Water Research 36 (2002) 617–624 Evaluation of a rapid physical–chemical method for the determination of extant soluble COD Zhiqiang Hu a , Kartik Chandran a , Barth F. Smets a , Domenico Grasso b, * a Environmental Engineering Program, University of Connecticut, Storrs, CT 06269-2037, USA b Picker Engineering Program, Smith College, 51 College Lane, Northampton, MA 01063, USA Received 1 May 2000; received in revised form 1 May 2001; accepted 1 May 2001 Abstract Characterization of total chemical oxygen demand (COD) in wastewater is critical for accurate modeling of constituent biotransformation steps. We evaluated the accuracy and precision of a commonly used soluble COD determination technique (coagulation using ZnSO 4 at pH 10.5) in relation to three other physical–chemical separation techniques: destabilization with a non-hydrolyzing trivalent cation (LaCl 3 ), sequential filtration, and ultracentrifuga- tion. Samples of deionized water and domestic wastewater were spiked with aliquots of synthetic soluble COD and recoveries using the different separation methods were compared. Although mechanisms of coagulation using LaCl 3 and ZnSO 4 are different, the mean COD recoveries using these methods were in close agreement. Further, sorption of soluble COD onto zinc hydroxide precipitate flocs appeared to be negligible. The two coagulation methods yielded statistically different ðp ¼ 0:05Þ soluble COD values when applied to nine independent wastewater samples (obtained on nine different days), but the difference was less than 10%. The COD quantified by the coagulation techniques corresponded most closely with the o1000Da molecular weight fraction defined as ‘‘truly soluble COD’’. Centrifugation of wastewater samples amended with mercuric chloride (HgCl 2 ) at 10 mg/L overestimated the soluble COD concentration. Our results confirm that coagulation using either ZnSO 4 or LaCl 3 is appropriate for the rapid determination of soluble COD fraction in wastewater matrices. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coagulation; Centrifugation; Filtration; Soluble COD; Wastewater 1. Introduction Increasingly complex mechanistic models describing biological wastewater treatment processes necessitate detailed characterization of wastewater composition. It is often important, at a minimum, to distinguish between soluble and particulate fractions because they are subject to different biochemical reactions [1]. Practical differ- entiation of soluble chemical oxygen demand (COD) in wastewater is contingent on the analytical method employed. Generally, soluble constituents are defined as possessing a characteristic dimension less than 1 nm, while colloidal particles are defined as having a characteristic dimension between 1 nm and 1 mm [2,3]. Common operational definitions reported in the litera- ture further segregate wastewater composition into dissolved (o1nm), colloidal (1nm–1 mm), supracolloi- dal (1–100 mm), and settleable (>100 mm) fractions [4]. Although there is continual debate in the physical– chemical community regarding the characterization of macromolecules as dissolved, colloidal, or particulate moieties, from a microbial physiological perspective, it should be recognized that soluble molecules must be sufficiently small to pass through transmembrane porins (typical internal pore diameter around 1nm) for transport across cell membranes [5]. As a result, *Corresponding author. Tel.: +1-413-585-7000; fax: +1- 413-585-7001. E-mail address: dgrasso@smith.edu (D. Grasso). 0043-1354/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0043-1354(01)00273-1