Front. Environ. Sci. Engin. China 2007, 1(1): 57-62 DOI10.1007/s11783-007-0011-7 RESEARCH ARTICLE Classification of contaminants and treatability evaluation of domestic wastewater WANG Xiaochang(<), JIN Pengkang', ZHAO Hongmei, MENG Lingba I School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China 2 Beishiqiao Wastewater Purification Center, Xi'an 710003, China C Higher Education Press and Springer-Verlag 2007 Abstract Long-term sampling and analysis were conducted particle separation [4], utilization of inorganic coagulants in a domestic wastewater treatment plant for the investigation and polymers [5], post filtration [6], and post disinfection [7 on the characteristics of the representative contaminants inin the enhanced primary treatment process. Chemically raw sewage such as SS, COD, BODs, TP, and TN. All these enhanced primary treatment is also recommendable as the constituents were classified into dissolved and suspended first step of wastewater treatment prior to biological treatment groups by using a 0. 45-um membrane filter, and the concen- to achieve higher removal of organics and nutrients [8, 9 tration of each constituent in each group was analyzed. As a In general, organic and inorganic substances in the result, almost 100% of the Ss was found to be suspended domestic sewage may include both suspended and dissolved matter, as well as about 65% of COD, 60% of BOD,, 50% fractions, and the suspended fraction can be easily removed of P, and 20% of N. All these could be easily removed by by physical and/or physiochemical processes under most sedimentation or coagulation/sedimentation. A treatability conditions. However, some dissolved substances may attach evaluation diagram was proposed for a rational selection of on to the suspended particles. Therefore, as long as the suspen- wastewater treatment process in accordance with raw water ded particles can be effectively removed, the originall dissolved matter may also be removed substantially [1, 10 Regarding a selection of treatment technology, especially Keywords domestic wastewater, dissolved matter, sus- when primary or enhanced primary treatment is considered as pended matter, treatability evaluation the sole process for achieving certain water quality goals there is often a dispute on the evaluation of the treatment effect due to discrepancies of past experiences of different 1 Introduction people dealing with different wastewaters. The lack of a theo- retic base for contaminant classification and treatability eval- The contaminants in domestic sewage can be divided into uation has influenced a rational selection and optimization of three categories: suspended solids (SS),organic matter the wastewater treatment process [1l (chemical oxygen demand or biochemical oxygen demand), In this paper, the authors used the domestic sewage at and nutrients(nitrogen and phosphorus), which are the sub- the Beishiqiao Wastewater Purification Center, Xi'an,China stances to be removed by conventional and/or advanced treat- as an example, and through long-term water quality analysis ment for the purposes of discharge or treated water reuse. investigated the distribution of contaminants of different conventional treatment processes often consist of primary categories and different fractions. On this basis, a water treatment(physical process)and secondary treatment(bio- quality matrix was formulated and the treatability of con- logical process). In recent years, enhanced primary treatment taminants by primary, enhanced primary, and secondary that utilizes a chemical coagulant to assist the removal of sus- treatments was evaluated pended and dissolved contaminants, has drawn wide attention for wastewaters that are not amenable to conventional bio logical treatment[1], especially in developing countries [2, 3]. 2 Materials and methods Many studies have been conducted on the optimization of 2.1 Raw domestic sewage [译自:给水排水] Raw domestic sewage used for this study was from the inlet
