Study On Granulation Of Aerobic For The Treatment Of Aniline-containing Organic Wastewater

Aniline and its derivatives, which are used as important intermediates in the production of synthetic organic chemicals and polymers including the polyurethanes, rubber additives, dyes, pharmaceuticals, pesticides and herbicides, are present in industrial wastes and are typical toxic persistent pollutants legislative controlled by the environmental protection agency of U.S., Europe and China. They can exist in the environment for a long time because of their biotoxicity, hydrophobicity, chemical stability and bioaccumulation. Up to now, many physico-chemical methods, such as photodecomposition, oxidation, electrolysis, ozone oxidation and resin adsorption, are traditionally used for the toxic pollutants removal, but they do not remove harmful substances incompletely and cost too much. Otherwise, the biological methods are more advance on investment and removal efficiency than the physico-chemical ones, however conventional biodegradation process can be used in handle tow-strength toxic organic wastewater. Compared to conventional activated sludge, aerobic granules are known to have a compact and dense microbial structure, excellent settleability and synergetic effect of multifunctional micro-flora. This implies that aerobic sludge granulation can improve significantly biomass retention and enhance the performance and stability of the bioreactor. The major objective of the present work was to investigate the feasibility of developing granulation of aerobic sludge for degrading aniline (AN) in a sequencing airlift bioreactor (SABR), and investigated a acclimation strategy including organic loading, sludge settling time and hydrodynamic shear force for improving the performance and stability of aerobic granular bioreactor.The SABR was fed with aniline as sole carbon, nitrogen and energy source. Small granules were observed in half past a month after start-up. By the decreased sludge settling time and gradually increased COD and AN loadings, the granulation of aerobic sludges was develeped after subsequent 15 days. Operated at AN Ioadings of 1.44 kg·m^-3·d^-1, SABR reached a steady state, as evidenced by constant COD and CAs removal efficiencies of above 91% and 93%, respectively. The SABR was deteriorated by continuously operated at a high AN loading of above 1.4 kg·m^-3·d^-1 for two months, the aerobic granules becoming sludge flocs, and the removal efficiencies of AN and COD were both decreased to below 90%.In order to evaluate the recovering performance of the granular bioreactor, the performance of the SABR gradually recovered from this disturbance after decreasing AN concentration to about 600 mg·L^-1. At the end of the experiment, the biomass concentration of the granular SABR finally reached 4.4±0.1 gVSS·L^-1. The process of sludge granulation could be divided into three different stages, i.e., Start-up, Aerobic granules formation and Granular reactor recovery period, based on the changes of sludge morphology, minimal settling velocity, SVI and AN-degrading performance.Mature granules had the average diameter of 1.2±0.2 mm, SVI of 45.5 mL·g^-1, settling velocity of 35 m·h^-1. SEM showed that the aerobic granules had dense structure and consisted of a wide variety of heterotrophic bacteria, including rods-like and cocci-like, which interlaced with extracelluar polymers. The SOUR of the granular sludge was 91.5 mgDO·gVSS^-1·h^-1, higher than that of conventional activated sludge. The activity of catechol 2,3-dioxygenase (C23O) was increased gradually during the whole sludge granulation and reached the highest value of 1230±90 nmol.mgProtein^-1·min^-1 at the AN loading of 0.9 kg·m^-3·d^-3, but the activity of catechol 1,2-dioxygenase (C12O) maintained a lower level all along, indicating the aerobic granules degraded AN mainly via the meta-cleavage pathway.Evidence showed that the sludge settling time and hydrodynamic shear force are two key hydraulic selection pressures in the aerobic sludge granulation. Results showed that the aerobic sludge displayed floc-like when the sludge settling time was 15rain, and the granulation rate was increased along with the settling time shortened. At the same time, high superficial air velocity favored the formation of the aerobic granules with regular appearance, compact structure and good settling characteristics.Degradation kinetics for all tests were well described by the typical substrate inhibition pattern as predicted by the Haldane equation, and the corresponding kinetic coefficients of V_max, K_s and K_i were 1.40±0.42 g·gVSS^-1d^-1, 124.87±34.69 mg·L^-1 and 1428.5±53.1 mg·L^-1, respectively.