Effect of COD/N Ratio on Growth and Adaptation of Nitrifying Bacteria

Acclimation of nitrifying bacteria using inorganic carbon source (bicarbonate) and organic carbon source (bicarbonate + glucose) was performed in this study. Sequential batch reactors were inoculated by activated sludge (mixed culture of heterotrophic and nitrifying bacteria), and fed by synthetic wastewater contain high concentration of ammonium (1000 mg·L). Adapted nitrifying bacteria, only using bicarbonate as carbon source, caused nitrification at a rate of 41 mg L− h−. The second reactor, which using bicarbonate and glucose as carbon source due to high C/N ratio resulting high growth of heterotrophic bacteria and result revealed a minor effect on nitrifying bacteria, and complete removal of ammonium occurred. Keywords: Wastewater, nitrification, acclimatization, heterotrophic bacteria 1. Introduction Inappropriate discharge of wastewater containing ammonia causes eutrophication in water bodies and toxicity to aquatic culture [1]. Biological nitrogen removal as a promising method for eliminating ammonia from wastewater generally occurs in two steps, namely, nitrification and denitrification[2]. Nitrification is known as a controlling step that consists of two consecutive familiar reactions, nitritation (Eq. (1)) and nitratation (Eq. (2)), which are carried out by autotrophic bacteria in two main groups — ammonia-oxidizing bacteria (AOB) and nitriteoxidizing bacteria (NOB), respectively [3]. Mousavi, Ibrahim/American Journal of Oil and Chemical Technologies 6 (2014) 189-­‐196 190 First step: NH4 + +1.5O2→ NO2 − +2H +H2O (1) Second step: NO2 − +0.5O2→ NO3 (2) The efficiency and sustainability of nitrification processes depend on the activities of nitrifying bacteria, and the rate of nitrification is affected by many factors, such as nitrogen concentration, organic carbon, and microorganism population, including the ratio of heterotrophs to autotrophs, sludge age, and environmental conditions (e.g., temperature and pH) [4]. Among different controlling factors, the substrate concentration (carbon/nitrogen, C/N) determines the heterotroph/autotroph population ratio in such wastewater treatment systems as single reactor activated sludge, SBR, and biofilm systems. When the concentration of organic carbon is high (high ratio of C/N), heterotrophic bacteria dominate the denitrifying bacteria, thereby reducing the nitrification rate [5]. Therefore, researchers have attempted to determine the C/N ratio, which acts as a limiting factor in the nitrification process. As a result, full nitrification of sewage treatment plants occurs at a loading rate lower than 0.15 g-COD g-VSS L− d− [6]. Dinçer & Kargi (2001) reported that a C/N ratio less than 0.25 is essential to carry out effective nitrification. Furthermore, Okabe et al.(1996) found that a C/N ratio of 1.5 has unfavorable effects on the performance of the nitrification process in a biofilm system [7]. Wu. G. et al. (2008) investigated the effect of different C/N ratios (1.5, 0.7, and 0.4) on the enrichment of nitrifying bacteria in a nitrification reactor as well as in a nitrification and organic removal reactor. Their results demonstrated potential enrichment in both reactors but that the specific nitrification rate in the nitrification reactor was higher than that in the other reactor. Researchers have recommended using two biological units, with the nitrification process occurring separately in the second unit, to overcome the above-described shortcomings [6, 8]. However, although a separate unit for nitrification may enhance the potential of nitrification, it increases the initial capital and maintenance cost of the entire procedure. Other researchers have thus focused on the development of energy-saving nitrogen removal systems and increasing the nitrification rate by applying cost-effective processes in the treatment of reject water [9], such as SHARON [10], Anammox, and a combination of the two [11]. Partial nitrification (PN) occurs via AOB according to Eq. (1) limit the growth of NOB and enrich AOB, which cause nitrite accumulation [2, 9, 12]. Savings of 25% in aeration costs using low concentrations of dissolved oxygen only to enrich AOB and a reduction of 40% of the external carbon source needed during denitrification by limiting NOB in PN have been reported [13, 14]. This study evaluated the role of the C/N ratio as a controlling factor in the nitrification process. In addition, the possibility of partial nitrification as a cost-effective process was investigated during enrichment of nitrifying bacteria with a high concentration of ammonium. Mousavi, Ibrahim/American Journal of Oil and Chemical Technologies 6 (2014) 189-­‐196 191 2. Materials and methods 2.1. Experimental set-up and feeding Tow laboratory-scale sequencing batch reactors (SBRs) with a working volume of 5 L and sequencing stages of 23 h reaction, 50 min settling, 5 min decanting, and 5 min filling have been used in this study (Figure1). The operating conditions in this research are summarized in Table 1. 1. Controller 2. DO probe 3. pH probe 4. Exhaust of air 5. Mixer 6Sam 6. Sampling and feeding pipe Figure 1. Schematic diagram of the experimental apparatus Activated sludge from an wastewater treatment plant (WWTP) in Pantai Dalam, Kuala Lumpur, Malaysia with an initial mixed-liquor suspended solid (MLSS) concentration of 2 g·L was inoculated in two sequencing batch reactors. The reactors were fed with synthetic wastewater consist of different C/N ratio with 1000 mg·L of (NH4)2SO4 as a source of nitrogen; 200 mg·L KH2PO4; 3000 mg·L NaHCO3 and 1 ml/L trace elements: MgSO4 (60mg/L), EDTA (10mg/L), ZnSO4.7H2O (2.2mg/L), CoCL2.6H2O (3.2mg/L), MnCl2.4H2O (10.2mg/L), CuSO4.5H2O (0.22mg/L), (NH4)6Mo7O24.4H2O (2.2mg/l), CaCl2.2H2O (1.1mg/L), FeSO4.7H2O (10mg/L), H3BO3 (0.3mg/L), NiSO4.6H2O (1mg/L). O2 .... mg/L pH ..... ORP ..... mv 2 3 1