Effects of Al on the microstructure and bioflocculation of anoxic sludge.

The aluminum ions generated from mining aluminum, electrolytic aluminum and the industrial production of aluminum-based coagulants (such as AlCl and Al(SO)) enter sewage treatment plants and interact with activated sludges. An anaerobic/anoxic/oxic (AO) process was used to reveal the effects of Al on the pollutant removal efficiencies, bioflocculation and the microstructure of sludge. The results showed that a low concentration of Al improved the pollutant removal efficiencies and increased the sludge particle size. However, a high concentration of Al hindered microbial flocculation and reduced the pollutant removal efficiencies. With a 10 mg/L Al addition, the chemical oxygen demand (COD), total nitrogen (TN) and NH-N increased by 3%, 16% and 27%, and reached as high as 68%, 60% and 87%, respectively. At the same time, the dehydrogenase activity, flocculation ability (FA) and contact angle of the sludge reached their maximum levels at 41.3 mg/L/hr, 45% and 79.63°, respectively. The specific surface area of the sludge decreased to 7.084 m/g and the sludge pore size distribution shifted to concentrate in the mesoporous range. Most of Al was adsorbed on the surface of sludge, changing the physicochemical properties and physical structure of the sludge.