Proc. Nordic Conf. Nitrogen removal and biological phosphate removal, Oslo, Norway 2.-4. February 1999 The influence of wastewater characteristics on choice of wastewater treatment method Hallvard odegaard Faculty of Civil and Environmental Engineering, Norwegian University of Science and Technology, N-7034 Trondheim -NTNU E-mail: hallvard odegaard@byggntnu.no Introduction In advanced wastewater treatment the effluent standards are currently aiming at removing particles (suspended solids), organic matter(BOD, COD, TOC)and nutrients(nitrogen and phosporous Various treatment methods may be used, physical, chemical and biological as well as combinations of these. Among these main treatment methods, there are many reactor and process alternatives Traditionally the choice of treatment method has mainly been based on the effluent standard set in individual cases as well as practical experience with the various methods. This has resulted in the fact that some methods and processes are more favoured in some places than in others. In UK fo instance, biological treatment has been favoured even in cases where chemical treatment for phosphorous removal (in combination with biological treatment)might have been the right choice from the receiving water point of view. In norway, on the other hand the good experience with chemical treatment for phosphorous removal has lead to the use of chemical treatment even in cases where phosphorous is not the key element from the receiving water point of view After the introduction of biological nitrogen and phosphorous removal this picture has changed it is being realised, however, that in addition to effluent standard and traditional experience, onlesow somewhat but we can still see tendencies towards"preferred"solutions in the Nordic countries solution. In this paper wastewater characteristics will be discussed in this perspective onomiaay has to take account of the characteristics of the wastewater in order to arrive at the most ec Wastewater characteristics There are several ways of characterising wastewater 2. According to the state, such as soluble, colloidal, particulate, gaseous ef nutr 1. According to bulk parameters, such as ded matter, organic matter, nutrients, bacteria etc 3. According to its treat-ability such as biodegradability, ability to separate etc Normally wastewater treatment plant owners restrict themselves to characterising in terms of bulk parameters since this is the way the effluent standards are given, but also because other characterisation methods may be costly and difficult to perform. Bulk parameters do not, however, give sufficient information with respect to optimal selection of treatment method/process and optimisation of operatic
Proc. Nordic Conf. : Nitrogen removal and biological phosphate removal, Oslo, Norway 2.-4.February 1999 The influence of wastewater characteristics on choice of wastewater treatment method Hallvard Ødegaard Faculty of Civil and Environmental Engineering, Norwegian University of Science and Technology, N-7034 Trondheim - NTNU E-mail : hallvard.odegaard@bygg.ntnu.no Introduction In advanced wastewater treatment the effluent standards are currently aiming at removing particles (suspended solids), organic matter (BOD, COD, TOC) and nutrients (nitrogen and phosporous). Various treatment methods may be used, physical, chemical and biological as well as combinations of these. Among these main treatment methods, there are many reactor and process alternatives. Traditionally the choice of treatment method has mainly been based on the effluent standard set in individual cases as well as practical experience with the various methods. This has resulted in the fact that some methods and processes are more favoured in some places than in others. In UK for instance, biological treatment has been favoured even in cases where chemical treatment for phosphorous removal (in combination with biological treatment) might have been the right choice from the receiving water point of view. In Norway, on the other hand, the good experience with chemical treatment for phosphorous removal has lead to the use of chemical treatment even in cases where phosphorous is not the key element from the receiving water point of view. After the introduction of biological nitrogen and phosphorous removal this picture has changed somewhat but we can still see tendencies towards "preferred" solutions in the Nordic countries. Now it is being realised, however, that in addition to effluent standard and traditional experience, one has to take account of the characteristics of the wastewater in order to arrive at the most economical solution. In this paper wastewater characteristics will be discussed in this perspective. Wastewater characteristics There are several ways of characterising wastewater : 1. According to bulk parameters, such as suspended matter, organic matter, nutrients, bacteria etc 2. According to the state, such as soluble, colloidal, particulate, gaseous etc 3. According to its treat-ability such as biodegradability, ability to separate etc Normally wastewater treatment plant owners restrict themselves to characterising in terms of bulk parameters since this is the way the effluent standards are given, but also because other characterisation methods may be costly and difficult to perform. Bulk parameters do not, however, give sufficient information with respect to optimal selection of treatment method/process and optimisation of operation
