Previous studies on the water quality of surface runoff often focused on the chemical\noxygen demand (COD), nitrogen, phosphorus, and total suspended solid (TSS), but little is known in\nterms of the inorganic suspended solids (ISS). This research investigated the effects of ISS carried\nby surface runoff on the treatment efficiency of the pretreatment facilities and the ratio of mixed\nliquor volatile suspended solid to mixed liquor suspended solid (MLVSS/MLSS) of the activated\nsludge in a wastewater treatment plant (WWTP) with the anaerobic-anoxic-oxic (AAO) process in\nChongqing city, China. The results showed that the surface runoff had a long-lasting impact on the\ngrit removal capacity of the grit chamber, affecting the normal operation after the rainfall. In contrast,\nthe primary sedimentation tank showed strong impact resistance with higher removal rates of COD,\nTSS, and ISS. Nonetheless, the primary settling tank aggravates the removal of organic carbon in\nsewage during rainfall, having a negative impact on subsequent biological treatment. The ISS in the\nsurface runoff could increase the sludge concentration and decrease the MLVSS/MLSS ratio. After\nrepeated surface runoff impact, the MLVSS/MLSS ratio in the activated sludge would drop below\neven 0.3, interrupting the normal operation of WWTP.
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