دانلود رایگان مقاله انگلیسی ویژگی های ساختار لجن فعال ته نشین شده در مرحله نابودی شتابدار - الزویر 2019

ABSTRACT

The sedimentability of the activated sludge can be affected by the presence of a large variety of coagulants and polymers from a previous physical-chemical process. In this paper, the activated sludge settling process in industrial wastewater treatment plants where the sludge does not settle in a conventional way is studied. The two observed constant hindered settling velocity stages and the instant the intermediate sludge acceleration period occurs are described. A variation of the Richardson and Zaki model is used to characterize the two stages of constant settling velocity. The concentration of suspended solids, where a sudden decrease of hindered settling velocity was observed, is calculated. Finally, a new hypothesis to explain the processes triggering the collapse of the initial homogeneous sludge structure and the existence of an acceleration period is formulated.

4. Conclusions

Two different stages with constant hindered settling velocity and an intermediate acceleration process are observed during the settling tests. The defined delay time allows estimating the instant when the sludge collapse-acceleration process occurs. The delay time increases exponentially with increasing X concentration and decreases linearly with increasing initial hindered settling velocity. Richardson and Zaki model lets characterize and classify the activated sludge sedimentability in a settling test with a sludge acceleration stage. It also allows estimating the X concentration where a sudden decrease of final hindered settling velocity, not previously described in the literature, occurs. The existence of two constant hindered settling velocity stages and a sludge acceleration stage can be explained by the fracture of the initial homogeneous sludge structure and the flocculation of the generated flocs. The proposed physical-chemical mechanisms of fragmentation and the initial sludge structure explain the sudden drop observed in the final settling velocity and the delay in the start of the sludge acceleration phase. The additives added in the previous physical-chemical treatment play an important role in the sedimentation process and in the fragmentation of the initial homogeneous sludge structure.