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Journal of Fisheries science and Technology Special issue - 2015 WASTEWATER TREATMENT IN THE HYBRID BIOLOGICAL REACTOR: EFFEECTS OF HYDRAULIC RETENTION TIME AND OXYGEN SUPPLY ON TOTAL NITROGEN REMOVAL AND NITRIFICATION Nguyen Thị Ngoc Thanh1, Pham Khac Lieu2 ABSTRACT A hybrid biological reactor (HBR) which contained both suspended- and attached-growth biomass was developed by introducing sponge materials into a regular activated sludge unit and used for the treatment of seafood processing wastewater. The HBR was operated over 170 days, to assess the effect of hydraulic retention time (HRT) and oxygen supply conditions on total nitrogen (TN) removal efficiency. The HBR was supplied with oxygen at three different levels: 1, 1.5 and 2 L/min, and HRT was maintained for 8 h. Results indicate oxygen levels of 1 and 1.5 L/min, had higher efficiencies (60% of total nitrogen was removed) than an oxygen level of 2 L/min (48%). The HRT was changed at 8, 10 and 12 h and the rate of oxygen supply was maintained at 1.5 L/min. Each run was operated for at least 15 days. In general, the percentage of TN removed increased linearly with increasing HRT. More than 65% TN could be removed up to a HRT of about 10 - 12 h, which indicates that this hybrid system was highly effective. In this experiment, the nitrification rates were very high, more than 90%, and the best result of nitrification was (94.9%) at HRT = 12 h, and an oxygen supply of 1.5 L/min. Keywords: Hybrid biological reactor, Wastewater treatment, Activated sludge process, Nitrification I. INTRODUCTION Wastewater treatment via aerobic processes has some problems that remain unsolved for the conventional activated-sludge process, such as a large space requirement, complicated system operation and large excess sludge production [6-7]. The expense for excess sludge treatment has been estimated to be half of the total cost of wastewater treatment plant operations [11]. A hybrid reactor consists of both suspended and attached (biofilm) growth in a single reactor [3-4]. As such, the level of its total biomass can be maintained at much higher than 3000 mg/L without encountering subsequent sludge settling problems [3-4-7-10]. This greatly increases the treatment capacity of a biological system. In addition, the sludge age of the attached growth is much longer than that of the suspended growth. This allows nitrifiers to be maintained in the biofilm even if the suspended growth is operated at 1 to 2-day short sludge age. The biomass of shorter sludge age (1 to 2 days) has a much higher bioactivity as compared to that of an older sludge age (as in conventional activated sludge system) [10]. Thus, a hybrid reactor can significantly increase the treatment capacity. Hybrid systems, which combine suspended and attached growth have been widely applied Institute for Biotechnology and Environment, Nha Trang University, Nha Trang, Vietnam, nguyenngocthanh.84@gmail.com 2 Department of Environmental Science, Hue University of Sciences, Hue, Vietnam 1 NHA TRANG UNIVERSITY • 93 Journal of Fisheries science and Technology to wastewater treatment. The contribution of biofilm and suspended sludge in a hybrid sequencing batch reactor for N transformation and N2O emission for a simultaneous nitrification, denitrification and phosphorus removal process were investigated by Ingwei (2010), it shown that for the hybrid system, nitrification occurred mostly in the suspended sludge, and the biofilm played the major role in denitrification. The interaction of the biofilm and the suspended sludge in the same reactor resulted in a better overall nitrogen removal performance with simultaneous nitrification and denitrification. N2O emission was the main end product of nitrogen removal for the hybrid system and it was N2 for the biofilm [4]. A shortcut biological nitrogen removal (SBNR) process converts ammonium directly through nitrite to nitrogen gas, thus requiring less aeration and carbon [5]. According to Jinwook (2007), a hybrid SBNR (HSBNR) reactor is considered consisting of an anoxic tank followed by an aerobic tank and a settling tank. The aerobic tank was filled with polyvinyl alcohol sponge media (20%, etc) to attach and retain ammonium oxidizers. Two configurations of the HSBNR reactor were tested for treating a wastewater with high strength ammonium and organic electron donor [5]. The HSNBR reactors accumulated nitrite stably for 1.5 years and maintained a high free ammonia (FA) concentration (20–25 mg/L) and a low dissolved oxygen (DO) concentration (<1 mg/L) in the aerobic tank. Apparently, the biofilm carriers increased the solids retention time (SRT) for ammonium oxidizers, while high FA and low DO selected against nitrite oxidizers and promoted direct denitrification of nitrite in the aerobic tank. The significant amount of chemical oxygen demand (COD) was removed by shortcut denitrification of nitrite in the anoxic tank [5]. 