Composting and Anaerobic Digestion: Promising Technologies for Organic Waste Management

Sourabh Manuja* Suneel Pandey*, Shabana Mehmood Kazi*, and Vaibhav Rathi#

*The Energy and Resources Institute, India#GIZ India

Organic waste which comprises nearly 52 percent of the nation’s municipal solid waste, poses around 15,065 Gg CO2 -eq of greenhouse gas emissions to our climate through disposal sites [1]. However, as per the Planning Commission’s report of 2014, the potential of our country to convert this waste to wealth is about 5.4 million tonnes of compost, and about 72 MW of electricity from biogas [2]. The two promising technologies identified under Nationally Appropriate Mitigation Action1 phase 1 project for India targeting organic waste are also bio-methanation and composting[3]. This paper further discusses the pros and cons of both aerobic composting and anaerobic digestion and also brings forth the economics involved with both technologies. The purpose of this paper is to educate stakeholders generating waste, particularly bulk waste generators, to choose the correct technology and benefit themselves, further making cities more efficient and sustainable.

Aerobic composting is a bio-oxidative process whereby a large portion of the degradable organic carbon is converted into carbon dioxide and water in the presence of oxygen [4]. The end product from the composting process is nutrient rich manure, which helps substitute chemical fertilizer [5]. On the other hand, anaerobic digestion is a fermentation process that breaks down organic matter in the absence of oxygen to produce biogas and a digestate. Biogas can be used as a fuel after purification, and the digestate, as manure. The extent of volume reduction is a function of the nature of the waste and compost time.

Composting is basically of three types (a) Windrow composting (b) Vermi composting and (c) In vessel composting. Depending on the type of environmental conditions and the quantity of waste to be handled, any of the above technologies can be adopted by a waste manager. Though the cost of windrow composting is the lowest, yet they show various drawbacks like uncontrolled and undesirable emissions and odours resulting from the lack of monitoring and process control. Vermi compost technology on other hand requires a necessary control on the environmental conditions for earthworms to survive and can only be practiced with limited quantities of waste. The third composting technology which provides a strong control on odour and undesirable emissions is, in vessel composting, which again has an investment involved in terms of capital and requires semi-skilled manpower to manage the waste effectively. Yet the reduction in volumes of municipal refuses have been reported as 30-35% and garden debris in range of 60-65% [6].

Further, though the advantages of converting waste to compost cannot be ruled out, yet another powerful technology which can even manage organic waste with high moisture content in a very effective way, is anaerobic digestion 1 Phase two of the NAMA project, titled “Pilot Implementation of Waste NAMA” is an extension of the project “Development and Management of NAMA in

India” where TERI as a partner of the implementing agency Deutsche Gesellschaft für International Zusammenarbeit (GIZ) GmbH India is facilitating the implementation of low carbon measures to reduce emissions from the solid waste sector in cities of Panaji and Varanasi. This project is part of the International Climate Initiative (IKI). The Federal Ministry for the Environment, Nature Conservation, and Nuclear Safety (BMU) supports this initiative on the basis of a decision adopted by the German Bundestag.

(also called bio methanation). The organic wastes are digested in a digester which reduces the volume of waste to be handled to about 90% and provides biogas, which is a renewable source of energy. Anaerobic digestion has several more merits: First, the emission of CO2 from anaerobic digestion tends to be 25% to 67% less than that from composting[7]; second, anaerobic digestion is better at treating waste with high moisture content than direct combustion and landfilling. Similarly, cooking oil is better treated in an anaerobic digestion process than through composting; third, anaerobic digestion requires less space than aerobic composting; fourth, the biogas generated from bio-methanation can be cleaned and used as a fuel source or for producing electricity in case no direct consumption is available. The digestate from anaerobic digestion contains about 7.6 kg per tonne nitrogen and 1.1 kg per tonne phosphorus (dry weight), which is not as effective a fertilizer as compost residue[8]. Whereas, after the composting process, compost can be used as fertilizer. Compost contains approximately 8.3 kilograms of nitrogen per dry tonne waste and 2.0 kilograms of phosphorus per dry tonne waste [9]. For farmlands that are depleted through agricultural practice over multiple years, compost with a large amount of organic matter is an ideal soil amendment.

For further understanding, readers are encouraged to read Table 1, which highlights the comparison between aerobic composting and anaerobic digestion in terms of actual processes, outputs and the resultant emissions.

Table 1: Considerations for composting and anaerobic digestion process

Parameter Composting Anaerobic Digestion (Bio methanation)

Process Decomposition of organic matter occurs in the presence of oxygen

Decomposition of organic matter occurs in an enclosed structure without oxygen

Output High quality compost/manure Gas plus nutrient rich liquid manure

GHG emissions Carbon dioxide and nitric oxide released into the atmosphere

Biogas is collected and purified for use as fuel. Methane converted to carbon dioxide is released into atmosphere.

