Effects of starvation conditions on biomass behaviour for minimization of sludge production in membrane bioreactors

The behaviour of an activated sludge system in starvation conditions was examined in batch according to substrate impulses defined by different S₀/X₀ ratio. The answer was characterised by an exogenous phase followed by a starvation one. If at high S₀/X₀ ratio, bacterial cell multiplication was the main synthesis process during exogenous phase, at low S₀/X₀ ratio the observed phenomenon was compound storage. In starvation conditions, for the lowest S₀/X₀ ratio, a rapid decrease in the MLVSS without soluble proteins production was observed. No bacterial lysis occurred and this phenomenon was due to consumption of the storage compounds with a decrease rate equal to 0.74 d⁻¹. For high S₀/X₀ ratio, as soon as the exogenous phase was completed, a decrease of the MLVSS simultaneously to a soluble protein production was observed. An immediate bacterial lysis occurred with a decay rate equal to 0.53 d⁻¹. Because MBR systems work generally in low F/M conditions, the activity of the present microbial population is close to the one observed in starvation phase. This work points out that these conditions do not allow net bacterial growth and cells just use lysis products to satisfy their maintenance requirements. These assumptions confirm the feasibility of a decrease of the net biomass production in a MBR when high sludge retention time is operated.