Front. Environ. Sci. Engin. China 2007, 1(1): 57–62 DOI 10.1007/s11783-007-0011-7 RESEARCH ARTICLE Classification of contaminants and treatability evaluation of domestic wastewater WANG Xiaochang ( )1 , JIN Pengkang1 , ZHAO Hongmei1 , MENG Lingba2 1 School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China 2 Beishiqiao Wastewater Purification Center, Xi’an 710003, China © Higher Education Press and Springer-Verlag 2007 Abstract Long-term sampling and analysis were conducted in a domestic wastewater treatment plant for the investigation on the characteristics of the representative contaminants in raw sewage such as SS, COD, BOD5, TP, and TN. All these constituents were classified into dissolved and suspended groups by using a 0.45-µm membrane filter, and the concentration of each constituent in each group was analyzed. As a result, almost 100% of the SS was found to be suspended matter, as well as about 65% of COD, 60% of BOD5, 50% of P, and 20% of N. All these could be easily removed by sedimentation or coagulation/sedimentation. A treatability evaluation diagram was proposed for a rational selection of wastewater treatment process in accordance with raw water quality. Keywords domestic wastewater, dissolved matter, suspended matter, treatability evaluation 1 Introduction The contaminants in domestic sewage can be divided into three categories: suspended solids (SS), organic matter (chemical oxygen demand or biochemical oxygen demand), and nutrients (nitrogen and phosphorus), which are the substances to be removed by conventional and/or advanced treatment for the purposes of discharge or treated water reuse. A conventional treatment processes often consist of primary treatment (physical process) and secondary treatment (biological process). In recent years, enhanced primary treatment that utilizes a chemical coagulant to assist the removal of suspended and dissolved contaminants, has drawn wide attention for wastewaters that are not amenable to conventional biological treatment [1], especially in developing countries [2,3]. Many studies have been conducted on the optimization of particle separation [4], utilization of inorganic coagulants and polymers [5], post filtration [6], and post disinfection [7] in the enhanced primary treatment process. Chemically enhanced primary treatment is also recommendable as the first step of wastewater treatment prior to biological treatment to achieve higher removal of organics and nutrients [8,9]. In general, organic and inorganic substances in the domestic sewage may include both suspended and dissolved fractions, and the suspended fraction can be easily removed by physical and/or physiochemical processes under most conditions. However, some dissolved substances may attach on to the suspended particles. Therefore, as long as the suspended particles can be effectively removed, the originally dissolved matter may also be removed substantially [1,10]. Regarding a selection of treatment technology, especially when primary or enhanced primary treatment is considered as the sole process for achieving certain water quality goals, there is often a dispute on the evaluation of the treatment effect due to discrepancies of past experiences of different people dealing with different wastewaters. The lack of a theoretic base for contaminant classification and treatability evaluation has influenced a rational selection and optimization of the wastewater treatment process [11]. In this paper, the authors used the domestic sewage at the Beishiqiao Wastewater Purification Center, Xi’an, China as an example, and through long-term water quality analysis, investigated the distribution of contaminants of different categories and different fractions. On this basis, a water quality matrix was formulated and the treatability of contaminants by primary, enhanced primary, and secondary treatments was evaluated. 2 Materials and methods 2.1 Raw domestic sewage Raw domestic sewage used for this study was from the inlet of the Beishiqiao Wastewater Purification Center, Xi’an, Translated from Water & Astewater Engineering, 2004, 30(9): 38–41 [译自: 给水排水] E-mail: xcwang@xauat.edu.cn
58 China. During the experimental period, the average values of were compared with that of the secondary treatment in the the main water quality items were as follows: the concentra- wastewater treatment plant where Kruger Oxidation Ditch tions ofSS, COD, BOD, TN, NH3-N, NO3-N, and TP were(BioDenipho Process) was applied 162.3mg/L,275.8mgL,134.7mg,388mg/L,262mg/L, 0.48 mg/L, and 8.16 mg/L, respectively, pH was 7.6 2.4.1 Primary treatment 2.2 Classification of suspended and dissolved substances The primary treatment experiment was conducted using a 1-L measuring cylinder where the raw sewage sample was placed In this study, each substance in the raw sewage as well as to settle for I h, and then the supernatant was collected for in the treated water was roughly classified into suspended analysis and dissolved fractions using a 0. 45-um membrane filter Those that could pass through the 0. 