Impact of influent quality on the mixed liquor volatile suspended solids/mixed liquor suspended solids of mixed liquor from treatment plant

In order to study the reason of mixed liquor volatile suspended solids/mixed liquor suspended solids (MLVSS/MLSS) decline in wastewater treatment plant (WWTP), a WWTP in Chongqing City was chosen as the object, and the relationship between influent quality and MLVSS/MLSS of mixed liquor was investigated. After tracking and monitoring the WWTP for 1 y, the average concentrations of influent for chemical oxygen demand (COD), suspended solid (SS), inorganic suspended solid (ISS), total nitrogen (TN) and total phosphorus (TP) were 635; 1,123.2; 809.2; 67.72 and 9.3 mg/L, respectively. After the pretreatment of grit chamber and primary settling tank, the average concentrations of influent for COD, SS, ISS, TN and TP were decreased to 277, 317.6, 231.9, 45.08, and 4.8 mg/L, respectively. And the volume average grain diameter was decreased from 60.93 to 49.56 mu m. Besides, the concentrations of influent for SS, ISS, COD and MLSS were decreased in the dry season, while the concentrations of influent were higher concentration in rainy season. The annual variation rule of MLVSS/MLSS for mixed liquor was ranging from 0.24 to 0.57. According to the coefficient analysis obtained by Pearson correlation, the COD, ISS, TP and particle size for influence were the most important factor on MLVSS/MLSS of mixed liquor. The COD and particle size were positively correlated with MLVSS/MLSS, and the correlation coefficients were 0.525 and 0.25, respectively. While, ISS and TP were negatively correlated with MLVSS/MLSS, and the correlation coefficients were -0.635 and -0.557, respectively. Finally, a linear regression model between MLVSS/MLSS (Y) and COD (X1), ISS (X2), TP (X3) and particle size (X4) was established by a backwards method, that was Y = 0.488X1 - 0.521X2 - 0.154X3 + 0.165X4.