Wastewater treatment is of great importance to environment-protecting. The application of moving bed biofilm reactors (MBBR) greatly improved the treatment capacity and efficiently eliminated organic substrates and nitrogen from wastewater. Microbial carrier is one of key elements of the MBBR, and the packing rates of microbial carriers affect the organics and nitrogen removal efficiencies as well as the magnitude of the investment, but previous studies only reported in the range of 10%-30% of the reactor volume.

Researchers and students with Institute of Process Engineering (IPE) made a further study on the effects of packing rates on treatment performance in continuous MBBRs. They chose three packing rates of 20%, 30% and 40% to construct three reactors in parallel for the future application with an aim to systematically investigate the effects of the packing rate of Polyurethane Foam (PUF) carriers on organics and nitrogen removal in MBBRs.

In their work, the relationship between the chemical oxygen demand (COD) volume loading of carriers and the amount of attached biofilm was identified. They measured the DO and nitrate micro profiles in biofilms with different densities using a microelectrode. The diversity of microbial communities in biofilms with different packing rates was analyzed by using a pyrosequencing method.

The effects of packing rates (20%, 30%, and 40%) of the PUF to the removal of organics and nitrogen were investigated by continuously feeding artificial sewage in three aerobic MBBRs. The results indicated that the packing rate showed good performances in COD removal.

But, the packing rate significantly affected biofilm structure and nitrification. Ammonium removal was great influenced. A high ammonium removal efficiency of 96.3% at a hydraulic retention time of 5 h was achieved in 40% packing rate reactor. The DO, nitrate transfer process and SEM results demonstrated that a dense biofilm blocked DO transfer and limited nitrifier growth space and nitrate, glucose transfer.

The packing rate had little influence on the dominant phyla, however, the Nitrosomonas and Nitrospira bacteria in 30% and 40% packing rate reactor were higher than those in 20%, which might have affected the nitrification performance in the respective reactors. Pyrosequencing analysis of the biofilm showed that Proteobacteria, Bacteroidetes and Verrucomicrobia were the three most abundant phyla, but the proportions of the microbial community varied with the packing rate of the PUF carriers.

This study will contribute insights into developing MBBRs filled with PUF carriers for domestic wastewater treatment in practical applications. This work was supported by the National High Technology Research and Development Program (863 Plan) and the National Science and Technology Support Program. The paper was published in Bioresource Technology.