Treatability of organic fractions derived from secondary effluent by reverse osmosis membrane

doi:10.1016/j.watres.2003.08.002

Water Research

Volume 37, Issue 19, November 2003, Pages 4801-4809

https://doi.org/10.1016/j.watres.2003.08.002 Get rights and content

Abstract

Dissolved organic matters (DOMs) from two batches of secondary effluent collected from a local water reclamation plant were fractionated using column chromatographic method with non-ionic resins XAD-8, AG MP-50 and IRA-96. Seven isolated fractions were obtained from the fractionation study and these fractions were quantified using DOC, UV₂₅₄ and SUVA values. The fractionation study revealed that the secondary effluent samples comprised about 47.3–60.6% of hydrophobic and 39.4–52.7% of hydrophilic solutes. The treatability of each isolated fraction was investigated by subjecting each fraction to reverse osmosis (RO) treatment individually. It was noted that RO process could achieve high DOC rejections for acid and neutral fractions (ranging from 80% to 98% removal) probably due to the negative charge of RO membrane. The results obtained also indicated that hydrophobicity of DOMs is significant in determining treatability of organic species by RO process. The performance of RO in terms of DOC rejection of un-fractionated secondary effluent was also investigated to assess possible effects of interactions among organic fractions on their treatability by RO process. It was noted that DOC rejection associated with the un-fractionated secondary effluent was generally higher (ranging from 2% to 45%) than the corresponding rejection obtained from each individual fraction isolated from the secondary effluent. This finding suggested there is a beneficial interaction among the fractions that in turn has contributed towards a better overall DOC rejection performance by RO treatment.

Introduction

Membrane technologies have been used for over 20 years in seawater desalination and advanced wastewater treatment. The efficient removal of organic substances from aqueous solutions by reverse osmosis (RO) process has also received considerable attention [1]. The mechanism of RO separation for organic matters is not just a physical process based on size difference of solute and solvent, but also based on processes relating to their sizes and shapes, their ionic charges and their interactions with the membrane itself.

The characteristics of natural organic matters (NOMs) in terms of molecular weight distribution, hydrophobicity and functional groups are important in determining RO performance with respect to permeate quality. It has been shown that large, branched, poly-functional, and sterically complex molecules are better retained by various membranes [2], [3], [4]. In contrast, low molecular weight, non-electrolytes and non-polar water-soluble species with hydrogen bonding capabilities are usually not readily rejected and tend to permeate through RO membranes.

To study organic rejection by RO, many approaches for systematically examining the structural characteristics and the reactivity of organic matters often require isolation of specific organic mater from source water. However, it is not practical to analyze for each individual chemical compound present in dissolved organic matters (DOMs) as it is both time consuming and a great scientific challenge that could not be overcome till this juncture. In view of this constraint, surrogate characterization methods have often been sought for investigating interrelationships between DOMs and their treatability. An ideal process of such isolation should provide rapid extraction of large quantities of DOMs from water without chemical alteration, and/or other losses [5]. It is critical that the isolation process could provide large recoveries and preserve the original reactivity of the DOMs present in the source to enable a realistic characterization effort and reactivity studies. A review of literature revealed that resin adsorption chromatographic method represents the most commonly used surrogate characterization method for studying organic matters present in an aqueous environment. Several advantages of resin adsorption chromatographic method have been documented [16]. These include quantitative isolation by DOC analysis as well as simple and rapid method for large volume of water. Although fractionation method has been commonly applied for characterization of water samples [6], [7], the treatbility of the isolated fractions are not well studied.

In view of the above, the objective of this study was to investigate the rejection characteristics of organic fractions isolated from treated secondary effluent by using RO process. Understanding treatment efficiencies of RO process at organic fractions level (with respect to each of the isolated fractions derived from the secondary effluent) would facilitate development of novel treatment system for removing organics that are precursors to disinfection-by-products (DBPs) formation and subsequently preventing and controlling their presence in treated water. In addition to comprehensively assess the removal efficiencies of organic fractions by RO, it was also the aim of this study to relate fraction characterization data to the treatability of organics in a treatment train.

Section snippets

Resin

The resins used for the fractionation procedure adopted in this study comprised of Amberlite XAD-8 resin (Rohm and Haas), AG MP-50 cation exchange resin (Bio Rad) and Amberlite IRA-96 anion exchange resin (Rohm and Haas). The resins were pre-purified using the Soxhlet extraction method described by Leenheer [8].

Water samples

Treated secondary effluent taken from a local water reclamation plant was used in this study. The water samples were collected in 30 L jerry cans and transported back to the laboratory

Characteristics of the secondary effluent

Two batches of secondary effluent from a local water reclamation plant were collected and analyzed. The characteristics of the secondary effluent are summarized in Table 2.

It is seen from Table 2 that Sample 1 contained more hydrophobic materials and aquatic humic substances per unit amount of organic carbon than Sample 2 by using the specific ultraviolet absorbance (SUVA, UV₂₅₄ to DOC ratio) guidelines proposed by Edzwald and Tobiason [12]. It was noted in this study that UV Abs. results

Conclusions

Fractionation using column chromatography method revealed that treated secondary effluent comprised 47.3–60.6% of hydrophobic solutes and 39.4–52.7% of hydrophilic solutes.

The DOC rejection efficiency by RO process for various isolated fractions ranged from 49.7% to 95.5%. Highest DOC removal of HPO-A suggested that RO process could remove most of the hydrophobic acid organic fraction from the secondary effluent. It was confirmed that hydrophobicity of dissolved organic matters was a

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Treatability of organic fractions derived from secondary effluent by reverse osmosis membrane

Abstract

Introduction

Section snippets

Resin

Water samples

Characteristics of the secondary effluent

Conclusions

Water Res

Water Res

Water Res

Chemosphere

Water Sci Technol

Water Research.

Separation of hazardous organics by reverse osmosis membranes

Environ Progress

Removing trace organics by reverse osmosis using cellulose acetate and polyamide membranes

J Am Water Works Assoc