Optimization of the treatment and disposal of sewage sludge in the ATO of Como: options and scenarios assessment.

The problem of sludge management can be framed at a general level, highlighting the following points:

• To deal with the problem of disposing/recycling of sludge, which is taking on an increasing weight, it is necessary to act simultaneously in several directions, all equally important and effective;

• Following the basic principles of European legislation on waste, such directions can be identified in a scale of priority interventions aimed at minimizing the production of sludge, interventions aimed at recovery (matter and energy) and finally disposal in safety;

• As regards the minimization of the production, there are interesting prospects in technological field: some processes are already commercially available and many promising systems are in the testing phase. The reliability, the cost-effectiveness, the disadvantages and indirect damages resulting from the application of these processes (e. g. changes in the characteristics of effluent quality, adverse effects on biomass, etc.) should be evaluated very carefully on a case by case basis. In particular, some of the technologies appear to be interesting and intended to be conveniently applied in the near future (there are already signs in this direction through the first applications, e. g.: thermolysis), working in the "sludge line", as they appear "safer" (as no risk to the purification process is present) than those that operate in the "water line." It should also be noted that even within the treatments already present on the plants there is certainly room for improvement that can be achieved through careful management, which would lead to a reduction of the production of sludge;

• As regards the recovery of material it is essentially referred to the use of sludge in agriculture (possibly after composting). In this case, there are many initiatives launched primarily by control or standardization bodies, designed to test the effects of this practice and to "renew" the existing rules, to ensure that the recovery of the resource material can be conducted to minimize the potential risks to humans and the environment;

• In the field of thermal treatment, alternatives seem interesting to allow energy recovery, as on the one hand ensure the control of noxious emissions, and on the other allow the exploitation of the resource. Also in this area, next to conventional processes, such as incineration, and applications that use dried sludge as alternative fuel in industrial furnaces (e. g. cement plants), the study of alternative systems continues (gasification, pyrolysis, thermocatalysis etc..) and there are also important industrial applications;

• Finally, with regard to the disposal in landfills, in the regional scenario, and also with a view to recent standards, it will necessarily be limited to the residues from the treatment.

Under the current state of the system it is clear that the use of traditional technologies such as liquid sludge reuse in agriculture after appropriate pre-treatment to obtain qualitative characteristics that comply with the regulations could still be a viable alternative, albeit with a progressive increase in the the cost of disposal. The regional guidelines for the sludge provide most stringent requirements with regard to the quality of the sludge and thus a lower availability of areas ready to receive them. In addition, the delta between the minimum and maximum cost for disposal is still a strong impact on the economies in general. The analysis of the quantities of sludge produced in recent years outline the scenario of storage and subsequent use in agriculture as a predominant fate after mechanical dewatering of sludge inside the plants. As said so far, and for the greater difficulty of checking the sludge for agricultural use, it can definitely be said that the sludge disposal as agriculture is no longer the only hypothesis plausible for the future. As shown previously, it seems desirable to determine the kinds of treatment that are innovative within the Integrated Water Service, in principle built and managed by the single Manager as a result of the agreement signed with the Autorità d’ambito for the ATO of reference.
The management of sludge produced from the treatment plants of medium-small size results in common practice greatly simplified, for obvious reasons of a technical-economic development. The most common form of disposal is the conferment to companies that operate the reuse in agriculture. It is not considered appropriate, at least at first, to "force" the small plants to procedures of management different from those of today.