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Application of microwave radiation to biofilm heating during wastewater treatment in trickling filters
Bioresource Technology 127 (2013) 223–230
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Application of microwave radiation to biofilm heating during wastewater treatment in trickling filters Marcin Zielin´ski a, Magdalena Zielin´ska b,⇑, Marcin De˛bowski a a b
Department of Environmental Engineering, University of Warmia and Mazury in Olsztyn, Warszawska 117, 10-709 Olsztyn, Poland Department of Environmental Biotechnology, University of Warmia and Mazury in Olsztyn, Słoneczna 45G, 10-709 Olsztyn, Poland
h i g h l i g h t s " Microwaves (MW) and convection (CH) were used to heat biofilm. " Efficiency of pollutant removal from wastewater by biofilm was tested in 20–40 °C. " Continuous MW dosage resulted in lower biomass yield than intermittent MW or CH. " Continuous MW gave 10% higher nitrification efficiency than periodic MW or CH. " Selective heating by MW makes the system less energy-consuming than CH.
a r t i c l e
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Article history: Received 25 July 2012 Received in revised form 22 September 2012 Accepted 26 September 2012 Available online 5 October 2012 Keywords: Microwave heating Trickling filters Nitrogen removal
a b s t r a c t The purpose of this study was to demonstrate the potential for improving wastewater treatment by the application of microwave radiation (MW) compared to convective heating (CH) of trickling filters. Microwaves were delivered to the biofilm in a continuous and intermittent way to obtain temperatures of 20, 25, 35 and 40 °C. Although there was no effect of MW on organic removal, the observed yield coefficient was lower during the continuous MW supply compared to the periodic dosage and CH. The presence of organic compounds in the influent and continuous biofilm exposure to MW resulted in ca. 10% higher efficiency and ca. 20% higher rate of nitrification compared to intermittent MW dosage and CH. Independent of the method of reactor heating, the absence of organic carbon in the influent induced a significant increase in ammonium oxidation efficiency at 20–35 °C. Despite the aerobic conditions in trickling filters, nitrogen loss was observed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The rate of biochemical degradation of pollutants is connected with the activity of microorganisms and particularly with the rate of enzymatic reactions which depend on environmental conditions, including temperature. An increase in temperature induces an acceleration of the biochemical reaction rates according to the van’t Hoff–Arrhenius equation; on the other hand, due to the protein nature of enzymes, an excessive temperature leads to denaturation and a cessation of the process. Most of the bacteria involved in wastewater treatment processes are mesophilic microorganisms. Ilies and Mavinic (2001) proved that autotrophic bacteria such as nitrifiers are more sensitive to temperature drops than heterotrophic bacteria. They stated that at 10 °C, nitrification effi⇑ Corresponding author. Tel.: +48 89 523 41 85; fax: +48 89 523 41 31. E-mail addresses: [email protected] (M. Zielin´ski), magdalena. [email protected] (M. Zielin´ska), [email protected] (M. De˛bowski). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.09.102
ciency can reach 10–30%. However, it is widely believed that only at above 15 °C does ammonia oxidation proceed effectively. Temperature affects the growth rate of both ammonia- (AOB) and nitrite-oxidizing bacteria (NOB), while AOB have superior growth rates at higher temperatures than NOB (Hellinga et al., 1998). Hence, ensuring the optimal temperature for nitrification is the key action to obtain an effective course of the cascade of reactions leading to nitrogen removal from wastewater. The precise control of thermal conditions inside microbial structures in bioreactors can be achieved by the application of microwaves (MW). Microwaves are part of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with a corresponding frequency from 300 MHz to 300 GHz. Due to the low energy of microwave radiation quanta, interaction with the substances occurs at the molecular level and is based on the effect between electromagnetic field and particles. The action of the electromagnetic field of a microwave frequency (leading to an increase in the temperature of some substances) is connected with its dielectric
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was investigated in order to improve nitrification efficiency. According to Remya and Lin (2011), the installation and operational costs of an MW system are high. However, based on literature data that prove the reduction of the time required for MW-supported treatment and high efficiency of reactions and on our earlier experience (Zielin´ski et al., 2007), it is assumed that the application of microwave radiation in technological systems of wastewater treatment can be advantageous, particularly in nitrification proceeding in a temperate climate. Due to the fact that nitrification reactions occur in the biofilm, not in the bulk liquid (Moreau et al., 1994) and the energy of microwaves is directly transferred into the biomass, the proposed solution allows for a cost reduction compared with convection heating. The aim of this research was to determine if the application of microwave heating had a thermal or non-thermal impact on the efficiency and rate of carbon and nitrogen transformations in trickling filters. 2. Methods 2.1. Characteristics of the trickling filters The biofilm developed on polyethylene granules with a shape similar to a sphere: the average, minimal and maximal diameters were 3.00, 2.88 and 3.12 mm, respectively. These particles were characterized by a sphericity of 0.98 and a porosity of 0.38. The biofilm-covered particles were placed in the bioreactors with active volumes of 0.5 L made of plastic material which allowed for penetration of radiation (Fig. 1). The specific surface area and active area of the bed equaled 202.0 m2/m3 and 0.101 m2, respectively. The bioreactors were operated in a batch mode. Once a day, raw wastewater was introduced into the retention tank with a volume of 0.5 L and was pumped from this tank into the down-flow bioreactor in an intermittent mode: 15 s of flow and 45 s of break. From the bioreactor, wastewater was returned to the retention tank. The ø 60 mm
220 mm
Influent
180 mm
features, i.e. its ability to conduct electricity. Dipole molecules vibrations and, to a lesser extent, migration of ions (only for solutions) have basic significance in an increase in temperature due to microwave radiation (Ponne and Bartels, 1995). As a result of microwave interaction, only the growth of the interior energy of substances containing polar molecules (e.g. water) occurs. Energy transport on a molecular level allows for selective heating (Perreux and Loupy, 2002). When using MW, for example, in reactors with an attached biomass, it is possible to apply energy directly into the biofilm (a substance absorbing radiation). Energy losses due to absorption by the particles of the reactor bed can be avoided since particles made of plastic are transparent to microwaves. In addition, turning off the radiation immediately stops the heating process (Remya and Lin, 2011). The way of heating by microwaves is called a ‘‘volumetric’’ process, which is characterized by a shorter time of reaction and a saving of energy (Gabriel et al., 1998) compared to convection heating which is realized due to convection and heat conduction from the surface of the material. The effects of microwaves on living organisms are divided into two groups: thermal (connected with temperature increase) and non-thermal (athermal), which describes the phenomenon induced by microwaves but not observed during convection heating (Banik et al., 2006). The thermal effect is related to the heat generated by the absorption of microwave energy by water and other polar molecules, both characterized by a permanent or induced polarization (Porcelli et al., 1997). Non-thermal effects describe the changes of the chemical, biochemical or physical behaviour of the system while the temperature remains unaltered. Parker et al. (1996) proved the positive athermal effects of microwave, heating lipase suspended in substrate to 50 °C using microwaves (2.45 GHz) and convection. They obtained a 2- to 3-fold higher rate of enzymatic reaction during microwave heating. The absorption of radiation by DNA is a significant problem involving athermal effects of microwaves on living organisms. It has been proven that DNA absorbs microwaves directly (Takashima et al., 2006) but the mechanism has not yet been recognized. Since the activation energy of microwaves equals ca. 10 5 eV, microwaves cannot disrupt hydrogen bonds (0.04–0.44 eV) or covalent bonds (5 eV) in DNA. Interactions between microwaves and molecules of both DNA and proteins can sometimes influence chemical bonds. Experiments conducted by Kakita et al. (1999) showed that microwave radiation was capable of extensive fragmentation of a DNA strand, whereas external heating at the same temperature left it intact. This could indicate the occurrence of an alternative mechanism of microwave interaction (athermal) leading to bond destructions in DNA. (Edwards et al., 1985) (cited by Ponne and Bartels, 1995) stated that radiation at the frequency of 1 GHz can limit DNA reproduction, whereas Gabriel et al. (1989) (cited by Ponne and Bartels, 1995) documented no evidence in the range 1–10 GHz, as did Bigio et al. (1993) (cited by Ponne and Bartels, 1995) in the range 5–20 GHz. It is considered that zones of highly increased temperature, so called ‘‘hot-spots’’, formed in cells as a result of microwave heating, can cause denaturation of proteins (Hill and Marchant, 1996). In our research, we analyzed the nitrogen removal from wastewater of a relatively low concentration of pollutants. Sewage from recirculating aquaculture systems could be an example of such wastewater. Ammonium is a major metabolic waste excreted by fish and originated from the fish feeding in aquaculture systems (Ali et al., 2005). Since ammonium is toxic to most fish species at relatively low concentrations, the nitrification process is the main objective in the operation of aquaculture systems. For this purpose, bioreactors with a long bacterial cell residence time are required, such as reactors with attached biomass (Chen et al., 2006), which have started to become key components in aquaculture engineering. In our research, microwave heating of the trickling filters
20 mm
224
Effluent
Fig. 1. Scheme of the biofilm reactor.
