Total phosphorus detection technology began in the 1950s with chemical methods, such as the Nessler reagent method and ammonium molybdate reduction method. In the 1980s, automatic analyzers and spectrometers were applied to total phosphorus detection, enabling online monitoring. Later, detection methods with higher sensitivity and faster analysis, such as chemiluminescence and biosensing technology, were developed. [10]

Through technological development, modern analyzers adopt high-pressure digestion, intelligent temperature control, 5G, edge computing, and AI algorithms, helping to achieve high-precision detection and real-time early warning. [11][32] In recent years, intelligent unattended surface water quality automatic monitoring stations have begun to be applied, integrating IoT, artificial intelligence, and big data technology. Intelligent monitoring stations usually have full-process unattended operation and maintenance capabilities, with operation and maintenance frequency reduced by more than 80% and single maintenance time shortened by more than 70%. [5]

The system can automatically execute quality control measures such as weekly checks and monthly linear calibration, and combined with dynamic pretreatment technology can handle environmental interference such as turbidity to improve the reliability of monitoring data. In terms of early warning functions, the equipped AI algorithm can identify abnormal water quality changes, push abnormal information to the provincial monitoring platform, and generate fault diagnosis suggestions. The monitoring station can monitor parameters such as total phosphorus in real time. [5]

Total Phosphorus (TP) is a general term, mainly used to describe the sum of inorganic and organic phosphorus in water bodies, with the unit mg/L. [8] Phosphorus is an active element that does not exist in a free state in nature, but exists in the form of organophosphorus, inorganic phosphorus compounds, and reduced-state PH₃. [13][23] Phosphorus in water can exist in the form of elemental phosphorus, orthophosphate, condensed phosphate, pyrophosphate, metaphosphate, and organic-bound phosphate. Total phosphorus is the result measured after various forms of phosphorus in the water sample are converted to orthophosphate by digestion. [8][23] Total phosphorus is a key indicator for evaluating water eutrophication, and its content directly determines the risk of algal blooms. [6]

Total phosphorus entering environmental water bodies mainly comes from human activities and is a key pollutant causing water eutrophication. [13-14][23] In recent years, total phosphorus has become the primary pollutant in areas such as the Yangtze River basin. [16][18] According to the nature of pollution sources, it can be mainly divided into domestic sources, agricultural sources, and industrial sources. [13-14][23][26]

Agricultural sources are the main source of total phosphorus non-point source pollution, mainly including the application of chemical fertilizers (especially phosphate fertilizer), livestock and poultry breeding manure discharge, and farmland runoff. [9][13-14][23] Among them, livestock and poultry breeding has the largest total phosphorus discharge, accounting for 34.44% of the total phosphorus discharge in the entire Yangtze River basin; crop farming total phosphorus discharge accounts for 29.18% of the basin’s total phosphorus discharge. Agricultural non-point source pollution total phosphorus discharge is about 60% of the basin’s total discharge, and is currently the focus of total phosphorus control in the Yangtze River basin. [16][18][26]

Industrial sources are an important source of point source pollution, among which the discharge from "three phosphorus" (phosphate mining, phosphorus chemical enterprises, and phosphogypsum warehouses) enterprises is an important reason for exacerbating regional phosphorus pollution. [15-16][18][26] In addition, wastewater discharged from industries such as chemical, papermaking, printing and dyeing, food processing, rubber, dye, pesticide, coking, petrochemical, fermentation, and pharmaceutical often contains a large amount of phosphorus. [13-14][23] Industrial source total phosphorus has a high river-entry coefficient and more directly affects water bodies. [26]

In addition to the above main anthropogenic sources, soil erosion and discharge from wastewater treatment facilities (such as effluent from urban wastewater treatment plants) are also pathways for total phosphorus to enter water bodies. The flood season from June to September each year is the main period for total phosphorus pollutants to enter rivers and lakes in the Yangtze River basin. [16][18][26] Sudden environmental events such as phosphogypsum yard leakage can also cause serious total phosphorus pollution. [41]

In addition, actual engineering applications also need to consider multiple factors.

Total phosphorus is a key limiting factor for water eutrophication. According to China’s "Environmental Quality Standards for Surface Water" (GB 3838-2002), when the total phosphorus concentration in lakes and reservoirs exceeds 0.1 mg/L (corresponding to Class V water), there is a eutrophication risk; if it continues to be higher than 0.2 mg/L, it indicates that the water body is in a severe eutrophic state and is very likely to break out cyanobacterial blooms. [6][21]

In addition, from the perspective of industrial development, market demand is also driving technological progress.

