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Geografia d'Europa |
Laurent Bontoux, Miguel Vega and Demosthenes Papameletiou,
IPTS
Issue: The implementation of the European Urban Wastewater Treatment Directive (91/271/EEC) is leading to a rapid multiplication of waste water treatment plants across Europe, producing increasing quantities of sludge. At the same time, disposal at sea is being banned, the landfill directive project restricts the possibility of landfilling organic material, and environmental and health concerns lead the farming profession to become more and more reluctant to spread sludge on land. To date, these were the main disposal routes for sludge. A way forward needs to be found.
Relevance: The simultaneous implementation of various
areas of European environmental policy combined with environmental and
public health fears could create a large problem for the management of
sludge from wastewater treatment. The issue carrys important public health
and environmental risks, striking sensitive chords now that the fear of
pervasive and insidious chemical pollution is rising. Therefore, a combination
of appropriate policy and technical responses at European level are rapidly
needed.
The most common municipal wastewater treatment technology applied in Europe is the activated sludge process, a biological process consuming large amounts of energy and generating large amounts of organic sludge. This sludge, separated from the treated water in the last stage of the process, contains more than 90% water and is highly biodegradable. Dewatering processes are usually applied to facilitate sludge handling and disposal. Until now, the main disposal routes were landfilling, spreading on land, disposal at sea (mostly in the UK), and incineration.
However, times are changing and limitations are appearing on all the sludge disposal routes. Restrictive waste disposal legislation linked with concerns on the potential health and environmental risks of spreading sludge in crop-fields are rendering the sludge disposal problem more acute. Simultaneously, activated sludge wastewater treatment plants keep being built in compliance with the wastewater directive and will foreseeably continue to function as "sludge factories" in the long-term with an unstoppable output. It is therefore essential to find safe, affordable and sustainable outlets for sewage sludge.
As of the beginning of 1999, disposal of wastewater treatment sludge at sea will be banned. The current restrictions being proposed for landfilling aim at excluding any organic waste from this disposal route. Incineration is expensive because of the amount of water to be eliminated from the sludge. Potentially, the most attractive option could be spreading on agricultural land because it may recycle nutrients and have a useful agronomic value. However, because of the physical-chemical processes involved in activated sludge wastewater treatment, the sludge tends to concentrate heavy metals and poorly biodegradable trace organic compounds (e.g. pesticides, household chemicals, etc...) present in the wastewater. This raises both public health and environmental issues. Another issue is the availability of enough agricultural land near sludge production centers to avoid transportation costs.
Given this lack of total safety, there is an increasing trend among farmers to be reluctant to spread sludges on their land. More and more "niche markets" (e.g. health food, baby food, vegetable canning) expressly require their suppliers to forbid the use of sewage sludges.
What is often overlooked is the cost of handling the huge quantities of sludge produced by the large treatment plants necessary to treat the enormous volumes of wastewater, and how the sludge can be managed in a safe, practicable and sustainable manner. In some cases, sludge treatment and disposal account for up to half of the overall wastewater treatment costs.
We need to take a fresh and serious look at the issue. Quality standards need to be achieved by the sludge to be spread on land and alternative treatment and disposal technologies need to be considered for other types of sludge. This highlights once more the need for good coordination of the various areas of European environmental policy.
The activated sludge process and product
The concept of "secondary treatment" of wastewater is based on the activated sludge process. In this process, organic matter from sewage is oxidized and transformed into microbial biomass by a wide range of organisms. This is generally performed in a large aerated tank where sewage and microorganisms remain in contact for a few hours. This mixture then overflows into a settling tank where the microbial flocs (aggregates) fall to the bottom and the treated wastewater flows over the weir. The flocs accumulated at the bottom of the tank are then extracted as sludge. Part of it is recycled to the aeration tank to maintain the process, while the excess sludge, produced by microbial growth, must be eliminated.
In most cases, this process is preceded by a primary settling operation which also generates organic sludge, albeit of a slightly different nature. This primary sludge must be eliminated with the excess secondary sludge. Another important element to mention is the fact that sludge quality is not constant. It varies according to the design characteristics of each plant, the type of wastewater treated, the industries connected, the season, the weather, the location of the plant, etc... It is the elimination of this excess sludge which constitutes the topic of this article.
