(166g) Strategies for Improvement of Anaerobic Digestion from Agricultural Waste: A Critical Review

Safenaz Shaaban, Doaa Bassuney, and Mahmoud Nasr*

Sanitary Engineering Department, Faculty of Engineering, Alexandria University, 21544, Alexandria, Egypt

*Corresponding author: Dr. Mahmoud Nasr

(+20) 100 63 90 400

mahmmoudsaid@gmail.com

mahmoud-nasr@alexu.edu.eg

https://orcid.org/0000-0001-5115-135X

Abstract

Agricultural wastes, also known as agro-wastes, refer to the residual products discarded from various farming practices. Agro-wastes consist of multiple organic and non-biodegradable substances, and they include large quantities of crop residues such as rice straw, sugarcane bagasse, wheat shell, soybean hulls, and corn stalk. Agricultural wastes also comprise a wide range of animal residues such as livestock feces and urine, poultry litter and excreta, and carcasses (or parts) of animals, as well as, agro-industrial byproducts such as sugarcane bagasse ash (a byproduct of sugar factories), cassava bagasse, soy hulls, and sugar-beet pulp. Moreover, agricultural wastes contain nitrogen and phosphorus species released from crop fertilization, as well as, toxic elements due to the utilization of pesticides (herbicides and insecticides). Furthermore, agricultural wastes include lignocellulosic biomass such as wood and straw residues, which is considered a renewable resource for the 2^nd generation of biofuels production. The fractions of cellulose, hemicellulose, and lignin vary among agricultural wastes (e.g., corn stover, rice husk, wheat straw, and sweet sorghum stalk). Based on the nature of farming systems, agro-wastes can be collected in the forms of solid, liquid, or slurry. The annual quantity of agricultural wastes has been estimated as 998 million tonnes, in which the organic fraction can reach up to 80% of the total solid wastes.

Anaerobic digestion has found successful applications as an environmentally friendly approach for the conversion of agro-wastes into valuable products and renewable fuels. The input agro-wastes (i.e., as a substrate) include wet organic substances, animal slurries, biosolids, and livestock manure (pig and cattle). The anaerobic bioconversion process reduces the volume and mass of agro-wastes, and the residual matter after digestion can be used as a soil amendment and as secondary fertilizer compounds. In anaerobic digestion, biodegradable materials are stabilized and broken down by the action of microbial organisms in an oxygen-deprived environment. The microbial community includes fermentative bacteria, acetogens, and methanogens. The biological degradation reactions are complex, consecutive, and parallel, as they undergo multiple steps of hydrolysis, acidogenesis, acetogenesis, and methanogenesis. During these processes, the waste feedstock is decomposed into simple chemical components such as sugars, which are converted to organic acids. Finally, the organic acids are converted to biogas, viz., about 55–75% CH₄, CO₂, and traces of other gases. The percentage of CH₄ varies according to the operating temperature and the amount of organic fraction in the feed substrate. These steps can be summarized as follows:

In the hydrolysis step, hydrolytic bacteria convert the complex organic compounds (e.g., proteins, lipids, and carbohydrates) to monomers (low-molecular-weight matters) including amino acids, long chain fatty acids, and simple sugars.

In the acidogenesis phase, acid bacteria stimulate the fermentation process, producing intermediary products such as volatile fatty acids (VFAs), alcohols, hydrogen, and carbon dioxide.

In the acetogenesis process, acid-forming bacteria or acetogens utilize VFAs to obtain acetic acid, carbon dioxide, and hydrogen.

In the methanogenesis phase, methanogenic bacteria transform the intermediary products to CH₄ gas via two pathways:

Pathway-I: Methanogenic bacteria, converting CH₃COOH to CH₄ and CO₂.

Pathway-II: Methanogenic bacteria, converting CO₂ and H₂ to CH₄ and H₂O.