Particle size Feed with optimum size of 25-75 mm Feed in the form of slurry or with an optimum size between 25-75 mm

Temperature Should be between 50-55 °C for first few days and between 55-60 °C for the rest of the days

Should be between 55-60 °C.

pH Above 8.5 Needs to be adjusted to about 6.7

Moisture content

Should be between 50-60%. Optimum value is about 55%.

Optimum moisture content to be 70%

C/N Ratio Between 30/1–50/1 for efficient composting

Optimum ratio should be 20/1–30/1

Figure 1: Pit composting

Contd...

Parameter Composting Anaerobic Digestion (Bio methanation)

Advantages • Lower initial capital investment • Requires semi-skilled manpower• It produces a solid output only

called manure which is rich in phosphorous and nitrogen

• Shorter period for ROI (return on investment.) with technologies yielding liquid fertilizer and biogas

• Biogas produced can be used for cooking or production of electricity

• It also produces nutrient dense liquid manure.• Provides good odour control

Disadvantages • Requires more landspace• It is necessary to pasteurize the

composted material to ensure that all infectious agents have been effectively removed

• Composting requires quite large energy inputs to fuel and operation of the equipment needed to aerate and turn the compost piles

• If run inefficiently composting can cause an odour nuisance

• High initial investment cost• Semiskilled / skilled manpower is required for

operations• It is not suitable for treating mixed municipal

solid waste • If run inefficiently, anaerobic digestion can cause

an odour nuisance• Requires less land space compared to large scale

composting units

Another important aspect in choosing the technology for handling organic waste is Environmentally Sound Technologies (EST). EST for organic waste refers to cost effective and energy efficient technologies, which do not pollute the ecosystem’s vital components such as air, land or water. Any EST can be selected based on the following general criteria like affordability, validity and sustainability.

For better understanding of readers, the section below highlights a comparison between anaerobic digestion and composting being carried out in the jurisdiction of the Corporation of the City of Panaji (CCP).

CCP operates around 30 pit composting units that handle about 750 kgs of waste every day (25 kg/pit). Six of these units (handling 150 kgs of waste per day) are compared to a bio-digestor (Rhino technology) of 150 kg/day capacity and comparisons have been illustrated in Table 2 below.

Table 2 : Comparative analysis of decentralized composting vs. decentralized bio-digestion

S.No Headers Cost per year2

Pit Composting Bio-digester (Rhino) Bio-digester (Rhino)

Scenario 1 - with sale of liquid fertiliser

Scenario 2- without sale of liquid fertiliser

1 Input Feed 150 kg/day 150 kg/day 150 kg/day

2 Capital Cost Rs. 3,30,000 Rs. 10,00,000 Rs. 10,00,000

3 Space 36 sq.m 22.5 sq.m 22.5 sq.m

4 O&M Total Cost Rs. 5,26,147 Rs. 1,93,180 Rs. 1,93,180

4.1 O&M Rs 87,600 Rs. 1,00,000 Rs. 1,00,000

4.2 Manpower Rs 4,10,625 Rs. 65,8053 Rs. 65,8052

4.3 Transportation Rs. 54750 Nil Nil

2 Excluding depreciation of 40% per year allowed (by IT department) on solid waste recycling and resource recovery systems3 Manpower @120Rs/hr and 1.5hr requirement per day

4 Considering a cylinder of 19kgs for about 1152 Rs each.

In case increased capacities are considered, decentralized composting will require more manpower depending on number of pits to be handled, whereas digesters will not require manpower proportionately.

If we also consider depreciation allowance of 40% per year (as stated by Income Tax department), the net incomes will go up due to income tax benefits. The graphs (Figure 2,3, and 4) depict the cumulative income and cumulative cost lines for the two technologies over a life span of 15 years.

Figure 2 depicts that the breakthrough point on investment and income will never be reached with the composting technology.

Figure 3 depicts that if gas is saved and liquid fertiliser is sold, the breakthrough point will be achieved after the first year itself. Over a life of 15 years cumulatively, the plant can save a cost of around Rs 65.15 lakhs.

