Biogas Plant

(Basic Anaerobic

Digester)

Muhammad Muzammil

Akbar Hussain Lodhi

Zeshan Khalil

Ali Suleman

Course: Renewable Energy Resources

Instructor: Engr Khuda Bukhsh

By

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CONTENTS

Pg no:

Abstract 2

Introduction 2

The brief 3

Biogas 4-5

Scaling the digester 6

Basic summary of materials required 7

Discussion of the design 7

Summary 8

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ABSTRCT

With environmental issues such as the greenhouse effect and correct waste disposal methods

gaining much attention throughout the community, the concept of controlled anaerobic

digestion is perhaps a much overlooked example of a way to reduce green house gas

emissions and provide a better waste disposal method for organic waste.

INTRODUCTION

Controlled anaerobic digestion is by no means a radical or new concept. Large scale industrial

digesters and small domestic digesters are in operation in many places around the world. The purpose

of all these digesters is to produce combustible biogas which can be burned to provide energy for a

whole range of uses. Here in Pakistan, there is quite a bit of ideological interest in anaerobic digestion

and biogas production, particularly from intensive farmers, but there are not many examples of

digesters in operation. These farmers are interested in this topic primarily as an alternative energy

source (biogas), and secondly, as part of an efficient effluent waste disposal system for the farm.

Somehow there seems to be a problem in finding ways to put controlled anaerobic digestion into

practice on the average Pakistani farm. There is almost a small library of information from all over the

world on this topic, but this information doesn't seem to be reaching the average intensive farmer with

some interest in this topic. Why isn't this concept being utilized more? There could be a number of

possible reasons for this including the capital cost of setting up an anaerobic digester project, a lack of

working models and / or a lack of a source of ideas to base individual projects on, i.e. - trouble

shooting and project development at a technical 'on farm' level. The purpose of this project was to

develop a small scale working prototype possibly suited to operate on the average farm. The focus of

this project was the production of usable (combustible) biogas. This project is definitely not supposed

to be revolutionary or radically new, but rather to be a starting point for further research and

development in this area. The purpose of this report is not to provide a method for the fabrication of

the project produced (although a basic materials list will be provided). Rather, this report will aim to

identify key aspects of the design, concentrating on their function and the theory behind their function.

Therefore, the aim of this report is to provide the reader with a basic explanation of the mechanics of a

small, continuous flow anaerobic digester.

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THE BRIEF

The purpose of this project was to design and build an anaerobic digester to meet the following

criteria.

The design should:

attempt to maximize the amount of biogas produced per unit time.

be simple and easy to understand so that the average person is able to grasp the function and

theory behind each component of the design with only a small amount of guidance. The idea

here is to encourage people looking at the design to think and understand the requirements for

controlled anaerobic digestion and the continuous flow model.

be a durable, compact, versatile design which is capable of being shifted around if necessary to

be displayed.

be operated with a minimum of monitoring, regulating, and adjusting (in other words, be easy

to operate).

attempt to reduce time and money costs associated with maintenance

attempt to minimize the cost of setting up and running the digester without compromising the

performance of operation or the other specifications of the brief

look aesthetically pleasing as another mechanism to effectively sell the concept!

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BIOGAS

Biogas is somewhat lighter than air and has an ignition temperature of approximately 700 °C (diesel

oil 350 °C; petrol and propane about 500 °C). The temperature of the flame is 870 °C.

Biogas consists of about 60 % methane (CH4) and 40 % carbon dioxide (CO2). It also contains small

proportions of other substances, including up to 1% hydrogen sulphide (H2S).

The methane content and hence the calorific value is higher the longer the digestion process. The

methane content falls to as little as 50% if retention time is short. If the methane content is

considerably below 50 %, biogas is no longer combustile. The first gas from a newly filled biogas

plant contains too little methane. The gas formed in the first three to five days must therefore be

discharged unused.

The methane content depends on the digestion temperature. Low digestion temperatures give high

methane content, but less gas is then produced.

The methane content depends on the feed material. Some typical values are as follows:

Cattle manure 65%

Poultry manure 60%

Pig manure 67%

Farmyard manure 55%

Straw 59%

Grass 70%

Leaves 58%

Kitchen waste 50%

Algae 63%

Water hyacinths 52%

THE DIGESTION PROCESS

Biogas is produced by putrefactive bacteria, which break down organic material under airless

conditions. This process is called "anaerobic digestion".

The digestion process consists of two main phases:

acid formation,

methane formation.

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In the first phase, protein, carbohydrate and fat give rise to fatty acids, amino acids and alcohols.

Methane, carbon dioxide and ammonia form in the second phase. The slurry becomes somewhat

thinner during the process of digestion.

The better the two phases merge into each other, the shorter the digestion process. The conditions for

this are particularly favorable in the "fermentation channel" arrangement (Fig. 27,b). The following

types of digestion are distinguished according to the temperature in the digester:

psychrophilic digestion (10-20 °C, retention time over 100 days),

mesophilic digestion (20-35 °C, retention time over 20 days),

thermophilic digestion (50-60 °C, retention time over 8 days).

