Need as soon as possible Define biochemical oxygen demand (BOD) and explain how
ID: 1073755 • Letter: N
Question
Need as soon as possible Define biochemical oxygen demand (BOD) and explain how this is different than chemical oxygen demand (COD) Why perform the five-day BOD test and what useful information does it give you? Why do we apply a temperature correction factor when calculating the BOD rate constant? List and briefly describe the three steps in wastewaster treatment: Draw out the cross section of a typical landfill liner system List five (5) major air pollutants (+1 additional bonus point for each additional)Explanation / Answer
1 BOD :
It is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.
Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water. BOD also measures the chemical oxidation of inorganic matter i.e., the extraction of oxygen from water via chemical reaction.
COD :
chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution. Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers) or wastewater, making COD a useful measure of water quality.
2 The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C and is often used as a surrogate of the degree of organic pollution of water.Incubation periods of 1, 2, 5, 10 and 20 days were being used into the mid-20th century. Keeping dissolved oxygen available at their chosen temperature, investigators found up to 99 percent of total BOD was exerted within 20 days, 90 percent within 10 days, and approximately 68 percent within 5 days.
Pristine rivers will have a 5-day carbonaceous BOD below 1 mg/L. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/L. Rivers may be considered severely polluted when BOD values exceed 8 mg/L. Municipal sewage that is efficiently treated by a three-stage process would have a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L in Europe and as low as 200 mg/L in the U.S.
3 The assumption of a first order kinetic can be made as ;
( dL / dt ) = ( - K 1 ) ( L )
and ;
L = ( L O ) [ 10 ( - k 1 ) ( t ) ]
where L and L O are the carbonaceous BOD remaining at time t and the ultimate carbonaceous BOD, respectively, and K 1 and k 1 are the reaction rate constants to base " e " and base " 10 ", respectively. The change in BOD with temperature is two fold :
( a ) k 1 changes with temperature as ;
k T = ( k 20 ) ( THETA T - 20 )
where THETA is the temperature correction factor ( usually = 1.047 ).
( b ) L O also changes ;
T L O = ( 20 L O ) [ 1 + ( 0.02 ) ( T - 20 ) ]
The combination of the two effects of temperature changes can cause considerable changes in the total load to receiving waters.
4 STAGE ONE: SCREENING
Screening is the first stage of the wastewater treatment process. Screening removes large objects like, diapers, nappies, sanitary items, cotton buds, face wipes and even broken bottles, bottle tops, plastics and rags that may block or damage equipment.
Special equipment is also used to remove grit that gets washed into the sewer.
STAGE TWO: PRIMARY TREATMENT
This involves the separation of organic solid matter (or human waste) from the wastewater. This is done by putting the wastewater into large settlement tanks for the solids to sink to the bottom of the tank. The settled solids are called ‘sludge’. At the bottom of these circular tanks, large scrappers continuously scrape the floor of the tank and push the sludge towards the center where it is pumped away for further treatment. The rest of the water is then moved to the Secondary treatment.
STAGE THREE: SECONDARY TREATMENT
The water, at this stage, is put into large rectangular tanks. These are called aeration lanes. Air is pumped into the water to encourage bacteria to break down the tiny bits of sludge that escaped the sludge scrapping process.
STAGE FOUR: FINAL TREATMENT
Next, the ‘almost’ treated wastewater is passed through a settlement tank. Here, more sludge is formed at the bottom of the tank from the settling of the bacterial action. Again, the sludge is scraped and collected for treatment. The water at this stage is almost free from harmful substances and chemicals. The water is allowed to flow over a wall where it is filtered through a bed of sand to remove any additional particles.
The filtered water is then released into the river. These are the stages of waste water treatment.
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