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1、BIODEGRADABILITY AND CHANGE OF PHYSICALCHARACTERISTICS OF PARTICLES DURING ANAEROBICDIGESTION OF DOMESTIC SEWAGEAbstract: At the high-rate anaerobic treatment of domestic sewage, both biological and physical processes play an important role. Therefore, the anaerobic biodegradability of raw, paper-fi
2、ltered and membrane-filtered sewage and black water has been investigated in batch experiments. Additionally, the effect of anaerobic digestion on physical characteristics, like particle size, surface tension and zeta-potential, of the present particles is studied. The biodegradability of domestic s
3、ewage and black water at 308C is almost similar (7174%). Moreover, a high methanogenesis of the colloidal fraction in domestic sewage (86_3%) is achieved, showing that the low removal of colloidal particles in continuous high-rate anaerobic reactors is due to low physical removal rather than biodegr
4、adability. The lowest biodegradability is demonstrated for the dissolved fraction (62%). The results show that after anaerobic digestion the average radius of particles with diameter54.4 and50.45 mm increased for domestic sewage, while it decreased for black water. Part of the surface-active compone
5、nts in domestic sewage is not biodegraded during anaerobic batch digestion, as indicated by the development of the surface tension. The negative zeta-potential of all particles hardly changes during digestion, showing that colloidal interactions were not affected by anaerobic digestion.Key words: an
6、aerobic treatment, domestic sewage, black water, biodegradability, particle size, surfactants, zeta-potentialINTRODUCTIONSeveral authors have shown that particles represent the major part, up to 85%, of the total COD (CODt) in domestic sewage (Levine et al., 1985; Zeeman et al., 1997). The separatio
7、n of particulate and dissolved compounds in domestic sewage is usuallymade by filtration through a membrane filter with a pore diameter of approximately 0.45 mm (Nielsen and Harremoes, 1995). The particles are often separated in a suspended and a colloidal part, with a particle size of respectively
8、larger than 4.4 mm and between 0.45 and 54.4 mm, although the size range for colloidal particles is not in agreement with the definition as used in colloid chemistry.At treatment under anaerobic conditions, colloidal COD (CODcol) from domestic sewage is removed to a lower degree than under aerobic o
9、r micro aerophillic conditions and represents 6080% of the e.uent CODt of an anaerobic reactor (Yoda et al., 1985; Wang, 1994; Wang et al., 1995). The removal of CODcol in batch recirculation experiments at long etention times, indicates however that colloidal articles are biodegradable (Last and Le
10、ttinga, 992; Wang, 1994). So far this has never been proven.At the high-rate anaerobic treatment of domestic sewage, both biological and physical processes play an important role. Particles can only be converted via hydrolysis, after being physically removed by adsorption, settling or entrapment in
11、the sludge bed. Particle characterization based on the biological and physical aspects are therefore of the same importance.The presence of surfactants in domestic sewage, which are known to adsorb at both solid/liquid and liquid/air interfaces, may affect the anaerobic biodegradability of particles
12、. Surfactants have the ability to emulsify poorly soluble hydrophobic compounds in water, thus potentially improving the accessibility of these substrates to microorganisms (Rouse et al. ,1994). On the other hand, the emulsifying effect might prevent the physical removal of the particles. Moreover i
13、nhibition of anaerobic biodegradation of organic compounds in the presence of surfactants have also been reported (Wagener and Schink, 1987; Rouse et al., 1994). Boller (1993) mentioned that the surfactant concentration in Zu rich City wastewater was 1722mg l1 and the non-ionic and anionic surfactan
14、ts represent the main part (9194%). Linear alkylbenzene sulphonates constitute the major anionic surfactant fraction in households (Holt et al.,1998) with average concentrations of 4 and 3mg in domestic sewage in The Netherlands (Water and Feijtel, 1995) and in UK (Holt et al., 1998), respectively.
15、At low concentrations, surfactants are present as monomers. Micelles are formed above the critical micelle concentration (CMC). The CMC of dodecyl benzene sulphonate amounts to 264 mgl1 (Mukerjee and Mysels, 1971). Therefore, surfactants in domestic sewage seem to be present as monomers. The size of
16、 particles in domestic sewage affects both biological and physical processes (Levine et al., 1985). Gravitational and drag forces predominate over colloidal forces (van der Waals attraction and electrostatic repulsion) for larger particles, while colloidal forces are more important for particles les
17、s than a few mm (Gregory, 1993). The zeta-potential, which represents the potential on or just outside the Stern layer, is an important physical parameter for colloidal particle separation because electrostatic interactions of colloidal particles are mainly related to the zeta-potential. This resear
18、ch aims for the determination of the anaerobic biodegradability of the suspended, colloidal and dissolved fraction of domestic sewage and black water. Moreover, the change in physical characteristics, like particle size, zeta-potential and surface tension as a result of biodegradation is determined.
