Recovery of sludge-paper sludge and sludge from sewage treatment plants through co-digestion with the WWTP sludge itself


Financed by:

Financing program Demonstration Projects in Circular Economy



This project was financed through the Ihobe-Basque Government in the 2014

Project Aid Demonstration of Waste Recovery and Circular Economy program.


  • CONAQUA (Leader)
  • Ceit-IK4
  • Añarbe waters - Añarbeko urak
  • Papelera Gipuzkoana de Zicuñaga S.A. (Iberpapel)


In a significant number of urban WWTPs, there is extra capacity for anaerobic digestion (DA) of sludge that is not exploited due to insufficient monitoring, an operation with excessively high retention times and / or due to oversized design. Specifically, the WWTP that treats wastewater from the Donostialdea region in Loiola (Gipuzkoa), has three 6,000 m3 digesters, of which only two are currently operating. Therefore, an interesting option to take advantage of this extra capacity is co-digestion, that is, the joint treatment of various residues with the sludge. The DA allows to stabilize the sludge, reduce its volume, improve its dehydration capacity and produce biogas, which can be used for energy use in boilers or cogeneration engines. Likewise, it allows to produce a product (digestate), with potential to be applied to soils, and thus recycle the organic matter contained in it.

The PRODIGEST project focused on the transformation of pulp-paper sludge into methane and fertilizer thanks to its co-treatment with urban WWTP sludge, developing for this purpose a co-digestion pilot plant. This plant, equipped with different sensors, would allow to optimize the operating conditions and resolve critical aspects, ensuring scalability to real conditions. In case of success, the possibility of taking advantage of the existing urban WWTP digesters was considered in order to maximize their treatment capacity and to take advantage of underused facilities, expanding the co-digestion alternative to additional waste streams.

Anaerobic co-digestion is a waste management solution that has been investigated in many countries in Europe, leading to successful examples of full-scale implementation. The urban WWTPs, mostly public, could use their surplus DA capacity by co-digestion with biodegradable streams that require an alternative management solution, thus contributing to progress towards a circular economy in the Basque Country.


The overall objective of the PRODIGEST project was to carry out the demonstration in a laboratory pilot prototype of the technical-economic-environmental feasibility of a protocol for the recovery of pulp-paper sludge originating from biological treatment through co-digestion with WWTP sludge. which are valued in the WWTP itself by anaerobic digestion or biomethanization.

Ceit's role in the project

Ceit played an important role in the project since it was in charge of carrying out the technical tasks of the project. The activities of Ceit within PRODIGEST are indicated here:

  • Initial experimental technical study of the selected sludge that included: laboratory infrastructure setup, sludge characterization and methanogenic potential tests.

  • Implementation of the experimental protocol for the co-digestion of sludge, control and monitoring on a laboratory scale, including: the commissioning of the pilot plant on a laboratory scale and the experimental campaign for mono-digestion tests of urban sludge and co -digestion of urban mud and sludge-paper.