Activated Sludge Process

The activated sludge process is a suspended growth treatment system where a mixture of microorganisms is utilized for the degradation of organic pollutants. It consists of a pretreatment system, an aeration tank where biological treatment occurs, sedimentation to separate water from the SS, and, finally, sludge treatment. A portion of sludge from the sedimentation is recycled back to the aeration tank and the rest is removed from the system as waste sludge for further treatment and disposal. A recommended complete activated sludge process is given in Figure 30.6.

According to the flow characteristics, the activated sludge process can be a plug-flow reactor (PFR), a completely stirred tank reactor (CSTR), or a sequencing batch reactor (SBR). In order to maintain the plug-flow behavior, the reactor length to width ratio should exceed 10:1. When it comes to the CSTR, the main benefit is the higher buffer capacity due to complete mixing. Therefore, the CSTR is capable of handling toxic wastewater compared to the PFR. However, it should be noted that the PFR requires a lower volume compared to the CSTR in order to achieve the same effluent quality. The SBR contains five steps: fill, react, settle, draw, and idle. All of the steps are conducted in the same reactor. Due to the complex nature of operation, the SBR is suitable if the wastewater flow is low.

Other than the configuration of the bioreactor, the performance of activated sludge processes is affected by influent characteristics and operational parameters. Influent characteristics are waste-water flow rate, COD and BOD, nutrient compositions (nitrogen and phosphorus), FOG, alkalinity, heavy metals, toxins, pH, and temperature. Operational parameters in the treatment are biomass concentration (mixed liquor volatile suspended solids (MLVSS) and VSS, organic load, food to biomass ratio (F/M), dissolved oxygen (DO), sludge retention time (SRT), HRT, sludge recycle ratio, and surface hydraulic flow load. Among these, SRT and DO are the most important control parameters and significantly affect the treatment results. A suitable SRT can be achieved by judicious sludge wasting from the final clarifier. DO in an aeration tank should be maintained at a level slightly above 2 mg/L.

The activated sludge process for meat processing wastewater treatment was reported by Annachhatre and Bhamidimarri.37 When a model reactor was operated at an HRT of 5-15 h, a COD removal of more than 85% was achieved. The COD loading for the above performance was 3.2 kg COD/(m3 d). An SRT of 13 d resulted in almost complete nitrification. El-Gohary et al.30 reported that the activated sludge process reduced the BOD and COD of a potato-chips factory wastewater by 86% and 84%, respectively. The organic loading rate and HRT were reported as 8.9 kg BOD/ (m3 d) and 6 h, respectively.

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