In a world turning towards higher consumption and production levels, resources become more and more scarce. It is from this point of view that we believe water and energy use must be managed to stay both competitive and sustainable. This paper provides a short introduction on the role of anaerobic treatment in paper mill effluent treatment and reuse. Anaerobic treatment provides large benefits compared to only aerobic treatment in terms of sludge production, power consumption and environmental footprint. In practice, primary treatment followed by an anaerobic/aerobic biological treatment and tertiary treatment with solids removal, provides a configuration in which both water can be reused and energy can be generated (we call it energy reuse). Challenges are there, like granular sludge formation, high or low COD (Chemical Oxygen Demand) loads and concentrations, high calcium concentrations, high sulfate / sulfide concentrations and toxicity. Proper data gathering, together with in-depth knowledge of biological processes, provide tools to deal with these challenges and identify solutions. Water reuse is possible, but only to a certain extent. Closed loop paper mills are in practice almost-closed loop mills, but the best available now.
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