ICS 27.100  
 
Reference number  
DRS 306: 2016  
© RSB 2023  
  
 RWANDA 
STANDARD  DRS 
306 
 
Firstedition  
2016 -06-30 
Reaffirmed 2023 -mm-dd 
Domestic biogas plant —Design, 
construction and operation —Code of 
practice  
 
   
DRS 306: 2016  
©RSB 2023 - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition 
 
 
 
 
 
 
 
 
© RSB2023  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reproduce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw  DRS 306:2016  
iii                ©RSB 2023 - All rights reserved  
 Contents  Page 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and definitions  ................................ ................................ ................................ ...........................  1 
4 Classification of digesters according to digestion processes  ................................ .........................  4 
5 Selection of construction site  ................................ ................................ ................................ ..............  5 
6 Pre-requisites for setting up a biogas plant  ................................ ................................ .......................  6 
7 Determination of the plant sizes  ................................ ................................ ................................ ..........  6 
8 Design of biogas plant  ................................ ................................ ................................ ..........................  6 
9 Construction of biogas plant  ................................ ................................ ................................ ...............  7 
10 Production process of biogas  ................................ ................................ ................................ ............  15 
11 Operational requirements of the plant  ................................ ................................ ..............................  16 
12 Safety considerations  ................................ ................................ ................................ .........................  17 
13 Sludge management  ................................ ................................ ................................ ...........................  17 
Annex  A (informative)  Layout of biogas plant ................................ ................................ ..............................  19 
Annex  B (informativ e)  Lay -out of pipeline  ................................ ................................ ................................ .... 21 
 DRS 306: 2016  
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 Foreword  
Rwanda Standard sareprepared by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national st andards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by RSB Board of Directors members for publication and 
gazettement as Rwanda Standards.  
DRS306 was prepared by  a Joint  Technical Committee RSB/TC 49 , Renewable energy, RSB/TC 10, 
Electrical installation and protection against electrical shock and RSB/TC 50, Electrical generation, 
transmission and distribution systems.   
In the preparation of this standard, reference was made to the following standard:  
IS 9478:1989 (Reaffirmed 2 005) Family size biogas plant –‒ Code of practice  
The assistance de rived from the above source is hereby acknowledged with thanks.  
The first edition (RS  306: 2016 ) was reaffirmed  by the Board ondd-mm-2023 . 
Committee membership  
The following organizations were represented on Joint Technical Committee RSB/TC 49, Renewable energy , 
RSB/TC 10,Electrical installation and protection against electrical shock andRSB/TC 50, Electrical generation, 
transmission and distribution systems  in the preparation of this standard.  
AGROPLAST  
ANCHORECO  
EGCC Ltd  
Enterprise Multiservices (EMS)  
Geni Green Solutions (GGS) Ltd  
LIKANO  
Ministry of Infrastructure (MININFRA)  
Quality Engineering Company Ltd  DRS 306:2016  
v                ©RSB 2023 - All rights reserved  
 REDO  
REGREPOWER  
RMH  
Ruliba Clays Ltd  
Rwanda Energy Group Energy Development Corporation Ltd (REG/EDCL)  
SUN ALLIANCE  
TGS Ltd  
University of Rwanda - College of Science and Technology (UR -CST)  
Yes shop Ltd  
Rwanda Standards Board (RSB) – Secretariat  
 RS 306: 2016  
1                ©RSB 2023 - All rights reserved  
 Domestic biogas plant  — Design, construction and operation — Code of 
practice  
1 Scope  
This Draft Rwanda Standard covers the requirements for the design, construction and operation of a domestic 
fixed dome biogas plant.  
2 Normative references  
The following The following referenced documents are indispensable for the applicat ion of this document. For 
dated refere nces, only the edition cited applies. For undated references, the latest edition of the 
referenceddocument (in cluding any amendments) applies . 
RS EAS 18 -1, Cement –‒ Part 1: Composition, specification and conformity criteria for common cements  
RS EAS 54, Burnt clay bricks –‒ Specification  
RS 107, Building sand –‒ Specification  
RS EAS 12, Potable water –‒ Specification  
3 Terms and definitions  
For the purposes of this standard, the follow ing terms and definitions apply . 