Total phosphorus pollution has become a prominent environmental problem in the Yangtze River basin and is the primary pollutant in the basin. [15-16] The proportion of sections using total phosphorus as the water quality classification factor reaches about 51%; in 2022, the proportion of sections where total phosphorus was the primary exceeding-standard factor reached 57.3%. [16][18]

The distribution of total phosphorus concentration in the Yangtze River basin generally shows the characteristics of higher in the upper and lower reaches and slightly lower in the middle reaches. [16] The total phosphorus exceeding-standard areas are mainly distributed in provinces such as Yunnan, Guizhou, Sichuan, and Hubei in the middle and upper reaches of the Yangtze River. [15]

Total phosphorus pollution mainly comes from non-point source pollution, "three phosphorus" and other industrial enterprise discharges, wastewater treatment facility discharges, and soil erosion. Agricultural non-point source pollution total phosphorus discharge accounts for about 60% of the basin’s total discharge; the sum of agricultural and domestic source total phosphorus discharge accounts for as high as 96.92%. [16][18][26] Industrial enterprise discharges such as "three phosphorus" are an important reason for exacerbating regional phosphorus pollution and have a direct impact on local water bodies. [15-16]

At the same time, the relevant supporting processes and equipment are also continuously optimized and upgraded.

According to China’s "Environmental Quality Standards for Surface Water" (GB 3838-2002), the total phosphorus limit for lakes and reservoirs is stricter than that for rivers. Taking the Class III standard limit as an example, it is 0.2 mg/L for rivers and 0.05 mg/L for lakes and reservoirs. [35] When the total phosphorus concentration of lakes and reservoirs exceeds 0.1 mg/L (corresponding to Class V water), there is a eutrophication risk; when it continues to be higher than 0.2 mg/L, it usually indicates that the water body is in a severe eutrophic state. [6] In addition, the reference range of total phosphorus background concentrations in different natural water bodies also varies: rivers and lakes are usually between 0.02–0.1 mg/L, offshore and inland seas are between 0.01–0.05 mg/L, and deep seas are between 0.001–0.005 mg/L. [23]

Total phosphorus discharge from various sewage outlets is strictly limited. [3-4] In industrial wastewater discharge standards, the permitted discharge concentration limit for total phosphorus (as P) from major discharge outlets is commonly 2–3 mg/L. [25][33] For aquaculture tailwater, Jiangxi Province’s "Aquaculture Tailwater Discharge Standard" (DB 36/1993-2004) stipulates: for discharge into Class I, II, III waters, the first-class standard applies, with total phosphorus ≤0.4 mg/L; for discharge into Class IV, V waters, the second-class standard applies, with total phosphorus ≤0.8 mg/L. [34] Some places have formulated stricter control schemes for key basins, such as the "Total Phosphorus Pollution Control Scheme for the Yangtze River Basin (Yunnan Section)," which requires that by 2025, the total phosphorus concentration at the point where the main stream of the Yangtze River (Jinsha River) and its important tributaries enter the main stream be controlled below 0.1 mg/L. [19]

Total phosphorus pollution sources are extensive. [1] According to the census, at the basin scale, agricultural and domestic sources are the main contributors to total phosphorus, and their discharge can account for more than 96% of the basin’s total discharge. Industrial sources (especially phosphate mining, phosphorus chemical, and other "three phosphorus" enterprises) account for a small proportion of discharge but have a direct impact on local water bodies. [16][26] Excessive total phosphorus concentration will lead to water eutrophication, massive algae reproduction consuming dissolved oxygen in the water, causing the death of fish and other aquatic organisms and destroying the aquatic ecosystem. [21] Some organophosphorus pesticides are highly toxic and can inhibit nervous system function. [13]

The "Lake Nutrient Criteria" is a reference value for nutrient concentrations based on scientific laws, used to evaluate lake eutrophication, and has no legal binding force, but can provide a scientific basis for the formulation and revision of water quality standards. [7] In terms of monitoring technology, the Ministry of Ecology and Environment released the "Technical Requirements and Test Methods for Water Quality Total Phosphorus Automatic Monitor" (HJ 103—2026) to standardize the performance of monitoring equipment and ensure data accuracy. [28]

In addition, this technology also has wide application and practice in related fields.