45 um filter were taken 2.4.2 Enhanced primary treatment as dissolved substances, and those that were retained by the 0.45 um filter were taken as suspended substances The enhanced primary treatment was conducted using a standard jar-tester with polyaluminium chloride(PAc, 23% 2.3 Chemical analysis aS Al,O3)as coagulant. The optimum Pac dose was pre- determined by comparing the residual COD in the coagulated Chemical analysis in this study was conducted regarding Ss, and settled water as 70 mg/L(dry weight)or 8.5 mg/L (alumi COD, BODs, TN, NH*-N, NO3-N, NO2-N, and TP of the num ion). The operational condition was set as: rapid mixing raw sewage and treated water. Methods utilized are shown in (120 r/min) for 1 min, slow mixing(45 r/min)for 15 min, and Table 1 (the numbers in the brackets are the codes of Chinese settling for 30 min National Standards) Table 1 Chemical analysis for the raw sewage and treated water 3 Classification of contaminants in the raw sewage gravimetric method(GB 11901-89 Dichromate method (GB 11914-89) 3.1 Suspended solids 5-day Bod test Alkaline potassium per-sulfate digestion-UV After filtration using the 0. 45-um filter, the Ss concentration spectrophotometric method( GB11894-89) of the filtrate was almost zero, indicating that all the solid Nessler's reagent colormetric method(GB7974-87) particles in the raw sewage were above 0.45 um in size (GB7480-87) 3.2 COD and BOD NO --N EDTA spectrophotometric method( GB7973-87) mmonium molybdate spectrophotometric method (GB11893-89) Figures I and 2 show the fractions of dissolved and suspended substances in the raw sewage regarding COD and BOD respectively. From the variations of the concentrations of the 2.4 Experimental methods total, dissolved and suspended parts and their average values it is understood that of the organic substances represented In order to evaluate the treatability of various contaminants by COD and BOD,, 66% and 62% were retainable by the in the raw sewage, experiments of primary treatment and 0.45-um filter either due to their own size distribution or their enhanced primary treatment were conducted. The results attaching to the filterable suspended particles D Dissolved Ave. 93.7 mg/L I 234567891012131415161718192021222324252627282930313233 Fig. 1 Suspended and dissolved fractions of COD in the raw sewage
58 China. During the experimental period, the average values of the main water quality items were as follows: the concentrations of SS, COD, BOD5, TN, NH3 + –N, NO3 − –N, and TP were 162.3 mg/L, 275.8 mg/L, 134.7 mg/L, 38.8 mg/L, 26.2 mg/L, 0.48 mg/L, and 8.16 mg/L, respectively; pH was 7.6. 2.2 Classification of suspended and dissolved substances In this study, each substance in the raw sewage as well as in the treated water was roughly classified into suspended and dissolved fractions using a 0.45-μm membrane filter. Those that could pass through the 0.45 μm filter were taken as dissolved substances, and those that were retained by the 0.45 μm filter were taken as suspended substances. 2.3 Chemical analysis Chemical analysis in this study was conducted regarding SS, COD, BOD5, TN, NH3 + –N, NO3 − –N, NO2 − –N, and TP of the raw sewage and treated water. Methods utilized are shown in Table 1 (the numbers in the brackets are the codes of Chinese National Standards). Table 1 Chemical analysis for the raw sewage and treated water Items Methods SS Gravimetric method (GB 11901-89) COD Dichromate method (GB 11914-89) BOD5 5-day BOD test TN Alkaline potassium per-sulfate digestion—UV spectrophotometric method (GB11894-89) NH3 + –N Nessler’s reagent colormetric method (GB7974-87) NO3 − –N phenoldisulfonic acid spectrophotometric method (GB 7480-87) NO2 − –N EDTA spectrophotometric method (GB7973-87) TP Ammonium molybdate spectrophotometric method (GB 11893-89) 2.4 Experimental methods In order to evaluate the treatability of various contaminants in the raw sewage, experiments of primary treatment and enhanced primary treatment were conducted. The results were compared with that of the secondary treatment in the wastewater treatment plant where Kruger Oxidation Ditch (BioDenipho Process) was applied. 2.4.1 Primary treatment The primary treatment experiment was conducted using a 1-L measuring cylinder where the raw sewage sample was placed to settle for 1 h, and then the supernatant was collected for analysis. 