Total Phosphorus and Total Nitrogen Removal Technology Routes: From Process Principles to Engineering Selection

Total Phosphorus and Total Nitrogen Removal Technology Routes: From Process Principles to Engineering Selection

In municipal sewage plant upgrading and industrial wastewater treatment projects, the simultaneous removal of total phosphorus (TP) and total nitrogen (TN) is a technical challenge and a key indicator of environmental regulation. Total phosphorus mainly comes from detergents in domestic sewage, phosphates in industrial wastewater, and pesticide residues. Total nitrogen consists of ammonia nitrogen, nitrate nitrogen, organic nitrogen, etc. Both are key factors causing water eutrophication.

For engineering contractors and system integrators, understanding the applicable scope and boundary conditions of various phosphorus and nitrogen removal technologies is the foundation for process design and equipment selection.

1. Total Phosphorus Removal Technology Routes

Total phosphorus exists in water in the forms of orthophosphate, polyphosphate, and organic phosphorus. Different forms correspond to different removal mechanisms.

1.1 Chemical Precipitation Method

Chemical precipitation is the mainstream process for phosphorus removal. It involves adding metal salts or lime to convert phosphate radicals into insoluble precipitates.

Calcium salt precipitation method: Adding lime (CaO) or calcium chloride to phosphorus-containing wastewater generates hydroxyapatite precipitate under alkaline conditions. The reaction equation is as follows:

5Ca²⁺ + 7OH⁻ + 3H₂PO₄⁻ → Ca₅(OH)(PO₄)₃↓ + 6H₂O

Engineering parameters: The best phosphorus removal effect is achieved when pH is controlled at 10.5–12.5, and the effluent phosphate concentration can be lower than 0.5 mg/L. Reaction time is 10–15 minutes. Cooperating with PAM flocculant can improve sedimentation efficiency.

Aluminum/iron salt precipitation method: Adding aluminum sulfate, polyaluminum chloride or ferric chloride generates aluminum phosphate or iron phosphate precipitates. Its advantage is that it can operate under near-neutral pH (6.5–7.5) conditions, suitable for post-biochemical system phosphorus removal.

It should be noted that the chemical precipitation method mainly targets inorganic phosphorus and has limited effect on organic phosphorus removal. Process verification is required according to actual water quality.

1.2 Biological Phosphorus Removal

Biological phosphorus removal relies on the excessive phosphorus uptake behavior of phosphorus accumulating organisms (PAOs) in alternating anaerobic/aerobic environments. In the anaerobic stage, polyphosphate is hydrolyzed to release phosphate; in the aerobic stage, excessive phosphate is absorbed and stored in cells as polyphosphate, and phosphorus is removed through the discharge of excess sludge.

When the enhanced biological phosphorus removal (EBPR) system operates stably, the effluent total phosphorus can reach below 0.5 mg/L. However, biological phosphorus removal is sensitive to influent carbon sources and operating conditions, and efficiency decreases significantly under low temperature or low carbon-to-nitrogen ratio conditions.

1.3 Adsorption Method

Utilizing the adsorption effect of porous materials to remove phosphate from water. Studies have shown that sludge-based activated carbon has a phosphorus adsorption capacity of 7.3 mg/g under pH 6 and adsorption time of 360 minutes, which is better than commercial activated carbon's 4.1 mg/g. Biochar prepared from date palm fiber can achieve 81%–91% total phosphorus removal rate.

The adsorption method is suitable for deep treatment of low-concentration phosphorus-containing wastewater or as a tertiary treatment unit, but the regeneration and replacement costs of adsorbents need to be included in the engineering economic evaluation.

2. Total Nitrogen Removal Technology Routes

The core of total nitrogen removal is to convert various forms of nitrogen into nitrogen gas and release it from the water body. Mainstream technical paths include biological denitrification and physicochemical methods.

2.1 Biological Denitrification Process

Biological denitrification is completed by coupling nitrification and denitrification processes.

Nitrification: Under aerobic conditions, ammonia nitrogen is sequentially oxidized to nitrate nitrogen by nitrite bacteria and nitrifying bacteria.

Denitrification: Under anoxic conditions, denitrifying bacteria use organic matter as electron donors to reduce nitrate nitrogen to nitrogen gas.

Common processes in engineering include:

• AAO process (Anaerobic-Anoxic-Oxic): Standard three-stage structure, achieving simultaneous nitrogen and phosphorus removal. The improved AAO process, through refined control of internal reflux ratio and carbon source addition, can achieve effluent TN<11 mg/L and TP<0.5 mg/L when the influent C/N ratio is 3–4.

• Bardenpho process: An additional post-anoxic stage is added to make full use of endogenous carbon sources for deep denitrification. Studies have shown that under carbon source dosage of 38.63 mg/L and internal reflux ratio of 300%, the effluent TN is 8.15 mg/L and TP is 0.3 mg/L.

• AOA-SBR process: Anaerobic/oxic/anoxic sequencing batch reactor. Under low temperature of 15.7℃, when influent TN is 45.5 mg/L and TP is 3.9 mg/L, the effluent is reduced to 10.9 mg/L and 0.1 mg/L respectively.

2.2 External Carbon Source Addition Strategy

Low carbon-to-nitrogen ratio (C/N<4) is a typical feature of municipal sewage in northern regions. Insufficient carbon sources will limit denitrification efficiency. In actual projects, external carbon sources such as sodium acetate are used for supplementation.