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40 30 1512 (1498)
20-fold pumping of the whole volume of wastewater from the tank into the bioreactor per day corresponded to a hydraulic loading rate of 0.15 m3/(m2 h).
25 20 1008 (989)
35 25 1260 (1189)
2.2. Organization of the experiment
S1
Continuous
20 15 756 (725)
Series
Dosage of microwave
Temperature (°C) Power of microwave (W) Consumption of electric energy (Wh/d)
Numbers in brackets indicate the values obtained for the convection heated reactors. a Time (in seconds) of microwave supplying/time (in seconds) of pause; microwave generator worked periodically with a constant power of 600 W.
With organic carbon Type of wastewater
25 20 1008 (989)
35 25 1260 (1189)
40 30 1512 (1498)
20 15 789 (725)
25 20 1129 (989)
35 29 1462 (1189)
40 55 2772 (1498)
20 15 756 (725)
Continuous 15/600a
Intermittent
S2
15/450a
15/300a
15/150a
S3
Without organic carbon
V2 V1 Variant
Table 1 Schematic diagram of the technological system.
225
The organization of the lab-scale technological system is shown in Table 1. The reactors were fed with the synthetic wastewater (prepared according to Coelho et al. (2000) after modifications) containing ammonium chloride, hydrogenphosphates, carbonates, bicarbonates, macro- and microelements. In variant 1 (V1), wastewater was supplied with sodium acetate, resulting in the COD concentration in the influent of 400 ± 10 mg/L and BOD5/COD = 0.9. In variant 2 (V2), raw wastewater did not contain organic compounds. In both variants, the ammonium concentration in the influent equaled 40 ± 2 mg/L. The bioreactors placed inside the thermostatic chambers were heated using microwave radiation (MW). Radiation generated by a magnetron (Plazmatronika, Poland) was transferred to the thermostatic chamber with the use of a waveguide. Because a magnetron uses resonant structures to generate an electromagnetic field, it is capable of generating a field of constant frequency. The microwave generator permitted smooth power regulation in the range from 0 to 600 W. The frequency of the generated radiation equaled 2.45 GHz. During 1 s the microwave radiation emitted by a magnetron caused a temperature increase of 1 L of distilled water by 0.1 °C. The specific heat of distilled water is 4186.8 J/(kg K), hence, a growth of temperature by 0.1 °C required 418.68 J of energy as microwave radiation converted to heat energy. The efficiency of the magnetron, calculated on this basis, equaled 52% and was consistent with the efficiency for this type of device as presented in the literature data (Thostenson and Chou, 1999). MW was supplied into the reactors in two ways. First, radiation was dosed continuously. Therefore, the changes in the frequency of the current introduced to the cathode of magnetron were done which resulted in a decrease in the intensity of the emitted microwaves. The power of the applied radiation was adjusted to obtain the desired temperature (Table 1). Second, radiation was delivered sequentially during 15 s exposures separated by a few-minute pauses, depending on the temperature required, leading to the limitation of microwave energy supplied. In this case, a magnetron was operated with a stable power of 600 W and alternating periods of radiation and non-radiation resulted in maintaining the expected temperature. Simultaneously, a microprocessor controlled the pump responsible for the wastewater flow into the reactors, cutting off the flow before starting the microwave generator. Parallel control technological systems were operated with the same temperature (but obtained due to convection). Such an experimental design allowed for a conclusive analysis to be made of the potential non-thermal effects of microwaves. Convection heating (CH) was realized with the use of thyristor resistors and air circulation was also enforced. The temperature inside the bioreactors was maintained at the levels of 20, 25, 35 and 40 °C. In both systems (MW and CH), four trickling filters were placed inside a thermostatic chambers made from metal to ensure safety work and to prevent the penetration of microwaves outside the system. To ensure constant ambient temperature, a temperature of 15 ± 1 °C was maintained inside both thermostatic chambers. Both experimental sets were equipped with the system of continuous multi-point temperature measurements. Simultaneously, the temperature was measured inside the reactors, inside the retention tank and in the chamber outside the reactors (ambient) with the use of a four channel HI 98801 thermometer (Hanna Instruments, UK). The results were collected every 30 s in the measuring device memory. A precise microprocessor system (Schneider Electric, France) for temperature measurement and
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adjustments was applied which allowed for hysteresis of 0.5 °C. Dissolved oxygen (DO) concentrations in the retention tanks were measured using an oxygen meter 2BA301 Oxi 3310 (WTW, Germany) with oxygen sensor CellOx 325. The measurements of total electric energy consumed were conducted separately for each system (MW and CH), using a power meter Energy Check 3000 (Voltcraft, Germany). In the MW system, a total consumption involved the heating and operation of the pumps, whereas in the CH system – power of electric heaters, a ventilator and the pumps. The consumption of electric energy is shown in Table 1.
dized nitrogen forms, kinetic research was conducted: 10 ml samples from the retention tank were collected every 2 h for 12 h, the last sample – after 24 h and analyzed with the colorimetric method by HACH for NH4+–N, NO2 –N and NO3 –N. The obtained data were tested for significance by one-way ANOVA, at the significance level of p < 0.05. The normality of the distribution was confirmed by Shapiro–Wilk’s test, whereas the hypothesis of the homogeneity of variances across the groups was verified on the basis of Levene’s test. The differences between the mean values derived from particular groups were examined by Tukey’s test. Statistica 8.0 PL (StatSoft) was used.
2.3. Analytical methods Raw wastewater and wastewater collected from the retention tank were assayed daily for determination of COD, NH4+–N, NO2 –N, NO3 –N, and TSS, according to APHA (1992). In order to calculate the rates of ammonium removal and the reduction of oxi-
3. Results and discussion In variant 1 (V1) of the current research, wastewater introduced into the trickling filters was characterized by the presence of eas-
Fig. 2. The average effluent concentrations of COD (a), ammonium (b), nitrate (c), nitrite (d) and TSS (e).
´ ski et al. / Bioresource Technology 127 (2013) 223–230 M. Zielin SERIES 1
Nhien (1995) investigated municipal wastewater treatment in immersed biofilm reactors at 30–55 °C and observed that an increase in turbidity at the highest temperature levels resulted from cell decay. In the current research, the lowest values of observed yield coefficient (Y) (0.21–0.34 g TSS/g COD) were obtained at 20 and 25 °C but the significantly smallest Y values were observed under the conditions of continuous MW dosage compared to intermittent MW dosage and CH (Table 2). The rise in temperature to 40 °C caused an increase in Y to over 0.4 g TSS/g COD, independent of the method of reactor heating. At this high temperature, the activity of heterotrophic bacteria was limited, whereas the nitrifier growth and activity was completely inhibited (Table 2, Fig. 4). This could have been a result of temperature-induced cell lysis. Partially biodegradable products of cell lysis can accumulate on biomass particles, increase the mass of cells in the reactor and result in higher observed yield (Lishman et al., 2000). Microwave radiation had much more effect on nitrogen reactions than on organic removal from wastewater. The conducted research showed the dependence between the method of biofilm heating and observed efficiency of nitrogen transformations. In both experimental variants, nitrogen loading rates were the same and equalled 0.198 g N/(m2 d). Ammonium concentration in the influent averaged 40 ± 2 mg/L. The concentrations of particular nitrogen forms in the effluents are given in Fig. 2. In variant 1, with easily degradable organic compounds in the influent, the efficiency of ammonium oxidation reached 87% in MW reactors and 82% in reactors heated by hot air, and was the highest at 25 °C (Fig. 4). In the temperature range from 20 to 35 °C, nitrification efficiency was higher by ca.10% in reactors exposed to continuous MW radiation in contrast to those with intermittent microwave heating, despite using more power in the latter case (Table 1). The continuous delivering of MW also resulted in ca. 10% higher effectiveness of ammonia oxidation than in the control series with convective heating, suggesting an athermal effect of MW on ammonia-oxidizing bacteria activity. There were, however, no significant differences in the process efficiency between control systems and the reactors heated by periodic dosage of microwaves. The athermal effects can be explained by the impact of microwave radiation on microbial communities in biofilm that was proved in our earlier experiments. In the bioreactors exposed to the radiation, total bacterial diversity (Zielin´ski et al., 2007) and the abundance of ammonia-oxidizing bacteria (Zielin´ski and Zielin´ska, 2010) were significantly higher as compared to the reactors heated by convection. It is known that the wastewater treatment systems with highly diversed bacterial consortia are more resistant to environmental stress due to their functional redundancy and alternative ways to use the flow of energy (Fernandez et al., 2000). For this reason, the operational parameters should provide the conditions for the development of bacterial communities characterized by high richness. In variant 2 (with a lack of organic carbon in the influent in the range 20–35 °C) a very high efficiency of ammonia oxidation was achieved (above 98%), independent of the method of biomass
SERIES 2
EC (%)
100 80 60 40 20 0 20
25
35
40
20
25
35
40 0
temperature ( C) MW
227
CH
Fig. 3. The efficiency of COD removal from wastewater during continuous (series 1) and intermittent (series 2) dosage of microwaves (MW) compared to convection heated reactors (CH).
