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 sewage58 China. During the experimental period, the average values of the main water quality items were as follows: the concentra￾tions 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 pre￾determined by comparing the residual COD in the coagulated and settled water as 70 mg/L (dry weight) or 8.5 mg/L (alumi￾num 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