Question

Anaerobic digestion for biogas production or wastewater treatment

• Intro: • Why we need alternative fuel/energy sources and why biomethane?

• Why wastewater must be treated (link to biogeochemical cycles/decomposition) significance/context (20%)

• Microbiology of AD (50%) • Reactor types used for biogas production or wastewater treatment and how they are applied

– case studies or examples welcome (30%)

Microbiology

Answer 1

Increased burning of fossil fuels such as coal, oil and gasoline has resulted in increased emission of greenhouse gases in our environment. To protect the environment from this damage there's a need to switch to alternate source of energy. Biogas production is a boon for the improvement of current global situation. It definitely ensures to minimize the use of fossil fuels to some extent.

It is imperative to understand that almost all the developing countries are heavily relied on non-renewable sources of energy and stocks of these sources are finite in our environment. Alternate energy source offers ample of benefits for example apart from providing renewable and clean source of energy but also it keeps us and our environment in a healthy state.

Biogas being a renewable source of energy is gaining momentum across the globe in providing energy for various purposes such as cooking etc. Also it has zero percent emission of greenhouse gases which is certainly the best option to reduce greenhouse effect. It cuts down the reliance on fossil fuels such as oil and coal to a great extent. It essentially makes use of readily available raw material such as Vegetables wastes, crops etc. which makes it a highly sustainable option as well. It also reduces water and soil pollution.

Emissions from fossil fuels also pollute water bodies and has deleterious effects on the lives of marine life and unfit for human use. Wastewater treatment is very important to protect marine life, to prevent soil and groundwater pollution and most importantly to protect public health.

Anaerobic microorganisms play a crucial role in biogas production. Microorganisms which are involved in biogas production are essentially facultative as well as strict anaerobic bacteria. Most important among them are methanogenic archaebacteria, represented by Methanobacterium. The other bacteria involved are Bacillus, Clostridium, Ruminococcus, Eubacterium and CCellulomonas. These bacteria are commonly found in anaerobic sludge which is formed during sewage treatment and these bacteria are also found in rumen of cattle where they are involved in breakdown of cellulose.

Types of reactors which are used for biogas production are Anaerobic activated sludge process, Anaerobic contact process, Continuous stirred tank reactor ( CSTR), Expanded granular sludge bed digestion (EGSBD) etc.

Biogas is produced inside a biogas plant which are usually seen built in rural parts. It consists of a tank covered by a lid. Various raw materials are put into the tank along with cattle dung. After filling, it is covered by a lid which keeps on rising as the gas is produced. Biogas is produced by action of anaerobic microbes and the gas thus produced is supplied through pipes to nearby houses.

A case study was done in a state of Chattisgarh in India where randomly several biogas plants were selected in different districts of the state and their efficiency was tested. Overall efficiency was about 75% and the maximum being 83% and minimum 71%. The overall biogas plant efficiency reduced with increase in plant size.