The pulp and paper industry carries high costs in wastewater treatment. Anaerobic digestion can be a useful supplement to aerobic treatment with benefits such as a reduction of sludge production, reduced power consumption, lower nutrient/chemical requirements, and the production of energy-rich methane gas.

We have previously showed that the fibrous fraction of Kraft mill wastewater contains high amounts of organic matter easily accessible for methane production. Typically, anaerobic digestion of fibrous waste would be carried out in a conventional continuously stirred tank reactor (CSTR), but the long hydraulic retention time needed to obtain a stable mesophilic process (normally 15-20 days) would mean the need of large reactor volumes in order to reach biogas production volumes interesting for commercial use. By de-coupling the hydraulic retention time from the sludge retention time, the advantages of treating solid substrate could be retained at higher volumetric loadings.

In my work I’m showing that fiber sludge from pulp and paper mills can be successfully digested at low hydraulic retention times using a CSTR with sludge recirculation. By returning concentrated sludge to the reactor, washout of microorganisms is prevented and substrate digestion time prolonged, while still allowing for treatment of large volumes of wastewater.