Proceedings of the UNESCO - University of Tsukuba International Seminar on Traditional Technology for Environmental Conservation and Sustainable Development in the Asian-Pacific Region, held in Tsukuba Science City, Japan, 11-14 December, 1995.
Editors: Kozo Ishizuka, D. Sc. , Shigeru Hisajima, D. Sc. , Darryl R.J. Macer, Ph.D.
Tetsuya Yamamoto, Hiroshi Tsubota, Osafumi Ito, Elvira G. Suto, University of Tsukuba,
Japan;
Kazuhiro Shinabe, Kubota Co., Ltd., Japan;
Ryuji Uno, Kimiaki Yasuda, Biomaterial Co., Ltd., Japan
Key words : nitrification, denitrification, porous carrier, immobilization, hydrodynamic jet flow, effectiveness factor
Elimination of eutrophication requires the development of a compact but highly efficient system of nitrification and denitrification. In the biological nitrogen removal system, the nitrification process generally becomes the limiting step because of the extremely low growth rate of obligate autotrophic nitrifying bacteria. Previous works conducted to improve nitrogen removal rates include the use of immobilized cells accomplished by entrapment. However, in these cases, most of the cells existed only in the narrow region near the surface of the gel beads due to the limited oxygen transfer rate inside them. Consequently, this resulted in the poor volume efficiency of the gel volume. When the entrapment method was applied to immobilization of denitrifying bacteria, floatation of gel beads caused by the evolution of nitrogen gas resulted. The entrapment method also requires specialized equipment and sufficient time to prepare an adequate number of gel beads for wastewater treatment.
The purpose of this research work is to establish a practical system of nitrification and denitrification which is suitable for small scale point sources of pollution. In this study the effectiveness of a positively-charged macro-porous carrier in the immobilization of nitrifying and denitrifying bacteria by adsorption was investigated. This carrier, "AQUACEL", is made of foamed cellulose with continuous macro-pores. This structure is intended to allow convection of the liquid inside the carrier resulting in oxygen transfer rates increase. The increase in oxygen transfer rate leads to high cell concentration inside the carrier. More stable adhesion can be attained as a result of electrostatic forces. To counter the problem of carrier floatation in the denitrification process, a new reactor equipped with a hydrodynamic jet flow drive was developed. This new reactor distributes the floating carriers in the reactor homogeneously without any mechanical damage to the carriers. The performance of this novel denitrification system was evaluated by using an actual wastewater discharged from an electroplating factory.
The surface of the AQUACEL carrier was treated with polyethyleneimine (PEI) to provide an ion exchange capacity (IEC) of 0.8-1.3 meq/g stabilized by crosslinking. The effective surface area is high (3-7 m2/g by BET adsorption isotherm method), a void fraction of 97%, and an apparent density of about 1.05 kg-wet/dm3. The optimum structure of AQUACEL carrier in cubic form was determined by changing the particle and pore sizes to about 1-5 mm and 100-1260 m, respectively. Cubic PVF (polyvinylformal) carriers with homogenous pore sizes ranging from 60 to 350 m and a particle size of 3 mm were also employed to study the possibility of enhancement of oxygen transfer into the porous carriers by convection.
Reactors for nitrification and denitrification
Two different reactors were used for denitrification. Fig. 2 shows the reactor with a hydrodynamic jet flow drive. The floating carriers were suctioned into a pipe set at the center of the reactor by downward liquid flow which was induced by driving jet flow through an ejector. The suctioned carriers were transferred to the bottom part of the reactor, and then discharged tangentially against the vessel wall. This tangential flow caused rotation of the liquid in the reactor, which enhanced homogenous distribution of carriers. The working volume of this reactor was 12.5 dm3, with a 20% volume fraction of the carriers. This laboratory scale reactor was mainly used for basic research works to determine the optimum operating conditions for denitrification. The other reactor with a working volume of 500 dm3 employed for denitrification of actual wastewater discharged from an electroplating company was a mixing tank equipped with an elbow at the end of the downflow tube (Fig. 3). The floating carriers in this reactor flew into the downflow tube together with the downward liquid flow induced by centrifugal force of the elbow pipe.
Fig. 3. Schematic diagram of mixing tank reactor with downflow tube.
The basic composition of the synthesized feed medium for nitrification and denitrification are shown in Table 1. Sodium bicarbonate at a concentration of 937.2 mg/l is equivalent to an alkalinity of 534 CaCO3 mg/l and the ratio of alkalinity to ammonium nitrogen (alkalinity/N ratio) was fixed at 14.0 throughout this research work. When the concentration of ammonium ion in feed medium was increased, the concentration of NaHCO3 was also increased according to this ratio to avoid alkali deficiency. Denitrification requires several trace elements and a carbon source. In this work, four different carbon sources, namely, acetate, ethanol, methanol and isopropanol were used as electron donor for the denitrification. The concentrations of these carbon sources were fixed at 1.5 times the theoretical requirement calculated from the stoichiometric relationship between reduction of nitrate and the hydrolysis of these carbon sources. To avoid contamination of the feed medium inside the storage tank, the carbon source and the salt solution were separately supplied. These solutions were fed into the reactor at the same flow rate.
