Methods to Test Wastewater and Drinking Water for Organic Pollutants
There are several methods to test wastewater and drinking water for organic pollutants to assess water quality, including Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic Carbon (TOC). BOD and COD differ from TOC as both measure the amount of oxygen that is depleted by organic species in wastewater, while TOC is a measure of all carbon (organic and inorganic) in drinking water. Specific to COD, it measures the reactive fraction of the TOC, also known as oxidizability in the European Union.
Advantages and Disadvantages
Biochemical Oxygen Demand (BOD)
The BOD method complies with APHA and ISO standard methods and is widely recognized in water testing.
However, BOD requires a 5 or 7-day incubation period making it not ideal for rapid results or effluent control. Results are often inconsistent due to precision errors and reproducibility, and microbial activity can be disrupted by interferences in the sample. It is also worth noting that BOD is highly matrix dependant, which can inhibit microbial activity.
Chemical Oxygen Demand (COD)
COD by dichromate (CODcr) also complies with APHA and ISO standard methods and is valued for its faster process compared to BOD. It offers a useful measure of oxidizable organics across many types of water samples.
Despite these advantages, COD suffers from several drawbacks. This method requires 2–4-hour digestion and cooling time while relying on harmful chemicals that are toxic and environmentally hazardous. Dichromate is a known carcinogen, reproductive toxin and mutagen that can cause serious health issues. COD is not applicable to natural or drinking waters because of high detection limits. COD does not oxidize all organic species, nor does it differentiate between inorganic and organic carbon. It is prone to interferences from halides, peroxide, and nitrates. Vial explosions during digestion and cooling impose a serious safety risk.
Total Organic Carbon (TOC)
TOC is rapid, less than 10 minutes for analysis, and is not subject to interferences. This method also complies with APHA and Standard Methods.
TOC does not measure oxidizability, only total carbon. It fails to distinguish between reactive and non-reactive fractions which means it does not quantify overall reactivity of the organics which form the disinfection by-products. Results may vary depending on whether samples are measured warm or at room temperature. Furthermore, this method struggles to measure change even when reactivity of organics increases, while carbon loading remains at the same concentration. The TOC method incurs a high capital cost, cost per sample, and produces a complicated analysis, which makes it less accessible for regular monitoring. Its scale of response is smaller than COD, limiting its usefulness for certain applications.
PeCOD
PeCOD (Photoelectrical Chemical Oxygen Demand) offers a modern alternative that addresses the many limitations and drawbacks of traditional methods. Using titanium dioxide as a photocatalyst due to its oxidation potential, it fully oxidizes a wide range of organic species without the need for harmful chemicals like mercury or dichromate. It conforms to ASTM International Method D8084 and can detect COD in the range of 0.7 ppm to 15,000 ppm.
Compared to BOD and CODcr tests, PeCOD’s primary benefit is its speed; producing an analysis for wastewaters within 10 minutes, and 3-5 minutes for natural and drinking water. Furthermore, it eliminates the use of hazardous chemicals associated with CODcr tests, which greatly reduces safety risks. PeCOD also shows a strong correlation to BOD5 and aligns with standard COD method for many matrices.
While TOC only measures total carbon, PeCOD is the first analyzer to directly measure the reactive fractions of TOC empirically. This makes PeCOD data more actionable for process control in wastewater treatment.
PeCOD can be shipped directly, set up, and used within 60 minutes.
To view this article as a PDF, please click here.
Author:
Laura Martin, Quality Control and Research Chemist