94 • NHA TRANG UNIVERSITY Special issue - 2015 Refer to another new hybrid biological reactor (HBR) using mineral powders as a bio-carrier was developed, and compared to conventional activated sludge (AS) with increasing substrate organic loading rate (OLR) from 1.0 to 4.0 kg COD/m3 per day [3]. The HBRs employed different powders of bentonite and clinoptilolite, respectively, and its concentration in an aeration basin was sustained at 500 and 4000 mg/l, separately [3]. Biomass concentrations in the HBRs were greatly increased by biofilm formation attached on the powder, which were linearly augmented with OLR [3]. The HBRs operated under 4000 mg/l of minerals and highly improved removal efficiencies of organic compounds as well as organic removal rates, although no significant advancement was found in the HBRs run under 500 mg/l, compared to conventional AS. This was probably attributed to high biomass concentration (the maximum value of 7100 mg MLVSS/l) by biofilms formed on the minerals. The efficiencies of total Kjeldahl nitrogen (TKN) removal and nitrification were decreased with increasing substrate COD to TKN (C/N) ratio. However, the HBR with 4000 mg/l of clinoptilolite concentration showed higher nitrification efficiency rather than that in other reactors, little depending on substrate C/N ratio varied. It could be concluded that high ammonium exchange capacity of the clinoptilolite enabled nitrfiers to colonize favorably in the biofilms on this mineral, which increased nitrification [3]. II. MATERIALS AND METHODS A hybrid biological reactor (HBR) was developed and involved the introduction of a new phase of attached-biomass into a regular suspended-growth system (activated sludge process) by addition of carriers to the aeration tanks. Journal of Fisheries science and Technology In this hybrid biological reactor system, the activated sludge process was combined with a fixed film biomass process. The attached-biomass was developed on porous carriers. The porous Polyvinyl Alcohol forms (obtained by cutting randomly like tiny cube pomegranate seed-shaped) had a size of 5–10 mm and floated under unloaded conditions. The quantity of the foam carriers varied was 15% (v/v) of the tank volume [5-7] (17.2 L * 15/100 = 2.58 L of the carriers). The semi-suspended aerobic experimental system consisting of reaction tank (it is supplemented by materials to change into agglutinate semi-suspended operation mode), accumulation tank, quantitative pumps (inlet wastewater and returned sludge), compressed air, air measured meters, input and output wastewater barrels. The end section of reaction tank was placed by protection grid in order to stop material particle going to accumulation tank, so only suspended sludge and water pass through. The experimental scheme used in this study is shown in Fig. 1. Reaction tank was designed by mica, totally 3 valves, including 1 valve is for wastewater inlet, another valve is for returned Special issue - 2015 sludge flow at the bottom of tank, and 1 valve for wastewater outlet on the top. The tank volume is 17.2L. At the outlet valve of reaction tank is placed protection grid to stop material particle passing through. Accumulation tank was also made by mica with the volume of 8,5L, having of one central pipe in the middle with dimension (3cm x 3cm x 24cm). Moreover, in accumulation tank was additionally placed by sinking stirring system at the bottom of tank in order to stir sludge not to form biomass, and help the returned sludge flow is maintained. The stirring blade has semicircle, the length is 7cm, the stirring speed is 12 circles/minute. The experimental technological parameters are summarized in Table 1. The specifications of the foam cubes used in this study are shown in Table 2. The experiment can be divided into five stages. The hydraulic retention times (HRT) were changed at 8, 10 and 12 h (by changes of pumping speeds of wastewater inlet), the oxygen supplies were at 1 mg/L, 1.5 mg/L and 2 mg/L (the speed of air supply is changed and controlled by adjusting of air flow meters). The system is operated continuously, each stage is run during about 15 days. Fig. 1. Experiment scheme for the hybrid biological reactor NHA TRANG UNIVERSITY • 95 Journal of Fisheries science and Technology Special issue - 2015 Table 1. Technological parameters of experimental systems Parameter Unit Value Volume of aerobic tank 1 17.2L Volume of clarifier 1 8.5L Recirculation