Wastewater sewage sludge management: evaluation of the thermal treatment option

The sewage sludge production coming from the wastewater treatment is continuously increasing; taking also into account the high difficulty normally encountered in the sludge disposal, the individuation of innovative strategies or the optimization of the conventional existing systems for its final management seems to be a very important task. The produced sludge, chiefly if it has been subjected to thermal drying, is suitable for energetic valorization processes as, for example, incineration, gasification and pyrolysis. These processes present the very important positive aspect to lead to a correct management for the disposal problem; at the same time they can represent an useful energy source. From this point of view it is important to consider that, in opposition to the continuously increasing difficulty to generate energy from traditional fossil sources, the possibility to obtain energy from biomass (as for example from the sewage sludge), represents an interesting tool both for the reason that this source is certainly renewable and corresponding to the directives of the Kyoto protocol, and also for economical aspects (in fact the energy that is derived from traditional sources presents an always increasing cost). On the basis of these considerations the aim of the present work is the individuation of the optimal solution (in particular with a thermal treatment approach), for the correct disposal of the sewage sludge deriving from wastewater treatment, with particular reference to a specific large-scale example. In particular, in this work we examined, both from the technological and from the environmental point of view, the thermal solutions (direct combustion in incineration plant first of all, but also gasification), for the sludge deriving from the main Italian wastewater plant (plant of the metropolitan area of Torino). The plant actually produces about 100.000 t/y of sludge (38.9 % d.s.), in three main streams: centrifuged sludge, filtered sludge and thermal dried sludge (the water content of the streams is obviously different, the indicated humidity is a mean value). In order to arrive to propose a solution a first analysis was required in order to define the main characteristics of the produced sludge (flow rates, composition); afterwards we analyzed in detail the potential plant design solutions, from the technological and the environmental point of view, by arriving at the end to some perspective indications.