In this paper it will be focused much on the need and benefit to know and understand wastewater characteristics in terms of the state in which the various compounds are present, that is if they are present as soluble, colloidal or particulate matter. This is knowledge can be acquired quite easily and cheaply. Nevertheless is tells a lot about the treat-ability of the wastewater and therefore also about the most economical choice of method with respect to both investment and operation. The various compounds may be present in wastewater as soluble(d I um). Suspended matter, normally determined by filtering through a 1 um filter, contains the particulate but not the colloidal matter. It has been demonstrated(Levine et al 1985 )that most of the mass in colloids will be included when a 0, 1 um filter is used Especially since nitrogen removal came into operation, the need to characterise the wastewater according to biodegradability has become evident and even more so since biological phosphorous removal has come into use. Various characterisation techniques have been introduced such oxygen uptake rate(OUR), nitrogen uptake rate(NUR)various C/N-ratios etc Specific analyses such as VFA(volatile fatty acids have also been used as well as various interpretations of COD- analyses, such as BSCOD(biodegradable, soluble COD). The state in which especially organic matter is present in the wastewater is very important and therefore characterisation with respect to particulate fractions may shed some light also on biodegradability A survey on wastewater characteristics with special emphasis on particulate content There has not been any surveys carried out in order to analyse the presence of particulate matter in Scandinavia, but experiences from Norway and Sweden have indicated that the particulate fraction may be even higher in some Scandinavian plants than elsewhere. It was, thetefore, decided to try to evaluate the situation in the Scandinavian countries. Unfortunately it was only possible to collect relevant data from only one Danish plants. The analysis will concentrate, therefore on data from Sweden, Finland and Norway. It is mostly data from those plants that have to remove nitrogen that are included As expected, it was not standard procedure to analyse on filtered samples. Those plants that had carried out such analyses either in connection with special projects or as a routine, normally used a standard I um filter. In some cases the data are based on yearly averages and in some cases on single day samples. The main issue has been to evaluate to what degree organic matter(BOD and COD)and nutrients(P and N)are connected to suspended matter in Scandinavian wastewater One will see that there are great variations in wastewater characteristics from plant to plant and from country to country. Based on the data collected, it will generally be seen that the norwegian Finnish wastewater was found to be quite concentrated indicating more comprehensive use of he wastewater was found to be much more dilute than that in both Sweden and Finland Especially separate sewerage systems in Finland
2 In this paper it will be focused much on the need and benefit to know and understand wastewater characteristics in terms of the state in which the various compounds are present, that is if they are present as soluble, colloidal or particulate matter. This is knowledge can be acquired quite easily and cheaply. Nevertheless is tells a lot about the treat-ability of the wastewater and therefore also about the most economical choice of method with respect to both investment and operation. The various compounds may be present in wastewater as soluble (d 1 µm). Suspended matter, normally determined by filtering through a 1 µm filter, contains the particulate but not the colloidal matter. It has been demonstrated (Levine et al, 1985) that most of the mass in colloids will be included when a 0,1 µm filter is used. Especially since nitrogen removal came into operation, the need to characterise the wastewater according to biodegradability has become evident and even more so since biological phosphorous removal has come into use. Various characterisation techniques have been introduced such as oxygen uptake rate (OUR), nitrogen uptake rate (NUR) various C/N-ratios etc. Specific analyses such as VFA (volatile fatty acids) have also been used as well as various interpretations of CODanalyses, such as BSCOD (biodegradable, soluble COD). The state in which especially organic matter is present in the wastewater is very important and therefore characterisation with respect to particulate fractions may shed some light also on biodegradability. A survey on wastewater characteristics with special emphasis on particulate content There has not been any surveys carried out in order to analyse the presence of particulate matter in Scandinavia, but experiences from Norway and Sweden have indicated that the particulate fraction may be even higher in some Scandinavian plants than elsewhere. It was, thetefore, decided to try to evaluate the situation in the Scandinavian countries. Unfortunately it was only possible to collect relevant data from only one Danish plants. The analysis will concentrate, therefore on data from Sweden, Finland and Norway. It is mostly data from those plants that have to remove nitrogen that are included. As expected, it was not standard procedure to analyse on filtered samples. Those plants that had carried out such analyses either in connection with special projects or as a routine, normally used a standard 1 µm filter. In some cases the data are based on yearly averages and in some cases on single day samples. The main issue has been to evaluate to what degree organic matter (BOD and COD) and nutrients (P and N) are connected to suspended matter in Scandinavian wastewater. One will see that there are great variations in wastewater characteristics from plant to plant and from country to country. Based on the data collected, it will generally be seen that the Norwegian wastewater was found to be much more dilute than that in both Sweden and Finland. Especially the Finnish wastewater was found to be quite concentrated indicating more comprehensive use of separate sewerage systems in Finland
