OPTIMISED PARTICLE SEPARATION IN THE PRIMARY STEP OF WASTEWATER TREATMENT Hallvard odegaard Faculty of civil and Environmental Engineering, Norwegian University of Science and Technology(NTNU, N-7034 Trondheim, Norway ABSTRACT Since the pollutants in wastewater to such a large extent are associated with particles, enhanced particle separation in the primary step of wastewater treatment should be aimed at. Space restrictions demand pretreatment options with a small foot-print. In this paper it is shown that coagulation with metal salts very efficient but leads to excessive sludge production. It is demonstrated how the use of cationic polymers may reduce the sludge production considerably without ruining the Ss-removal efficiency and also how improved flocculation, either chemically, by the addition of flocculants or physically, by improvement of settling tank design, may lead to better particle separation as well as smaller foot-print of the plant. As an Itration in coarse, floating filters is discussed as well KEYWORDS Wastewater; primary treatment; coagulation; polymers; sludge production; coarse media filtration INTRODUCTION Wastewater treatment is to a very large extent a matter of particle separation. This is a result of the fact that most of the pollutants in wastewater exist on particulate or colloidal form or are transformed to this form in the course of the treatment process. This has lead to the wastewater treatment strategy of removing particulate and colloidal matter in the primary step and thereafter deal with soluble compounds that need to be transformed to colloidal and particulate matter(e.g. bacteria) before they are separated(Odegaard, 1992). Traditionally particle separation in the primary step is carried out by settling. This paper will concentrate on how direct particle separation in the primary step may be enhanced and how this may be chieved at a smaller plant foot print". The basis for this discussion must be the characteristics of raw wastewater that to a large extent is determined by the processes that take place in the wastewater during its flow through the sewer network WASTEWATER CHARACTERISTICS AND ITS INFLUENCE ON TREATMENT Already in the fifties there were studies carried out in order to fractionate the organic contaminants in wastewater in different size fractions and demonstrate the difference in biodegradability of these fractions Balmat, 1957; Heukelekian and Balmat, 1959; Richert and Hunter, 1971; Munch et al, 1980). Generally it was found in these studies that about 25 of the Cod was on a soluble form(defined as compounds/
OPTIMISED PARTICLE SEPARATION IN THE PRIMARY STEP OF WASTEWATER TREATMENT Hallvard Ødegaard Faculty of Civil and Environmental Engineering, Norwegian University of Science and Technology (NTNU), N-7034 Trondheim, Norway ABSTRACT Since the pollutants in wastewater to such a large extent are associated with particles, enhanced particle separation in the primary step of wastewater treatment should be aimed at. Space restrictions demand pretreatment options with a small ”foot-print”. In this paper it is shown that coagulation with metal salts is very efficient but leads to excessive sludge production. It is demonstrated how the use of cationic polymers may reduce the sludge production considerably without ruining the SS-removal efficiency and also how improved flocculation, either chemically, by the addition of flocculants or physically, by improvement of settling tank design, may lead to better particle separation as well as smaller ”foot-print” of the plant. As an alternative to primary settling, primary filtration in coarse, floating filters is discussed as well. KEYWORDS Wastewater; primary treatment; coagulation; polymers; sludge production; coarse media filtration INTRODUCTION Wastewater treatment is to a very large extent a matter of particle separation. This is a result of the fact that most of the pollutants in wastewater exist on particulate or colloidal form or are transformed to this form in the course of the treatment process. This has lead to the wastewater treatment strategy of removing particulate and colloidal matter in the primary step and thereafter deal with soluble compounds that need to be transformed to colloidal and particulate matter (e.g. bacteria) before they are separated (Ødegaard, 1992). Traditionally particle separation in the primary step is carried out by settling. This paper will concentrate on how direct particle separation in the primary step may be enhanced and how this may be achieved at a smaller plant ”foot print”. The basis for this discussion must be the characteristics of raw wastewater that to a large extent is determined by the processes that take place in the wastewater during its flow through the sewer network. WASTEWATER CHARACTERISTICS AND ITS INFLUENCE ON TREATMENT Already in the fifties there were studies carried out in order to fractionate the organic contaminants in wastewater in different size fractions and demonstrate the difference in biodegradability of these fractions (Balmat, 1957; Heukelekian and Balmat, 1959; Richert and Hunter, 1971; Munch et al, 1980). Generally it was found in these studies that about 25 % of the COD was on a soluble form (defined as compounds/
