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Types of Aqueous Emissions & Standard Test Methods for Water…
Types of Aqueous Emissions & Standard Test Methods for Water Contamination (Wan Mohamad Syameer bin Wan Suhaimi, 25247)
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Chemical Oxygen Demand
- In the chemical oxidation demand (COD) test, oxidation with acidic potassium dichromate is used.
- A catalyst (silver sulfate) is required to assist oxidation of certain classes of organic compounds.
- COD results are generally higher than BOD5, since the COD test oxidizes materials that are only slowly biodegradable.
- Only certain organic compounds are oxidized only
slowly, or not at all.
- The ratio of BOD5 to COD varies according to the contamination (0.05 to 0.8)
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Total Oxygen Demand
- measures the oxygen consumed when a sample of wastewater is oxidized in a stream of air at high temperature (up to 1200°C) in a furnace.
- Under these harsh conditions, all the carbon is oxidized to CO2.
- The oxygen demand is determined from the change in oxygen in the carrier gas.
- The resulting value of TOD embraces oxygen required to oxidize both organic and inorganic substances present.
- The relationship between BOD5, COD and TOD for the same organic waste is in the order,
BOD5<COD<TOD
Total Organic Carbon
- The total organic carbon (TOC) test measures the carbon dioxide produced when a sample of waste water is subjected to a strongly oxidizing environment.
- One option is to oxidize the sample in a stream of air at a high temperature (around 1000°C) in a furnace,
- similar to the TOD test, but measuring the change in CO2 rather than the change in O2.
- Rather than using high temperature in a furnace, other strongly oxidizing environments can also be used.
- For example, UV light with sodium persulfate as a digesting reagent can be used.
- To obtain the TOC requires that inorganic carbon compounds be removed prior to test, or results corrected for their presence.
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