Introduction :

DEVELOPMENT OF BIOGAS TECHNOLOGY

INTRODUCTION

Organic matters such as animal and human excreta, agricultural and industrial waste, water hyacinth etc. when fermented under an anaerobic condition produce a combustible gas called Biogas.  It is a renewable source of energy, can be used as fuel for cooking, lighting, running vehicles and generators, etc.  Other natural resources like oil, gas etc. are limited and will be exhausted in course of time.  That is why, the developed countries consider their natural resources very precious and are cautious about extracting those.  In Bangladesh neither the decision-makers nor the experts pay due importance on proper extraction and use of natural resources.  With the present rate of consumption, natural energy resources like gas will be exhausted shortly and this is high time to derive policy and practice for exploration and use of alternative renewable sources if we want to meet energy crisis in near future.  Biogas has been found to be a proven renewable energy option.

POTENTIALS OF BIOGAS IN BANGLADESH

Cattle dung available from 22 million cows and buffaloes is nearly 220 million kg. One kg. Of dung can produce 0.037 cum of Biogas. i. e. available cattle dung can produce 2.97XIO⁹  cum of gas which is equivalent to 1.52X10⁶ tons of kerosene or 3.04X10⁶ tons of coal. Besides, a substantial amount of Bio-gas can be produced from human and animal excreta, garbage and water hyacinth.  If all the household can be brought under Biogas technology, it is possible to get 1.03X 10⁹ cum of gas from the filth of human being only. A conversion of 5% of different raw materials should produce an estimated 2X10⁸ cum/day of Biogas.

At present bangladesh meet 46.15% of its energy need by agricultural residue, 10.5 % by fule wood and 33% bt tree residue. Biogas will reduce energy deficit 15 %.    

       INVOLVEMENT OF LGED IN BIOGAS TECHNOLOGY

LGED has been created mainly to provide technical support to the local government institutions. This organization can play an important role in disseminating appropriate technology all over the country as it has setup at Thana level.  LGED constructed it's first Biogas plant in 1986 in Kurigram and organized a seminar on Biogas technology there. By now LGED constructed 300 of Biogas plants in different districts of Bangladesh, of which 106 are on night soil, 15 on garbage, 3 on poultry droppings, 2 on water hyacinth and the rests on cow dung.  Initially LGED constructed 15 floating dome type plants.  Due to corrosion and leakage on steel domes, these plants did not work for long  time. In June 1992, LGED constructed two fixed dome type Chinese model plants in Noakhali. These plants are working without any problem.  After close monitoring of success of these two plants, LGED started wide application of this.  Subsequently, based on local condition and practical situation, LGED developed its own model.

First Biogas plant based on night soil was constructed by LGED in Faridpur Muslim Mission.  Before construction of this plant, load there was an apprehension about its acceptability.  But after completion of the plant there was no social and cultural barrier.  This plant could drew attention of all concerned. Small bore sewerage system, in conjunction with Biogas plant installed by LGED in Bauniabad slum in Dhaka proved to be cheaper and better solution for sewage disposal.  Solid waste and water hyacinth based Biogas digesters of LGED also created great interest among the users.  LGED has so far trained 70 professional engineers on Biogas technology, 4 of whom received higher training in China.  To provide technical support to the beneficiaries, gradually all the engineers of LGED will be trained on Biogas technology.

Types of Biogas Digester :

There are three types of basic designs of biogas plants tried in Bangladesh :

(i)    floating cover digester, (ii) fixed cover digester, and (iii) plastic cover digester.

(i) Floating cover digester : It works on the principle of constant pressure, changing volume. The disgester, cylindrical well, commonly made from brick and cement, is covered with a floating steel cylinder with an open bottom (Fig. A). As the cylinder has a contant weight, it moves up when gas production is higher than consumption and comes down under the reverse conditions.

