The effect of fine grits and fine debris concentrations on the MLVSS/MLSS ratio of an activated sludge system.

This study embarks on an explorative investigation into the effects of typical concentrations and varying particle sizes of fine grits (FG, the involatile portion of suspended solids) and fine debris (FD, the volatile yet unbiodegradable fraction of suspended solids) within the influent on the mixed liquor volatile suspended solids (MLVSS)/mixed liquor suspended solids (MLSS) ratio of an activated sludge system. Through meticulous experimentation, it was discerned that the addition of FG or FD, the particle size of FG, and the concentration of FD bore no substantial impact on the pollutant removal efficiency (denoted by the removal rate of COD and ammonia nitrogen) under constant operational conditions. However, a notable decrease in the MLVSS/MLSS ratio was observed with a typical FG concentration of 20 mg/L, with smaller FG particle sizes exacerbating this reduction. Additionally, variations in FD concentrations influenced both MLSS and MLVSS/MLSS ratios; a higher FD concentration led to an increased MLSS and a reduced MLVSS/MLSS ratio, indicating FD accumulation in the system. A predictive model for MLVSS/MLSS was constructed based on quality balance calculations, offering a tool for foreseeing the MLVSS/MLSS ratio under stable long-term influent conditions of FG and FD. This model, validated using data from the BXH wastewater treatment plant (WWTP), showcased remarkable accuracy.

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

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.