ratio (Qreturned sludge = Qwastewater inlet) - 1.0 Temperature 0 C 22-30 Volumetric portion of porous carriers % 15-30 Table 2. Characteristic parameters of porous carriers Item Material Density (g/cm3) Size (mm) Porosity Value PolyvinylAlcohol-PVA 0.0847 5-10 High The aerobic sludge and wastewater used in this experiment were obtained from a wastewater treatment plant at the F17 seafood company, Khanh Hoa, Viet Nam. The most general properties of the wastewater are listed in Table 3. Table 3. Characteristics of wastewater Samples were taken at regular intervals Parameter Value (2-3 days/time) from the influent and effluent of pH Suspended solids, mg/L BOD5, mg/L COD, mg/L TN, mg/L NH4-N, mg/L NO2-N, mg/L NO3-N, mg/L Temperature (0C) 6.4 – 7.4 410 - 583 1249 - 2803 1953 - 4623 128 – 476 19.5 – 220.0 0.42 – 1.18 1.61 – 5.46 18-30 each stage for analyses. The chemical oxygen demand (COD), ammonium nitrogen (NH4+), total nitrogen (TN), mixed liquor suspended solids (MLSS) were determined according to APHA standard methods [1]. A steady stage was assumed when these parameters did not change considerably for several days. Table 4 shows the different stages and parameters at hybrid biological reactor system Table 4. The different stages and parameters at HBR system Stages IV Operating day 8 6 103 1 0 3 117 11 8 131 1 3 2 145 1 4 6 159 V 1 6 0 173 Ph 7.2 7.1 7.0 7.1 7.3 7.2 Organic loading rate (kgCOD/m3/d) 3.063 2.754 2.775 2.141 1.758 4.566 Influent COD (mg/L) 1021 918 925 892 881 2283 Influent TN (mg/L) 115 106 112 116 120 294 HRT (h) 8 8 8 10 12 12 Oxygen supply (L/min) 1 1.5 2 1.5 1.5 1.5 Flow rate (L/h) 2.15 2.15 2.15 1.72 1.43 1.43 Recirculation ratio (%) 100 100 100 100 100 100 III. RESULTS AND DISCUSSION 1. Effects of hydraulic retention time on TN removal According to the theory and practice of earlier studies, the treatment ability of total nitrogen in the 96 • NHA TRANG UNIVERSITY Journal of Fisheries science and Technology HBR system was higher than activated sludge process (ASP) [3-7]. There are 3 ways to remove nitrogen [6]: - Nitrification and denitrification processes were taken via biofilms and activated sludge; Special issue - 2015 - Nitrogen was removed via biomass; - NH4-N was evaporated (in the form of NH3) by prolonged aeration. The TN removal efficiency observed at different HRT is illustrated in Fig. 2 and Fig. 3. Fig 2. Changes in the concentration and efficiency of the HBR and ASP systems Fig 3. Changes in the concentration and efficiency of the HBR system at different HRTs From the graphs, the TN removal efficiency wasn’t high in the early days (at the first five-day), this may be because the bacteria needed longer time to adapt to the conditions and develop the cycle of nitrification. After that, efficiency was higher and more stable. More than 60% TN could be removed via HBR system, which was higher than the ASP system (16%) (Fig 2). The high attachment of biomass captured in this hybrid reactor, via the immobilization of microorganisms on the carrier particles, contributed to system efficiency. TN removal efficiency increased with increasing HRT. With HRT = 8 h, 60.2% of TN was removed, at HRT = 10 h and 12 h, removal reached 66.1% and 65.2%, respectively (Fig 3). These results indicate that high concentrations of nitrifying biomass can be maintained by biofilm formation, which was attached to the carrier particles in the HBR. TN removal efficiency increases with increasing HRT may be due to anoxic conditions. Thus, the process of nitrogen removal by nitrification and denitrification in the same reactor has been formed. The anoxic conditions were made by reduction in flow rate, reducing disturbance in tank, and storing wastewater in tanks for longer time periods to develop a biofilm. The value of pH increases during operational time and with longer HRT, pH increased greatly. NHA TRANG UNIVERSITY • 97 Journal of Fisheries science and Technology 2. Effects of oxygen supply on TN removal TN removal efficiencies of oxygen supply rates of 1 and 1.5 L /min were equivalent and high, at nearly 60% (Fig 4). At the oxygen supply level of 2 L/min, TN removal efficiency dropped, reaching 48.5%. The decreasing level of oxygen supply would create anoxic conditions affecting the function of nitrifying Special issue - 2015 of the HBR system was high and stable. At the different HRTs and oxygen supply rates, more than 50% TN could be removed, and the highest efficiency of 66% was obtained at HRT=10 h, while ASP system removed only 40% of TN removal. Therefore, this technology is suitable for type of wastewater with high concentration of nitrogen and organic matters. bacteria, especially attached bacteria. During processing, pH was