In an anaerobic process to break down and ferment plant wastes, the wastes are first inoculated with water derived especially from a post-fermentation de-watering press. The inoculation water is devoid of recirculated solids. The fresh organic wastes are first shredded before being mixed with aerated water drawn from a holding tank (40) drained from the press. Aerated water promotes oxidative break-down of inhibitors and toxic substances. Prior to its use as a wetting agent, the expelled water is treated to adjust one or more parameters to a given value. The parameters are pH, buffer capacity, hydrogen sulfide content, ammonium content, fatty acid content and short-chained carbonic acid content. The adjustments are made using acids, alkalis, buffer substances, micro-nutritional elements or trace elements, and macro-nutritional elements.; In the start-up phase the inoculation water is itself first inoculated with a culture of fermentation bacteria, acetogenic bacteria, and methanogenic bacteria. Also claimed is an assembly (1) with a fermentation vessel (10), a water press (13), and a solids mixer (11) with inoculation inlet (44). Solids are introduced via an inlet (12,28). Expelled water is held in the tank (40) compartments with heaters, mixers and/or ventilators.