Organic matter In table I are summarised the data reported on suspended solids and organic matter Table I Average values on organic matter in raw wastewater from Scandianvian plants Country COD COD Fract. BOd BOFr Fraction BOD/COD CODss BODss Tot Filtr Sweden 17 243 47157 0.66 0.320.38 +87 123+79 0.10 47 +0.12 0.12+0.10 Norway 12 143 0.66 +39 +69+30 0,11±28 +9 +0,11 +0,21+0,17 Finland 7 378 559 164 0.71 266 0.71 0460,43 +144+161+2 Number of plants included The data-base is better for Cod than for BOD. Nevertheless it is quite remarkable that even though the concentrations of organic matter is vastly different from one plant to the other, and from one country to the other, the fractions of suspended COd and bod are generally high and quite similar in the three countries. Table 1 indicates that one can expect that the suspended organic matter(both as COd and bOd) in the wastewater of these countries is close to 70 of the total. And this does not include the colloidal fraction that can be estimated to be in the range of 10-15 % This means that only a fraction of 15-20 of the total COd is truly soluble Figure 1 demonstrates that there are big differences, but that the particulate fraction does not seem to be dependent on the COD-concentration of the wastewater or country. In many cases the fraction of suspended COd is higher than 75%, in some even higher than 80%. It can be noticed in figure 1 that the norwegian wastewater seem to have generally a lower total COD-and BOD-concentration than the Swedish and especially the Finnish wastewater, even though the fraction of soluble COD seem to be about the same ,9 0,9 0,8 0,7 0.7 0.6 05 0,6 0,5 0,4 E 0, 3 o Sweden 0.3。 Sweden B02№ay 0, 2 f- Norway 0.1 ▲ Finland 200 400 0 800 Total coD Total BOD Fig 1 Fraction of suspended COd and BOd versus total COD and bod
3 Organic matter In table 1 are summarised the data reported on suspended solids and organic matter. Table 1 Average values on organic matter in raw wastewater from Scandianvian plants Country N BOD/COD 1 SS COD CODf Fract. CODSS BOD BOFf Fraction BODSS Tot Filtr Sweden 17 243 +87 477 +123 157 +79 0,68 +0,10 171 +72 63 +47 0,66 +0,12 0,32 0,38 +0,12 +0,10 Norway 12 143 +39 233 +69 81 +30 0,66 +0,11 113 +28 33 +9 0,71 +0,11 0,48 0,48 +0,21 +0,17 Finland 7 378 +144 559 +161 164 +22 0,71 +0,06 266 +78 81 +27 0,71 +0,06 0,46 0,43 +0,08 +0,05 1 Number of plants included The data-base is better for COD than for BOD. Nevertheless it is quite remarkable that even though the concentrations of organic matter is vastly different from one plant to the other, and from one country to the other, the fractions of suspended COD and BOD are generally high and quite similar in the three countries. Table 1 indicates that one can expect that the suspended organic matter (both as COD and BOD) in the wastewater of these countries is close to 70 % of the total. And this does not include the colloidal fraction that can be estimated to be in the range of 10-15 %. This means that only a fraction of 15-20 % of the total COD is truly soluble. Figure 1 demonstrates that there are big differences, but that the particulate fraction does not seem to be dependent on the COD-concentration of the wastewater or country. In many cases the fraction of suspended COD is higher than 75 %, in some even higher than 80 %. It can be noticed in figure 1 that the Norwegian wastewater seem to have generally a lower total COD- and BOD-concentration than the Swedish and especially the Finnish wastewater, even though the fraction of soluble COD seem to be about the same. O d B nde p e s u s on of cti Fr a 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0 200 400 600 800 Total COD Fr a ction of s u s p ended C O D Sw eden Norw ay Finland 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0 100 200 300 400 Total BOD D Sw eden Norw ay Finland Fig 1 Fraction of suspended COD and BOD versus total COD and BOD
How large the fraction of biodegradable organic matter is in a given case, is dependent upon (1)Origin of the wastewater-for instance influence of food industry wastewater, (2) Infl uence o f leakage water-storm water and infiltration water and (3)Characteristics of sewer system It seems more and more clear that processes taking place in the sewer network play an important role. These processes may be physical(settling), chemical(precipitation) and biological biodegradation). Of special importance with respect to organic matter are biodegradation processes and therefore availability of oxygen in the sewer system. If the wastewater carries an ample concentration of oxygen, aerobic biodegradation will take place in biofilms and in bioflocs. The most easily biodegradable matter will rapidly be converted into bacterial cells, i.e. into particulate, less biodegradable matter. On the contrary, if anaerobic conditions prevail in the sewer system, biodegradation of soluble organic matter is very slow. Hydrolysis of the particulate fraction of the organic matter may be significant, however, and result in an increase in the soluble fraction and a decrease in the particulate fraction. This may explain some of the regional differences. In Norway, flow speed, many pumping stations, large water surface-to-volume ratio etc). In most of the high for instance, there are many wastewater systems that carry oxygen-rich wastewater(caused by Norwegian plants reported, the soluble COD-concentration is well below 100 mg/l. It is experienced that the non-biodegradable soluble COd in these kind of waters is in the order of 30-40 mg/l. This may vary from country to country, but