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The COVID-19 pandemic and the related measures brought a change in daily life that affected the characteristics of the municipal wastewater and further, of the biological activated sludge. The activated sludge process is the most widely used biological wastewater treatment process in developed areas. In this paper, we aim to show the situation of specific investigations concerning the variation of the physicochemical parameters and biological composition of the activated sludge from one conventional wastewater treatment plant from a metropolitan area. The investigations were carried out for three years: 2019, 2020 and 2021. The results showed the most representative taxa of microorganisms: Microtrix, Aspidisca cicada, Vorticella convallaria, Ciliata free of the unknown and Epistylis and Rotifers. Even if other microorganisms were found in the sludge flocs, their small presence did not influence in any way the quality of the activated sludge and of the wastewater treatment process. That is why we conclude that protozoa (especially Flagellates and Ciliates) and rotifers were the most important. Together with the values and variation of the physicochemical parameters, they indicated a good, healthy, and stable activated sludge, along with an efficient purifying treatment process, no matter the loading conditions.

The objective of this work is to give the fundamental information that should be known about sludge drying methods. The purpose of all types of sludge processing is to reduce the volume, stabilise the sludge, remove water and kill pathogenic organisms. It is processed in stages that contain a sequence of treatments such as thickening, dewatering and drying. Sludge drying in tunnels using photovoltaic panels is a continuous operation from the sludge treatment which reduces the water content of sludge by heating it for short periods. Dried product not only reaches granular formation with 92-95% DS but also gets stabilized. This is a complicated process with contemporary heat and mass transfer attended by physical-chemical transformations. Drying occurs as a result of vaporization of liquid by supplying heat to wet feedstock.