particles with size <0,08 Hm). 15 of the organic matter was found to be appearing as colloidal(0,08 1,0 um), about 25 as supracolloidal (1-100um)and about 35% as settleable( 100 um) particles Based on a thorough study of particle size distributions in the primary effluents of some US plants, Levine et al (1985)concluded that organic contaminants in municipal wastewater could effectively be classified as being either greater than or smaller than 0, 1 um. It was found that 63-70 of the toc was associated with particles >0, 1 um. Recently Neis and Thiem(1996) carried out size distribution analysis in primary effluents in some German plants. They found that among particles >0,lum, 25-30 was 1 um, 70-85% <8 um, 85-95%<32 um and 100 % 100um From 48 to 69 of the Cod was found to be associated with particles >0, 1 um while from 7 to 18 was associated with particles >8 um in primary effluents When evaluating data from Scandinavian plants, Odegaard (1992)reported that the filtered fraction(1 um filter) in raw water samples was typically 20-30 of the total COD, 30-40 %of tot P and 75-85 %of tot n. With respect to other contaminants, like organic and inorganic micropollutants, it is quite clear that heavy metals as well as PCB s and Pah's are strongly associated with particles(odegaard, 1987) An important conclusion from these investigations, is that the smaller organic fractions biodegrade more rapidly than the larger fractions. Even if this should be pretty obvious from a microbiological point of view, it is not taken much into consideration in the planning and design of wastewater treatment plants This means that not only will the load of organic matter on bioprocesses be reduced by enhanced primary treatment, but the rate at which the bioprocess will perform, will be increased as well Transformations of wastewater composition in the sewer network. One of the main reasons for the large differences with respect to how organic matter(as well other pollutants)appear in the wastewater that is to be treated, is the fact that biological and physical/chemical processes are taking place in the sewer network before the wastewater is reaching the treatment plant. Only lately, have we become interested in the sewer network processes and this has resulted in the organising of conferences on the topic(Hvitved-Jacobsen et al, 1995). The interest has primarily been driven by the fact that researchers dealing with biological nitrogen and phosphorous removal have been concerned about the fate of the organic acids of the wastewater. There are several processes taking place in the sewer network depending on the prevailing conditions, such as aerobic/anaerobic, laminar/turbulent, oxidising/reducing etc and on the composition of individual discharges to the sewer such as amount and type of industrial discharge, sewer corrosion, inclination of sewers, the extent of sewage pumping and so on One can characterise two extreme situations that to a large extent may explain transformations of organic matter in sewers. Typical for the first situation is that the landscape is flat and consequently the sewers are laid with a low inclination. Sewage pumping is necessary and the sewer pipe is filled most of the time Anaerobic conditions prevail in the wastewater and in the biofilm that will establish itself on the wall of the sewer. Particulate organic matter will be captured/adsorbed in the biofilm and hydrolysed biologically The end result will be that the amount of soluble, easily biodegradable organic matter(organic acids)is increased while the amount of particulate/colloidal, heavily biodegradable organic matter is decreased Typical for the other situation is that the landscape is hilly, the sewers are laid with a steep inclination and the wastewater flows fast in sewers that function as channels. Oxygen is driven into the wastewater because of turbulence. Aerobic conditions prevail in the wastewater and in the biofilm that will establish itself on the wall of the sewer. The end result will be decreased amounts of soluble, easily biodegradable organic matter(organic acids) and increased amounts of particulate/-colloidal, heavily biodegradable organic matter in the form of biomass. More will be gained by enhanced particle separation in this situation than in the one described above The possibilities for improvement of the efficiencies of the primary treatment step
