Biodegradability and Denitrification        Potential of Settleable Chemical Oxygen           Demand in Domestic Wastewater...
Tas et al.Table 1—Conventional characterization of domestic wastewater.                                                   ...
Tas et al.Table 1—(Extended)                                Domestic wastewater in _                                      ...
Tas et al.                                                                          ¨Figure 1—Statistical distribution of ...
Tas et al.Table 2—Significant parameters in domestic wastewater                       Table 3—Removal ratios of the signific...
Tas et al.Table 4—Effect of settling and filtration on significant ratios for domestic wastewater.                          ...
Tas et al.                                             ¨Table 6—The ratios of fixed solids in the Atakoy domestic wastewate...
Tas et al.                                                       ment plant. Soluble and particulate COD fractions were de...
Tas et al.Figure 4—Schematic representation of COD fractionation and size distribution in domestic wastewater.sludge into ...
Tas et al.Table 8—Kinetic and stoichiometric coefficients used for the modeling of aerobic activated sludge systems.       ...
Tas et al.35%, respectively, at 0.5 days of hX. A lower maximum hydrolysis         Table 9—Kinetic and stochiometric param...
Tas et al.                                                                        References                              ...
Tas et al.Kabdasli, I.; Tunay, O.; Orhon, D. (1993) The Treatability of Chromium           Orhon, D.; Uslu, O.; Meric, S.;...
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Biodegradability potential of settleable dqo

  1. 1. Biodegradability and Denitrification Potential of Settleable Chemical Oxygen Demand in Domestic Wastewater ¨ ¨¸ ¨ _ Didem Okutman Tas1*, Ozlem Karahan1, Guclu Insel1, Suleyman Ovez1, ¨ ¨ 2 3 Derin Orhon , Henri SpanjersABSTRACT: The effect of settling on mass balance and biodegradation et al. (1986), chemical oxygen demand (COD) has been adopted ascharacteristics of domestic wastewater and on denitrification potential was the main parameter to quantify organic carbon. The biodegradablestudied primarily using model calibration and evaluation of oxygen COD conveniently establishes the electron balance betweenuptake rate profiles. Raw domestic wastewater was settled for a period of substrate used, biomass generated, and electron acceptor (dissolved30 minutes and a period of 2 hours to assess the effect of primary settling on oxygen in aerobic systems) consumed. Substantial research haswastewater characterization and composition. Mass balances in the systemwere made to evaluate the effect of primary settling on major parameters. been conducted to identify and assess the biodegradation character-Primary settling of the selected raw wastewater for 2 hours resulted in the istics of different COD fractions in domestic wastewater (Henze,removal of 32% chemical oxygen demand (COD), 9% total Kjeldahl 1992; Orhon et al., 1997; Sollfrank and Gujer, 1991) and differentnitrogen, 9% total phosphorus, and 47% total suspended solids. Respiro- industrial wastewaters (Kabdasli et al., 1993; Orhon et al., 1995;metric analysis identified COD removed by settling as a new COD fraction, Orhon, Tasli, and Sozen, 1999). Respirometry has been a significantnamely settleable slowly biodegradable COD (XSS), characterized by a asset for the experimental assessment of these fractions (Spanjershydrolysis rate of 1.0 day21 and a hydrolysis half-saturation coefficient of and Vanrolleghem, 1995), which are now incorporated as model0.08. A model simulation to test the fate and availability of suspended (XS) components to major activated sludge models (Gujer et al., 2000;and settleable (XSS) COD fractions as carbon sources for denitrificationshowed that both particulate COD components were effectively removed Henze et al., 1987, 1995).aerobically at sludge ages higher than 1.5 to 2.0 days. Under anoxic con- Particle size is an integral component of COD fractionation.ditions, the biodegradation of both COD fractions was reduced, especially Wentzel et al. (1999) stated that the biodegradation potential definedbelow an anoxic sludge retention time of 3.0 days. Consequently, modeling for different COD fractions would have a correlation with physicalresults revealed that the settleable COD removed by primary settling could categorization, in terms of particle size and physical state in waste-represent up to approximately 40% of the total denitrification potential of the water. In wastewater characterization, one particle size (0.45-lmsystem, depending on the specific configuration selected for the nitrogen membrane or 1.2-lm glass-fiber filter size) is commonly used toremoval process. This way, the results showed the significant effect of roughly differentiate soluble and particulate ranges. Soluble inertprimary settling on denitrification, indicating that the settleable COD fraction COD (SI), readily biodegradable COD (SS), and rapidly hydrolyz-could contribute an additional carbon source in systems where the denitri-fication potential associated with the influent becomes rate-limiting for the able COD (SH) are associated with the soluble range, while slowlydenitrification efficiency. Water Environ. Res., 81, 715 (2009). biodegradable COD (XS) and particulate inert COD (XI) are evalu- ated within the particulate range. Recently, a settleable COD frac-KEYWORDS: biodegradation, chemical oxygen demand fractions, de- tion (XSS) with a slower biodegradation rate was defined within thenitrification potential, domestic wastewater, primary settling, respirometricmodeling, settleable chemical oxygen demand. particulate COD for domestic wastewater (Okutman et al., 2001). In a study involving detailed particle size analysis, Dulekgurgen et al.doi:10.2175/106143009X425942 (2006) reported that, for domestic wastewater, most of the COD was in the size ranges above 0.45 lm, and only a relatively small part was in the soluble range.Introduction A detailed characterization is a prerequisite for understanding and Assessment of biodegradation characteristics of different organic interpreting the fate of pollutants in wastewaters. It should cover,fractions in wastewaters should be considered as one of the major aside from COD and its significant fractions, all relevant parametersresearch milestones in environmental science and technology. and especially particulate solids—total suspended solids (TSS) andBecause of the pioneering work of Dold et al. (1980) and Ekama volatile suspended solids (VSS)—in a way to establish basic mass balances defining useful ratios, such as COD/VSS, COD/N/P, and1 Department of Environmental Sciences and Engineering, Istanbul VSS/TSS, for treatment system design and operation (Orhon et al.,Technical University, Istanbul, Turkey. 