Sewage sludge generated in urban WWTP (Wastewater Treatment Plants) is a complex heterogeneous mixture of micro-organisms, inorganic materials and organic substances, which is classified as a non-hazardous waste. The organic substances that are typically found in sewage sludge include composite molecules like proteins, lipids, polysaccharides, plant macromolecules with phenolic structures or aliphatic structures, along with different organic micro-pollutants.
In terms of resource recovery, the two components in sludge that can be recycled in a technically and economically feasible way are nutrients (primarily nitrogen and phosphorus) and energy. There are several technologies available to recover resources from sewage sludge, being the most typical anaerobic digestion (AD) and co-digestion with biogas exploitation, incineration and co-incineration with energy recovery, pyrolysis, gasification, and supercritical (wet) oxidation. Biogas produced by means of anaerobic decomposition of sludge is the main source of energy at municipal WWTPs, being commonly exploited by means of cogeneration. The cost for electricity in a WWTP can account in some cases for up to 80% of the operational costs, and the energy obtained thanks to methane released in the anaerobic degradation of sludge can cover close to half of this cost.
In WWTPs, the disposal of sludge can represent up to 50% of its management costs.  Sewage sludge must undergo some treatment in order to reduce its volume, to improve its characteristics (e.g. odor elimination or organic matter content reduction) and to reduce the associated health problems (i.e. hygienization). Stabilization with chemicals (e.g. lime) or biological stabilization is different means to achieve the aforementioned objectives. Regarding sludge stabilization in EU, anaerobic digestion (AD) is most commonly used in Spain, UK, Italy, Finland and Slovakia. The general treatment scenario for sludge from urban WWTPs is in situ low-solids or wet anaerobic digestion followed by a dewatering step, in such a way that energy is recovered from sludge and stabilization is attained. Dewatered sludge is then used for several purposes, including land application (agricultural use or not) and the production of construction materials, or otherwise, it is disposed of in landfills. Concerning the management of sludge generated in medium-small sized urban WWTPs (<100,000 p.e. or population equivalents), a quite common scenario consists of dewatering and final disposal in landfill or land application, with no stabilization, no hygienization or no energy recovery by using AD.
There is a number of medium to small sized urban WWTPs in some Spanish territories that currently apply these management strategies. For example, sludge produced in medium to small sized WWTPs in the region of Murcia represents close to 70% of the total production of 130,000 t/year and it is applied directly on land with no hygienization treatment. According to environmental legislation in force in EU and in terms of environmental sustainability, this management scenario could be improved by means of anaerobic digestion under dry or high-solids conditions prior to disposal.
In this sense LIFE-ANADRY project (Dry anaerobic digestion as an alternative management & treatment solution for sewage sludge) in mesophilic (35ºC) and thermophilic (55ºC) conditions represents an innovative challenge for the treatment of sludge produced in medium to small sized WWTPs (Figure below). This situation represents an environmental problem that can be extrapolated to up to 27% of urban WWTPs with similar characteristics in other countries across Europe.
Schematic description of the environmental problem (current scenario) and the hypothetical scenario (LIFE-ANADRY scenario)