By Salman Zafar, Renewable Energy Expert

1. Introduction

The generation and disposal of organic waste without adequate treatment result in significant environmental pollution. Besides health concerns for the people in the vicinity of disposal sites, degradation of waste leads to uncontrolled release of greenhouse gases (GHGs) into the atmosphere. Conventional means, like aeration, is energy intensive, expensive and also generates a significant quantity of biological sludge. In this context, anaerobic digestion offers potential energy savings and is a more stable process for medium and high strength organic effluents. Waste-to-Energy (WTE) plants, based on anaerobic digestion of biomass, are highly efficient in harnessing the untapped renewable energy potential of organic waste by converting the biodegradable fraction of the waste into high calorific gases. Apart from treating the wastewater, the methane produced from the biogas facilities can be recovered, with relative ease, for electricity generation and industrial/domestic heating.

Anaerobic digestion plants not only decrease GHGs emission but also reduce dependence on fossil fuels for energy requirements. The anaerobic process has several advantages over other methods of waste treatment. Most significantly, it is able to accommodate relatively high rates of organic loading. With increasing use of anaerobic technology for treating various process streams, it is expected that industries would become more economically competitive because of their more judicious use of natural resources. Therefore, anaerobic digestion technology is almost certainly assured of increased usage in the future. Further, anaerobic digestion can play a crucial role in achieving three major international environmental policy objectives. These are:

• Kyoto Protocol / Global Warming
• Renewable Energy
• Water Pollution

2. Utilization of Biogas and Digestate

An anaerobic digestion plant produces two outputs, biogas and digestate, both can be further processed or utilised to produce secondary outputs. Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. A combined heat and power plant system (CHP) not only generates power but also produces heat for in-house requirements to maintain desired temperature level in the digester during cold season. CHP systems cover a range of technologies but indicative energy outputs per m3 of biogas are approximately 1.7 kWh electricity and 2.5kWh heat. The combined production of electricity and heat is highly desirable because it displaces non-renewable energy demand elsewhere and therefore reduces the amount of carbon dioxide released into the atmosphere.

In Sweden, the compressed biogas is used as a transportation fuel for cars and buses. Biogas can also be upgraded and used in gas supply networks. The use of biogas in solid oxide fuel cells is being researched.

The surplus heat energy generated may be utilized through a district heating network. Thus, there is potential scope for biogas facilities in the proximity of new housing and development areas, particularly if the waste management system could utilise kitchen and green waste from the housing as a supplement to other feed stock.

Digestate can be further processed to produce liquor and a fibrous material. The fiber, which can be processed into compost, is a bulky material with low levels of nutrients and can be used as a soil conditioner or a low level fertilizer. A high proportion of the nutrients remain in the liquor, which can be used as a liquid fertilizer.

3. Benefits of Anaerobic Digestion

Anaerobic digestion provides a variety of benefits. These may be classified into three groups viz. environmental, economic and energy benefits:

The environmental benefits include:

1. Elimination of malodorous compounds.
2. Reduction of pathogens.
3. Deactivation of weed seeds.
4. Production of sanitized compost.
5. Decrease in GHGs emission.
6. Reduced dependence on inorganic fertilizers by capture and reuse of nutrients.
7. Promotion of carbon sequestration
8. Beneficial reuse of recycled water
9. Protection of groundwater and surface water resources.
10. Improved social acceptance

Anaerobic digestion is advantageous in terms of energy in the following manner:

1. Anaerobic digestion is a net energy-producing process.
2. A biogas facility generates high-quality renewable fuel.
3. Surplus energy as electricity and heat is produced during anaerobic digestion of biomass.
4. Anaerobic digestion reduces reliance on energy imports.
5. Such a facility contributes to decentralized, distributed power systems.
6. Biogas is a rich source of electricity, heat, and transportation fuel.

The economic benefits associated with a biomass-to-biogas facility are:

1. Anaerobic digestion transforms waste liabilities into new profit centers.
2. The time devoted to moving, handling and processing manure is minimized.
3. Anaerobic digestion adds value to negative value feedstock.
4. Income can be obtained from the processing of waste (tipping fees), sale of organic fertilizer, carbon credits and sale of power.
5. Power tax credits may be obtained from each kWh of power produced.
6. A biomass-to-biogas facility reduces water consumption.
7. It reduces dependence on energy imports.
8. Anaerobic digestion plants increases self-sufficiency.

4. Feedstock for Anaerobic Digestion Plants

A wide range of feedstock is available for anaerobic digesters. In addition to MSW, large quantity of waste, in both solid and liquid forms, is generated by the industrial sector like breweries, sugar mills, distilleries, food-processing industries, tanneries, and paper and pulp industries. Out of the total pollution contributed by industrial sub-sectors, nearly 40% of the total organic pollution is contributed by the food products industry alone. Food products and agro-based industries together contribute 65% to 70% of the total industrial wastewater in terms of organic load. Poultry waste has the highest per tonne energy potential of electricity per tonne but livestock have the greatest potential for energy generation in the agricultural sector.