1997; Rossle and Pretorius, 2001). From a practical standpoint, ¨2 Turkish Academy of Sciences, Ankara, Turkey. accurate assessment of influent composition is essential for the3 Lettinga Associates Foundation, Wageningen, Netherlands. design and operation of biological treatment units. In this respect,* Department of Environmental Sciences and Engineering, Istanbul Technical the removal rate in plain settling sets is another important factorUniversity, 80626 Ayazaga, Maslak, Istanbul, Turkey; e-mail: within the particulate range, to estimate the fate of different CODokutmand@itu.edu.tr. fractions and other parameters before biological treatment.July 2009 715
  2. 2. Tas et al.Table 1—Conventional characterization of domestic wastewater. This study Domestic wastewater in _ Istanbul (Orhon et al., 1997) Atakoya ¨ Baltalimania ¨ Kadikoy K.Cekmece ¸Parameter Mean 70% Range Mean 70% Range Mean Range Mean RangeCOD (mg/L) 406 445 295 to 535 353 368 314 to 408 450 220 to 775 400 345 to 480TKN (mg/L) 41 43 36 to 47 35 38 30 to 40 49 22 to 73 42 38.6 to 46.7NH3-N (mg/L) 27 28.8 19 to 34 20.4 23 11 to 26.0 30.5 25 to 39 24.78 22.4 to 30.4Total phosphorus (mg/L) 8.3 9.1 6.0 to 11.6 7.1 8.4 4.4 to 10.2 8.1 5.0 to 15 7.4 6.1 to 9.6TSS (mg/L)b 190 210 122 to 247 184 195 151 to 262 310 140 to 930 200 165 to 270VSS (mg/L)b 178 198 118 to 227 148 160 126 to 165 210 130 to 395 103 100 to 105Total dissolved solids (mg/L) 2874 2920 2600 to 3300 4486 4510 4200 to 4630 474 425 to 495 616 520 to 680pH 7.6 7.7 7.2 to 7.9 7.4 7.5 7.1 to 7.7 7.2 — 7.68 7.6 to 7.7 a Summer season, composite samples. b 1.2 lm. Biodegradation characteristics of different COD fractions also Materials and Methodsprovide essential information for activated sludge systems designed Survey Site. The study was conducted as part of a comprehen-for biological nutrient removal. In fact, in these systems, an impor- sive survey on the treatability-oriented characterization of domestictant role is attributed to organic carbon. Basically, denitrification wastewater within the metropolitan area of Istanbul, Turkey, nowrelies on the same principles as organic carbon removal under housing more than 12 million residents. The Istanbul Metropolitanaerobic conditions, except for the final different electron acceptors Area is located on the northern coast of the Marmara Sea and lies onutilized. However, the two systems have totally opposite objectives both sides of the Bosphorus strait, connecting the Black Sea to thewhen evaluated in terms of the corresponding treatment processes; Marmara Sea. The area has been the subject of similar studies as thein conventional aerobic activated sludge systems, in which the sole major polluting source in the Marmara basin (Gorgun et al., 1996;purpose is the organic carbon removal, the typical approach is to Orhon et al., 1997; Orhon, Sozen, and Ubay, 1994; Orhon, Uslu,reduce the organic load by means of primary settling, which Meric, Salihoglu, and Filibeli, 1994). The study was mainly carriedremoves, aside from suspended solids, the settleable fraction of the out at the Atakoy treatment plant, serving a population equivalent of ¨influent COD. In denitrification, which occurs under anoxic condi- 45 000 residents, now under extension for nutrient removal, wheretions, the main objective is to remove the final electron acceptor— a statistically significant number of composite samples (i.e., morenitrate—and system design should ensure that a stoichiometrically than 30 samples) were collected from the influent of the plant. Thesufficient amount of biodegradable COD is present for the reduction composite samples were collected in summer (sampling from 8 a.m.and removal of nitrate in the anoxic reactor. In this respect, organic until 5 p.m. each day), during dry weather, for a period of 3 monthscarbon assumes a different function as an essential ingredient for (July to September). The sampling sequence was arranged todenitrification. characterize all days of the week. In addition to the summer The function of biodegradable COD is best evaluated in terms characterization, five grab samples also were taken and analyzedof the denitrification potential (NDP), a parameter that reflects in December, to compare the summer and winter wastewater com-the nitrogen equivalent that may potentially consume nitrate positions. The study also included evaluation of wastewater samplesunder anoxic conditions. Basic process stoichiometry and process collected during the same summer period from the Baltalimanimodeling are commonly used for the assessment of NDP (Artan treatment plant, another significant wastewater discharge stationet al., 2002; Ekama and Marais, 1984; Sozen et al., 2002). The within the metropolitan area along Bosphorus. Analytical Measurements. All analyses were performedefficiency of denitrification greatly depends on the balance between according to Standard Methods (APHA et al., 1998). As prescribedNDP and the extent of available nitrate (NA) introduced to the anoxic in current activated sludge modeling, COD was used to characterizereactor volume. The merit of the removing settleable COD by wastewater organic matter. The soluble and particulate organicprimary settling requires serious consideration and reevaluation, matter was differentiated by filtration, using 0.45-lm celluloseespecially in cases where NDP becomes rate-limiting and additional acetate membrane filters. The COD measurements were performedorganic carbon becomes a significant asset. Obviously, the magni- as described in ISO6060 (International Organization for Standard-tude of the settleable COD fraction alone is not enough for this ization, 1986). Whatman (Kent, England) GF/C glass-fiber filtersevaluation, which basically depends on the biodegradation charac- (1.2 lm) were used for TSS and VSS measurements. In addition,teristics of all COD fractions involved. 0.45-lm cellulose acetate membrane filters were used to quantify In this context, the main objective of the study was to establish TSS and VSS in domestic wastewater, to compare the results witha conceptual basis for the assessment of the biodegradability and the soluble parameters that are defined as filtrate from 0.45-lmdenitrification potential of settleable COD. The conceptual approach cellulose acetate membrane filters.was illustrated using data derived from the domestic wastewater at Respirometric Modeling and Performance Simulation.Atakoy, Istanbul, Turkey. Detailed characterization, emphasizing ¨ Oxygen uptake rate (OUR) measurements were conducted with athe effect of primary settling on COD fractionation, and model Manotherm RA-1000 continuous respirometer (Nazareth, Belgium)simulation were also carried out as the necessary tools to conduct with a personal computer connection (Orhon and Okutman, 2003).this examination. Three sets of composite samples collected from the influent of716 Water Environment Research, Volume 81, Number 7