different investigations in the three countries(Mattsson 1997), (Carlsson et al, 1997),(Jeppson, 1997),(Rautiainen, 1995)indicate that this is a level of non biodegradable Cod that is typical in Scandinavia When the level of soluble COD is low in the first place and the level of non-biodegradable COD seem to be at a certain level (30-40 mg COD/D), the level of biodegradable COD can be expected to be particularly low in Norway as compared to Sweden and particularly Finland. In figure 2 the BSCOD-concentration for the different plants is plotted, assuming that the non-biodegrable soluble COD-concentration is 35 mg/l in all cases. The average BSCOD-levels calculated this way is 122, 4+ 79, 2 for the Swedish plants, 129, 3+22, 0 for the Finnish plants and only 46, 7+ 28, 3 for the norwegian plants. There are large variations, especially at higher COD-concentrations, where the assumption about a constant concentration of non-biodegradable COd may not hold. The figure demonstrates well. however that the concentration of biodegradable soluble cod in the norwegian wastewater can be expected to be very low, in some instances close to zero o Sweden 200 600 800 Figure 2BSCOD-concentration versus total COD-concentration
4 How large the fraction of biodegradable organic matter is in a given case, is dependent upon; (1) Origin of the wastewater - for instance influence of food industry wastewater, (2) Influence of leakage water - storm water and infiltration water and (3) Characteristics of sewer system. It seems more and more clear that processes taking place in the sewer network play an important role. These processes may be physical (settling), chemical (precipitation) and biological (biodegradation). Of special importance with respect to organic matter are biodegradation processes and therefore availability of oxygen in the sewer system. If the wastewater carries an ample concentration of oxygen, aerobic biodegradation will take place in biofilms and in bioflocs. The most easily biodegradable matter will rapidly be converted into bacterial cells, i.e. into particulate, less biodegradable matter. On the contrary, if anaerobic conditions prevail in the sewer system, biodegradation of soluble organic matter is very slow. Hydrolysis of the particulate fraction of the organic matter may be significant, however, and result in an increase in the soluble fraction and a decrease in the particulate fraction. This may explain some of the regional differences. In Norway, for instance, there are many wastewater systems that carry oxygen-rich wastewater (caused by high flow speed, many pumping stations, large water surface-to-volume ratio etc). In most of the Norwegian plants reported, the soluble COD-concentration is well below 100 mg/l. It is experienced that the non-biodegradable soluble COD in these kind of waters is in the order of 30-40 mg/l. This may vary from country to country, but different investigations in the three countries (Mattsson, 1997), (Carlsson et al, 1997), (Jeppson, 1997), (Rautiainen, 1995) indicate that this is a level of non biodegradable COD that is typical in Scandinavia. When the level of soluble COD is low in the first place and the level of non-biodegradable COD seem to be at a certain level (30-40 mg COD/l), the level of biodegradable COD can be expected to be particularly low in Norway as compared to Sweden and particularly Finland. In figure 2 the "BSCOD"-concentration for the different plants is plotted, assuming that the non-biodegrable soluble COD-concentration is 35 mg/l in all cases. The average BSCOD-levels calculated this way is 122,4 + 79,2 for the Swedish plants, 129,3 + 22,0 for the Finnish plants and only 46,7 + 28,3 for the Norwegian plants. There are large variations, especially at higher COD-concentrations, where the assumption about a constant concentration of non-biodegradable COD may not hold. The figure demonstrates well, however, that the concentration of biodegradable, soluble COD in the Norwegian wastewater can be expected to be very low, in some instances close to zero. Figure 2 "BSCOD"-concentration versus total COD-concentration 0 20 40 60 80 100 120 140 160 180 200 0 200 400 600 800 Total COD, mg O/l "B S COD", m gO/l Sweden Norway Finland
It is obvious that also some of the suspended organic matter is biodegradable. It is probably more slowly biodegradable, however, since hydrolysis has to be involved in order to this organic matter to be biodegraded. We may analyse this matter by looking at the bOD/COD-ratio (see figure 3). There were, however, fewer BOD-data than COD-data and therefore the analysis becomes a bit uncertain 0,8 o Swed 0,8 ▲ Finland ▲ Finland 0,6 0,6 0.4 0.4 0 0 200 400 600 800 400 00 800 Total COD, mg/ Total COD, mg/ Figure 3 Ratio bOd/Cod and bod/CODe versus total COD Again we can see that the variation between the different countries is not so great and that th fraction is around 0, 4 in both cases. There is a tendency, however, that the fraction of biodegradable organic matter to that of total organic matter increases with increasing total COD. This is particularly evident in the filtered samples. This is also reasonable, since it may be expected that the amount of organic matter that is biodegraded in the network is relatively independent upon the concentration of biodegradable COd as long as there is no limitation with respect to the presence of biodegradable matter Nitrogen and phosphorus Nitrogen appears in wastewater mainly as ammonium and therefore the particulate fraction of nitrogen is quite low. In the survey, some plant owners have reported data on tot n on filtered samples, but most reported data on NH4-N and NO3-n(or only NH4-N)in addition to tot N. It was demonstrated in a survey of Norwegian plants(Osterhus, 1991)(Odegaard, 1992), that the organic N ily exi ded form(see table 2) Table 2 Nitrogen in wastewater from 10 Norwegian, chemical plants(Odegaard, 1992) Parameter Inlet Outlet Variation range Average Variation range 24.8 14,6-45,0 11,8-348 Tot N 19,6 11,7-326 NHA-N 9,1-45,0 18,3 6,5-326 0,1 0,1 NO2-N 0.3 0-14 0.6
