In both developed and developing countries, centralised and on-site wastewater treatment systems have been developed for a long time. The main function of the wastewater treatment plants (WWTPs) should be removal of pollutants including organic carbon, nitrogen and phosphorus. Technologies applied to WWTPs are mostly with biological functions such as activated sludge, stabilization reservoir, oxidation ponds, rotating biological contactor (RBC), waste stabilization pond (WSP, normal, hybrid and complete), continuous stirred tank reactor (CSTR), stabilisation lagoon, wastewater stabilization reservoir, aerated lagoon, and constructed wetland.
One of the problems which can still be found in the WWTPs is that evaluations of the WWTPs are sometimes conducted only based on the effluent concentrations, e.g. 20 mg l-1 biological/biochemical oxygen demand (BOD), 10 mg l-1 total nitrogen (TN) or 1 mg l-1 total phosphorus (TP). However, even an advanced WWTP has been constructed, under the conditions of insufficient wastewater treatment planning, low BOD influent concentration, i.e. 20–40 mg l-1 could be found in WWTP management data. It might be well known that there are appropriate influent concentrations of the pollutants for the biological WWTPs, however, it is sometimes not well achieved in some specific WWTPs. Huge amounts of public money have been invested in wastewater treatment.