ily-degradable organic compounds (BOD5/COD = 0.9). The loading of organic compounds equaled 1.98 g COD/(m2 d). In V1, the influent COD concentrations averaged 400 ± 10 mg/L, in V2 wastewater was devoid of organic carbon compounds. The concentrations of COD in the effluents are depicted in Fig. 2. Because of wastewater composition in V2, the effluents did not contain organic compounds expressed as COD or suspended solids. The highest effectiveness of COD removal reached over 98% and was observed at 25 °C (Fig. 3). In all temperature conditions applied (20–40 °C), there was no statistically significant impact of the microwave radiation (compared to convection heating) on the efficiency of organic compound removal. The reactions of microorganisms depend on temperature more at lower levels <15 °C than at the optimal range of 20–35 °C, where a temperature change only slightly affects organic removal Kadlec and Reddy (2001). Mayo and Noike (1996) proved that there was no difference in heterotrophic colonies forming at 20 and 35 °C, and heterotrophic bacteria abundance was not affected by temperature variations in the range 10– 20 °C. Wu et al. (1983) claimed that above 15 °C the efficiency of carbon removal from wastewater is not correlated with temperature, and above 25 °C the efficiency declines. In our research (in the temperature range 20–35 °C), the differences in organic removal were insignificant. Similar results were obtained by Pabello et al. (1992) who noted a 21% increase in organic removal efficiency with a temperature rise from 10 to 20 °C in a biofilm reactor, whereas further growth to 30 °C resulted in only a 13% increase in the effectiveness. In the current research, a further temperature increase to 40 °C resulted in a decline in process efficiency to the lowest value of 73% during periodic dosage of microwave energy. The organic loading in the effluent was significantly higher than at lower temperatures. This observation was done independent of the method of heating; hence, it was not connected with the activity of microwaves but with an excessively high temperature. Similarly, Visvanathan and
Table 2 Operational results. Series
S1
T (°C)
20
25
35
40
S2 20
25
35
40
S3 20
25
35
40
Y (g TSS/g COD) Y (g TSS/g NOx) rNH4+–N (mg/(L h)) rNOx (mg/(L h))
0.27 (0.34) n.m. 5.7 (4.3) 4.7 (3.9)
0.21 (0.28) n.m. 6.4 (5.0) 5.6 (4.6)
0.34 (0.36) n.m. 4.7 (4.1) 4.3 (3.6)
0.42 (0.41) n.m. 0.0 (0.0) 0.0 (0.0)
0.32 (0.34) n.m. 4.1 (4.3) 3.6 (3.9)
0.27 (0.28) n.m. 4.6 (5.0) 4.3 (4.6)
0.35 (0.36) n.m. 4.4 (4.1) 2.3 (3.6)
0.43 (0.41) n.m. 0.0 (0.0) 0.0 (0.0)
n.m. 0.15 (0.16) 7.6 (7.2) n.m.
n.m. 0.17 (0.18) 7.3 (7.1) n.m.
n.m. 0.09 (0.09) 7.2 (7.0) n.m.
n.m. 0.00 (0.00) 0.0 (0.0) n.m.
Numbers in brackets indicate the values obtained for the convection heated reactors. T – temperature; Y – observed yield coefficient; rNH4+–N – ammonium oxidation rate; rNOx – rate of the reduction of oxidized nitrogen; n.m. – not measured.
´ ski et al. / Bioresource Technology 127 (2013) 223–230 M. Zielin
228
ENITR (%)
SERIES 3
SERIES 2
SERIES 1
100 80 60 40 20 0 20
25
35
40
20
25
35
40
20
25
35
40 0
temperature ( C) MW
CH
Fig. 4. Nitrification efficiency during continuous (series 1 and series 3) and intermittent (series 2) dosage of microwaves (MW) compared to convection heated reactors (CH).
heating. In contrast to variant 1, an increase in temperature up to 35 °C did not result in a failure of nitrification. The results showed that, at optimal temperature conditions, the composition of wastewater, especially the COD/N ratio, is the most important factor determining nitrification efficiency – not the method of biofilm heating. At 40 °C, a complete inhibition of ammonium oxidation was observed, independent of the method of system heating and the composition of wastewater. Our results and literature data indicate a high dependence between nitrification efficiency and thermal conditions which was confirmed by temperature coefficients from 1.11 to 1.37 at a temperature below 10 °C, from 1.07 to 1.16 at 10–15 °C and from 1.06 to 1.12 at 15–20 °C (Kadlec and Reddy, 2001). Our observations are consistent with those obtained by Lapara and Alleman (1999) who stated that nitrification does not occur under thermophilic conditions. In addition, the direction of temperature variations is significant. Sudarno et al. (2011) proved that after the ‘‘cold shock’’ by increasing the temperature from 6 to 22.5 °C, a full recovery of ammonium and nitrite oxidation rates were obtained, whereas no recovery was achieved after the ‘‘heat shock’’ when the temperature was reduced from 50 to 22.5 °C. In our research, based on changes in ammonium concentration during the reaction time, nitrification rate (rNH4+–N) was calculated after taking into account the assimilation of nitrogen for biomass synthesis. A decrease in ammonium concentration proceeded in accordance with a zeroth-order reaction. In variant 1, the highest rNH4+–N equaled 6.4 mg/(L h) was obtained at 25 °C (Table 2). From its initial value of ca. 40 mg/L, ammonium was completely oxidized during 6 h of the reaction. In the range from 20 to 35 °C, a continuous microwave dosage resulted in 20% higher nitrification rates compared to the conditions of convection heating, and 22% higher rates compared to the series with intermittent MW. The rNH4+–N was almost the same during periodic MW radiation and convective heat transfer, and the time for complete ammonium oxidation ranged 8–9 h. An increase in temperature to 35 °C entailed a slowdown of ammonium oxidation to ca. 4.4 mg/(L h), independent of the method of biofilm temperature growth, and the period needed for total ammonium oxidation equaled ca. 8 h. In variant 2, as a result of substrate conditions favoring nitrification, ca. 5 h reaction was sufficient to completely oxidized ammonium present in the influent. The nitrification rate was over 30% higher than under parallel conditions in variant 1 and equaled ca. 7.4 mg/(L h). The rate depended neither on the investigated temperature range nor on the type of reactor heating. Grunditz and Dalhammar (2001) reported that the highest rate of ammonium oxidation was achieved at 35 °C, whereas the highest rate of nitrite oxidation was at 38 °C. A further temperature in-
crease induced a sharp decline in the process rate and at 50 °C neither the activity of Nitrosomonas nor Nitrobacter was observed. Similarly, Fontenot et al. (2007) showed that with increasing temperature, nitrification rates increased to a maximum at 30–37.5 °C, whereas above 37.5 °C the rates decreased. Our results are consistent with this thesis. At 40 °C, the ammonium oxidation rates dropped to zero (Table 2). In biofilm systems, the effect of temperature on nitrification kinetics is more complicated to describe due to other factors such as the limitations of oxygen and reduced mass diffusion (Chen et al., 2006). The nitrification rate is in an equilibrium between substrate demand, created by the growth of bacteria biomass and the rate of substrate supply determined by diffusion transport limitation (Rasmussen and Lewandowski, 1998). According to Zhu and Chen (2002), the impact of temperature on nitrification at DO limitation is different from that achieved at total ammonium limitation. The authors indicated that a temperature increment at 20 °C resulted in a nitrification rate increase of 1.108% per °C and 4.275% per °C under DO and total ammonium limited conditions, respectively. Due to diffusion limitations in attached biomass processes, the impact of temperature on the nitrification rate is greatly reduced compared with that of suspended growth processes. When oxygen is limited, the decrease in saturation DO as temperature increases results in a negative temperature impact upon the nitrification rate. In the current research, synthetic wastewater introduced into the retention tank was characterized by DO of ca. 10 mg/L that corresponded to the maximum saturation at temperature maintained in the thermostatic chamber. After 24 h of reaction, as the resultant of oxygen consumption for biochemical processes and natural aeration of the trickling filters, DO in the retention tank averaged 8.2 ± 0.4, 6.7 ± 0.3, 5.9 ± 0.5 and 4.8 ± 0.3 mg/L, respectively at 20, 25, 35 and 40 °C in every series. The results suggested that DO was not a limiting factor in oxidation processes. In variant 1, the highest total efficiency of nitrogen removal (over 60%) was noted at 25 °C. The application of continuous microwave heating induced a significantly higher process effectiveness than biofilm exposure to the intermittent