particles with size < 0,08 µm). 15 % of the organic matter was found to be appearing as colloidal (0,08– 1,0 µm), about 25 % as supracolloidal (1-100µm) and about 35 % as settleable (> 100 µm) particles. Based on a thorough study of particle size distributions in the primary effluents of some US plants, Levine et al (1985) concluded that organic contaminants in municipal wastewater could effectively be classified as being either greater than or smaller than 0,1 µm. It was found that 63-70 % of the TOC was associated with particles > 0,1 µm. Recently Neis and Thiem (1996) carried out size distribution analysis in primary effluents in some German plants. They found that among particles > 0,1µm, 25-30 % was < 1 µm, 70-85 % < 8 µm, 85-95 % < 32 µm and 100 % < 100µm. From 48 to 69 % of the COD was found to be associated with particles > 0,1 µm while from 7 to 18 % was associated with particles > 8 µm in primary effluents. When evaluating data from Scandinavian plants, Ødegaard (1992) reported that the filtered fraction (1 µm filter) in raw water samples was typically 20-30 % of the total COD, 30-40 % of tot P and 75-85 % of tot N. With respect to other contaminants, like organic and inorganic micropollutants, it is quite clear that heavy metals as well as PCB’s and PAH’s are strongly associated with particles (Ødegaard, 1987). An important conclusion from these investigations, is that the smaller organic fractions biodegrade more rapidly than the larger fractions. Even if this should be pretty obvious from a microbiological point of view, it is not taken much into consideration in the planning and design of wastewater treatment plants. This means that not only will the load of organic matter on bioprocesses be reduced by enhanced primary treatment, but the rate at which the bioprocess will perform, will be increased as well. Transformations of wastewater composition in the sewer network. One of the main reasons for the large differences with respect to how organic matter (as well other pollutants) appear in the wastewater that is to be treated, is the fact that biological and physical/chemical processes are taking place in the sewer network before the wastewater is reaching the treatment plant. Only lately, have we become interested in the sewer network processes and this has resulted in the organising of conferences on the topic (Hvitved-Jacobsen et al, 1995). The interest has primarily been driven by the fact that researchers dealing with biological nitrogen and phosphorous removal have been concerned about the fate of the organic acids of the wastewater. There are several processes taking place in the sewer network, depending on the prevailing conditions, such as aerobic/anaerobic, laminar/turbulent, oxidising/reducing etc and on the composition of individual discharges to the sewer such as amount and type of industrial discharge, sewer corrosion, inclination of sewers, the extent of sewage pumping and so on. One can characterise two extreme situations that to a large extent may explain transformations of organic matter in sewers. Typical for the first situation is that the landscape is flat and consequently the sewers are laid with a low inclination. Sewage pumping is necessary and the sewer pipe is filled most of the time. Anaerobic conditions prevail in the wastewater and in the biofilm that will establish itself on the wall of the sewer. Particulate organic matter will be captured/adsorbed in the biofilm and hydrolysed biologically. The end result will be that the amount of soluble, easily biodegradable organic matter (organic acids) is increased while the amount of particulate/colloidal, heavily biodegradable organic matter is decreased. Typical for the other situation is that the landscape is hilly, the sewers are laid with a steep inclination and the wastewater flows fast in sewers that function as channels. Oxygen is driven into the wastewater because of turbulence. Aerobic conditions prevail in the wastewater and in the biofilm that will establish itself on the wall of the sewer. The end result will be decreased amounts of soluble, easily biodegradable organic matter (organic acids) and increased amounts of particulate/-colloidal, heavily biodegradable organic matter in the form of biomass. More will be gained by enhanced particle separation in this situation than in the one described above. The possibilities for improvement of the efficiencies of the primary treatment step
Traditionally primary settling has been used for particle separation in the first step of wastewater treatment. With the overflow rates normally used(around 2 m/h) and the densities of typical wastewater particles, it can be calculated from Stokes law that particles down to about 30-50 um will be settled out This means in practice that around 50 of the suspended solids and 30 of the organic matter is removed by primary settling. If particles down to around 0, 1 um could be removed, considerable Improvement would be gained particles More and more the"foot print"of the wastewater treatment plant is a factor that has to be taken into spacious and more compact, " area-efficient"separation units are being looked tor y settling tanks are too consideration. Compared to what is achieved in terms of treatment, the primar In order to improve the particle separation- and area efficiency, there are three possible ways to go, none of which excludes the others 1. To pretreat the wastewater before settling in order to increase particle size and/or particle density 2. To improve the design of the settling tank so that better efficiency can be achieved at a smaller 3. To use another more efficient particle separation method IMPROVED EFFICIENCY BY PRETREATMENT