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 Finland2 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 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 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