In the following:
This paper considers the self-heating process which occurs within a compost pile using a one-dimensional spatially-dependent model incorporating terms that account for self-heating due to both biological and oxidation mechanisms. As the moisture content within a compost pile and the air flow through it are the two crucial factors in the degradation process, we use a model which incorporates four mass-balance equations, namely, energy, oxygen, vapour and liquid water concentrations, to investigate the behaviour of a compost pile when these two factors interact. Analyses of different initial water contents and air-flow velocities within a compost pile show that they can determine the efficiency of the biodegradation process. For an intermediate water content range and air-flow rate, the biological reaction is shown to be at its optimal value but there is also a possibility of spontaneous ignition within the compost pile.
T. Luangwilaip, H.S. Sidhu, and M.I. Nelson. One-dimensional spatial model for self-heating in compost piles: Investigating effects of moisture and air flow. Food and Bioproducts Processing, 108: 18--26, 2018. http://dx.doi.org/10.1016/j.fbp.2017.12.001.
The activated sludge process was discovered by Ardern and Lockett in the years 1913--1914. In the slightly more than 100 years since its discovery, it has become the most widely used process for the biological treatment of both domestic and industrial wastewaters in developed and developing countries. At its most basic, the process consists of an aerated reactor basin connected to a settling unit. The effluent stream leaving the reactor enters the settling unit where particulate matter settles under the action of gravity to the bottom of the unit. From here, it can be recycled into the reactor unit. The recycling of particulate matter is the key to improving the efficiency of the process, as enmeshed within it are micro-organisms. This particulate matter is known as sludge and consequently sludge is good. However, too much sludge is bad; disposal of excess sludge can account for between 50 and 60% of the typical operating costs of the activated sludge process. This chapter provides a historical overview of the activated sludge process and two methods for reducing the amount of sludge: disintegration through the use of a sludge disintegration unit and a biological approach based upon the use of predators that graze upon the sludge.
M.I. Nelson. Reducing sludge formation in the activated sludge process. In R.S. Anderssen, P. Broadbridge, Y. Fukumoto, K. Kajiwara, M. Simpson, and I. Turner, editors, Agriculture as a Metaphor for Creativity in All Human Endeavors, pages 53--66. Springer, Singapore, 2018. "https://doi.org/10.1007/978-981-10-7811-8_7.
Alcohol based biofuels, such as bio-butanol, have considerable potential to reduce the demand for petrochemical fuels. However, one of the main obstacles to the commercial development of biological based production processes of biofuels is end-product toxicity to the biocatalyst. We investigate the effect of end-product toxicity upon the steady-state production of a biofuel produced through the growth of microorganisms in a continuous flow bioreactor. The novelty of the model formulation is that the product is assumed to be toxic to the biomass. The increase in the per-capita decay rate due to the presence of the product is assumed to be proportional to the the concentration of the product. The steady-state solutions for the model are obtained, and their stability determined as a function of the residence time. These solutions are used to investigate how the maximum yield and the reactor productivity depend upon system parameters. Unlike systems which do not exhibit toxicity there is a value of the feed concentration which maximises the product yield. The maximum reactor productivity is shown to be a sharply decreasing function of both the feed concentration and the toxicity parameter. In conclusion, alternative reactor configurations are required to reduce the effects of highly toxic products.
M.I. Nelson. A mathematical model for end-product toxicity. Chemical Product and Process Modelling, Volume 13, Number 3, 2018. 2018. http://dx.doi.org/10.1515/cppm-2017-0061.
Before wastewaters can be released into the environment, they must be treated to reduce the concentration of organic pollutants in the effluent stream. There is a growing concern as to whether wastewater treatment plants are able to effectively reduce the concentration of micropollutants that are also contained in their influent streams. We investigate the removal of micropollutants in treatment plants by analysing a model that includes biodegradation and sorption as the main mechanisms of micropollutant removal. For the latter a linear adsorption model is used in which adsorption only occurs onto particulates.
The steady-state solutions of the model are found and their stability is determined as a function of the residence time. In the limit of infinite residence time, we show that the removal of biodegradable micropollutants is independent of the processes of adsorption and desorption. The limiting concentration can be decreased by increasing the concentration of growth-related macropollutants. Although, in principle, it is possible that the concentration of micropollutants is minimized at a finite value of the residence time, this was found not to be the case for the particular biodegradable micropollutants considered.
For nonbiodegradable pollutants, we show that their removal is always optimized at a finite value of the residence time. For finite values of the residence time, we obtain a simple condition which identifies whether biodegradation is more or less efficient than adsorption as a removal mechanism. Surprisingly, we find that, for the micropollutants considered, adsorption is always more important than biodegradation, even when the micropollutant is classified as being highly biodegradable with low adsorption.
Mark I. Nelson, Rubayyi T. Alqahtani and Faisal I. Hai. Mathematical modelling of the removal of organic micropollutants in the activated sludge process: a linear biodegradation model. The ANZIAM Journal, 60(2), 191-229, 2018. https://doi.org/10.1017/S1446181118000226.
Fires at industrial composting facilities, particularly those storing industrial waste products such as municipal solid waste and landfills, are fairly common. Although, in most cases, they are manageable and not sufficiently destructive to attract attention beyond these facilities, over the years, there have been a few notably devastating ones.
Industrial compost piles contain large volumes of bulk organic materials and, typically, two sources of heat generation, the oxidation of cellulosic materials and heat release due to biological activity. Biological heating, which occurs at lower temperatures than oxidation, may increase the temperature sufficiently to kick-start the oxidation reaction and is known to be important in large-scale composting operations.
This paper considers the self-heating process that occurs in a compost pile using a one-dimensional spatially- dependent model incorporating terms that account for self-heating due to both biological and oxidative mechanisms. As the moisture content in a compost pile is a crucial factor in its degradation process, four mass-balance equations, namely, energy, oxygen, vapour and liquid water concentrations, are included to investigate the behaviours of compost piles containing moisture.
The results of a detailed examination of the effects of the initial water content in a compost pile and relative humidity when an air flow is present are discussed. The optimal conditions for the biodegradation process and the regions in the compost pile which may possibly spontaneously ignite are identified.
T. Luangwilai, H.S. Sidhu and M.I. Nelson. Understanding the effects of ambient humidity on the self-heating of compost piles. In, Proceedings of the Australasian Chemical Engineering Conference, Chemeca 2018, Engineers Australia, 2018.