Elevated Oil Content Readings in Industrial Wastewater: A Technical Analysis of China's HJ637-2018 Standard

In analytical laboratories, the seemingly high data on oily wastewater conceal a technical reality that the industry must confront.

In routine environmental monitoring, many industrial enterprises have discovered a perplexing phenomenon: the test results for oily wastewater are often higher than expected. This is not only a technical problem but also involves environmental compliance and discharge costs.

In fact, this phenomenon is closely related to my country's environmental protection standard HJ637-2018, "Water Quality - Determination of Petroleum and Animal and Vegetable Oils - Infrared Spectrophotometric Method," which was officially implemented on January 1, 2019. This standard replaced the original HJ 637-2012, and while promoting international compliance, it also brought new technical considerations to oily wastewater testing.

I. Standard Changes and the Replacement of Extractants

The implementation of the HJ637-2018 standard was not accidental, but a concrete manifestation of my country's commitment to international environmental responsibilities. The extractant carbon tetrachloride used in the original standard HJ 637-2012 is a Class II controlled substance under Annex B of the Montreal Protocol on Substances that Deplete the Ozone Layer.

As a responsible major country, my country pledged to cease the use of carbon tetrachloride for laboratory purposes from January 1, 2019. Therefore, the relevant standards were revised.

The biggest change in the revised HJ637-2018 standard is the use of tetrachloroethylene as an alternative extractant.

However, while this change is beneficial to global ozone layer protection, it has brought a series of technical challenges in actual testing, becoming one of the important reasons for the higher-than-expected indicators in industrial oily wastewater testing.

II. Multiple Factors Contributing to High Industrial Wastewater Test Results

2.1 Instability of the Extractant

Tetrachloroethylene differs significantly from the previously used carbon tetrachloride in its chemical properties. Studies have shown that  Tetrachloroethylene (PCE) exhibits significantly lower chemical stability than carbon tetrachloride. It is prone to decomposition, particularly at elevated temperatures. This instability accelerates in warmer months, necessitating strict refrigerated and light-protected storage. Crucially, opened PCE can degrade within days, even hours, directly leading to inflated measurement results.

2.2 Increased Method Detection Limit

The method detection limit of the HJ637-2018 standard is 0.06 mg/L, and the lower limit of determination is 0.24 mg/L (when the sample volume is 500 ml, the extraction liquid volume is 50 ml, and a 4 cm quartz cuvette is used).

Compared with the original standard, the revised method detection limit has actually increased, which means it renders it unsuitable for monitoring Class I-III surface water (per GB 3838-2012), where the regulatory limit (0.05 mg/L) is below the method's detection capability.

Due to this technical limitation, the HJ637-2018 standard explicitly specifies its scope of application: it is mainly applicable to the detection of industrial wastewater and domestic sewage, but not to the monitoring of clean water samples such as Class I-III surface water.

2.3 Differences in Measurement Process and Calculation

The measurement process of the HJ637-2018 standard includes sample transfer, extraction, and colorimetric analysis. The standard first measures total oil, then removes animal and vegetable oils by adsorption, then measures petroleum oils, and finally calculates animal and vegetable oils based on the difference between the two.

This process yields results for three indicators: total oil, petroleum oil, and animal and vegetable oil.

Due to the instability of tetrachloroethylene and the separate measurement of the extraction liquid, sometimes the petroleum oil concentration may be slightly higher than the total oil concentration. For example, it can lead to logically inconsistent results, such as a measured petroleum hydrocarbon concentration (e.g., 1.82 mg/L) exceeding the total oil concentration (e.g., 1.80 mg/L).

2.4 Complex Composition of Industrial Wastewater

Industrial wastewater comes from a wide range of sources, including chemical, pharmaceutical, printing and dyeing, and electroplating industries. These wastewaters usually contain high concentrations of organic matter, inorganic salts, heavy metals, and other toxic and harmful substances.

The complex matrix may contain substances that respond to all three characteristic absorption peaks at 2930 cm⁻¹, 2960 cm⁻¹, and 3030 cm⁻¹, leading to higher results during infrared spectrophotometry.

