Multiple Factors Affecting Total Phosphorus Removal in Wastewater Treatment
The removal of phosphorus in wastewater treatment involves the release of phosphorus by polyphosphate-accumulating organisms (PAOs) under anaerobic conditions and the excessive uptake of phosphorus under aerobic conditions. The phosphorus is then removed by discharging the phosphorus-rich residual sludge.
The factors affecting the total phosphorus removal efficiency involve several aspects, mainly:
1. Temperature
The effect of temperature on phosphorus removal is less significant than on biological nitrogen removal. Within a certain temperature range, as long as the temperature variation is not too large, biological phosphorus removal can still operate successfully. Tests indicate that the temperature for biological phosphorus removal should be above 10°C, as polyphosphate bacteria grow slower at low temperatures.
2. pH Value
When the pH is between 6.5-8.0, the phosphorus content and phosphorus uptake rate of polyphosphate microorganisms remain stable. When the pH drops below 6.5, the phosphorus uptake rate decreases sharply. A sudden drop in pH causes a rapid increase in phosphorus concentration both in the anaerobic and aerobic zones, with the release being greater as the pH decreases. This suggests that the phosphorus release caused by pH reduction is not a physiological-biochemical response of polyphosphate bacteria to pH changes but a pure chemical "acid dissolution" effect. Moreover, the lower the pH, the lower the aerobic phosphorus uptake capacity, indicating that pH-induced release is destructive and ineffective. Phosphorus uptake slightly increases when pH rises.
3. Dissolved Oxygen (DO)
Each milligram of oxygen can consume 1.14 mg of biodegradable COD, inhibiting the growth of polyphosphate bacteria and making it difficult to achieve the expected phosphorus removal effect. The anaerobic zone should maintain a low dissolved oxygen level to promote acid production by anaerobic bacteria, thereby enhancing phosphorus release by polyphosphate bacteria. Additionally, low dissolved oxygen helps reduce the consumption of biodegradable organic matter, allowing polyphosphate bacteria to synthesize more PHB.
In the aerobic zone, higher dissolved oxygen levels are needed to help polyphosphate bacteria break down stored PHB for energy to absorb soluble phosphates from the wastewater and synthesize cellular polyphosphate. The DO in the anaerobic zone should be controlled below 0.3 mg/l, while in the aerobic zone, it should be above 2 mg/l to ensure smooth anaerobic phosphorus release and aerobic phosphorus uptake.
4. Nitrate Nitrogen in the Anaerobic Tank
The presence of nitrate nitrogen in the anaerobic zone consumes organic substrates and inhibits PAO’s phosphorus release, affecting polyphosphate bacteria’s phosphorus absorption under aerobic conditions. Additionally, nitrate nitrogen can be utilized by Pseudomonas species as an electron acceptor for denitrification, which impairs the ability of polyphosphate bacteria to ferment and produce acids, thus inhibiting phosphorus release and uptake. Each milligram of nitrate nitrogen can consume 2.86 mg of biodegradable COD, suppressing anaerobic phosphorus release. Typically, nitrate nitrogen should be controlled below 1.5 mg/l.
5. Sludge Age
As biological phosphorus removal systems mainly remove phosphorus through the discharge of residual sludge, the amount of residual sludge determines the phosphorus removal efficiency. The sludge age directly affects the quantity of residual sludge and the ability of the sludge to absorb phosphorus. A shorter sludge age generally leads to better phosphorus removal because it increases the discharge of residual sludge and the phosphorus removal in the system, reducing the phosphorus content in the effluent. However, for biological processes that remove both phosphorus and nitrogen, the sludge age is often controlled longer to meet the growth requirements of nitrifying and denitrifying bacteria, which is why phosphorus removal efficiency is sometimes unsatisfactory. For phosphorus removal-only biological treatment systems, the sludge age is generally controlled between 3.5 and 7 days.
