Comparison and Selection of Wastewater Phosphorus Removal Processes
- Chemical phosphorus removal
1.1 Principle of Chemical Phosphorus Removal
Chemical phosphorus removal is mainly achieved through a chemical precipitation process. Chemical precipitation refers to the reaction of inorganic metal salts added to wastewater with soluble salts (such as phosphates) in the wastewater to form particulate, insoluble substances. In fact, after the addition of chemical agents, not only precipitation but also chemical flocculation occurs in the wastewater, where the small, insoluble solids bind together to form larger flocs.
Wastewater sedimentation can be simply understood as the process by which dissolved substances in water, mostly ionic substances, are converted into non-dissolved, particulate forms. Flocculation, on the other hand, is the process by which small, non-dissolved solids bind together to form larger shapes. Therefore, flocculation is not a phase transfer process. Flocculation is used to improve the sedimentation effect in sedimentation tanks, while sedimentation is used to remove dissolved phosphorus from wastewater.
1.2 Chemical phosphorus removal agents
To generate insoluble phosphate compounds, the main chemical agents used for phosphorus removal are metal salts and calcium hydroxide. Many high-valence metal ion agents, when added to wastewater, combine with soluble phosphorus ions to form poorly soluble compounds. However, for economic reasons, the metal salts used for phosphorus precipitation are mainly Fe3+, Fe2+, and Al3+ salts, which are used in solutions and suspensions. Besides metal salts, calcium hydroxide is also used as a precipitation agent, reacting to form water-insoluble calcium phosphate.
Table 1 Commonly Used Chemicals for Wastewater Treatment
| type | name | Molecular formula | state |
| Aluminum salts | Aluminum sulfate | Al2(SO4)3·18H2O | solid |
| Al2(SO4)3·14H2O | liquid | ||
| nAl2(SO4)3·xH2O+mFe2(SO4)3·yH2O | solid | ||
| Aluminum chloride | AlCl3 | liquid | |
| AlCl3+FeCl3 | liquid | ||
| Polyaluminum chloride | [Al2(OH)nCl6-n]m | liquid | |
| Ferrous salts | Ferrous sulfate | FeSO4·7H2O | solid |
| FeSO4 | liquid | ||
| Ferrous salts | Ferric chloride sulfate | FeClSO4 | Liquid (approximately 40%) |
| Ferric chloride | FeCl3 | Liquid (approximately 40%) | |
| slaked lime | calcium hydroxide | Ca(OH)2 | Approximately 40% emulsion |
Coagulation and sedimentation of aluminum salts
Al 2 (SO 4 ) 3 + 6H 2 O----2Al(OH) 3 +3SO 4 2- +6CO 2
Al 2 (SO 4 ) 3 + 2PO 4 ----2AlPO 4 +3SO 4 2-
At a pH of 6.0–6.5, 1.5–3.0 mol of aluminum are required for every 1 mol of phosphorus. If the water is alkaline, the pH should be lowered before adding aluminum to reduce Al(OH) ₃ precipitation.
Coagulation and precipitation of iron salts
Fe 2 (SO 4 ) 3 + 3HCO 3 ----Fe(OH) 3 +2SO 4 2- +3CO 2
Fe 3+ + PO 4 3- ---FePO 4 ↓ pH=5～5.5
For every 1 mol of phosphorus, 1.5–3 mol of iron (Fe³⁺) needs to be added, with an optimal pH of 5.0.
For secondary treated water with a phosphorus content of around 5 mg/L, adding 100–200 mg/L of ferric chloride (FeCl₃ · 6H₂O ) can achieve a phosphorus removal rate of over 90%.
Metal hydroxides form large flocs, which is beneficial for the flocculation of precipitation products. They also adsorb colloidal substances and fine suspended particles. It is important to note that the precipitation removal of organic matter in chemical precipitation reactions aimed at chemical phosphorus removal is secondary, but the coagulation of organic colloids and suspended matter in the flocs during separation is the decisive process.
Precipitation efficiency is affected by pH, as is the solubility of metal phosphates. The optimal pH range for iron salts is 5.0–5.5, and for aluminum salts, it is 6.0–7.0, because FePO4 or AIPO4 exhibits the lowest solubility within these pH ranges. Furthermore, the use of metal salts can bring benefits to wastewater and sludge treatment, such as reducing the sludge index and facilitating biogas desulfurization.
The addition of metal salt reagents can increase the concentration of Cl- or SO42- ions in the effluent from wastewater treatment plants. Special attention should be paid if the precipitant solution also contains acid.
Adding metal salts will lower the alkalinity of the wastewater, which may negatively impact purification. When using ferric sulfate in a simultaneous precipitation process, its effect on the nitrification reaction must be considered.
In addition, if wastewater treatment plant sludge is used for agriculture, the impact of aluminum or iron load on agriculture must be considered when using metal salt agents for phosphorus removal.
Coagulation and sedimentation of lime
5Ca 2+ + 4OH - + 3HPO 4 2- ---Ca 5 OH(PO 4 ) 3 + 3H 2 O
To achieve a phosphorus removal rate of over 90%, the pH value needs to be adjusted to above 10.5-11.0. The Ca/P weight ratio should be above 2.2:1.
During the precipitation process, the main factor in the formation of insoluble calcium phosphate is not Ca2+, but OH- ions, because the solubility of calcium phosphate decreases as the pH value increases. The pH value required for phosphorus removal using Ca(OH)2 is above 8.5.
However, within a pH range of 8.5 to 10.5, in addition to calcium phosphate precipitation, calcium carbonate will also be produced, which may lead to scaling on the walls of pools, channels, or pipes. The reaction formula is...
