Renewable Energy

Biogas digestate: This high-value byproduct deserves more attention

Digestate is underutilised due to lack of regulatory guidelines, absence of fixed procurement prices and poor marketability by fertiliser companies

By Rahul Jain, Kaifee Jawed
Published: Monday 13 February 2023
Currently, India has the potential to generate 62 million metric tonnes of compressed biogas (CBG) and 370 million metric tonnes of bio manure per year from various feedstocks. Representative photo: iStock.

Biogas can help minimise our dependence on fossil fuels and reduce greenhouse gas emissions. It is produced by the anaerobic digestion of organic waste by microbes.

During the production of biogas, two products are mainly obtained: biogas and digestate. Biogas mainly comprises gases like 50-65 per cent of methane, 35-44 per cent carbon dioxide, 1-2.5 per cent hydrogen sulphides and traces of moisture.

Also read: Budget kick: Keep these factors in mind while setting up a Bio-CNG plant

Digestate amounts to approximately 20 per cent by weight of the initial input feedstock. 

Currently, India has the potential to generate 62 million metric tonnes of compressed biogas (CBG) and 370 million metric tonnes of bio manure per year from various feedstocks.

When it comes to the final utilisation of CBG, it has excellent market viability and is also promoted through various programmes like Sustainable Alternative Towards Affordable Transportation (SATAT), where oil marketing companies ensure the offtake of CBG.

As of October 2022, a total of 38 plants have been commissioned, according to the SATAT portal. These plants have sold about 9,019 tonnes of CBG, which clearly indicates its good marketability.

On the other hand, the end use and marketability of digestate pose a big challenge. Though digestate is rich in carbon and nitrogen and has the potential to improve soil health by improving water retention capacity and soil structure, it is underutilised due to lack of regulatory guidelines, absence of fixed procurement prices and poor marketability by fertiliser companies.

“One tonne of chemical fertiliser replaced with biogas digestate saves one tonne of oil, 108 tonnes of water and seven tonnes of CO2 emissions,” according to Anaerobic Digestion and Bioresources Association.

Also read: GOBAR-Dhan: Scheme announced in budget a welcome step, but challenges ahead

The Ministry of Agriculture and Farmers Welfare is the apex body for the formulation and administration of the rules and regulations related to agriculture and fertilisers.

On July 13, 2020, it issued an order designating biogas digestate as fermented organic manure (FOM)with the following standards:

Sl.No Parameters FOM
1 Moisture % by weight, maximum 30-40%
2 NPK  Nutrients- Total  N,  P2O5  and  K2O  nutrient  should  not  be less than 1.2%
3 Total Organic Carbon(minimum) 14%
4 C:N Ratio <20
5 Practical Size Minimum 90% material should pass through   4.0  mm IS Sieve
6 pH 6.5-8.0
7 Pathogens NIL
8 Conductivity  (as dSm-1) not more than 4
9 Heavy metal content, (as mg/kg),maximum  
10 Arsenic as (As2O3) 10.0
 11 Cadmium (as Cd) 5.0
 12 Copper (as Cu) 50.0
 13 Chromium (as Cr) 300.0
 14 Mercury (as Hg) 0.15
 15 Nickel (as Ni) 50.0
 16 Lead (as Pb) 100.0
 17 Zinc (as Zn) 1000.0

It has been found that solid and liquid fragment separation and upgradation by adding crop-specific nutrients can help improve digestate transportability and marketability.

The German Biogas Association has additionally advised the following techniques to modify digestate in order to increase its quality, supply chain and portability:

Pelletisation of the solid component: While considering the utilisation of digestate, the solid fraction is considered to be the main fraction, as the liquid component is utilised in the next digestion cycle.

The solid part can be processed by further drying and pelletisation to reduce the volume and make it more transportable for marketing and utilisation.

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The bulk density of loose dried digestate is 250-350 kilograms per cubic metre (kg/m3), which increases to 700-750 kg/m3 after pelletisation and thus helps increase storage suitability and decrease transport cost.

Upgradation by adding nutrients: The specific nutritional needs of the crop can also be used as a benefit for marketability and usage by improving the digestate in a way that can enhance its overall applied value.

For instance, Govardhannathji Energies LLP CBG plant in Gujarat has upgraded its biogas digestate and developed an organic fertiliser brand, UTPANN.

Apart from Bio-CNG marketing, the plant had set an excellent example of an organic fertiliser demand supply chain by enriching and developing digestate into crop-specific different category products.

Integrating Organic Farming with CBG plants: Integrating the CBG plant with organic farming can be explored as a potential solution in order to utilise the digestate onsite and save on transportation costs.

Way forward

Incentivising Bio-CNG plants: Infrastructure is necessary for the improvement and enrichment of digestate. The government must take action to support both higher and lower-capacity plants by providing incentives, or common enrichment facilities at subsidised cost so that digestate can be upgraded either in situ or ex-situ.

Uniform pricing mechanisms: Like Bio-CNG, the government must announce a uniform price for digestate that meets the necessary standards to make marketing transparent or effective.

It is important to take into account the centralised monitoring system, which enables access to data on digestate production and consumption at the state or district level, regardless of feedstock, on a single platform.

Research and development: The qualities of the digestate are directly affected by changes in the type of feedstock used in biogas generation.

Given this, one of the essential study objectives should be a comparison of digestates from various biogas plants for plant growth. This would allow for a more precise assessment of the favourable impact on yield and the soil ecosystem.

Awareness among farmers: In terms of digestate consumption, the farmers are the primary stakeholders who need to be aware of the positive aspects of its utilisation and organic farming. 

This can be done by demonstrating the farmer with digestate benefits, sharing the success stories and capacity building at the village level.

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