By Dr. Lirone Friedman

There is growing recognition of Anaerobic Digestion (AD) as the most efficient and sustainable solution to maximize the recovery of energy and nutrients from organic waste.

Organic waste, such as food waste, manure, and wastewater sludge, is commonly stabilized by concealment (landfills), solidification, composting, or treatment via Anaerobic Digestion (AD). There is growing recognition of AD as the most efficient and sustainable solution to maximize the recovery of energy and nutrients from organic waste.

Anaerobic digestion is a simple and humble process: when organic waste is sealed in a definite volume, anaerobic conditions support microbial degradation, followed by the production of biogas which can further be used to generate energy – heat or electricity.

The efficient separation between both valuable products, the biogas and the effluent, termed digestate, makes the process of AD relatively easy to maintain and operate. The digestate is a nutrient-rich stream. Concentrations range between 0.5-5 gr/l-N of nitrogen, 0.2-1 gr/l of phosphorus, and 0.5-3 gr/l of potassium, which makes it an ideal biofertilizer.

In small farms (less than two hectares), the common fertilization practice is either direct application of the organic waste, after some level of stabilization, or the use of synthetic fertilizer. However, farmers who use synthetic fertilizer are reliant on its continuous supply. In addition, the decentralized, widespread nature of farm locations forces higher distribution costs of synthetic fertilizer, in comparison to larger farms. Further, synthetic fertilizers are produced by the Haber-Bosch process using natural gas, leaving farmers susceptible to fluctuations in natural gas prices caused by market turmoil resulting from global conflicts. The war in Ukraine, for example, has resulted in fertilizer prices more than doubling over the last few months, significantly increasing the global risk of food security.

In terms of the carbon footprint along the supply chain of synthetic fertilizers, the Haber-Bosch process generates about 1.5 tons of CO2 eq/ ton of ammonia and is responsible for 2.4% of global greenhouse gas emissions. Another consideration is CO2 emissions from transportation, which vary with distance. Moreover, the stabilization of organic waste is a major source of methane and CO2 emissions, from storage to treatment to land application, generating a significant carbon footprint. In cases of reduced soil fertility over time, farmers tend to over-fertilize which can result in the loss of 30-70% of the nitrogen by evaporation, with significant N2O emissions.

Although digestate management could be a limitation for large and industrial-scale AD, in general, careful management of digestate is highly advantageous over any other fertilization practice, especially for small-scale farms.

ADVANTAGES OF DIGESTATE MANAGEMENT

• Using secure, locally-produced digestate lessens farmers’ dependency on synthetic fertilizers and cuts one of their biggest costs.
• The relatively high content of organic material and micronutrients remaining from the digestion increases soil fertility by stimulating the soil’s fauna, permitting better nutrients and water management. Thus, recovery of infertile soils is also possible.

From a life cycle perspective, applying digestate as a biofertilizer significantly reduces carbon emissions and the climate impact along the life cycle of AD in comparison to other fertilization practices.

Various countries worldwide are exploring policies to support the usage of digestate, from taxing imported fertilizers (Denmark, The Netherlands, and Austria) to certifying digestate as an organic fertilizer (Ecuador) to providing governmental incentives to support farmers in more sustainable use of digestate (UK). Nevertheless, one of the limitations of sustainable development is a lack of awareness regarding digestate as a substitute for synthetic fertilizers which also benefits the climate.

One of the most significant gaps in the wider application of AD is its economies of scale and its long-distance distribution to decentralized farms. The greatest potential for the application of AD is treating the organic waste of smallholder and decentralized farms which are 80% of the world’s farmers and are responsible for about 35% of the global food production (Figure 1 and 2). These communities are among the most vulnerable to changes in energy and fertilizer prices. The remaining challenge for overcoming these limitations is developing systems that allow for the simple, safe, and feasible installation and operation of small-scale AD.

Few companies are already addressing the market of small-scale systems and presenting easy-to-apply, available technologies that show feasibility and efficiency. Prioritizing financial investment in wider application of such AD systems will have a significant impact on farmers’ resilience to financial risks, increase their independence, notably reduce global carbon emissions, accelerate the transition to circular fertilization, and secure food production.

About the Author

Lirone Friedman, Ph.D., has over 16 years of experience in environmental engineering, both in industry and academia, with a focus on water, wastewater, and waste treatment technologies. He conducted his Ph.D. research at Columbia and Tel Aviv University and published in leading journals in the field.

He is developing designs to address nutrient shortages, energy recovery, and water scarcity through state-of-the-art practices such as virtual biokinetics modeling and life cycle assessment. He is now focusing on improving biogas systems to aid vulnerable communities worldwide and increase food security through more sustainable agricultural practices. Liron is part of government committees and industry associations aimed at implementing policy guidelines and promoting the adoption of circular practices.

This article will be published in the May-June 2022 issue of Smart Water & Waste World Magazine.
e-Magazine link will be available here soon.

© Smart Water & Waste World. Send us your editorial contributions at mayur@smartwww.in