This work was to give a comprehensive estimation for the chemical compositions contributable to COD of the produced
water treatment system. For this purpose, the wastewater samples were collected from an onshore wastewater
treatment plant. The chemical compositions of the wastewater were investigated, and the COD contributed by each component
was estimated. The results showed that the COD levels of O&G and SS presented decreasing trends during the
whole process and achieved total removal percentages of 95.1% and 62.3%, respectively. The final COD of organic acids
and low-molecular-weight carbonyl compounds were respectively lowered to nearly 64% and 35% of their initial levels,
and no regular trends were found for the COD of these chemicals during the whole treatment process. The COD of inorganic
components presented minor variations at all sampling spots. The majority of COD was originated from O&G in
raw wastewater. The COD contributed by O&G decreased greatly with continuous treatment and finally was lower than
17% of measured COD. At each sampling spot, the ratios of COD contributed by SS did not exceed 7.6% of measured
COD. Other measured chemicals, including organic acids, carbonyl compounds, volatile phenols, reductive anions, metals
and TDP were not the main sources of COD during the whole treatment process, and the ratio of COD was below 9%
at each sampling spot. Most of the soluble components contributable to residual COD were still unknown after biological
treatment, and the COD contributed by these components was greater than 57% of measured COD.
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