19、MATERIALS AND METHODSAnaerobic digestion of domestic sewage and black waterAnaerobic batch digestion has been carried out in duplicate series of serum bottles of 120 ml each at each temperature. To each bottle 100 ml of wastewater is added. The biogas composition in the headspace of each bottle is m
20、onitored in time. For each bottle, COD fractions, volatile fatty acids (VFA), and surface tension were determined.Two series of experiments have been performed. In the first run, raw and paper-filtered sewage was digested at 4, 20 and 30C and bottles were monitored after 8, 15, 23 and 43 days. The a
21、im of the second run was to confirm the results of the first run and to find the maximum conversion of wastewater to methane (biodegradability). The second run was performed with raw, paper- and membrane-filtered (not in duplicate) sewage and black water at 20 and 30C and monitoring was carried out
22、after 15, 28 and 135 days. In the second run also the average particle radius and zeta-potential were measured.AnalysisCOD was analysed using the micro-method as described by Jirka and Carter (1975). Raw samples were used for CODt, 4.4 mm folded paper-filtered (Schleicher & Schuell5951/2, Germany) s
23、amples for CODp and 0.45 mm membrane-filtered (Schleicher & Schuell ME 25, Germany) samples for dissolved COD (CODdis). The suspended COD (CODss) and CODcol were calculated by the differences between CODt and CODp, CODp and CODdis, respectively. In the experiments it was dicult to have representativ
24、e samples for CODt due to the formation of large ocs during anaerobic digestion. Therefore, only CODp and CODdis are presented. VFA were determined from membrane-filtered samples by gas chromatography. The chromatograph (Hewlett Packard 5890A, Palo Alto, USA) was equipped with a 2m_2mm (inner diamet
25、er) glass column, packed with Supelco port (100120 mesh) coated with 10% Fluorad FC 431. Operating conditions were: column, 1308C; injection port, 2008C; ame ionization detector, 2808C. N2 saturated with formic acid at 208C was used as a carrier gas (30 ml/min). The biogas composition CH4, CO2, N2 a
26、nd O2 was determined in a 100 ml sample using Fisons Instrument gas chromatography model GC 8000 series, equipped with columns connected in parallel (split 1 : 1)(1.5m_2mm) Teon, packed with chromosorb 108, (6080 mesh), and a (1.2m_2mm) stainless steel, packed with molecular sieve 5A, (6080 mesh). H
27、elium was used as carrier gas (45 ml min1). The oven, detector and injection temperatures were 40, 100 and 1108C, respectively. All measurements were performed in duplicate.An indication of the presence of surface-active components can be obtained by measuring the surface tension with the Wilhelmy-p
28、late method. It is likely that in domestic sewage, surfactants will strongly contribute to the lowering of the surface tension of the aqueous solution. After the CMC, surface tension becomes almost independent of the overall concentration. However, one should be aware that also other surface-active
29、components can contribute to the lowering of the surface tension and therefore the qualitative interpretation can be presented. In run 1, the surface tension was measured for the original samples without filtration. The formation of large ocs during run 1 increased the standard deviations of the sur
30、face tension measurements. Moreover, the surface tension of wastewater slightly increases after paper filtration (from the results of run 1). Therefore, the surface tension in run 2 was measured for all samples after paper and membrane filtration.The hydrodynamic particle radius was determined with
31、dynamic light scattering. Measurements were carried out in a 2 ml cylindrical quartz cell using an ALV5000 system with a Lexel 150mW multiline Ar-laser. Particles with a radius between 2.5nm and 5 mm can be detected. The measurements were performed for both paper- and membrane- fitered samples. For
32、each sample, the average particle radius was measured seven times at an angle of 90Electrophoretic mobilities were determined with a Malvern Zetasizer III. Zeta-potentials were calculated from the Smoluchowski equation. Measurements were performed at constant ionic strength (0.02MKCl) and samples we
33、re paper filtered to remove big particles.CalculationsThe total CH4 production in each serum bottle was the summation of the CH4 in the headspace and the dissolved CH4. The dissolved CH4 was calculated according to Henrys law. Percentage of hydrolysis (H), acidification (A) and methanogenesis (M) we
34、re calculated according to equations (1), (2) and (3) respectively.H, A andM of CODcol for domestic sewage in run 2 were calculated by subtracting the results of membrane-filtered sewage from the results of paper-filtered sewage and applying equations (1), (2) and (3), respectively. Similarly, H, A