3.1 
biogas plant  
plant used to process substrate to produce biogas and sludge consisting of an inlet mixing tank, digester, 
gaschamber and outlet/sludge tank  
3.2 
fixed dome biogas plant  
closed digester with an immovable, rigid gas chamber and a displacement pit  
3.3 
collecting tank/inlet  tank  
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 chamber where substrate and water are collected, stored and separated from heavy and non -biodegradable 
materials before feeding them into the digester  
3.4 
inlet pipe  
serves as conveyor of the substrate -water mixture or slurry from the mixing tank to the digester  
3.5 
digester/biodigester/bio -reactor/anaerobic reactor  
any water and air tight container designed for the process of anaerobic microbiological degradation of organic 
matter into which the slurry is introduced for digestion and methanization  
3.6 
stirrer/mixer/agitator  
manual or mechanical device inside the digester used to stir the slurry  
3.7 
gas chamber/gas holder  
space inside or outside the digester for the collection and storage of biogas  
3.8 
effluent  
residue that comes out at the outlet after the substrate is digested/processed inside the digester  
3.9 
backfill  
layer of compacted soil and gravel to support the digester wall  
3.10 
substrate  
organic material used to produce biogas  RS 306: 2016  
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 3.11 
slurry  
mixture of substrate and water  
3.12 
retention time  
average period that a given quantity of slurry is retained in the digester for digestion  
3.13 
methanogens  
anaerobic bacteria that act upon organic materials and in the process, produce biogas  
3.14 
methanization/digestion  
various processes th at take place among the methanogens, non -methanogens and substrate fed into the 
digester as inputs  
3.15 
biogas  
mixture of gas (composed of 50 % - 70 % methane and 30 % - 40 % carbon dioxide) produced by 
methanogenic bacteria  
3.16 
gas production rate  
amount  of biogas produced per day per cubic meter of slurry  
3.17 
loading rate  
amount of slurry fed per unit volume of digester capacity per day  
3.18  
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 scum  
layer of floating material (mainly fibrous) on the slurry  
3.19 
sludge  
settled portion or precipitate of the slurry; a mud -like, semi -solid mass  
3.20 
outlet pipe  
serves as conveyor where the effluent or the slurry is forced out  
3.21 
filling line  
level of slurry when the digesters is at full load  
3.22 
mesophilic temperature range  
temperature range of 20 °C - 40 °C where mesophilic bacteria operate  
4 Classification of digesters according to digestion processes  
4.1 Thermophilic fermentation  
These should be placed where the average ambient temperature in is 30 °C and above.  
4.2 Mesophilic fermentation  
These should be placed where the average ambient temperature is 20 °C - 30 °C.  RS 306: 2016  
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 4.3 Ambient temperature  
These should be placed where the average ambient temperature in is 10 °C - 20 °C.  
5 Selection of construction site  
5.1 Factors for site selection  
Selection of construction sites s hould be governed by the following main factors:  
a) site should facilitate easy construction works;  
b) selected site should be such that the construction cost is minimised;  
c) selected site should ensure easy operation and maintenance activities like feedi ng of plant, use of main  
gas valve, composing and use of slurry, checking of gas leakage, draining condensed water from  
pipeline; and  
d) site should guarantee plant safety.  
5.2 Considerations for site selection  
Based upon the factors mentioned in 5.1, plant location should be selected based upon the following 
considerations:  
a) for effective functioning of biodigesters, right temperature (20 °C - 35 °C) shall be maintained inside the 
digester. Damp and cool places should be avoided. Therefore, sunny sit e should be preferable.  
b) area to construct the plant should have even surface;  
c) site should be in slightly higher elevation than the surrounding to avoid water logging and also ensure  
free flow of slurry from overflow outlet to the composting pit;  
d) make plant easier to operate and avoid wastage of raw materials, especially the dung, plant should be as 
close as possible to the cattle shed;  
e) mix dung and water, considerable quantity of water shall be required;  
f) well or ground water source should be at least 10 m away from the biodigester especially the slurry pit to 
avoid the ground water pollution;   
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 g) main gas valve which is fitted just above the gas holder should be opened and closed before and after 
theuse of biogas. Therefore, the plant should be  as near to the point of application as possible;  
h) edge of plant should be at least 2 m away from the foundation of house or any structure;  
i) There should be enough space for compost -pit(s) as these are integral parts of the biodigester;  
j) site should be at sufficient distance from trees to avoid damage of biodigester from roots;  
k) type of soil should have enough bearing capacity to avoid the possibility of sinking of structure;  and 
l) when space is a problem, the cattle shed can be constructed on top of the plant after proper backfilling.  
