Wastewater contains several undesirable elements which need to be removed before it can be discharged safely. Suspended solids, biodegradable organic compounds, pathogenic microbes, large amounts of nutrients such as nitrates and phosphates etc. are some of elements present in wastewater effluents. Presence of these have consequences of their own. For example, suspended solids can clog water outlets, rivers etc. while biodegradable organic compounds can lead to high biological oxygen demand thus depleting oxygen concentration in water. This negatively affects organisms such as fish etc. which live in water and depend on oxygen to survive. Pathogenic microbes in water pose serious health hazard if such water is consumed. Further, high nutrient content gives rise to undesirable algal blooms in water. To remove such different types of undesirable elements,  different water treatment solutions are needed in different stages of wastewater treatment.

After preliminary treatment of wastewater involving removing large solids and grit using different types of screens, the wastewater is ready for primary treatment. Primary treatment stage involves  reducing suspended solids in wastewater. Mechanical methods and chemicals, which can help enhance sedimentation, are generally used in this stage of treatment. The process mainly takes place in clarifiers or settling tanks where the wastewater or sewage is transferred after preliminary treatment. In clarifiers relative calm conditions promotes settling down and sedimentation of relatively larger suspended solids under gravitational force. The leads to forming of primary sludge at the bottom of the clarifiers. The relatively smaller suspended solids along with dissolved organic matter do not settle down so easily and are usually dealt with in other stages of wastewater treatment. Oil and grease, being lighter than water, form the top layers of wastewater in the tank. This can be skimmed off and recovered and reused for other purposes like making soap etc. The settled primary sludge is pumped out and sent for further treatment.

For wastewater to be fit for reuse, primary treatment is not enough. After primary treatment, the wastewater is subjected to secondary treatment to minimize biological waste and subsequently to  tertiary treatment after which it can be reused or discharged safely into natural water bodies. Advanced wastewater treatment is useful for producing relatively higher quality water from wastewater in the treatment process. This is constituted of three treatment categories which are tertiary treatment, physico-chemical treatment and combined bio-physical treatment.

In the final stages of wastewater treatment (usually tertiary treatment), portable water treatment comes in handy when water needs to be reused. This can be done through boiling, filtration, activated charcoal absorption, chemical disinfection etc. Membrane filter water treatment can help remove particles larger than 0.2 micrometer including infectious microbes like giardia and cryptosporidium. Further, disinfection may be necessary in many cases and this can be done with chlorine treatment or ultraviolet treatment or ozone treatment as part of potable water treatment process. Coagulation and flocculation are also important parts of potable water treatment. During any stage, if water is acidic then adding neutralizers, soda ash or sodium hydroxide are used to raise the pH from acidic towards neutral for acid water treatment.

Other sources of waste water, like that from oil and gas fields need special treatment. For example, for such produced water treatment, sorption of organic substances by certain zeolites, air stripping etc can be a cheap and effective treatment method.

Thus, wastewater treatment process is a long process in which primary treatment plays the initial critical part so that wastewater is ready for further treatments in secondary and tertiary stages for safe reuse or discharge.