a water body. As such, TSS can be used to calculate sedimentation rates, while turbidity cannot .

Water Clarity

Water clarity is strictly relative to sunlight penetration. While this is usually determined by the amount of suspended solids in water, it can also be affected by CDOM and other dissolved solids . Water clarity is the most subjective measurement of the these three parameters, as it is usually determined by human observation .

Total suspended solids, as a measurement of mass are reported in milligrams of solids per liter of water (mg/L) . Suspended sediment is also measured in mg/L . The most accurate method of determining TSS is by filtering and weighing a water sample . This is often time consuming and difficult to measure accurately due to the precision required and the potential for error due to the fiber filter .

JTU

Turbidity, on the other hand, is most often measured with a turbidity meter. Turbidity is reported in units called a Nephelometric Turbidity Unit (NTU), or a Jackson Turbidity Unit (JTU) . The JTU was the original turbidity unit based on the visibility of candlelight in a tube (Jackson Candle Turbidimeter) . However, this method is considered out of date and inaccurate in comparison to newer methods.

NTU

While some organizations consider the two units to be approximately equal, there are some specific differences . In particular, NTU is more precise and has a wider range (JTU cannot measure above 25 JTU/NTU) . In addition NTU is the standard unit of many broadband output (400-680 nm wavelength) turbidit meters. Nephelometric refers to the measurement technology used. This technology method requires the photodetector in the meter to be placed at a 90 degree angle from the illumination source . As light bounces off the suspended particles, the photodetector can measure the scattered light.

FNU

The USGS also suggests the use of the Formazin Nephelometric Unit (FNU) if a turbidity meter only has a monochrome/infrared output, as opposed to the white/broadband output . This applies to instruments that are in compliance with the European drinking-water protocol, including most submersible turbidity meters . Both NTU and FNU will show equal measurements when calibrating as they both use nephelometric technology, but may operate differently in the field due to the different light source . Turbidity meters that use FNU units are able to compensate for dissolved colored materials (such as humic stain), while NTU turbidity meters cannot .

In most situations, a total suspended solids concentration below 20 mg/L appears clear, while levels over 40 mg/L may begin to appear cloudy . In comparison, a turbidity reading below 5 NTU appears clear, while a reading of 55 NTU will start to look cloudy and a reading over 500 NTU will appear completely opaque .

It is important to note that this is dependent on the size and nature of the suspended solids. Typical turbidity and TSS levels are difficult to quantify due to their natural variation by season, local geology, water flow and weather events. During a low-flow period, most rivers and lakes are fairly clear with a turbidity reading below 10 NTU. These readings can easily jump into the hundreds due to runoff during a rainstorm, snowmelt or a dredging project .