Principles and Process Technologies of Biological Phosphorus Removal

I. Forms of Phosphorus in Wastewater Phosphorus in wastewater primarily exists in three forms: Inorganic phosphates: H₂PO₄⁻, HPO₄²⁻, PO₄³⁻ Polyphosphates Organic phosphorus In wastewater treatment, all bacteria uptake a certain amount of phosphorus from the environment to meet their growth requirements. However, a specific group of bacteria known as Polyphosphate Accumulating Organisms (PAOs), also referred to as phosphate-accumulating bacteria or phosphorus-removing bacteria, can uptake phosphorus in excess of their physiological needs, storing it intracellularly in large quantities as polyphosphates. By discharging this phosphorus-rich sludge, phosphorus removal from the system is achieved. II. Core Principles of Biological Phosphorus Removal The essence of biological phosphorus removal lies in driving PAOs through an "anaerobic phosphorus release – aerobic phosphorus uptake" cycle via alternating anaerobic and aerobic environments.

1. Anaerobic Phosphorus Release Phase Under anaerobic conditions, the following key processes occur: (1) Organic matter in the wastewater is converted into Volatile Fatty Acids (VFAs) by fermentative acidogenic bacteria. (2) Under these unfavorable anaerobic conditions, PAOs decompose their intracellular polyphosphate reserves. The generated energy is partially used for cell maintenance and partially to actively uptake VFAs, which are then converted and stored intracellularly as PHB (poly-β-hydroxybutyrate). (3) The inorganic phosphate produced from polyphosphate decomposition is released back into the wastewater — this constitutes "anaerobic phosphorus release."
2. Aerobic Phosphorus Uptake Phase Upon entering the aerobic environment: (1) PAOs aerobically metabolize the stored PHB, releasing substantial energy to fuel proliferation and other physiological activities. (2) A portion of this energy drives the active uptake of phosphates from the wastewater, which are accumulated intracellularly as polyphosphates in excessive amounts — this constitutes "aerobic phosphorus uptake." (3) The waste activated sludge, containing PAOs that have taken up excess phosphorus, represents the phosphorus removed from the wastewater. Key Data: Conventional activated sludge processes achieve only about 12%–20% phosphorus removal through assimilation. In contrast, systems designed for biological phosphorus removal can achieve sludge phosphorus contents of 5%–6% (dry weight), with removal rates generally sufficient to meet discharge standards. III. Factors Affecting Biological Phosphorus Removal
3. Dissolved Oxygen (DO) Anaerobic Zone: Must be strictly anaerobic; even nitrates (NO₃⁻) must be absent. Aerobic Zone: Must maintain sufficient dissolved oxygen to ensure adequate phosphorus uptake by PAOs.
4. Sludge Retention Time (SRT) SRT significantly impacts phosphorus removal efficiency — shorter SRT yields better performance: SRT = 30 days → ~40% removal rate SRT = 17 days → ~50% removal rate SRT = 5 days → up to 87% removal rate
5. Temperature Optimal range: 5–30°C.
6. pH Value Optimal range: 6–8.
7. BOD₅ Loading A BOD₅/TP ratio > 20 is a fundamental requirement for effective phosphorus removal. Readily biodegradable low-molecular-weight organic compounds induce stronger phosphorus release; more complete anaerobic release leads to greater subsequent aerobic uptake.
8. Nitrate Concentration Nitrate levels in the anaerobic zone should be maintained below 2 mg/L. When COD/TKN > 10, the inhibitory effect of nitrate is mitigated.
9. Oxidation-Reduction Potential (ORP) 表格 Zone ORP Range Aerobic Zone +40 to +50 mV Anoxic Zone -160 to -5 mV IV. Biological Phosphorus Removal Processes
10. Anaerobic-Oxic Process (A/O Process) Process Characteristics: Hydraulic Retention Time (HRT): 3–6 h Mixed Liquor Suspended Solids (MLSS) in aeration tank: 2,700–3,000 mg/L Phosphorus Removal Efficiency: ~70%; effluent TP < 1 mg/L Sludge Phosphorus Content: ~4% (high fertilizer value) SVI < 100; good settleability, resistant to bulking
11. Phostrip Process (Combined Biological-Chemical) The Phostrip process integrates biological and chemical phosphorus removal: (1) Mainstream flow undergoes conventional biological treatment (anaerobic-aerobic). (2) A sidestream of return activated sludge enters a stripping tank where phosphorus is released under anaerobic conditions. (3) Lime is added to the phosphorus-rich supernatant for chemical precipitation. (4) The stripped sludge is returned to the mainstream system. Phostrip Process Characteristics: Excellent phosphorus removal; effluent TP typically < 1 mg/L High sludge phosphorus content: 2.1%–7.1% Low lime consumption SVI < 100; sludge settles, thickens, and dewaters easily; high nutrient/fertilizer value; resistant to bulking Conclusion The core of biological phosphorus removal lies in the cyclic "release-uptake" mechanism of PAOs driven by alternating anaerobic-aerobic conditions. The extent of anaerobic phosphorus release directly determines the capacity for subsequent aerobic uptake; therefore, strict maintenance of the anaerobic environment (absence of both dissolved oxygen and nitrates) is critical to successful biological phosphorus removal. Additionally, appropriate SRT, sufficient carbon source, and suitable pH are essential factors for ensuring high removal efficiency.