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United Nations Environment Programme
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

Priority issue: Sludge stabilization and utilization

Sludge stabilization and utilization is a priority issue in Western Europe because of the high degree of sewage treatment achieved aggravated by high population densities, scarcity of land and rigid legislation for safeguarding human health and the environment against hazards arising from waste material. The state of the art is summarized in some of the handbooks referred to in Section 6.0.6 (e.g. CIWEM 1995a and b, 1996 and 1999 & ATV 1997).

Technology development was dominated to large measure by changes in industrial technology, equipment and machinery rather than traditional wastewater engineering. Accordingly, technology is highly sophisticated, bounded by technological inventions and patents, and, increasingly, combined into integrated packages offered by manufacturers. The leeway which planners and consulting organizations usually have in the field of wastewater engineering is limited when technology for sludge stabilization and handling is chosen.

The ultimate disposal of sewage sludge is a key technical and regulatory problem. Where will it end up? And in which form?

Figure 6.6 is an answer with respect to the first of these two questions; the second answer is more complex. The Figure shows the trend and forecast in the then 12 Member States of EU in 1995. Over the span of 20 years, the total amount of sludge will almost double. There is an upward trend in the use of sludge in agriculture, a definite downward trend in sludge deposition and an upward trend in incineration. As of 2005, the estimated percentage of sludge used will be 45% whereas 38% will be incinerated and only 17% deposited (dumped).

Figure 6.6: Sludge disposal in 12 EU countries

Source: ATV, 19996

 

The percentages vary considerably between the counties. The but limited comparative study referred to above in "Priority issue: nitrogen and phosphorus removal" provides some insight (Fink et al. 1998). Use in agriculture was highest in France with 58% and lowest in The Netherlands (26%) and Germany (27%). Incineration highest in Switzerland (25%) and Denmark (25%) and lowest in Germany (14%). Deposition is highest in Germany (54%), Italy (55%) and The Netherlands (51%) and lowest in Denmark (20%). The trend in agricultural use is of particular interest: in Denmark it increased by 80% from 1987 to 1990 and in Germany by 50% while it decreased in The Netherlands. In considering the use of sludge in agriculture, methods of composting are given considerable attention in Europe. This will be discussed below. It must also be understood that disposal into the sea will soon end by international agreement.

As regards the agricultural use of the sludge, and the concentration of the hazardous material it may contain, Directive 86/278 of the EU on the use of sludge in agriculture and the protection of the environment and of soil (EEC 1986) sets forth minimum requirements only because prior practice in the countries was very much differentiated. It is based on the maximum permissible concentration of certain metals in the soil; once the concentrations shown in Table 6.10 have been reached, no more sludge may be used on the land. The values for chromium have not yet been agreed upon, and the allowable annual tonnage is not yet included. In some countries, values are more stringent while others have made use of their prerogative to set standards at a lower level. In some countries, additional substances have been added to the list, e.g. arsenic, fluorides, molybdenum and selenium in the UK, parts of Austria, and Switzerland. Germany has added limits for dioxins, PCBs and AOX.

Not contained in the EU Directive is the permissible sludge load. As shown in Table 6.11, countries have chosen to legislate it either before or after the date the EU Directive came into effect.

Table 6.10: Limits for metals in soil in accordance with EU Directive 865/278/EEC

Metal

Limit

Lead
Cadmium
Chromium
Cooper
Nickel
Mercury
Zinc

50-300
1-3
100-150 (tentative)
50-140
30-75
1-1.5
150-300
Source: EEC, 1986

 

Table 6.11: Permissible load

Country
 
 
Average
Annuial Load,
(t/ha)
Years of Single
Annual Load
(years)
Maximum
Single Load
(t/ha)
Austria
Belgium
Denmark
Finland
France
Germany
Ireland
Italy
Netherlands
Norway
Sweden
Switzerland
2,5
1-4
10
1
3
1.66
2
2.5-5
1-10
2
1
1.66

2
3
10
4
10
3
1
3
1
10
5
3

5
3-12
100
4
30
5
1
7.5-15
1-10
20
5
5
Source: ATV, 1996

The average amount of primary sludge produced in Western European countries is about 1.80 liters per-capita and day. This figure corresponds to an amount of 45 gr total solids per-capita and day and a moisture of 97.5%. Where combined systems exist, the mean weekly volume may be 25% higher. Secondary sludge from trickling filters is in the order of 0.63 liters per-capita and day and the excess sludge of activated sludge plants is 5,00 with the total solids and the moisture respectively 25 gr per-capita and day and 96.0% for trickling filter secondary sludge and 35 gr per-capita and day and 99.3 %for activated sludge plants.