5 It is obvious that also some of the suspended organic matter is biodegradable. It is probably more slowly biodegradable, however, since hydrolysis has to be involved in order to this organic matter to be biodegraded. We may analyse this matter by looking at the BOD/COD-ratio (see figure 3). There were, however, fewer BOD-data than COD-data and therefore the analysis becomes a bit uncertain. 0 0,2 0,4 0,6 0,8 1 0 200 400 600 800 Total COD, mg/l BO D/CO D Sweden Norway Finland 0 0,2 0,4 0,6 0,8 1 0 200 400 600 800 Total COD, mg/l BO Df/C O Df Sweden Norway Finland Figure 3 Ratio BOD/COD and BODf/CODf versus total COD Again we can see that the variation between the different countries is not so great and that the fraction is around 0,4 in both cases. There is a tendency, however, that the fraction of biodegradable organic matter to that of total organic matter increases with increasing total COD. This is particularly evident in the filtered samples. This is also reasonable, since it may be expected that the amount of organic matter that is biodegraded in the network is relatively independent upon the concentration of biodegradable COD as long as there is no limitation with respect to the presence of biodegradable matter. Nitrogen and phosphorus Nitrogen appears in wastewater mainly as ammonium and therefore the particulate fraction of nitrogen is quite low. In the survey, some plant owners have reported data on tot N on filtered samples, but most reported data on NH4-N and NO3-N (or only NH4-N) in addition to tot N. It was demonstrated in a survey of Norwegian plants (Østerhus, 1991)(Ødegaard, 1992), that the organic N primarily exist on suspended form (see table 2). Table 2 Nitrogen in wastewater from 10 Norwegian, chemical plants (Ødegaard, 1992) Parameter Inlet Outlet Average Variation range Average Variation range Tot N Tot Nf NH4-N NO3-N NO2-N 24,8 19,6 19,1 0,1 0,3 14,6 - 45,0 11,6 - 45,0 9,1 - 45,0 0 - 0,2 0 - 1,4 20,9 20,4 18,3 0,1 0,6 11,8 - 34.8 11,7 - 32,6 6,5 - 32,6 0 - 0,2 0 - 2,4
It can be seen that the particulate fraction of the tot n was around 20%, and that the filtered Tot N was about equal to the inorganic N. Consequently the organic n was mainly on particulate form This can also be seen from the fact that the amount of n left after coagulation was very close to the sum of the inorganic N In table 3 the results of the survey of the Scandinavian plants are shown with respect to nitrogen as well as phosphorus Table 3 Average values on N and p in raw wastewater from Scandianvian plants Country N TotN NH -N F-Nss Tot P PO -P F-Pss Sweden 1733,124,4 0, 6.14 8.1+7.6 +0.13 +165+142 +0.15 12 0 13.7 0,29 3.00 1.10 0.65 +4.2 +0.08+1.12+0.62+0.09 Finland 743828,7 0.33 7.473.29 +10.4+7.6 +007 +134 Based on the assumption that the organic n is suspended 2 Based on the assumption that the soluble p is equal to soluble PO4-p As it is shown i figure 4, the suspended fraction of tot n varies between 0, 2 and 0, 5, but is in most cases around 30 % It seems to decrease a bit with increasing tot N concentration. Generally it seems from the data that the plants with a higher fraction of soluble COd had a lower fraction of particulate N. This seems reasonable, since the particulate n to a large extent is organically N bound in biomass In oxygen- rich systems with considerable biodegradation, inorganic, soluble N(NH4-N is used in the assimilation process producing biomass, i. e particulate, organic N 日0.8 ay 08 0.6 ▲ Finland 0.6 0.4 o Swede ▲ Finland 0 0102030405060 2 6 Total N, mg N/ Total P, mgP/ igure 4 Fraction of N and P on suspended form In the survey, some plant owners gave data for filtered tot P and some only on PO4-P. The difference was not great. However, filtered tot P is on average somewhat higher than PO4-P. There were large variations from plant to plant with respect to the particulate fraction of phosphorous(30 80%). This may be caused by the fact that the sludge water may influence in some cases on inlet concentrations since the inlet sampling point in some cases may be downstream the introduction point of the sludge water. It was a bit surprising, however, that in most of the plants the fraction of particulate P was higher than 50 %. Again, it is the plants with a low fraction of soluble COD, that has a high fraction of particulate P. As mentioned for nitrogen, this indicates again that a considerable fraction of the particulate P is in fact present in biomass
6 It can be seen that the particulate fraction of the tot N was around 20 %, and that the filtered Tot N was about equal to the inorganic N. Consequently the organic N was mainly on particulate form. This can also be seen from the fact that the amount of N left after coagulation was very close to the sum of the inorganic N. In table 3 the results of the survey of the Scandinavian plants are shown with respect to nitrogen as well as phosphorus. Table 3 Average values on N and P in raw wastewater from Scandianvian plants Country N Tot N NH4-N F-NSS 1 Tot P PO4-P F-PSS 2 Sweden 17 33,1 +8,1 24,4 +7,6 0,28 +0,13 6,14 +1,65 3,26 +1,42 0,49 +0,15 Norway 12 22,0 +6,2 13,7 +4,2 0,29 +0,08 3,00 +1,12 1,10 +0,62 0,65 +0,09 Finland 7 43,8 +10,4 28,7 +7,6 0,33 +0,07 7,47 +1,34 3,29 +1,36 0,56 +0,19 1 Based on the assumption that the organic N is suspended 2 Based on the assumption that the soluble P is equal to soluble PO4-P As it is shown i figure 4, the suspended fraction of tot N varies between 0,2 and 0,5, but is in most