(ii) Fixed cover digester : It works according to the principle, constant volume, changing pressure. When the rate of gas production is higher than that of gas consumption pressure inside the digester rises and expels some digester contents into the outlat compartment. When the consumption is higher than production, pressure inside the digester falls and the expelled materials in the outlet compartment run back to the disgester (Fig. B)

(iii) Plastic Cover digester :         A long cylindrical polythene/PVC bag, half-buried longitudinally in the ground, is fed with fresh cow-dung slurry at one end and discharged at the other. With the formation of gas, the bag swells like a balloon and the gas is led out to the point of use through a pipe by putting pressure on the balloon form outside (Fig. C)

In early 80’s, the floating type design  was used for biogas plant. But due to corrosion of the steel dome, the gas leakage problem happened and it could not be removed. Later on BCSIR tried with fixed done type design and it has been successful in all biogas plants. The plastic bag type design is not used in Bangladesh It is used in China. In this project, the fixed dome type design has been proposed.

Gas Generation Rate

Daily input in the digester is 221xO.5 kg. =110.05 kg.  Assuming water content to be 83% the quantity of suspended solid will be 18,71 kg.  For optimum gas generation, the percentage of suspended solid should be 8%.  To attain this percentage, water and night soil mixing proportion should be 1:1.  But in practice the users generally use more then 3 liters of water for the cleansing purpose, as a result the percentage of solid reduced to 2.5. This causes drastic reduction in gas production.  This problem is no more acute where there is no water supply reduction in gas production.  This problem is no more acute where there is no water supply system.  To improve this situation, we have decided to add kitchen waste with night soil.

Sector-wise Energy Consumption

Comparison between the Organic Fertilizer with cow dung & city waste:

Disposal of city waste is common problem all over the world .  In Developed countries 75 % of the city waste is are inorganic. But in Bangladesh 75 % of the city waste are organic, which can easily be used in a biogas digester. In Dhaka alone about 3000 tons of garbaze is being dumped every day which could meet the need of organic fertilizer and enrich our soil nutrient contents of City waste as compared to cow dung is shown below :

SOME EXAMPLES

A)    Biogas plants in Madrasa orphanage / other institution LGED has constructed 61 Biogas plants in different Madrasas, orphanages, hospitals, school/college hostels and mosques for solving the sanitation problems and getting biogas as an alternative energy source.  One of these important plants is in Faridpur Muslim Mission.  There are 250 students and staff in Faridpur Muslim Mission.  For their night  soil disposal they needed to construct a septic tank for 250 users with Tk. 60,000.LGED in mid 1992 constructed a Biogas plant with Tk. 16,000 which served the purpose of septic tank as well as a source of gas and fertilizer.  This has reduced the investment by Tk. 34,000 and the mission has been saving Tk. 25,000 against fuel cost per year.

B)     Bauniabad small bore sewer system The project is an innovation of environment friendly energy source and sanitation system and described in some detail elsewhere.

C)    Ganaktuli Sweeper colony, Dhaka There are five buildings for sweepers and 40 families reside in each building.  The latrines of the buildings were not connected with any septic tank or sewerage system.  Previously, night soil was passing through surface drain creating health hazard.  To connect the latrines of building No-I to the nearby sewer line Tk. 60,000 and to construct a septic tank for 221 users of the building Tk. 50,000 were necessary.  Instead, LGED constructed a Biogas plant for Tk. 20,000 in 1993 which is working till today without any problem giving sufficient gas to meet fuel need of 3 families.  There was some maintenance for leakage and Tk. 300 was spent. Observing the performance of the plant, residents of other 4 buildings created pressure on the city corporation to construct similar plant in those buildings. By now Biogas plants have been constructed in all the remaining 4 buildings.

  D)   'Ecological Village' Amgram / Uttar Hogla in Madaripur LGED took up 'Ecovillage' project on experimental basis as pilot programme with the objectives to make the villagers aware about environment and technology, create clear, healthy and acceptable environment in the villages and inspire the people around the villages to accept such project.