  3. 3. Tas et al.Table 1—(Extended) Domestic wastewater in _ Istanbul (Orhon et al., 1997) Domestic wastewater in Europe (Pons et al., 2002) Baltalimani Yenikapi France Austria Netherlands Sweden Norway Finland GermanyParameter Mean Range Mean Range Mean Mean Mean Mean Mean Mean MeanCOD (mg/L) 340 265 to 645 680 280 to 1480 634 526 450 477 233 559 548TKN (mg/L) 35 23.9 to 57 66 27 to 92 52 44 42 33.1 22 43.8 59NH3-N (mg/L) 19.9 10 to 26.3 37.74 24 to 48.8 — — — — — — —Total phosphorus (mg/L) 6.8 5 to 8.63 7 3.6 to 13 9.3 7.1 6.7 6.14 3 7.47 8TSS (mg/L)b 140 85 to 318 480 110 to 820 302 — 237 243 143 378 208VSS (mg/L)b 125 120 to 135 65 65 to 69 — — — — — — —Total dissolved solids (mg/L) 435 335 to 537 — — — — — — — — —pH 7.4 7.2 to 7.5 7.24 7.1 to 7.3 — — — — — — —the Atakoy treatment plant were used for the assessment of COD ¨ YNHD 5 net anoxic yield coefficient for heterotrophs (mgcell-fractions and evaluation of kinetic coefficients, as previously COD/mgCOD),described (Orhon et al., 2002). A 30-L composite sample was bHD 5 anoxic endogenous decay rate for heterotrophs (day21),collected from the influent of the treatment plant and subjected to and2 hours of gravity settling in a cylindrical reactor to simulate the hX 5 total sludge age (days).quality of fresh settled wastewater. Approximately 2 L of the settled The anoxic endogenous decay rate (bHD) is calculated by reducingwastewater fraction was withdrawn from the bottom of the reactor the aerobic endogenous decay coefficient with the endogenousfor the OUR measurements. The experiments were conducted at decay correction factor (gE), as follows:room temperature (228C). The pH was kept in the range 7.0 to 8.0,which is suitable for biological activity. Aeration was supplied bHD ¼ bH ÁgE ð2Þcontinuously during OUR measurements to maintain a sufficientdissolved oxygen concentration. The OUR data were collectedonline with a sampling frequency per minute. Experimental assess-ment of the kinetic coefficients was performed by model calibration Results and Discussion Conventional Characterization. Conventional characteriza-using the experimental OUR data. The COD fractionation and tion results of the domestic wastewater investigated in this studysoluble and particulate COD components of the wastewater were during the summer season are outlined in Table 1. The table in-determined according to the methods previously described (Ekama cludes previously reported literature data based on surveys con-et al., 1986; Insel et al., 2003; Orhon and Okutman, 2003; Orhon ducted on the domestic wastewater from important wastewateret al., 2002). discharge stations of Istanbul (Orhon et al., 1997) and representative A simulation study was performed to illustrate the biodegradation domestic wastewater characteristics for selected countries in Europecharacteristics of particulate slowly biodegradable COD (XS, XSS) (Pons et al., 2002). The results indicated that the average com-under different sludge ages (hX). The steady-state simulations were position of domestic wastewater at the Atakoy treatment plant, ¨performed using a conventional activated sludge system having which served as the focal point of the study, could be expressed asa single aerobic continuous stirred-tank reactor (CSTR) and a final 406 mg/L total COD, 190 mg/L TSS, 178 mg/L VSS, 41 mg/L totalclarifier. A simulation study was conducted using average waste- Kjeldahl nitrogen, 27 mg/L ammonia nitrogen (NH3-N), and 8.3water characterization and influent COD fractionation. Sludge age mg/L total phosphorus. The average pH of the domestic wastewater(hX) and hydraulic retention time (hH) were changed in parallel to was 7.6. Slightly lower values were observed for all the parameterskeep the mixed liquor suspended solids (MLSS) concentration at in the Baltalimani treatment plant, the other station considered inapproximately 4.0 kgSS/m3 in all simulations. The final clarifier this study (Table 1). Both stations represent residential areaswas assigned as a point settler, in which the mixed liquor is physi- generating municipal wastewater with no significant effect fromcally separated from the clarified effluent. The solids separation industrial activities. The mean values obtained for the Atakoy and ¨efficiency of the final clarifier was assumed to be 100%. The return Baltalimani wastewaters represent typical domestic wastewateractivated sludge rate was adjusted to unity. The AQUASIM prog- quality, which has not changed significantly over time. The strongerram developed by Reichert et al. (1998) was used in all simulations. character of the Yenikapi wastewater reported in the previous study The simulation results were processed to obtain the contributions indicates the significant effect of industrial discharges on wastewa-of different COD fractions to the denitrification potential (NDP) of ter quality. The appreciable quality spectrum observed among thethe system at different operating conditions and different anoxic different wastewater characteristics given in Table 1 underlines thevolume ratios (VD/V). The following simplified equation was used specific nature of wastewater quality affected by local conditions.to calculate the denitrification potential of both total and settleable The statistical distribution of major parameters for the Atakoy ¨COD: treatment plant influent is plotted in Figure 1. The plots exhibit ÁCS a regular trend for all parameters with a range of 5 to 10% deviation NDP ¼ ð1 À YNHD Þ and YNHD ¼ YHD =ð1 þ bHD ÁhX Þ ð1Þ 2:86 between mean and 70th-percentile values. The 70th-percentile values, representing the upper threshold level of 70% of the samplesWhere analyzed, are also indicated in Table 1. The 70th-percentile values CS 5 concentration of biodegradable COD (mg/L), are generally recommended and adopted for design, as they couldJuly 2009 717