6 Pre-requisites  for setting up a biogas plant  
The following requirements shall be looked into before setting up a biogas plant:  
a) individual or the family shall have sufficient source of substrate ; 
b) sufficient space shal l be available for constructing biogas plant and for  the location of effluent pits, 
composting or drying of effluent. This space shall preferably be near the source of substrate andinvariably 
be close to the place where gas shall be used. As there is no pr oblem of fly breeding or foulodour, the 
gas plant may be located near the residence ; and  
c) sufficient quantity of water shall be available for mixing with fresh bio -mass before feeding.  
7 Determination  of the plant sizes  
The sizes of the biogas plant shall dep end on the quantity and kind of available biomass and retention time of 
the material in the biogas system for which the plant is designed as specified in clause 8.  
8 Design of biogas plant  
8.1 Components  of a biogas plant  
The biodigester shall have the following  six main parts:  
a) inlet/mixing chamber for cattle dung fed plant);  
b) digester (digestion chamber);  
c) gas holder (storage chamber);  
d) outlet (displacement chamber);  RS 306: 2016  
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 e) gas conveyance system; and  
f) slurry compost pit(s).  
8.2 Sizes of biogas plant components  
8.2.1  The size of the digester, that is the digester volume (Vd) shall be determined on the basis of the 
chosen retention time (RT) and the daily substrate input quantity (Sd).  
Vd = S d × R T [ m3] = m3/day × number of days ]  
where 
Vd is in m3;  
Sd in m3/day;  
RT in Days;  
Biomass/ Organic material in kg and Water in L . 
8.2.2  The retention time, in turn, is determined by the chosen/given digesting temperature. For an 
unheatedbiogas plant, the temperature prevailing in the digester can be assumed as -272.15 °C to +271.1 5 °C 
abovethe soil temperature.  
Substrate input (S d) = biomass (B) + water (W) [m3/d] 
The ratio shall be 1:1  
8.2.3  The sizes of biogas plant components are shown on Figure A.1 and described in Table A.1.  
9 Construction  of biogas plant  
9.1 Selection of constructi on materials  
9.1.1  Cement  
The cement to be used in the plant construction shall comply with the requirements of RS EAS 18 -1. It shall 
be fresh, without lumps and stored in a dry place. Bags of cement should never be stacked directly on the 
floor or against the w alls to protect the cement from absorbing moisture before use.  
9.1.2  Sand  
Sand for construction purpose shall be clean. Sand containing more than 3 % impurities by volume shall be 
washed. The quantity of impurities especially mud in the sand can be determined by  a simple test using a 
bottle and clean water. For the test, the bottle shall be half -filled with sand, filled with clean water, and then  
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 stirred vigorously. The bottle shall be allowed to sit stationary to allow the sand to settle. The particles of sand 
will settle first while mud particles will settle last. After 20 min - 25 min, the thickness of the mud layer shall be 
compared to the sand inside the bottle. The percentage of mud should be less than 3 % of the overall volume. 
Course and granular sand can be used for concrete work, whereas fine sand complying with the requirements 
of RS 107 shall be used for plastering work.  
9.1.3  Gravel  
Individual gravel diameter should not be greater than 25 % of the thickness of the slab where it is used. As the 
slabs and the top of the dome shall not be greater than 8 cm thick, gravel should not be larger than 2 cm in 
size. The gravel shall be clean. If it is dirty, it should be washed with clean water.  
9.1.4  Water  
Water to be used for preparing the mortar for masonry work, conc rete work, plastering work, soaking 
bricks/stones before using them, washing sand and aggregates shall be potable water complying with the 
requirements of RS EAS 12 to avoid an adverse effect on the strength of the structure.  
9.1.5  Bricks  
Burnt clay bricks shall  comply with the requirements of RS EAS 54. When hitting two bricks together, the 
sound shall be crisp or clean. They shall be well baked and regular in shape. Before use, bricks shall be 
soaked for at least 10 m in clean water to prevent them from soaking  moisture from the mortar after laid in 
place.  
9.1.6  Cobble  stone  
Cobble -sized stones of 7.5 cm - 30 cm in diameter should be used for masonry work. They shall be clean and 
solid.  
9.2 Plant layout  
The plant layout shall be marked on the ground surface with wooden st akes, rocks, chalk or other materials. 
To mark the plant, a small peg shall be stuck in the ground at the planned centre of the digester. A cord 
indicating the radius of the digester shall be attached to the peg. The circumference can be marked by rotating  
the end of the cord in circular fashion. A suitable arrangement shall then be marked for the inlet tank, inlet -
pipe(s), outlet -chamber, compost -pits and gas piping.  
