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 <Inm), colloidal (1 nm <d< 1 um)or particulate matter(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 <1nm), colloidal (1 nm < d < 1 µm) or particulate matter (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