848 titrino plus

The carbonate system was manipulated by adding calculated amounts of NaHCO₃ and HCl in a closed system following Schulz et al. [21] . Alkalinity was measured by potentiometric titration following Dickson et al. [22] , using a Metrohm Titrino Plus 848 equipped with a 869 Compact Sample Changer, and calibrated with certified reference material supplied by A. Dickson. The pH was measured using a glass electrode (WTW, pH 340i) and calibrated with a TRIS seawater buffer, supplied by A. Dickson.
Carbonate chemistry was calculated from measured temperature, salinity, silicate and phosphate concentrations, and pH and TA using CO2sys [23] , with the equilibrium constants determined by Mehrbach et al. [24] as refitted by Dickson and Millero [25] .

AV and PV were evaluated using a potentiometric auto-titrator, Titrino Plus 848 (Metrohm, Herisau, Switzerland), in accordance with the American Oil Chemists’ Society (AOCS) official procedures Cd 3d63 [60 ] and Cd 8b-90 [61 ].

Produced water samples were characterized immediately after receipt according to salinity, sulfate content, pH and microbial concentration. Salinity (chloride content) was determined by potentiometric titration in a Titrino Plus 848 (Metrohm, Switzerland). The equipment was connected to a silver electrode, and 0.01 mol L⁻¹ AgNO₃ solution was used as the standard. Sulfate quantification in water samples was carried out by the ICP-OES Agilent 5100 technique (Agilent, USA). A sulfur standard was used to determine the sulfate content. pH was determined with a pH meter (Edge, Hanna Instruments). The microbial concentration was determined by ATP quantification using a luminometer (Photonmaster, Luminultra) and specific reagent kit (ATP Quench Gone Aqueous Kit, Luminultra).

Seawater pH values were continuously monitored by a pH-stat system, which electrodes were calibrated using NBS solutions (Seawater National Bureau of Standards) and adjusted every day to the desired pH_T (Total Scale) using a pH meter with a glass electrode (Eutech Instruments EcoScan) calibrated with Tris/HCl referenced solutions ([37 ]; standards provided by A.G. Dickson, batch 13). Mean pH_T were calculated from hydrogen ion concentrations of each measurement and then re-converted back to pH [37 ]. Total alkalinity was measured twice a week on water samples collected in glass bottles, filtered at 0.45 μm (GF/F Whatman) and stored in the dark at 4°C to avoid biological alteration. The pH was measured at 0.1 ml increments of 0.01 N HCl at 25°C using a Metrohm titration system (848 Titrino Plus). Three replicated 20 mL sub-samples were analyzed. Total alkalinity (A_T) was calculated from the Gran function applied to pH variations from 4.2 to 3.0 as mEq L^-1 from the slope of the curve HCl volume versus pH. Titrations of A_T standards provided by A.G. Dickson (batch 121) were within 0.85 μmol kg^-1 of the nominal value. Mean A_T of seawater was 2.341 ± 0.047 mmol kg^-1 (n = 40). CO₂ and saturation states of aragonite (Ω_ara) were calculated from pH_T, mean A_T, temperature, and salinity using the free access CO₂ Systat package.

Effluent samples for each reactor were analyzed twice per week. Soluble COD, TSS, and VSS were determined according to the Standard Methods for the Examination of Water and Wastewater [23 ]. The effluent concentration of short-chain volatile fatty acids (VFA), expressed as acetic acid (mg HAc L⁻¹), were measured from centrifuged samples using a titrator (848 Titrino Plus, Metrohm, Herisan, Switzerland). The solvent concentration was measured in a gas chromatograph (Agilent GC 7890A, Las Rozas, Spain) equipped with a Restek Rtx-VMS column (30 m × 0.25 mm × 1.4 mm) and a flame ionization detector and operated with 1.3 mL min⁻¹ of helium as a carrier. The injector and detector temperatures were set to 190 °C and 240 °C, respectively. The oven temperature was set to 60 °C for 14 min followed by a ramp up of 25 °C min⁻¹ to 110 °C. Biogas composition was analyzed in a gas chromatograph (Agilent GC 7820A, Las Rozas, Spain) equipped with a thermal conductivity detector and two columns connected in series, HP-Plot/U (30 m × 0.32 mm × 10 mm) and HPMolisieve (30 m × 0.32 mm × 12 mm), and operated with 3.5 mL min⁻¹ of helium. The injector, oven, and detector temperatures were 200 °C, 40 °C, and 250 °C, respectively. Methane production was monitored by the volumetric gas meter of the AMPTS II (Bioprocess Control, Lund, Sweden)).