The re-use of Waste-Activated Sludge as part of a “zero-sludge” strategy for wastewater treatments in the pulp and paper industry

•Total reuse of Waste Activated Sludge from pulp and paper industry was studied.•The WAS hydrolysis solubilises over 90% of tCOD and hygienises the solid residue.•It is possible to use solid residue of WAS as a raw material in cardboard production.•Biogas can be produced from solubilised portion of WAS in an UASB reactor.•Presented “zero-sludge” concept produces renewable energy and raw materials. The possibility of introducing the thermo-alkali hydrolysis of Waste-Activated Sludge (WAS) was investigated, in order to enable the use of its solid residue as a raw material in cardboard production and the use of its liquid portion for anaerobic digestion in an UASB reactor. The evaluation of the hydrolysis at pH>12 and T=70°C showed that the microbe cells were disrupted with more than 90% efficiency in less than 2h. The solid portion was hygienised, therefore making it possible to integrate it into the cardboard production as a raw material for less demanding cardboards. Up to 6% addition of the liquid portion of hydrolysed WAS to wastewater decreased the specific biogas production in a pilot-scale UASB from 0.236 to 0.212m3/kgCOD, while the efficiency of the COD removal decreased from 80.4% to 76.5%. These values still guarantee an adequate treatment of the wastewater and an increased biogas production by 16%.