THE FERMENTATION SLURRY

All feed materials consist of

organic solids,

inorganic solids,

water.

The biogas is formed by digestion of the organic substances. The inorganic materials (minerals and

metals) are unused ballast, which is unaffected by the digestion process.

Adding water or urine gives the substrate fluid properties. This is important for the operation of a

biogas plant. It is easier for the methane bacteria to come into contact with feed material which is still

fresh when the slurry is liquid. This accelerates the digestion process. Regular stirring thus speeds up

the gas production.

Slurry with a solids content of 5-10% is particularly well suited to the operation of continuous biogas

plants.

Example:

Fresh cattle manure is made up of 16 % solids and 84% water. The cattle dung is mixed with water in

the proportions of 1:1. The prepared fermentation slurry then has a solids content of 8% and a water

content of 92%.

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SCALING OF THE DIGESTER

The size of the digester - the digester volume (VD) - is determined by the length of the retention

time (RT) and by the amount of fermentation slurry supplied daily (Sd). The amount of

fermentation slurry consists of the feed material (e.g., cattle dung) and the mixing water.

Example:

30 l dung + 30 l water = 60 l fermentation slurry

The digester volume is calculated by the formula

VD(l) = Sd(l/day) x RT (days)

Example:

Daily supply (Sd): 60 l, Retention time (RT): 80 days, Digester volume (VD): 60 l/day x

80 days = 4800 1 (4.8 m³)

For a specific digester volume and a known amount of fermentation slurry, the actual retention

time is given by the formula

RT(days) = VD (l) ÷Sd (l/day)

Example:

Digester volume (VD): 4800 l, Daily supply (Sd): 60 l/day, Retention time (RT): 4800 l ÷

60 l/day = 80 days

If the digester size is given and a specific retention time is required, the daily amount of feed is

calculated by the formula

Sd (l/day) = VD (l) . RT(days)

Example:

Digester volume (VD): 4800 l, Retention time (RT): 80 days, Daily fermentation slurry

requirement (Sd): 4800 l ÷ 80 days = 60 l/day

If a biogas plant is loaded not daily but at relatively long intervals, the daily supply (Sd) decreases

although the fermentation slurry proportion (S) remains the same. The retention time is

correspondingly prolonged.

Example: Digester volume (VD): 4800 l, Fermentation slurry proportion (S): 60 l

1. Daily loading, i.e. Sd = S = 60 l/day: Retention time (RT): 4800l ÷ 60 l/day = 80 days

2. Loading every other day, i.e. Sd=S 2=30Q/day: Retention time (RT): 4800 l ÷ 30 £/day

= 160 days

3. Loading twice a week, i.e. Sd = S x 2/7 = 17.2 l/day: Retention time (RT): 4800 l ÷ 17.2

l/day = 279 days

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BASIC SUMMARY OF MATERIALS REQUIRED

1 x 301t plastic drum suitable to act as a digester;

1 x tyre tube (with two nozzles as the only openings) to be used for the gas collector

various PVC fitting including glue, primer, various diameter pressure pipes and their associated

joining fittings

guttering silicon

1 x 50mm ball valve fore solids outlet

1 x 10mm ball valve for gas outlet

1 x (0-5)Psi pressure gauge

DISCUSSION OF THE DESIGN

As the gas produced in an anaerobic digester is hopefully combustible, there are safety issues to firstly

be considered when designing and operating the digester. Adequate ventilation is required and be best

way to ensure that the environment around the digester is well ventilated is to position the digester in

an open area, preferably outdoors. This means that the digester must be weather proof. Any possible

ignition sources should be kept well away from the digester.

For the purpose of this project, we decided that the best way to make the digester unit transportable

was to build the whole unit on a single drum. From the area available on the top of the drum (lid), we

formed the basic layout and determined the location of the digester inlet and the gas collector nozzle.

The first thing we had to do was to process the drum for the PVC pipe and gas fittings to sit on by

drilling hole(s) in the drum and the lid respectively. The digester was made from a 301t plastic drum

with a 'clip-on' lid, held on with a compression ring and guttering silicon safety seal. The lid of the

drum was then replaced with the PVC inlet pipe, gas collector nozzle and the pressure gauge fittings

added to it. Guttering silicon was used to add for another safety seal in spite of the pre fitted rubber kit

in the lid to minimize the chance of gas leaks through the lid.

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SUMMARY

From the above description of the design features of this project, we believe that we have succeeded in

meeting the requirements of the brief and are hopeful that the explanations above have given some

incite to the design, theory and operation of an anaerobic digester of this type. As mentioned in the

introduction, this design is not meant to be revolutionary or radical, it is only aiming to present this

concept in a simple, easy to understand format.

Sullery DigesterAnaerobic digestion BIOGAS