9.3 Excavation  
9.3.1  The excavation work should only be started after deciding the location of  manhole and outlet tank. For 
safety, the pit walls should be vertical and stepped from the ground surface by one meter away from the 
centre of the excavation for each meter in depth excavated.  
9.3.2  Excavated soil should be placed at least one meter away from the edge of the dig, so it does not fall 
inside the pit during construction. The pit bottom shall be levelled and the earth shall be untouched.  
9.3.3  If the design depth cannot be achieved because of hard rock or high groundwater, the design should 
be modified to a smaller plant or wider digester or combination of both. The biogas plant should not be RS 306: 2016  
9                ©RSB 2023 - All rights reserved  
 constructed at or below the ground water table elevation. The earth base of the excavation shall then be 
compacted using mechanical or manual tools.  
9.4 Const ruction  of digester main chamber  
9.4.1  The digester foundation base shall be placed using cobbles and gravel as aggregate then filled with 
concrete and compacted. The foundation should be 15 cm thick. The upper portion of the foundation shall 
have a smooth  plastered surface to prevent the seepage.  
9.4.2  At the centre of the pit, a straight rod or pipe shall be placed in an exact vertical position. The vertical 
pipe shall be used during the construction as a field -expedient guide to ensure symmetry of the biogas plant. 
At ground level, a rigid pole, pipe or cord shall be placed horizontally across the diameter of the pit. The 
vertical pipe shall be secured to the horizontal pipe, pole or cord. After securing, the vertical pipe should be 
checked to ensure that it is still in the plumb/vertical position.  
9.4.3 After the foundation has cured for at least two days, the round wall shall be constructed. The first two 
rows of bricks shall be positioned side by side so that 23 cm wide base is made. First row shall be placed on a 
firm, untouched and level foundation . Subsequent rows of bricks shall be positioned on their lengths so that 
the wall thickness is maintained at 23 cm wide.  
9.4.4  The backfilling between wall and pit -side shall be compacted with great care. Backfilling should be done 
not sooner than 12 h fol lowing brick course placement to allow mortar to cure. Earth should be well 
compacted by adding water and gentle ramming along the circumference of the digester.  
NOTE  Poor compaction leads to cracks in round -wall and dome.  
9.4.5  The cement mortar used can be one part of cement to four parts of sand (1:4) up to one part of cement 
to six parts of sand (1:6) depending on the quality of the sand. The height of the round -wall is detailed on the 
drawing in Figure 1 under dimension 'H' as measured from the finishe d floor. The substrate inlet pipe (and 
toilet pipe, if installed) shall be placed in position when the round -wall is 30 cm - 36 cm high. To reduce the 
risk of blockages, the inlet pipe(s) shall be placed as vertical as practically possible.  
9.4.6  Exactly t o the opposite of the main substrate inlet pipe, a 60 -cm wide opening shall be left in the round -
wall that serves as a manhole. The digested slurry will flow to the outlet tank through this opening. Additional 
inlet pipes should be placed as close as possi ble to the main substrate inlet pipe with a maximum distance of 
45° from the inlet -centre -manhole line. When the round -wall has reached the correct height, the inside shall 
be plastered with a smooth layer of cement mortar with mix of 1:3 cement -sand.  
9.5 Cons truction  of dome  
9.5.1 When the round wall of the digester is complete, the dome shall then be constructed. Before filling the pit 
with earth to make the mould for dome, backside of the round wall should be filled with proper compacted 
back -fill. If this i s not done, the pressure of earth for the mould can lead to cracks in the round -wall. 
9.5.2  The dome shall be constructed using a mould or cast technique. This can be accomplished by 
constructing a timber frame then placing the earth -cast top for proper ar ch design. Once the dome is cast, the 
timber frame and earth will be removed through the outlet chamber manhole.  
9.5.3  Mark on the vertical centre pipe the distance “J‟ from the finished floor noted on drawing in Figure A.1. 
The vertical pipe should remain  in place as the mould is constructed.   
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 9.5.4  The earth of the mould shall be well compacted. If the earth is furth er compressed after casting the 
dome, by its own weight and that of the concrete, it can lead to cracks in the dome. When the earth mould 
hasthe exact shape of the guide, a thin layer of fine sand shall be spread on the mould -top by gently patting it 
onthe surface. The sand layer shall prevent the earth from adhering to the cast. The earth used for the 
mouldshall be damp to prevent dry earth  from soaking up water from freshly casted concrete.  