Fine control strategies link carbon source dosage with nitrate nitrogen concentration in the anoxic section, which can reduce carbon source consumption per ton of water from 60 g to 20 g (calculated as pure sodium acetate), saving about 70% in carbon source costs.

2.3 Physicochemical Denitrification Methods

For high-concentration nitrogen-containing wastewater that cannot be biochemically treated (such as landfill leachate and industrial wastewater), the following methods can be used:

• Ion exchange: Selective adsorption of ammonium ions using materials such as zeolite

• Membrane separation: Reverse osmosis or electrodialysis to achieve nitrogen concentration and separation

• Ammonia nitrogen remover: Only applicable to wastewater dominated by ammonia nitrogen, converting it into nitrogen gas through chemical reactions. Note that ammonia nitrogen removers are ineffective against nitrate nitrogen and organic nitrogen. The total nitrogen removal effect depends on the proportion of ammonia nitrogen in total nitrogen.

3. Process Combination and Engineering Selection Reference

4. Process Synergy: Engineering Difficulties in Simultaneous Nitrogen and Phosphorus Removal

There are process contradictions in the simultaneous removal of total nitrogen and total phosphorus:

• Biological phosphorus removal requires sufficient carbon sources (anaerobic section), while biological denitrification also consumes carbon sources (anoxic section)

• Phosphorus removal relies on excess sludge discharge, while denitrification relies on longer sludge age

Engineering achieves balance through the following strategies:

• Adopt staged water inlet process to distribute carbon sources to anaerobic and anoxic sections

• Sludge age controlled at 10–15 days, taking into account the growth needs of nitrifying bacteria and phosphorus accumulating bacteria

• When effluent TP requirements are strict (<0.3 mg/L), supplement chemical post-precipitation unit

5. Monitoring and Process Control

For system integrators, online monitoring of total phosphorus and total nitrogen is the foundation of process regulation. Currently, total phosphorus monitoring uses the ammonium molybdate spectrophotometric method (detection limit 0.01–1.0 mg/L), and total nitrogen uses the alkaline potassium persulfate digestion-ultraviolet spectrophotometric method. In the AAO process, the nitrate nitrogen concentration in the anoxic section is the key parameter for controlling internal reflux ratio and external carbon source dosage.

FAQ

Q1. Is chemical phosphorus removal effective on organic phosphorus?

Ineffective. The chemical precipitation method is only effective on inorganic phosphate ions. Organic phosphorus needs to be first converted into inorganic phosphorus through oxidation or biodegradation before precipitation removal.

Q2. Why is denitrification difficult in low carbon-to-nitrogen ratio wastewater?

The denitrification process requires organic matter as an electron donor. When C/N is lower than 4, the raw water carbon source is insufficient to support complete denitrification, and external carbon sources (such as sodium acetate) or multi-point water inlet strategies need to be supplemented.

Q3. How to choose between biological phosphorus removal and chemical phosphorus removal?

Biological phosphorus removal is used for the main process (low operating cost, but efficiency is affected by water quality fluctuations); chemical phosphorus removal is used as a safeguard measure (stable reaction, but high chemical cost). In engineering, a combination mode of biology as the main and chemistry as auxiliary is often adopted.

Q4. When total nitrogen compliance is difficult, which parameters should be adjusted first?

Check in sequence: internal reflux ratio (should be 200%–400%), dissolved oxygen in anoxic section (should be <0.5 mg/L), carbon source dosage and dosing point (should be added at the entrance of the anoxic zone), sludge age (nitrifying bacteria require longer sludge age).

Q5. To reduce total phosphorus from 1.0 mg/L to 0.3 mg/L, which process is the most economical?

When the influent TP has been reduced to about 1.0 mg/L, the potential of biological phosphorus removal is close to the upper limit. Using aluminum or iron salts for chemical post-precipitation (dosage about 10–30 mg/L) is an economical and feasible deep phosphorus removal solution.

Q6. How long is the saturation cycle of activated carbon adsorption for phosphorus removal?

It depends on the influent phosphorus concentration and carbon type. Regeneration is required when the phosphorus adsorption reaches 60%–70% of the saturated adsorption capacity. Taking coconut shell activated carbon as an example, when treating secondary effluent with phosphorus 1–2 mg/L, the saturation cycle is about 2000–5000 bed volumes.

Q7. Can integrated wastewater treatment equipment simultaneously meet TP and TN standards?

Yes, it can be configured. It is necessary to adopt processes with anaerobic, anoxic, and aerobic segmented control (such as AAO integrated equipment), and be equipped with chemical dosing units and online monitoring instruments. Suitable for decentralized wastewater treatment scenarios of 50–500 m³/d.

Summary:

The removal of total phosphorus and total nitrogen is a systematic challenge in wastewater treatment engineering. Total phosphorus removal is mainly based on chemical precipitation and biological phosphorus removal, while total nitrogen removal is centered on biological nitrification/denitrification. There is a game between the two in carbon source distribution and sludge age control.

Engineering contractors need to clarify the characteristics of influent water quality (C/N ratio, phosphorus form, temperature range) in the design stage, select matching main processes, and reserve chemical dosing and deep filtration units as compliance guarantees. System integrators should focus on the configuration of online monitoring instruments (nitrate nitrogen, ammonia nitrogen, TP/TN analyzers) to provide data support for refined process regulation.

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