  4. 4. Tas et al. ¨Figure 1—Statistical distribution of characteristic parameters in Atakoy domestic wastewater: (a) COD, (b) suspendedsolids, (c) total Kjeldahl nitrogen, and (d) total phosphorus (¤ 5 total, 5 30 minutes settled, Á 5 2 hours settled, andu 5 soluble).provide more meaningful and useful information compared with total COD or 43% of the particulate COD. After settling, 188 mg/Lmean values (Kayser, 1989; Orhon et al., 1998). Therefore, in of suspended or colloidal COD remained in the effluent, withthis study, evaluations for different parameters presented in the a different composition of 38% soluble COD and 62% particulatefollowing sections are based on the 70th-percentile values. COD. Thirty minutes of settling was roughly half efficient, only Comparison of the wastewater composition in the summer and providing 17% COD removal. The 0.45 to 1.2 lm size range waswinter seasons is shown in the following results, in terms of the observed to contain 7% of the COD content of domestic waste-suspended solids and COD concentrations that can be expected to water, which is a statistically significant amount that should bechange as a function of the seasonal activities. Because a significant considered for the evaluation of wastewater characterization.difference for nitrogen and phosphorus parameters was not expected Similar results also were observed in the winter season.during summer and winter seasons under dry-weather conditions, The soluble fraction of the total nitrogen was assessed as 78%. Athese two parameters were not investigated in the winter season. similar fraction of 80% was calculated for total phosphorus. Two Effect of Settling and Filtration on Wastewater Characteristics. hours of settling provided only 9% nitrogen removal, which cor-The nature and size distribution of significant conventional parameters responded to 42% of the initial particulate nitrogen. Similarly, 9%have been studied for the following four indices associated with of the total phosphorus and 44% of the particulate phosphorus wasdifferent size thresholds: (1) 30 minutes of settling, (2) 2 hours of removed. The removal efficiencies decreased to 5% for nitrogensettling, (3) 1.2-lm filtration, and (4) 0.45-lm filtration. The first two and 7% for phosphorus when the settling time was reduced tohave practical significance, as they illustrate the effect of primary 30 minutes. The specific nature of wastewater characteristics wassettling typically implemented before biological treatment of domestic confirmed, as the results of this study were only partially supportedwastewater. Filtration through 0.45-lm filters is commonly used to by similar observations reported in the literature. Odegaard (1997)differentiate soluble and particulate fractions and roughly approx- indicated that the filtered fraction (1 lm) in raw wastewater samplesimates the effect of chemical treatment (Henze et al., 2000). Filtration was typically 20 to 30% of the total COD, 30 to 40% of the totalthrough 1.2-lm filters is also a common analytical technique for phosphorus, and 75 to 85% of the total nitrogen. Tiehm et al. (1999)wastewater characterization. stated that, in raw wastewater and primary effluent, 45% of The results of settling and filtration experiments conducted on the COD and 35 to 80% of the phosphorus was associated withall samples and displayed in Table 2 indicated that 74 to 78% of suspended solids.the total COD could be considered of particulate nature based on Similarly, in the summer samples, based on quantification witha 0.45-lm size threshold for both the summer and winter seasons. membrane filters (0.45 lm), settling periods of 30 minutes and ofIn the summer samples, settling for 2 hours removed 32% of the 2 hours resulted in the removal of 33 and 47% TSS and 36 and 49%718 Water Environment Research, Volume 81, Number 7
  5. 5. Tas et al.Table 2—Significant parameters in domestic wastewater Table 3—Removal ratios of the significant parameters inafter settling and filtration (70% statistical values).a the primary settling (2 hours). Supernatant Percent removal after settling Filtrate TotalParameters (mg/L) Total 30 minutes 2 hours 1.2 lm 0.45 lm COD TKN phosphorus TSS (%) (%) (%) (%)Organics and nutrients Summer seasonb Atakoy wastewaters ¨ 32 9 9 47* (this study) COD 445 370 305 149 117 South African wastewaters 40 15 to 20 15 to 20 60 Total nitrogen 43 41 39 NM 33.5 (Rossle and Pretorius, 2001) ¨ Total phosphorus 9.1 8.5 8.3 NM 7.3 Riva/Istanbul wastewater 26 7 15 — Winter seasonc (Orhon, Sozen, and Ubay, 1994) COD 510 350 300 150 115 Kadikoy/Istanbul wastewater 33 9 25 63 (Orhon et al., 1997)TSS and VSS ATV131 (2000) 33 9 11 64 Summer seasonb (1.2 lm) TSS 210 144 114 NA NA * 0.45 lm. VSS 198 142 110 NA NA Summer seasonb (0.45 lm) TSS 230 153 123 NA NA rates for South African wastewater. These results merit further VSS 200 128 103 NA NA attention in terms of the settling properties of domestic wastewater Winter seasonc (1.2 lm) as it relates to the removal of a significant portion of the available TSS 195 168 129 NA NA organic carbon source. VSS 148 109 87 NA NA Assessment of Characteristic Parameters. Interpretation of Winter seasonc (0.45 lm) conventional characterization, in terms of significant ratios of sel- TSS 207 152 132 NA NA ected parameters, such as N/COD, P/COD, and VSS/COD, is quite VSS 160 131 102 NA NA useful, as it could be used for prediction of the biodegradability of a domestic wastewater. As reported in Table 4, the N/COD ratio NA 5 not applicable, NM 5 not measured. for Atakoy was calculated to be in the range 0.07 to 0.15 mgN/ ¨ b Composite samples. c mgCOD, with an average value of 0.11 mgN/mgCOD. The cor- Grab samples. responding value for Baltalimani was 0.10 mgN/mgCOD. These values coincide with the average N/COD ratios of 0.10 to 0.11VSS, respectively. Although measured concentrations were lower, mgN/mgCOD previously observed for Istanbul wastewater (Orhonsimilar removal ratios were obtained based on quantification with et al., 1997; Orhon, Sozen, and Ubay, 1994). They also are con-1.2-lm glass-fiber filters. In the winter season, although TSS con- sistent with the range 0.087 to 0.115 mgN/mgCOD reported forcentrations were very similar to those measured in the summer, VSS different domestic wastewaters in Europe (Pons et al., 2002). Theconcentrations were relatively lower. Based on a 0.45-lm analytical N/COD ratio is an important index for predicting