Waste water produced by industrial and urban activities are collected for treatment through physical, chemical and biological processes. There are two major outputs from a waste water treatment plant (WWTP):
Clean water can be safely discharged to the environment
High quality treated sludges become Biosolids
Biosolids refers to treated sludge that meets quality standardsfor safe land application.
To guarantee the quality of municipal Biosolids, the discharge of effluent to the sewer is highly regulated. For some industries it is therefore mandatory to install pre-treatment equipment to remove potential hazardous contaminants before they can reach the wastewater treatment plant. The waste water treatment operators are constantly monitor the processes that results in Biosolids.
The main components of Biosolids are water, organic matter, macro and micro nutrients and potential toxic elements such as metals and organic compounds.
The following diagram represents the different components of the Biosolids on a dry matter basis.
This organic matter is generated from the suspended solids that are contained in the effluent discharged to the waste water treatment plant, and from the excess microorganisms produced by the biological part of the treatment process.
The intrinsic characteristics of organic matter in Biosolids are very similar to animal manure; this means that the nutrients bound to the organic matter will be readily available.
For sewage sludge, the main types of species found are bacteria, viruses, helminths, protozoa and fungi. Sludge produced by the food industries such as dairy, abattoir or meat processing factories also contain pathogens directly related to their specific activities (e. g. listeria for dairy sludge or fecal coliform and streptococcus or pneumonia for abattoir sludge).
Sludge treatment, such as thermophilic digestion, liming or composting can produce Class A Biosolids that contain no detectible levels of pathogens and therefore can be used without restriction.
In any case the survival of pathogens after land application is limited in time and most will decline below detectable limits within 3 months’ except for for helminth ova which will take up to 2 years.
Their presence in Biosolids is due to industrial discharge to the sewer network but also to the use of domestic or personal care products.
These substances can be persistent into the soil. However, experiments have shown that the transfer of these organic compounds to the crops is extremely limited.
As for heavy metals, limit values have been defined by European Member States to ensure the safety of Biosolids land application.
Biosolids also contain micronutrients such as Copper, Magnesium, Zinc which are essential in small quantities for the different chemical reactions that take place in the plant i.e. they are essential for healthy plant growth.
The main heavy metals that are contained into Biosolids are Cadmium, Mercury, Nickel, Lead and Chromium. Total concentration for these elements is representing in average less than 200 mg per kg of DS.
Biosolids are not solely responsible for the presence of heavy metals in the soils. Heavy Metals are naturally present in soil but can also come from atmospheric deposition, land application of animal manure or mineral fertilisers.
To avoid unacceptable risk to public health, European Member States have set up legal threshold limits for different metals in sludge as well as in soils. Most of the time, the amount of heavy metals in the Biosolids spread on land are insignificant when compared to regulatory thresholds.
Depending on their final destination, additional treatments of the Biosolids may be implemented.
There are three main types of Biosolids treatment
These treatments (thickening, dewatering or Drying) aim to increase the dry matter content in the Biosolids to improve their physical characteristics and to facilitate their transport, storage and landspreading.
These treatments (liming, composting, anaerobic digestion) aim to reduce the biodegradability of Biosolids in order to eliminate odours.
These treatments aim to destroy pathogens. Pasteurisation and stabilisation of the Biosolids are often carried out simultaneously.
According to the implemented treatments, different types of Biosolids are produced : thickened, dewatered , dried,, digested, limed, composted…
The Biosolids fertilising properties are also different depending on the chosen treatment.
The choice of the Biosolids treatment must be based on identified agricultural outlets.
Biosolids can be routed to three outlets
Biosolids can be either incinerated into a dedicated incinerator or in conjunction with Municipal Solid Waste.
Incineration significantly reduces the volume of Biosolids but produces an ash that needs to be disposed to landfill. Sophisticated systems are required for the treatment of stack gases. When properly operated incineration represents little risk for public health.
Contrary to what one might think, incineration of Biosolids does not produce energy as most Biosolids are not sufficiently dewatered to achieve self-combustion.
As for landfill, the public opposes the development of incineration facilities as all the beneficial constituents of Biosolids are definitively lost.
Incineration can be justified in heavily populated areas or where there is limited access to suitable landbank. In addition, incineration is the most expensive option for Biosolids. Handling and the corresponding costs weigh significantly on the price of water.
Landfilling is the easiest option to dispose of Biosolids .
When appropriately managed this outlet presents very limited risks for public health. The major inconvenience to landfilling is public acceptance and the fact that all the beneficial constituents such as organic matter, nitrogen and phosphorus are definitively lost.
The global impact on the environment is negative therefore this solution, which is not sustainable, shall be limited to sludge that does not meet the specification for land application.
As an alternative to disposal by landfilling or incineration, landspreading seeks to beneficially reuse the organic matter and the nutrients that are contained in the Biosolids.
Contrary to landfilling or incineration land application requires overall management of the waste water treatment processes from the effluent discharge to the sewer to the production of Biosolids.
The use of Biosolids on land is heavily regulated to control the potential risks to human health and the environment.
Major issues concerning Biosolids land application are potential odour emission and public acceptance.
The only sustainable solution for Biosolids recovery is land application. This requires significant treatment and regular quality monitoring.
Biosolids land application is the most favorable outlet for the environment.
Regulation in Europe
The use of sludge is regulated by three main European framework directives
The Sewage Sludge Directive 86/278/EEC
The purpose of this Directive is to encourage sludge landspreading and to regulate this practice in such a way as to prevent harmful effects on soil, crops, animals and to humans.
The Directive lays down limit values for concentrations of heavy metals in the soil, in sludge and for the maximum annual quantities of heavy metals which may be introduced into the soil.
The Member States have taken the national regulatory measures necessary to ensure the good application of this directive
The Waste Framework Directive
This directive 75/442/EEC, published in 1975, provides for the establishment of proper waste control regimes, and requires that the designated national competent authorities draw up a waste management plan. This text was revised and the new directive was published in November 2008.
The Directive aims to set a framework for waste management in the EU, promoting both reuse and recycling, including recycling/reclammation of organic substances activities and its use within a revised waste management hierarchy. Member states have a deadline of 2 years to translate the directive into national law.
The new directive contains provisions to define end of waste criteria that provide a high level of environmental protection.
The Nitrates Directive
The Council Directive 91/676/EEC concerning the protection of water from pollution caused by nitrates from agricultural sources was adopted on December 12th 1991.
This Directive aims to reduce or prevent the pollution of water caused by the application and storage of inorganic and organic fertiliser on farmland.
It is designed both to safeguard drinking water supplies and to prevent wider ecological damage in the form of the eutrophication of freshwater and marine waters generally.