35、and M of CODss for domestic sewage in run 2 were calculated by subtracting the results of paper-filtered sewage from the results of raw sewage.Fig. 1. The course of the total CH4 production during the anaerobic batch digestion of raw sewage (),paper-filtered sewage (&), membrane-fitered sewage (n) a
36、nd black water (*) in run 2 at temperature of 20 and 30C.Fig. 2. The course of the surface tension during the anaerobic batch digestion of raw () and paper-filtered (&) sewage in run 1 at temperature of 4, 20 and 30 C.RESULTS AND DISCUSSIONBiodegradabilityTable 1 summarizes the calculated percentage
37、s of hydrolysis, acidification and methanogenesis for each wastewater sample after 43 and 135 days of digestion, in runs 1 and 2, respectively. The results of run 1 show that 43 days of batch digestion are not sucient for complete anaerobic digestion even at 30C. The VFA concentration exceeds 100mg
38、COD1l at all applied conditions. The results of the total CH4 production in run 2 (Fig. 1) show that the maximum conversion of the domestic sewage fractions and black water is achieved after about 80 days at 30C. Figure 1 the anaerobic digestion has a characteristic lag-phase period depending on the
39、 temperature and the size of the particles. At 20C only raw sewage reached the maximum conversion after 135 days of digestion. The maximum methanogenesis for raw sewage was similar at temperatures of 20 and 30C indicating that anaerobic treatment is not only a promising technique in tropical but als
40、o in moderateareas.The biodegradability of black water, raw sewage and paper-filtered sewage at 30C is approximately the same, viz, 7174%, while that of the membranefiltered fraction was relatively low (62%). No reported data are available to compare with the presented results.Table 2 presents the m
41、aximum hydrolysis, acidification and methanogenesis of the CODss and CODcol fraction of domestic sewage at 30C. The maximum hydrolysis for CODss and CODcol is similar, while the maximum acidification and metha nogenesis are higher for CODcol as compared to CODss. Hydrolysis of suspended particles se
42、ems to produce more non-degradable CODdis than hydrolysis of colloidal particles. Last and Lettinga (1992) reported a lower maximum removal of CODdis of 54% during batch recirculation of pre-settled sewage of the same origin as used in the here presented experiments. This lower biodegradability migh
43、t be due to the production of non-degradable COD disfrom the hydrolysis of particles present in pre-settled sewage.Surface tensionFig. 2. The course of the surface tension during the anaerobic batch digestion of raw () and paper-filtered (&) sewage in run 1 at temperature of 4, 20 and 30 C.The surfa
44、ce tension is a measure for the presence of surface-active compounds, such as detergents. The development of the surface tension in time during both runs 1 and 2, was almost similar for temperatures of 20 and 308C (Figs 2 and 3). The maximum surface tension was however much lower at 48C as compared
45、to that at temperatures of 20 and 308C. A plateau value on the surface tension is obtained between 15 and 25 days. This period is much shorter than the characteristic anaerobic digestion time. It seems that a small amount of highly surface-active components are rapidly decreased. Degradation of the
46、detergents seem much slower leading to a maximum surface tension much lower than that of water.The initial surface tension of paper-filtered black water was higher than that for paper-filtered domestic sewage (Fig. 3), probably due to the fact that hardly any detergents are added to black water. Aft
47、er 135 days batch digestion at 20 and 30C the maximum surface tension of black water almost reached that of water. Part of the surfactants was retained during filtration as shown by the higher initial surface tension of paper-filtered as compared to raw sewage (Fig. 2), while that of membrane-ltered
48、 sewage is higher than that of paper-filtered sewage (Fig. 3). As the initial surface tension of the membrane-filtered sewage was still lower than that of water, some surfactants were remaining, even after 135 days batch digestion (Fig. 3). The surfactants, remaining after digestion, are however rem
49、oved by a repeated membrane filtration prior to measurement (Fig. 3), which indicates that they are mainly adsorbed to particles produced during the digestion process.Fig. 3. The course of the surface tension during the anaerobic batch digestion of raw sewage (), paper-filtered sewage (&), member-filtered sewage (n) and black water (*) in run 2 at temperature of 20 and 30 C.Average radiusFigure 4 shows that the average initial radius of particles after paper or me
