Environmental Engineering Reference
In-Depth Information
can be relatively easily dewatered for disposal. The secondary sludge consists predominantly
of excess biomass produced during the biological process (Ramalho, 1983), and about half of
the incoming organic pollution load is converted into secondary sludge, containing 0.5 to 2%
solids (Winkler, 1993). Compared with the primary sludge, the secondary sludge is far more
difficult to dewater.
The management of municipal and industrial wastewater sludges has been a long-standing
challenge for many utilities. For example, Canadian municipalities spend $12-15 billion
annually for sewage sludge treatment (Buberoglu and Duguay, 2004). Normally, sludges are
disposed by landfilling and incineration (Reid, 1998), which have, however, suffered from their
inherent drawback of poor economics due to many reasons including (1) the high cost
associated with dewatering the sludge to 20-40% solids or higher so as to meet the requirements
of landfilling or incineration, and (2) the significant energy loss in evaporating the sludge-
containing water in incineration or combustion of the sludges in a recovery boiler. The sludge
disposal/management costs can be as high as 60% of the total wastewater treatment plant
operating costs (Canales et al., 1994). In recent years, due to rapidly shrinking landfill space and
the secondary pollution issues associated with the conventional sludge disposal approaches as
well as the increasingly stringent environmental regulations, the disposal of sludges continues to
be one of the major challenges for the municipal wastewater plants and most pulp and paper
mills (Mahmood and Elliott, 2006). This together with record high oil prices have contributed to
a need to examine methods of converting secondary sludge waste into energy. For instance, the
percentage of pulp/paper sludges disposed by landfills has constantly decreased in Europe in
recent years, as shown in Fig. 1, dropping 40% in 1990 to 20% in 2002. In the meantime, the
percentage of pulp/paper sludge used as a raw material in other industries and other applications
(e.g., agriculture as soil improvers, in road construction, land reconstruction) and for energy
recovery has steadily increased.
Figure 1. Disposal methods for the pulp and paper residues in Europe (adapted from Monte et al., 2008
and CEPI, 2004)
