What are the methods for phosphorus removal from wastewater?

The last edit to this post was made by zhaolijun on 2015-7-6 at 19:46. It is generally divided into two categories: Chemical phosphorus removal: One approach involves using chemical methods to remove phosphorus from single streams of wastewater with high phosphorus content; this method requires relatively simple facilities； Second is phosphorus removal before discharge; at this stage, inorganic phosphorus generally turns into orthophosphates, ensuring thorough removal. Biological phosphorus removal: A2/0, SBR, CASS, and oxidation ditches all exhibit good phosphorus removal efficiency.

There are two processes for phosphorus removal: chemical phosphorus removal and biological phosphorus removal. Biological phosphorus removal is a relatively economical method, but since this process cannot currently ensure stable compliance with the discharge standard of 0.5 mg/l, chemical phosphorus removal measures are often required to meet such standards. Chemical phosphorus removal is accomplished through a chemical precipitation process. Chemical precipitation involves adding inorganic metal salt agents to wastewater; these agents combine with soluble salts in the wastewater, such as phosphates, to form particulate, insoluble substances. This process involves what is known as a phase transfer reaction, and an example of the reaction equation is given in Equation 1. In fact, after the addition of chemical agents, not only precipitation reactions occur in the wastewater, but also chemical flocculation reactions take place; therefore, it is necessary to distinguish between chemical precipitation and chemical flocculation. FeCl3 + K3PO4 → FePO4↓ + 3KCl. Equation 1: The sedimentation reaction in wastewater can be simply understood as the process by which most of the dissolved substances in water, which are in ionic form, are converted into an insoluble, particulate form. Flocculation, on the other hand, is the process by which small insoluble solid particles stick together to form larger particles; therefore, flocculation is not a phase transition process. In wastewater treatment processes, flocculation and sedimentation are both extremely important; however, flocculation is used to improve the sedimentation efficiency in sedimentation tanks, while sedimentation is used to remove soluble phosphorus from wastewater. If phase conversion is achieved using the precipitation process, then when soluble metal salt agents are added to the wastewater, on the one hand, soluble phosphorus is converted into insoluble metal phosphate salts; at the same time, insoluble hydroxides are also formed (depending on the pH value). On the other hand, as precipitates increase and smaller insoluble solids accumulate to form larger ones, the stable colloids become destabilized. Through velocity gradients or diffusion processes, these destabilized colloids come into contact with each other and form flocs. Finally, through a solid-liquid separation step, purified wastewater and a solid-liquid concentrate (chemical sludge) are obtained, achieving the goal of chemical phosphorus removal. Based on the principles of chemical precipitation reactions, to produce phosphate compounds, the chemical agents used for chemical phosphorus removal are mainly metal salt agents and calcium hydroxide (slaked lime). Many high-priced metal ion agents, when added to wastewater, combine with the soluble phosphorus ions in the wastewater to form insoluble compounds. For economic reasons, the metal salt reagents used for phosphorus precipitation are mainly Fe3+, Al3+, and Fe2+ salts along with lime. These agents are used in solution and suspension form. Ferrous salts can be used only when oxygen is present in the wastewater, allowing them to be oxidized to ferric salts. In practice, in order to enable oxidation, Fe2+ is often added to aeration sedimentation tanks or, using a simultaneous precipitation process, to aeration tanks; its effect is the same as that of Fe3+, with the reaction equations given in Equations 2 and 3. Al3++PO43-→AlPO4↓ at pH=6～7, Equation 2; Fe3++PO43-→FePO4↓ at pH=5～5.5, Equation 3. The reactions that compete with these precipitation reactions are those between metal ions and OH-, so for various metal salt products, it is important to pay attention to the amount of metal ions present; the reaction equations are given in Equations 4 and 5. Al3++3OH-→Al(OH)3↓ Equation 4; Fe3++3OH-→Fe(OH)3 Equation 5. Metal hydroxides form large flocculent aggregates, which is beneficial for the flocculation of the precipitated products; moreover, they can adsorb colloidal substances as well as fine suspended particles. It should be noted that the precipitation and removal of organic matter in chemical precipitation reactions aimed at chemical phosphorus removal is secondary, but during separation, the coagulation of organic colloids and suspended solids into flocs is a decisive process. The precipitation effect is influenced by pH, and the solubility of metal phosphates is also affected by pH. The optimal pH range for iron salts is 5.0–5.5, while for aluminum salts it is 6.0–7.0, as the solubility of FePO4 or AlPO4 is lowest within these pH ranges. Additionally, the use of metal salt agents