The fate of dissolved organic carbon (DOC) in the wastewater treatment process and its importance in the removal of wastewater contaminants |
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Authors: | Athanasios Katsoyiannis Constantini Samara |
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Institution: | (1) Department of Chemistry, Environmental Pollution Control Laboratory, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece;(2) EU — Joint Research Center, Physical and Chemical Exposure Unit, Institute for Health and Consumer Protection, Ispra (Va), TP-281, Via E. Fermi 1, 21020, Italy |
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Abstract: | Goal, Scope and Background Dissolved organic carbon (DOC) constitutes a parameter of organic pollution for waters and wastewaters, which is not so often
studied, and it is not yet regulated by directives. The term ‘DOC’ is used for the fraction of organics that pass through
a 0.45 μm pores’ size membrane. The type of wastewater plays an important role in the quality of DOC and it has been shown
that DOC may contain aquatic humic substances, hydrophobic bases, hydrophobic neutrals, hydrophilic acids, hydrophilic bases
and hydrophilic neutrals. The quality of the DOC is expected to affect its fate in a wastewater treatment plant (WWTP), since
a considerable fraction of DOC is not biodegradable, and it may be released in the aquatic environment together with the treated
effluent.
In the present study, the occurrence of DOC during the wastewater treatment process is investigated and its removal rates
during primary, secondary and overall treatment are being estimated. Furthermore, a correlation is being attempted between
DOC and the concentrations of selected Persistent Organic Pollutants (POPs) and Heavy Metals (HMs) in the dissolved phase
of wastewaters, to examine whether there are common sources for these pollution parameters in WWTPs. Also, DOC is being correlated
with the partition coefficients of the above-mentioned pollutants in wastewater, in order to examine the effect of ‘solubility
enhancement’ in WWTPs and to evaluate the result of this phenomenon in the efficiency of a WWTP to remove organic pollutants.
Methods For the purposes of this study, 24-h composite wastewater samples were collected from the influent (raw wastewater, RW), the
effluent of primary sedimentation tank (primary sedimentation effluent, PSE) and the effluent of secondary sedimentation tank
(secondary sedimentation effluent, SSE). Samples were analyzed for the presence of 26 POPs (7 PCBs and 19 organochlorine pesticides),
8 HMs and DOC.
Results and Discussion Mean concentrations of DOC in RW and PSE were at similar levels (∼ 70 mg l−1), suggesting that primary treatment has a minor effect on the DOC content of wastewater. DOC concentrations in SSE were significantly
lower (∼ 19 mg l−1) as a result of the degradation of organic compounds in the biological reactor. Calculated removals of DOC were 0.8% in the
primary treatment, 63% in the secondary treatment, and 69% in the overall treatment, exhibiting large differences from other
organic pollution parameters, such as BOD and COD. The overall DOC removal was found to be independent from the DOC concentration
in raw wastewater. Poor correlation was also observed between the DOC content and the concentrations of wastewater contaminants,
such as persistent organic pollutants (POPs) and heavy metals (HMs), probably suggesting that their occurrence in WWTPs is
due to different sources. A good negative linear relationship was revealed between DOC concentrations and the logarithms of
the distribution coefficients (K
d) of various POPs and HMs between the solid and the liquid phases of wastewater. This relationship suggests that DOC facilitates
hydrophobic pollutants to remain in the dissolved phase thus causing lower removal percentages during the treatment process.
Conclusion DOC was measured at three stages of a municipal WWTP that receives mainly domestic wastewater and urban runoff. DOC concentrations
in untreated and primarily treated wastewater were almost equal, and only after the secondary sedimentation there was a decrease.
Concentrations and removal rates of DOC were in the same levels as in other WWTPs that receive municipal wastewater. The origin
of DOC was found to be different to the one of POPs and of HMs, as no correlation was observed between the concentrations
of DOC and the concentrations of these pollutants. On the contrary, DOC was found to have significant negative correlation
with the K
d of all pollutants examined, suggesting that it plays an important role in the partitioning of those pollutants between the
dissolved and the sorbed phase of wastewaters. This effect of DOC on partitioning can affect the ability of WWTPs to remove
toxic pollutants, and that way it facilitates the discharge of those chemicals in the aquatic ecosystems together with the
treated effluent.
Recommendation By the results of this work it is shown that the presence of DOC in wastewaters can significantly affect the partition of
hazardous pollutants between the dissolved and the sorbed phase. It is therefore of importance that this parameter is controlled
more in wastewaters, since it can cause a decrease in the efficiency of WWTPs to remove quantitatively persistent pollutants. |
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Keywords: | Dissolved organic carbon distribution coefficient DOC heavy metals persistent organic pollutants POPs wastewater treatment plant |
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