heating at given temperature. However, no differences between nitrogen removal efficiencies in biofilm reactors heated by microwaves and by hot air were observed (Fig. 5). Variable dosage of microwaves into the system affected both unit processes involved in nitrogen removal. During a continuous dosage of microwaves, the reduction of oxidized nitrogen forms into gaseous nitrogen accounted for over 80% (and biomass synthesis – almost 20%) of the participation in nitrogen elimination from wastewater. At a given temperature, both intermittent application of microwaves and convection heating generated higher participation of biomass synthesis in nitrogen removal. These results showed that microwave radiation, due to its volumetric nature, changes the conditions in biofilm. At 40 °C, there was no nitrification, hence an absence of oxidized nitrogen forms as substrates for denitrification caused the biomass synthesis to be responsible for only a small level of nitrogen removal (ca. 9%). Despite the fact that trickling filters were operated in aerobic mode, denitrification occurred at 20–35 °C. If biofilm consists of both nitrifiers and denitrifiers and the amount of oxygen allows for nitrification and is low enough to provide denitrification, these processes can occur simultaneously, as is widely documented in literature for the attached biomass (Watanabe et al., 1992). The rate of the reduction of oxidized nitrogen (rNOx) was determined based on the time course of the concentrations of reduced nitrogen, calculated as the difference between the estimated concentration of oxidized nitrogen and the experimental sum of nitrite and nitrate concentrations. The changes in NOx concentration proceeded in accordance with first-order reaction. In variant 1, the
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SERIES 1
229
SERIES 2
SERIES 3
100% 90% 80% 70% 60% 50% 40% 30% 20% 10%
biomass synthesis
)
40
(C H
)
W )
(C H
(M
35
40
)
W )
(C H
(M
25
35
)
W )
(C H
(M
20
25
)
W ) (M
(C H
40
20
)
W )
(C H
(M
35
denitrification
40
)
W )
(C H
(M
25
35
)
W )
(C H
(M
20
25
)
W )
(C H
(M
40
20
)
W )
(C H
(M
35
40
)
W )
(C H
(M
25
35
)
W ) (M
25
(C H
(M 20
20
W )
0%
N removal efficiency
Fig. 5. The contribution of biomass synthesis and denitrification in nitrogen removal and the efficiency of total nitrogen elimination from wastewater during continuous (series 1 and series 3) and intermittent (series 2) dosage of microwaves (MW) compared to convection heated reactors (CH).
highest rNOx, equaled 5.60 mg/(L h), and was achieved during continuous microwave heating to 25 °C (Table 2), whereas both intermittent microwave dosage and convection heating resulted in a significantly lower denitrification rate. In variant 2, a lack of organic carbon in the influent was the reason for the very low efficiency of nitrogen removal at the level of 1–12%, independent of the type of biomass heating (Fig. 5). Under these substrate conditions, heterotrophic denitrification could not have occurred. The biomass yield was low and equaled from 0.09 to 0.17 g TSS/g NOx. Therefore, the assimilation of nitrogen for biomass growth had a maximum of 23% participation (CH, 25 °C) in total removal of this nutrient. Due to the fact that the reaction environment (ca. 7.5 pH) did not allow nitrogen stripping, the remaining nitrogen loss could have been a result of other processes. It is difficult to evaluate which processes are responsible for nitrogen elimination from wastewater in biofilm reactors. Depending on DO, ammonium can be oxidized to nitrites or – in the case of limited DO – anaerobic ammonium reactions to gaseous nitrogen with nitrites as an electron acceptor can proceed. The second process is called Anammox and can be conducted by autotrophic microorganisms in the inner layers of biofilm (Hao et al., 2002). The current research showed the effect of microwave radiation on the effectiveness of wastewater treatment, particularly on nitrogen transformations. Such a result could have been induced by the volumetric nature of microwave radiation resulting in its uniformity. Microwaves heat simultaneously the whole substance to their penetration depth. In the current research, at the MW frequency of 2.45 GHz the calculated value of the distance of the radiation penetration was ca. 12 cm. It meant that the whole biofilm volume was exposed to MW at the same time, in contrast to CH where the heat is transferred from the outer warmer layers to inner ones due to convection. In addition, the research proved that the continuous MW dosage induced better effects than the intermittent one and used significantly less energy (Table 1). Taking into consideration the technological results and energy consumption, the application of microwaves for the heating of trickling filters is promising, particularly in the treatment of cold wastewater with a high ammonium load, e.g. for aquaculture sewage purification. Due to the fact that water absorbs microwaves almost ideally, it can be supposed that microwave application for suspended biomass would require the heating of both the activated sludge and huge volume of wastewater resulting in high operational cost. Further research should focus on the scalability that is a significant problem in the systems with radiation. The issues concerning the arrangement and the number of waveguides leading radiation from a generator inside the full-scale reactors should be explored.
4. Conclusions The knowledge gained in this study can be used to provide an insight into the application of heated trickling filters to improve nitrification during wastewater treatment, especially in cold climates. The continuous biofilm exposure to microwaves resulted in ca. a 10% increase in nitrification efficiency compared to the convective heating or the intermittent microwave dosage, despite the fact that periodic radiation induced higher energy consumption. Acknowledgements We are grateful for the financial support of the Ministry of Scientific Research and Information Technology of Poland (Project No. 1 T09 D 058 30). References Ali, N., Mohammad, A.W., Jusom, A., Hasan, M.R., Ghazali, N., Kamaruzaman, K., 2005. Treatment of aquaculture wastewater using ultra-low pressure asymmetric polyethersulfone (PES) membrane. Desalination 185, 317–326. APHA, 1992. Standard Methods for the Examination of Water and Wastewater, 18th ed. APHA, AWWA and WEF, Washington. Banik, S., Bandyopadhyaya, S., Ganguly, S., Dan, D., 2006. Effect of microwave irradiated Methanosarcina barkeri DSM-804 on biomethanation. Bioresour. Technol. 97, 819–823. Bigio, I.J., Gosnell, T.R., Mukherjee, P., Saffer, J.D., 1993. Microwave absorption spectroscopy of DNA. Biopolymers 23, 147–150. Chen, S., Ling, J., Blancheton, J.P., 2006. Nitrification kinetics of biofilm as affected by water quality factors. Aquacult. Eng. 34, 179–197. Coelho, M.A.Z., Russo, C., Araujo, O.Q.F., 2000. Optimization of a sequencing batch reactor for biological nitrogen removal. Water Res. 34, 2809–2817. Edwards, G.S., Davis, C.C., Saffer, J.D., Swicord, M.L., 1985. Microwave-field-driven acoustic modes in DNA. Biophys. J. 47, 799–807. Fernandez, A.S., Hashsham, S.A., Dollhopf, S.L., Raskin, L., Glagoleva, O., Dazzo, F.B., Hickey, R.F., Criddle, C.S., Tiedje, J.M., 2000. Flexible community structure correlates with stable community function in methanogenic bioreactor communities perturbed by glucose. Appl. Environ. Microbiol. 66, 4058–4067. Fontenot, Q., Bonvillain, C., Kilgen, M., Boopathy, R., 2007. Effects of temperature, salinity, and carbon:nitrogen ratio on sequencing batch reactor treating shrimp aquaculture wastewater. Bioresour. Technol. 98, 1700–1703. Gabriel, C., Grant, E.H., Tata, R., Brown, P.R., Gestblom, B., Noreland, E., 1989. Dielectric behaviour of aqueous solutions of plasmid DNA at microwave frequencies. Biophys. J. 55, 29–33. Gabriel, C., Gabriel, S., Grant, E.H., Halstead, B.S.J., Mingos, D.M.P., 1998. Dielectric parameters relevant to microwave dielectric heating. Chem. Soc. Rev. 27, 213– 223. Grunditz, C., Dalhammar, G., 2001. Development of nitrification inhibition assays using pure cultures of Nitrosomonas and Nitrobacter. Water Res. 35, 433–440. Hao, X., Heijnen, J.J., Van Loosdrecht, M.C.M., 2002. Model-based evaluation of temperature and inflow variations on a partial nitrification–anammox biofilm process. Water Res. 36, 4839–4849. Hellinga, C., Schellen, A.A.J.C., Mulder, J.W., van Loosdrecht, M.C.M., Heijnen, J.J., 1998. The sharon process an innovative method for nitrogen removal from ammonium rich waste water. Water Sci. Technol. 37, 135–142.
