Proceedings

Mixing optimization of gas-mixed anaerobic digestion using CFD

Dapelo, D.1 and Bridgeman, J.1

1Department of Civil Engineering, University of Birmingham, UK

(free)

Abstract

In this paper, an Euler-Lagrange model for computational fluid dynamics was employed to simulate a full-scale gas-mixed digester. A novel method to assess mixing quality was described. The method defines a passive tracer to assess the relative mixing quality of different system configurations by comparing the respective tracer distributions. The method was used to assess the effectiveness of four different mixing strategies. Better mixing quality and less mixing time can be achieved by switching biogas injection between two different nozzle series at regular time intervals, and smaller time intervals resulted to be more effective in reducing the mixing time.

Keywords

Anaerobic Digestion, CFD, Lagrangian, Mixing, Non-Newtonian, Sludge, Tracer

Introduction

Every day, over 10 billion litres of wastewater are treated in UK in more than 9,000 wastewater plants (WaterUK, 2012). A number of stages in the wastewater process result in sludge production: in 2010—2011, the wastewater plants in the UK produced about 1.5 million tonnes of sewage sludge (WaterUK, 2012). The whole wastewater treatment process, including sludge treatment and disposal, is an energy-intensive operation. Data returned by the EU Member States suggest energy consumption exceeds 23,800 GWh per annum, and further increases of 60% are forecast in the next 10-15 years, primarily due to tightened regulation of effluent discharges. Predictions show that by 2030 the world will have to produce 50% more food and energy and provide 30% more water, while mitigating and adapting to climate change. Therefore, the “explicit link between wastewater and energy” must be addressed.

Mesophilic anaerobic digestion is the most widespread technology for sludge treatment (Bridgeman, 2012). Sludge is mixed with anaerobic bacteria at temperatures between 22 and 41 ◦C, and biodegradable material is broken down into more stable compounds. One of the most interesting aspects of anaerobic digestion is that biogas, which is prevalently methane, is produced during the process. Biogas, in turn, is increasingly harnessed as a renewable energy by means of combined heat and power technology (Bridgeman, 2012). According to (Owen, 1982) mixing is responsible for about 17—73% of the total energy consumption of an industrial digester, and yet, current practice in digester design is still rooted in “rule of thumb rather than science” (Dapelo and Bridgeman, 2015). Therefore, the only practicable strategy to reduce the energy consumption of a digester consists of reducing the level of mixing without compromising, and indeed enhancing, biogas production.

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