BASED ON COAGULATION The most well-known and commonly used method of pretreatment in order to enhance particle separation in primary treatment, is the addition of coagulants. In primary coagulation(sometimes also referred to as primary precipitation) a coagulant is added to the raw wastewater resulting in destabilisation of colloids The small aggregates of primary destabilised particles are flocculated and separated by settling(most common), flotation or filtration. The coagulant is normally based on aluminium or iron resulting in precipitation of phosphate as well as coagulation of colloids. In fact, where primary precipitation plants are frequently used, as in Norway, the primary treatment goal has been phosphate elimination. In Norway the conditions in the sewer network can frequently be described by the situation where aerobic conditions prevail, for which primary coagulation is very favourable, since a major part of the organic matter appears in the form of particles/colloids. This is demonstrated in Table 1 where the average treatment result from Norwegian primary precipitation plants, taken from two different investigations of larger and smaller plants respectively(Odegaard, 1992 and Odegaard and Skrovseth, 1995)are given Table 1. Average treatment results in 23 larger (2.000 pe)(Odegaard, 1992)and 35 smaller(<2000 pe) (Odegaard and skrovseth, 1995) primary precipitation plants in Norway Parameter Average inlet Average outlet Average treatment concentration concentration efficiency Large plants Small plants Large plants Small plants Large plants Small plants Ss(mg/)233±171226±15017,3±10,022,3±16,6 COD(mg/)505±243494+90 108+40 121+72 786 75,5 TotP(mg/)5,40±3,015,33±2,260,28±0,140,50±0,46 94,8 It is demonstrated that very good efficiencies in SS-removal can be obtained even at small plants with large variations in flow, demonstrating the operational stability of the process. These good SS-removal results in a reduction of the COD-load on proceeding processes with more than 75%. And salt is used as the coagulant(as in this case)excellent phosphate removal is obtained as wel when a metal
Traditionally primary settling has been used for particle separation in the first step of wastewater treatment. With the overflow rates normally used (around 2 m/h) and the densities of typical wastewater particles, it can be calculated from Stokes law that particles down to about 30-50 µm will be settled out. This means in practice that around 50 % of the suspended solids and 30 % of the organic matter is removed by primary settling. If particles down to around 0,1 µm could be removed, considerable improvement would be gained. More and more the "foot print" of the wastewater treatment plant is a factor that has to be taken into consideration. Compared to what is achieved in terms of treatment, the primary settling tanks are too spacious and more compact, ”area-efficient” separation units are being looked for. In order to improve the particle separation- and area efficiency, there are three possible ways to go, none of which excludes the others : 1. To pretreat the wastewater before settling in order to increase particle size and/or particle density 2. To improve the design of the settling tank so that better efficiency can be achieved at a smaller space 3. To use another more efficient particle separation method IMPROVED EFFICIENCY BY PRETREATMENT BASED ON COAGULATION The most well-known and commonly used method of pretreatment in order to enhance particle separation in primary treatment, is the addition of coagulants. In primary coagulation (sometimes also referred to as primary precipitation) a coagulant is added to the raw wastewater resulting in destabilisation of colloids. The small aggregates of primary destabilised particles are flocculated and separated by settling (most common), flotation or filtration. The coagulant is normally based on aluminium or iron resulting in precipitation of phosphate as well as coagulation of colloids. In fact, where primary precipitation plants are frequently used, as in Norway, the primary treatment goal has been phosphate elimination. In Norway the conditions in the sewer network can frequently be described by the situation where aerobic conditions prevail, for which primary coagulation is very favourable, since a major part of the organic matter appears in the form of particles/colloids. This is demonstrated in Table 1 where the average treatment result from Norwegian primary precipitation plants, taken from two different investigations of larger and smaller plants respectively (Ødegaard, 1992 and Ødegaard and Skrøvseth, 1995) are given. Table 1. Average treatment results in 23 larger (>2.000 pe)(Ødegaard, 1992) and 35 smaller (<2.000 pe) (Ødegaard and Skrøvseth, 1995) primary precipitation plants in Norway Average inlet concentration Average outlet concentration Average treatment efficiency Parameter Large plants Small plants Large plants Small plants Large plants Small plants SS (mg/l) 233 + 171 226 + 150 17,3 + 10,0 22,3 + 16,6 92,0 90,1 COD (mg/l) 505 + 243 494 + 90 108 + 40 121 + 72 78,6 75,5 Tot P (mg/l) 5,40 + 3,01 5,33 + 2,26 0,28 + 0,14 0,50 + 0,46 94,8 90,6 It is demonstrated that very good efficiencies in SS-removal can be obtained even at small plants with large variations in flow, demonstrating the operational stability of the process. These good SS-removal results in a reduction of the COD-load on proceeding processes with more than 75 %. And when a metal salt is used as the coagulant (as in this case) excellent phosphate removal is obtained as well
