4. Results and discussion
Since the modified method was developed after many
trials, the results obtained at every stage of the development
of the method constitute an important piece of
information and thus have been presented here to bring
out the importance of variation in each parameter.
4.1. Effect of method parameters
Table 1 gives the comparison of COD values obtained
with the sample in the powder and the chip forms
for original cattle dung slurry from three different anaerobic
reactors. For all samples, the coefficient of
variation is found to be less than 2% with sample in chip
form which is much lower than the CV of more than 6%
with sample in powder form reflecting a higher consistency
in results with chip form.
Table 2 shows the effect of different combinations of
reagents (with same ratio of K2Cr2O7 and H2SO4
reagents as recommended in APHA–AWWA–WPCF method) on consistency of results. It can be seen that
with 20 ml K2Cr2O7 +60ml H2SO4, quite consistent resultswere obtained with the lowest value of coefficient
of variation among the combinations tried. It needs to be pointed out that in this experiment the original slurry
was from different reactors and hence the mean value of COD from each combination is different.
The effect of reflux time on consistency of COD is presented in Table 3. All these results are for the same
original sample. It can be seen that COD values obtained are higher as well as more consistent for 4 h of
reflux time as compared to 2 h, clearly indicating that oxidation was not complete in 2 h. The COD values
did not change significantly with further increase in reflux time.
For studying the effect of chip thickness on the results, anaerobic sludge sample obtained from an operational
upflow anaerobic sludge blanket (UASB) reactor at New Delhi were used. Different volumes of anaerobic
sludge samples were dried in hot air oven for obtaining chip of different thickness. It was difficult to measure the
exact thickness of the chip, thus observations reported are for approximate values of the chips thickness.
Table 4 shows that for the range of chip thickness tried, the results did not vary very significantly though
a small reduction in COD values is observed at higher thickness. Based on these results, it is recommended that
chip thickness should be less than 1.0 mm.
4.2. Comparison of APHA–AWWA–WPCF method and modified method
Modified method was finalized after a series of COD experiments carried out by using different samples of
cattle dung slurry from different bioreactors in operation.
Further experiments were carried out by preparing the slurry from fresh cattle dung in the laboratory by
mixing fresh cattle dung and water in different proportions so as to get original slurry samples of different
solids content. The prepared samples were then tested for COD using APHA–AWWA–WPCF method as well
as modified method for comparison.
A comparison of the estimated values of COD from the two methods is presented in Table 5. It is evident
from the results that as the solids content of slurry samples increased, APHA–AWWA–WPCF method failed
to produce consistent and representative results due to the reasons discussed earlier. For samples containing
70.90 g/l TS, the value of SD and % CV in case of APHA–AWWA–WPCF method with 4 h of refluxing
time were found to be ±9910.50 g/l and 19.28% respectively, while the modified method gave SD and % CV
value of ±438.75 g/l and 0.54% respectively. Also, the values of COD obtained using new method are much
higher as compared to those obtained with APHA–AWWA–WPCF method indicating incomplete oxidation
of these samples in the latter.