The Effect Of Adding Polyaluminum Chloride On The Properties Of Activated Sludge

Chemical flocculation is commonly used for the treatment of conventional wastewater.Polyaluminum chloride (PAC) is an efficient inorganic polymer flocculant developed in the 1930s and officially put into industrial production and use in the late 1960s. It is widely used in processes such as water supply and drainage, recycling water, industrial wastewater treatment, and sludge dewatering.

At present, wastewater treatment plants also widely use PAC as a coagulant for synchronous chemical phosphorus removal. After PAC reacts with pollutants in wastewater, the residual amount will re-enter the biological tank with reflux, leading to the accumulation of PAC in the biological tank and the production of a certain concentration of residual aluminum. This may affect the microbial performance of activated sludge in the biological tank, thereby affecting the biochemical treatment effect of wastewater, When the residual aluminum content in water is high, discharge can cause certain harm to the water body.

This article takes the wastewater and PAC agents at the end of the biological tank of a large sewage treatment plant in Sichuan as the research objects, explores the effect of different PAC dosages on residual aluminum concentration in wastewater, and its impact on microorganisms in activated sludge, providing reference for the stable operation of biological treatment systems in sewage treatment plants.

The design treatment capacity of the large-scale municipal sewage treatment plant in this experimental model after upgrading is 980000 m3/d. The treatment process is shown in Figure 1, and the effluent indicators are better than the Class A discharge standard of the Urban Sewage Treatment Plant Pollutant Discharge Standard (GB18918-2002).

The factory adds PAC to the effluent of the biological tank according to the actual situation to achieve chemical phosphorus removal. However, after the reaction between PAC and pollutants in wastewater, the residual amount will re-enter the biological tank with reflux, leading to the accumulation of PAC in the biological tank, which may have an impact on the microorganisms in the activated sludge in the biological tank, directly affecting the water treatment effect. Moreover, discharge when the residual aluminum content in the water is high can cause certain harm to the water body.

Experimental process: The effluent from sedimentation tank A and the sludge from aeration tank B were mixed according to the actual ratio (simulating the B-stage aeration of AB process), and PAC was added at 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, and 60 mg/L, respectively. Aeration 4 formed two flocculation activated sludge aeration systems (referred to as PAC activated sludge aeration system here). Simultaneously set up an activated sludge aeration system without flocculants, namely a non flocculant aeration system, and operate the three systems under the same conditions after 4 hours of aeration. Compare the changes in the effluent quality.

Experimental results: (1) When the dosage of PAC is 30 mg/L, the activated sludge is not affected. When the PAC dosage is>30 mg/L, the effluent COD is significantly higher than that of the non coagulant activated sludge aeration system, indicating that the activated sludge is inhibited or poisoned. This may be because excessive PAC can cause the gel nucleus to indicate the adsorption of excess positive ions. The previously negatively charged colloid in water becomes a positively charged colloid, causing the colloid to stabilize again and transferring colloidal particles from the sludge to the liquid phase, resulting in a decrease in the concentration of suspended solids in the sludge.

The sludge under different PAC dosages exhibits rich biota and a relatively diverse variety of microbial species. Microorganisms play an indicative role in activated sludge systems. When protozoa such as bell shaped insects and rotifer are present, it indicates that the activated sludge has good performance, good sewage treatment efficiency, stable microbial community structure, and good biological activity. This indicates that the accumulation of PAC in activated sludge systems has no significant impact on the types of microbial communities.

The results of Ferron's hourly complexation colorimetric method showed that with the increase of PAC dosage, the concentration of residual aluminum also increased. When PAC>100mg/L, the rate of increase in residual aluminum increases, and the impact on activated sludge increases; When the dosage of PAC is below 60mg/L, the residual aluminum concentration in the liquid phase is very low, and 98% or more of the aluminum is transferred to the sludge for discharge, without producing strong toxic effects on the bacteria in the activated sludge.