Thermal treatments of sewage sludge

The management of sewage sludge produced by wastewater treatment plants is a problem strongly influenced by the legislation. The gradual phasing out of landfilling and severe limitations on agricultural use has prompted the public water management sector to search for sustainable technologies which must be feasible from an economic, environmental and social point of view. In this context, for the last few years in the European Union countries there has been a trend towards thermal treatments of sewage sludge (thermal destruction, pyrolysis and gasification).

Sludge disposal options: ongoing evolution and how legislation affects it

Mono-incineration

Pyrolysis

Gasification

PerFORM WATER 2030 and future challenges for the public water management sector


Sludge disposal options: ongoing evolution and how legislation affects it

The increasing volume of sewage sludge produced by waste water treatment plants is becoming a global problem, also due to the difficulty of finding safe and sustainable disposal methods that take into account the high content of organic and inorganic pollutants.
The problem is particularly felt in Italy, where legislation is evolving towards the limitation of the most practiced solution until very recent times: direct agronomic use and composting. A considerable part of the sludge produced by many wastewater treatment plants does not meet the requirements that make it suitable for agronomic use, according to the most restrictive recent limits, no longer limited to heavy metals and pathogenic organisms, but also extended to organic substances (e.g.: hydrocarbons).
The solution of landfill disposal is not feasible both because of the costs and because the European and national regulations are gradually pushing towards its phasing out the disposal of any waste rich in organic substances (including sewage sludge).
Water Service Companies must therefore consider alternative ways, based on consolidated and sustainable technologies under the economic, environmental and social point of view.

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A scheme of sewage sludge treatment options is shown in the figure above (source: Oladejo et al., 2018, A Review of Sludge-to-Energy Recovery Methods, Energies 2019, 12 (1), 60; ).

The following figure shows the sludge disposal options practiced in the countries of the European Union and their evolution between 2005 (left) and 2015 (right; source Eurostat). You can see that the current trend is towards the reduction of agronomic use and the progressive increase of the fraction subjected to thermal valorization.

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The main thermal sludge treatment technologies are thermal destruction, pyrolysis and gasification. A fourth wet process called hydrothermal carbonization does not currently have full-scale applications, even if the process has passed the research and development phase.

The main advantage offered by all the different kinds of thermal treatment of sludge is energy recovery. In addition to energy recovery, incineration also offers other advantages:

  • reduction of the sludge volume to 10% of that after mechanical dehydration;

  • thermal destruction of toxic organic compounds;

  • absence of odor emissions;

  • production of ashes rich in phosphorus, which is recoverable with different technologies (to learn more about the topic go to the fact sheet dedicated to phosphorus recovery).


Mono-incineration

In the case of thermal destruction, the calorific value developed by the thermal oxidation (combustion) of the organic substance must be sufficient for the evaporation of the water contained in the sludge. The lower calorific value (LCV, which is the useful thermal energy) of the sewage sludge with a dry content percentage of 35 - 40% (the remainder is water) and a fuel fraction of 70-80% on the dry content (the rest is inert material), is about 7-8 MJ/kg, a value that allows incineration without auxiliary fuels, which are used only for start-up and shut-down of the furnace. The LCV of the dry fraction is generally between 16 and 25 MJ/kg, depending on the content of the fuel fraction and on whether the sludge is stabilized or not. Stabilized sludge (i.e. that has undergone a process that reduces the content of biodegradable organic substance, aerobically or anaerobically) contains a lower fuel fraction than non-stabilized sludge.

Incinerators are generally designed and managed for process temperatures between 850°C e 950°C. The limit set by Italian standards for incineration is 850°C as temperatures below that value can cause emissions of odors and product of incomplete combustion, while temperatures above 950°C can cause ash melting. The residence times of the gas are usually in the order of 2 seconds or a little more.

The most popular technologies for the incineration of sludge alone (mono-incineration) are the fluidized bed furnace, the multiple hearth furnace and, recently developed for sludge, the grate incineration furnace, fit to burn dried and pelleted sludge.

The most widespread type is the fluidized bed furnace, consisting of a vertical cylindrical chamber filled with sand, which rests on a perforated platform from which the primary combustion air is introduced. The sludge, about 40% dry content, is fed over the sand bed; the combustion air flows upwards and fluidizes the mixture of hot sand and sludge.
The following figure shows an example of a schematic of a fluid bed incinerator for wet sludge (dry about 25-35%) without energy recovery. Auxiliary fuel is required for dry content <30%.

