Cj Brouckaert

A critical review of experimental and predicted methane generation from anaerobic codigestion

Anaerobic digestion is increasingly being considered as a treatment option for an extensive range of waste biomass, due to the potential for energy recovery, in the form of methane production, and lower sludge volumes relative to aerobic treatment processes. Furthermore, when two substrates are codigested (i.e. digested together), added benefits are foreseeable, such as increased methane production and detoxification of toxic compounds via cometabolic degradation pathways. The objectives of this study were to compare experimental and predicted methane production from codigestion literature studies in order to objectively evaluate digester performance. Two predictive methods were used, both assuming methane yields are additive: literature values for digestion of single substrates and a stoichiometric method using model substrates to represent different substrates. Waste sources included in the analysis were primary sewage sludge, waste activated sludge, cow manure, waste paper, grease trap sludge, fat oil and grease and algal sludge. It was found that methane production could approximately be predicted using both methods, with literature methane yields from the same study being the most accurate predictor. One important finding from this study was that the assumption that methane yields are additive is a reasonable one. Furthermore, both predictive methods may be usefully employed as a screening tool to compare methane yields between different types and blends of substrates.

Quantitative geochemical modelling using leaching tests: Application for coal ashes produced by two South African thermal processes

The present work focuses on the reactivity of coal fly ash in aqueous solutions studied through geochemical modelling. The studied coal fly ashes originate from South African industrial sites. The adopted methodology is based on mineralogical analysis, laboratory leaching tests and geochemical modelling. A quantitative modelling approach is developed here in order to determine the quantities of different solid phases composing the coal fly ash. It employs a geochemical code (PHREEQC) and a numerical optimisation tool developed under MATLAB, by the intermediate of a coupling program. The experimental conditions are those of the laboratory leaching test, i.e. liquid/solid ratio of 10 L/kg and 48 h contact time. The simulation results compared with the experimental data demonstrate the feasibility of such approach, which is the scope of the present work. The perspective of the quantitative geochemical modelling is the waste reactivity prediction in different leaching conditions and time frames. This work is part of a largest research project initiated by Sasol and Eskom companies, the largest South African coal consumers, aiming to address the issue of waste management of coal combustion residues and the environmental impact assessment of coal ash disposal on land.

Leak Identification in a Water Distribution Network using Sparse Flow Measurements

A Linear Programming technique is proposed to balance the flows in a network, matching flow measurements where available. Where necessary, a “leak-out” flow is invoked on a pipe section in order to achieve the balance. Usually, multiple solutions are possible, and these are sounded out by progressively increasing an integrity weight for each pipe section. A feature of the method is that it overlays “snapshots” of the network at a series of points in time, in order to progressively narrow down the part of the network which can commonly account for all observations.

Modelling of ionic interactions with wastewater treatment biomass

Titration data for samples of mixed salts with glycine, bakers yeast cell mass and anaerobic digester sludge were obtained and compared to a speciation model of weak acid-base interaction in aqueous solutions. The effect of glycine on the buffer intensity of the solution could be precisely described by the speciation model but did not represent the proton exchange characteristics of either bakers yeast or anaerobic sludge well. A model component, UKZiNe, consisting of carboxylic acids, phosphate and amine groups described the bakers yeast well, and a combination of UKZiNe and carbonate-yielding inorganic solids described anaerobic digester sludge. The effect of biomass on buffer intensity in the pH range 6.5 to 8 was small for the concentration ranges tested.

Multi-objective optimisation of the operation of a water distribution network

The aim was to move water through the reservoir network in such a way as to meet consumer demands and level constraints, minimise the cost of electricity, and minimise the loss of chlorine. This was to be achieved by choosing the switching intervals of reservoir inlet pumps and valves, at the same time complying with the allowed minimum interval size of each device. Flows were balanced by Linear Programming. The genetic algorithm gave confidence in the near-optimality of its solutions, through well-defined Pareto fronts between the competing objectives. The method was applied to a 16-reservoir water distribution system in Durban, South Africa. Comparison with an equivalent “dead-band” control showed a 30% improvement in a weighted objective.