Browsing by Author "Dold, Peter Lorimer"

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    The COD/VSS ratio of the volatile solids in the activated sludge process
    (1983) Schroeter, Wolfgang Dietrich; Dold, Peter Lorimer
    In the kinetic analysis of the activated sludge process behaviour, a vital requirement is that a COD balance near unity is obtained. The COD/VSS ratio of the mixed liquor forms an integral part of such a balance by reason that the practice has arisen, prompted by convenience, to determine the VSS rather than the COD of the mixed liquor and to convert the VSS to COD by accepting a COD/VSS ratio. Evidently good experimental COD recoveries will depend on the correctness of the COD/VSS ratio. Dold, Ekama and Marais (1980), in an analysis on the dynamic behaviour of the aerobic activated sludge process, observed that no COD balances on their experimental data near unity were possible for a 2,5 day and 20 day sludge age at 12Ā°C. They hypothesized that under the conditions above a possible cause for the lack in COD balances was that the COD/VSS ratio was higher than that 'normal' at the longer sludge ages at 20Ā°C i.e. COD/VSS = 1,48 mgCOD.mgVSSā»Ā¹; then, in a mass balance if the lower value was used the apparent loss of COD recovery would be observed. Such a high COD/VSS ratio could very likely arise from an accumulation of solidified fats (derived from the influent) at the lower temperature i.e. 12Ā°C. Upon solidification, these fats could be relatively resistant to biodegradation thereby leading to a substantial accumulation on the sludge mass. As the average COD/VSS ratio of fats is of the order of 2,6 mgCOD.mgVSSā»Ā¹ it is clear that, upon accumulation, these could increase the COD/VSS ratio. At 20Ā°C, it was hypothesized that the fats, most probably, are in an emulsified form and therefore amenable to biodegradation; a substantial accumulation of fats is therefore not expected at this temperature. To test the above hypothesis, aerobic activated sludge units at a 20, 8 and 3 day sludge age at 12Ā°C and 20Ā°C treating a raw domestic effluent were operated and tested. These experiments indicated a slight increase in the COD/VSS ratio at a 3 day sludge age at 12Ā°C (relative to the other units) but insufficient to explain the discrepancies in the COD balances observed by Dold et al. (1980). During the above experiments it was established that a significant error (up to 30 percent) in the COD balance can occur due to oxygen transfer from the atmosphere into the liquid mass giving rise to an under estimation of the 'true' oxygen consumption rate of the mixed liquor. By sealing the liquid surface from contact with the atmosphere during an oxygen consumption rate measurement, (particularly at the short sludge ages and low temperatures) good COD recoveries were achieved. It was concluded that the most likely cause for the discrepancies between the observed and predicted response in Dold et al's (1980) experiments was due to this error in the oxygen consumption rate measurement; this was verified by repeating the experiments under the same conditions, after Dold et al. (1980), and obtaining COD mass balances of close to 100 percent. To enquire whether fats could be a significant factor in raising the COD/VSS ratio value above that of 'normal' activated sludge (treating a 'normal' municipal effluent) at the short sludge ages, a set of experiments was inaugurated at a 3 day sludge age at both 12Ā°C and 20Ā°C - two units treating only raw domestic sewage and the other two treating raw domestic sewage spiked with up to 33 percent of influent COD by mass with oleic acid. It was observed that the COD/VSS ratio for the units treating artificially added fat (in the form of oleic acid) increased above the ratio in the units treating only raw domestic sewage thereby indicating that high concentrations of fat can increase the COD/VSS ratio under these conditions. However, with the 'normal' fat concentrations expected in a municipal effluent, the effect on the COD/VSS ratio should be negligible for sludge ages of 3 days and longer between -1 temperatures of 12Ā°C and 20Ā°C; a COD/VSS ratio of 1,48 mgCOD.mgVSS, as proposed by Ekama and Marais (1978) appears to be acceptable for the design of activated sludge processes at 3 days sludge age or longer and temperatures ranging from 12Ā°C to 20Ā°C.
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    D.O. control and O.U.R. estimation in the activated sludge process
    (1987) Bocken, Stephan; Dold, Peter Lorimer; Braae, Martin
    The objective of this investigation was to evaluate Holmberg's method for simultaneous dissolved oxygen control and parameter estimation in a completely mixed diffused air activated sludge reactor.
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    Design and control of equalization tanks.
    (1982) Dold, Peter Lorimer; Marais, Gerrit van Rooyen
    The objective of this investigation was to develop a control strategy for the operation of an equalization tank upstream of a waste water treatment plant which utilizes the available equalization hold-up volume in such a manner that it reduces, optimally, diurnal fluctuations in both influent flow and load rates. The influent to a wastewater treatment plant generally exhibits wide diurnal variations in both flow rate and concentration, and consequently in load rate (defined as the product of flow rate and concentration). Deviations of these parameters from steady state cause plant operating problems in areas such as aeration control (due to load rate fluctuations) settling tank overloading due to flow rate fluctuations) and/or over- or under-aeration which affects settling properties, and others.
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    Modelling techniques for biological reaction systems: 1 Mathematical description and model representation
    (Water Research Commission, 1988) Billing, Alison Emslie; Dold, Peter Lorimer
    This paper is the first in a series of three which deals with modelling and numeric techniques for biological reaction systems. A matrix forĀ­ ma rov1des a usef\11. method for model presentation. The matrix ensures clarity as to the compounds, processes, reaction terms and s01chio.metry compnsmg the model. It allows ready comparison of different models and facilitates incorporating the model in a computer slffiulauon program.
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    Modelling techniques for biological systems
    (1987) Billing, Alison Emslie; Dold, Peter Lorimer
    The objective of this investigation has been to develop and evaluate techniques which are appropriate to the modelling and simulation of biological reaction system behaviour. The model used as the basis for analysis of modelling and simulation techniques is a reduced version of the biological model proposed by the IAWPRC Task Group for mathematical modell ing in wastewater treatment design. This limited model has the advantage of being easily manageable in terms of analysis and presentation of the simulation techniQues whilst at the same time incorporating a range of features encountered with biological growth applications in general. Because a model may incorporate a number of different components and large number of biological conversion processes, a convenient method of presentation was found to be a matrix format. The matrix representation ensures clarity as to what compounds, processes and react ion terms are to be incorporated and allows easy comparison of different models. In addition, it facilitates transforming the model into a computer program. Simulation of the system response first involves specifying the reactor configuration and flow patterns. With this information fixed, mass balances for each compound in each reactor can be completed. These mass balances constitute a set of simultaneous non-linear differential and algebraic eQuations which, when solved, characterise the system behaviour.