cases around 30 %. It seems to decrease a bit with increasing tot N concentration. Generally it seems from the data that the plants with a higher fraction of soluble COD had a lower fraction of particulate N. This seems reasonable, since the particulate N to a large extent is organically N bound in biomass. In oxygen-rich systems with considerable biodegradation, inorganic, soluble N (NH4-N) is used in the assimilation process producing biomass, i.e. particulate, organic N. Figure 4 Fraction of N and P on suspended form 0 0,2 0,4 0,6 0,8 1 02468 1 Total P, mgP/l Fr a ction s u s p e nded P Sw eden Norw ay Finland 0 0 0,2 0,4 0,6 0,8 1 0 10 20 30 40 50 60 Total N, mg N/l Fr a ctionsuspended N Sw eden Norw ay Finland In the survey, some plant owners gave data for filtered tot P and some only on PO4-P. The difference was not great. However, filtered tot P is on average somewhat higher than PO4-P. There were large variations from plant to plant with respect to the particulate fraction of phosphorous (30- 80 %). This may be caused by the fact that the sludge water may influence in some cases on inlet concentrations since the inlet sampling point in some cases may be downstream the introduction point of the sludge water. It was a bit surprising, however, that in most of the plants the fraction of particulate P was higher than 50 %. Again, it is the plants with a low fraction of soluble COD, that has a high fraction of particulate P. As mentioned for nitrogen, this indicates again that a considerable fraction of the particulate P is in fact present in biomass
C/N-ratios and C/P-ratios In table 4 are given calculated C/N-and C/P-ratios. The C/N-ratio is often taken as an indication of the availability of carbon source for pre-denitrification. One has to remember, however, that it is the biodegradable organic matter that counts, and that particulate nitrogen may be hydrolysed during the process(extent depending on which process). The C/N-ratios that tell us most, therefore, is probably the bod tot N-ratio or the bscod tot N-ratio Table 4 Average values calculated C/N-and c/P-ratios from Scandinavian plants Country N COD/ CODf/ BOD/ BOD/ BSCOD/ BSCOD/ COD/ COD/ NHg-N NHA-N Tot n NHI-N TP POk-P Sweden 17 14.7 2.6 79,555,5 3.3 +10 6 +4.0 23.9 Norway 1211,5 6.3 3,3 86.7 93,2 +14 +23 +249+48.3 Finland 7 13.5 5.3 2.8 75.4 +5,0±1,8 +14 +1,3 +0.9 +1.6 29.3+52.1 It is interesting to note that even of there are great variations between the C/N- ratios when comparing the three countries, the differences are smaller than one could expect, even though the Norwegian waters generally seem to be less suitable for pre-denitrification than the ones in Swedish and Finland. Based on the COD/Tot N-ratio alone one might draw the conclusion that the available carbon source is more than sufficient for pre-denitrification in most cases. When considering the cases(see figure 4), especially when there is not time available for hydrolysis(as in some biofilm BSCOD/Tot N-ratio, however, one may fear that the available carbon source is insufficient in many processes ). One has to remember, however, the carbon source that will be needed for oxygen assimilation. Especially in the Norwegian wastewater the oxygen concentration in the inlet water is normally high, making this wastewater even more unfavourable for pre-denitrification o Sweden a Norw ay a Norw ay 64▲ Finland ▲ Finland 0 200 600 800 Total COD, mg/l mg Po4-P/ Figure 5"BSCOD"/TotN versus Figure 6"BSCOD"/PO4-P ratio versus COD concentration PO4-P concentration
7 C/N-ratios and C/P-ratios In table 4 are given calculated C/N- and C/P-ratios. The C/N-ratio is often taken as an indication of the availability of carbon source for pre-denitrification. One has to remember, however, that it is the biodegradable organic matter that counts, and that particulate nitrogen may be hydrolysed during the process (extent depending on which process). The C/N-ratios that tell us most, therefore, is probably the BOD/Tot N-ratio or the BSCOD/Tot N-ratio. Table 4 Average values calculated C/N- and C/P-ratios from Scandinavian plants Country N COD/ TN CODf/ NH4-N BOD/ TN BODf/ NH4-N BSCOD/ Tot N BSCOD/ NH4-N COD/ TP CODf/ PO4-P Sweden 17 14,7 +3,3 7,1 +4,2 5,2 +2,1 2,6 +1,0 3,8 + 2,6 5,5 + 4,0 79,5 +7,2 55,5 +23,9 Norway 12 11,5 +1,4 6,3 +1,4 5,2 +2,3 2,2 +0,7 2,1 + 0,8 3,3 + 1,3 86,7 +24,9 93,2 +48,3 Finland 7 13,5 +5,0 5,3 +1,8 6,1 +1,4 3,3 +1,3 2,8 + 0,9 4,2 + 1,6 78,2 +29,3 75,4 +52,1 It is interesting to note that even of there are great variations between the C/N-ratios when comparing the three countries, the differences are smaller than one could expect, even though the Norwegian waters generally seem to be less suitable for pre-denitrification than the ones in Swedish and Finland. Based on the COD/Tot N-ratio alone one might draw the conclusion that the available carbon source is more than sufficient for pre-denitrification in most cases. When considering the BSCOD/Tot N-ratio, however, one may fear that the available carbon source is insufficient in many cases (see figure 4), especially when there is not time available for hydrolysis (as in some biofilm processes). One has to remember, however, the carbon source that will be needed for oxygen assimilation. Especially in the Norwegian wastewater the oxygen concentration in the inlet water is normally high, making this wastewater even more unfavourable for pre-denitrification. 0 1 2 3 4 5 6 0 200 400 600 800 Total COD, mg/l "B S COD"/ T ot N Sw eden Norw ay Finland 0 20 40 60 80 100 0 2 4 6 8 10 mg PO4-P/l "B S COD"/ P O4- P Sw eden Norw ay Finland Figure 5 "BSCOD"/TotN versus Figure 6 "BSCOD"/PO4-P ratio versus COD concentration PO4-P concentration