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In grate furnaces, which are derived from waste-to-energy plants for the thermal treatment of solid wastes, the sludge is dried beforehand to a residual water content of 15% and compressed into pellets. The dried and pelleted sludge is fed onto the grid under which the primary combustion air is introduced. As the sludge is fed in the form of pellets, in this type of furnaces the combustion process produces compact ashes, which are collected mostly below the grate. These ashes do not contain sand and are suited for subsequent material recovery options.

On the European Commission website the best available technologies (BAT) for incineration have recently been published (see https://ec.europa.eu/jrc/en/news/new-eu-environmental-standards-waste-incineration): although designed for waste incineration plants, they also apply to the waste-to-energy treatment of sludge.


Pyrolysis

Pyrolysis consists of the thermochemical decomposition of organic materials in the absence of oxygen. During pyrolysis, complex organic molecules are decomposed into smaller and simpler molecules in the gas phase (pyrogas), liquid (oily phase), and solid (char).

  • The gas phase, pyrogas, is made up of non-condensable vapors (mainly CO, CO2, H2, CH4 other low carbon atom hydrocarbons); represents 15 to 30% by weight of the initial product, with an increasing percentage incidence with the process temperature. The gaseous mixture cannot be stored or transported, but must be burned on site for the production of heat and electricity.

  • The liquid (oily) phase is obtained by condensing the vapors and represents, on average, 50 to 60% by weight of the starting material; it contains significant levels of humidity (up to 60 - 80%) and is made up of complex organic substances such as alcohols, ketones and condensable hydrocarbons of various kinds.

  • The solid residues, char, represent about 20-30% by weight of the starting material, and have an average calorific value between 5,000 and 6,000 kcal/kg: they are made up of carbonaceous substances, similar to bituminous coal, if obtained by low temperature pyrolysis (400-500°C) or anthracite if obtained at higher temperatures (800-900°C). The carbon phase, if combusted, gives rise to ashes similar to those deriving from waste-to-energy, which constitute a potential source of fertilizers (phosphates). Char can also be turned into activated carbon.

Pyrolysis products can have various uses, according to the type of material treated, although the most frequent use is as fuel for the production of energy. The characteristics of the materials obtained and their relative quantities depend on the type of material treated, on the operating conditions with which the pyrolysis is conducted, in particular the temperature and the process time.

In Europe, only a few plants for the pyrolysis of sewage sludge have been put into operation, all on a pilot scale. A plant, fed with 65% dry sludge, operated at 600°C, generating char and pyrogas. The combustion of the latter at 1.100°C produced thermal energy to activate the pyrolytic process (which requires energy) and to mechanically dry the sewage sludge from 20 to 65% of dry matter. The process had conduction problems that caused chimney emission limits to be exceeded for toxic and carcinogenic compounds.


Gasification

During the gasification process the volatile organic substances are converted into a gaseous mixture called syngas (or synthesis gas) while the inert material is converted into ashes.
Gasification is carried out with sub-stoichiometric oxygen and in some cases also with additional external heat. Since for the gasification process it is necessary to use 75-90% of dry substance, the sewage sludge must be previously subjected to mechanical dehydration; the necessary energy is obtained by using the syngas produced by gasification as fuel.
Examples of full-scale plants operating with this technology are in Koblenz, Mannheim e Balingen.


PerFORM WATER 2030 and future challenges for the public water management sector

The management of the sewage sludge produced by the wastewater treatment plants and their disposal represent a complex challenge for public urban water management; therefore within the PerFORM WATER 2030 project an entire research line, the Biosolid valorization line, was devoted to this complex topic.

The important possibility of optimizing processes by reducing the production of biological sludge was not neglected in the studies (to learn more about the topic go to the fact sheet dedicated to minimization of biological excess sludge through ozonolysis), while, on the other hand, two complementary activities have been dedicated to thermal treatments: the field experimentation of the technology of incineration on a mobile grate of dried sludge and the theoretical analysis and modeling of the phenomenon to optimize its yield.

Design & construction of a grate-combustion plant for sludge

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Theoretical modelling of a grate-combustion plant for sludge

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