In actual practice, however, the amount and general characteristics of the sludge varies greatly depending on the general composition of waste produced per-capita, the population equivalent of industrial sludge discharged into communal sewers, the system of stormwater collection and discharge, and the processes applied for the treatment of wastewater. The per-capita values and general composition of sludge from industry has been the subject of long-term records and the information has been published extensively. The impact of the system for the collection and discharge of stormwater has been referred to in Section 6.2.2. As regards the fourth determinant, the following should be noted:

  • Primary sedimentation has been suppressed in some cases and countries for reasons of economy and as a measure to enhance the denitrification and aerobic stabilization of the sludge.
  • Nitrification/de-nitrification is accompanied by a reduction in the volume and the organic content of the sludge.
  • Phosphorus removal increases the amount of sludge.
  • The choice of aerobic vs anaerobic sludge stabilization will greatly impact on both the amount and the composition of the sludge.

The disposal of raw sludge into the environment is no longer practiced in Europe. As of the end of 1998, the dumping of raw sludge or pumping onto land or into surface water is prohibited, and the disposal of sludge from municipal sewage treatment plants is regulated and is subject to registration and consent. An EU Directive concerning the deposition and incineration of sludge is in preparation.

There is considerable variation among the technologies chosen for the stabilization of sludge in Europe. The anaerobic digestion of sludge has been standards practice for a long period of time, especially for large plants in Germany, the Netherlands and Switzerland before legislation called for tertiary treatment. But aerobic stabilization aerobic stabilization is now an alternative especially for medium-size and smaller STPs. For large installations with more than one or several hundreds of thousands pe, the efficiency of anaerobic digestion has been greatly increased and the cost reduced by more effective processes, engineering design, and the equipment for the heating of digesters and the circulation of the sludge. The production of methane gas is an important factor in Western European countries where the cost of energy is high, and where the gas is commonly used as the primary source of energy. Similarly, where strict hygienic requirements exist, including for protecting workers of STPs, the anaerobic mesophilic process is considered advantageous.

Aerobic co-stabilization by the activated sludge process is chosen for an increasing number of STPs when nitrification/de-nitrification is applied. In these cases, the units for nitrification/de-nitrification are used additionally for the stabilization of sludge. For instance, in the but limited sample of STPs investigated in the course of the study referred to above, none of the facilities in France used anaerobic digestion. A new technology has been investigated for thermophilic aerobic stabilization but application of the process is slow, and research continues into the basic mechanisms involved, the most effective supply of oxygen, engineering design of the reactors, and operation and maintenance. The process may take place in the liquid or de-watered phase with essentially the same microbiological reactions involved. The process is also called "composting" though, generally, that term implies that the water content of the sludge would be lower than 92 to 95%. If de-watered further, the sludge is composted in bioreactors of various types with or without the addition of organic additives such as saw dust, bark, paper, or garbage. Composting is exhibited in Figure 6.7. On the left side of the Figure, composting in the liquid phase is depicted involving, as the first step, thermophilic aerobic stabilization followed by anaerobic finishing or mechanical de-watering or none. On the right side of the Figure, mechanically de-watered sludge undergoes composting in two stages with the addition of additives. In both cases, the final product is used in agriculture or landscaping.

Figure 6.7: Composting of sewage sludge (larger image)

Source: ATV, 1996

 

For de-watering, a variety of machinery is available but sludge drying beds are still being used in Europe. Filter presses and high performance centrifuges are now very common. They produce a sludge with 25 to 30% solids. If STPs serve less than 10 000 pe, stationary machinery may not be economical; in such cases, the sludge may be stored intermittendly and the de-watering accomplished by mobile equipment. For latest technologies, reference is made to the information presented at IFAT 99 (IFAT 1999).

 

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