2.4.2 Enhanced primary treatment The enhanced primary treatment was conducted using a standard jar-tester with polyaluminium chloride (PAC, 23% as Al2O3) as coagulant. The optimum PAC dose was predetermined by comparing the residual COD in the coagulated and settled water as 70 mg/L (dry weight) or 8.5 mg/L (aluminum ion). The operational condition was set as: rapid mixing (120 r/min) for 1 min, slow mixing (45 r/min) for 15 min, and settling for 30 min. 3 Classification of contaminants in the raw sewage 3.1 Suspended solids After filtration using the 0.45-μm filter, the SS concentration of the filtrate was almost zero, indicating that all the solid particles in the raw sewage were above 0.45 μm in size. 3.2 COD and BOD Figures 1 and 2 show the fractions of dissolved and sus pended substances in the raw sewage regarding COD and BOD, respectively. From the variations of the concentrations of the total, dissolved and suspended parts and their average values, it is understood that of the organic substances represented by COD and BOD5, 66% and 62% were retainable by the 0.45-μm filter either due to their own size distribution or their attaching to the filterable suspended particles. Fig. 1 Suspended and dissolved fractions of COD in the raw sewage
a Dissolved Ave 51.3 mg/L 2345678910I12131415l6171819202122 Fig 2 Suspended and dissolved fractions of BOD, in the raw sewag d Dissolved Ave 4.06 mg/ D Suspended Ave, 4. 10 mg/L IAnihhnllbhilnhhi Fig 3 Suspended and dissolved fractions of total phosphorus in the raw sewage 3.3 Total phosphorus and total nitrogen 3.4 Composition of nitrogen Figure 3 shows the fractions of dissolved and suspended Figure 5 shows the composition of nitrogen in the raw phosphorus in the raw sewage. In most measurements, the sewage. Inorganic nitrogen took 68.8% of the tn while concentration of the suspended fraction was higher than that organic nitrogen took 31.2%. Of the inorganic nitrogen, the of the dissolved fraction, while they were about the same dissolved and suspended fractions were 87.8% and 12.2%, on average(50.2% and 49.8%, respectively ) Regarding total respectively. And of the organic nitrogen, the dissolved and nitrogen(TN), the dissolved fraction was much larger than suspended fractions were 59.6% and 40.4%, respectively the suspended fraction in each water sample( Fig 4), while on The inorganic nitrogen mostly took the form of NH3-N average, the percentage of the dissolved fraction and that of(about 98%)with the other as no3-N. There was no NO2-N the suspended fraction were 79.6% and 20.4%, respectively. detected from the raw sewage O Dissolved Ave 30.9 mg/ D Suspended Ave. 7.9 mg/ 1234567891011213141516171819202 Sample number Fig.4 Suspended and dissolved fractions of total nitrogen in the raw sewage
59 3.3 Total phosphorus and total nitrogen Figure 3 shows the fractions of dissolved and suspended phosphorus in the raw sewage. In most measurements, the concentration of the suspended fraction was higher than that of the dissolved fraction, while they were about the same on average (50.2% and 49.8%, respectively). Regarding total nitrogen (TN), the dissolved fraction was much larger than the suspended fraction in each water sample (Fig. 4), while on average, the percentage of the dissolved fraction and that of the suspended fraction were 79.6% and 20.4%, respectively. 3.4 Composition of nitrogen Figure 5 shows the composition of nitrogen in the raw sewage. Inorganic nitrogen took 68.8% of the TN while organic nitrogen took 31.2%. Of the inorganic nitrogen, the dissolved and suspended fractions were 87.8% and 12.2%, respectively. And of the organic nitrogen, the dissolved and suspended fractions were 59.6% and 40.4%, respectively. The inorganic nitrogen mostly took the form of NH3 + –N (about 98%) with the other as NO3 − –N. There was no NO2 − –N detected from the raw sewage. Fig. 2 Suspended and dissolved fractions of BOD5 in the raw sewage Fig. 3 Suspended and dissolved fractions of total phosphorus in the raw sewage Fig. 4 Suspended and dissolved fractions of total nitrogen in the raw sewage
口 Inorganic Ace.68.8% D Organic Ace. 31.2% 4567891012131415161718192021 Sample number Fig 5 Inorganic and organic fractions of total nitrogen in the raw sewag Dissolved 045pm Organic D-O Fig. 6 Classification of pollutants by a 2 x 2 water quality matrix 4 Treatability evaluation of contaminants in ecause a 0. 45-um filter was used for the classification domestic sewage of suspended and dissolved substances, the existence of upended solid particles in the raw sewage might have much 4. 