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Hill, J.M., Marchant, T.R., 1996. Modelling microwave heating. Appl. Math. Modell. 20, 3–15. Ilies, P., Mavinic, D.S., 2001. The effect of decreased ambient temperature on the biological nitrification and denitrification of a high ammonia landfill leachate. Water Res. 35, 2065–2072. Kadlec, R.H., Reddy, K.R., 2001. Temperature effects in treatment wetlands. Water Environ. Res. 73, 543–557. Kakita, Y., Funatso, M., Miake, F., Watanabe, K., 1999. Effects of microwave irradiation on bacteria attached to the hospital white coats. Int. J. Occup. Med. Environ. Health. 12, 123–126. Lapara, T.M., Alleman, J.E., 1999. Thermophilic aerobic biological wastewater treatment. Water Res. 33, 895–908. Lishman, L.A., Legge, R.L., Farquhar, G.J., 2000. Temperature effects on wastewater treatment under aerobic and anoxic conditions. Water Res. 34, 2263–2276. Mayo, A.W., Noike, T., 1996. Effects of temperature and pH on the growth of heterotrophic bacteria in waste stabilization ponds. Water Res. 30, 447–455. Moreau, M., Liu, Y., Capdeville, B., Audic, J.M., Calvez, L., 1994. Kinetic behavior of heterotrophic and autotrophic biofilms in wastewater treatment processes. Water Sci. Technol. 29, 385–391. Pabello, L.V., Alardo-Lubel, A., Durana-de-Bazua, C., 1992. Temperature effects on ciliates diversity and abundance in rotating biological reactor. Bioresour. Technol. 39, 55–60. Parker, M.C., Besson, T., Lamare, S., Legoy, M.D., 1996. Microwave radiation can increase the rate of enzyme catalysed reaction in organic media. Tetrahedron Lett. 37, 8383–8386. Perreux, L., Loupy, A., 2002. A tentative rationalization of microwave effects in organic synthesis according to the reaction medium, and mechanistic considerations. Tetrahedron 57, 9199–9223. Ponne, C.T., Bartels, P., 1995. Interaction of electromagnetic energy with biological material – relation to food processing. Radiat. Phys. Chem. 45, 591–607.
Porcelli, M., Cacciapuoti, G., Fusco, S., Massa, R., Ambrosio, G., Bertoldo, C., Rosa, M.D., Zappia, V., 1997. Non-thermal effects of microwaves on proteins: thermophilic enzymes as model system. FEBS Lett. 402, 102–106. Rasmussen, K., Lewandowski, Z., 1998. Microelectrode measurements of local mass transport rates in heterogenous biofilms. Biotechnol. Bioeng. 59, 302–309. Remya, N., Lin, J.G., 2011. Current status of microwave application in wastewater treatment – a review. Chem. Eng. J. 166, 797–813. Sudarno, U., Winter, J., Gallert, C., 2011. Effect of varying salinity, temperature, ammonia and nitrous acid concentrations on nitrification of saline wastewater in fixed-bed reactors. Bioresour. Technol. 102, 5665–5673. Takashima, Y., Hirose, H., Koyama, S., Suzuki, Y., Taki, M., Miyakoshi, J., 2006. Effects of continuous and intermittent exposure to RF fields with a wide range of SARs on cell growth, survival, and cell cycle distribution. Bioelectromagnetics 27, 392–400. Thostenson, E.T., Chou, T.W., 1999. Microwave processing: fundamentals and applications. Composites Part A 30, 1055–1071. Visvanathan, C., Nhien, T.T.H., 1995. Study on aerated biofilter process under high temperature conditions. Environ. Technol. 16, 301–314. Watanabe, Y., Masuda, S., Ishiguro, M., 1992. Simultaneous nitrification and denitrification in micro-aerobic biofilms. Water Sci. Technol. 26, 511–522. Wu, Y.C., Smith, E.D., Chen, C.Y., 1983. Temperature effects on RBC scale-up. J. Environ. Eng. 109, 321–331. Zhu, S., Chen, S., 2002. The impact of temperature on nitrification rate in fixed film biofilters. Aquacult. Eng. 26, 221–237. Zielin´ski, M., Ciesielski, S., Cydzik-Kwiatkowska, A., Turek, J., De˛bowski, M., 2007. Influence of microwave radiation on bacterial community structure in biofilm. Process Biochem. 42, 1250–1253. Zielin´ski, M., Zielin´ska, M., 2010. Impact of microwave radiation on nitrogen removal and quantity of nitrifiers in biofilm. Can. J. Civ. Eng. 37, 1–6.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Application of microwave radiation to biofilm heating during wastewater treatment in trickling filters Marcin Zielin´ski a, Magdalena Zielin´ska b,⇑, Marcin De˛bowski a a b
Department of Environmental Engineering, University of Warmia and Mazury in Olsztyn, Warszawska 117, 10-709 Olsztyn, Poland Department of Environmental Biotechnology, University of Warmia and Mazury in Olsztyn, Słoneczna 45G, 10-709 Olsztyn, Poland
h i g h l i g h t s " Microwaves (MW) and convection (CH) were used to heat biofilm. " Efficiency of pollutant removal from wastewater by biofilm was tested in 20–40 °C. " Continuous MW dosage resulted in lower biomass yield than intermittent MW or CH. " Continuous MW gave 10% higher nitrification efficiency than periodic MW or CH. " Selective heating by MW makes the system less energy-consuming than CH.
a r t i c l e
i n f o
Article history: Received 25 July 2012 Received in revised form 22 September 2012 Accepted 26 September 2012 Available online 5 October 2012 Keywords: Microwave heating Trickling filters Nitrogen removal
a b s t r a c t The purpose of this study was to demonstrate the potential for improving wastewater treatment by the application of microwave radiation (MW) compared to convective heating (CH) of trickling filters. Microwaves were delivered to the biofilm in a continuous and intermittent way to obtain temperatures of 20, 25, 35 and 40 °C. Although there was no effect of MW on organic removal, the observed yield coefficient was lower during the continuous MW supply compared to the periodic dosage and CH. The presence of organic compounds in the influent and continuous biofilm exposure to MW resulted in ca. 10% higher efficiency and ca. 20% higher rate of nitrification compared to intermittent MW dosage and CH. Independent of the method of reactor heating, the absence of organic carbon in the influent induced a significant increase in ammonium oxidation efficiency at 20–35 °C. Despite the aerobic conditions in trickling filters, nitrogen loss was observed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The rate of biochemical degradation of pollutants is connected with the activity of microorganisms and particularly with the rate of enzymatic reactions which depend on environmental conditions, including temperature. An increase in temperature induces an acceleration of the biochemical reaction rates according to the van’t Hoff–Arrhenius equation; on the other hand, due to the protein nature of enzymes, an excessive temperature leads to denaturation and a cessation of the process. Most of the bacteria involved in wastewater treatment processes are mesophilic microorganisms. Ilies and Mavinic (2001) proved that autotrophic bacteria such as nitrifiers are more sensitive to temperature drops than heterotrophic bacteria. They stated that at 10 °C, nitrification effi⇑ Corresponding author. Tel.: +48 89 523 41 85; fax: +48 89 523 41 31. E-mail addresses: [email protected] (M. Zielin´ski), magdalena. [email protected] (M. Zielin´ska), [email protected] (M. De˛bowski). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.09.102
ciency can reach 10–30%. However, it is widely believed that only at above 15 °C does ammonia oxidation proceed effectively. Temperature affects the growth rate of both ammonia- (AOB) and nitrite-oxidizing bacteria (NOB), while AOB have superior growth rates at higher temperatures than NOB (Hellinga et al., 1998). Hence, ensuring the optimal temperature for nitrification is the key action to obtain an effective course of the cascade of reactions leading to nitrogen removal from wastewater. The precise control of thermal conditions inside microbial structures in bioreactors can be achieved by the application of microwaves (MW). Microwaves are part of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with a corresponding frequency from 300 MHz to 300 GHz. Due to the low energy of microwave radiation quanta, interaction with the substances occurs at the molecular level and is based on the effect between electromagnetic field and particles. The action of the electromagnetic field of a microwave frequency (leading to an increase in the temperature of some substances) is connected with its dielectric
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was investigated in order to improve nitrification efficiency. According to Remya and Lin (2011), the installation and operational costs of an MW system are high. However, based on literature data that prove the reduction of the time required for MW-supported treatment and high efficiency of reactions and on our earlier experience (Zielin´ski et al., 2007), it is assumed that the application of microwave radiation in technological systems of wastewater treatment can be advantageous, particularly in nitrification proceeding in a temperate climate. Due to the fact that nitrification reactions occur in the biofilm, not in the bulk liquid (Moreau et al., 1994) and the energy of microwaves is directly transferred into the biomass, the proposed solution allows for a cost reduction compared with convection heating. The aim of this research was to determine if the application of microwave heating had a thermal or non-thermal impact on the efficiency and rate of carbon and nitrogen transformations in trickling filters. 2. Methods 2.1. Characteristics of the trickling filters The biofilm developed on polyethylene granules with a shape similar to a sphere: the average, minimal and maximal diameters were 3.00, 2.88 and 3.12 mm, respectively. These particles were characterized by a sphericity of 0.98 and a porosity of 0.38. The biofilm-covered particles were placed in the bioreactors with active volumes of 0.5 L made of plastic material which allowed for penetration of radiation (Fig. 1). The specific surface area and active area of the bed equaled 202.0 m2/m3 and 0.101 m2, respectively. The bioreactors were operated in a batch mode. Once a day, raw wastewater was introduced into the retention tank with a volume of 0.5 L and was pumped from this tank into the down-flow bioreactor in an intermittent mode: 15 s of flow and 45 s of break. From the bioreactor, wastewater was returned to the retention tank. The ø 60 mm