The EU Council Directive concerning urban wastewater treatment (91/271/EEC) requires, by the end of 2005 at the latest, every agglomeration of more than 2,000 population-equivalent discharging to surface fresh water and estuaries, and of more than 10,000 equivalent inhabitants discharging to coastal waters, to apply at least secondary treatment to its wastewater before discharge.
To comply with this directive, most municipalities choose the well-known activated sludge technology for three main reasons: compactness, reliability and efficiency (if it is properly operated and maintained). However, this technology produces large amounts of sludge. The current estimate for the EU is 6.5 million tonnes per year, expected to reach 15 to 20 million tonnes by 2005. The corresponding increases in each EU Member State range from 40% to 300%!
The waste sludge must be treated to facilitate handling and to avoid potential problems from odor and pathogens. These treatments modify the sludge properties making them more suitable for reuse or disposal. Among these processes are: thickening, disinfection, stabilization, conditioning, dewatering, thermal drying, composting and others. After treating the sewage sludge the following products are obtained: liquid sludge (stabilized or raw), solid sludge (stabilized or raw), dried sludge and compost. The sludge treatment and disposal costs accounts for up to half of the total costs of sewage treatment and they are likely to increase due to the tightening of European legislation.
Because of the physico-chemical characteristics of the activated sludge process, the sludge tends to accumulate a number of metals and organic compounds. This property is an advantage when one looks at the quality of the treated wastewater but it makes sludge quality dependent on essentially 4 main groups of contaminants:
Metals
Mainly zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), lead (Pb), mercury (Hg) and chromium (Cr). Their potential accumulation in human tissues and biomagnification through the food-chain create both human health and environmental concerns. Metals are always present at low concentrations in domestic wastewaters but concentrations of concern come mostly from industrial wastewaters.
Major plant nutrients
They are nitrogen and phosphorus. They are a concern because of their eutrophication potential for ground and surface waters. However, they can be viewed as a valuable fertilizer while their main agronomic value remains in their high organic matter content. In the identified sensitive areas, the wastewater directive requires tertiary wastewater treatment (nutrient removal). These treatments also produce sludge, always high in nutrient content and of a varying nature according to the processes used.
Organic contaminants
Pesticides, industrial solvents, dyes, plasticizers, surfactants and many other complex organic molecules, generally with low water solubility and high adsorption capacity, tend to accumulate in sludge. Even polynuclear aromatic hydrocarbons (PAH) from the combustion of fossil fuels are present in sewage sludge. These cause concern about their potential impacts on the environment and particularly on human health. A specific character of this category of contaminants, compared to the previous two, is their (varied) potential for biodegradation. Many of these molecules have a slow but measurable biodegradation potential. Therefore, biological wastewater treatment systems with longer residence times will have an increased power to biodegrade these undesirable compounds. Biodegradation can also occur after land spreading or during composting.
The WHO Working Group on the Risk to Health of Chemicals in Sewage Sludge Applied to Land concluded that "the total human intake of identified organic pollutants from sludge application to land is minor and is unlikely to cause adverse health effects". However, in spite of a recent increasing level of investigation, the ecotoxicological role of organic contaminants in the soil-plant-water system and in the food chain is still not clear.
Pathogenic agents
Bacteria (such as Salmonella), viruses (notably enteroviruses), protozoa, trematodes, cestodes and nematodes are the most significant water-borne pathogens found in sludge. As a result, any safe disposal of sludge requires the elimination, or at least the sufficient inactivation of these pathogens. A range of treatments can be applied to the sludge to this end, such as pasteurization, aerobic or anaerobic digestion, composting, lime stabilization, liquid storage and dewatering and dry storage.
Sludge disposal routes
The traditional disposal routes for sewage sludge are:
Landfill
Over the last decades, landfill has been (and still is, see Figure 1) a widely used disposal route for sludge. However the new proposal for a Council Directive on the Landfill of Waste is changing this perspective. This proposal aims at reducing gradually until the year 2010 the total amount of biodegradable wastes to landfill to less than 25% of the total amount (by weight) of biodegradable municipal waste produced in 1995. Such a strong reduction will probably not be accepted in the end (a figure of 35% has been proposed), but the general trend is set. This new situation will push forward the development of alternative sludge disposal routes.