Anaerobic digestion is sustainable management of agricultural waste as it converts the organic matter into renewable energy that can replace conventional fossil fuels. Anaerobic digestion has the advantage of producing biogas and fertilizer, leading to minimizing the net energy input of the system. Moreover, the amount of waste sludge yielded from the anaerobic digestion system is lower than that of the aerobic treatment mechanisms. However, anaerobic digestion requires a high level of investment for a commercial or large scale application. Moreover, the environmental conditions should be strictly optimized to avoid the accumulation of volatile fatty acids and the inhibition of bacterial growth. Additionally, the fluctuation of input variables (e.g., flowrate, organic load, and chemical composition) provides a negative impact on the efficiency, control, and stability of the process. Furthermore, a lack of knowledge about the activities of living organisms and the phenomena occurring in the bioprocess offers serious challenges for the wide implementation of anaerobic digestion.

Anaerobic digestion of agricultural wastes is influenced by various factors, which can be summarized as follows:

1. Carbon to nitrogen (C/N) ratio

A C/N ratio of 20 – 30 has been recognized to provide the optimum condition for most anaerobic digestion applications. The unbalanced C/N ratio results in the accumulation of ammonia, leading to the inhibition of microbial activities. Accordingly, different types of chemical components are mixed with the feedstock wastes to provide the favorable C/N ratio that can improve gas productivity.

2. Organic loading rate

Organic loading rate (OLR) refers to the mass of volatile solids (VS) or chemical oxygen demand (COD) per cubic meter of reactor’s volume per day. OLR is used to estimate the ability of a digester to convert the feedstock waste into biogas and other end products. The high OLR requires large digester’s size, leading to an increase in the initial cost of investment.

3. Medium pH

pH is an important environmental factor in the anaerobic digestion process, as it affects the growth rate of microorganisms and the biological conversion pathways. The optimum pH range for the biomethanation processes has been reported as 6.6 – 7.0.

4. Alkalinity

The accumulation of volatile fatty acids in the digester during the acidogenesis phase can lead to the deactivation of methanogenic bacteria. Hence, alkalinity (e.g., ability to resist acidification via the addition of calcium carbonate) is an essential factor in the anaerobic digestion process as it safeguards the buffer capacity and stability of the digester.

5. Inoculum (Seeding)

A stable methanogenic community is required for the efficient degradation of organic compounds to produce biogas. Hence, the anaerobic digestion system should be enriched with sufficient quantities of micro-organisms. The acclimatization stage allows the bacterial cells to adapt to the new environmental conditions.

6. Substrate pretreatment

Agricultural wastes may include a heterogeneous complex of biodegradables and non-biodegradables. Accordingly, the input substrate for anaerobic digestion should be subjected to a pretreatment process (e.g., physicochemical, and biological treatments). This stage is used to (a) form high amounts of readily biodegradable substrates, (b) reduce the formation of inhibitory products, (c) minimize operational costs (e.g., energy demands), and (d) avoid the loss of sugars.

7. Other factors

Anaerobic digestion of agricultural wastes and metabolic conditions for microorganisms’ growth are also influenced by several factors including activators (e.g., enzymatic additives), retention time, and temperature regime (mesophilic and thermophilic).

Because agricultural wastes are abundant (available), low cost, and renewable, they can be used as a valuable input for bioenergy generation. Hence, this study presents a comprehensive review on the application of anaerobic digestion for biogas production from agricultural wastes. The review illustrates the optimum operating conditions (e.g., culture temperature, medium pH, nutrients availability, organic load, and digestion time) that maximize biogas productivity and improve organic biodegradability. The reactor’s configurations, viz., suspended bed reactor, fixed bed reactor, fluidized bed reactor, two-stage processes, and hybrid reactors employed for the enhancement of digester stability are discussed. The types of agricultural wastes such as lignocellulose biomass, livestock manure, crop residues, poultry litter, and cattle wastes used for biomethane production are illustrated. These objectives are represented regarding recent literature studies. Recommendations for future works are also suggested.