Full-scale anaerobic digestion processes for organic solid waste are common in Europe but are generally unaffordable in Canada and the United States because of inadequate regulations to restrict cheaper forms of disposal, particularly landfill. We investigated the viability of solid-state anaerobic digestion (SS-AD) as an alternative that reduces the costs of waste pretreatment and subsequent wastewater treatment. A laboratory SS-AD digester, comprising six 10 L leach beds and an upflow anaerobic sludge blanket reactor treating the leachate, was operated continuously for 88 weeks, with a mass balance based on chemical oxygen demand (COD) of 100 ± 2% (CODout/CODin). The feed was a mixture of fibers (cardboard, boxboard, newsprint, and fine paper) with varying amounts of food waste added. The process remained stable throughout. The addition of food waste caused a synergistic effect, raising methane production from the fiber mixture from a low of 52.7 L kg-1 COD fibersadded at no food waste, to 152 L kg-1 COD fibersadded at 29% food waste, an increase of 190%. Substrate COD destruction efficiency reached 65%, and the methane yield reached 225 L kg-1 CODadded at 29% food waste on a COD basis, with a solids retention time of 42 days. This performance was similar to that of a completely stirred tank reactor digesting similar wastes, but with much lower energy input. Multiple factors likely contributed to the enhanced fiber destruction, including the action of hydrolytic enzymes derived from fresh food waste and continuous leachate recirculation between leach beds of different ages.