It is difficult to evaluate the C/P-ratios since biological phosphorous removal is totally dependent upon the availability of readily biodegradable organic matter (i.e. VFA). The C/P-ratios given in table 4 do not tell much, therefore In figure 6, the"BSCOD"/PO -ratio is plotted against the PO4-P concentration. This ratio is probably the one that tells most. It is obviously great differences among the plants with respect to suitability for bio-P removal nfluence of wastewater characteristics on optimal selection and operation of treatment processes Even though there are great variations in the characteristics of Scandinavian wastewater, it seems quite clear that the majority in this survey, could be characterised as having a high fraction of suspended organic matter, a considerable fraction of suspended phosphorus and a surprisingly high fraction of suspended nitrogen. It is also reason to believe that many plants have a very low fraction of organic matter on readily biodegradable form. Below we shall discuss what implications such wastewater characteristics may have on various choices that have to be made when selecting the total wastewater treatment method Conventional versus enhanced primary treatment Traditionally primary treatment by sedimentation has been used to remove suspended solids in raw wastewater. The removal efficiency is primarily governed by the size of the particles constituting the ended matter and the hydraulic surface load on the settling tank Typically removals of around 50% with respect to SS and around 30 % with respect to BOD at surface loading rates in the order of 2-2, 5 m/h are experienced, corresponding to removal of particles down to around 100 um. There may be different reasons for including a primary treatment step. In most cases, however, it is used to lower the organic loading on the proceeding biological step. Even at a BOD-removal of 30% primary treatment is very cost-effective If BOD-removal is considered the important factor for using primary treatment, one would gain a lot by enhancing the primary treatment step efficiency Enhancing particle separation by coagulation The traditional way of enhancing separation of colloidal matter and fine particles, is by coagulation/flocculation. Table 5 demonstrates what can be achieved by primary precipitation in wastewater with characteristics like the ones described above The downside of traditional chemical primary precipitation is the increased sludge production as compared to primary settling only, partly as results of improved SS-removal but mainly due to precipitated material. The sludge produced during coagulation consists basically of the suspende solids removed and the coagulated/precipitated matter, as described below(Odegaard, 1994) SP=SSin-SSout+ Kprec d sludge production(g SS/m) SSin, SSout =SS concentration in influent and effluent respectively(g SS/m) sludge production coefficient(g SS/g Me), around 4-5 for Fe and 6-7 for Al dosage of metal coagulant(g Me/m)
8 It is difficult to evaluate the C/P-ratios since biological phosphorous removal is totally dependent upon the availability of readily biodegradable organic matter (i.e. VFA). The C/P-ratios given in table 4 do not tell much, therefore. In figure 6, the "BSCOD"/PO4-ratio is plotted against the PO4-P concentration. This ratio is probably the one that tells most. It is obviously great differences among the plants with respect to suitability for bio-P removal. Influence of wastewater characteristics on optimal selection and operation of treatment processes Even though there are great variations in the characteristics of Scandinavian wastewater, it seems quite clear that the majority in this survey, could be characterised as having a high fraction of suspended organic matter, a considerable fraction of suspended phosphorus and a surprisingly high fraction of suspended nitrogen. It is also reason to believe that many plants have a very low fraction of organic matter on readily biodegradable form. Below we shall discuss what implications such wastewater characteristics may have on various choices that have to be made when selecting the total wastewater treatment method. Conventional versus enhanced primary treatment Traditionally primary treatment by sedimentation has been used to remove suspended solids in raw wastewater. The removal efficiency is primarily governed by the size of the particles constituting the suspended matter and the hydraulic surface load on the settling tank. Typically removals of around 50 % with respect to SS and around 30 % with respect to BOD at surface loading rates in the order of 2-2,5 m/h are experienced, corresponding to removal of particles down to around 100 µm. There may be different reasons for including a primary treatment step. In most cases, however, it is used to lower the organic loading on the proceeding biological step. Even at a BOD-removal of 30 %, primary treatment is very cost-effective. If BOD-removal is considered the important factor for using primary treatment, one would gain a lot by enhancing the primary treatment step efficiency. Enhancing particle separation by coagulation The traditional way of enhancing separation of colloidal matter and fine particles, is by coagulation/flocculation. Table 5 demonstrates what can be achieved by primary precipitation in wastewater with characteristics like the ones described above. The downside of traditional chemical primary precipitation is the increased sludge production as compared to primary settling only, partly as results of improved SS-removal but mainly due to precipitated material. The sludge produced during coagulation consists basically of the suspended solids removed and the coagulated/precipitated matter, as described below (Ødegaard, 1994): SP = SSin - SSout + Kprec.* D SP = sludge production (g SS/m3 ) SSin, SSout = SS concentration in influent and effluent respectively (g SS/m3 ) Kprec . = sludge production coefficient (g SS/g Me), around 4-5 for Fe and 6-7 for Al D = dosage of metal coagulant (g Me/m3 )