9.5.5  Before start of the cast work, sufficient labour and construction mate rials like sand, gravel, cement and 
water shall be staged on the site and ready for use. The casting shall be done as quickly a s possible 
andwithout interruptions as this will negatively affect the quality of the cast. A constant, adequate supply 
ofconcrete (mix: 1 cement, 3 sand, 3 gravel (1:3:3) shall be made for the mason. No concrete older than 30 
minshould be used. Special ca re should be taken to maintain the thickness of the dome while casting, that is 
thethickness near the outer edge should be greater than the thickness at the centre. For the 4 m3, 6 m3, 8 m3 
and10 m3 plant, the thickness at the edge should be 25 cm whereas  the thickness in the centre should be 7 
cm.Similarly, for 15 m3 and 20 m3 plants, the thickness at the edge should be 25 cm and the thickness in 
thecentre should be 8 cm and 9 cm respectively.  
9.5.6  An alternative to concrete cast construction technique sh all be the use of  baked clay brick in corbel -arch 
construction. The compression of the brick and mortar in a spherical shape shall support the dome. Theclay 
brick dome shall have a near -uniform thickness compared to the cast -concrete dome that thins toward sthe 
centre. A continuous application of mortar along the sand mould is necessary as the bricks are placed.  
The brick dome should be placed continuously and use a mortar mix of (1:4) of cement to sand. Once the 
bricks for the dome have all been placed, the  exterior is covered with (1:3) of cement to sand plaster.  
9.5.7  During the casting, the concrete shall be protected against strong sunlight by covering it with wetted 
burlap, jute bags or straw mats. This protection shall be left in place for at least one  week. The day after the 
casting, the turret shall be made. The turret shall be made with brick, 36 cm square and 50 cm tall. The turret 
shall be plastered with 1:3 of concrete. Any delays during dome construction can lead to leakage between 
main gas pipe and dome.  
9.5.8  Following completion of the dome, the structure shall be sprinkled with water 3 to 4 times a day during 
the curing period (up to one week).  
9.5.9  After the dome has cured for approximately one week, the timbers and earth of the mould can be  
removed through the manhole. When all earth is removed, the inside of the dome has to be thoroughly 
cleaned with a stiff brush and clean water. On the clean surface of the dome interior, the following plaster 
coats shall be applied to make the dome gas -tight from first to last coats:  
a) cement and water wash (1:1);  
b) 10 mm layer: one part of cement, two parts of sand (1:2);  
c) 5 mm layer: 1:1;  
d) cement/acrylic emulsion paint coating: (1.5) parts of paint, 20 parts of cement; and  
e) cement/acrylic emulsion paint coating : one part of paint, two parts of cement . RS 306: 2016  
11                ©RSB 2023 - All rights reserved  
 9.5.10  Allow at least 24 h between plaster coats. When a layer of plaster is applied, the work must be 
executed with great care and without interruptions. The performance of the biogas plant is dependent on the 
gas tightness of the dome.  
9.5.11  For proper insulation during the cold season and as counter weight against the gas pressure inside, 
minimum top cover of 40 cm compacted earth is required on the dome. If the top cover will be prone to 
erosion due to wind and  rain proper protection with gravel, circular wall, or straw matting should be applied.  
9.6 Construction  of outlet chamber  
9.6.1  The outlet chamber excavation and manhole shall be completed concurrently with the digester vessel 
and the manhole shall share a co mmon foundation with the digester vessel. The manhole of the outlet 
chamber should be near the digester wall. The depth of excavation shall be less than the digester vessel 
measured from the top of digester floor by taking the dimension 'I' minus the thick ness of the digester floor 
shall depend upon construction material used but generally “I‟ + 13 cm. The earth behind the manhole and 
under the outlet floor shall be well compacted to prevent cracks in outlet chamber walls.  
9.6.2  The inside dimensions of the  outlet can be found on the drawing under  A, B and D. The overflow level 
shall be at the top of dimension “D‟ and top of the outlet chamber walls shall be dimension “D‟ + 15 cm. 
Theprescribed dimensions shall be used as they determine the useful capacity o f the gasholder. For the 
samereason the outlet floor and the top of the walls shall be level. The walls shall be vertical and finished with 
a smooth layer of cement plaster mix: one part of cement, three parts of sand. Outside of the walls shall be  
support ed with sufficient compacted earth up to the overflow level to avoid cracks. The outlet chamber walls 
should slightly higher elevation than the surrounding ground to reduce chances of surface water entering the 
outlet during the rainy season.  
