A method and apparatus for anaerobic treatment of wastewater and sludge in a digestive reactor in which the length of stay of hydraulic components is decoupled from that of particulate components by discontinuous removal of a portion of the digesting sludge from the digestive reactor, enrichment of the extracted portion of the digesting sludge in a sapropel reactor, and feedback of the enriched digesting sludge into the digestive reactor.