thus disturbing alkalinity equilibria and eventually leading to reactor instability (Van Groenestÿn et al., 1995). Recovery of S is well feasible both from the stripping gas using catalytic scrubbing techniques (Jensen and Webb, 1995; Smet et al., 1998) or from the absorbent (see Section III.C.1). The extend to which stripping of H2S combined with gas scrubbing and gas recirculation is feasible for modern high-rate anaerobic reactors is still uncertain because of the large quanti-ties of air required for complete sulfide elimination (Figure 5).