Fig. 4. Experimental set-up for measuring effectiveness factor of immobilized cells.
Determination of effectiveness factor
The experimental set-up for the measurement of the effectiveness factor for the immobilized cells is shown in Fig.4. The air-saturated medium for nitrification was continuously fed from the medium storage tank (T) into the vessel (V) through the peristaltic pump (P). The carriers immobilizing cells and the medium in the vessel were stirred by a magnetic stirrer, and the DO concentration in the vessel was measured with a galvanic DO electrode (E). The oxygen uptake rate of the immobilized can be expressed as:
where VL= volume of the medium in the vessel, CL=DO concentration in the vessel, CL*=DO concentration in the inlet medium, F=flow rate of the medium, R=distance from the center of carrier to the surface, n=number of the carriers, De=effective oxygen diffusion coefficient in the carrier, CA=local DO concentration inside the carrier, and r=distance from the center of the carrier.
(NOTE EQUATIONS ARE NOT ON-LINE HERE)
At a steady state, VL(dCL/dt)=0, therefore
The effectiveness factor of the immobilized cells represents the ratio of oxygen uptake rate by the immobilized cells to that obtainable when there is no intraparticle resistance. This is represented as:
where CAS=DO concentration at the surface of the carriers, Vm=maximum oxygen uptake rate, and Km=Michaelis constant. From Eqs. 2 and 3, the effectiveness factor of the immobilized cells can be rewritten as:
where CL=CAS since the partition coefficient of oxygen was assumed to be 1.0. The effectiveness factor of the immobilized cells was thus calculated from
Analyical methods
A long term nitrification experiment with the AQUACEL carrier (particle size 3 mm, pore size 500 m, IEC 1.13 meq/g) which lasted for more than eighteen months was successfully conducted without any degradation of the carriers. The hydraulic retention time (HRT: 2.5-10 h), alkalinity (CaCO3 130-1340 g/dm3), dissolved oxygen (DO: 1.3 - 7.5 g/m3), and NH4-N influent concentration (40-60 g/m3) were varied. Figure 5 shows the time course of NH4-N concentration in the early phase after the start-up of continuous nitrification. Twenty days after the start-up, the inlet ammonium ions of 50 g/m3 was completely oxidized under a HRT of 5 h, and the ammonium oxidation rate attained the maximum value of 14 kg-N/m3-carrier within 60 days. After confirming the maximum ammonium oxidation rate, the alkalinity and DO were varied to investigate their effects on ammonium oxidation rate.
Comparing the results obtained with cells entrapped in polyethylene glycol cubes (PEG), the effect of DO on ammonium oxidation rate is shown in Fig. 6. In both cases, ammonium oxidation rate was obviously influenced by DO. It increased with increasing DO until it reaches a point where it levels off in spite of DO supply. The maximum ammonium oxidation rate of AQUACEL immobilized cells was more than twice that of cells entrapped in polyethylene cubes (PEG).
Temp. : 30 C
Effect of particle size on maximum ammonium loading rate
The maximum ammonium loading rates which allow the complete ammonium oxidation were measured during continuous nitrification using AQUACEL carriers with a pore size of 100 m and particle sizes of 1, 3, and 5 mm. The continuous operation was started with a NH4-N loading rate of 1 kg-N/m3-carrier/d and was gradually increased up to 25 kg-N/m3-carrier/d by increasing NH4-N influent concentrations up to 170 g/m3 under a constant HRT of 3 h. Loading rates lower than 5 kg-N/m3-carrier/d resulted in no differences in ammonium oxidation rate (Fig. 7). Loading rates higher than 8 kg-N/m3-carrier/d resulted in residual ammonium ion and accumulation of nitrite ion using carriers with particle sizes of 3 and 5 mm. Complete ammonium removal was attained in the 1 mm carrier even at a high loading rate of 12 kg-N/m3-carrier/d. At completion of the experiment, the number of cells was determined using the most probable number (MPN) method. The differences in the concentration of nitrifiers among the carriers used are shown in Table 2. The low cell concentration of nitrite oxidizers in the 3 and 5 mm carriers must be caused by the depletion of oxygen inside the carriers and the growth inhibition caused by high residual ammonia and accumulated nitrite.
Temperature 25 C
Relative alkalinity 14.4 kg-CaCO3 /kg-NH4-N
Carrier pore size 100 m
Fig. 7. Effect of carrier size on ammonium oxidation rate.
Table 2. Cell number of nitrifiers in the carriers.
Using the PVF carriers operated under an ammonium loading rate of 3 kg-N/m3-carrier/d, the effectiveness factors were measured at different rotating speed of magnetic stirrer under a constant DO of 3 mg/l (Fig. 9). More obvious influence of mixing intensity was observed in the effectiveness factor of the carrier with pore size of 360 m. This means that the oxygen transfer by convection may be expected for the carriers with pore size larger than 360 m.
Fig.9. Effect of agitation speed on effectiveness factor.
Fig.10. Effect of nitrate loading rate on
Fig.11. Time course of NO3-N and NO2-N concentration during the