Table 5. Average treatment results in 23 larger(2000 pe)(Odegaard, 1992)and 35 smaller (2.000 pe)primary precipitation plants in Norway(odegaard and skrevseth, 1997 Parameter Average inlet Average outlet Average treatment concentration concentration efficienc (mg/l) Large plants 233+171 17,3±10,0 92,0 Small plants 226+150 22.3+16.6 90.1 COD(mg/) Large plants 505+243 08 78,6 Small plants 494+90 121+72 75.5 Tot P(mg/l) Large plants 5,40±3,01 0,28±0,14 94,8 Small plants 5.33+ 0.50+046 90.6 experienced that the best phosphate removal takes Pa or d or both. Plant operators have One can reduce sludge production only by reducing K ce at ph around 6 and they add enough of the acid metal coagulant to get down to this pH. This results in overdosing considering the stoichiometric need and to precipitation of metal hydroxide, i.e. excessive sludge production. If particle removal is focused on, one may, however, lower the dosage without ruining coagulation efficiency by replacing part of the metal cation with an organic polymeric cation. The cation will not result in precipitation and only add very little extra sludge production caused by coagulation. This is demonstrated in figure 7 that gives the ratios between the amount of Ss produced(sludge production) and the amount of Ss removed (SS in-SSout)as well as removal efficiency(( u/SSin)100%)for situations where iron only or a low dose of iron(5, 5 mg Fe/l)combined with a cation polymer were used in jar-tests(Odegaard, 1998) In figure 7a it is shown that the sludge production caused by precipitation was increasin with metal dosage and was almost as high as that caused by the ss-removal at dosages above 20-25 mg Fer where removal efficiencies over 90 is achieved. In figure 7b. however where metal cation is replaced by polymer cation, it is shown that close to nothing was precipitated without loosing much on removal efficiencies at optimal dosages of polymer 2.5 100 E20 88 0.5 0.0 mg Fe/l 5, 5 mg Fe +mg DC 242/244 igure 7 Comparison of primary particle separation at different dosage scenarios9 Table 5. Average treatment results in 23 larger (>2.000 pe) (Ødegaard, 1992) and 35 smaller (<2.000 pe) primary precipitation plants in Norway (Ødegaard and Skrøvseth, 1997) Parameter Average inlet concentration Average outlet concentration Average treatment efficiency SS (mg/l) Large plants Small plants 233 + 171 226 + 150 17,3 + 10,0 22,3 + 16,6 92,0 90,1 COD (mg/l) Large plants Small plants 505 + 243 494 + 90 108 + 40 121 + 72 78,6 75,5 Tot P (mg/l) Large plants Small plants 5,40 + 3,01 5,33 + 2,26 0,28 + 0,14 0,50 + 0,46 94,8 90,6 One can reduce sludge production only by reducing K or D or both. Plant operators have experienced that the best phosphate removal takes place at pH around 6 and they add enough of the acid metal coagulant to get down to this pH. This results in overdosing considering the stoichiometric need and to precipitation of metal hydroxide, i.e. excessive sludge production. If particle removal is focused on, one may, however, lower the dosage without ruining coagulation efficiency by replacing part of the metal cation with an organic polymeric cation. The cation will not result in precipitation and only add very little extra sludge production caused by coagulation. This is demonstrated in figure 7 that gives the ratios between the amount of SS produced (sludge production) and the amount of SS removed (SS in – SSout) as well as removal efficiency ((1- SSout/SSin)100%) for situations where iron only or a low dose of iron (5,5 mg Fe/l) combined with a cation polymer were used in jar-tests (Ødegaard, 1998). In figure 7a it is shown that the sludge production caused by precipitation was increasin with metal dosage and was almost as high as that caused by the SS-removal at dosages above 20-25 mg Fe/l, where removal efficiencies over 90 % is achieved. In figure 7b, however, where metal cation is replaced by polymer cation, it is shown that close to nothing was precipitated without loosing much on removal efficiencies at optimal dosages of polymer. 0.0 0.5 1.0 1.5 2.0 2.5 0 1 2 3 5,5 mg Fe + mg DC 242/244/l S Sprod/S Srem 0 20 40 60 80 100 S S-rem oval ( %) SSp/SSr R 0.0 0.5 1.0 1.5 2.0 2.5 0 20 40 60 mg Fe/l SS pro d/S Srem 0 20 40 60 80 100 S S-rem oval ( %) SSp/SSr R Figure 7 Comparison of primary particle separation at different dosage scenarios