220 mm
Influent
180 mm
features, i.e. its ability to conduct electricity. Dipole molecules vibrations and, to a lesser extent, migration of ions (only for solutions) have basic significance in an increase in temperature due to microwave radiation (Ponne and Bartels, 1995). As a result of microwave interaction, only the growth of the interior energy of substances containing polar molecules (e.g. water) occurs. Energy transport on a molecular level allows for selective heating (Perreux and Loupy, 2002). When using MW, for example, in reactors with an attached biomass, it is possible to apply energy directly into the biofilm (a substance absorbing radiation). Energy losses due to absorption by the particles of the reactor bed can be avoided since particles made of plastic are transparent to microwaves. In addition, turning off the radiation immediately stops the heating process (Remya and Lin, 2011). The way of heating by microwaves is called a ‘‘volumetric’’ process, which is characterized by a shorter time of reaction and a saving of energy (Gabriel et al., 1998) compared to convection heating which is realized due to convection and heat conduction from the surface of the material. The effects of microwaves on living organisms are divided into two groups: thermal (connected with temperature increase) and non-thermal (athermal), which describes the phenomenon induced by microwaves but not observed during convection heating (Banik et al., 2006). The thermal effect is related to the heat generated by the absorption of microwave energy by water and other polar molecules, both characterized by a permanent or induced polarization (Porcelli et al., 1997). Non-thermal effects describe the changes of the chemical, biochemical or physical behaviour of the system while the temperature remains unaltered. Parker et al. (1996) proved the positive athermal effects of microwave, heating lipase suspended in substrate to 50 °C using microwaves (2.45 GHz) and convection. They obtained a 2- to 3-fold higher rate of enzymatic reaction during microwave heating. The absorption of radiation by DNA is a significant problem involving athermal effects of microwaves on living organisms. It has been proven that DNA absorbs microwaves directly (Takashima et al., 2006) but the mechanism has not yet been recognized. Since the activation energy of microwaves equals ca. 10 5 eV, microwaves cannot disrupt hydrogen bonds (0.04–0.44 eV) or covalent bonds (5 eV) in DNA. Interactions between microwaves and molecules of both DNA and proteins can sometimes influence chemical bonds. Experiments conducted by Kakita et al. (1999) showed that microwave radiation was capable of extensive fragmentation of a DNA strand, whereas external heating at the same temperature left it intact. This could indicate the occurrence of an alternative mechanism of microwave interaction (athermal) leading to bond destructions in DNA. (Edwards et al., 1985) (cited by Ponne and Bartels, 1995) stated that radiation at the frequency of 1 GHz can limit DNA reproduction, whereas Gabriel et al. (1989) (cited by Ponne and Bartels, 1995) documented no evidence in the range 1–10 GHz, as did Bigio et al. (1993) (cited by Ponne and Bartels, 1995) in the range 5–20 GHz. It is considered that zones of highly increased temperature, so called ‘‘hot-spots’’, formed in cells as a result of microwave heating, can cause denaturation of proteins (Hill and Marchant, 1996). In our research, we analyzed the nitrogen removal from wastewater of a relatively low concentration of pollutants. Sewage from recirculating aquaculture systems could be an example of such wastewater. Ammonium is a major metabolic waste excreted by fish and originated from the fish feeding in aquaculture systems (Ali et al., 2005). Since ammonium is toxic to most fish species at relatively low concentrations, the nitrification process is the main objective in the operation of aquaculture systems. For this purpose, bioreactors with a long bacterial cell residence time are required, such as reactors with attached biomass (Chen et al., 2006), which have started to become key components in aquaculture engineering. In our research, microwave heating of the trickling filters
20 mm
224
Effluent
Fig. 1. Scheme of the biofilm reactor.
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40 30 1512 (1498)
20-fold pumping of the whole volume of wastewater from the tank into the bioreactor per day corresponded to a hydraulic loading rate of 0.15 m3/(m2 h).
25 20 1008 (989)
35 25 1260 (1189)
2.2. Organization of the experiment
S1
Continuous
20 15 756 (725)
Series
Dosage of microwave
Temperature (°C) Power of microwave (W) Consumption of electric energy (Wh/d)
Numbers in brackets indicate the values obtained for the convection heated reactors. a Time (in seconds) of microwave supplying/time (in seconds) of pause; microwave generator worked periodically with a constant power of 600 W.
With organic carbon Type of wastewater
25 20 1008 (989)
35 25 1260 (1189)
40 30 1512 (1498)
20 15 789 (725)
25 20 1129 (989)
35 29 1462 (1189)
40 55 2772 (1498)
20 15 756 (725)
Continuous 15/600a
Intermittent
S2
15/450a
15/300a
15/150a
S3
Without organic carbon
V2 V1 Variant
Table 1 Schematic diagram of the technological system.
225
The organization of the lab-scale technological system is shown in Table 1. The reactors were fed with the synthetic wastewater (prepared according to Coelho et al. (2000) after modifications) containing ammonium chloride, hydrogenphosphates, carbonates, bicarbonates, macro- and microelements. In variant 1 (V1), wastewater was supplied with sodium acetate, resulting in the COD concentration in the influent of 400 ± 10 mg/L and BOD5/COD = 0.9. In variant 2 (V2), raw wastewater did not contain organic compounds. In both variants, the ammonium concentration in the influent equaled 40 ± 2 mg/L. The bioreactors placed inside the thermostatic chambers were heated using microwave radiation (MW). Radiation generated by a magnetron (Plazmatronika, Poland) was transferred to the thermostatic chamber with the use of a waveguide. Because a magnetron uses resonant structures to generate an electromagnetic field, it is capable of generating a field of constant frequency. The microwave generator permitted smooth power regulation in the range from 0 to 600 W. The frequency of the generated radiation equaled 2.45 GHz. During 1 s the microwave radiation emitted by a magnetron caused a temperature increase of 1 L of distilled water by 0.1 °C. The specific heat of distilled water is 4186.8 J/(kg K), hence, a growth of temperature by 0.1 °C required 418.68 J of energy as microwave radiation converted to heat energy. The efficiency of the magnetron, calculated on this basis, equaled 52% and was consistent with the efficiency for this type of device as presented in the literature data (Thostenson and Chou, 1999). MW was supplied into the reactors in two ways. First, radiation was dosed continuously. Therefore, the changes in the frequency of the current introduced to the cathode of magnetron were done which resulted in a decrease in the intensity of the emitted microwaves. The power of the applied radiation was adjusted to obtain the desired temperature (Table 1). Second, radiation was delivered sequentially during 15 s exposures separated by a few-minute pauses, depending on the temperature required, leading to the limitation of microwave energy supplied. In this case, a magnetron was operated with a stable power of 600 W and alternating periods of radiation and non-radiation resulted in maintaining the expected temperature. Simultaneously, a microprocessor controlled the pump responsible for the wastewater flow into the reactors, cutting off the flow before starting the microwave generator. Parallel control technological systems were operated with the same temperature (but obtained due to convection). Such an experimental design allowed for a conclusive analysis to be made of the potential non-thermal effects of microwaves. Convection heating (CH) was realized with the use of thyristor resistors and air circulation was also enforced. The temperature inside the bioreactors was maintained at the levels of 20, 25, 35 and 40 °C. In both systems (MW and CH), four trickling filters were placed inside a thermostatic chambers made from metal to ensure safety work and to prevent the penetration of microwaves outside the system. To ensure constant ambient temperature, a temperature of 15 ± 1 °C was maintained inside both thermostatic chambers. Both experimental sets were equipped with the system of continuous multi-point temperature measurements. Simultaneously, the temperature was measured inside the reactors, inside the retention tank and in the chamber outside the reactors (ambient) with the use of a four channel HI 98801 thermometer (Hanna Instruments, UK). The results were collected every 30 s in the measuring device memory. A precise microprocessor system (Schneider Electric, France) for temperature measurement and
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adjustments was applied which allowed for hysteresis of 0.5 °C. Dissolved oxygen (DO) concentrations in the retention tanks were measured using an oxygen meter 2BA301 Oxi 3310 (WTW, Germany) with oxygen sensor CellOx 325. The measurements of total electric energy consumed were conducted separately for each system (MW and CH), using a power meter Energy Check 3000 (Voltcraft, Germany). In the MW system, a total consumption involved the heating and operation of the pumps, whereas in the CH system – power of electric heaters, a ventilator and the pumps. The consumption of electric energy is shown in Table 1.
