Efficiency of both activated sludge system and its variations in removing the chemical oxygen demand from industrial waste: systematic review and meta-analysis

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The activated sludge system is widely used worldwide to treat complex waste types, given its high efficiency level. The aim of the present study is to identify the efficiency of both activated sludge systems and their variations in removing the chemical oxygen demand (COD) from industrial waste, based on systematic review and meta-analysis. The search was carried out in Scopus and ScienceDirect databases; two independent researchers selected studies published between 2016 and 2020 to gather the data. The industrial waste types presented great initial COD variation, even within the same segments and typologies; furthermore, some activated sludge systems presented pre- or post-treatment. The systematic review and the meta-analysis allowed presenting the global efficiency of removal of chemical oxygen demand in industrial effluents by activated sludge systems and their variants used globally, which reaches 83.00% (CI = 82.00 to 93.00). In addition, the current study is the first on this topic; therefore, it will work as a reference for future studies addressing this subject.