Anaerobic Digestion: An Option for Organic Waste Management

 

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Municipal Solid Waste Composition

At present, most of the municipalities are facing the soaring challenge of municipal solid waste management. As per the study conducted by the ADB in 2013, the average municipal solid waste (MSW) generation of 58 municipalities in Nepal was 317 grams per capita per day and 56 % of this generated MSW comprises of organic waste. Several technologies like gasification, pyrolysis, and incineration of solid wastes have already been developed to solve the waste management problem but these technologies require high temperature and more energy than what they can produce. Among various approaches available, anaerobic digestion is considered a promising option in the management of organic waste because of its techno-economic viability and environmental sustainability.

 

 

Anaerobic digestion is the process of microbial decomposition of organic materials in absence of oxygen. Bioslurry and biogas which mainly contains methane (55% to 70%) and carbon dioxide (30% to 45%) are the end products of the anaerobic digestion. However, the percentage of composition of biogas are affected by the type of feedstock used. The amount of biogas generation depends on several factors.

  • pH Value: In biogas generation, anaerobes can be divided into two groups: acidogens and methanogens. The optimum pH is 5.5 – 6.5 for acidogens and 7.8 – 8.2 for methanogens. Thus, pH should be maintained between 6.8 to 7.4 for the optimum production of biogas.
  • Temperature: Methane production can occur over a wide range of temperatures. However, methanogenic bacteria are passive in extreme high and low temperature. Satisfactory gas production takes at 35 °C.
  • Loading Rate: If loading rate is high, acids accumulation will inhibit methane production and if loading rate is low, gas production will also decrease
  • Dilution and Consistency of Inputs: For proper mixing of organic materials, the ratio between domestic wastes and water should be 1:1 on a unit volume basis (i.e. equal volume of wastes and water). If the slurry mixture is too diluted, solid particles can precipitate at the bottom of the digester and if it is a too thick flow of gas can be blocked. In both conditions, gas production will be less than optimum
  • Carbon-Nitrogen (C: N) Ratio: The optimum C: N ratio, for anaerobic digestion, is around 20-30:1. A high value of C: N ratio decreases the rate of digestion due to rapid consumption of nitrogen. The low value of C: N ratio liberates the nitrogen and accumulates in the form of ammonia, which is toxic under certain conditions. Combination of materials with high C: N ratio and low C: N ratio helps to bring the average ratio of the composite input to a desirable level

A wide range of feedstocks is suitable for anaerobic digestion. In general, biomass containing fats, carbohydrates, proteins, cellulose as main components can be used for biogas production. The theoretical CH4 content of biogas for various substrates are as follows:

Substrate Methane, % of Total Gas
Fat 72
Protein 63
Carbohydrate 50

Proteins and Fats are rich in energy source and produce a lot of methane during material decomposition. In general, methane content in biogas ranges from 55% to 65% and cannot be higher than 70%, even if all the substrate used are fats and vegetable oils. A high concentration of fats and protein inhibit the anaerobic digestion process through the accumulation of volatile fatty acids and long chain fatty acids.

Among different kinds of substrate available, kitchen waste and maize straw are better feeding material for biogas generation.

Table 1: Biogas production from different feedstocks

Feedstock Biogas production (m3/ton dry solids)
Cattle manure 200-300
Pig manure 250-500
Chicken manure 310
Sheep manure 300-400
Human excreta 380
Rice straw 550-620
Maize silage 600-700
Maize straw 400-1000
Kitchen waste 400-1000

(source: Surendra et al., 2013)

The feedstock can either be utilized as a single substrate in biogas generation or can be co-digested. Co-digestion results in an improved nutrient balance of total organic carbon, nitrogen and phosphorous, which results in the stable digestion process and good fertilizer.

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