Waste water is any water which has been used in an industrial process or household. This includes water used in the treatment of sewage or storm water. To ensure the water does no harm to people or the environment when it is released, waste water must be treated to reduce the level of contaminants to acceptable levels. Contaminants may include heavy metals, nutrients, organic substances or particles.

Different biological, chemical and physical methods can be used when treating waste water. Normally a combination of physiochemical and biological methods is required to clean the waste water. Aluminium and iron coagulants are used in many of these applications.

Inorganic coagulants are particularly effective in:

Bulking sludge
Activated sludge is a process which is used to treat sewage and industrial waste water. Aeration and micro-organisms are typically used. However, the process depends on good flocculation and settling. In many cases, coagulants are used to improve the conditions in the activated sludge process, ensuring that the waste water meets treatment targets.

Energy optimisation
Coagulants can help to optimise the use of energy in the treatment of waste water. For example, using an inorganic coagulant before biological water treatment reduces oxygen demand in the activated sludge tank. There is a corresponding decrease in the amount of electricity needed.
Using inorganic coagulants in the pre-treatment phase increases the amount of energy-rich primary sludge. Part of that sludge can be transferred to anaerobic digesters to produce biogas (methane). The biogas can be used to create electricity and provide heating. The energy produced has a very low carbon footprint. Iron coagulants can also directly affect the efficiency of an anaerobic digester by precipitating hydrogen sulphide.

Particle removal
The muddiness (turbidity) of water is caused by suspended particles. The smaller the particles, the harder they are to separate by sedimentation or filtration. Coagulants are used to agglomerate the smaller particles into larger ones.
Particles typically have a negative charge. To enable them to agglomerate, a sufficient quantity of positively charged coagulant is introduced to the water in order to neutralise the negative charge. The aluminium or ferric hydroxide flocs which are formed are usually separated from the water using sedimentation. The performance of filtration and flotation technologies are significantly enhanced.

Phosphorus reduction
Phosphorus is an essential nutrient for living organisms. Yet an excess of phosphorus in water can cause a dramatic increase in the growth of algae (known as eutrophication). Using aluminium or iron coagulants, phosphorus (in the form of ortho-phosphate) can be separated from the waste water. The precipitated phosphorous is separated from the waste water using a physical process such as filtration, flotation or sedimentation.

Sludge dewatering
Sludge dewatering involves the separation of solid particles from the liquid. Utilising an inorganic coagulant during sludge dewatering normally improves efficiency. When inorganic coagulants are used in combination with polymers or lime, the polymer dose can be significantly reduced. The level of dry solid content is increased and less sludge is produced.