Overview
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- Part 4: Aligning the modelled and measured aqueous phases
- Water Research Group, Department of Civil Engineering, University of Cape Town, Rondebosch 7700, Cape Town, South Africa
- phase property that can be used to track aqueous-phase changes caused by physical, chemical and biological Received: 22 February 2016
- processes. Alkalinity is a stoichiometry property of the components in solution (i.e., a linear function of the Accepted: 29 December 2021
- using the 5-point titration method is described. While several alkalinity titration based methods are available 5-point titration
- for anaerobic digestion bioprocess monitoring, only the 5-point titration is sufficiently accurate for aqueous-
- In Part 3 (Ekama and Brouckaert, 2022) of this series, the alignment of the modelling and (CC BY 4.0)
- is, in turn, represented by species. As pointed out in Part 1 (Brouckaert et al., 2021a), the rate at
- methanogenesis (Bioprocess 1 in Table 1 of Part 2 – Brouckaert et al., 2021b) in anaerobic digestion
- the aqueous phase. Note that in this series of papers, species are italicised (e.g. CO32−), while their
- This paper, Part 4, discusses the measurements needed to characterize the aqueous phase for
- perspective in Part 1 (Brouckaert et al., 2021a).
- As discussed in Part 1, speciation modelling, which determines dissolved species concentrations
- 5 (Brouckaert et al., 2022).
- The weak acid/base system components are CO32−, NH4+, PO43−, HS− and Ac− (CH3COO−). (The
- ISSN (online) 1816-7950 21
- analyser, with which it is difficult to prevent errors due to CO2 loss. affect the aqueous-phase speciation, and hence pH.
- component concentrations (Eqs 19a and 19b in Part 1 – because equilibrium constant values are well known (unlike the
- Brouckaert et al., 2021a). bioprocesses, which are usually held in a non-equilibrium state
- • As discussed in Parts 1 and 2, the changes in aqueous-phase by kinetic factors which require calibration for different reactor
- direct application as a control parameter for pH-sensitive models (Batstone et al., 2002; Brouckaert et al., 2010; Ikumi et al.,
- processes such as anaerobic digestion. 2011; Lizarralde et al., 2015; Part 5 – Brouckaert et al., 2022).
- (Bioprocess 5a in Table 1 of Part 2 – Brouckaert et al., 2021b), and leaving the system, the progress and performance of the
- sulphide is the e− donor, and is transformed to sulphate; and nitrate bioprocess(es) can be monitored. In Part 3 (Ekama and Brouckaert,
- is the e− acceptor, and is transformed to nitrogen gas. Sulphate and 2022) this flux/mass balance principle was applied to characterise
- in the aqueous phase – almost all are in the un-protonated NO3− the characterisation of the aqueous phase through measuring the
- or SO42− form. The free (H2S) and saline (HS−, S2−) sulphide (FSS) material content entering and exiting bioprocesses, and inferring
- and free (NH3) and saline (NH4+) ammonia (FSA) are weak acid/ solution composition in terms of model components, is considered.
- (NH4+, H2S) and partially protonated (HS−, S2−, NH3) forms co- LINKING THE MODELLING AND MEASUREMENT
- are selected for modelling (Part 1 – Brouckaert et al., 2021a,) and
- Details of the component selection is outlined in Part 5 (Brouckaert original sample. The usual case is that there is a single substance
- et al., 2022). The measurement of the weak acid/bases, in particular of interest, the measurement response is assumed linear, and no
- the inorganic carbon (IC) system via the H2CO3 alkalinity, aligned specific information about the matrix is sought. In the titration
- As mentioned in Part 1 (Brouckaert et al., 2021a), alkalinity is concentration (CT):
- pH endpoint, which can be either a fixed value of 3.7, or to
- H2CO3 alk is then determined by subtracting the alkalinity
- refer to carbonate alkalinity (H2CO3 alk), phosphate alkalinity
- (H2PO4− alk), sulphide alkalinity (H2S alk), etc. Note that H2CO3,
- as discussed in Part 1 (Brouckaert et al., 2021a). Although the • A two-point titration with pH end-points 5.75 and 4.3, to
- means that they have limited practical application in solution • The 5-point titration that provides more accurate values
- Characterizing mixed weak acid/base samples procedures, using the solution specified in Table 1 as an example.
- In aqueous samples with a mixture of n (say 6) weak acid/bases Speciated alkalinity and buffer capacity vs. pH curves for the
Methods (brief)
- for anaerobic digestion bioprocess monitoring, only the 5-point titration is sufficiently accurate for aqueous-
- methanogenesis (Bioprocess 1 in Table 1 of Part 2 – Brouckaert et al., 2021b) in anaerobic digestion
- processes such as anaerobic digestion. 2011; Lizarralde et al., 2015; Part 5 – Brouckaert et al., 2022).
- Details of the component selection is outlined in Part 5 (Brouckaert original sample. The usual case is that there is a single substance
- Characterizing mixed weak acid/base samples procedures, using the solution specified in Table 1 as an example.
- In aqueous samples with a mixture of n (say 6) weak acid/bases Speciated alkalinity and buffer capacity vs. pH curves for the
- it, one for each weak acid/base system, including the water components during the preparation of the sample for titration is an
- So in a water sample with a mixture of six weak acid/bases, viz.
- When the IC system is the only weak acid/base present in a water sample can be calculated by subtracting the subsystem alkalinities
- sample or the contribution of other weak acid/base systems from Alkt, i.e.:
- of around 4.5. However, in mixed weak acid/base systems such sample ‘in-situ’ pH (7.00 in Table 1) and the measured total
- by titrating a sample from the in-situ pH to 5.75 and the TA was
- from 50 to 4 400 mg/L as CaCO3 (CT from 140 to 1 250 mgC/L) points of the titration are the sample ‘in-situ’ pH, two pH points
- only for control of ADs, but also for characterizing water samples
- water samples containing the five (six counting water) weak
- water sample (Poinapen et al., 2009).
- The 5-point titration method for sample with pH < 6.7
- 5-point titration programme will be assigned to the H2CO3 If the AD sample has a pH below the pH of the first pH titration
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