Chemical Oxygen Demand (COD)

Background

Chemical Oxygen Demand (COD) is a test that measures the amount of oxygen required to chemically oxidize the organic material and inorganic nutrients, such as Ammonia or Nitrate, present in water. The earliest methods for quantification of COD were developed ~150 years ago and involved recording colour changes of a permanganate solution mixed when mixed with a water samples. There was, however, significant variability between samples using this compound. The use of the dichromate procedure was pioneered and perfected for wastewater in 1949. COD is measured via a laboratory assay in which a sample is incubated with a strong chemical oxidant for a specified time interval and at constant temperature (usually 2 h at 150°C). The most commonly used oxidant is potassium dichromate, which is used in combination with boiling sulphuric acid. It is important to note that the chemical oxidant is not specific to organic or inorganic compounds, hence both these sources of oxygen demand are measured in a COD assay. Furthermore, it does not measure the oxygen-consuming potential associated with certain dissolved organic compounds such as acetate. Thus, measurements are not directly comparable to Biochemical Oxygen Demand (BOD) but can be used to compliment (though is sometimes used as surrogate measure).

Why is it Important?

COD is an important water quality parameter and is used in a wide range of applications, including:

• to confirm wastewater discharge and the waste treatment procedure meets criteria set by regulators (see Table 2);
• to quantify the biodegradable fraction of wastewater effluent - ratio between BOD and COD;
• COD or BOD measurements are also used as an indicator of the size of a wastewater treatment plant required for a specific location.

Challenges associated with COD monitoring

Despite the test being entrenched in legislation there are numerous problems and challenges associated with use of the test:

• There is a lag until results are available (transportation to lab + 2h for test), hence environmental damage can occur before the data is available;
• the test is time consuming and expensive;
• the test involves dangerous chemicals that need careful disposal and are potentially harmful to operators;
• it fails to recreate natural processes (i.e. the test involves an artificial incubation with a strong oxidising agent);
• it is imprecise and has a high minimum detection limit thus is not applicable to clean/uncontaminated river samples;

It is clear that a move from traditional laboratory testing to in-situ (real-time) monitoring would help to alleviate some of the problems outlined above. It would immediately address points I - III and would help to improve spatial temporal resolution of monitoring that would be directly beneficial to basin managers, water companies and legislators alike.

Proteus the real-time solution for COD monitoring

The Proteus is a new product launched by Proteus Instruments providing users with a robust, repeatable, low maintenance sensor platform for measuring COD in real-time. The The Proteus is underpinned by comprehensive research exploring the use of in-situ fluorescence as a technique for real-time COD measurement. The Proteus (See Fig. 1) is a multi-parameter instrument that can incorporate a range of optical sensors. For COD measurement the standard configuration includes a tryptophan-like fluorescence (TLF) sensor, turbidity sensor and thermistor and can provide real-time measurement of reactive dissolved organic matter found in sewage and slurry, negating the need for COD laboratory analysis. Using a robust correction algorithm the tryptophan signal is corrected, in real time, for temperature interference. The result is a repeatable measurement that can provide instantaneous COD measurement with a simple site specific calibration for turbidity and TLF relationship with COD.

The Science...

Fluorescence spectroscopy is a selective and sensitive optical technique enabling in-situ, real-time measurement of dissolved organic matter. Molecules absorb light of a specific wavelength and orbiting electrons are excited to a higher energy state .The electrons then emit light of a specific wavelength to return to the base state.

The dissolved organic matter pool can be mapped in optical spaced based on its fluorescent properties (see Fig. 2). The TLF peak (red) represents a mixture of free amino acids, peptides and proteins. This is associated with microbial activity and human/animal waste contamination. Numerous published studies have correlated TLF with COD and our site specific calibrations can provide users with accurate and highly repeatable measurements (see Fig. 3).

Applications

• Monitoring for compliance
• Optimization of wastewater treatment processes
• Development of process control algorithms
• Identification of cross-connected sewers
• Identification of pollution sources
• Rapid assessment of incident severity
• Advanced treatment monitoring & protection
• Effluent pollution