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6.3.2 Off-site systems

Overview and future development

The recent report of the EEA shows that the percentage of the population served by wastewater treatment varies from about 50% in the Southern to about 80% in the Northern and Western countries (EEA, 1999). It concludes that wastewater treatment has improved in many countries during the past 10 to 15 years, especially in the South of Europe where the backlog was large. A larger proportion of the population has been connected to treatment plants and the treatment level has changed. There has been a pronounced change from primary to secondary treatment and with it, a substantial reduction of the organic degradation of European rivers. Moreover, in Western and Northern Europe, the introduction of tertiary treatment, usually with phosphorous removal, has grown substantially in the past decade.

Table 6.8 and Figure 6.4 show that treatment is most advanced in the North of the region with 57% tertiary and another 23% biological treatment achieved. Tertiary treatment is found in the Nordic countries, Germany, Switzerland, Austria and the Netherlands, while most of the wastewater in the Unite Kingdom and Luxembourg is treated in plants with secondary treatment (EEA, 1999). In the south of the region, 29% is still discharged without treatment and only 43% receive secondary treatment. In France and Italy, more than half of the wastewater is treated in secondary plants. Over the past 15 years, reductions of 50% to 80% in organic matter discharged and 60% to 80% in phosphorous have been observed in the Northern EU countries but only small a reduction in nitrogen has been achieved; few countries have yet upgraded their wastewater treatment to include nitrogen removal.

Figure 6.4: Wastewater treatment of the combined load from households and indirect industrial discharges in EU10 Member States

Source: EEA, 1999

Table 6.9 contains information on the percentage of the population connected to wastewater treatment plants on a country-by-country basis. Unfortunately, the Table is not complete and to some degree at variance with Table 6.8. Nevertheless, it is shown here because of the evolution it exhibits between 1990 and 1995 in terms of population percentages. However, of greater interest is Table 6.8 because it is more detailed, more up-to-date and exhibits information with respect to the combined load from both households and indirect industrial discharges.

The further development of wastewater treatment in Western Europe is governed by EU Directive 91/271. The Directive and the progress expected are reviewed in Section 6.6.1, Table 6.15 and Figure 6.9.

Priority issue: nitrogen and phosphorus removal

The stringent requirements set forth in the European Council's Directive 91/271 concerning the collection, treatment and disposal of urban wastewater imply that in the so-called "sensitive" areas of Western Europe a very high degree of treatment must be provided whenever the Population equivalent is greater than 10000 pe (see Section 6.6.1). Figure 6.8 exhibits the extent of the "sensitive" areas. The Directive stipulates that in these areas the concentrations of total nitrogen and total phosphorus in effluent must be extremely low: total phosphorus must be 2 and 1 mg/L, and total nitrogen 15 and 10 mg/L in the effluent from locations with, respectively, between 10000 and 100000 pe and with more than 100000 pe. Percentage reduction rates for phosphorus must be 80% and for nitrogen 70 to 80%. BOD5 and COD must be, 25 and 125 mg/L, respectively, with a minimum percentage reduction of 70-90% and 75%, respectively. Total solids must be below 35 mg/L with a reduction rate of 90%. This implies that in Western Europe tertiary (or advanced) sewage treatment is more common than in other parts of the world. The terms tertiary and advanced treatment are not always used consistently. No comprehensive comparative survey has been undertaken as to the type, processes, design, equipment, and operation of the facilities for advanced treatment in the countries of the EU but a great deal of technical information has been published and is conveniently summarized some of the handbooks referred to in Section 6.0.6 (e.g. CIWEM 1994 & ATV 1997a)

The work horse for advanced treatment is the activated sludge process but trickling filters, rotating biological contactors and fixed bed reactors are also in use either because they exist or as a final stage. Phosphorus removal is usually combined with denitrification and almost always depends on chemical action though in some cases biological processes are used. In the design of the process, the method for the removal of phosphorus is decided first. The methods used are either precipitation prior to sedimentation, or precipitation simultaneously with nitrogen during removal by the activated sludge process, or thereafter. Floculants most commonly used include chlorites, chlorosulfates and sulfates of iron, aluminum sulfate, poly-aluminum chlorite, sodium aluminate, or a mixture of these.

Activated sludge units operate in one or two stages. This is depicted in Figure 6.5. Several options are in use. Nitrification always occurs in the aerobic zone. Denitrification takes place before nitrification in single stage plants, or between two aerobic stages in the two stage process. In the latter case, external carbon sources may be added such as acetates, alcohols, or starches, or either industrial effluent rich in these substances or in internal carbon source like hydrolyzed sludge. There are also schemes which provide for simultaneous denitrification. In these cases, surface aerators create aerobic conditions close to their operating points but when the water flows away from the aerators, oxygen depletion occurs and denitrification can take place. A case study is contained in Section 6.11.2 with further details.

Figure 6.5: Removal of nitrogen

Source: ATV, 1997

Satisfactory results cannot be obtained unless the system is carefully monitored and controlled (ATV 1997b). In the North of Europe, temperature is an important factor; it may be 10 degree Celsius and lower. Parameters to be monitored include, in addition to inflow, detention period, outflow, the amount and the age of sludge: pH and temperature, the organic and nutrient load in the system and its units both in the liquid and sludge phase, recirculation rates of liquid and sludge, and the secondary sludge produced. Effective monitoring is assured by on-line monitors for BOD, COD, TC, ammonia and nitrites and nitrates, and phosphorus and the physical parameters indicated. Automatic systems for monitoring and control are available.

Distribution of the methods used for advanced treatment is not even throughout Europe, nor is the compliance effluent with the effluent standards contained in EU Directive 91/271 concerning nitrogen and phosphorus met uniformly. As indicated in Figure 6.4, there are more plants in the West than East, and also more in the North than the South. In the North and some parts of Germany and in Switzerland, there was a boom in the introduction of nutrient removal during the late 1980s and early 1990s. The protection of some of the costal waters was a motivating factor in such countries like Denmark and The Netherlands where nutrient removal is almost universal. But as a whole, the countries of the EU are probably leading in nutrient removal world-wide.

One of the few comparative studies (Fink et al. 1998) conducted in 6 EU countries shows that plants are more generously designed in countries like Denmark where aeration and secondary sedimentation tanks are larger than elsewhere (540 L/pe). Temperature and higher effluent standards are among the reasons, but also that in Denmark, like in France, plants are laid out for aerobic stabilization, even for up to 300 000 pe. In France, loads are high and with 205 L/pe, aeration tanks small. Small aeration tank are also found in Switzerland and Italy where they have been designed for lower rates of nitrogen removal. There are also differences as regards secondary sedimentation. In France and Italy, the tanks are smaller than in Germany whereas the surface load is the same.

Low loads on the activated sludge process produce removal rates of nearly 90% of total nitrogen in Denmark, France, Germany and The Netherlands. The load may be between 0.044 to 0.064 kg/d in the plants surveyed in the sample. In Italy and Switzerland, the loads of the plants surveyed are higher and removal rates are between 60 and 70%. Phosphorus removal rates are as high as 93% in the plants surveyed in Denmark and The Netherlands, while as low as 70% in some of the other countries.

Case Study 2 (see Section 6.11.2) describes nutrient removal in the wastewater treatment plant Zuerich-Werdhoelzli, Switzerland.

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