There are 662 people in the village in 123 families.  Besides other socio economic activities, the following facilities were also provided:

  There was no latrine in the village and 95 latrines were constructed. .As an alternative source of energy 15 Biogas plants were installed, three persons were trained in the construction of Biogas plant. The Biogas plants are providing energy input to the villagers.

Acceptability

It was apprehended people would not use gas from night soil for cooking purpose.  During construction of the plant in Faridpur Muslim Mission, there was hesitation among the students and staff.  When they found that there is no bad smell, they readily accepted the technology.  In Ganaktuli and other places LGED faced no such problem of acceptability.

Affordability

Biogas is a proven technology, There is no risk of failure if proper design and supervision can be ensured.  Most of the commercial banks are now convinced and took decision to provide loan for the construction of Biogas plants.  Most of the urban poor can not afford gas connection as it costs Tk  200 to Tk. 400 per month.  Instead they can install Biogas plants with. bank loan and repay the loan out of their fuel savings 'domestic' size Biogas plant of 100 cft capacity cost Tk. 15,000 to Tk. 18,000 and can meet the cooking energy need for a five- member family.

Impact on health and environment

Large scale bioenergy development in Bangladesh could bring significant environmental benefits. Sustainable bioenergy development could:

·        Reduce higher lavel of deforestation

·        reduce net greenhouse gas emissions

·        improve air quality and reduce acid deposition

·        improve soil quality and reduce erosion

·        reduce landfilling by adding value to residues

·        reduce agricultural chemical runoff

·        Improve sanitation condition

·        Improve habitat for native wildlife and improve biodiversity.

·        Outlining sustainable land use and improved air quality.

·        Improved habitat for wildlife and reduced use of fertilizers and insecticides compared with lands used for row crops, protection of riparian areas, and erosion protection for sensitive land areas.

·        Reduction of Greenhouse Gases from biomass power takes place because the carbon dioxide released during combustion is absorbed by the plants as they grow.

biomass .could play a role in reducing C0₂ emissions in both of these sectors.  As the slurry remains in the digester for 30-40 days in anaerobic condition, the effluent becomes pathogen free and the output is smell-free combustible gas and organic fertilizer improving the environment and preventing diseases. At present Bangladesh meet 46.15% of its energy need by agricultural residue, 10.5% by cow dung, 12.9% by fuel wood and 33% by tree residue.  This means that to meet our energy need we are depriving, ourselves from organic fertilizer and creating environmental imbalance by deforestation.  One pragmatic estimate puts that the use of Biogas will reduce energy deficit  by 15%.

BACKGROUND

A need assessment on Bauniabad slum identified that increase in supply of adequate and safe water, improvement of sanitation and solid waste services and improvement of knowledge about related hygienic practices should be addressed immediately.  On the basis of this study, a pilot project was undertaken to introduce the small bore sewer system in combination with Biogas plant to solve the sanitation problem in one portion of the Bauniabad slum.

OBJECTIVE

The prime objective of this intervention is to utilise Biogas technology as low cost sanitation option for the residents.  At the same time a number of families within the slum  avails the advantage of using Biogas for cooking purpose by rotation.  Specific objectives are to provide improved community latrine services and develop , financing operation method, optimize resource recovery from human excreta through Biogas generation and minimize environmental pollution through efficient waste disposal techniques.