Minimising sludge production The downside of traditional chemical primary coagulation is the considerably increased sludge production as compared to primary settling only, partly as a results of improved SS- removal but partly also due to precipitated material. The sludge produced during chemical coagulation consists basically of the suspended solids removed and the coagulated/precipitated matter, as described below(Odegaard, 1994) SP= SSin-SSout t Kprec. D where SP sludge production(g SS/m) SSin, SSout =suspended solids concentration in influent and effluent respectively(g SS/m) sludge production coefficient(g SS/g Me), typically 4-5 for Fe and 6-7 for Al dosage of metal coagulant(g Me/m) If the primary goal is optimised particle removal (low SSout), one can reduce sludge production only by reducing K or D or both. The level of the dosage in primary precipitation plants in practice, is very much determined by the need for phosphate removal since this, to a large extent, is governed by the ph of coagulation. 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 and a considerable precipitation of metal hydroxide, i.e. excessive sludge production If, on the other hand, phosphate removal is not the important issue, but rather particle removal, one could 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 would only add very little extra sludge production caused by coagulation. This effect is demonstrated in Figure 1 showing that reducing the Al-dosage and replacing this with cationic polymer could considerably lower sludge production Fig. 1. Example of reduced sludge production as a consequence of replacement of some of the meta cation dose with a polymer cation dose in order to reach a given SS-concentration in effluent Coagulant Pax XL-60. Polymer: Fennofix 40(Odegaard and Karlsson, 1994 Such low-metal dose coagulation, not intended for phosphate removal, has especially been used in the US under the name of"Chemically enhanced primary treatment"(CEPT(Morrissey and Harleman, 1992) The least sludge production would be obtained by coagulation with polymeric cation alone. In principle the Kprec -value would then be close to zero. Experience have shown, however, that it is difficult, with polymer alone, to obtain as good particle removal as with the use of metal coagulant. A comparison is shown in Figure 2 where jar-test results from various dosing situations are compared
Minimising sludge production The downside of traditional chemical primary coagulation is the considerably increased sludge production as compared to primary settling only, partly as a results of improved SS-removal but partly also due to precipitated material. The sludge produced during chemical coagulation consists basically of the suspended solids removed and the coagulated/precipitated matter, as described below (Ødegaard, 1994): SP = SSin - SSout + Kprec.* D where SP = sludge production (g SS/m3 ) SSin, SSout = suspended solids concentration in influent and effluent respectively (g SS/m3 ) Kprec . = sludge production coefficient (g SS/g Me), typically 4-5 for Fe and 6-7 for Al D = dosage of metal coagulant (g Me/m3 ) If the primary goal is optimised particle removal (low SSout), one can reduce sludge production only by reducing K or D or both. The level of the dosage in primary precipitation plants in practice, is very much determined by the need for phosphate removal since this, to a large extent, is governed by the pH of coagulation. 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 and a considerable precipitation of metal hydroxide, i.e. excessive sludge production. If, on the other hand, phosphate removal is not the important issue, but rather particle removal, one could 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 would only add very little extra sludge production caused by coagulation. This effect is demonstrated in Figure 1 showing that reducing the Al-dosage and replacing this with cationic polymer could considerably lower sludge production. Fig. 1. Example of reduced sludge production as a consequence of replacement of some of the metal cation dose with a polymer cation dose in order to reach a given SS-concentration in effluent. Coagulant Pax XL-60. Polymer: Fennofix 40 (Ødegaard and Karlsson, 1994). Such low-metal dose coagulation, not intended for phosphate removal, has especially been used in the US under the name of "Chemically enhanced primary treatment" (CEPT)(Morrissey and Harleman, 1992). The least sludge production would be obtained by coagulation with polymeric cation alone. In principle the Kprec.-value would then be close to zero. Experience have shown, however, that it is difficult, with polymer alone, to obtain as good particle removal as with the use of metal coagulant. A comparison is shown in Figure 2 where jar-test results from various dosing situations are compared