6. COD/TP Ratio
In biological phosphorus removal, the ratio of organic substrates and nutrients required by microorganisms in the anaerobic zone, relative to the phosphorus content in wastewater, is an important factor affecting phosphorus removal. Phosphorus release and uptake differ when different organic substrates are used.
Easily degradable organic matter with low molecular weight (such as volatile fatty acids) is readily utilized by polyphosphate bacteria, which break down stored polyphosphates and release phosphorus. These substances have a stronger ability to induce phosphorus release compared to high molecular weight, difficult-to-degrade organics. The more thorough the anaerobic phosphorus release, the greater the aerobic phosphorus uptake.
Additionally, the energy generated from phosphorus release during the anaerobic phase is mainly used by polyphosphate bacteria to absorb low molecular organic substrates for survival under anaerobic conditions. Therefore, whether the influent contains enough organic matter is critical for the survival of polyphosphate bacteria in anaerobic conditions. Generally, the influent COD/TP ratio should be greater than 15 to ensure adequate substrate for polyphosphate bacteria and achieve optimal phosphorus removal.
7. RBCOD (Readily Biodegradable COD)
Studies have shown that when easily degradable carbon sources, such as acetic acid, propionic acid, and formic acid, are used as phosphorus release substrates, the release rate of phosphorus is higher. The release rate is not dependent on the substrate concentration but on the concentration of activated sludge and microbial composition. The release of phosphorus from such substrates follows zero-order reaction kinetics. Other organic matter must first be converted into small, easily degradable carbon sources before polyphosphate bacteria can utilize it for metabolism.
8. Glycogen
Glycogen is a large branched polysaccharide made up of glucose units, which serves as an intracellular storage form of sugar. As shown in the figure, glycogen is formed under aerobic conditions in polyphosphate bacteria and is metabolized in anaerobic conditions to synthesize PHAs (polyhydroxyalkanoates). This process generates NADH, which serves as the raw material for PHB synthesis and provides energy for the metabolism of polyphosphate bacteria. In cases of delayed aeration or over-aeration, the phosphorus removal effect is poor, as excessive aeration depletes some of the glycogen in polyphosphate bacteria, leading to insufficient NADH for PHAs synthesis under anaerobic conditions.
9. Hydraulic Retention Time (HRT)
For well-operated urban wastewater biological nitrogen and phosphorus removal systems, phosphorus release typically requires 1.5–2.5 hours, while phosphorus uptake requires 2.0–3.0 hours. Generally, phosphorus release is more critical, so greater attention is paid to the HRT in the anaerobic zone. A short HRT in the anaerobic zone will not allow for effective phosphorus release. Furthermore, facultative acidifying bacteria in the sludge will not sufficiently degrade macromolecular organic matter into low fatty acids, affecting phosphorus release. However, excessively long HRT is unnecessary, increases capital investment, operational costs, and may lead to side effects.
In summary, phosphorus release and uptake are interrelated processes. Polyphosphate bacteria can better absorb phosphorus during the aerobic phase only if they release sufficient phosphorus during the anaerobic phase. Proper control of these processes creates a positive cycle. Based on practical operations, our data suggests that an anaerobic HRT of 1 hour 15 minutes to 1 hour 45 minutes and an aerobic HRT of 2 hours to 3 hours 10 minutes are optimal.
10. Return Sludge Ratio (R)
A key factor in ensuring the phosphorus removal effect in A/O processes is ensuring that the system sludge carries enough dissolved oxygen into the secondary sedimentation tank. This prevents phosphorus release due to anaerobic conditions in the tank. However, if the sludge cannot be quickly discharged and the sludge layer is too thick, even high DO levels cannot prevent phosphorus release. Therefore, the return ratio in A/O systems should not be too low and must maintain a sufficient return ratio to quickly discharge sludge from the secondary tank.
Excessively high return ratios increase energy consumption in the return and aeration systems, and shorten the actual retention time of sludge in the aeration tank, affecting BOD5 and phosphorus removal. The return ratio is generally maintained between 50-70%.