brings benefits to wastewater and sludge treatment, such as reducing the sludge index and facilitating the desulfurization of biogas. The addition of metal salt agents increases the concentration of Cl- or SO2-4 ions in the effluent from wastewater treatment plants. Special attention is required if the precipitation reagent solution also contains acid. The addition of metal salt agents will correspondingly reduce the alkalinity of the wastewater, which may have an adverse effect on purification. When ferric sulfate is used in the synchronous precipitation process, the impact on the nitration reaction must be considered. Furthermore, if wastewater treatment plant sludge is used in agriculture, the impact of aluminum or iron loads on agriculture must be considered when using metal salt agents for phosphorus removal. In addition to metal salt reagents, calcium hydroxide is also used as a precipitation reagent. During precipitation, it is not Ca2+ but OH- ions that play a key role in the formation of insoluble calcium phosphate; as the pH value increases, the solubility of calcium phosphate decreases, and a pH level of 8.5 or higher is required when using Ca(OH)2 for phosphorus removal. The formation of calcium phosphate occurs according to reaction equation 6: 5Ca2+ + 3PO43- + OH- → Ca5(PO4)3OH↓. This process takes place at pH values of ≥8.5. However, in the range of pH 8.5 to 10.5, in addition to the precipitation of calcium phosphate, calcium carbonate is also formed, which may lead to scaling on the walls of tanks, channels, and pipes; the relevant reaction is shown in equation 7. The reaction Ca2+ + CO32- → CaCO3, Equation 7, for phosphate precipitation with calcium is influenced not only by the pH value but also by the bicarbonate concentration (alkalinity). Under certain pH conditions, the amount of calcium added is proportional to the alkalinity. For soft or moderately hard wastewater, calcium precipitation requires only a small amount of calcium to achieve the desired pH value; in contrast, wastewater with strong buffering capacity demands a larger amount of calcium to be added. The chemical precipitation process is classified based on the location where the precipitation agents are added. Commonly used methods in practice include pre-precipitation, simultaneous precipitation, and post-precipitation, or flocculation and filtration after biological treatment. (1) Pre-sedimentation: The characteristic of the pre-sedimentation process is that the sedimentation agent is added in the grit chamber, or in the inlet channel (pipe) of the primary sedimentation tank, or in a Venturi channel (using vortexes). It generally requires the installation of devices to generate vortices or the supply of energy to meet the mixing requirements. The resulting precipitated products (large flocculent masses) are then separated by precipitation in a primary sedimentation tank. If a biological filter is used for the biological section, Fe2+ chemicals are not allowed to be used in order to prevent damage to the packing (the formation of yellow rust). The pre-sedimentation process (as shown in Figure 2) is particularly suitable for the retrofitting of existing wastewater treatment plants (to add chemical phosphorus removal measures), as this process step not only allows for the removal of phosphorus but also reduces the load on the biological treatment facilities. The commonly used precipitation agents are mainly lime and metal salt agents. The phosphate content remaining after precipitation prior to menstruation is 1.5–2.5 mg/L, which fully meets the phosphorus requirements for subsequent biological treatment. (2) Synchronous precipitation is the most widely used chemical phosphorus removal process, accounting for about 50% of all chemical phosphorus removal processes abroad. The process involves adding the precipitation agent to the effluent from the aeration tank or to the water entering the secondary sedimentation tank; in some cases, the agent is also added to the water entering the aeration tank or to the return sludge channels (pipes). It is currently used in many sewage treatment plants; for example, the third phase of the Datansha Sewage Treatment Plant in Guangzhou employs synchronous sedimentation, and chemical dosing has little impact on the activated sludge. (3) Post-sedimentation: Post-sedimentation involves carrying out sedimentation, flocculation, and the separation of the flocculated substances in a facility separate from the biological facility; hence, it is also referred to as a two-stage process. Generally, the sedimentation agent is added to a mixing tank (Tank M) located after the secondary sedimentation tank, followed by a flocculation tank (Tank F) and a sedimentation tank (or air flotation tank). For receiving water bodies with less stringent requirements, lime slurry can be used in the post-sedimentation process, but it is necessary to control the pH value of the effluent, for example by using CO2 from biogas for neutralization. The use of air flotation tanks allows for better removal of suspended solids and total phosphorus compared to sedimentation tanks, but the operating costs are higher due to the need for a constant supply of air.

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