Figure 1: Disposal of sewage sludge in the EU in 1994
Source: S.R. Smith; Agricultural Recycling of Sewage Sludge and the Environment, CAB International, UK, 1996
Land spreading (agriculture)
The application of sewage sludge to farmland is possibly often the cheapest sludge disposal option. It can be compared to what is traditionally done with a wide range of organic wastes spread on land such as manure or livestock wastes. It provides an opportunity to recycle beneficial plant nutrients and organic matter for crop production. Moreover, it seems that the application of sludge to soil may in many cases improve the physical properties of soil, leading to increased crop productivity.
However, care must be taken that chemical or pathogenic contaminants present in sludge do not cause adverse effects. For example, heavy metals concentrations in sludge are mostly larger than those in the soil and these elements may be retained indefinitely in the cultivated soil layers. Therefore repeated applications of sludge will gradually increase the trace element content of soil. According to the application rate of the sludge and the metals concentrations, a time can be calculated (usually 70 to 80 years) after which the maximum permissible concentrations for each of the regulated elements in the soil are reached. After that period, sludge can no longer be applied safely. Zn, Cu and Hg are the principal elements limiting sludge recycling to agricultural land while Cd raises specific questions due to its toxicity and variable mobility.
The Cation Exchange Capacity (CEC) is the main soil property controlling the retention and toxicity of metals in sludge-treated soil. Consequently, regulations for spreading sludge on agricultural land must take into account the establishment of different sludge application limits for toxic metals. The CEC depends on pH, organic matter content and soil texture. However, the absorption capacity of plants depends on soil properties and farming practices. Spinach, celery lettuce and carrot are likely to be the crops most at risk from accumulation.
Incineration
Although it is the most expensive outlet, is frequently used simply because it reduces sludge volume by more than 90% while producing a mostly mineral ash (< 5% organic matter) that can be landfilled. In spite of specific environmental concerns, incineration is widely expected to increase due to the restriction on the organic content of landfilled material.
Disposal at sea
The disposal of sludge at sea has been one of the most popular routes in both the UK and Spain, but will be banned at the beginning of 1999. The risk of repetition of problems such as the Minamata affair, in which hundreds of people were affected by excessive concentrations of methyl mercury in the marine food chain, have pushed to ban this outlet. The issue concerned was the biomagnification potential of certain elements present in sludge.
All European countries have been relying on these various disposal routes in different respects. Table 1 gives the respective statistics for 1992.
Table 1: Disposal of sludge within the EU in 1992
|
|
(x1000 dry tonnes per year) |
(%) |
(%) |
(%) |
(%) |
(e.g. forestry) (%) |
|---|---|---|---|---|---|---|
| Austria |
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| Belgium |
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| Denmark |
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| Finland |
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| France |
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| Germany |
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| Greece |
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| Ireland |
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| Italy |
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| Luxembourg |
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| Netherlands |
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| Norway |
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| Portugal |
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| Spain |
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| Sweden |
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| Switzerland |
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| UK |
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* Disposal to surface waters
Source: Adapted from P. Matthews and K.-H. Lindner, "European Union" in "A Global Atlas of Wastewater Sludge and Biosolids Use and Disposal", P. Matthews (ed.), Scientific and Technical Report nº4, IAWQ, UK, 1996
Alternative disposal routes already exist or are being developed. They fall into four categories:
Land application
Low organic matter content (either natural or through losses) is a major problem in ensuring the maintenance of good water retention properties in soil. Sludge solids may be used to maintain, restore or create soil fertility, as well as appropriate soil-structure in degraded land. As mentioned above, heavy metals can have detrimental effects on crops and human health if allowed to accumulate beyond established safe limits. The potential risks are reduced in the soils from arid areas because they are mostly alkaline and minimize crop uptake of many elements such as heavy metals. A major sludge reuse study, funded in the frame of the Mediterranean Environmental Technical Assistance Program (METAP) by the European Investment Bank, and promoted by the Cairo Wastewater Organization, began in 1995.
Forestry has a huge potential to absorb sludge in the future. Its main advantage is the possibility to require less stringent standards than agricultural applications. Nevertheless this disposal route highly depends on regulatory support.
