Decentralized wastewater treatment system (DEWATS) are widely used for the treatment of wastewater originating from residences, institutes and municipalities, specifically in South India. Most of these STPs are denounced owing to failures on several fronts including design, operation and maintenance, installation and monitoring. A comprehensive review and evaluation of STPs was timely, in order to derive sound conclusions and recommendations for future wastewater management strategies. The objective of the present study was to conduct an independent evaluation of already existing decentralized STPs in South India. The technologies assessed were Aerated lagoon (AL), Extended aeration (EA), Anaerobic filter/Vortex put forward by Centre for Scientific Research (CSR VORTEX), Constructed Wetland (DEWATS others), Membrane bioreactor (MBR) and Moving bed Biofilm reactor (MBBR). Among the various technologies evaluated, MBR exhibited the highest total COD, BOD and solids removal efficiency. Pathogen count was lowest in MBR, followed by MBBR and AL. Nutrient removal in terms of ammoniacal nitrogen and nitrate nitrogen was highest in DEWATS. Effective hours of continuous operation enabled improved plant performance. In case of natural treatment technology such as DEWATS, energy requirement is quite low, whereas conventional treatment technologies such as EA necessitate considerably high demand of energy, requiring few personnel to operate the system. Innovative high cell density systems such as MBBR and MBR entail significant power consumption and elaborate maintenance, requiring large number of skilled professionals. The major reasons for failure of STPs were related to mechanical, electrical and labour problems. Regular monitoring and maintenance is required with due diligence in all the treatment technologies for proper functioning.

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