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 scenarios
9 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
When considering the fact that the soluble fraction of phosphate seem to constitute only about 50% (or less)of the total phosphate, there are reasons to believe that most plants using primary precipitation are overdosing, thus creating excessive sludge production. Combining a low metal cation dose with a cationic polymer dose may lead to acceptable SS, COD and P-removal at minimised sludge production Enhancing particle separation by coarse filtration Another approach that may be used, is too choose a physical treatment method that is able to remove smaller particles than can be expected by primary settling. Sand filtration would obviously yield better SS-removal than primary sedimentation but cannot be used because of the low sludge carrying capacity(low porosity). With a coarser filter with a high porosity, such problems might be overcome. This has lead to the development of coarse, floating filters(Odegaard et al, 1998) biofilm carriers as filter medium, are reported elsewhere(Odegaard et al, 1998(odegaard anlydnes Results from experiments carried out with the so called Kaldnes floating filter, that uses the Kaldne Helness, 1998 ). Here only a table showing typical performances from a pilot plant operated on preseved (1, 5 mm or 0, 3 mm sieve opening) raw wastewater under various cationic polymer dosing conditions shall be included(see table 5) Table 5 Results and operating conditions for primary treatment in a coarse floating filter with kmt biofilm carriers as filter bed N Pre- Polym Filter Run SS COD COD Runs treat rate time in Mm 173137333757344 68,3 54.6168 73,2 348 13461.5 07,597,115228,1 429072,175,1 27,2 249 63.174.7 14.7181 78.934093.5 72.5 7.5 17 16.2 90.6 283 9 80.6 It was found that the SS-removal efficiency could be higher than 75 at filtration rates at 7, 5 m/h or lower without the use of coagulating chemicals and that SS-removal efficiencies over 80 could be obtained when cationic polymers, that did not contribute to sludge production by precipitation were used The main mechanisms that are expected to contribute to the removal of suspended matter in such a floating filter, are flocculation/sedimentation and adhesion to biofilms within the filter Chemical versus biological secondary treatment In order to be able to nitrify, the organic matter concentration has to be reduced to such low levels that the autotrophs are not out-competed by the heterotrophs in their struggle for oxygen. with wastewater characteristics like the one dominating in Scandinavia, it is not evident that biological treatment is more favourable than chemical coagulation for pre-treatment before nitrification
10 When considering the fact that the soluble fraction of phosphate seem to constitute only about 50 % (or less) of the total phosphate, there are reasons to believe that most plants using primary precipitation are overdosing, thus creating excessive sludge production. Combining a low metal cation dose with a cationic polymer dose may lead to acceptable SS, COD and P-removal at minimised sludge production. Enhancing particle separation by coarse filtration Another approach that may be used, is too choose a physical treatment method that is able to remove smaller particles than can be expected by primary settling. Sand filtration would obviously yield better SS-removal than primary sedimentation but cannot be used because of the low sludge carrying capacity (low porosity). With a coarser filter with a high porosity, such problems might be overcome. This has lead to the development of coarse, floating filters (Ødegaard et al, 1998). Results from experiments carried out with the so called Kaldnes floating filter, that uses the Kaldnes biofilm carriers as filter medium, are reported elsewhere (Ødegaard et al, 1998)(Ødegaard and Helness, 1998). Here only a table showing typical performances from a pilot plant operated on presieved (1,5 mm or 0,3 mm sieve opening) raw wastewater under various cationic polymer dosing conditions shall be included (see table 5). Table 5 Results and operating conditions for primary treatment in a coarse floating filter with KMT biofilm carriers as filter bed N Runs Pretreatm. Mm Polym dose mg/l Filter rate m/h Run time hrs SS in mg/l SS out mg/l SS % COD in mg/l COD out mg/l COD % 2 1,5 none 7,5 17,3 137 33,3 75,7 344 109 68,3 1 0,3 none 7,5 54,6 168 45,0 73,2 348 134 61,5 1 0,3 3,0 7,5 97,1 152 28,1 81,4 290 72,1 75,1 1 0,3 5,0 7,5 25,1 147 27,2 81,5 249 63,1 74,7 3 1,5 3,3 7,5 14,7 181 38,3 78,9 340 93,5 72,5 3 1,5 5,5 7,5 5,3 172 16,2 90,6 283 54,9 80,6 It was found that the SS-removal efficiency could be higher than 75 % at filtration rates at 7,5 m/h or lower without the use of coagulating chemicals and that SS-removal efficiencies over 80 % could be obtained when cationic polymers, that did not contribute to sludge production by precipitation were used. The main mechanisms that are expected to contribute to the removal of suspended matter in such a floating filter, are flocculation/sedimentation and adhesion to biofilms within the filter. Chemical versus biological secondary treatment In order to be able to nitrify, the organic matter concentration has to be reduced to such low levels that the autotrophs are not out-competed by the heterotrophs in their struggle for oxygen. With wastewater characteristics like the one dominating in Scandinavia, it is not evident that biological treatment is more favourable than chemical coagulation for pre-treatment before nitrification
