The role of activated sludge in wastewater treatment is a dynamic system of complex microbial ecosystems and physical and chemical processes, and its core mechanism can be deeply analyzed from micro metabolism to macro process level
1, The collaborative metabolic mechanism of microbial communities
Hierarchical division of functional microbial communities
-Advantageous bacterial community: Mainly composed of heterotrophic bacteria (such as Pseudomonas and Zygomycetes), responsible for the primary degradation of organic matter, secreting extracellular enzymes to hydrolyze large organic molecules into absorbable small molecules (such as polysaccharides → glucose, proteins → amino acids).
-Functional microbiota:
-Nitrifying bacteria (nitrite bacteria, nitrate bacteria): under aerobic conditions, oxidize NH ∝ - N to NO ₂⁻ and NO ∝⁻.
-Denitrifying bacteria (such as Pseudomonas): Under anaerobic conditions, they use organic matter as an electron donor to reduce NO ∝⁻ to N ₂.
-Polyphosphate accumulating bacteria (such as Acinetobacter): Excessive uptake of phosphorus in anaerobic aerobic alternating environments (releasing energy to absorb phosphorus during aerobic conditions and releasing phosphorus to obtain carbon sources during anaerobic conditions).
Energy allocation in microbial metabolism
-Decomposition metabolism: Organic matter oxidizes and releases energy (about 40% converted to ATP, 60% lost as heat energy).
-Anabolism: Energy is used for microbial cell proliferation (sludge production), and the remaining energy is consumed through endogenous respiration.
2, The strengthening effect of physical and chemical processes
The linkage effect of adsorption flocculation precipitation
-Adsorption stage: Microorganisms rapidly capture organic matter through the viscous network of EPS (extracellular polymeric substances) (adsorption rate can reach more than 10 times the degradation rate).
-Flocculation mechanism:
-Biological flocculation: Polysaccharides and proteins in EPS secreted by microorganisms act as biological flocculants to promote floc formation.
-Charge neutralization: By using Ca ² ⁺ and Mg ² ⁺ ions to reduce the negative charge on the colloidal surface and decrease repulsion.
-Precipitation efficiency: Good flocs (SVI=100~150 mL/g) achieve sludge water separation in the secondary sedimentation tank, and the concentration of returned sludge can reach 3000~5000 mg/L.
Mass transfer and diffusion control
-Dissolved oxygen gradient: Uneven distribution of dissolved oxygen in the aeration tank forms a microenvironment (aerobic anoxic interface), promoting synchronous nitrification and denitrification.
-Substrate diffusion: The transfer rate of organic matter from the aqueous phase to the surface of microbial cells affects degradation efficiency, which can be optimized by increasing stirring intensity.
3, Control logic of process parameters
Key control parameters
-Sludge age (SRT): determines the microbial population structure (e.g. long SRT promotes the growth of nitrifying bacteria, short SRT inhibits filamentous bacteria).
-Sludge load (F/M): High load (0.3~0.6 kgBOD/kgMLSS · d) accelerates organic matter degradation but can easily cause sludge swelling; Low load (<0.15 kgBOD/kgMLSS · d) is beneficial for nitrification.
-Reflux ratio (R): affects the sludge concentration and treatment efficiency of the aeration tank (usually 20%~100%).
Optimization direction of typical processes
-A/O process: Phosphorus removal is achieved through anaerobic aerobic alternation, and the ORP in the anaerobic zone needs to be controlled at -150~-250 mV.
-A ²/O process: Increase the anoxic stage to enhance denitrification, requiring balanced carbon source allocation (priority denitrification, followed by phosphorus removal).
-SBR process: Multi functional integration achieved through time series control, requiring optimization of aeration intensity and sedimentation time.
4, Challenges and coping strategies during operation
Common problem analysis
-Sludge swelling: Excessive proliferation of filamentous bacteria leads to SVI>200 mL/g, which can be inhibited by adding Fe ³ ⁺ or adjusting F/M.
-Sludge aging: Long term low load operation leads to flocculation, requiring sludge discharge or increased load to activate metabolism.
-Denitrification efficiency is limited: when the carbon source is insufficient, methanol/sodium acetate can be supplemented, or MBR process can be used to extend SRT.
Intelligent control technology
-Online monitoring: Real time feedback of process status through DO, pH, ORP sensors.
-Model prediction: Apply ASM (activated sludge model) to simulate metabolic processes and optimize aeration and reflux strategies.
5, Technological innovation and cutting-edge directions
New process development
-Short range nitrification denitrification: oxidize NH ∝ - N to NO ₂⁻ and directly denitrify, saving 25% aeration and 40% carbon source.
-Granular sludge technology: By self agglomeration to form millimeter sized particles, it enhances the ability to resist impact loads.
resource utilization
-Anaerobic digestion of sludge: converting organic matter into biogas (containing 60%~70% CH4) to achieve energy recovery.
-Phosphorus recovery: Extracting slow-release fertilizer from sludge through bird droppings (MgNH ₄ PO ₄ · 6H ₂ O) crystallization technology.
summarize
The activated sludge system achieves full chain control from organic mineralization to nutrient cycling through the coupling of microbial metabolism and physicochemical processes. The future development trend will focus on low-carbon and energy-saving processes, intelligent regulation, and resource recovery to meet the demand for upgrading sewage treatment under the goal of carbon neutrality. In practical applications, it is necessary to flexibly adjust process parameters based on water quality characteristics (such as toxic substances in industrial wastewater and metabolic inhibition in low-temperature environments) to ensure stable and efficient operation of the system.
