The management of sewage sludge: technologies, critical issue and perspectives

The management of sludge produced by urban wastewater treatment plants has long been a problem for plant managers. This is due both to the growing costs for their disposal, and to the possible consequences that incorrect management may have on the performance of the wastewater treatment plant and on the environment. Further difficulties arose following the evolution of the European and national legislations, which regulates the management of sludge and its possible agronomic uses that introduced further checks and restrictions for this use. Several studies have been underway since last decades to reduce the extent of the problem. These focus on alternative objectives being aimed on the one hand, at minimizing the production of sludge, and on the other, at taking advantage of the sludge production by promoting its valorization. Minimization of sludge production can be achieved in different ways: minimizing the sludge production in the water line, improving stabilization in the sludge line or reducing the sludge moisture; techniques for sludge minimization are based on mechanical, biological, chemical, and thermal processes, or a combination of the above. Sludge valorization can bee achieved by recovering material, through agronomic reuse or recovery of value‐added products, as well as energy, by enhancing methane production. In this paper, after a brief reference to the management and regulatory framework relating to the recovery and disposal of sewage sludge, the main applications currently available for both the minimization and enhancement strategies are discussed.