1. Co-Digestion of Biodegradable Municipal Waste with Agricultural Residues - a Developing Concept and Market
Tahir Mahmood a*, Zaki-ul-Zaman Asam b, Abdul-Sattar Nizami c
a* Norwegian University of Life Sciences (UMB), Norway. Phone:
b National University of Ireland Galway (NUI), University Rd. Galway, Ireland
c Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada
INTRODUCTION
Biodegradable municipal waste mainly consists of food and garden waste from domestic, commercial and street cleanings. A significant portion of this waste has a dry solids (DS) content of 20-30%. It is the main cause of the smell and nuisance of municipal solid waste (MSW) and is responsible for most of the environmental hazards associated with municipal waste management, such as the formation of polluting leachate and methane gas under anaerobic conditions. Anaerobic digestion (AD) is a promising technology for the management of such wastes with bioenergy recovery. But, problems with long term continuous mono-digestion of biodegradable municipal waste have been reported. Thus for long term operation, there is a need to co-digest the biodegradable municipal waste with agricultural residues like crop residues and grass silage. Co-digestion can lead to benefits such as increased buffering capacity and microbiological stability. It will also provide an approach to improve the rural economy and environment.
CONCEPT
Anaerobic digestion (AD) is a process where organic waste and lignocellulosic biomass are converted into biogas as a biofuel and digestate for value added products (Figure 1).
The organic wastes include slaughterhouse waste, agricultural residues and the organic fraction of municipal solid waste (OFMSW).
The biodegradable municipal waste and agricultural residues are the base substrates, which are most often utilized and available for AD process.
The inter relationship between the components of anaerobic digester inputs and of outputs are highlighted in Figure 2.
Problems with Mono-Digestion of Feedstock
The long term mono-digestion results in low C:N ratios, which lead to increased levels of ammonia within the digester. This inhibitor (ammonia) reduces biogas yields.
The trace elements such as molybdenum and cobalt are reduced in long term operation, which lead to failure of AD process.
Benefits of Co-Digestion
The digestion process is improved, thus the biogas rates.
The micro-biological stability and buffering capacity is increased.
The inhibitors to the digestion process such as ammonia and H2S is lowered.
The co-digestion guide to scale up the AD process from laboratory scale to farm scale and to industrial scale.
The biogas production per unit mass is maximized.
The deficiency of nitrogen in the process is reduced.
The creation of opportunities for the use of biodegradable wastes.
The nutrient deficiency of the AD process is reduced.
It will improve the rural economy and environment.
Factors Determining Biogas Yield of an Anaerobic Digester *
Figure 2
A Way Forward in Co-Digestion
Hydrolysis with co-digestion of biodegradable municipal waste with agricultural residues should be investigated.
There should be a buildup of data on feed ratio for the various substrates. This data will reduce the chances of AD process failure. *
AS Nizami, NE Korres, JD Murphy. 2009. A review of the integrated process for the production of grass biomethane. Environmental Science and Technology, 43(22), 8496–8508.
Future Vision
Figure 1
Biodegradable Municipal Waste and Agricultural Residues
Anaerobic Digestion (AD) process
Biofuels and Bioproducts