the efficiency ofquantification, settling periods of 30 minutes and of 2 hours resulted denitrification in biological treatment. The overall mean N/CODin the removal of 27 and 36% TSS and 18 and 36% VSS, respec- ratios measured in this study represent the limit value of 0.1 mgN/tively. Thus, higher removal efficiencies for suspended solids were mgCOD, below which, a single sludge predenitrification system hasobtained in the summer samples as opposed to the winter samples. the potential of providing high nitrogen removal efficiency (OrhonThe difference in removal efficiencies can be attributed to different and Artan, 1994). Pitman (1991) and Randall et al. (1992) reportedcharacteristics of particulate pollutants during the wet season; to that, if the N/COD ratio is higher than 0.11 and the volatile fattyhigher temperatures in the summer season, which may decrease thefraction of XSS in the sewer because of the faster hydrolysis; and tovariation in the sampling method, which was composite sampling inthe summer season and grab sampling in the winter season. In general, filtration through 0.45-lm filters is used to differen-tiate soluble and particulate fractions. Although the use of 1.2-lmfilters is a common analytical technique, especially for the quantifi-cation of suspended solids, it could be more appropriate to use0.45-lm filters for all quantifications, to compare all the parameterson the same basis. The effect of primary settling, approximated by 2-hour settlingexperiments, on the removal of significant parameters, is outlined inTable 3, with similar results in the literature. A mass balance forthese parameters associated with primary settling is schematicallyillustrated in Figure 2. The removal efficiencies are generally com-patible with the results of similar studies in the Istanbul area, except Figure 2—The mean values of mass balance for COD,for phosphorus (Orhon, Sozen, and Ubay 1994; Orhon et al., 1997). nutrients, and suspended and volatile suspended solidsRossle and Pretorius (2001) have reported slightly higher removal ¨ in the wastewater after primary settling (*0.45 lm).July 2009 719
  6. 6. Tas et al.Table 4—Effect of settling and filtration on significant ratios for domestic wastewater. ¨ Atakoy Baltalimani Mean Standard deviation Range Mean Standard deviation RangeN/COD Total 0.11 0.02 0.07 to 0.15 0.10 0.01 0.09 to 0.11 30 minutes settled 0.13 0.02 0.1 to 0.2 0.12 0.01 0.12 to 0.13 2 hours settled 0.14 0.02 0.1 to 0.21 0.12 0.02 0.1 to 0.15 Soluble (0.45 lm) 0.31 0.05 0.2 to 0.41 0.17 0.02 0.15 to 0.2P/COD Total 0.021 0.004 0.017 to 0.028 0.020 0.006 0.013 to 0.027 30 minutes settled 0.026 0.005 0.013 to 0.037 0.025 0.006 0.017 to 0.032 2 hours settled 0.029 0.004 0.021 to 0.037 0.027 0.006 0.022 to 0.033 Soluble (0.45 lm) 0.067 0.018 0.027 to 0.089 0.012 0.036 0.015 to 0.043VSS/TSS Total 0.93 0.09 0.57 to 0.98 0.84 0.19 0.51 to 0.96 30 minutes settled 0.97 0.01 0.94 to 0.99 0.86 0.16 0.58 to 0.96 2 hours settled 0.97 0.02 0.92 to 0.99 0.89 0.16 0.61 to 0.96acid (VFA) content is low (,50 mg/L), an external carbon source commonly differentiated using filters with pore sizes ranging fromshould be used or prefermentation should be implemented. As ex- 0.45 to 1.8 lm (Henze et al., 2000). In this study, only a 0.45-lmpected, settling increased this ratio, as a result of significantly lower filter was used to quantify COD during the summer season,nitrogen removal compared with COD. After 2 hours of settling, the whereas, during the winter season, both 0.45- and 1.2-lm pore-mean N/COD ratio was measured as 0.14 mgN/mgCOD for Atakoy ¨ sized filters were used for particulate COD assessment. As shown inand 0.12 mgN/mgCOD for Baltalimani wastewater, which are both Table 5, iX was calculated as 0.66 mgVSS/mgCOD for both rawabove the limit for efficient predenitrification. and settled wastewater during the summer survey (0.45 lm). In the The mean value of the P/COD ratio for both the Atakoy and ¨ winter season, this ratio was found to increase to 0.79 mgVSS/Baltalimani wastewater was 0.02 mgP/mgCOD, which is also con- mgCOD for the same type of filter and to 0.70 for a 1.2-lm filter. Insistent with the range 0.014 to 0.019 given for European wastewaters the winter season, particulate VSS/COD ratio ranged from 0.73 to(Pons et al., 2002). Settling for 2 hours resulted in an increase 0.79 for the same type of filters (0.45 lm). According to the VSSof approximately 50%. Similarly, an increase has been reported in and COD measurements with 1.2-lm filters, the particulate VSS/the P/COD ratio from the range 0.015 to 0.025 to the range 0.02 to COD ratios were relatively small compared with the measurements0.03 in the settled sludge (Water Research Commission, 1984). with 0.45-lm filters. Although no difference was observed in the iX The VSS/SS ratio was 0.93 mgVSS/mgSS for Atakoy and 0.84 ¨ level in summer, a significant decrease was observed as a functionmgVSS/mgSS for Baltalimani, as summarized in Table 4, indicating of settling time in the winter samples. These values compare wellthat 7 to 16% of the suspended solids were of an inorganic nature. with the typical value of 0.75 mgVSS/mgCOD suggested for settledAs expected, settling was more effective in removing the inorganic wastewater in Activated Sludge Model No. 3 (ASM3) (Gujer et al.,suspended solids, as a result of higher settling velocities associated 2000). The particulate N/COD ratio, iXN, was similarly calculatedwith this fraction. as 0.03 mgN/mgCOD for both raw and settled wastewater for the The ratios of particulate fractions of significant parameters are summer samples, a level consistent with the default value of 0.04also quite important for system design and process modeling (Gujer mgN/mgCOD of ASM3, after primary settling (Gujer et al., 2000).et al., 2000). Table 5 outlines the values of two ratios, namely the The inorganic portion of the suspended solids, called fixed solids,ratio of VSS to particulate COD, iX, and of particulate nitrogen to XFS, also is an important parameter for the accurate estimation ofparticulate COD, iXN, for the Atakoy wastewater. In conventional ¨ biomass under different operating conditions. The effect of settlinganalysis, soluble and particulate components of the wastewater are on XFS and on the fixed solids fraction, fXFS, for the Atakoy ¨ ¨Table 5—Particulate nitrogen and COD fractions as a function of settling time in the Atakoy domestic wastewatertreatment plant. ix (g-VSS/g-COD) ixN (g-N/g-COD) Total 30 minutes settled 2 hours settled Total 30 minutes settled 2 hours settledSummer season 0.45 lma 0.66 0.66 0.03 0.03 0.03Winter season 0.45 lmb 0.79 0.76 0.73 — — — 1.2 lmb 0.70 0.68 0.63 — — —Literature ASM3 (Gujer et al., 2000) 0.75 0.04 a Composite samples. b Grab samples.720 Water Environment Research, Volume 81, Number 7
