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
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
Table 6.11: Permissible load
|Years of Single
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
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).