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124 New Trends in Eco-efficient and Recycled Concrete

5.2 Production of sewage sludge ashes

The thermal processing of SS is the valorisation of its energy content. In such
energy recovery, it may be taken into account that SS has a considerable water con-
tent and dewatering/drying processes may strongly influence the energy balance
and total cost (Samolada and Zabaniotou, 2014). Thermochemical technologies,
mainly incineration, pyrolysis and gasification, were found to be promising alterna-
tive ways for the valorisation of SS (Samolada and Zabaniotou, 2014).
The pyrolytic process consists of the thermal decomposition of the organic
fraction into inert atmosphere. Thus, gas and liquid fractions with potential energy
content are recovered and a solid residue (char) is obtained as a by-product.
Pyrolysis can be considered a zero-waste process, because char can be used as an
adsorbent of H 2 SorNO x in gaseous streams.
Gasification, in contrast to pyrolysis, is a thermal treatment in the presence of a
limited reactive atmosphere. Thus, a combustible gas (syngas, which is a mixture of
CO, H 2 and other gases) is obtained. This process has several advantages with
respect to simple combustion: no supplementary fuel is required, low emissions of
SO x and NO x are produced and there is a limited production of chlorinated dibenzo-
dioxins and dibenzofurans.
Finally, incineration/combustion of SS is the most-used thermal technology. Wet
or dried SS is combusted in a fluidised bed reactor. In some cases, fuel is required,
especially when high-water content SS is used. Dry SS has an important calorific
value, ranging in 12 20 MJ/kg, which is a similar value to lignite. Usually fly ash
(FA) particles are produced during thermal treatment in the fluidised bed reactor
which are trapped by means of mechanical and electrostatic filters. This ash, the SS
ash (SSA), is a powdered solid material, in some cases characterised as inert dust
and in others as a hazardous solid waste. In some countries (China, Cyprus, Spain)
(Zhang et al., 2013) the co-incineration of SS with coal as supplementary fuel in
cement kilns, power plants and brick kilns has been proposed.
The European production of SS (OECD Organization for economic co-operation
and development, 2017) in 2012 was estimated to be 10 Mt. Taking into account
(Lynn et al., 2015) that incineration processes reduce the volume of the waste by
90% and its mass by 70%, the potential production of SSA would be about 3 Mt. In
2012, 2.3 Mt of SS was incinerated, mainly from Germany, France and the
Netherlands, generating 0.7 Mt of SSA. This amount will probably increase in the
future (Stasta et al., 2006) because of a 50% 55% decrease in the agriculture
disposal option and the increase in price for landfilling.

5.3 Characteristics of sewage sludge ash

SSA is a material which has been widely researched by the scientific community.
Of all the research, five uses stand out and these depend fundamentally on the
physico-chemical characteristics of the ashes used: