Wastewater which is a leading environmental pollutant results from numerous industrial processes as well as domestic operations. There is an increase focus on tougher legislation and policies to protect the environment from all forms of pollution, wastewater treatment and recycling have been seen as a promising solution.
Among the several available wastewater treatment methods, treatment by algae-bacterial systems has in particular attracted significant attention of many researchers since it does not require any external aeration. Unfortunately, the low efficiency of algal-bacterial reactants limits their performances. For instance, the low algal concentrations and reactor activities are a consequence of poor retention of the suspended algae in the reactors. Therefore, significant efforts have been directed towards increasing the efficiency of algal-bacterial reactors. For example, a single reactor has been used to eliminate oxygen chemical demand and ammonium from wastewater. However, it never worked well with phosphate. Thus, increasing phosphate removal efficiency will enhance their performance.
Presently, sludge retention time has been applied to reduce the ammonium oxidation bacteria concentration. This enables algae to access more ammonium thus resulting in more production of oxygen and removal of phosphate. Consequently, due to the different growth rates between algae and ammonium oxidation bacteria, sludge retention time will determine the efficiency of the algal-bacterial reactors. However, the impact of sludge retention time on the performance of the algal-bacterial reactors have not been fully explored.
A group of researchers led by Professor Jixiang Yang at Chinese Academy of Science investigated the impact of sludge retention time on algal-bacterial reactors. They constructed two photo bioreactors consisting of dynamic filters. Membrane A and B were applied at a sludge retention time of 10 and 20 days respectively to provide ideal condition control. Their work is published in the research journal, Chemical Engineering Journal.
The authors observed that reactor B generally exhibited better performance in chemical oxygen demand ammonium removal as compared to reactor A. For instance, the removal efficiencies were 50% and 60% at a sludge retention time of 10 and 20 days for reactor A and B respectively without an external oxygen supply. However, slight aeration resulted in a significant increase in the chemical oxygen demand and ammonium removal efficiencies. In addition, the presence of phosphate accumulation organisms was confirmed. Furthermore, reactor B exhibited high phosphate removal efficiency as compared to reactor A due to the fact that algae had enough access to ammonium.
The study successfully investigated the impacts of sludge retention time on algae ecology that further influences the overall performance of an algal-bacterial bioreactor. Thus, it will not only enhance cost-effective treatment of domestic wastewaters but also has the potential of application in the treatment of industrial wastewater. As such, it will help in reducing environmental pollution which has become a global concern.
The research showed that phosphate was not fully removed by the algal-bacterial reactor. Nevertheless, much influent phosphate concentrations of practical domestic wastewater are merely around 3 mg/L that is much lower than the concentration applied in the research. This indicates that the efficient removal of organic matters, ammonium as well as phosphate can be achieved in a single reactor in which wastewater is continuously pumped into. Furthermore, expensive external oxygen supply is saved. Therefore, we expect that this technology is highly potential for future wastewater treatment.
H.H. Tsang, S. Raza. Impacts of sludge retention time on the performance of an algal-bacterial bioreactor. Chemical Engineering Journal, 343, 37-43.Go To Chemical Engineering Journal