DESCRIPTION

The project connects individual household pit latrines with a grid pipe line through which the sewage flows and finally accumulate in a main digester.  From the main digester the effluent passes through secondary tank to the drain connected to outfall. A portion (96 houses) of the Bauniabad slum was selected for this purpose.  The households of this slum were spaced in a planned manner.  Since all the components of fixed dome Biogas plant like, inlet chamber, di-ester, hydraulic chamber / secondary treatment tank are installed below ground level, a 15 ft wide internal road was selected as the site for the plant.  The latrines of 96 households, situated symmetrically with respect to the site for the plant were selected.  The pit latrines were joined through 6" dia concrete pipe and ended into a bigger pipe.  The pipe network ended up at the inspection pit.  A big inlet pipe leads the human excreta from the latrines through the inspection pit towards the Biogas digester.  The fermentation takes place within the Biogas digester.  Temperature rises in the digester during the fermentation and most of the pathogenic bacteria die.  A pipe is connected with the Biogas digester to two nearby kitchens.  After fermentation, the digested residue passes into the secondary treatment tank and remains there for a longer time period.  The design retention time within the secondary tank is about 30 days.  The purpose of the secondary treatment tank is to destroy highly resistant group of bacteria making the effluent almost safe for human being.  An outlet from the sedimentation tank is attached to the nearby drain which leads towards the outfall into the river Turag.  When the secondary treatment tank will be filled up it should be cleaned.  This solid residue can be used as soil conditioner.

USE OF GAS

Twelve to fifteen families are using two burners for their cooking purposes which in effect means a saving of Tk. 4000 per month and use of clean odourless gas.

Biogas digester Generator

Generator Description:

 The biogas technology employs the technology of anaerobic digestion. It is based on the natural breakdown of organic material under bacteriological attack in the absence of oxygen, producing a combustible mixture of gas with up to 70 % methane. Biogas can be used in cooking stoves, piglet heaters, boilers, dryers and direct thermal heat processes. In larger systems it can run engines for mechanical power and generate electricity. The system consists of primary, secondary and post-treatment:

I)                  Primary treatment occurs in a low rate channel digester. The digester also functions as a separator for the liquid and solid fractions of the wastewater. For the solid fraction, a hydraulic retention time in the low rate digester from 20 to 30 days is calculated. The liquid fraction (80-90 % of wastewater) goes to the Upflow Anaerobic Sludge Blanket reactor (a high rate reactor).

II)               Secondary treatment incorporates two components: an Upflow Anaerobic Sludge Blanket (UASB) reactor and a Slow Sand Bed Filter (SSBF). The liquid wastewater fraction, containing a high soluble organic matter content, is treated in the UASB reactor. The ratio of digester volume between the low rate biogas digester and the UASB is 2-3 : 1, depending on the condition of the wastewater. After passing the UASB, a remaining COD of 800-1000 mg/l is regularly achieved. The SSBF is connected to the low rate channel digester and receives the discharged fermented solid fraction slurry. The solid waste from the SSBF can be dried further or composted and sold as organic fertilizer to crop farms, landscaping businesses and golf courses.

III)            Post-treatment. For this final treatment, the water is released into a natural oxidation system. This system features a small stabilization/buffer pond and a series of wetlands to enhance natural treatment of the effluent. A fishpond at the end of the wetland serves as a life indicator - providing sign of water treatment quality. Final chemical oxygen demand (COD) is less than 200-400 mg/l with biological oxygen demand (BOD) less than 60 mg/l. (The standard limits are 60-100 mg/l for pig farms and 20-60 mg /l for industrial plants. Often, untreated livestock farms have discharge with 8000-13,000 mg/l of BOD.) After flowing through the complete system, the treated water can be used to clean stables or can be released directly into a channel or river.

Major

Load(s) 1: generator  Major

Load(s) 2: heater  Major

Load(s) 3: motors   Major

Load(s) 4: cooking  

Technical Performance: It is expected that 50,000 cu.m. of digesters constructed during phase 1 and 2 of the program will annually produce 9,125,000 cu.m. of biogas, 10,950 MWh of electricity, 31,536 tons of bio-fertilizer, and displace 4,198 tons of LPG.

DESIGN OF BIOGAS PLANT

1.1 Introduction:

          Biogas can be obtained from any organic materials after anaerobic          
          fermentation by three main phases.

1.2 Mechanism of biogas fermentation:

A) Groups of Biogas microbes-

B) Groups of microbes involved in the 3 stages of biogas fermentation-

1.3 Design perameter:

          A) Selection of materials.

