Where is the Water?

We were all taught at school that the Earth’s surface is over 70% water and its no wonder that the late great astronomer Carl Sagan described us all inhabiting a pale blue dot. The vast majority of the Earth’s water is salt water found in the sea’s and ocean’s, with only around 2.5% as freshwater, but where is that? Of the 2.5% of total freshwater, only 1.2% of this supply is in the form of surface water and 21.4% of this is in the held in lakes and rivers. This means that of the total volume of water on earth, only 0.01% is freshwater in rivers and lakes! But fear not, water treatment engineers have been very busy devising methods to provide us with more fresh water.

In a litre of average ocean water, there is about 35g of salt. But it does vary. The Atlantic Ocean is saltier than the Pacific Ocean, for instance. The saltiest water in the world is found in Antarctica in a small lake named Don Juan Pond.

What’s in the water?

You can say that water has a very attractive personality and attracts all kinds of contaminants, ions, bacteria and other organics and this mixture is highly influenced by where the water is located. The source of and use you have for the water tends to create the focus on which components you are looking for, which in turn dictates the treatment processes required. In addition to the more natural sources of water, there are also less natural sources such as waste water form our civilisation, industrial effluents and storm water. These other sources of water are likely to have much different compositions. The table below shows a summary of what can be found in water by category, some typical components and test methods.

Disolved Salt (Ions)

The worlds natural water sources are principally categorised by how much salt they contain, but what is a salt and where does it come from? A salt is an ionic compound formed between positively charged ions (cations) and negatively charged ions (anions). When dissolved in water, salts dissociate into their component ions. Salts can be either mono or multivalent, where mono is formed of a single ionic bond between ions (e.g common table salt Sodium Chloride NaCl) and multi is of course a larger molecule with multiple bonds (e.g Calcium Chloride CaCl₂).

Salts eventually make their way to the sea and their ions are created by natural processes such as the weathering of rocks, volcanic outgassing and hydrothermal vents. The table below shows some of the categories of water and some commonly used measurements.

While both Total Dissolved Solids (TDS) and salinity are measures of dissolved substances in water, they differ in what they measure and how they are expressed. For desalination design, TDS is usually used. Salinity specifically refers to the concentration of dissolved salts in water and expressed in percentage (%), parts per thousand (PPT), or sometimes in ppm. Salinity measurements often focus on the specific ionic contribution of salts using conductivity, refractometers, or ion-selective electrodes.

Salinity % =(Mass of dissolved salts / Mass of water sample) ×100

TDS refers to the total concentration of all dissolved substances (inorganic and organic) in water. These substances can include dissolved salt ions, but also minerals, metals, and organic matter that is measured by electrical conductivity.

Biological Contaminants

Biological contaminants are naturally present in aquatic environments, but their levels can increase dramatically due to pollution, wastewater discharge, and runoff from agriculture. Bacteria are amongst the oldest lifeforms on Earth and they were crucial in shaping the Earth’s atmosphere and environment.Viruses may have evolved around the same time as the first cells, possibly as parasitic entities or genetic material that escaped from cellular organisms, with origins that are still debated by scientists. Protozoa evolved from the first complex cells and diversified into a wide variety of forms, playing important roles in ecosystems and disease transmission. They are all harmful to public health and are some of primary concern to water engineers.

Nutrients

A nutrient refers to a chemical element or compound that is essential for the growth of living organisms, particularly plants and microorganisms. The most common found in water are Nitrogen (N) and Phosphorus (P). They can be bonded with Carbon within living or decaying organisms (Organic) or bonded with other elements (inorganic). When they are present in excessive amounts in water, they can lead to eutrophication, causing the overgrowth of algae and other aquatic plants. This process can lead to oxygen depletion in water bodies, causing harm to aquatic life and creating dead zones.

The most common form of P in water is Orthophosphate (PO₄³⁻), which is utilised by plants. It comes from agricultural runoff, wastewater, fertilisers, and detergents.

Nitrogen naturally undergoes a continuous transformation in nature through the nitrogen cycle and many forms are found in water.

Emerging Contaminants

Whilst many of the items above have been around in our water for a very long time, there are new kids on the block that are rightly getting much attention and this list is likely to ever keep expanding as we continue to progress with our industrialised civilisations.

First up is PFAS, which stands for Per- and Polyfluoroalkyl Substances. These are human made chemicals and have been used in various industries since the 1940s and can be as small as 0.001 micrometer. They are known as forever chemicals and for very good reason. They are made with carbon-fluorine bonds which are amongst the strongest chemical bonds in organic chemistry, have great heat resistance, repel water and oil and are highly resistant to environmental degradation. Their properties make them useful for modern life and they are commonly used for non stick cookware (Teflon), for firefighting foams and for water proofing clothing (Goretex). They can bioaccumulate in water, soil, and living organisms, including humans and can have been linked with multiple health issues such as cancer, disruption of natural hormone systems and organ failure. They can travel long distances from their original sources through the air and water. Its for these reasons that their removal from water is becoming much more of a requirement.

Plastics were first created in the early part of the 20th century from long chains of polymers, which are synthetic organic molecules derived from petrochemicals. An example of a plastic is PE or Polyethylene (C₂H₄), which is used for bottles, food packaging and pipework in construction projects. Plastics of this size do pose huge challenges with their disposal and we are all sadly accustomed of seeing such debris in the environment. However, it is micro plastic that primarily poses a challenge to water treatment, which range in size between 5mm to 0.001 micrometer. Some micro plastics are deliberately manufactured for use in some exfoliating scrubs and toothpastes, but most are the product of slow degradation of larger items in the environment. These particles tend to accumulate in the sea and can also be washed into sewers during heavy rainfall and many pass through waste water treatment processes. They accumulate in fresh water, the soil and also unfortunately inside of animals and humans causing health issues due to the toxicity and also physical harm to our internal systems.

What are the Treatment Options?

A wide variety of treatment methods are available and their selection depends upon the composition of the influent, concentration of products, the form they are presented and the desired effluent specifications.

Dissolved Salts. Desalination methods such as; Reverse Osmosis (RO), Ion Exchange, thermal process such as Multistage Flash (MSF).
Suspended Solids. Solids seperation methods such as; Granular Activated Carbon (GAC), sand filtration, Dissolved Air Floatation (DAF).
Dissolved Organics. GAC, Advanced Oxidation, Coagulation.
Biological Contamination. Chlorination, UV Disinfection, Ozonation, Ultrafiltration (UF), Microfiltration (MF).
Dissolved Gases. De-gasification, aeration, or chemical treatment (pH adjustment).
Trace Elements. Chemical precipitation, ion exchange, or RO.
PFAS. RO, GAC.
Microplastics. UF, RO.
Nutrients. Enhanced Biological Phosphorus Removal (EBPR), RO, Ion Exchange, Biological Treatment, Chemical Precipitation.

Water Composition