dized nitrogen forms, kinetic research was conducted: 10 ml samples from the retention tank were collected every 2 h for 12 h, the last sample – after 24 h and analyzed with the colorimetric method by HACH for NH4+–N, NO2 –N and NO3 –N. The obtained data were tested for significance by one-way ANOVA, at the significance level of p < 0.05. The normality of the distribution was confirmed by Shapiro–Wilk’s test, whereas the hypothesis of the homogeneity of variances across the groups was verified on the basis of Levene’s test. The differences between the mean values derived from particular groups were examined by Tukey’s test. Statistica 8.0 PL (StatSoft) was used.
2.3. Analytical methods Raw wastewater and wastewater collected from the retention tank were assayed daily for determination of COD, NH4+–N, NO2 –N, NO3 –N, and TSS, according to APHA (1992). In order to calculate the rates of ammonium removal and the reduction of oxi-
3. Results and discussion In variant 1 (V1) of the current research, wastewater introduced into the trickling filters was characterized by the presence of eas-
Fig. 2. The average effluent concentrations of COD (a), ammonium (b), nitrate (c), nitrite (d) and TSS (e).
´ ski et al. / Bioresource Technology 127 (2013) 223–230 M. Zielin SERIES 1
Nhien (1995) investigated municipal wastewater treatment in immersed biofilm reactors at 30–55 °C and observed that an increase in turbidity at the highest temperature levels resulted from cell decay. In the current research, the lowest values of observed yield coefficient (Y) (0.21–0.34 g TSS/g COD) were obtained at 20 and 25 °C but the significantly smallest Y values were observed under the conditions of continuous MW dosage compared to intermittent MW dosage and CH (Table 2). The rise in temperature to 40 °C caused an increase in Y to over 0.4 g TSS/g COD, independent of the method of reactor heating. At this high temperature, the activity of heterotrophic bacteria was limited, whereas the nitrifier growth and activity was completely inhibited (Table 2, Fig. 4). This could have been a result of temperature-induced cell lysis. Partially biodegradable products of cell lysis can accumulate on biomass particles, increase the mass of cells in the reactor and result in higher observed yield (Lishman et al., 2000). Microwave radiation had much more effect on nitrogen reactions than on organic removal from wastewater. The conducted research showed the dependence between the method of biofilm heating and observed efficiency of nitrogen transformations. In both experimental variants, nitrogen loading rates were the same and equalled 0.198 g N/(m2 d). Ammonium concentration in the influent averaged 40 ± 2 mg/L. The concentrations of particular nitrogen forms in the effluents are given in Fig. 2. In variant 1, with easily degradable organic compounds in the influent, the efficiency of ammonium oxidation reached 87% in MW reactors and 82% in reactors heated by hot air, and was the highest at 25 °C (Fig. 4). In the temperature range from 20 to 35 °C, nitrification efficiency was higher by ca.10% in reactors exposed to continuous MW radiation in contrast to those with intermittent microwave heating, despite using more power in the latter case (Table 1). The continuous delivering of MW also resulted in ca. 10% higher effectiveness of ammonia oxidation than in the control series with convective heating, suggesting an athermal effect of MW on ammonia-oxidizing bacteria activity. There were, however, no significant differences in the process efficiency between control systems and the reactors heated by periodic dosage of microwaves. The athermal effects can be explained by the impact of microwave radiation on microbial communities in biofilm that was proved in our earlier experiments. In the bioreactors exposed to the radiation, total bacterial diversity (Zielin´ski et al., 2007) and the abundance of ammonia-oxidizing bacteria (Zielin´ski and Zielin´ska, 2010) were significantly higher as compared to the reactors heated by convection. It is known that the wastewater treatment systems with highly diversed bacterial consortia are more resistant to environmental stress due to their functional redundancy and alternative ways to use the flow of energy (Fernandez et al., 2000). For this reason, the operational parameters should provide the conditions for the development of bacterial communities characterized by high richness. In variant 2 (with a lack of organic carbon in the influent in the range 20–35 °C) a very high efficiency of ammonia oxidation was achieved (above 98%), independent of the method of biomass
SERIES 2
EC (%)
100 80 60 40 20 0 20
25
35
40
20
25
35
40 0
temperature ( C) MW
227
CH
Fig. 3. The efficiency of COD removal from wastewater during continuous (series 1) and intermittent (series 2) dosage of microwaves (MW) compared to convection heated reactors (CH).
ily-degradable organic compounds (BOD5/COD = 0.9). The loading of organic compounds equaled 1.98 g COD/(m2 d). In V1, the influent COD concentrations averaged 400 ± 10 mg/L, in V2 wastewater was devoid of organic carbon compounds. The concentrations of COD in the effluents are depicted in Fig. 2. Because of wastewater composition in V2, the effluents did not contain organic compounds expressed as COD or suspended solids. The highest effectiveness of COD removal reached over 98% and was observed at 25 °C (Fig. 3). In all temperature conditions applied (20–40 °C), there was no statistically significant impact of the microwave radiation (compared to convection heating) on the efficiency of organic compound removal. The reactions of microorganisms depend on temperature more at lower levels <15 °C than at the optimal range of 20–35 °C, where a temperature change only slightly affects organic removal Kadlec and Reddy (2001). Mayo and Noike (1996) proved that there was no difference in heterotrophic colonies forming at 20 and 35 °C, and heterotrophic bacteria abundance was not affected by temperature variations in the range 10– 20 °C. Wu et al. (1983) claimed that above 15 °C the efficiency of carbon removal from wastewater is not correlated with temperature, and above 25 °C the efficiency declines. In our research (in the temperature range 20–35 °C), the differences in organic removal were insignificant. Similar results were obtained by Pabello et al. (1992) who noted a 21% increase in organic removal efficiency with a temperature rise from 10 to 20 °C in a biofilm reactor, whereas further growth to 30 °C resulted in only a 13% increase in the effectiveness. In the current research, a further temperature increase to 40 °C resulted in a decline in process efficiency to the lowest value of 73% during periodic dosage of microwave energy. The organic loading in the effluent was significantly higher than at lower temperatures. This observation was done independent of the method of heating; hence, it was not connected with the activity of microwaves but with an excessively high temperature. Similarly, Visvanathan and
Table 2 Operational results. Series
S1
T (°C)
20
25
35
40
S2 20
25
35
40
S3 20
25
35
40
Y (g TSS/g COD) Y (g TSS/g NOx) rNH4+–N (mg/(L h)) rNOx (mg/(L h))
0.27 (0.34) n.m. 5.7 (4.3) 4.7 (3.9)
0.21 (0.28) n.m. 6.4 (5.0) 5.6 (4.6)
0.34 (0.36) n.m. 4.7 (4.1) 4.3 (3.6)
0.42 (0.41) n.m. 0.0 (0.0) 0.0 (0.0)
0.32 (0.34) n.m. 4.1 (4.3) 3.6 (3.9)
0.27 (0.28) n.m. 4.6 (5.0) 4.3 (4.6)
0.35 (0.36) n.m. 4.4 (4.1) 2.3 (3.6)
0.43 (0.41) n.m. 0.0 (0.0) 0.0 (0.0)
n.m. 0.15 (0.16) 7.6 (7.2) n.m.
n.m. 0.17 (0.18) 7.3 (7.1) n.m.
n.m. 0.09 (0.09) 7.2 (7.0) n.m.
n.m. 0.00 (0.00) 0.0 (0.0) n.m.
Numbers in brackets indicate the values obtained for the convection heated reactors. T – temperature; Y – observed yield coefficient; rNH4+–N – ammonium oxidation rate; rNOx – rate of the reduction of oxidized nitrogen; n.m. – not measured.