          B) Total solid(TS) contains calculations of organic materials Organic materials-

Solid part :-  Total solid contained in a certain amount of materials is usually used as the material unit to indicate the biogas- producing rate of the materials. Most favourable TS value desired is 08%

Liquid part:  As per Annexure-I

          C) Favourable temperature, P^H value & C/N ratio for good fermentation-

Temperature:- Mesophilic; 20^o  c to 35 ^o  c ( Annexure-II).

                   P^H value :- Neutral P^H   and  ranges 6.8 to 7.2

                   C/N ration :- Ranges from 20:1 to 30:1 ( Annexure- VI )

 D) Table showing discharge per day, TS value of fresh discharge and water to  

be added to make favorable TS condition-

          E) Hydraulic retention time ( HRT)-

For Mesophilic digestion where temp. varies from 20^o c to 35^o C and HRT is greater than 20 days.

1.4 Relationship between temperature, HRT & TS value of 8% :

1.5 Cross-section of  a digester:

a. Volume of gas collecting chamber        = Vc

b. Volume of gas storage chamber            = Vgs

c. Volume of fermentation chamber          = V_f

d. Volume of hydraulic chamber               =V_H

e. Volume of sludge layer                          =Vs

Total volume of digester V   =Vc+Vgs+V_f+Vs

1.6 Geometrical dimensions of the cylindrical shaped biogas digester body:

1.7 Assumptions:

1.8 Volume calculation of digester and hydraulic chamber:

          A)Volume calculation of digester chamber-

Given :

6 cows of body weight 200 Kg each.

 Temp. = 30^OC (average)

Solⁿ:

Let HRT   = 40 days ( for temp. 30^O C )

                             Total discharge   =  10 kg X 6 = 60 Kg/day

                   TS of fresh discharge    = 60 kg X 0.16 = 9.6 Kg.

In 8% concentration of TS ( To make favourable condition )

8 Kg. Solid    = 100 Kg. Influnt

1 Kg. Solid    =  100 / 8 Kg influent

9.6 Kg Solid = 100 x 9.6/ 8 = 120 Kg. Influent.

Total influent required = 120 Kg.

Water to be added to make the discharge 8% concentration of TS

=120 Kg - 60 Kg. = 60 Kg.

Working volume of digester  = Vgs + V_f

Vgs + Vf = Q.HRT

                                   = 120 Kg/day X 40 days

                                   = 4800 Kg.   ( 1000 Kg = 1 m³ ) = 4.8 m³.

From geometrical assumptions:

                    Vgs + V_f = 0.80 V  

Or  V= 4.8/0.8 = 6.0  m³.  (Putting value Vgs + Vf = 4.8 m³)                                                    

&   D = 1.3078 V ^1/3 = 2.376 m @  2.40 m.

Again

(Putting V₃=0.3142D³)

                   Say H = 1.00m            

Now we find from assumption as we know the value of 'D' & 'H'

f₁           = D/5 = 2.40 /5 = 0.480 m

f₂           = D/8 = 0.30m

R ₁         = 0.725 D = 1.74 m

R₂          = 1.0625 D = 2.55 m

V₁          = 0.0827 D³ = 1.143 m³

Vc          = 0.05V = 0.3 m³

          Now the dimension of digester chamber is kown & drawn below-

B) Volume calculation of hydraulic chamber-

From assumptions:

Vc   = 0.05 V = 0.3 m³

Vgs = 0.50 x (Vgs + V_f + Vs) x K ( Where K = Gas production rate per 

                                                          m³digeste vol./day )

                  = 0.5 x 5.7x 0.4  =1.14 m^{3  --------------------------------------------------------------------} (A)

Again,

Vgs  = 50% of daily gas yield

                   = 0.5 x TS x gas producing rate  per Kg TS

                   = 0.5 x ( 60 kg x 0.16 ) x 0.28 m³/kg TS ( See Annex- III )

                   = 1.344 m³ ------------------------------------------------------------- (B)

From A & B  let Vgs = 1.344 m³.

