Question

There are different types of biogas plants based on the type of digesters used. Only two types were discussed during the course. Explain about the following two other

types:

i. Earth-pit Plants

ii. Ferro-cement Plants

Answer 1

Biogas refers to a gas produced by anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste, energy crops, or other biodegradable feedstock. In biogas plant; biogas digester is a large tank, inside which biogas is produced. Decomposition/breakdown of organic matter occurs inside this through anaerobic digestion. It's called as digester because organic material is eaten and digested by bacteria to produce biogas.

Following are different types of biogas plants based on the type of digesters used:

1. FIXED DOME BIOGAS PLANTS. Closed, dome shaped digester. Rigid and immovable gas holder and displacement pit. Digester volume can be up to 200meter cube. Gas storage and pressure : Internal gas storage is large ; between 60 and 120mbar Materials : cement and ferro-cement. Types: Chinese fixed dome plant Janata model Deenbandhu AKUT fixed dome plant AKUT Maendaleo CAMARTEC MODEL Advantages: Low initial cost Durability very high as no rusting parts involved Self agitated by biogas pressure High methane emission Underground construction save space. Disadvantages: Gas pressure fluctuates substantially. Special sealants are required for masonry Gas leak can occur. Skilled gas tight construction required

2. FLOATING DRUM PLANT. Underground digester that can be cylindrical or dome shaped Moving gas holder which floats either in slurry or water jacket Gas collected in gas drum which moves up during gas production and goes down as gas is consumed. Digester size can be 20-100 meter cube. (Industrial); 5-15meter cube( farm size) Gas storage and pressure: small; upto 20mbar Materials: Steel Lifetime : 15 years Advantages: Used for digesting animal and human feces helps in farming Disadvantages: Durability weak Rusting chance high Regular maintenance needed Tendency to get stuck by gas holder Methane emission is medium Manual steering is needed for agitation

3 LOW COST POLYETHYLENE TUBE DIGESTER. Bended tubular polyethylene film at 2 ends, around PVC drainpipe draped with rubber straps(recycled tire) Digester size is 1/4 of total capacity. Gas storage and pressure: Low, around 2mbar Materials: polyethylene, rubber. Advantages: Low cost Easy installation Tubular polyethylene is flexible Durability is medium and depends. Disadvantages: Low gas flow rate Self ignition not possible(plug flow) Methane emission is low

4. BALLOON PLANT. Heat sealed plastic or rubber bag Combined with gas holder and digester Digester size : up to 6 meter cube(depend on size of bag used) Lifetime: 2-5years Gas storage and pressure: Small, lower than 2mbar Materials: Reinforced plastic, Red mud plastic, Trevira, butyl Advantages: Low cost Simple construction Transportation is easy Suitable for high ground water areas Maintenance is uncomplicated Disadvantages: Chances of mechanical damages are high. UV resistant material is needed. Sometimes gas pumps required during low gas pressure. Requires manual steering.

5. EARTH-PIT PLANTS A pit in earth lined by thin layer of cement Wire mesh used to prevent seepage Metal or plastic gas holder Materials used : Cement, wire mesh Advantages : Low cost Easy installation Self maintenance Less space only required Disadvantages. Short lifespan Soil is major factor

6. FERRO -CEMENT PLANTS Self supporting shell or an earth pit lining construction. Cylindrical gas holder vessel Digester size : 6meter cube Advantages: Cost effective Easy construction Disadvantages: Uses substantial amount of cement Wire mesh will be expensive Efficacy is not adequately tested Special sealing required for gas holder