  7. 7. Tas et al. ¨Table 6—The ratios of fixed solids in the Atakoy domestic wastewater treatment plant as a function of settling time,where XFS15 [mg TSS/L - mg VSS/L] and fXFS5 [mg TSS/L - mg VSS/L]/[mg TSS/L]. Raw wastewater 30 minutes settled wastewater 2 hours settled wastewaterThis study XFS1 fXFS XFS1 fXFS XFS1 fXFSSummer season 0.45 lma 15 0.09 6 0.03 5 0.02 1.2 lma 11 0.09 4 0.03 3 0.02Winter season 0.45 lmb 52 0.16 25 0.14 20 0.14 1.2 lmb 36 0.16 20 0.15 11 0.12Literature Gujer and Kayser (1998) (1.2 lm) 0.2 to 0.3 Orhon et al. (1997) (1.2 lm) 100 0.3 — — 17 0.15 a Composite samples. b Grab samples.wastewater is shown in Table 6. The fixed solids fraction, fXFS, was specific biodegradation characteristics of settled COD. This requirescalculated as 0.09 for summer and 0.16 for winter samples. These COD fractionation primarily in relation with particle size rangesvalues are relatively lower than the 0.2 to 0.3 range suggested by (Dulekgurgen et al., 2006) and experimental assessment of the bio-Gujer and Kayser (1998). Settling induced a significant decrease in degradation characteristics of different COD fractions (Henze et al.,this ratio, to a level of 0.02 in summer and to 0.12 to 0.14 in winter. 2000; Okutman et al., 2001).Similar to these results, Orhon et al. (1997) reported a decrease in In this study, calibration of the OUR profile, now a widelythe fixed solids from 30 to 15% after 2 hours of settling. accepted and tested experimental instrument, was used for the Effect of Settling on Chemical Oxygen Demand Fractionation. assessment of COD fractions and corresponding biodegradationThe results of this study indicated that more than 30% of the COD kinetics. The ASM1, modified for the endogenous decay processcontent of the Atakoy domestic wastewater could be removed by ¨ and for the separate identification and hydrolysis of rapidlyprimary settling. While this removal could be quite beneficial in hydrolyzable COD (SH) and settleable COD (XSS), was adopteddecreasing the organic load of biological treatment, it also may pose as the basis for model calibration (Orhon, Cokgor, and Sozen, 1999;the problem of reducing the available organic carbon necessary for Orhon et al., 2002). The schematic description of the modifieddenitrification, where applicable. An accurate evaluation of the merit ASM1 in a matrix format is given in Table 7. The experiments wereof COD removal by primary settling can only be made in terms of carried out with a composite sample taken from the Atakoy treat- ¨Table 7—Matrix representation for endogeous decay model.Processes SO SNO SNH SS SH XS XSS XH XA SP XP Reaction rateHeterotrophs SH =XH Hydrolysis of SH 1 21 kh XH KX þ SH =XH XS =XH Hydrolysis of XS 1 21 khXS XH KXXS þ XS =XH XSS =XH Hydrolysis of XSS 1 21 khXSS XH KXXSS þ XSS =XH 1 À YH 1 ^ SS Aerobic growth À 2iXB À 1 lH XH YH YH KS þ SS 1 À YHD 1 ^ SS Anoxic growth À 2iXB À 1 gÁ lH XH 2:86ÁYHD YHD KS þ SS Aerobic endogenous decay 2(12fES2fEX) 21 fES fEX bHXH ð1 À fES À fEX Þ Anoxic endogenous decay À 21 fES fEX gÁbHXH 2:86Autotrophs 4:57 À YA 1 1 ^ SNH Aerobic growth À ÀiXB À 1 lA XA YA YA YA KNH þ SNH Aerobic endogenous decay 2(12fES2fEX) iXB(12fES2fEX) 21 fES fEX bAXAJuly 2009 721
  8. 8. Tas et al. ment plant. Soluble and particulate COD fractions were determined by means of model calibration and simultaneous evaluation of four different OUR profiles obtained from raw, settled, filtered waste- water and settled COD derived from the composite sample. Two parallel tests were carried out with two different initial food-to- microorganism (S0/X0) ratios for all types of samples. The con- ceptual basis of the evaluation procedure was previously explained, in detail, by Orhon et al. (2002). As illustrated in Figure 3, four different OUR profiles could be successfully calibrated with the same set of model coefficients. Modeling results indicated that the Atakoy wastewater also was quite typical, in terms of COD frac- ¨ tionation, as it involved 77% biodegradable COD (CS), with a soluble inert COD fraction (SI) of 7% and a particulate inert COD fraction (XI) of 16%. As expected, a small fraction—only 9%—was readily biodegradable (SS). The rest of the biodegradable COD was composed of 13% rapidly hydrolyzable COD (SH), 26% suspended slowly biodegradable COD (XS), and 29% settleable slowly bio- degradable COD (XSS). The correlation between conventional size distribution and COD fractionation is better visualized in Figure 4. The figure shows that the sum of SI 1 SS 1 SH does not extend beyond the 0.45 to 1.2 lm range, justifying the term soluble slowly biodegradable commonly adopted to define SH. Figure 4 also indicates that the settled COD fraction for this particular experiment was 37%, which is slightly higher than the average of all samples tested. Based on the COD fractionation for settled COD, settleable slowly biodegradable COD (XSS0) and settleable inert COD (XI0) represented 29 and 8% of the raw domestic wastewater COD, respectively. These results agree with previous observations, where approximately 180 mg/L of the particulate slowly biodegradable COD was of a settleable nature (Orhon et al., 2002). Thus, only 78% of the COD removed by primary settling was biodegradable in nature. Mass balance and the effect of primary settling on the COD fractions are shown in Figure 5. The concentrations and corre- sponding percent fractions reported in this figure are the average values of three different sets of experiments—one in this study and the other two previously reported by Okutman et al. (2001) and Orhon et al. (2002) for the same wastewater. The kinetic and stoichiometric coefficients used for modeling for the three sets of experiments are given in Table 8. The results in this table show good agreement between the OUR tests conducted on different samples of the Atakoy wastewater and provide a clear indication ¨ for the need of differentiating three slowly biodegradable COD fractions—the first in the soluble range, SH, the second of a suspended particulate nature, XS, and the third within the settled COD, XSS—associated with clearly different hydrolysis rates of 3.5, 1.7, and 1.0 day21, respectively. The biodegradation kinetics of the settled COD fraction also