Analysis of nitrogenous and algal oxygen demand in effluent from a system of aerated lagoons followed by polishing pond

In this descriptive-analytical study, nitrogenous and algal oxygen demand were assessed for effluent
from a system of facultative partially mixed lagoons followed by the polishing pond using 120 grab
samples over 1 year. Filtered and non-filtered samples of polishing pond effluent were tested in
the presence and absence of a nitrification inhibitor. Effective factors, including 5-day biochemical
and chemical oxygen demand (BOD and COD), total suspended solids (TSS), dissolved oxygen,
chlorophyll A, and temperature, were measured using standard methods for water and wastewater
tests. The results were analyzed using repeated measures analysis of variance with SPSS version 16.
Findings show that the annual mean of the total 5-day BOD in the effluent from the polishing
pond consisted of 44.92% as the algal carbonaceous biochemical oxygen demand (CBOD), 43.61% as
the nitrogenous biochemical oxygen demand (NBOD), and 11.47% as the soluble CBOD. According to
this study, the annual mean ratios of algal COD and 5-day algal CBOD to TSS were 0.8 and 0.37,
respectively. As the results demonstrate, undertaking quality evaluation of the final effluent from the
lagoons without considering nitrogenous and algal oxygen demand would undermine effluent quality
assessment and interpretation of the performance of the wastewater treatment plant.