1 Formulation of a water quality matrix influenced the fractions of BOD, cod, TP, and TN, as shown in Fig. 6. if we consider that even dissolved substances might Based on the results mentioned above, a water quality matrix be attached to the surface of the suspended particles [9] was formulated as shown in Fig. 6. The figure is a 2 x2 Therefore, Fig. 6 might not represent the real state of the matrix and can thus classify each of the contaminants into individual substance existing in the sewage. However, from the viewpoint of treatment, such a water quality matrix 4 groups, namely dissolved-organic(D-O), suspended-organ- was still useful to assist the selection of suitable wastewater ic(S-O), dissolved-inorganic(D-I)and suspended-inorganic (S-I)substances. The differences of organic SS. COD and treatment processes total phosphorus(TP) from inorganic ones were conducted principally by ignition method 4.2 Treatability of contaminants of suspended and As shown in Fig. 6, in the raw sewage investigated, dissolved fractions SS included both organic and inorganic fractions at about 45% and 55%, respectively. BOD, represented biodegradable Table 2 compares the removals of the total, suspended, and organics of which 62% belonged to the suspended fraction dissolved fractions by the primary, enhanced primary, and and of which the remaining 38% belonged to the dissolved secondary treatment processes based on this study. By the fraction. COD was composed of both organic substances and primary treatment experiment, i.e., plain sedimentation, reductive inorganic substances that consumed oxygen. The the average removal of Ss was 59.8%. Regarding COD percentages of the suspended and dissolved fractions of Cod and BODs, the overall removals were 39.6% and 38.9%, were 66% and 34%, respectively, which are very similar to respectively, and all the removed contaminants were found that of BODs. Regarding TP, about 60% was inorganic, and to belong to the suspended fraction. Considering that the its suspended and dissolved fractions were almost equal Of suspended COD and BODs were about 66% and 62%, the TN, the inorganic and dissolved fraction took most parts. respectively, as shown in Fig. 6, the removal percentages
60 4 Treatability evaluation of contaminants in domestic sewage 4.1 Formulation of a water quality matrix Based on the results mentioned above, a water quality matrix was formulated as shown in Fig. 6. The figure is a 2x2 matrix and can thus classify each of the contaminants into 4 groups, namely dissolved-organic (D-O), suspended-organic (S-O), dissolved-inorganic (D-I) and suspended-inorganic (S-I) substances. The differences of organic SS, COD, and total phosphorus (TP) from inorganic ones were conducted principally by ignition method. As shown in Fig. 6, in the raw sewage investigated, SS included both organic and inorganic fractions at about 45% and 55%, respectively. BOD5 represented biodegradable organics of which 62% belonged to the suspended fraction and of which the remaining 38% belonged to the dissolved fraction. COD was composed of both organic substances and reductive inorganic substances that consumed oxygen. The percentages of the suspended and dissolved fractions of COD were 66% and 34%, respectively, which are very similar to that of BOD5. Regarding TP, about 60% was inorganic, and its suspended and dissolved fractions were almost equal. Of the TN, the inorganic and dissolved fraction took most parts. Because a 0.45-μm filter was used for the classification of suspended and dissolved substances, the existence of suspended solid particles in the raw sewage might have much influenced the fractions of BOD5, COD, TP, and TN, as shown in Fig. 6, if we consider that even dissolved substances might be attached to the surface of the suspended particles [9]. Therefore, Fig. 6 might not represent the real state of the individual substance existing in the sewage. However, from the viewpoint of treatment, such a water quality matrix was still useful to assist the selection of suitable wastewater treatment processes. 4.2 Treatability of contaminants of suspended and dissolved fractions Table 2 compares the removals of the total, suspended, and dissolved fractions by the primary, enhanced primary, and secondary treatment processes based on this study. By the primary treatment experiment, i.e., plain sedimentation, the average removal of SS was 59.8%. Regarding COD and BOD5, the overall removals were 39.6% and 38.9%, res pectively, and all the removed contaminants were found to belong to the suspended fraction. Considering that the suspended COD and BOD5 were about 66% and 62%, respectively, as shown in Fig. 6, the removal percentages Fig. 5 Inorganic and organic fractions of total nitrogen in the raw sewage Fig. 6 Classification of pollutants by a 2x2 water quality matrix
