Glossary and Terms

An Introduction to some terms

Water treatment is a complex process that is constantly changing and improving to deal with an ever-increasing list of pollutants. A more organised way of looking at water treatment would be to group lists of pollutants based on how they change the characteristics of wastewater through various indicators; and then to discuss technologies, stakeholders, and policies associated with these indicators.

As a disclaimer, some pollutants affect multiple indicators or none of those listed! New indicators are constantly being defined by different sources to better define the parameters a treatment facility needs to adequately treat their wastewater stream. For the purpose of this blog, here’s a list of wastewater indicators that I’ve compiled with reference to the Code of Practice for Pollution Control SS593 Annex H and the Sewerage and Drainage Act.

Indicator Description Examples of associated pollutants(not exhaustive)
Chemical Oxygen Demand (COD) A measure of the amount of oxygen used to completely oxidise all chemicals in a wastewater sample. It indicates the concentration of pollutants that can be chemically oxidised. A very wide list of pollutants and naturally occurring chemicals found in all water bodies affect COD. Laws normally provide an acceptable level of COD allowable after water treatment.

–        Terrible at determining if the water is safe for release into natural ecosystems for being too general

–        Great for indicating whether a sample of water has oxidisable chemicals in excessive amounts


–        Metals that can be oxidised (Iron, Zinc, Mercury, to name a few)

–        Any carbon-based molecule other than CO2 (paper, food waste, oil, pesticides, cyanide, etc.)

–        Any other compound or element that can be further oxidised (Nitrogen, Phosphorus, Potassium to name a few)

Biological Oxygen Demand (BOD) More specific than COD, BOD indicates the concentration of compounds that can be oxidised by biological processes. This value is always lower than COD. An acceptable amount is often allowed when releasing treated wastewater into natural systems.

–        Useful for indicating if a wastewater stream is in need of biological treatment.

–        A large disparity between COD and BOD values is a good indication that there are biocidal chemicals or chemicals that cannot be oxidised by biological processes present.

–        May even be an indicator of pathogens that kill the bacteria used for the test of BOD

–        Does not account for chemicals that can be metabolised by organisms that do not involve oxygen in their metabolic processes (sulphur-reducing and methanogenic bacteria found in mangroves)

–        Pathogens that bacteria cultures used in biological treatment prey on.

–        Biological waste found in domestic sewers (faecal matter, urine, food waste, used oil, etc)


Total Suspended Solids (TSS) An indication of how much solid matter is not dissolved in water, but will not settle out in any appreciable time. Visually indicated by water turbidity.

–        High TSS indicates that wastewater will have to undergo physiochemical treatments to force these solids to settle out of suspension.

–        Fine sand, stones.

–        Metal particles

–        Microplastics

Total Dissolved Solids (TDS) An indication of how much solid matter has dissolved in water. There is a grey area between TSS and TDS since TDS does not require molecules to form chemical interactions with water as with the connotation of the term “dissolved” used in chemistry.

This opens up a variety of chemicals to being classified as TDS as long as they below a stipulated size.

Water with high TDS often will be off-colour, smell strange, or have corrosive properties due to their pH levels being higher or lower than 7(neutral pH)

–        Useful as an indicator to determine if water needs to be treated by distillation techniques, membrane techniques, or hybrid techniques.

–        Shares a large variety of chemicals that also affect COD

–        Aqueous pesticides and fertilisers

–        Mother liquor (spent chemical solutions used in the formation of crystals, like sugar)

–        Smaller microplastics

–        Salts that can occur in nature (magnesium, potassium, sodium, ammonium salts)

Settleable Solids Solids that do not fall under the previous two categories.

–        Large solids are separated by filter gates

–        Smaller solids that pass through are left to settle in a clarifier.

–        Rags

–        Plastic bags

–        Bottles

–        Sawdust

–        Sand

Heavy Metals An indicator of the presence of any forms of chemical compound or element that consists of metallic elements with high atomic mass. In water treatment, this usually refers to any metallic elements or metalloids above atomic number 40, as well as selected metallic elements below that.

Selection criteria for these lighter exceptions are due to their ability to cause serious issues with public health or natural systems.

–        Due to the transitional nature of many heavy metallic elements and their many oxidative states, they may fall under COD as well

–        Copper

–        Chromium

–        Zinc

–        Nickel

–        Cobalt

–        Mercury, caesium, cadmium

Volatile Organic Compounds and Semi-Volatile Organic Compounds (VOCs and SVOCs) A troublesome indicator of a large group of carbon-based organic compounds. Even in water treatment, their definitions vary among sources. One common trait is that they cannot be solid at room temperature and pressure.

For the purpose of this blog,

–        VOCs will be used to classify organic compounds that have significantly lower boiling points and higher volatility than water.

–        SVOCs will be used to classify organic compounds that have similar or higher boiling points, and similar or lower volatility as water.

–        Volatility is the degree of vapourisation of the VOC or SVOC.

–        VOCs: CFCs, Benzene, Chloroform

–        SVOCs: Polycyclic Aromatic Hydrocarbons (PAH), Polychlorinated Biphenyls (PCB), pharmaceuticals, organometallic complexes, oils and grease

These are the few indicators that I will be referencing to in my blog posts. This is certainly a non-exhaustive list of indicators, some of which I deliberately left out (like radionuclides) since my blog will be focused heavily around Singapore which currently does not have a nuclear industry. However, I won’t be discounting radioactive water treatment as a possible topic to discuss!

Process terms I will be using are:

Primary Treatment – Treatment of TSS and Settleable Solids through physiochemical means

Secondary Treatment – Reduction of BOD through biochemical means

Preliminary/Tertiary Treatment – Balance of pH, and reduction of non-biodegradable COD, VOCs & SVOCs, Heavy Metals, and TDS through physiochemical means. Used as a preparatory step for Secondary Treatment or as a finishing step for the next receiver of the treated wastewater.