a FeCl3(high dose)only b FeCl3(high dose) only 2,5 50 mg Fe/ mg Fe/ a、.,+:+m自 8 E9 0,0 0.0 0 g D 5,5 mg Fe + mg DC 242/244/ c Cationic polymer only d FeCl3(low dose)+ cationic polymer Fig. 2. Comparison of primary particle separation at different dosage scenarios In Figure 2 a, c and d 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-SSouSSin )100 are given versus dosage of coagulant. In Figure 2a only FeCl3 was used as coagulant. The ratio of sludge production relative to the amount of ss that is removed, increases with the iron dosage from I to about 2 since matter is precipitated The Ss-removal efficiency increases with dosage to its maximum at dosages over around 15 mg Fe/ Figure 2b shows that the difference between the amount of sludge produced and the amount removed increases with dosage, demonstrating the amount of sludge production caused by precipitation. It is interesting to note that the curve crosses the x-axis at around 5 mg Fe/l under which no precipitation seems In Figure 2c the results from the use of a low molecular weight, strongly cationic polymer, as the only coagulant, are shown. Two different polymers were tested(Sepco DC 242 and Sepco DC 244). They showed little difference in performance, so the results from the use of both are included in the same figure Now the sludge production is demonstrated to be close to equal to the amount of Ss removed independent upon coagulant dosage. This means that close to nothing is precipitated. The SS-removal efficiency increases with dosage but reaches a maximum of about 80 at dosages over 4 m High SS-removal efficiency, combined with low sludge production can, however, be obtained when combining a low dosage of metal coagulant with a relatively low dosage of cationic polymer,as demonstrated in Figure 2d. From this figure it seems that the low iron dosage does not result in precipitation but rather in enhancement of the coagulation of particles There have been objections to the use of synthetic, polymeric cations because of the possibility of monomer formation. Therefore, we are undertaking research into the area of using natural, cationic biopolymers, such as chitosan. Some results are shown in Figure 3, where varying dosages of the chitosan biopolymer Profloc 240 was added to a wastewater at two different aluminium-dosages
a. FeCl3 (high dose) only b. FeCl3 (high dose) only 0,0 0,5 1,0 1,5 2,0 2,5 0 20 40 60 mg Fe/l S Sprod/S Srem 0 20 40 60 80 100 S S-removal (%) SSp/SSr R 0 50 100 150 200 250 300 0 20 40 6 mg Fe/l S P-S Srem (mg S S/l) 0 em S Sr od/ Spr S 0,0 0,5 1,0 1,5 2,0 2,5 0 2 4 6 mg DC 242/244/l 0 20 40 60 80 100 S S-removal (%) SSp/SSr R 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-removal (%) SSp/SSr R c. Cationic polymer only d. FeCl3 (low dose) + cationic polymer Fig. 2. Comparison of primary particle separation at different dosage scenarios In Figure 2 a, c and d 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 are given versus dosage of coagulant. In Figure 2a only FeCl3 was used as coagulant. The ratio of sludge production relative to the amount of SS that is removed, increases with the iron dosage from 1 to about 2 since matter is precipitated. The SS-removal efficiency increases with dosage to its maximum at dosages over around 15 mg Fe/l. Figure 2b shows that the difference between the amount of sludge produced and the amount removed, increases with dosage, demonstrating the amount of sludge production caused by precipitation. It is interesting to note that the curve crosses the x-axis at around 5 mg Fe/l under which no precipitation seems to occur. In Figure 2c the results from the use of a low molecular weight, strongly cationic polymer, as the only coagulant, are shown. Two different polymers were tested (Sepco DC 242 and Sepco DC 244). They showed little difference in performance, so the results from the use of both are included in the same figure. Now the sludge production is demonstrated to be close to equal to the amount of SS removed independent upon coagulant dosage. This means that close to nothing is precipitated. The SS-removal efficiency increases with dosage but reaches a maximum of about 80 % at dosages over 4 mg/l. High SS-removal efficiency, combined with low sludge production can, however, be obtained when combining a low dosage of metal coagulant with a relatively low dosage of cationic polymer, as demonstrated in Figure 2d. From this figure it seems that the low iron dosage does not result in precipitation but rather in enhancement of the coagulation of particles. There have been objections to the use of synthetic, polymeric cations because of the possibility of monomer formation. Therefore, we are undertaking research into the area of using natural, cationic biopolymers, such as chitosan. Some results are shown in Figure 3, where varying dosages of the chitosan biopolymer Profloc 240 was added to a wastewater at two different aluminium-dosages