The fate of dissolved organic carbon (DOC) in the wastewater ...

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The quality of the DOC is expected to affect its fate in a wastewater treatment ... In the present study, the occurrence of DOC during the wastewater treatment ...
DOC in Wastewater Treatment

Subject Area 2.2

Subject Area 2.2: Analysis and monitoring of natural and industrial products in the aquatic environment Research Article

The Fate of Dissolved Organic Carbon (DOC) in the Wastewater Treatment Process and its Importance in the Removal of Wastewater Contaminants Athanasios Katsoyiannis*1,2 and Constantini Samara1 1 Aristotle

University of Thessaloniki, Department of Chemistry, Environmental Pollution Control Laboratory, 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 * Corresponding author ([email protected]) DOI: http://dx.doi.org/10.1065/espr2006.05.302 Please cite this paper as: Katsoyiannis A, Samara C (2007): The Fate of Dissolved Organic Carbon (DOC) in the Wastewater Treatment Process and its Importance in the Removal of Wastewater Contaminants. Env Sci Pollut Res 14 (5) 284–292 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 com-

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pounds 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 (Kd) 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 Kd 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. Keywords: Dissolved organic carbon; distribution coefficient;

DOC; heavy metals; persistent organic pollutants; POPs; wastewater treatment plant

Env Sci Pollut Res 14 (5) 284 – 292 (2007) © 2007 ecomed publishers (Verlagsgruppe Hüthig Jehle Rehm GmbH), D-86899 Landsberg and Tokyo • Mumbai • Seoul • Melbourne • Paris

Subject Area 2.2

DOC in Wastewater Treatment

Introduction

Dissolved organic carbon (DOC) constitutes a parameter of organic pollution of natural waters and wastewaters, which is less studied and yet not regulated by directives. The term 'DOC' is used for the fraction of organics that pass through a 0.45 μm pore size membrane (Dignac et al. 2000). It has been shown that DOC may contain aquatic humic substances, hydrophobic bases, hydrophobic neutrals, hydrophilic acids, hydrophilic bases and hydrophilic neutrals (Imai et al. 2002). DOC is characterized by some authors as a 'third phase' in aquatic environments (Gao et al. 1998b), along with the aqueous solution and the sediment solids, in which a chemical is distributed (Hegeman et al. 1995). Warren et al. (2003) have suggested the DOC / water distribution (or partition) coefficient, Kdoc, expressed by the equation Kdoc = Cdoc / Cw. The same authors have noted that hydrophobic organic compounds, such as DDTs, PAHs, PCBs and dioxins, are expected to partition to DOC to such an extend that the solid – water partition coefficient, Kd, will be affected. Several investigators have proposed the expression of Kdoc as a function of the octanol – water partition coefficient, Kow (Kdoc = 100.71 x Kow0.8, Lee and Kuo 1999, log Kdoc = 0.82 x log Kow + 0.1923, Lindsey and Tarr 2000). Wastewater's DOC is not expected to be removed during the wastewater pretreatment and primary treatment, since these steps basically remove particulate matter, but is expected to be reduced in the biological process (Escalas et al. 2003). The type of wastewater plays an important role in the quality of DOC (Nishijima et al. 2003).The quality of DOC is expected to affect its fate in a wastewater treatment plant (WWTP), since a considerable fraction is not biodegradable, and may be discharged in the aquatic environment with the treated effluent. DOC is undesirable in the treated effluent because it causes color and odor in water (Chow et al. 2003), it is a precursor of trihalomethane formation (Chow et al. 2003, Nishijima et al. 2003), and can play an important role in controlling the speciation and toxicity of trace metals in the aquatic environments (Ma et al. 2001). Also, the presence of DOC can enhance the water solubility of hydrophobic organic pollutants (Warren et al. 2003) and that way facilitate their transport and bioavailability (Gao et al. 1998a).

In the present study, the occurrence of DOC during the wastewater treatment process is examined and its removal rates during primary, secondary and overall treatment are estimated. Moreover, the influence of DOC to the removal efficiency of specific wastewater contaminants, such as persistent organic pollutants (POPs) and heavy metals (HMs), is investigated. POPs can derive in WWTPs as a component of urban or agricultural runoff or drainage into the sewerage system, including wet and dry deposition from the atmosphere (Blanchard et al. 2001), and via the contribution of industrial discharges. Major urban inputs of HMs to sewage water include household effluents, drainage water, business effluents (e.g. car washes, dental uses, other enterprises, etc), atmospheric deposition, and traffic related emissions (vehicle exhaust, brake linings, tires, asphalt wear, gasoline/ oil leakage, etc) transported with storm water into the sewerage system (Karvelas et al. 2002, Sorme and Lagervist 2002, Gagnon and Saulnier 2003, Vosyliene et al. 2003). WWTPs are expected to control the discharge of heavy metals to the environment. However, biological wastewater treatment systems are chiefly designed for removal of organic matter by activated sludge microorganisms, and removal of heavy metals in these systems may be regarded as a sidebenefit. In this work, the relationship of DOC with the partition coefficients of the above-mentioned pollutants between the solid and the dissolved phases of wastewater is examined in order to reveal the effect of 'solubility enhancement'. 1 1.1

Materials and Methods Plant description

The WWTP of the city of Thessaloniki is located 12 km far from the city and serves about 1 million residents by treating daily 120000–150000 m3 of raw wastewaters. About 5– 10% of the total flow is the contribution of industry. The plant also receives the greatest part of the local urban runoff, mainly composed of atmospheric deposition, and traffic-related emissions deposited on the road surface. The treatment process includes screening, grit removal, primary sedimentation without use of chemical coagulants, conventional activated sludge treatment and effluent disinfection using Cl2. The treated wastewater is discharged into the Thermaikos Gulf via a channel. The flow chart of the plant is shown in Fig. 1.

Fig. 1: Flow chart of the WWTP of Thessaloniki

Env Sci Pollut Res 14 (5) 2007

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DOC in Wastewater Treatment

1.2

Subject Area 2.2

Sampling

Sampling was conducted through two weekly campaigns during the period November 2001–December 2001. In each campaign, wastewater samples were collected over five consecutive days, excluding Saturday and Sunday. 24-h composite, flow-proportioned samples of wastewater were collected from three different points along the treatment system, namely, the influent of the plant (raw wastewater, RW), and the effluents of primary and secondary sedimentation tanks (PSE and SSE, respectively).

DOC analysis was performed in centrifuged/filtered wastewater samples by employing a TOC analyzer (Shimadzu – VCSH). Conventional organic pollution parameters (BOD5 and COD) were determined by standard methods (APHA, 1989). 1.4

Descriptive statistics and correlation analysis of data were performed using the SPSS 11.5 statistical software. 2

1.3

Sample processing and analysis

Data analysis

Results and Discussion

2.1

The experimental procedure employed for sample processing and analysis for the determination of POPs has been described in details elsewhere (Katsoyiannis and Samara 2002). Briefly, wastewater samples were collected in brown glass vessels with Teflon caps. Upon receipt in the laboratory, samples were centrifuged at 4500 rpm for 20 min and the supernatants were further filtrated through 0.45 μm nitrate cellulose membranes (Schleicher & Schuell). Liquid and solid phases were kept refrigerated (4°C, storage period