Prediction of the effect of fine grit on the MLVSS/MLSS ratio of activated sludge

• The correlation between the particle size and the suspension ratio was measured.
• A model to predict the effect of fine grit on the MLVSS/MLSS ratio was developed.
• The concentration of fine grit is proportional to its suspension ratio.
• WWTPs should adjust MLSS based on MLVSS/MLSS to ensure the stability of MLVSS.

This paper investigated the suspension properties of fine grit with different particle sizes in a bioreactor and developed a model to predict its effect on the ratio of mixed liquor volatile suspended solids to the mixed liquor suspended solids (MLVSS/MLSS) of activated sludge. The experimental results revealed that a smaller particle size corresponds to a larger suspension ratio, defined as the proportion of fine grit brought in by influent that is suspended in the activated sludge, and a smaller MLVSS/MLSS ratio. The model demonstrated that the effect of fine grit on the MLVSS/MLSS ratio is related to the fine grit concentration and chemical oxygen demand in influent and the observed sludge yield. However, fine grit has no influence on the activity of microorganisms. Wastewater treatment plants (WWTPs) can adjust MLSS based on the MLVSS/MLSS ratio to ensure the stability of MLVSS, which can achieve the stable operation of WWTPs.