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.