Vc + Vgs =  0.3 m³  +1.344 m³   = 1.644 m³ 

Again    V₁ = [{(Vc + Vgs) - {p D²  H₁}/4 ]

                         = [1.644 - {3.14 x (2.4)² x H₁}/4] 

              Or, H₁ = 0.110m

We have  fixed h = 800 mm water volume ( I mm = 10 N/ m² )

h = h₃ + f₁ + H₁ 

Or, h₃ = 0.210m.

Again we know that

Vgs = V_H  

              Or, 1.344 m³= 3.14 x (D_H)² x h₃/4 

              Or, D_H = 2.85m

Now we know the dimension of hydraulic chamber. Moreover keeping  h = 800 mm, we can choose or re-arrange the dimension considering availability of site and construction suitability. For most suitable dimensions we can select the drawing of Annexure- IX for 60 Kg cow dung per day as raw material.

Note: For ready reference 4 family type Biogas plant's drawing is shown(standared dimension's) in Annexure-VIII, Annexure-IX, Annexure-X & Annexure-X1 where 40 Kg, 60Kg, 80Kg and 100Kg cow dung is considered as raw marterial per day respectively. If bigger size plant is required ,it can be designed keeping all safety considerations in design and construction.

Annexure- 1

TABLE-1: THE TOTAL SOLID CONTENT OF COMMON FERMENTATION MATERIALS IN RURALAREAS  (APPROXIMATELY)

Annexure-II

TABLE-2: BIOGAS-PRODUCING RATES OF SOME COMMON

FERMENTATION MATERIALS ATDIFFERENT

TEMPERATURES (m³/Kg TS )

Experimental conditions:- The fermentation  period  of the  excrement materials lasts  

                                           60  days  and  that  of  the  stalk  type  lasts  90  days.  The 

                                            fermentation material concentration (  total solid content ) 

                                            is 6%.

                                                                                                       Annexure- III

TABLE-3:  BIOGAS  PRODUCING  RATES  OF  SOME  FERMENTATION 

                    MATERIALS AND THEIR MAIN  CHEMICAL COMPONENTS.

Annexure- IV

TABLE-4  BIOGAS- PRODUCING RATES OF SEVERAL SUBSTANCES

Note: - The fermenting temperature is 30^oC. It is batch-fed fermentation.            YpCMDV refers to the average yield of biogas per cubic meter of the digester volume during the period of normal fermentation ( m³/ m³d) YpkgM refers to the yield of biogas per kilogram of the fermentation material (m³/kg TS)

Annexure-V

TABLE-5 : THE SPEED OF BIOGAS PRODUCTION WITH COMMON FERMENTATION MATERIALS.

* Biogas production is at the highest speed.

** Amount of  Biogas produced to more than 90% of the total yield of a fermentation period.

Experimental conditions: - Fermenting  temperature  35^oC, the  total length of fermentation period being

60 days for the excrement material and 90 days for the stalk type, the materials concentration; total solid content of the fermentative fluid being 6%.

ANNEXURE- VI

TABLE -6. CARBON-NITROGEN RATIOS OF SOME COMMON FERMENTATION MATERIALS (APPROX.)

ANNEXURE- VII

TABLE-7  AMOUNT OF HUMAN AND ANIMAL WASTES DISCHARGED PER DAY  ( APPROX.)

Note: The annual amount of excrements collected accounts for 80% of that discharge.

CONSTRUCTION PROCEDURE OF BIOGAS PLANT

1.1  SELECTION OF SITE

First choose the site which is nearest to the place of raw materials as well as appliances to be used for the following reasons.

- Extra labour requires to feed the digester if it is too far from the source

   of raw  materials.

- Longer length of gas distribution pipe line will reduce the design

   pressure in a   considerable amount.

- Construction cost will be higher.

- Condensate could be trapped if any sag in long gas pipe line which 

  may block the gas pipe line.

