Study On The Influence Of Ca²⁺ Concentration On Nitrongen Removal And Sludge Characteristics In The Activated Sludge System

This paper studies the influence of calcium ion (Ca2+) concentration on nitrogen removal efficiency and sludge characteristics in the activated sludge system. The variations of the total nitrogen (TN) and ammonia nitrogen (NH4+-N) removal rate, the nitrite nitrogen (NO2--N) and nitrate nitrogen (NO3--N) accumulation, and the nitrite bacteria, nitrobacteria, denitrifying bacteria number measured by the Most Probable Number (MPN) method were investigated through increasing Ca2+concentration step by step with a single sequencing batch reactor (SBR) and synthetic calcium wastewater, in which the chemical oxygen demand (COD) concentration was680mg/L, NH4+-N concentration was20mg/L. The influence of different Ca2+concentrations on the COD removal rate and sludge characteristics were also studied during the process of the calcium wastewater treatment, such as mixed liquor volatile suspended solid (MLVSS), mixed liquor suspended solid (MLSS), sludge volume index (SVI), sludge growth rate, and sludge morphology.It was detected that the SVI was25mL/g that made the compactness of sludge granules larger, the order of magnitude of nitrate bacteria and denitrifying bacteria was4.0-5.0, the TN and NH4+-N removal rate was more than90%, and the NO2--N accumulation was about0mg/L at basically and NO3--N accumulation was less than0.25mg/L in the system when Ca2+mass concentration was at480-1000mg/L. The results indicate that simultaneous nitrification and denitrification (SND) is the main reason for the higher nitrogen removal rate in the system. It was also detected that the COD removal rate decreased from98.1%to69.23%, MLSS increased from5000～5500mg/L to24200mg/L rapidly, MLVSS reduced fastly from5400mg/L to1600mg/L, SVI decreased sharply from55mL/g to14mL/g, and the structure of sludge flocs turned from loose to compact, and biofacies changed from epistylis to cytocyst from the microscopic observation with the increasing of Ca2+concentration when the Ca2+mass concentration was0～4000mg/L. Which reveals that active microbes reduce and inorganic components increase gradually in the activated sludge, and that the increase of Ca2+concentration makes the carbonate balance equilibrium in system move to the rightwards, and the majority of Ca2+turns into refractory carbonates. Then these carbonates attach to the sludge flocs and meanwhile compress them, which leads to the rapid increase of compactness of sludge granules and MLSS. So a large amount of sludge is produced.