Measurement of pH, alkalinity and acidity in ultra-soft waters

Conventional characterisation of low alkalinity waters via pH measurement and titration of total alkalinity to a prescribed end-point invariably leads to large errors. These errors result from instability of the pH probe and an unknown titration end-point. In this paper two indirect methods (termed the "double Gran function" and the "blend" method) for the characterisation of such waters are evaluated critically. A blend composed of the raw water, sodium chloride (to increase conductivity), and standard bicarbonate (to increase buffering capacity) was titrated with standard strong acid in two pH regions: 6.3 pH < 4.0. In both methods, total alkalinity was determined using the latter set of points, and the first Gran function. In the double Gran function method the upper set of titration points was used to determine CO2 acidity using the second Gran Function. In the "blend" method, equilibrium chemistry data were used to calculate total acidity for each point based on the known total alkalinity, pH reading, temperature and ionic strength. The two methods gave excellent results (in terms of both repetition and accuracy) as compared to characterisation based on total alkalinity and inorganic carbon analysis. A detailed procedure for the execution of the two approaches is given in an appendix. WaterSA Vol.27(4) 2001: 423-43

Short communication Sulphate measurement in organic-rich solutions: Carbonate fusion pretreatment to remove organic interferences

Sulphate measurement using a barium sulphate turbidimetric method in solutions with high concentrations of organic material is shown to be problematic. The organics give background colour, which introduces a positive error to the measured absorption, and inhibit the barium sulphate precipitate, which results in a negative error. A carbonate fusion pretreatment of the sample results in the removal of the organic matter and associated interferences. With this pretreatment, excellent sulphate recoveries were obtained (100%). Rigorous testing of the method shows that reproducible and accurate results are obtainable. Water SA Vol. 31 (2) 2005: pp.267-27

Integrated chemical/physical and biological processes modeling Part 2 - Anaerobic digestion of sewage sludges

The development and validation of a two phase (aqueous-gas) integrated mixed weak acid/base chemical, physical and biological processes kinetic model for anaerobic digestion (AD) of sewage sludge are described. The biological kinetic processes for AD are integrated into a two phase subset of the three phase mixed weak acid/base chemistry kinetic model of Musvoto et al. (1997, 2000a,b,c). The approach of characterising sewage sludge into carbohydrates, lipids and proteins, as is done in the International Water Association (IWA) AD model No 1 (ADM1, Batstone et al., 2002), requires measurements that are not routinely available on sewage sludges. Instead, the sewage sludge is characterised with the COD, carbon, hydrogen, oxygen and nitrogen (CHON) composition. The model is formulated in mole units, based on conservation of C, N, O, H and COD. The model is calibrated and validated with data from laboratory mesophilic anaerobic digesters operating from 7 to 20 d sludge age and fed a sewage primary and humus sludge mixture. These digesters yielded COD mass balances between 107 and 109% and N mass balances between 91 and 99%, and hence the experimental data is accepted as reasonable. The sewage sludge is found to be 64 to 68% biodegradable (depending on the kinetic formulation selected for the hydrolysis process) and to have a C,sub>3.5H7O2N0.196 composition. For the selected hydrolysis kinetics of surface mediated reaction (Contois), with a single set of kinetic and stoichiometric constants, for all retention times free and saline ammonia (FSA), short chain fatty acids (SCFA), H2CO3* alkalinity and pH of the effluent stream, and CO2 and CH4 gases in the gas stream. The measured composition of primary sludge from two local wastewater treatment plants ranged between C3.38H7O1.91N0.21 and C3.91H7O2.04N0.16. The predicted composition is therefore within 5% of the average measured composition providing persuasive validation of the model. Keywords: anaerobic digestion, weak acid/base chemistry, kinetic modelling, sewage sludge Water SA Vol. 31(4) 2005: 545-56

Fundamental study of a one-step ambient temperature ferrite process for treatment of acid mine drainage waters: rapid communication

A novel approach towards the removal of iron and heavy metals from South African acid mine drainage (AMD) waters is presented. The approach involves the controlled oxidation of ferrous-containing AMD water at ambient temperatures in the presence of magnetite seed. The resulting oxidation product is the ferrite (M13+2M22+O4) magnetite (Fe3O4), which has the capacity for nonferrous metal removal, and which forms a stable sludge that is easily separated from the effluent. Sludge characterisation studies (XRD, SEM and dissolution tests) show that oxidation of ferrous solutions under controlled pH and oxidation conditions (pH 10.5, air flow rate = 0.05 &#8467;/min) in the presence of magnetite seed (initial seed: ferrous ratio = 7: 1) yields almost pure magnetite at ambient temperature. It was found that magnetite seed channels the end products of the AMD oxidation reaction towards magnetite. Under identical conditions, but in the absence of magnetite seed, a poorly characterised mixture of largely amorphous iron oxides are formed with magnetite comprising not more than 17% of the total iron. The kinetics of the reaction under the investigated conditions were found to be very favourable, with magnetite forming at a rate of 12.8 mg Fe/&#8467;/min. The total iron concentration in the effluent was always less than 1 mg/l representing an iron removal efficiency of 99.9%. The precipitant settled well (SVI 8 m&#8467;/g) and showed substantial stability at pH 3 (dissolution of 1.1% after 120 h). An outline for a one-step ambient temperature ferrite process is presented. WaterSA Vol.27(2) 2001: 277-28