1.2  LAYOUT

First go through the drawings of the plant in detail. For example let choose  6m³ volume digester's drawing as shown in Annexure-X.

Figure-1 shows the layout plan of 6m³ Volume plant. Erect the longitudinal center line pegs P₁ & P’₁ along the inlet, digester and hydraulic chamber as shown in fig-2. Erect the center point peg P₂ of the digester. Then erect transverse center line pegs P'₂ & P"₂  of the digester at a safe distance as shown. Similarly taking the distance from layout plan (fig-1) erect the transverse pegs of hydraulic chamber P₃ , P'₃ & P"₃ and also erect the Inlet's pegs P'₄ and P₄" as shown in fig-2.

Calculate the outer diameter of the digester from Annexure-1 or take the outer diameter from layout plan of fig-1. Then find the radius of the digester for earth cutting; outer diameter ¸ 2 = 2800 ¸ 2 = 1400mm. Measuring this radius from peg P₂ make a circle C₁ on the ground as shown in figure-3. Similarly mark the circle C₂ on the ground for hydraulic chamber as shown. Lastly mark the inlet on the ground accordingly. Now the layout is completed.

1.3  EARTHWORK IN  EXCAVATION:

 A. Digester (Fermentation Chamber)-

            Depth of earth excavation

=300+480+1000+300+100=2180mm as shown in figure-4.  fªù¡ ew-24

Excavate around the outer diameter upto the depth of cylindrical portion that is  300+480+1000+100 = 1880mm as shown by shaded portion fig-4.

From Annexure-1 radius of curvature of the bottom dome of the digester =2600mm. Take a bamboo pole B₁ of length 2660+100=2760mm as shown in figure-4.

Now erect 2 bamboo post B₂ & B'₂ along transverse center line of the digester up to a height which is 2760(;B₁) -2180 (; total depth of excavation) = 580mm from G.L. as shown in figure-4. Now fix a straight bamboo B₃ horizentally upon these posts B₂ &B'₂ . Mark the center point of the digester in bamboo B₃ and hang bamboo pole B₁ from this center point as shown in figure-4 and excavate the bottom dome as per drawing until the bamboo B₁ swing freely. Now extend the excavation horizentally as per  measurement of the drawing in Annexure-X by trowel which is shown by double dotted line in figure-4.

B)  Hydraulic chamber & its connections-

Depth of earth excavation of hydraulic chamber  = 657+75 =732mm

Depth of earth excavation of outlet hole (Inter connection of digester & hydraulic   chamber)  = 300+480+1000+100 = 1880 (see Annexure-1 & fig-5). Excavate hydraulic chamber around the outer circle C₂ (fig-2) upto the depth of 732mm as shown by hatched line in fig-5.

Now excavate  the outlet holes taking dia. = 600+(125)x2 wall thickness + (075)x2 extra clearance = 1000mm  upto the depth 1880 & dia.950+(125)x2 wall thickness +(075)x2 extra clearance = 1350mm  upto the depth 1283 as shown in drawing by highlighted in fig-5.

Now excavate for inlet connection as the dimension  & drawing of  Annexure-X as shown in fig-5. Now earthwork in excavation is completed.

            1.4  CONSTRUCTION WORK:

     A)  Cement concrete -

Complete the casting of cc works  at the bottom of the digester and at the bottom   of the outlet hole as shown by highlighted in figure-6.

      B)  Brick work-

Fix a MS Rod-R (dia 6/8") straight upward through the center point of the digester (fermentation Chamber) as shown in fig-6. Take a longitudinal slice of bamboo-S and tie with MS rod-R as shown in fig-6 so that its  length is equal to inner radius of the digester and this bamboo slice-S can rotate around the MS Rod-R. Now start  brick work of cylindrical shape by rotating this bamboo slice-S. During brick work fix inlet R.C.C. Pipe as per requirement. Keep opening of required dimensions for outlet hole (Inter connection of digester & hydraulic chamber) and complete the brick work of cylindrical portion of the digester body Similarly complete the brick works of outlet hole & hydraulic chamber.