Ca²⁺ + CO₃²⁻ → CaCO₃
The phosphate precipitation reaction with calcium is affected not only by pH value but also by the concentration of bicarbonate ions (alkalinity). Under certain pH conditions, the amount of calcium added is directly proportional to the alkalinity.
For soft or medium-hard wastewater, the amount of calcium required to achieve the desired pH value when using calcium precipitation is very small. Conversely, wastewater with strong buffering capacity requires a larger amount of calcium.
1.3 Chemical phosphorus removal process
Chemical phosphorus removal processes can be classified according to the location of chemical agent addition. In practice, the commonly used methods are: pre-phosphorus removal, simultaneous phosphorus removal, and post-phosphorus removal.
The characteristic of pre-treatment phosphorus removal is that chemical agents are added to the grit chamber, the inlet channel (pipe) of the primary sedimentation tank, or the Venturi channel (using eddies). It generally requires a device to generate eddies or an energy supply to meet the mixing needs. The resulting precipitates (large flocculent particles) are separated by sedimentation in the primary sedimentation tank. If a biological filter is used in the biological stage, iron salt agents are not allowed to prevent damage to the packing material (causing yellow rust). Pre-treatment phosphorus removal, because it only adds chemical phosphorus removal measures at the front end of the existing process, is well-suited for the renovation of existing wastewater treatment plants. This process not only removes phosphorus but also reduces the load on the biological treatment facilities. Commonly used chemical agents are mainly lime and metal salt agents. After pre-treatment, the residual phosphate content is controlled at 1.5-2.5 mg/L, which fully meets the phosphorus requirements of subsequent biological treatment.
Simultaneous phosphorus removal is currently the most widely used chemical phosphorus removal process, accounting for approximately 50% of all chemical phosphorus removal processes internationally. The process involves adding chemical reagents to the effluent of the aeration tank or the influent of the secondary sedimentation tank; in some cases, the reagents are also added to the influent of the aeration tank or the return sludge channel (pipe). It is currently established that simultaneous chemical phosphorus removal can be used for activated sludge processes and biological rotating disc processes, but whether the reagents can be added to the influent of the secondary sedimentation tank in biological filter processes remains to be explored.
Post-phosphorus removal involves separating precipitation, flocculation, and the flocculated material in a facility separate from the biological treatment process; hence, it is also called a two-stage process. Generally, chemical reagents are added to a mixing tank after the secondary sedimentation tank, followed by a flocculation tank and a sedimentation tank (or flotation tank).
For receiving water bodies with less stringent requirements, lime slurry can be used in the post-phosphorus removal process, but the pH value of the effluent must be controlled, such as by using CO2 for neutralization.
Flotation tanks can remove suspended solids and total phosphorus better than sedimentation tanks, but they are more expensive to operate because they require a constant supply of air.
Table 2 Comparison of various chemical phosphorus removal processes
| Process type | advantage | shortcoming |
| Pre-phosphorus removal process | 1) It can reduce the load on biological treatment structures and balance load fluctuations, thereby reducing energy consumption; compared with simultaneous phosphorus removal, the organic content in activated sludge will not increase;
|
1) Total sludge production increased; 2) It affects the denitrification reaction (excessive substrate decomposition); 3) It is detrimental to improving the sludge index. |
| Simultaneous phosphorus removal process |
|
|
| Post-phosphorus removal process | 1) The precipitation of nitrates is separated from the biological treatment process, and the two processes do not affect each other; 2) The dosage of the pesticide can be controlled according to changes in the phosphorus load; 3) The generated phosphate sludge can be discharged separately and utilized. | Post-phosphorus removal processes require large investments and have high operating costs, but when building new wastewater treatment plants, using post-phosphorus removal processes can reduce the size of the secondary sedimentation tank in biological treatment. |
2. Biological phosphorus removal
The principle of biological phosphorus removal is that certain bacteria alternate between anaerobic and aerobic conditions. Under anaerobic conditions, the bacteria absorb low-molecular-weight organic matter and store it in their cells as poly(β-hydroxybutyric acid) (PHB), while simultaneously releasing polyphosphates from their cell protoplasm as orthophosphates. At this time, the phosphorus content in the wastewater increases, while the BOD content decreases. Then, under aerobic conditions, the bacteria oxidize and decompose the absorbed organic matter (PHB), providing energy, while simultaneously absorbing a large amount of phosphorus from the wastewater and storing it as polyphosphates. The amount absorbed exceeds the amount released, resulting in a significant reduction in the phosphorus content in the wastewater. By discharging the remaining sludge from the system, the phosphorus ingested by the bacteria is also removed, thus achieving the goal of phosphorus removal.
Common biological phosphorus removal processes include: A/O process, A2 / O process, and SBR process.
The A/O process is currently the simplest biological phosphorus removal method. Raw wastewater or primary sedimentation tank effluent is mixed with returned sludge in an anaerobic tank. This process requires no nitrification reaction. Generally, when the hydraulic retention times in the anaerobic and aerobic zones are 0.5–1 h and 1–3 h, respectively, good phosphorus and organic matter removal effects can be achieved. Since biological phosphorus removal systems primarily remove phosphorus by discharging excess sludge, the amount of excess sludge determines the system's denitrification efficiency. Systems with shorter sludge ages generally produce more excess sludge and achieve higher phosphorus removal efficiency. Reports indicate that the phosphorus removal rate is 40% at a sludge age of 30 days; 50% at 17 days; and can reach 87% at 5 days.
Summarize
While chemical precipitation is a relatively efficient phosphorus removal process, it consumes chemical reagents and produces a large amount of chemical sludge, making the treatment cost relatively expensive. Traditional biological treatment processes are simple to operate, but their phosphorus removal efficiency is low and they are difficult to meet effluent requirements.