increased and the highest increase was 0.56 units at an oxygen concentration of 1.5 L/min. This also means to occur denitrification, and denitrification process will increase alkaline environment and as a result, pH in water will be increased after treatment, TN removal efficiencies thereby considerably increased. The lowest increasing pH was in oxygen supply condition of 2 L/min and at this condition, TN removal was also reached the lowest level when compared with different oxygen supply conditions were surveyed. In general, the percentage of TN removed decreased linearly with increased oxygen supply rates. It can be seen that the TN removal ability Fig 4. Changes in the concentration and efficiency of the HBR system at different oxygen supply rates Nitrification efficiency achieved is also very high, more than 90% and the highest level at HRT=12 h, aeration rate = 1.5 L/min (94.9%). The assumption was mainly NH4-N was being nitrified. Nitrification efficiencies at the different operating conditions are presented in Fig 5. Fig 5. The nitrification efficiency in different time and operating conditions Efficiencies of TN elimination and nitrification increased linearly with decreasing HRT in all operating conditions, but acclimation periods for stable nitrification efficiency were differently monitored in each condition. During operation of the HBR system, the value of pH increased, but depended on different HRT and oxygen conditions (Fig 6). 98 • NHA TRANG UNIVERSITY This is quite reasonable because the HBR system was a combination of suspended and attached bacteria. In addition, suspended sludge led to aerobic conditions, while biofilm created anaerobic conditions. Therefore, in the reactor, there were simultaneous nitrification and denitrification. The changes of pH are quite consistent with the efficiencies Journal of Fisheries science and Technology of TN, at the operating conditions with pH increased highly, the denitrafication occurs strongly so that the efficiency of TN removal will increase considerably. Special issue - 2015 hybrid system was highly effective of TN removal. The average concentration of sludge in the HBR tank reached the 5800 mg/L. In comparison, the ASP system achieves only about 3000 mg/L [6-8]. The efficiency of TN was enhanced by increasing HRT. At a HRT of 8 h, TN removal efficiency was 60.2% and at HRT of 10 h and 12 h, TN removal efficiency reached at 66.1% and 65.2%, respectively. At the oxygen supply level of 1 and 1.5 L/min, TN removal was quite high (i.e., 60%), while increasing aeration (2 L/min) resulted in Fig 6. The values of pH at different conditions IV. CONCLUSION By addition of carriers to the aeration tanks, a HBR was created. Results indicate that this decreased efficiency (48%). The efficiency of nitrification was over 90%, during the operating model, with the highest efficiency achieved 94.9%, during conditions of HRT of 12h and oxygen supply level of 1.5 L/min. REFERENCES 1. APHA, AWWA, WPCF., 1999. Standard methods for the Examination of Water and Wastewater, 20th Edition, Washington, D.C. 2. Bitton Gabriel., 2005. Wastewater Microbiology, Third Edition, John Wiley & Sons Inc., USA. 3. Hyung S.L., Se J.P., Tai I.Y., 2002. “Wastewater treatment in a hybrid biological reactor using powdered minerals: effects of organic loading rates on COD removal and nitrification”, Process Biochemistry, 38, pp.81-88. 4. Ingwei W.L., Kwang V.L., Don S.M., Dean S., William R., 2010. “Contributions of biofilm and suspended sludge to nitrogen transformation and nitrous oxide emission in hybrid sequencing batch system”, Journal of Environmental Sciences, 22 (7), pp. 953–960. 5. Jinwook C., Wookeun B., Yong W.L., Bruce E.R., 2007. “Shortcut biological nitrogen removal in hybrid biofilm/suspended growth reactors”, Process Biochemistry, 42, pp. 320–328. 6. Metcaft & Eddy, Inc., 2003. Wastewater Engineering treatment and reuse, 4th Edition, Mc Graw Hill. 7. Wang J., Shi H., Qian Y., 2000. “Wastewater treatment in a hybrid biological reactor (HBR): effect of organic loading rates”, Process Biochemistry, 36, pp. 297-303. 8. WES (Water Environment Association)., 1987. Activated Sludge, Manual of Practice No.9. 9. WHO., 1993. Assessment of Sources of Air, Water and land pollution, Part 1, Geneva. 10. Yingjun C., Yusuke W., Sen Q., Toichiro K., Kenji F., 2006. “Comparison of treatment capacities of swim-bed and activated sluge processes for domestic wastewater”, Journal of Water treatment Biology, 42 (3), pp. 129-137. 11. Yingjun Cheng., 2006. “Advanced wastewater treatment using acyle-resin fibber biomass carrier”, PhD thesis, Graduate School of Science and Technology, Kumamoto University, Japan NHA TRANG UNIVERSITY • 99