9.6.3  The con crete slabs for the outlet chamber should be constructed at the same time of dome casting. It 
should be easy to make the additional concrete at this time and the slabs shall be well cured before they are 
placed on the outlet. The slabs shall be 8 cm thick with proper reinforcement (re -bar) 3 cm from the bottom 
side. The number of slabs should be designed that they can be handled by four people without great difficulty. 
Installing re -bar loop handles on the slabs may be useful for the occasional handling of the slabs.  
9.6.4  When preparing the slab casts, a smoothed, clean sand base and lumber shall be used for the frame. 
Special care shall be taken for the placement of the concrete to prevent small holes that can expose the steel 
reinforcement to corrosive va pour from the slurry in the outlet chamber which can cause corrosion and 
ultimately lead to the slab collapse. If holes form in the slab these should be filled with plaster layer. The outlet 
cover slabs are essential to protect people and animals from fall ing inside and to avoid excessive evaporation 
of the slurry in dry season.  
9.7 Construction  of inlet tank  
9.7.1  The inlet tank shall be constructed to mix substrate and water. This can be constructed with or without a 
mixing device. Installation of a mixing device should be preferable because not only it makes plant operation 
easier, but it also improves the qualit y of mix. The installed mixer shall be firmly attached to the structure, 
easyto operate, effective in the mixing process and the steel parts in contact with the substrate should be 
galvanized. The top of the structure should not be more than one meter abov e ground level and both inside 
and outside of the tank shall be covered with a smooth layer of plaster (Mix: 1:3 of cement, sand). The 
finished bottom of the inlet tank shall be at least 5 cm above the outlet chamber overflow level. The position of 
the inl et pipe shall be such that a pole or rod can be inserted through it to the digester vessel without 
obstructions. This shall allow the operator to clear blockages in the inlet pipe. For the same reason the inlet 
pipe shall be without bends. Even if a mixing  device is not installed, the inlet pit should be round in shape as 
this is a more economical use of material and easier for hand mixing.   
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 9.7.2 For plants  that receive from a toilet, a cleanout valve shall be constructed between the water -trap and 
digester.  The toilet inlet pipe should enter the digester tank no more than 45° from the centreline of the main 
inlet pipe. Additionally, the toilet pan level should be at least 25 cm above the outlet overflow level.  
9.8 Lay-out of pipeline  
9.8.1  The gas pipe conveying  the gas from the plant to point of user is vulnerable to damages by people, 
domestic animals and rodents. Therefore, only light quality galvanized iron pipe should be used which shall 
be, where possible, buried 30 cm below ground level. Fittings in the pi peline shall be sealed with zinc putty, 
teflon tape or jute and paint. Any other sealing agent, like grease, paint only, soap shall not be used. The use 
of fittings, especially unions, should be kept to a minimum to reduce the risk of leakage. No fittings should be 
placed between the main gas valve and the dome gas pipe. The pipe size, inside diameter should be between 
10 cm and 20 cm. Pipe size shall be determined by the size of the digester, (amount of gas produced) and 
amount of gas required in the house . 
9.8.2  The biogas generated from the digester is saturated with water vapour. This water vapour can 
condense on the walls of the pipeline. If the condensate is not removed regularly, it can ultimately clog the 
pipeline. For this reason, a water drain or t rap shall be installed in the pipeline. The position of the water drain 
should be vertically below the lowest point of the pipeline so that water flows by gravity to the trap. Water shall 
be removed by opening the drain (Figure B.1). As this has to be done  periodically; the drain shall be easily 
accessible and protected in a well -maintained drain pit.  
9.8.3  Neoprene and rubber hose should be used to connect burners to gas pipelines. Other biogas 
appliances should be mounted and connected to the galvanized iron pipe. As soon as there is gas production, 
all joints and taps shall be inspected for leakage by applying a thick soap solution and observing for foam 
movement.  
9.9 Compost  pits 
9.9.1  A minimum of two compost pits shall be dug near to the outlet  chamber overflow in such a way that 
the slurry can run freely into the pits. Enough earth body shall remain. However, at least 1 m, between the pits 
and the outlet chamber shall be kept to avoid cracking of the chamber walls. The total volume of the compo st 
pits shall be at least equal to the plant volume. The earth excavated from the compost pits should be used for 
backfilling of the inlet and outlet chamber and for top filling on the dome.  