 C) Ring beam-

                        Complete the casting of ring beam ( which is shown in figure-7).

D)  Brick work of the top dome & the plastering works-

First complete the plastering and neat cement finishing works inside the completed portion of the digester before starting the top dome.

Now place a brick touching ring beam with required cement mortar giving  direct support by bamboo-B₄ as shown in fig-7. Here bamboo B₄ = radius of curvature of the top dome =1780 mm. Subsequently place a MS hook(1/4" dia) with a counter weight brick which is placed at the other end of the hook as shown in fig-7. Now remove the bamboo-B₄. Brick will remain in its position by the help of MS hook and counter weight brick. Now similarly place the adjacent brick and proceed until the circle is completed. Now remove all hooks and first layer bricks of the top dome will remain as it is placed .

Similarly complete all the circular layers of bricks & at the center of the top dome keep a 1" dia.  G.I. pipe with one end threaded as shown in Annexure-1. Now complete the plastering work with neat cement finishing inside (Special type plaster = in 3 layers; 1^st layer in 1:4 proportion, 2^nd layer in 1:3 proportion and 3^rd  layer in 1:2 proportion. Normal plaster = 1 layer in 1:4 proportion ) & outside (normal plaster)  Then complete the wax polishing inside as required.

 FEW CONSTRUCTION STEPS IN PICTURE : 

 Photo : Inside Plustering of Digester

Recent Improvements

•          Mixing Device

•          Turret

•          Pipeline

•          Hydraulic Chamber

•          Burner

•          Slurry Processing

•          Water Drain

•          Cooking Practice

 A Complete Plant (Old)

 A Complete Plant (New)

 Digester Bottom and Wall

 Digester Wall and Outlet Passage

 Top Dome

Inlet (Old)

 Mixing Device

 Inlet (New)

Inlet (New)

Center Pipe (Old)

Turret

Turret and Valve

Pipe Line

Hydraulic Chamber

Hydraulic Chamber (New)

  Slurry Pit

 MoU on Slurry Processing and Marketing 

Slurry Processing 

 Water Drain

Burner

.

Cooking with Biogas

1.5    MISCELLANEOUS WORK:

            Complete inlet chamber and the cover of hydraulic chamber.

            Complete back filling over the top dome.

            Install gas pipe ling.

Charge the digester with raw materials so that it is full without any void & keep close the burner until gas is produce.

1.6 INITIAL CHARGING OF BIOGAS PLANT :

            First keep the gas valve open. Put a mark "M" on the inside wall of the hydraulic chamber as shown in fig. below with the help of water level so that the position of this mark is the same level of the top most point of the top dome from inside .

Now charge the digester until raw material touches the mark "M". Close the valve & it is aneorobic now. Keep the burner's or appliance's knob or gas valve close after in use to keep the digester always anearobic.

2.0 Economic Worth of Bio-Gas

Bio-gas is not traded, and, therefore, has no market price.  Hence it has to be valued according to the equivalents that are traded.  Several fuels can be considered as alternatives, especially for cooking where cow-dung, firewood, charcoal, other gases, electricity and kerosene, and for lighting houses where kerosene, electricity have all been used.  Non-nal householders in the rural areas of Bangladesh use wood as a fuel in the oven and kerosene for lighting lamp or hajack.  Bio-gas can be used for both the above mentioned purposes.  So in this case the output of bio-gas plant could be used for cooking replacing the equivalent firewood requirement, for lighting replacing the equivalent kerosene requirement and also as fertilizer, meeting the equivalent need of urea of a household.  The prices of key inputs were estimated and the output of the bio-gas plant was estimated by considering the market price of firewood, kerosene and fertilizer.

DETAILED ESTIMATE OF FAMILY TYPE BIO GAS PLANT

PLANT TYPE-F2,DIA=2400mm,VOL=6m3FROM COW DUNG )