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Determining the organic load in wastewater is essential for designing effective treatment systems. The two primary parameters used are Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). Accurate measurement of these values helps in selecting appropriate treatment processes and sizing equipment.
Understanding BOD and COD
BOD measures the amount of oxygen consumed by microorganisms to decompose organic matter over a specified period, usually five days at 20°C. It indicates the biodegradable portion of organic pollutants. COD, on the other hand, quantifies the total amount of oxygen required to chemically oxidize both biodegradable and non-biodegradable organic substances in the wastewater.
Methods for Determining Organic Load
Sampling wastewater and conducting laboratory tests are standard procedures. BOD testing involves incubating a sample for five days and measuring oxygen consumption. COD testing uses a strong chemical oxidant, such as potassium dichromate, to oxidize organic compounds, with the oxygen demand calculated from the amount of oxidant consumed.
Calculating Organic Load for Design
The organic load is expressed in terms of mass per unit volume, typically milligrams per liter (mg/L). To determine the total load, multiply the concentration by the flow rate of wastewater. For example, if the BOD is 200 mg/L and the flow rate is 1000 m³/day, the daily BOD load is 200,000 grams or 200 kg.
- Collect representative wastewater samples.
- Perform BOD and COD laboratory tests.
- Calculate the organic load based on flow rates.
- Use the load data to select treatment processes.