9.9.2  The compost pits can also be filled with agricultural resid ues and the slurry from the plant for a 
nitrogen and nutrient rich fertilizer. Compost Pits are utilized by alternating discharge flow allowing the full pit 
to evaporate and leach into the soil while the other is being filled. Once the pit material desicca tes to the 
consistency of humus, the material could be removed for agricultural application as fertilizer.  
9.10 Collecting/mixing tank  
9.10.1  Appropriate channels (concrete, PVC pipes, HDPE pipes) shall be provided from the source of 
substrate to the collecting  tank with a minimum slope of 2 %.  
9.10.2  The tank should be concreted and a sluice gate should be provided to control or allow the proper 
mixture of water and manure.  
9.10.3  The floor of the mixing tank should be inclined from 8.5% - 17.5 % toward the inl et pipe and it should 
be elevated at least 0.2 m from the filling line.  RS 306: 2016  
13                ©RSB 2023 - All rights reserved  
 9.10.4  A cover made of steel sheet shall be provided to prevent undesired objects  
9.11 Inlet pipe 
Inlet pipe should be inclined 58 % with the digester wall. It should be sealed at the poin t of intersection with 
the digester wall.  
9.12 Digester  
9.12.1  Digesters should be made of metal, bricks, or reinforced concrete.  
9.12.2  For reinforced digester, reinforcement shall be a minimum of 10 mm diameter spaced at 0.15 m (both 
the curved and the horizo ntal bars) and the curved bars shall be anchored at the top beam. All reinforcement 
bars shall be secured and tied together with steel wire.  
9.12.3  The concrete walls of the digester shall be reinforced with steel wire mesh before plastering with mortar 
mixed with sealing compound or water -proofing compound. Plaster shall be applied in three layers (13 mm, 6 
mm, and 6 mm thick). Each layer shall be applied continuously and should be finished within one day. All 
comers of the digester shall be curved.  
9.12.4  Floors, beams and foundation shall withstand the maximum load and shall be made of reinforced 
concrete.  
9.12.5  Access to the digester should be through the manhole or through the outlet chamber. If a manhole is 
used as the access inside the digester, it s hould be constructed in the centre of the dome and it should be 
tightly sealed. Manhole cover should be 0.65 m in diameter and 0.125 m thick.  
9.13 Agitator  
Natural agitation should be performed for small biogas plant. Mechanical agitation should be provided for  
larger biogas, several times a day for the following reasons:  
a) avoid and destroy swimming and sinking layers,  
b) improve the activity of bacteria trough release of biogas and provision of fresh nutrients,  
c) mix fresh and fermenting substrate in order to inocula te the former, and  
d) arrive at an even distribution of temperature thus providing uniform conditions inside the digester.  
9.14 Gas chamber  
9.14.1  The concrete dome shall be reinforced with screen before plastering with mortar mixed with sealing 
compound. Plaster shall be applied in three layers (13 mm, 6mm and 6 mm thick). It should be applied 
continuously and should be finished within one day.  
9.14.2  The gas chamber shall be capable of withstanding an internal pressure of 0.15 bar.   
©RSB 2023 - All rights reserved                  14 
 9.14.3  The top part of a fixed -dome plant shall be painted with a gas -tight layer (water proofer', latex or 
synthetic paints).  
9.14.4  A weak -ring in the masonry of the digester should be provided to reduce the risk of cracking of the 
gas chamber. This should be placed between the lower (water -proof) and the upper (gas -proof) part of the 
hemispherical structure.  
9.15 Gas outlet pipe  
9.15.1  Gas outlet pipe shall be provided and it shall be connected to outgoing biogas valve.  
9.15.2  Ball valves or cock valves should be used. It should also be installed at all gas appliances as shutoff 
devices.  
9.15.3  Sealed T -joints should be connected before and  after the main valve to test the digester and the piping 
system for their gas -tightness separately.  
9.15.4  Gas escape valve prepared from three pieces of PVC pipes should be provided, one arm of the T -joint 
shall be connected from the gas outlet and the o ther arm shall link to the pipe which goes to the kitchen.  
9.15.5  The T -joint is inserted in the bottle and water shall be added to a depth of 40 mm - 50 mm above then 
lower point of the T. Sides of the bottle should be punched with small holes with a heig ht equal to the desired 
level of pressure to be maintained.  