Total phosphorus (TP) is a key indicator for evaluating water eutrophication degree and water quality, and its detection is of great significance for environmental monitoring, wastewater treatment, and industrial process control. [6][10] China’s mainstream national standard methods include the "Water Quality—Determination of Total Phosphorus—Ammonium Molybdate Spectrophotometric Method" (GB/T 11893-1989) and the "Water Quality—Determination of Total Phosphorus—Flow Injection-Ammonium Molybdate Spectrophotometric Method" (HJ 671-2013). [27][31]

The core principle of the above methods and national standard methods (such as the ammonium molybdate spectrophotometric method) is: after the sample is digested, various forms of phosphorus are converted to orthophosphate, and under acidic conditions, orthophosphate reacts with ammonium molybdate to generate phosphomolybdic heteropoly acid, which is reduced by a reducing agent such as ascorbic acid to generate a blue phosphomolybdenum blue complex; its color depth is proportional to the phosphorus content and can be measured by spectrophotometry at a wavelength of 700 nm. [6][27][29]

The key step in accurately determining total phosphorus is sample pretreatment, that is, converting organic phosphorus and insoluble inorganic phosphorus into measurable orthophosphate through digestion. [20] Common digestion methods include: dry digestion (high-temperature ashing), wet digestion (such as nitric acid-perchloric acid, nitric acid-sulfuric acid systems), high-pressure steam digestion (using potassium persulfate), and microwave digestion. [12][27] Microwave digestion is widely used in treating complex solid samples such as soil and sediment due to its high efficiency, low reagent consumption, and low blank value. [20]

In addition to spectrophotometry, with technological progress, a variety of total phosphorus detection technologies have emerged: continuous flow analyzers or flow injection analyzers can achieve automated, high-throughput detection; [10-11][20] inductively coupled plasma optical emission/mass spectrometry (ICP-OES/MS) is suitable for high-content or simultaneous multi-element analysis and has strong anti-interference ability; [12][20] detection methods based on chemiluminescence or biosensing technology are also under research, with characteristics such as high sensitivity; [10] in addition, online automatic monitors integrating intelligent algorithms can achieve real-time monitoring and early warning. [32]

Total phosphorus determination may be interfered with by various substances. Arsenic (As), chromium (Cr), sulfide, etc. will interfere with the color reaction. [20][24] Usually, arsenic can be masked with sodium thiosulfate, sulfide can be removed by passing nitrogen, and chromium can be reduced with sodium sulfite. In addition, the turbidity and color of the water sample will also affect the absorbance measurement, which can be corrected by preparing a turbidity-color compensation solution. [29] Suspended solids in the water sample should be treated by natural settling, centrifugation, or filtration before digestion. [20][30]

This standard specifies the method for determining total phosphorus by ammonium molybdate spectrophotometry after digesting unfiltered water samples using potassium persulfate (or nitric acid—perchloric acid) as the oxidant. Total phosphorus includes dissolved, particulate, organic, and inorganic phosphorus. This standard applies to surface water, sewage, and industrial wastewater. For 25 mL of test sample, the minimum detectable concentration of this standard is 0.01 mg/L, and the upper limit of determination is 0.6 mg/L. Under acidic conditions, arsenic, chromium, and sulfur interfere with the determination.

At the same time, the relevant supporting processes and equipment are also continuously optimized and upgraded.

(1) Potassium persulfate digestion: Add 4 mL of potassium persulfate to the sample, tightly cover the stoppered graduated tube, and tie the glass stopper tightly with a small piece of cloth and thread (or fix by other methods), place it in a large beaker, and heat it in a high-pressure steam disinfector. When the pressure reaches 1.1 kg/cm², the corresponding temperature is 120°C, maintain for 30 min, then stop heating. After the pressure gauge reading drops to zero, take it out and let it cool. Then dilute with water to the mark.

In addition, actual engineering applications also need to consider multiple factors.

② Arsenic greater than 2 mg/L interferes with the determination and is removed with sodium thiosulfate. Sulfide greater than 2 mg/L interferes with the determination and is removed by passing nitrogen. Chromium greater than 50 mg/L interferes with the determination and is removed with sodium sulfite.

In addition, actual engineering applications also need to consider multiple factors.

