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Chapter 2: Literature Review
Overview of Anaerobic Digestion (AD)
Anaerobic digestion (AD) of waste water is a widely practiced technique due to its sustainability aspects. Anaerobic wastewater treatment systems are biological systems operating in the absence of oxygen. AD is used for the stabilization of particulate organic matter and production of methane and carbon dioxide rich biogas that can be used as a source of green energy. The microbial communities in anaerobic operations are primarily prokaryotic, with members of both the bacteria and the archaea being involved. Thus the emphasis here would be on the complex and important interactions between the bacteria and the archaea that are fundamental to the successful functioning of the methanogenic communities.There are a variety of metabolic processes, inhibitory compounds, hydrolytic activities and syntrophic associations that occur during anaerobic digestion. In general, the progression of anaerobic degradation of organic matter moves through four principal reaction steps, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis, acidogenesis and acetogenesis are carried out by members of the domain bacteria while methanogenesis is carried out by methanogens, which are members of the domain archaea.The digestion process begins with bacterial hydrolysis of the input particulate materials in order to break down insoluble organic polymers such as carbohydrates, proteins and lipids into sugars, amino acids and long chain fatty acids respectively, and make them available for other bacteria. Acidogenic bacteria then convert the sugars, amino acids and long chain fatty acids into carbon dioxide, hydrogen, ammonia, and organic acids.Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. The production of hydrogen is very critical because it serves as one of the primary substrates for the production of methane (Grady et al., 1999). The final stage, methanogenesis results in the formation of methane (CH4). Two types of methanogenic archaea are involved in this process. Aceticlastic methanogens split acetic acid into methane and carbon dioxide. Hydrogen oxidizing archaea use hydrogen as an electron donor and carbon dioxide as an electron acceptor to produce methane (Grady et al., 1999, Appels et al., 2008). The multistep nature of anaerobic biochemical operations is depicted in Figure 2-1.Various factors govern the performance of the digester. These are pH, temperature, solids retention time (SRT), alkalinity, biogas production, accumulation of volatile fatty acids (VFAs), organic loading rate, total hydraulic loading and level of xenobiotic compounds.The optimum pH for anaerobic digestion is 6.8 – 7.4 (Grady et al., 1999). The methanogenic activity will slow down considerably with pH less than 6.3 and higher than 7.8 and this will reduce the biogas production (Leitao et al., 2006). A lower pH will result in the growth of filamentous bacteria and a high pH results in buildup of unionized ammonia (Grady et al., 1999).
ABSTRACT
ACKNOWLEDGEMENTS
TABLE OF CONTENTS
LIST OF TABLES AND FIGURES
CHAPTER 1: INTRODUCTION
CHAPTER 2: LITERATURE REVIEW
2.1 Overview of Anaerobic Digestion (AD)
2.2 Role of Cations, especially Iron, Magnesium and Aluminum
2.3 Odors from Dewatered Sludge Cakes
2.4 Phosphorus Removal and Recycling as Struvite
2.4.1 Phosphorus Removal from Wastewater
2.4.2 Struvite Precipitation Issues in Anaerobic Digestion
2.4.3 Interests in Controlling and Recovering Phosphorus as Struvite (Corre et al., 2009)
2.5 Iron Addition for Phosphorus Removal
2.6 THIOGUARD® in Sanitary Treatment Plants and Collection Systems (THIOGUARD® White paper)
CHAPTER 3: MATERIALS AND METHODS
3.1 Production and Dosing of Primary Sludge and Waste Activated Sludge Blend
3.2 Anaerobic Digester Setup and Operation
3.3 Analytical Methods
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Results
4.1.1 Phase I
(a) pH
(b) Solids
(c) Alkalinity
(d) Odors
(e) Ions
4.1.2 Phase II
(a) pH
(b) Solids
(c) COD
(d) Interactions Between the Above Parameters
(e) Alkalinity
(f) VFA
(g) Specific Gas Production
(h) Solution Ions
(i) Odors, Dewatering and Cake Solids
(j) Chemical Precipitation Experiment
4.2 Discussion.
4.3 Proposed Mechanisms of Mg(OH)2 Functioning
4.3.1 Dissolved Mg(OH)2 Can Neutralize the Volatile Fatty Acids Produced by the Acidogens, Resulting in a Better pH Environment for the Methanogens
(a) Increase in pH Leads to Enhanced Digestion Operability..
(b) Reduced Long Chain Fatty Acid Inhibition
4.3.2 Stimulatory Effects of Light Metal Cation Mg++
4.3.3 Phosphate Removal Efficiency and Struvite Formation Potential
CHAPTER 5: CONCLUSIONS
CHAPTER 6: ENGINEERING SIGNIFICANCE AND FUTURE RECOMMENDATIONS
REFERENCES
