Anaerobic Digestion

Technologies: Anaerobic Digestion, Bioethanol Production, Incineration

Introduction

Anaerobic digestion is the process of organic molecules being broken down in the absence of oxygen by various bacteria to simpler compounds with a resultant production of several common gases throughout the process.  As anaerobic digestion takes place in the presence of biodegradable material, it has been known for some time that the process is capable of breaking down organic municipal waste.  Towards the end of the process the main gas being produced is methane which allows the possibility of methane capture and subsequent use as a biofuel which can be used to generate power reducing the dependence upon fossil fuels.  The use of anaerobic digestion as a solution to waste disposal can be implemented from existing landfill sites which account for a high proportion of existing total methane release or as newly built reactors designed to maximise methane yields.     

The Process

The process begins with the seperation of organic and non-organic municipal waste.  The waste initially requires shredding, water addition, screening and pasteurisation before the waste is then biologically treated (1).

There are four main biological stages associated with the process of anaerobic digestion, these are:

·        Hydrylosis - Break down of proteins carbohydrates and fats by enzymes produced by hydrolytic bacteria, the products of this first process are sugars, fatty acids and amino acids

·    Acidogenesis - Decomposition and fermentation of sugars by acidogens produces carbon dioxide, hydrogen, ammonia and organic acids, gas levels are at about 80% carbon dioxide 20% hydrogen

·    Acetogenesis - Digestion of remaining molecules by acetogens to produce acetic acid, hydrogen and carbon diozide

·    Methanogenesis - Utilization of the remainaing products by methanogens to produce a mixture of methane (60%) and carbon dioxide (40%) [2]

Application of Anaerobic Digestion

There  are numerous journal articles on the topic of anaerobic digestion as a treatment to municipal waste.  The following information is an overview of the current research concerning anaerobic digestion.

There are several variables in the process of anaerobic digestion, different bacteria within the reactor require different temperature ranges producing mesophilic and thermophilic digestion, mesophilic digestion occurs at a temperature of 35C whereas thermophilic digestion occurs at 55-60C.  Conventional reactors use mesophilic digestion however thermophilic digestion provides faster reaction times, increased gas yield and higher pathogen removal.  Further research will be required to compare thermophilic and mesophilic gas yields and required energy input. [2] 

Anaerobic digestion can also be classified as 'wet' or 'dry' depending upon the fraction of dry matter within the waste.  It is generally accepted that dry systems are more flexible and reliable than wet, the bacteria involved in the anaerobic digestion process is sensitive and therefore susceptible to overloading and high toxicity levels which can occur under overly wet conditions. [3]

The efficiencies of the anaerobic digestion process vary greatly depending upon the type of waste being input to the digestion process.  From past research it has been observed that the methane yield and biodegradation vary signifcantly from three experimental waste materials.  The materials being compared were, food waste (FW), shredded organic fraction of municipal waste (SOFMW) and organic fraction of municipal waste (OFMW).  The methane yield of the food waste was found to be over three times that of either the organic waste and the shredded organic waste which produced similar values.  The values of biodegradability for each waste stock however showed the organic matter removal of the OFMW was 80% with SOFMW at 62% and FW with 16%.  Therefore the digestion of food waste produces significantly more methane than general organic waste however very little of the total solids degrade producing a substantial volume of digestate as a by-product. [4]      

Current Research

There  are numerous journal articles on the topic of anaerobic digestion as a treatment to municipal waste.  The following information is an overview of the current research concerning anaerobic digestion.

There are several variables in the process of anaerobic digestion, different bacteria within the reactor require different temperature ranges producing mesophilic and thermophilic digestion, mesophilic digestion occurs at a temperature of 35C whereas thermophilic digestion occurs at 55-60C.  Conventional reactors use mesophilic digestion however thermophilic digestion provides faster reaction times, increased gas yield and higher pathogen removal.  Further research will be required to compare thermophilic and mesophilic gas yields and required energy input. [2] 

Anaerobic digestion can also be classified as 'wet' or 'dry' depending upon the fraction of dry matter within the waste.  It is generally accepted that dry systems are more flexible and reliable than wet, the bacteria involved in the anaerobic digestion process is sensitive and therefore susceptible to overloading and high toxicity levels which can occur under overly wet conditions. [3]

The efficiencies of the anaerobic digestion process vary greatly depending upon the type of waste being input to the digestion process.  From past research it has been observed that the methane yield and biodegradation vary signifcantly from three experimental waste materials.  The materials being compared were, food waste (FW), shredded organic fraction of municipal waste (SOFMW) and organic fraction of municipal waste (OFMW).  The methane yield of the food waste was found to be over three times that of either the organic waste and the shredded organic waste which produced similar values.  The values of biodegradability for each waste stock however showed the organic matter removal of the OFMW was 80% with SOFMW at 62% and FW with 16%.  Therefore the digestion of food waste produces significantly more methane than general organic waste however very little of the total solids degrade producing a substantial volume of digestate as a by-product. [4]      

References
1-Waste, Advisers on Urban Environment and Development (2005) Fact Sheet Anaerobic Digestion; http://www.waste.nl/page/248
2-Thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste
3-Renewable methane from anaerobic digestion of biomass
4-Scale-up of anaerobic digestion of the biowaste fraction from domestic wastes



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