Besides, 2.145mg/L was the lowest value of DO at first days and it is
because the pollutant was not degraded by the microorganism.Even though the
degradation happened, the reduction was too little and it is because the
microorganism was tried to adapt to the new conditions and the number of
microorganisms was still in low amount.(Fleming,2014).
Typically, oxygen requirements are
met when the mixed-liquor dissolved oxygen is at 2 mg/L or more. However,
optimal dissolved-oxygen ranges can vary significantly from facility to
facility. Low dissolved-oxygen conditions have been observed in high-purity
oxygen processes, with dissolved-oxygen levels as high as 10 to 12 mg/L. It is
best to experiment with the system to determine what dissolved-oxygen
concentration is adequate.According to the BIS standard, DO of the effluent
should within range of 4 to 6 mg/lit.(Sarrafpour
et al., 2001)
5.5.7 Biochemical Oxygen Demand
The BOD of the
wastewater can be measured by BOD test which is also known as “BOD5” since its
based on the accurate measure of DO at the beginning and end of a five-day
period in which the sample is held in dark, incubated conditions at 200C.
The biological oxidation is very slow process during oxidation and organic
pollutants are oxidized by certain microorganisms into carbon dioxide and water
using dissolved oxygen. The BOD concentration in most wastewaters exceeds the
concentration of dissolved oxygen (DO) available in an air saturated sample.
Therefore, it is necessary to dilute the sample before incubation to bring the
oxygen demand and supply into appropriate balance. Make several dilutions of
the sample that will result in a residual DO of at least 1 mg/L and a DO uptake
of at least 2 mg/L after a 5-d incubation.(Fleming et al.,2014).
So in this study
needed to do a dilution first before incubation process. According to the
appendices in the table, A1 was shown the data of BOD testing for first days
sample. There is four dilution which is 25ml, 50ml, 100ml and 150ml and it was
shown the higher the dilution, the lower value of BOD5. It is
because when BOD levels are high, there is a decline in DO levels. This is
because the demand for oxygen by the bacteria is high and they are taking that
oxygen from the oxygen dissolved in the water. If there is no organic waste
present in the water, there won’t be as many bacteria present to decompose it
and thus the BOD will tend to be lower and the DO level will tend to be higher.
The BO5 is always lower than the COD. (Davies et al.,