The pulp and paper industry is a strategic economic sector in
Europe, which production is concentrated in a few countries [1].
Along with most of industrial sectors, it has suffered significantly
from the economic crisis in 2008 and 2009 and it has been largely
affected by the economic slowdown that began in mid-2011. Yet,
European pulp and paper industry remains an important contributor
to EU economic growth and job creation, with its performance
still stronger than other energy-intensive sectors in Europe [2]. As a
counterpart, pulp and paper production processes are very
demanding in terms of energy and water consumption and the pulp
and paper industry is considered one of the most polluting in the
world [3].
In pulp and paper mills, the high water usage results in the
generation of large wastewater volumes, which must be treated to
accomplish with environment regulations on effluents discharge
[4] and therefore sludge is produced as an unavoidable waste from
pulp and paper mill wastewater treatment. Globally, in a pulp and
paper mill, around 50 kg of dry sludge result from the production of
a tonne of paper, approximately 70% being primary sludge and 30%
secondary sludge [5]. Primary sludge is generated in the clarification
of process water treatments while secondary sludge is generated
in the clarifier of the biological units of the wastewater
treatment. Secondary sludge volumes are generally lower than
those corresponding to the primary sludge, since most of the heavy,
fibrous or inorganic solids are removed in the primary clarifier [6].
In any case, due to the large volumes of sludge generated, the
management of these wastes in an economically and environmentally
acceptable manner has been a critical issue for the mills