Energy recovery
The construction of the first commercial scale oil-from-sludge plant began in Australia in 1997. The process mimics nature by thermochemically converting sludge to oil, tar, gas and water. The oil produced is similar in nature to a middle distillate fuel and can be used to fuel both internal and external engines. The tar and gas produced are burned to dry the sludge prior to processing in the conversion reactor.
Anaerobic digestion of sludge is already widely used thanks to its ability to produce methane gas (for power production) and a more stable, easily dewatered sludge. Important trends in the anaerobic digestion towards the reactivation of the digestion process are: ultrasound treatment and special thickening centrifuge prior to digestion.
Mineral recovery
In Japan, recent legislation restricts the landfilling of ash that contains heavy metals (such as ash from incinerators). This lead to the development of the "sludge melting" technology. This process vitrifies the sludge in a combustion chamber at 1400ºC. This offers a means of stabilizing and minimizing the volume occupied by sludge, as well as offering the potential for reuse as a construction material (cement, glass ceramics, crystallized slag, etc).
In Europe, examples already exist of the use of wastewater treatment sludge in cement kilns. However, a new application is appearing: the fabrication of bricks for construction using sludge. A first full scale industrial project has started in Spain consuming about 18 tonnes of sludge solids per 200 tonnes of bricks produced.
Composting
The composting of sludge for inclusion into soil mixes for urban landscaping purposes is an important avenue for the disposal of sludge solids. However, these products are not allowed for home landscaping and cannot to be sold in bags to the general public. Another technology, vermicomposting is also promising. Development work at a number of plants in Poland has demonstrated the effectiveness of earth worms in degrading treated sludge to a non-odorous, humus-like material with high agricultural nutrient value.
Producing less sludge with alternative wastewater treatment processes
There are other biological processes than activated sludge that provide secondary wastewater treatment: trickling filters, rotating biological contactors, oxidation ditches, ponds, etc. They all produce sludge, albeit in smaller amounts than the classical activated sludge process. Each has its specific performance characteristics and none can fully replace the activated sludge process. However, more attention could be paid to them at the hour of choosing a new wastewater treatment facility in cases where sludge disposal is problematic. In the case of the activated sludge process, a low organic load will result in a smaller sludge production than a high organic load.
Other solutions have also been presented. In Japan, for example, a full-scale prototype activated sludge plant has been operating successfully for nine months without producing surplus sludge. This has been achieved by ozonating a portion of the returned activated sludge in the wastewater treatment plant, increasing its biodegradability and promoting biological oxidation within the aeration tank.
Sludge quality standards
Of course, each set of sludge quality standards must correspond to each disposal route. However, so far, the disposal of sludge on agricultural land is the only option which has seen any standard setting activity at the European level. These standards are stipulated in the European Directive 86/278/EEC concerning the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture.
This directive limits the total amount of several heavy metals depending on the pH of the soil. Nevertheless, it contains only minimum requirements permitting stricter national measures. An overall assessment of the land spreading outlet may bring together all aspects such as human health, crop yields, animal health, groundwater quality, surface water quality, air quality, soil fertility and natural ecosystems. Therefore, an extension of this directive could be desirable especially to other pollutants such as chromium, selenium, arsenic, fluoride, molybdenum, cobalt, dioxin, PCBs, AOX, PAH, chlorinated solvents and other organic chemicals. It may also be complemented by a Code of Practice for Agricultural Use of Sewage Sludge, a measure already taken by the UK government and called for by the European nitrates directive (91/676/EEC).
Directive 86/278/EEC limits the concentration values of heavy metals in both sewage sludge and soil (see Table 2).
Table 2: Limit values for potentially toxic elements
given by Directive 86/278/EEC
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|
(mg/kg dry soil) |
(mg/kg dry solids) |
| Cadmium |
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| Copper |
|
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| Nickel |
|
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| Lead |
|
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| Zinc |
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| Mercury |
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In spite of certain limits for organics set in Germany and Denmark (e.g. dioxins, PCB, AOX), there are no limit values set for trace organic contaminants in sludge or soils within the current European legislation concerning sewage sludge (in December 1997, France proposed a new legislation which includes provisions for some organics contaminants). Nevertheless, the US Environmental Protection Agency has already selected 18 organic pollutants - in the standards for the use or disposal of sewage sludge - for further evaluation by risk analysis of environmental exposures. The selection criteria considered were frequency of occurrence, aquatic toxicity, phytotoxicology, human health effects, domestic and wildlife effects, and plant uptake.
The scientific information available supports the conclusion that detrimental effects on human health arising from the agricultural use of sludge are unlikely. The main heavy metals of concern are Cd, Pb and Hg. Pb and Hg are not absorbed to any extent by crops and consequently do not pose a risk through the dietary intake of foods grown in sludge-amended soil. On the other hand, Cd is not subject to the soil-plant-barrier and can accumulate in crops to concentrations which may be potentially harmful.
Sludge application to land aims to take the maximum advantage of the soil’s capacity to assimilate, attenuate and detoxify pollutants. The World Health Organization (WHO) believes that when land application operations are managed properly, accumulation of pollutants in soil can be managed so that they will not reach levels harmful to human health.
Contamination of groundwater by leaching of nitrate from sludge-treated soil, is probably the most important impact arising from the agricultural utilization of sludge in the context of current environmental legislation. Nevertheless, assuming that the total amount of nitrate (from sludge and other sources) remains in accordance with the nitrogen requirement of the crops being grown, nitrate pollution of groundwater should stay minimal. The WHO concluded that no numerical limit should be set for the nitrogen content of sewage sludge.
An important concern currently arising is the presence of persistent organic contaminants in the sludge and their behavior in the soil-plant system. So far, the risk to human health from crops grown on sludge-treated soils appears to be small because there seems to be little or no plant intake of organic contaminants and no bioaccumulation in livestock.
Conclusion
In view of the increasing production of sludge in Europe and the simultaneous tightening of waste disposal practices, the likely preferred choices for sludge disposal in Europe for the medium-term will be chosen between land spreading (agricultural or other) and incineration. The practical choice will be made locally according to many cultural, economic and scientific parameters.
So far, there is general agreement that agricultural use can be a safe and viable option. It is certainly one of the more likely ways forward, although in some countries such as France, Germany, Sweden, and the Netherlands, fears about the effects on soils and crops are disabling this outlet. These countries, especially Germany, are developing new technologies for incineration of sludge. In other countries such as the UK, sludge use for land spreading is encouraged. In Sweden, life-cycle assessment studies of some wastewater treatment facilities identify sludge reclamation in agriculture as the best option in some cases.
In several Member States such as UK there has been a tremendous focus on developing robust control policies and practices to protect the security of the land spreading option. However, at the EU level there is a lack of a more coordinated approach to regulating sludge application to agricultural land. To date, it is up to each Member State to implement a secure and cost-effective disposal strategy for sludge.
This state of affairs highlights the need to ensure the safety of sludge spreading on agricultural land and to reassure those who already practice it or are susceptible to do so in the future. To this end, the quality of sludge is important, starting at the discharge of wastewater to the sewers, and an efficient control of this quality must be enforced. It may be useful, therefore, to encourage the treatment of certain industrial effluents separately from domestic wastewater in cases where this is not already done, or to pre-treat them before release to the sewers in order to remove excess concentrations of undesirable elements such as heavy metals.
Further research in the implication of the agricultural and incineration outlets for the environment is needed, as well as a sound scientific coordination to drive the current investigations.
It is vital that adequate information on sludge production,
treatment, outlets, products and markets be available. Therefore the time
for defining reasonable standards for sludge uses, as well as for wastewater
re-use, has come. The classification of sludges into well defined quality
classes, adequate for each disposal or re-use option must guarantee a safe
use and disposal. Sound environmental criteria should be devised for each
option. A possible basis for such standards need to include risk assessment.
Keywords
municipal wastewater treatment, sludge management, environmental legislation, european perspectives
The authors wish to thank D. Burgess (CEST), P. Magoarou
(DG XI) , L. Marmo (DG XI) and A. Piavaux (DG XII) for their constructive
comments.
About the author
Miguel Vega, IPTS
Tel: +34 95 4488211, e-mail: miguel.vega@jrc.es
Demosthenes Papameletiou, IPTS
Tel: +34 95 4488289, e-mail: demosthenes.papameletiou@jrc.es
Fuente:
http://www.jrc.es/iptsreport/vol23/english/ENV2E236.htm