I. Overview of Wastewater Biological Treatment Systems
Modern biological wastewater treatment is generally divided into three stages:
- Primary Treatment (Physical Treatment): Bar screen interception, primary sedimentation tank
- Secondary Treatment (Biological Treatment): Aeration tank, final sedimentation tank
- Tertiary Treatment (Biological/Physicochemical Treatment): Nitrogen and phosphorus removal
Sludge is separately delivered to the sludge digestion system.
Based on oxygen demand, biological treatment is classified into aerobic treatment (activated sludge process, oxidation pond, oxidation channel, trickling filter, rotating biological contactor, etc.) and anaerobic treatment (sludge digestion).
II. Microbial Community in Activated Sludge
The activated sludge process relies on heterotrophic ecosystems composed of clustered microorganisms including bacteria, fungi, protozoa and metazoa to purify wastewater. Heterotrophic bacteria and free-living heterotrophic fungi form the lowest trophic level. They are predated by heterotrophic protozoa, which are further consumed by secondary predators such as rotifers and nematodes, ultimately realizing wastewater purification.
(I) Bacteria – The Main Force for Organic Matter Degradation
Bacteria are the most critical microbial group in activated sludge, undertaking two core functions: organic matter degradation and floc formation.
Organic-degrading Bacteria Genera
Pseudomonas, Flavobacterium, Alcaligenes, Bacillus
- Dominant carbohydrate degraders: Pseudomonas
- Dominant protein degraders: Alcaligenes, Flavobacterium, Bacillus
(II) Mechanism of Floc Formation
Activated sludge flocs are formed via the synergistic effect of three components:
- Floc-forming bacteria (Zooglea ramigera): Rod-shaped, Gram-negative, capsulated and non-sporulating. They induce bacterial aggregation through extracellular polymers and intercellular substances.
- Moderate filamentous bacteria: Provide filamentous frameworks for flocs, enabling the attachment and growth of floc-forming bacteria, mainly including:
- Nitrifying bacteria
- Sphaerotilus (iron bacteria)
- Beggiatoa (sulfur-oxidizing bacteria)
- Thiothrix (sulfur-reducing bacteria)
- Ciliates: Secrete mucoproteins and polysaccharide polymers to form bridging structures and facilitate flocculation.
(III) Fungi – Degraders of Refractory Organics
Fungi can break down recalcitrant hard-to-biodegrade organics at a relatively low degradation rate. Moderate fungal filaments serve as floc cores and improve flocculation and sedimentation performance. However, excessive fungal proliferation leads to sludge bulking and poor settling properties.
Causes of abnormal massive fungal growth:
- Temperature drop
- Low pH value
- High C/N ratio of wastewater
- Excessive organic loading
- Presence of sulfides
(IV) Protozoa – Indicator Organisms for Water Quality
Common protozoa in activated sludge:
- Amoeboid protozoa
- Flagellates (phytoflagellates and zooflagellates)
- Ciliates: free-swimming ciliates and stalked ciliates
Indicator significance: The presence of ciliates is a key indicator of efficient biological treatment. Flagellates have weak competitiveness for food and only survive in high-concentration wastewater.
(V) Metazoa – Secondary Predators
Metazoa feed on bacteria and small protozoa as secondary predators. They extend the food chain and greatly reduce sludge yield. Representative species include rotifers and nematodes.
III. Three Stages of Sludge Formation
The organic matter removal process of activated sludge coincides with sludge formation, which consists of three sequential steps:
Step 1: Mass Transfer – Adsorption and Uptake
Return sludge instantly physically adsorbs colloidal substances and insoluble BOD from wastewater, followed by gradual uptake of soluble organics into microbial cells for metabolic degradation.
Step 2: Metabolic Transformation – Biodegradation
Organics undergo metabolic conversions including hydrolysis, glycolysis (carbohydrates), decarboxylation, deamination (proteins), and β-oxidation (lipids). The decomposed substrates then enter the TCA cycle and electron transport chain to generate energy, with carbon dioxide and water as final metabolic by-products.
Step 3: Flocculation – Generation of Well-settling Flocs
As organic concentration declines, microorganisms enter the endogenous decay phase. Polysaccharide polymers are secreted around cell surfaces to enhance cohesion, forming dense, settleable flocs that achieve solid-liquid separation in the final sedimentation tank.
IV. Environmental Conditions for Organic Matter Degradation
- Temperature: 20–30°C, regulates microbial enzymatic activity
- Nutrients: BOD : N : P = 100 : 5 : 1; trace elements K, Ca, Fe, Mg are additionally required
- Dissolved Oxygen (DO): 0.5–1 mg/L at tank inlet, 2–3 mg/L at outlet; metabolic rate drops sharply when DO is below 0.5 mg/L
- pH value: 6.5–7.5, optimal at slightly alkaline 7.2–7.4; microorganisms cannot survive at pH < 4 or pH > 10
- Toxic substances: Toxicity ranking of heavy metals: Ag > Hg > Sn > Cu > Cd > Cr