Take 7 stoppered graduated tubes and add 0.0, 0.50, 1.00, 3.00, 5.00, 10.0, 15.0 mL of phosphate standard solution respectively. Add water to 50 mL. Then process according to the determination steps. Using water as the reference, measure the absorbance. After deducting the absorbance of the blank test, draw the working curve with the corresponding phosphorus content.

At the national level, Article 46 of the "Yangtze River Protection Law of the People’s Republic of China" stipulates that provincial people’s governments in the Yangtze River basin shall formulate total phosphorus pollution control plans for their administrative regions and organize their implementation. [15] The Ministry of Ecology and Environment and 16 other departments jointly issued the "Action Plan for In-depth Fighting of the Yangtze River Protection and Restoration Battle," requiring strengthening of comprehensive phosphorus pollution control. [16][18] At the local level, the "Regulations on Total Phosphorus Pollution Prevention and Control of Poyang Lake Basin in Jiangxi Province" came into effect on January 1, 2024; [36] the "Total Phosphorus Pollution Control Plan for the Yangtze River Basin (Yunnan Section)" was issued in October 2023, with the goal of achieving obvious results in total phosphorus pollution control by 2025. [19][37]

In terms of technical standards, the "Environmental Quality Standards for Surface Water" (GB 3838-2002) sets total phosphorus standards for lakes and reservoirs stricter than those for rivers; [35] the "Integrated Wastewater Discharge Standard" (GB 8978-1996) involves total phosphorus discharge requirements; [15] the "Technical Requirements and Test Methods for Water Quality Total Phosphorus Automatic Monitor" (HJ 103—2026) was released in 2026, replacing HJ/T 103—2003, to standardize automatic monitoring equipment performance; [28] the Jiangxi local standard "Aquaculture Tailwater Discharge Standard" (DB 36/1993-2004) was fully implemented from February 1, 2026, stipulating total phosphorus discharge limits. [34] In addition, the "Lake Nutrient Criteria—Central and Eastern Lake Region (Total Phosphorus, Total Nitrogen, Chlorophyll a)" (2020 edition) released by the Ministry of Ecology and Environment provides a scientific basis for the formulation and revision of total phosphorus standards. [7]

To address total phosphorus pollution, the Ministry of Ecology and Environment and 16 other departments jointly issued the "Action Plan for In-depth Fighting of the Yangtze River Protection and Restoration Battle," requiring strengthening of comprehensive phosphorus pollution control and deepening the investigation and rectification of "three phosphorus" in the Yangtze River. The "Yangtze River Protection Law" stipulates that provincial people’s governments in the Yangtze River basin shall formulate and organize the implementation of total phosphorus pollution control plans for their administrative regions, and the Ministry of Ecology and Environment also issued the "Implementation Plan for the Special Investigation and Rectification Action of ‘Three Phosphorus’ in the Yangtze River" in 2019. [15-16]

Hunan Province implemented the "Dongting Lake Total Phosphorus Pollution Control and Reduction Action Plan (2022—2025)." In 2021, the total phosphorus concentration of Dongting Lake dropped to 0.063 mg/L, a decrease of 43.8% compared with 2015. [38]

In June 2023, a leak occurred at the main drainage channel connected to the pumping canal of the Fortune Cave of Wengfu Chemical Company in Fuquan City, Guizhou Province, causing the total phosphorus concentration in the Chonghuan River to exceed the standard by up to 45 times, with a total phosphorus leakage of about 45.53 tons, causing total phosphorus exceeding standards in about 237 km of downstream river channels. [39]

For the "three phosphorus" pollution problem in the Yangshui River basin in Guiyang City, a "5-step method" of "grasping demand—problem analysis—task decomposition—integrating results—landing application" was used for comprehensive treatment. After treatment, the total phosphorus concentration at the outflow section dropped from an average of 0.35 mg/L in 2018 to below 0.2 mg/L, and the water quality stably reached Class III standards. [17-18]

The latest research and whole-lake experiments show that eutrophication control should focus on phosphorus control; nitrogen control has limited effect on reducing total algae and may even induce nitrogen-fixing cyanobacterial blooms. In a multi-year comparative study of more than 40 lakes in the Yangtze River basin, it was found that regardless of the total nitrogen concentration, the total algae amount is determined by total phosphorus rather than total nitrogen. This view is confirmed by long-term whole-lake experiments in North America. [40]