Nitrogen removal in moving bed sequencing batch reactor using polyurethane foam cubes of various sizes as carrier materials
Highlights
► MBSBR with smaller-sized PU foam cubes achieved the highest total nitrogen removal. ► MBSBR performed better than SBR in TN removal due to the occurrence of SND process. ► Higher 8-mL PU foam cubes concentration in batch reactor led to higher TN removal.
Introduction
Nitrogen is widely found in wastewater streams, including municipal, industrial and agricultural wastewaters. Nitrogen removal can be achieved by nitrification under aerobic conditions and denitrification under anaerobic conditions. Among various biological treatment systems for nitrogen removal, sequencing batch reactor (SBR) systems possess many advantages including lower operational costs and less bulking. In addition, the cycle format can be flexibly configured to combine nitrification and denitrification phases in one reactor; thus, permitting the building of small treatment plant (Louzeiro et al., 2002, Kargi and Uygur, 2003, Yang et al., 2010). Over the years, many efforts have been made to modify the SBR system to improve the performance. Among others, the moving bed sequencing batch reactor (MBSBR) which incorporates both suspended-growth and attached-growth processes has attracted much interest among researchers in the field of wastewater treatment (Sirianuntapiboon and Yommee, 2006, Chen et al., 2008, Goh et al., 2009, Guo et al., 2010).
Three different type of support media, namely Kaldnes (polyethylene media), Liapor (ceramic media) and Linpor (plastic media with high porosity) were studied in nitrogen removal under sequencing batch operation (Valdivia et al., 2007). The results showed that the reactor packed with Linpor performed better in chemical oxygen demand (COD) and ammonium-nitrogen removal with organic loading rate over 3.0 g COD/m3 day. Polyurethane (PU) foam belongs to the Linpor type of support media which has a high porosity and is an ideal medium for biomass immobilization. Besides, it has good mechanical strength and is relatively low cost (Golla et al., 1994, Chu and Wang, 2011).
Nitrogen removal through simultaneous nitrification and denitrification (SND) process had been investigated in the previous studies by using various systems (Kotlar et al., 1996, Daniel et al., 2009, Chu and Wang, 2011). The SND process has generated much interest due to its potential to reduce the operational period in SBR system, resulting in the reduction of operation cost and time. The oxygen concentration and the availability of carbon source for denitrification were found to be the important parameters for the commencement of SND (Pochana and Keller, 1999). Previous studies had demonstrated the occurrence of decreasing dissolved oxygen (DO) gradient within the inner layer of biofilm and also deep inside the PU foam (Morper, 1994, Guo et al., 2010). Therefore, the creation of an anoxic condition allowed nitrogen removal via the SND process. Denitrification process will be inhibited due to the lack of carbon source especially when treating wastewater with low COD/N ratio. In some studies, biodegradable carrier materials were used not only to act as the biofilm carriers but also as an external carbon source for the denitrifiers (Walters et al., 2009, Chu and Wang, 2011). However, replacement of used biodegradable materials is required and therefore this leads to high operating cost. In the study of Guo et al. (2010), different sizes of inert PU foam were used as the carrier material and SND was observed in the anoxic zone of the PU foam. The occurrence of the SND without the addition of the external carbon source can be explained by the possibility of carbon storage in the deeper biofilm layers which was reported by several researchers (Morgenroth and Wilderer, 1999, Pastorelli et al., 1999, Gieseke et al., 2002). The carbon storage could be used for denitrification during the SND process.
Based on the results of previous studies, PU foam was found to be a good carrier material in MBSBR system. Its porous structure would allow the formation of attached-growth biomass, the establishment of anoxic zone and the storage of carbon which enhance the nitrogen removal via SND. To date, relatively little was reported on the effect of different sizes of the PU foam on the nitrogen removal in the MBSBR system. Thus, the objectives of this study are to investigate (i) the nitrogen removal efficiency in MBSBRs with different sizes of PU foam cubes as the carrier materials, (ii) the biomass growth onto the surfaces and into the interior porous structure of the various sizes of PU foam cubes and (iii) the role of the PU foam in the SND process in treating low COD/N ratio wastewater.
Section snippets
Carrier materials
The PU foam was used as the carrier material for the attached-growth biomass. The PU foam was cut into cube form with different sizes of 8, 27, 64 and 125 mL, respectively. The characterization of the PU foam was carried out and the surface morphology of the PU foam was examined by a scanning electron microscope (Leo Supra 50 VP) at the magnification of 40.
To determine the total internal and external surface areas of the PU foam cube, the pores of the PU foam were assumed to be spherical in
Characteristics of PU foam
The density and tensile strength of the PU foam were determined and found to be 87.4 ± 5.3 kg/m3 and 93 ± 10 kPa, respectively. The average pore diameter was found to be 661 ± 53 μm and the pore count was 42 ± 7 per 25 mm. Using the assumptions that the pores were spherical and the exposed pores were hemispherical in shape, the total surface was estimated to be 177 ± 3 m2/kg.
Characteristics of MBSBRs
Table 1 shows the characteristics of MBSBRs using varying sizes of PU foam cubes as the carrier materials. The physical degradation of
Conclusions
The presence of PU foam cubes as carrier media in MBSBRs which created an anoxic microenvironment and provided trapped carbon substrate stimulated the SND process. The percentage of TN removal increased with decreasing sizes of the PU foam cubes. This was due to the increasing amount of attached-growth biomass as the cubes became smaller. Higher concentrations of 8-mL PU foam cubes in batch reactors yielded higher percentage of TN removal, with 84% TN removal at carrier packing volume of 40%.
Acknowledgement
Financial support from the Universiti Sains Malaysia under the Research University (RU) Grant No: 1001/PKIMIA/815057 is gratefully acknowledged.
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