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ENITR (%)
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Fig. 4. Nitrification efficiency during continuous (series 1 and series 3) and intermittent (series 2) dosage of microwaves (MW) compared to convection heated reactors (CH).
heating. In contrast to variant 1, an increase in temperature up to 35 °C did not result in a failure of nitrification. The results showed that, at optimal temperature conditions, the composition of wastewater, especially the COD/N ratio, is the most important factor determining nitrification efficiency – not the method of biofilm heating. At 40 °C, a complete inhibition of ammonium oxidation was observed, independent of the method of system heating and the composition of wastewater. Our results and literature data indicate a high dependence between nitrification efficiency and thermal conditions which was confirmed by temperature coefficients from 1.11 to 1.37 at a temperature below 10 °C, from 1.07 to 1.16 at 10–15 °C and from 1.06 to 1.12 at 15–20 °C (Kadlec and Reddy, 2001). Our observations are consistent with those obtained by Lapara and Alleman (1999) who stated that nitrification does not occur under thermophilic conditions. In addition, the direction of temperature variations is significant. Sudarno et al. (2011) proved that after the ‘‘cold shock’’ by increasing the temperature from 6 to 22.5 °C, a full recovery of ammonium and nitrite oxidation rates were obtained, whereas no recovery was achieved after the ‘‘heat shock’’ when the temperature was reduced from 50 to 22.5 °C. In our research, based on changes in ammonium concentration during the reaction time, nitrification rate (rNH4+–N) was calculated after taking into account the assimilation of nitrogen for biomass synthesis. A decrease in ammonium concentration proceeded in accordance with a zeroth-order reaction. In variant 1, the highest rNH4+–N equaled 6.4 mg/(L h) was obtained at 25 °C (Table 2). From its initial value of ca. 40 mg/L, ammonium was completely oxidized during 6 h of the reaction. In the range from 20 to 35 °C, a continuous microwave dosage resulted in 20% higher nitrification rates compared to the conditions of convection heating, and 22% higher rates compared to the series with intermittent MW. The rNH4+–N was almost the same during periodic MW radiation and convective heat transfer, and the time for complete ammonium oxidation ranged 8–9 h. An increase in temperature to 35 °C entailed a slowdown of ammonium oxidation to ca. 4.4 mg/(L h), independent of the method of biofilm temperature growth, and the period needed for total ammonium oxidation equaled ca. 8 h. In variant 2, as a result of substrate conditions favoring nitrification, ca. 5 h reaction was sufficient to completely oxidized ammonium present in the influent. The nitrification rate was over 30% higher than under parallel conditions in variant 1 and equaled ca. 7.4 mg/(L h). The rate depended neither on the investigated temperature range nor on the type of reactor heating. Grunditz and Dalhammar (2001) reported that the highest rate of ammonium oxidation was achieved at 35 °C, whereas the highest rate of nitrite oxidation was at 38 °C. A further temperature in-
crease induced a sharp decline in the process rate and at 50 °C neither the activity of Nitrosomonas nor Nitrobacter was observed. Similarly, Fontenot et al. (2007) showed that with increasing temperature, nitrification rates increased to a maximum at 30–37.5 °C, whereas above 37.5 °C the rates decreased. Our results are consistent with this thesis. At 40 °C, the ammonium oxidation rates dropped to zero (Table 2). In biofilm systems, the effect of temperature on nitrification kinetics is more complicated to describe due to other factors such as the limitations of oxygen and reduced mass diffusion (Chen et al., 2006). The nitrification rate is in an equilibrium between substrate demand, created by the growth of bacteria biomass and the rate of substrate supply determined by diffusion transport limitation (Rasmussen and Lewandowski, 1998). According to Zhu and Chen (2002), the impact of temperature on nitrification at DO limitation is different from that achieved at total ammonium limitation. The authors indicated that a temperature increment at 20 °C resulted in a nitrification rate increase of 1.108% per °C and 4.275% per °C under DO and total ammonium limited conditions, respectively. Due to diffusion limitations in attached biomass processes, the impact of temperature on the nitrification rate is greatly reduced compared with that of suspended growth processes. When oxygen is limited, the decrease in saturation DO as temperature increases results in a negative temperature impact upon the nitrification rate. In the current research, synthetic wastewater introduced into the retention tank was characterized by DO of ca. 10 mg/L that corresponded to the maximum saturation at temperature maintained in the thermostatic chamber. After 24 h of reaction, as the resultant of oxygen consumption for biochemical processes and natural aeration of the trickling filters, DO in the retention tank averaged 8.2 ± 0.4, 6.7 ± 0.3, 5.9 ± 0.5 and 4.8 ± 0.3 mg/L, respectively at 20, 25, 35 and 40 °C in every series. The results suggested that DO was not a limiting factor in oxidation processes. In variant 1, the highest total efficiency of nitrogen removal (over 60%) was noted at 25 °C. The application of continuous microwave heating induced a significantly higher process effectiveness than biofilm exposure to the intermittent heating at given temperature. However, no differences between nitrogen removal efficiencies in biofilm reactors heated by microwaves and by hot air were observed (Fig. 5). Variable dosage of microwaves into the system affected both unit processes involved in nitrogen removal. During a continuous dosage of microwaves, the reduction of oxidized nitrogen forms into gaseous nitrogen accounted for over 80% (and biomass synthesis – almost 20%) of the participation in nitrogen elimination from wastewater. At a given temperature, both intermittent application of microwaves and convection heating generated higher participation of biomass synthesis in nitrogen removal. These results showed that microwave radiation, due to its volumetric nature, changes the conditions in biofilm. At 40 °C, there was no nitrification, hence an absence of oxidized nitrogen forms as substrates for denitrification caused the biomass synthesis to be responsible for only a small level of nitrogen removal (ca. 9%). Despite the fact that trickling filters were operated in aerobic mode, denitrification occurred at 20–35 °C. If biofilm consists of both nitrifiers and denitrifiers and the amount of oxygen allows for nitrification and is low enough to provide denitrification, these processes can occur simultaneously, as is widely documented in literature for the attached biomass (Watanabe et al., 1992). The rate of the reduction of oxidized nitrogen (rNOx) was determined based on the time course of the concentrations of reduced nitrogen, calculated as the difference between the estimated concentration of oxidized nitrogen and the experimental sum of nitrite and nitrate concentrations. The changes in NOx concentration proceeded in accordance with first-order reaction. In variant 1, the
´ ski et al. / Bioresource Technology 127 (2013) 223–230 M. Zielin
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)
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W )
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Fig. 5. The contribution of biomass synthesis and denitrification in nitrogen removal and the efficiency of total nitrogen elimination from wastewater during continuous (series 1 and series 3) and intermittent (series 2) dosage of microwaves (MW) compared to convection heated reactors (CH).
highest rNOx, equaled 5.60 mg/(L h), and was achieved during continuous microwave heating to 25 °C (Table 2), whereas both intermittent microwave dosage and convection heating resulted in a significantly lower denitrification rate. In variant 2, a lack of organic carbon in the influent was the reason for the very low efficiency of nitrogen removal at the level of 1–12%, independent of the type of biomass heating (Fig. 5). Under these substrate conditions, heterotrophic denitrification could not have occurred. The biomass yield was low and equaled from 0.09 to 0.17 g TSS/g NOx. Therefore, the assimilation of nitrogen for biomass growth had a maximum of 23% participation (CH, 25 °C) in total removal of this nutrient. Due to the fact that the reaction environment (ca. 7.5 pH) did not allow nitrogen stripping, the remaining nitrogen loss could have been a result of other processes. It is difficult to evaluate which processes are responsible for nitrogen elimination from wastewater in biofilm reactors. Depending on DO, ammonium can be oxidized to nitrites or – in the case of limited DO – anaerobic ammonium reactions to gaseous nitrogen with nitrites as an electron acceptor can proceed. The second process is called Anammox and can be conducted by autotrophic microorganisms in the inner layers of biofilm (Hao et al., 2002). The current research showed the effect of microwave radiation on the effectiveness of wastewater treatment, particularly on nitrogen transformations. Such a result could have been induced by the volumetric nature of microwave radiation resulting in its uniformity. Microwaves heat simultaneously the whole substance to their penetration depth. In the current research, at the MW frequency of 2.45 GHz the calculated value of the distance of the radiation penetration was ca. 12 cm. It meant that the whole biofilm volume was exposed to MW at the same time, in contrast to CH where the heat is transferred from the outer warmer layers to inner ones due to convection. In addition, the research proved that the continuous MW dosage induced better effects than the intermittent one and used significantly less energy (Table 1). Taking into consideration the technological results and energy consumption, the application of microwaves for the heating of trickling filters is promising, particularly in the treatment of cold wastewater with a high ammonium load, e.g. for aquaculture sewage purification. Due to the fact that water absorbs microwaves almost ideally, it can be supposed that microwave application for suspended biomass would require the heating of both the activated sludge and huge volume of wastewater resulting in high operational cost. Further research should focus on the scalability that is a significant problem in the systems with radiation. The issues concerning the arrangement and the number of waveguides leading radiation from a generator inside the full-scale reactors should be explored.
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