Most small-scale units such as tanneries, textile bleaching and dying, dairy, slaughterhouses cannot afford effluent treatment plants of their own because of economies of scale in pollution abatement. Recycling/recovery/re-use of products from the wastes of such small-scale units by adopting suitable technology could be a viable proposition. Generation of energy using anaerobic digestion process has proved to be economically attractive in many such cases. The urban municipal waste (both solid and liquid) – industrial waste coming from dairies, distilleries, pressmud, tanneries, pulp and paper, and food processing industries, etc., agro-waste and biomass in different forms – if treated properly, has a tremendous potential for energy generation. Fig 1 lists the possible feedstock for waste-to-energy plants based on anaerobic digestion of biomass.

Fig 1: Different feedstock for Anaerobic Digestion Plants

5. Anaerobic Digestion of Livestock Manure

The establishment of anaerobic digestion systems for livestock manure stabilization and energy production has accelerated substantially in the past several years. There are more than 111 digesters operating at commercial livestock facilities in the United States which generated around 215 million kWh equivalent of useable energy. Besides generating electricity (170 million kWh), some operations use the gas as a boiler fuel, some upgrade the gas for injection into the natural gas pipeline, and some flare gas for odor control. Many of the projects that generate electricity also capture waste heat for various in-house requirements.

A wide range of animal wastes can be used as sources of renewable energy. The most common are animal and poultry manures. In the past, this waste was recovered and sold as a fertilizer or simply spread onto agricultural land. The introduction of tighter environmental controls on odour and water pollution means that some form of waste management is necessary, which provides further incentives for biomass-to-energy conversion.

5.1 Important Factors to Consider

The main factors that influence biogas production from livestock manure are pH and temperature of the feedstock. It is well established that a biogas plant works optimally at neutral pH level and mesophilic temperature of around 35o C. Carbon-nitrogen ratio of the feed material is also an important factor and should be in the range of 20:1 to 30:1. Animal manure has a carbon - nitrogen ratio of 25:1 and is considered ideal for maximum gas production. Solid concentration in the feed material is also crucial to ensure sufficient gas production, as well as easy mixing and handling. Hydraulic retention time (HRT) is the most important factor in determining the volume of the digester which in turn determines the cost of the plant; the larger the retention period, higher the construction cost.

An emerging technological advance in anaerobic digestion that may lead to increased biogas yields is the use of ultrasound to increase volatile solids conversion. This process disintegrates solids in the influent, which increases surface area and, in turn, allows for efficient digestion of biodegradable waste.

5.2 Process Description of WTE Facility Based on Livestock Manure

The layout of a typical biogas facility using livestock manure as raw material is shown in Fig 2. The fresh animal manure is stored in a collection tank before its processing to the homogenization tank which is equipped with a mixer to facilitate homogenization of the waste stream. The uniformly mixed waste is passed through a macerator to obtain uniform particle size of 5-10 mm and pumped into suitable-capacity anaerobic digesters where stabilization of organic waste takes place.

In anaerobic digestion, organic material is converted to biogas by a series of bacteria groups into methane and carbon dioxide. The majority of commercially operating digesters are plug flow and complete-mix reactors operating at mesophilic temperatures. The type of digester used varies with the consistency and solids content of the feedstock, with capital investment factors and with the primary purpose of digestion.

Biogas contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H2S into elemental sulfur.

Gas is dried and vented into a CHP unit to a generator to produce electricity and heat. The size of the CHP system depends on the amount of biogas produced daily. The digested substrate is passed through screw presses for dewatering and then subjected to solar drying and conditioning to give high-quality organic fertilizer. The press water is treated in an effluent treatment plant based on activated sludge process which consists of an aeration tank and a secondary clarifier. The treated wastewater is recycled to meet in-house plant requirements. A chemical laboratory is necessary to continuously monitor important environmental parameters such as BOD, COD, VFA, pH, ammonia, C:N ratio at different locations for efficient and proper functioning of the process.

The continuous monitoring of the biogas plant is achieved by using a remote control system such as Supervisory Control and Data Acquisition (SCADA) system. This remote system facilitates immediate feedback and adjustment, which can result in energy savings.

Figure 2. Layout of Waste-to-Energy Plant based on Livestock Manure

6. CONCLUSIONS

Anaerobic digestion of biomass offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power. The growing use of digestion technology as a method to dispose off livestock manure has greatly reduced its environmental and economic impacts. Biomass-to-biogas transformation mitigates GHGs emission and harness the untapped potential of a variety of organic waste. Anaerobic digestion technology affords greater water quality benefits than standard slurry storage due to lower pollution potential. It also provides additional benefits in terms of meeting the targets under the Kyoto Protocol and other environmental legislations.

The livestock industry is a vitally important contributor to the economy of any country, regardless of the degree of industrialization. Animal manure is a valuable source of renewable energy; additionally, it has soil enhancement properties. Anaerobic digestion is a unique treatment solution for animal agriculture as it can deliver positive benefits related to multiple issues, including renewable energy, water pollution, and air emissions. Anaerobic digestion of animal manure is gaining popularity as a means to protect the environment and to recycle materials efficiently into the farming systems. There is an urgent need to integrate the digester with manure management systems for effective implementation of the anaerobic digestion technology to address associated environmental concerns and to harness renewable energy potential.