In contrast, HJ 970-2018 "Water Quality - Determination of Petroleum Hydrocarbons - Ultraviolet Spectrophotometric Method" can only determine olefin compounds containing conjugated double bonds, and cannot detect hydrocarbons that do not respond at 225 nm.

III. Technical Limitations Behind the High Results

3.1 Limitations of Standard Substances

The standard substances used in the HJ637-2018 standard are a mixture of n-hexadecane, isooctane, and benzene in different proportions.

Colorimetric measurement requires measurement at three wavenumbers, instead of just one wavelength as in ultraviolet spectrophotometry.

This method design allows infrared spectrophotometry to reflect a wider range of oil substances, but the method suffers from suboptimal recovery rates for animal and vegetable oils, and the measured values may not accurately reflect their true concentration in complex matrices like greasy wastewater.

3.2 Error Factors of Fully Automatic Oil Analyzers

To improve operational efficiency, the HJ637-2018 standard allows the use of automatic extraction devices instead of manual extraction.

Although domestic fully automatic oil analyzers have advantages such as reducing the health hazards of extractants to testing personnel and avoiding losses during the transfer process, there is still room for improvement and optimization in aspects such as separate preparation of standard curves and magnesium silicate column cleaning.

IV. Practical Impact of High Results

4.1 Data Comparability Issues

Due to changes in detection methods and extractants, the comparability between historical data and current data has been affected to some extent. Petroleum substances are important indicators for monitoring and evaluating the degree of pollution in natural water bodies and monitoring sewage and wastewater discharge.

When oil enters water bodies, it causes serious environmental pollution, and the aromatic hydrocarbons among them have "three effects" (teratogenic, carcinogenic, and mutagenic), which can threaten human health through the amplification effect of the food chain.

4.2 Increased Compliance Costs for Enterprises

For industrial enterprises, higher test results may mean higher pollution control costs and stricter emission requirements.

The treatment and discharge of industrial wastewater must strictly comply with relevant national laws, regulations, and emission standards. In the pollutant emission standards for industries such as petroleum, chemical, electroplating, coal, smelting, rubber, ceramics, automobiles, and ammunition, strict limits are stipulated for the emission of petroleum-based substances.

V. Response and Solutions

5.1 Standardized Reporting of Results

Facing potentially anomalous results, experts recommend standardized reporting: If the petroleum hydrocarbon concentration equals the oil concentration (e.g., oil concentration is 1.80 mg/L, and petroleum hydrocarbon concentration is also 1.80 mg/L), the animal and vegetable oil concentration should be reported as "<detection limit" (i.e., animal and vegetable oil < 0.06 mg/L).

This recommendation aims to address the rationality issues caused by the calculation method.

5.2 Method Selection and Complementarity

Different detection methods can be selected according to actual monitoring needs. HJ637-2018 (infrared method) is suitable for industrial wastewater and domestic sewage, while HJ970-2018 (ultraviolet method) is more suitable for surface water, groundwater, and seawater.

Both methods have their own characteristics: the ultraviolet method has high sensitivity, high equipment availability, and is easy to operate; the infrared method has high sensitivity and accurate qualitative and quantitative analysis, but the method detection limit is higher.

5.3 Quality Control Measures

Strengthening laboratory quality control is also an important means of reducing deviations. This includes regularly verifying the quality of the extraction agent – the quality of commercially available tetrachloroethylene varies considerably among different manufacturers and batches, so reagent acceptance testing should be performed upon arrival.

At the same time, storage conditions should be controlled; tetrachloroethylene needs to be stored in a cool, dark place, and its quality should be reconfirmed before use.

Companies do not need to be overly concerned about deviations in test results, but should actively understand the principles and limitations of standard methods, and ensure that monitoring data meets environmental protection requirements and accurately reflects pollution conditions through standardized operations, selecting appropriate detection methods, and strengthening laboratory quality control.

Accurately detecting the oil content in wastewater is not only a prerequisite for compliant emissions by enterprises, but also a social responsibility to protect the ecological environment and safeguard human health.