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Anaerobic Co-digestion as a Smart Approach for Enhanced Biogas Production and Simultaneous Treatment of Different Wastes

S. Bharathi and B. J. Yogesh

Bangalore University, The Oxford College of Science, Department of Microbiology, Sector 4, H.S.R Layout, Bangalore, 560102, India

1.1 Introduction

The world has witnessed tremendous growth over the past hundred years fueled by richness of earth's natural resources, but now we stare at the bleak prospects of exhaustion due to overutilization. With future economies balanced precariously on cost of fuel, with increasing demand for energy, ever-increasing annual fuel consumption, limited natural resources, volatility and disruption in fossil energy supplies, need of clean technologies has certainly driven us toward a pragmatic approach for optimized and proper use of natural resource for a sustainable ecosystem. Insightful planning and innovative methods are essential to enhance energy production in order to meet surge in future energy demands. Another scourge of the modern society is waste management; especially in the developing economies punctuated by improvement in individual purchase parity, it has led to tripling of waste generation per person just over the last one decade. An attempt is made in this chapter to link these two possible issues of fuel generation and waste management through a biotechnological intervention. The era of biotechnology as a futuristic technology strives to tap the service of the potential saprophytic microbes, which not only hastens the recycling of dead organic matter but can provide the fuel for running the future economy.

1.1.1 Biodegradation - Nature's Art of Recycling

The elemental components of our periodic table have finely blended the earth into molecules of infinite diversity. The organic forms of molecules are the basis of life existence in which the principal elements carbon, hydrogen, nitrogen, and oxygen have a subtle role in the formation of living system. The photosynthetic forms of life are one of the biggest producers of the organic matter, and it comes with an inherent clause of undergoing natural degradation over a period of time. This biodegradation is a very important invention of the nature, for, without recycling, a continuous existence of new life over millions of years would have been impossible. Microorganisms play a pivotal role in this process of biodegradation, without it recycling would have been unimaginably slower.

1.1.2 Anaerobic Digestion (AD)

Naturally existing anaerobic ecosystems such as paddy fields, swamps, lakes, ponds, intestine of ruminants, and ocean sediments rich in dead organic matter have paved way for microbes especially the archaeal obligate anaerobes-methanogens, mutual togetherness with other prokaryotic anaerobe leading to the production of methane. Though it can be attributed as a natural process, it leads to release of methane, a potential greenhouse gas capable of global warming far many times higher than carbon dioxide (CO2). Anaerobic digestion (AD) as a technology refers to a provision of a closed condition for efficient digestion of the organic waste and to collect the by-product, methane.

The benefits of AD are immense for both the economy and ecosystem:

• Firstly, the digestion takes place in a closed environment, thereby preventing air pollution from obnoxious gases or disease-spreading germs.
• There is no issue of leachate escaping into water bodies and thus prevents open water body pollution.
• No underground seepage and pollution of groundwater.
• Faster degradation of organic matter compared with composting (aerobic).
• The AD process can be easily monitored circumventing the problems, for example, seasonal variation in temperatures.
• A microbial consortium can be developed, and it would aid in continuous and efficient digestion of waste.
• Biogas production with a range of fuel applications.
• Downstream processing is not required as biogas collects in the head space and is siphoned off for clarification and usage.
• Further effluent treatment would not be necessary as the slurry can be used as organic manure.
• Pathogens are inactivated, thus rendering the digestate harmless and safe.

The drawbacks are few, but critical enough to be highlighted:

• Limited access to high-quality feedstock that is free of contamination
• Non-perennial aspects of feedstock
• Transportation costs
• Long-term sustainable biomethanation
• Unexpected digester failures
• Maintenance of high fuel quality
• Issues of multistakeholders (in case of co-digestion)

The first four issues are related to feedstocks and its management, while the last three issues are related to lack of good microbial inoculum. Thus in this chapter, these two aspects of feedstock and real-time monitoring of operational parameters are dealt in detail.

Technical issues could be overcome by reliable public-private partnership, government initiatives, financial supports followed by technological advancement.

1.1.3 Sustainable Biomethanation

Sewage water treatment plants mandatorily follow AD for sludge treatment, and the ensuing methane-based gas is used for running wastewater treatment plants (WWTPs), though this is in principle, but the scenario is that many WWTPs struggle to maintain sustainable digesters, which are progressively jeopardized by frequent reactor failures. Biogas plants were ideally found to be an alternate source for renewable energy and were operated widely in rural areas of India; however, over the last few decades, it has taken a back seat, partially attributed to:

• digester operational instability,
• nonhomogeneous substrate,
• lack of good microbial inoculum,
• promotion and easier availability of LPG,
• deeper reach of electricity to remote rural areas,
• dip in active promotion of AD and their significance, especially in rural areas.

Renewed interest in AD stems from the problems of rapid urbanization and urgent need of waste management. Running successful biogas digesters depends mainly on two important factors: nature of substrate and the quality of inoculum. Real-time monitoring emphasizes on the following factors:

• balanced micro- and macronutrients,
• efficient microbial inoculum,
• digester design optimization,
• optimized organic loading rate (OLR),
• efficient monitoring of critical parameters (pH fluctuations, temperature range, total solids (TSs) utilization rate, volatile solids (VSs) accumulation and dispersal rates, microbial profiling: that is, eubacterial versus archaeal load ratio),
• continuous evaluation of digester performance [rate of biogas production, methane percentage, reduction in total solids, reduction in chemical oxygen demand (COD)],
• Reducing inhibitor concentrations.

1.2 Anaerobic Co-digestion (AcD)

Biogas technology is a perfect example to emphasize on zero waste concept, conversion of waste into fuel, and even the final digested remnant slurry's immense value as organic manure, which is potentially free of pathogens. Mono-digestion refers to the classical way for biogas production from a single type of feedstock while a co-digestion refers to mixing of two different feedstocks in a digester for biogas production. Co-digestion was initially planned to balance a carbon-to-nitrogen (C/N ratio) content of the feedstocks, as few feedstocks are either rich in carbon (agricultural) or found to be rich in nitrogen (animal waste). High C/N ratio of feedstock will ultimately lead to reduction in microbial load due to overall nitrogen deficiency while lower C/N can result in ammonia poisoning that could particularly affect methanogens leading to lower biogas production. Excess of carbohydrates in feedstocks needs shorter retention time (RT) in digesters attributed by its quick oxidation, while excess protein content leads to lesser biogas production ascribed to accumulation of toxic levels of ammonia; on the other hand, excess lipids though results in higher biogas production but RT nearly doubles [1] further characterized by high concentrations of volatile fatty acids (VFAs) and low pH, thus leading to a consensus that excess of any nutrient cannot be beneficial for biogas production [2]. The anaerobic co-digestion (AcD) thus offers an opportunity to modify the composition of the waste to our need that suits our microbial consortium very well, and in this regard, C/N ratio can be altered to the optimum range. WWTPs around the world have increasingly opted for co-digestion to increase biogas output, and a WWTP in Mesa, USA, has successfully evaluated co-digestion of commercial solid food waste with sewage sludge in pilot-scale anaerobic digesters [3]. Lipid-rich restaurant waste has been co-digested with sewage sludge [4].

1.2.1 Zero Waste to Zero Carbon Emission Technology

The biogas as renewable energy can contribute in a big way to meet an overzealous future goal of zero emission economy by supplying fuel to major contributors of...