Table 2 Comparison of primary, enhanced primary, and secondary treatment processes for contaminants removal (% Primary treatment Enhanced primary treatment Secondary treatment Total Suspend Dissolved) Totala) Suspended) Dissolved 59.8 98.0 98.0 96.0 979 BODs 96.3 14.2 91.9 96.5 81.8 842 a)As the removal percentage from the total amount b)As the removal percentage from the suspended fraction c)As the removal percentage from the dissolved fraction could be calculated as 59.9% and 62.7% from the suspended 0.45-um membrane filter, the contaminants were classified fractions of COD and BOD, respectively. These were about into suspended and dissolved fractions the same as the ss removal. Such a result indicated that concentration of each constituent in each fraction and also the suspended fractions of COD and BOD, might be in coex- their chemical compositions, i.e., organic or inorganic matter istence with Ss in the sewage. Regarding TP and TN, the a 2 x 2 water quality matrix was formulated to characterize overall removals by plain sedimentation were 12.6% and the wastewater quality, which relates to the treatability of the 10.9%, respectively, and the calculated removal percentages contaminants. As a result, 65% of COD, 60% of BOD,, 50% from the suspended fractions were much lower than of TP, and 20% of TN were classified to the suspended frac- removal, indicating a different relation between suspended tion. We further analyzed the treatability of the contaminants phosphorus/nitrogen and suspended solids in the sewage in each fraction by conducting primary treatment(plain By enhanced primary treatment, i.e., coagulation/ sedimentation ), enhanced primary treatment(coagulation/ sedimentation, it was interesting that the removal percentages sedimentation ) and secondary treatment( Kruger Oxidation to that of SS. This supported again the assumption of coexis- suspended fractions could be easily removed by sedimenta- tence of the suspended fractions of Cod and BODs with Ss tion or coagulation/sedimentation. Therefore, the effect of the 68.2% and 65.2%, respectively, which include not only most primary or enhanced primary treatment would depend on of the suspended fractions but also part of the dissolved what percent of the contaminants in the raw sewage belonged fractions.TPremoval was high regarding both the suspended to the suspended fractions. The dissolved contaminants were difficult to remove by sedimentation or coagulation/ and dissolved fractions. It demonstrated that chemical pre- sedimentation. Therefore, biological treatment and other cipitation played an important role in phosphorus removal Regarding TN, about 75% of the suspended fraction advanced treatment may have to be applied in accorda removed but the removal of the dissolved fraction was very with the target water quality The amount of CoD and BOD. which were classified to The secondary treatment process, i.e., the Kruger Oxida- the suspended fraction, could be removed by a percentage, tion Ditch performed well in the wastewater treatment plant. almost the same as SS, when sedimentation or coagulation/ difficult to be coped with by the primary and enhanced pri- that the suspended COD and BODs were in coexistence with mary treatment, were effectively removed. The process also the suspended solids in the domestic sewage achieved a high removal of TN, of which most was dissolved Acknowledgements This study was supported by the National Natural matter Regarding suspended fractions of COD and BODs, as Science Foundation of China(Grant No. 50138020) well as SS, their removal by the enhanced primary treatment seemed to be more effective than by the secondary treatment. Chemical phosphorus removal seemed to be more effective References than biological phosphorus remova I. Semerjian L, Ayoub G M. High-pH-magnesium coagulation- focculation in wastewater treatment. Advances in Environmental 5 Conclusions Research,2003,7(2):389403 2. Harleman DR F, Murcott S. The role of physical-chemical The domestic sewage at Beishiqiao Wastewater Purification astewater treatment in the mega-cities of the developing world Water Sci. Technol., 1999, 40(4-5): 75-80 Center, Xi an was taken as an example, and the long-term 3. Harleman D, Murcott S An innovative approach to urban waste water sampling and analysis regarding Ss, COD, BOd ater treatment in the developing world. Water 21, 2001, February P, and TN in the raw sewage were conducted. By using a 2001:44-88