show that the carbon source directly derived from primary settling may not be very attractive for both nitrogen and phosphorus removal processes. Fermentation of this settled COD fraction, in sewers with long retention times or in the anaerobic zone of the wastewater treatment plant, or in prefer- menters, if properly controlled, is recognized as one of the cheapest ways of generating additional readily biodegradable COD by con- verting slowly biodegradable COD into more easily biodegradable components (Bannister and Pretorius, 1998; HatziconstantinouFigure 3—Model simulation of the OUR profiles: (a) et al., 1996; Moser-Engeler et al., 1998; Munch and Koch, 1999). ¨filtered wastewater (0.45 lm), F/M ratio 5 0.05 gCOD/ In a recent study involving a detailed investigation of the potentialgVSS; (b) settled wastewater, F/M ratio 5 0.07 gCOD/ of primary sludge fermentation for the generation of readily bio-gVSS; (c) raw wastewater, F/M ratio 5 0.1 gCOD/gVSS; degradable substrate, Cokgor et al. (2006) reported that un-and (d) settled COD, F/M ratio 5 0.2 gCOD/gVSS. controlled fermentation converted 22% of the initial VSS in the722 Water Environment Research, Volume 81, Number 7
  9. 9. Tas et al.Figure 4—Schematic representation of COD fractionation and size distribution in domestic wastewater.sludge into soluble biodegradable COD, and approximately 85% of orthis soluble COD was associated with the formation of short-chain hH ÁX SS X SSVFAs. 0 ¼ X SSin À À hH ÁkhXSS XH ð4Þ Effect of System Design on the Biodegradation of Settleable hX K XXSS ÁX H þ X SSChemical Oxygen Demand. In ASM models, the degradation of Whereparticulate organic matter is described using surface-saturation-typehydrolysis kinetics, as shown in Table 7. Basically, the hydrolysis Q 5 influent flowrate (L/d),rate is controlled by two kinetic parameters—(1) the maximum V 5 volume of biological reactor (L), and XH 5 active heterotrophic biomass (mgcellCOD/L).hydrolysis rate (kh), and (2) the half-saturation coefficient forhydrolysis (KX). The type of reaction becomes zero-order (khÁXH) Thus, to calculate the overall removal efficiencies by correcting forwhen the substrate (XS or XSS) is abundant in the bulk compared the accumulation effect resulting from sludge age, the particulatewith the active biomass (XH). On the other hand, the reaction rate biodegradable COD concentrations (XS, XSS) shown in Figure 6can be expressed as first-order (r 5 khÁXS) at low substrate levels were multiplied with the ratio hH/hX.(low XS/XH), where kH represents the kh/KX ratio. As shown in Figure 6a, significant removal of slowly biodegrad- A model simulation has been conducted to verify the effect of able COD (XS, XSS) can be obtained for hX levels above 2 days. Thesystem design on the biodegradation of the hydrolyzable COD hydrolysis reaction can be assumed to be first-order, because thefractions in a conventional activated sludge system. The simulation hydrolyzable COD concentration (XS, XSS) can be neglected com-was performed for a CSTR with the operating conditions, namely pared with the active biomass concentration (XH), where the kh andfor different sludge age (hX, days) and hydraulic retention time (hH, KX parameters both influence the hydrolysis rate. Under thesehours) couples. The hX/hH ratio was kept constant during the conditions, the removal efficiency was found to be approximatelysimulations. The sludge age and hydraulic retention time couples 90%, both for XS and XSS. At higher hX values, the correspondinghave been selected such that the hX/hH ratio maintains an averageMLSS concentration of 4.0 kgMLSS/m3 in the reactor. Figures 6 and 7 show the simulation outputs for the particulatebiodegradable COD fractions (XS, XSS) and the effluent quality(total soluble COD, ST), with respect to different hX and hH couplesunder both aerobic and anoxic conditions, respectively. In Figures 6and 7, the x-axis defines, with the sludge age, the correspondinghydraulic retention time, expressed in terms of hours, as explainedpreviously. The raw wastewater characteristics (as influent) andmodel parameters used in the simulations were adopted from Figure4 and Tables 8 and 9. Under steady-state conditions, a mass-balanceequation on XSS in a conventional activated sludge system can be Figure 5—The mean values of the COD fractions in thewritten as follows: wastewater after primary settling (reported values are the mean values of the three sets of experiments—one set in dX SS VÁX SS X SS this study and two sets from Okutman et al., 2001 and ¼ 0 ¼ QÁX SSin À À VÁkhXSS X H ð3Þ dt hX K XXSS ÁX H þ X SS Orhon et al., 2002).July 2009 723
  10. 10. Tas et al.Table 8—Kinetic and stoichiometric coefficients used for the modeling of aerobic activated sludge systems. bH kh khXS khXSS lHmax YHSet no. (day21) fE (day21) KX (day21) KXXS (day21) KXXSS KS (day21) (gcellCOD/gCOD)This study 0.2 0.2 3.2 0.04 1.4 0.28 1.0 0.10 3 3.5 0.671a 0.2 0.2 1.6 0.05 — — 0.7 0.05 3 4.2 0.672b 0.2 0.2 3.8 0.20 1.9 0.18 1.2 0.10 6 3.5 0.67Mean — — 3.5 0.12 1.7 0.2 1.0 0.08 — — — a Data taken from Orhon et al. (2002). b Data taken from Okutman et al. (2001).kh/KX ratios exhibit an effect on the hydrolysis of XS and XSS at values (,2 days). As shown in Figure 6a, the XSS accumulation wasnearly similar orders of magnitude. much higher than the XS level, because the low kh parameter played The differences in removal efficiencies with respect to XS and XSS a dominant role in the overall hydrolysis rate. For example, theare much more pronounced when the system is operated at lower hX COD removal efficiencies for XS and XSS corresponded to 60 andFigure 6—Steady-state simulation of the biodegradation Figure 7—Steady-state simulation of the soluble biodegrad-of particulate slowly biodegradable COD fractions under able COD components and the fate of total soluble COD(a) aerobic conditions and (b) anoxic conditions. under (a) aerobic conditions and (b) anoxic conditions.724 Water Environment Research, Volume 81, Number 7
