This work draws on the concept of circular economy (CE) to advance knowledge toward the obtention of low-cost and sustainable activated carbons from a biocollagenic waste from the leather industry, and using them in biogas upgrading for energy recovery. The research pursues a series of benefits across different industrial applications. The first is the production of sustainable carbon-based absorbents: using a CE approach, our experiments re-valorize solid wastes generated in the vegetable tanning from leather industry to obtain activated carbons for environmental applications. The second benefit is related to biogas energy recovery, which plays a key role in achieving the targets of the European Renewable Directive, since biogas is a renewable fuel which presents additional environmental value reducing the release of greenhouse gases in the atmosphere. Sustainable activated carbons were obtained from biocollagenic industrial waste by chemical activation (KOH, NaOH, K2CO3), with and without a previous pyrolysis step, at different activation temperature (750 °C and 900 °C) and different activating agent/precursor weight ratio. The microporous adsorbent materials obtained showed good chemical and textural properties, with BET specific surface area and total pore volume of up to 1600 m2 g−1 and 0.76 cm3 g−1, respectively. These efficient and low cost activated carbons showed a siloxane adsorption capacity of up to 500 mg g−1, higher than that of the commercial steam activated carbons supplied by adsorbent producers that reached values of up to 349 mg g−1. The industrial biocollagenic waste if vegetal tanning successfully performed as precursor for sustainable low-cost activated carbons, and the materials obtained were proved to be efficient for gaseous pollutants abatement applications. Thus, this work resulted in an efficient valorization method that assists the reduction of coal extraction by obtaining eco-friendly activated carbons that can be beneficially used for biogas upgrading in waste-to-energy applications in a circular economy scenario.