61 0.45-µm membrane filter, the contaminants were classified into suspended and dissolved fractions. According to the concentration of each constituent in each fraction and also their chemical compositions, i.e., organic or inorganic matter, a 2x2 water quality matrix was formulated to characterize the wastewater quality, which relates to the treatability of the contaminants. As a result, 65% of COD, 60% of BOD5, 50% of TP, and 20% of TN were classified to the suspended fraction. We further analyzed the treatability of the contaminants in each fraction by conducting primary treatment (plain sedimentation), enhanced primary treatment (coagulation/ sedimentation), and secondary treatment (Kruger Oxidation Ditch process). Almost all the contaminants belonging to the suspended fractions could be easily removed by sedimentation or coagulation/sedimentation. Therefore, the effect of the primary or enhanced primary treatment would depend on what percent of the contaminants in the raw sewage belonged to the suspended fractions. The dissolved contaminants were difficult to remove by sedimentation or coagulation/ sedimentation. Therefore, biological treatment and other advanced treatment may have to be applied in accordance with the target water quality. The amount of COD and BOD5, which were classified to the suspended fraction, could be removed by a percentage, almost the same as SS, when sedimentation or coagulation/ sedimentation was conducted. Therefore, it was assumable that the suspended COD and BOD5 were in coexistence with the suspended solids in the domestic sewage. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 50138020). References 1. Semerjian L, Ayoub G M. High-pH-magnesium coagulationflocculation in wastewater treatment. Advances in Environmental Research, 2003, 7(2): 389–403 2. Harleman D R F, Murcott S. The role of physical-chemical wastewater treatment in the mega-cities of the developing world. Water Sci. Technol., 1999, 40(4–5): 75–80 3. Harleman D, Murcott S. An innovative approach to urban wastewater treatment in the developing world. Water 21, 2001, February 2001: 44–88 could be calculated as 59.9% and 62.7% from the suspended fractions of COD and BOD5, respectively. These were about the same as the SS removal. Such a result indicated that the suspended fractions of COD and BOD5 might be in coexistence with SS in the sewage. Regarding TP and TN, the overall removals by plain sedimentation were 12.6% and 10.9%, respectively, and the calculated removal percentages from the suspended fractions were much lower than SS removal, indicating a different relation between suspended phosphorus/nitrogen and suspended solids in the sewage. By enhanced primary treatment, i.e., coagulation/ sedimentation, it was interesting that the removal percentages of the suspended COD and suspended BOD5 were also close to that of SS. This supported again the assumption of coexistence of the suspended fractions of COD and BOD5 with SS in the sewage. The total removals of COD and BOD5 were 68.2% and 65.2%, respectively, which include not only most of the suspended fractions but also part of the dissolved fractions. TP removal was high regarding both the suspended and dissolved fractions. It demonstrated that chemical precipitation played an important role in phosphorus removal. Regarding TN, about 75% of the suspended fraction was removed but the removal of the dissolved fraction was very low. The secondary treatment process, i.e., the Kruger Oxidation Ditch performed well in the wastewater treatment plant. The dissolved fractions of COD and BOD5, which were difficult to be coped with by the primary and enhanced primary treatment, were effectively removed. The process also achieved a high removal of TN, of which most was dissolved matter. Regarding suspended fractions of COD and BOD5, as well as SS, their removal by the enhanced primary treatment seemed to be more effective than by the secondary treatment. Chemical phosphorus removal seemed to be more effective than biological phosphorus removal. 5 Conclusions The domestic sewage at Beishiqiao Wastewater Purification Center, Xi’an was taken as an example, and the long-term water sampling and analysis regarding SS, COD, BOD5, TP, and TN in the raw sewage were conducted. By using a Table 2 Comparison of primary, enhanced primary, and secondary treatment processes for contaminants removal (%) Item Primary treatment Enhanced primary treatment Secondary treatment Total* Suspended** Dissolved*** Totala) Suspendedb) Dissolvedc) Totala) Suspendedb) Dissolvedc) SS 59.8 59.8 — 98.0 98.0 — 96.0 96.0 — COD 39.6 59.9 — 68.2 97.9 6.4 91.3 97.6 80.3 BOD5 38.9 62.7 — 65.2 96.3 14.2 91.9 89.9 96.5 TP 12.6 25.2 — 95.1 98.8 91.6 88.2 93.0 81.9 TN 10.9 54.5 — 15.8 74.8 1.7 81.8 72.1 84.2 a) As the removal percentage from the total amount b) As the removal percentage from the suspended fraction c) As the removal percentage from the dissolved fraction
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