  11. 11. Tas et al.35%, respectively, at 0.5 days of hX. A lower maximum hydrolysis Table 9—Kinetic and stochiometric parameters used inrate (kh 51.0 day21) of XSS caused more accumulation in the the calculation of NDP under anoxic conditions.reactor, despite the close influent COD levels of XS and XSS. Thesimulation also indicates that both XS and XSS became completely Parameters Symbol Value Unithydrolyzed and removed beyond the sludge age of 2.0 days. Figure Anoxic yield coefficient YHD 0.54 mgcellCOD/mgCOD6b shows the simulation results for the removal of biodegradable Endogenous decay rate* bH 0.20 day21particulate COD fractions under anoxic conditions. The removal of Anoxic growth correctionslowly biodegradable COD (XS, XSS) of above 95% can be obtained factor gg 0.80 — Anoxic hydrolysis ratefor hX levels above 3 days. As presented in the simulation results correction factor gH 0.60 —under aerobic and anoxic conditions, the degradation rate of slowly Endogenous decaybiodegradable COD fractions is relatively slower under anoxic correction factor gE 0.48 —conditions, as a result of the reduced rates of hydrolysis processes.The anoxic hydrolysis rates are 60% lower than the aerobic * Converted from death regeneration concept.hydrolysis rates because of the anoxic hydrolysis rate correctionfactor gH (Barker and Dold, 1997; Henze et al., 2000). Figure 7 illustrates the effluent quality simulations, with respect of XSS is achieved. Thus, the maximum contribution of theto different hX and hH couples, under both aerobic and anoxic settleable biodegradable COD (XSS) to the total NDP generated in theconditions, respectively. The dashed line indicates an SI level of system can be as high as 40% for the domestic wastewater studied.32 mg/L. The simulation study showed that nearly complete The maximum contribution of XSS is dependent on the design anddegradation of soluble biodegradable COD can be achieved for operating conditions. While the NDP of XSS is maximized at a totalhX.2 days under both aerobic and anoxic conditions. Considering sludge age of 6 days for a VD/V of 0.5, it can only be maximized ifthe soluble inert COD baseline, the difference of 10 mg/L COD is the system is operated at total sludge ages higher than 15 days andthe result of the contribution of rapidly hydrolyzable COD (SH) with at a VD/V of 0.2. In addition, it should be emphasized that, in theinert soluble microbial products (SP). Total effluent soluble COD presence of a primary settler as a system component, shorter settling(ST) rapidly increased as a result of the incomplete degradation times would introduce some XSS to the system, resulting in anof rapidly hydrolyzable COD- SH for hX levels below 1.0 day. The increase of NDP, depending on the settling efficiency and systemtotal effluent soluble COD (ST) and effluent rapidly hydrolyzable conditions. This may be needed for cases where NDP acts as theCOD (SH) are slightly higher under anoxic conditions, as a result of limiting factor for the denitrification efficiency.the slower degradation of SH, as given in Figure 7b. Effect of Settleable Chemical Oxygen Demand onDenitrification Potential. The denitrification potential (NDP) isa measure of the electron acceptor (nitrate) demand of the available Conclusionsorganic carbon under anoxic conditions. From a modeling standpoint, The results of the respirometric evaluations in this study enabledit is controlled by the readily biodegradable COD fraction, as nitrate the establishment of a mass balance for the significant CODdemand is associated with processes related to biomass growth and fractions of domestic wastewater. In this context, the effect ofdecay. Consequently, hydrolysis of slowly biodegradable COD primary settling was assessed, not only as overall COD removal, butcompounds is the rate-limiting step in the development of NDP, with also in terms of a new COD fraction, the settleable COD. This18 significant operating parameters related to the anoxic phase, such fraction, removed by primary settling, was identified as a slowlyas the sludge age (hX) and the anoxic volume ratio (VD/V). biodegradable substrate with a low hydrolysis rate of 1.0 day21, The model simulation results under anoxic conditions were used providing a clear differentiation from the other particulate slowlyto evaluate the relative contribution of the settleable biodegradable biodegradable COD components.COD fraction to the overall NDP of the system. The results have The efficiency of the primary settling is expected to be site-shown that the denitrification potential (NDP) increases with in- specific. Accordingly, the results related to the fate of conventionalcreasing VD/V ratios, as shown in Figure 8a. No significant increase parameters, reported in this study, relate only to the selectedin NDP is observed above an anoxic sludge age of 3 days (i.e., total domestic wastewater. However, the proposed approach introducedsludge age of 6 days for VD/V 5 0.5), where almost all biode- a generally applicable new concept of evaluating the settled CODgradable COD is consumed by heterotrophic biomass. The gradual as a separate entity, which is well-defined in terms of its bio-increase in NDP with increasing sludge ages above an anoxic sludge degradation characteristics. The settleable (biodegradable) CODage of 3 days is the result of a similar increase in electron acceptor (XSS) was incorporated to a multicomponent model as a new modelconsumption of decaying biomass (because the concentration of component, with its hydrolysis kinetics. The fate of this CODbiomass in the system increases with increasing sludge age). How- fraction could then be evaluated by means of model simulation,ever, for single sludge systems, denitrification at low sludge ages which indicated that it would be totally hydrolyzed and removedwill not be possible, because the nitrifiers will be washed out from at a sludge age of 2 days. Model simulation, accounting for thethe system under such short aerobic sludge ages. In addition, the biodegradation kinetics respirometrically determined for XSS, withsystem would be limited by the available amount of biodegradable similar characteristics of other COD components, identified a newCOD at anoxic sludge ages less than 3 days, as a result of incom- dimension of the settleable COD fraction as a possible source ofplete hydrolysis of XSS. The NDP generated by the degradation of additional organic carbon that could directly contribute to theXSS for different sludge ages and VD/V ratios is presented in Figure denitrification potential of the system, if the total sludge age and8b. The figure shows that the NDP contribution of the XSS com- the VD/V are adjusted to secure an anoxic sludge age of over 2 days.ponent increases with increasing VD/V ratios and an increasing The result challenges the function of primary settling, especially inanoxic sludge age of 3 days, where almost complete biodegradation activated sludge systems, where the denitrification potential asso-July 2009 725
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