Biogranular sludge for rubber processing wastewater in a sequencing batch reactor

Abstract

Rubber is one of the major agro-based industrial sectors that contributes to the development of the country. Malaysia is one of the leading producers of natural rubber in the world. The rubber industry consumes large volume of water, uses chemicals and other utilities, and produces an enormous amount of wastes and effluent. As rubber effluent contributes to highly polluted wastewater, the need to find an efficient and practical approach to preserve the environment is essential. Thus, this study aims to investigate the applicability of aerobic granular sludge technology in treating rubber processing wastewater using a laboratory scale sequencing batch reactor (SBR) system. Aerobic granular sludge was developed in a 1.6 L working volume of column reactor that operated with 3 hours of cycle time for 90 days. The reactor had a volumetric exchange ratio of 30% and the superficial upflow air velocity was set at 1.2 cm/s. The dissolved oxygen in the reactor was within the range of 6.0-7.5 mg/L. The SBR system was run at organic loading rate of 0.8-3.3 kg COD/m3 day and COD/N/P ratio was 100/19/8. Results showed that aerobic granules formed had an average diameter of 1.4 to 2.8 mm with settling velocity of 57.8 ± 3.5 m/h and sludge volume index (SVI) of 46.0 mL/g. These properties caused a significant increase in biomass concentration from 3.8 to 10.1 g/L, which was observed to be beneficial for the performance of the reactor system. The scanning electron microscope (SEM) examinations revealed that aerobic granular sludge consisted of non-filamentous cocci-shaped bacteria, tightly linked to one another to form a compact structure. The performances of aerobic granules that formed at three different cycle times of 3, 6 and 12 hours of SBR operation were studied. The highest cycle time favours the highest removal performances in removing organic and nutrients. 96.9% COD removal was achieved when the reactor was operated at high cycle time of 12 hours, while around 60.0% and 65.9% removal efficiencies were recorded for total nitrogen and total phosphorus in the granular SBR system for rubber processing wastewater treatment. The metagenome analysis was used to discover the microbial community that accumulated in aerobic granular sludge, which was potential in the granulation and biodegradation process. The abundance of COD degrading, denitrifying and polyphosphate bacteria such as Pseudomonas, Agrobacterium and Thauera bacteria were high in aerobic granules. Those bacteria had both capability in producing extracellular polymeric substances (EPS) and degrading waste. The characteristics of EPS of aerobic granular sludge were determined. Proteins (PN) were more dominant than polysaccharides (PS) in the EPS of aerobic granular sludge. The excitation-emission matrix (EEM) results also indicated the importance of aromatic protein-like substances, particularly tryptophan in maintaining the stable structure of granular sludge. Despite the different cycle times, aerobic granular sludge formation was successfully achieved for the treatment of high strength wastewater such as rubber effluent.