DO, BOD & COD

 Dissolved Oxygen

Dissolved oxygen is the oxygen present in a water body.  Fish and other aquatic animals depend on dissolved oxyge to live. The amount of dissolved oxygen  is dependent on the water temperature, the quantity of sediment , the amount of oxygen taken out of the system by respiring and decaying organisms, and the amount of oxygen put back into the system by photosynthesizing plants, stream flow, and aeration. Dissolved oxygen is measured in milligrams per liter (mg/l) or parts per million (ppm). The temperature of  water influences the amount of dissolved oxygen present; less oxygen dissolves in warm water than cold water. For this reason, there is cause for concern for streams with warm water. Most warm water fish need DO in excess of 2 mg/l.

The concentration of dissolved oxygen can be measured by the Winkler's method. Dissolved oxygen can also be determined using oxygen sensitive electrodes. However, electrodes are less reliable when oxygen concentrations are very low. For these reasons, the Winkler titration is often employed for accurate determination of oxygen concentrations in aqueous samples. 

Biochemical Oxygen Demand (BOD) 

Natural organic detritus and organic waste from waste water treatment plants, failing septic systems, and agricultural and urban runoff, acts as a food source for water-borne bacteria. Bacteria decompose these organic materials using dissolved oxygen, thus reducing the DO present for fish. Biochemical oxygen demand (BOD) is a measure of the amount of oxygen that bacteria will consume while decomposing organic matter under aerobic conditions. 

    BOD test

Biochemical oxygen demand is determined by incubating a sealed sample of water for five days and measuring the loss of oxygen from the beginning to the end of the test. Samples often must be diluted prior to incubation or the bacteria will deplete all of the oxygen in the bottle before the test is complete. It is primarily carbonaceous oxygen demand that is being measured (CBOD) 

There are two commonly recognized methods for the measurement of BOD.

1) Dilution method- 5-Day Biochemical Oxygen Demand-BOD₅ test 

The BOD₅ test is widely used to measure the pollutional characteristics of wastewater.  It is accepted as one of the primary measures of wastewater pollution. 

The BOD₅ test employs 300-ml glass bottles with glass stoppers. A series of 3 to 5 BOD bottles is used for each sample. The sample is diluted with with oxygen saturated dilution water, and if needed, inoculated with a fixed aliquot of microbial seed, the dissolved oxygen (DO) measured and then the sample sealed to prevent further oxygen dissolving in. The sample is kept at 20 °C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD.

BOD can be calculated by:

Undiluted sample: Initial DO - Final DO = BOD

Diluted sample: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor

Municipal wastewater samples normally have very high BOD₅ values making it  necessary to appropriately dilute the wastewater samples. The dilution water is prepared from distilled water or demineralized water.  A phosphate buffer can be added to the dilution water to keep the pH around 7.2 for good microbial growth. Ammonium chloride, magnesium sulphate, calcium chloride and ferric chloride are added to ensure sufficient nitrogen and key trace metals for  bacterial growth. Microbial seed containing a mixture of bacteria and protozoa which are acclimated to the wastewaters being tested can be added to ensure uniform reaction conditions. Municipal wastewaters normally do not need additional microbial seed. 

With excess ammonia nitrogen in municipal wastewaters, nitrification can occur in the BOD bottles under the right circumstances. To ensure that the BOD₅ data show only carbonaceous BOD (cBOD), add nitrification inhibitor eg: 2-chloro-6-trichloromethyl pyridine. The inhibitor prevents the oxidation of ammonia nitrogen.

 The average BOD₅ for domestic sewage: 100-300 mg/l. Higher in industrial sewage: 300-30,000 mg/l and should be around 20-50mg/l after biological treatment. 

Primary sedimentation removes about 30% of the BOD5; and biological treatment removes 85% to 95% of the raw wastewater BOD5.

 

Test Limitations

The test has many variables, limiting reproducibility.

Some wastes contain chemicals capable of suppressing microbiological growth or activity. Potential sources include industrial wastes, antibiotics in pharmaceutical or medical wastes, chlorination, disinfection used following conventional sewage treatment etc..Suppression of the microbial community oxidizing the waste will lower the test result.

2) Manometric method

This method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation. Ammonia oxidation is inhibited. 

The sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide (typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. 

Oxygen is consumed and carbon dioxide is released. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From the drop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen.

The main advantages of this method compared to the dilution method are:

·         simplicity: no dilution of sample required, no seeding, no blank sample.

·         direct reading of BOD value.

·         continuous display of BOD value during incubation

 Chemical oxygen demand (COD) 

It is a measure of the total quantity of oxygen required to chemically oxidize all organic material into carbon dioxide and water. It does not differentiate between biodegradable available and non-biodegradable organic matter, and. 

COD values are always greater than BOD values, but COD measurements can be made in a few hours while BOD measurements take five days. 

The COD test uses potassium dichromate with concentrated sulfuric acid, to oxidize organic matter to carbon dioxide, water, and ammonium sulfate. The COD results measure the total carbonaceous oxygen demand. The COD test uses a 2-hour boiling with silver sulfate and mercuric sulfate as catalysts for the oxidation reactions. The silver sulfate catalyst is essential for complete oxidation of acetate and long chain aliphatic compounds. The mercuric sulfate prevents the chemical oxidation of chlorides that occur in domestic wastewater.  

COD of domestic sewage: 300-500 mg/l and is higher in industrial sewage: 600-60,000 mg/l. COD After biological treatment- 50-200mg/l. COD values higher than BOD and comparatively very high values indicate large amounts of non-biodegradable organics. 

Unlike the BOD5 test, the COD test yields results in about 3 hours. 

The COD results measure both the biological oxygen demand and the non-biodegradable oxygen demand of the wastes.

 DO sag curve

 The drop and rise in DO levels downstream from a source of BOD is called the DO sag curve.   If effluent with high BOD levels is discharged into a stream or river, it will accelerate bacterial growth in the river and consume the oxygen levels in the river. The oxygen may diminish to levels that are lethal for most fish and many aquatic insects. As the river re-aerates due to atmospheric mixing and as algal photosynthesis adds oxygen to the water, the oxygen levels will slowly increase downstream.