S.No Headers Cost per year2

Pit Composting Bio-digester (Rhino) Bio-digester (Rhino)

Scenario 1 - with sale of liquid fertiliser

Scenario 2- without sale of liquid fertiliser

4.4 Input Power None Rs. 27,375 per year (Considering 15 kW-hr/day)

Rs. 27,375 per year (Considering 15 kW-hr/day)

5 Annual Net Savings (Revenue-O&M)

Rs -4,93,747 Rs 4,81,020 Rs 1,57,020

5.1 Finished Compost/ Fertilizer

10,800 kgs manure /year

32.4 Kiloliters Liquid fertilizer/year

32.4 Kiloliters Liquid fertilizer/year

5.2 Output Gas - 2463750 kg methane/year 2463750 kg methane/year

5.3 Revenues Rs 32,400 per year

Rs. 3,50,200 per year (304 commercial cylinders) from gas 4+ Rs 3,24,000 per year from liquid fertilizer

Rs. 3,50,200 per year (304 commercial cylinders) from gas

6 GHG emission reduction

23.025 MT CO2 -eq/year

30.135 MT CO2-eq/year 30.135 MT CO2-eq/year

*Can be even 1.6 times with only food waste [10]

Note: The cost calculations over here are based on estimates provided by the user and technology supplier and may vary from case to case based on site conditions. All the data here has been provided and verified by CCP and the Rhino Digester technology provider for their respective technologies. CCP area has 04 operational units of Rhino Digester from the past 4-5 months in various hotels and housing complexes. Hence a realistic data set (financial and technical) in context of Goa was available.

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Figure 2: Investment and savings over years from Pit Composting units (including tax benefit)

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Figure 3: Investment and savings over years from Rhino Digester (scenario 1) (including tax benefit)

Scenario 2 (Figure 4) depicts that in case liquid fertiliser is not sold by the plant, the breakthrough point will be reached in a tenure of around 4-5 years, and the plant will reap a profit of around Rs 16.5 lakhs in 15 years time period.

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Figure 4: Investment and savings over years from Rhino Digester (scenario 2) (including tax benefit)

ConclusionIt is well understood here from the above graphs that though the capital cost of bio-methanation process is high compared to composting process, the yield of liquid fertilizer and biogas will make the investor reap out its cost within a tenure of around 1-5 years. The added cost of transportation of waste would also be reduced. Further, the reduction in CO2-eq emissions from bio-methanation are more compared to composting, thus suggesting the technology to be more environmentally sound.

The environmental benefits of both technologies can not be overlooked and the choice of selecting the technology will completely lie on factors like availability of space, as well as options for utilisation of generated gas as heat source. In case there is uncertainty in the quantity of waste to be available for treatment, composting shall be a feasible technology to manage organic waste. Further, we should consider that as a first priority there is no wastage of food, secondly the wasted food is utilised for feeding the hungry, third priority should be to feed this wasted organics and food to animals, as a fourth option the organic waste generated from bulk waste generators should be composted or digested to harness energy.

Moreover, estimates suggest that biomethanation reduces 35.22% more GHG emissions compared to composting.Net GHGs reduced from bio-methanation plant of 150 kg per day capacity are about 31.135 metric tonnes CO2-eq per year. Bulk waste generators have a very important role to play in terms of not only helping the city administration in managing the waste generated, but also in reducing the emissions released during management of waste.

References [1] Second Biennial Update Report to the United Nations Framework Convention on Climate Change, Ministry of

Environment, Forest and Climate Change, December 2018. [2] Report of the Task Force on Waste to Energy, Volume I, Planning Commission, May 2014.[3] Feasibility Study for a Waste NAMA in India, GIZ-India, June 2015.[4] Berger, J., Fornés, L. V., Ott, C., Jager, J., Wawra, B., & Zanke, U. (2005). Methane oxidation in a landfill cover with

capillary barrier. Waste Management, 25(4), 369-373.

[5] Thompson, A. G., Wagner-Riddle, C., & Fleming, R. (2004). Emissions of N2O and CH4 during the composting of liquid swine manure. Environmental Monitoring and Assessment, 91(1-3), 87-104.

[6] Wilson, David Gordin (1977) Handbook of Solid Waste Management. USA, Van Nostrand Reinhold Company.[7] Mata-Alvarez, J., Mace, S., & Llabres, P. (2000). Anaerobic digestion of organic solid wastes. An overview of

research achievements and perspectives. Bioresource technology, 74(1), 3-16. [8] Finnveden, G., Johansson, J., Lind, P., & Moberg, Å. (2005). Life cycle assessment of energy from solid waste—

part 1: general methodology and results. Journal of Cleaner Production, 13(3), 213-229.[9] Moberg, Å., Finnveden, G., Johansson, J., & Lind, P. (2005). Life cycle assessment of energy from solid waste—

part 2: landfilling compared to other treatment methods. Journal of Cleaner Production, 13(3), 231-240. [10] Kigozi, R., Aboyade, A., & Muzenda, E. (2013). Biogas production using the organic fraction of municipal solid

waste as feedstock.

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As at February 2020

GIZ is responsible for the content of this publication On behalf of Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)

This paper has been developed in association with the Corporation of the City of Panaji.