Abstract
In the Mamais-Jenkins-Pitt method for determination of readily biodegradable COD (S S ), 2 alternatives were proposed for the intermediate determination of soluble inert COD (SI). When a full-scale treatment plant exists, influent S I = effluent truly soluble COD. When there is no full-scale plant, then the truly soluble COD of the effluent of a 24 h fill-and-draw batch reactor treating the wastewater is taken as influent S I. In this study, both S I methods were statistically compared on 24 wastewater samples from 2 municipal wastewater treatment plants (WWTPs). While average S I obtained for the 2 methods was the same, individual samples usually had very different SI values. In fact, virtually no correlation was found between the 2 methods. Also, the SS values obtained using both S I alternatives were statistically compared. A good correlation was observed, in spite of the poor S I correlation – low, dispersed SI values did not seriously affect the correlation between both SS determinations. A method was proposed for determination of the limit of detection and the limit of quantification (LOQ) for both S S methods. The LOQ resulted in 28.6 mg/l and 32.6 mg/l, respectively, for the full-scale and the laboratory-scale alternatives. Some assumptions of the original laboratory-scale (LS) method could potentially be sources of error in SI determination. Two modifications to the laboratory-scale method were implemented in order to avoid these potential problems: Washing biomass with tap water, and correcting SI in the fill-and-draw reactor by the S I of the original biomass suspension. These method modifications were tested on wastewater samples from the mentioned WWTPs. The fundamentals and results of both modifications are discussed in this paper, as well as the imprecision associated with estimating influent S I from effluent CODsol in all studied methods, and its impact on S S determination.
Highlights
Increased usage of BNR has promoted COD fractionation as a tool for wastewater evaluation and process design and control (Spérandio and Paul, 2000)
limit of detection (LOD) and limit of quantification (LOQ) for the low-range method were respectively 3.0 and 4.9 mg/l, below all COD or CODsol values obtained in this study, and well below most of them
The overall standard deviation of soluble inert organic matter (SI) by the FS method was computed for each wastewater treatment plants (WWTPs), resulting in 6.7 mg/l at Tanque Tenorio-Villa de Reyes plant (TTV) plant (50% relative standard deviation (RSD)), and 5.0 mg/l at Parque Tangamanga 1-B plant (PPT) plant (29% RSD). These s values were found statistically equal in an F-test, and a pooled overall standard deviation of 5.7 mg/l was used for (SI)FS. These results indicate that a high relative imprecision (36% RSD) was found associated to SI determination as CODsol of the FS plant effluent
Summary
Increased usage of BNR has promoted COD fractionation as a tool for wastewater evaluation and process design and control (Spérandio and Paul, 2000). IWA models for BNR use several COD fractions as state variables, so COD fractions must be evaluated for model initialisation, calibration and validation. Both ASM1 and ASM3 models consider 4 wastewater COD components, namely: readily biodegradable organic substrate (SS), soluble inert organic matter (SI), slowly biodegradable substrate (XS) and particulate inert organic matter (XI) (Henze et al, 2000). Though biomass can account for 10 to 20% of total organic matter in wastewater, not considering it in raw wastewater would not affect the modelling considerably (Henze, 1992) In this case, biomass would be included in the slowly hydrolysable organic fraction, according to the same author. Readily biodegradable COD is a basic and useful COD fraction, and several respirometric and physicochemical methods have been developed for SS determination
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