9.15.6  Piping system connects the biogas plant to the gas appliances or to the gas reservoir. Gas reservoir 
should be made of plastic or steel. Piping system should be made of steel pipes or PVC p ipes.  
9.15.7  PVC pipes are susceptible to UV radiation and can easily be damaged; hence, PVC pipes should be 
placed underground. If the site is located in an area with high intensity of sunlight, steel pipe should be used.  
9.15.8  PVC should be laid at leas t 0.25 m deep underground. It should be placed in a sand bed and be 
covered with sand or fine earth.  
9.15.9  Table B1 shows the recommended pipe diameters depending on the flow -rate of biogas through the 
pipe and the distance between biogas digester and gas  appliances.  
9.15.10  If there are turns and bends in the piping system and used for indoors, heavy -duty hose with ply 
should be used. The minimum diameter should be 13 mm and if used in outdoor, it should be protected from 
high sunlight exposure.  
9.15.11  Piping system should be laid out in a way that allows a free flow of condensation water from moisture 
saturated biogas back into the digester.  
9.15.12  If depressions in the piping system cannot be avoided, water traps shall be installed at the lowestpoint 
of depressions with a minimum inclination of 1 %.  
9.15.13  Flame arrester shall be provided. It should be made of a ball or roll of fine copper wire mesh inserted 
in the gas line and it should be located near the digester and near to the point of gas use.  RS 306: 2016  
15                ©RSB 2023 - All rights reserved  
 9.16 Outlet pipe  
9.16.1  Reinforced concrete pipe should be used with a minimum diameter of 200 mm and it should be inclined 
58 % with digester wall.  
9.16.2  The upper end of the outlet pipe should be level with the bottom of the auxiliary chamber to allow the 
draw ing back of slurry when the pressure decreases.  
9.16.3  The outlet pipe shall be sealed.  
9.17 Outlet  tank 
9.17.1  The chamber should be made of concrete with smooth finish. Steel reinforcement may be used.  
9.17.2  Overflow should be provided with the height of at least 100 mm lower than the lower surface of the gas 
chamber. It should flow into farmland of the plant owner or flow into the lagoon for further treatment.  
9.17.3 The height of the floor of the chamber from the filling line shall be at least equal to the operating 
pressure for appliances using biogas or the height should be at least 0.2 m from the filling line plus 15 % 
freeboard.  
9.18 Ground  water drainage  
9.18.1  If the water seeps from bottom of the excavated pit, blind drain shall be constructed. It should b e filled 
with gravels or chip of tiles, with central sump and take water away manually or by pump.  
9.18.2  If water seeps from wall of the excavated earth bank, circular drain outside the location of digester wall 
shall be constructed to lead water away to sump or some lower places.  
9.18.3  In case groundwater is in great quantity, deep wells should be constructed 2 m away from the 
excavated pit, with a depth 0.8 m - 0.1 m lower than the pit, then pump water away from wells.  
9.18.4  In case of too much water and shifting sand sunken barrel sh all be used.  
10 Production  process of biogas  
10.1 The required quantity of substrate and water is mixed in the inlet tank and the slurry is discharged to the 
digester vessel for digestion. The gas produced through methanogenesis bacteria in the digester is co llected 
in the dome.  
10.2 The digested slurry flows to the outlet tank through the manhole.  
10.3 The slurry then flows through the overflow opening in the outlet tank to the compost pit.  
10.4 The gas is supplied from the dome to the point of application through a turret and pipeline.   
©RSB 2023 - All rights reserved                  16 
 10.5 When a biogas plant is underfed the gas production will be low ; in this case, the pressure of the gas 
might not be sufficient to fully displace the slurry in the outlet chamber. It is important to design the plant 
keeping hydrostatic pressure higher at the inlet tank than the outlet tank.  
10.6 The hydrostatic pressur e from slurry in the inlet and outlet tanks will pressurize the biogas accumulated 
in the dome. If too much material is fed into the digester and the volume of gas is consumed, the slurry may 
enter the gas pipe and to the appliances.  
10.7 The mix of substr ate and water (mixed in inlet or mixing chamber) shall pass through the inlet pipe to the 
digester. The mixer shall produce gas through digestion process in the digester and the produced gas shall be 
stored in the gas holder (top of dome).  
NOTE  In most bio gas plants, the mixing ratio for dung (cattle and / or pigs) and water (B:W) is 1:1.  
10.8 The digested slurry shall pass out from the digester to outlet tank (displacement chamber) and shall flow 
out to the compost pits through the overflow opening in the outlet tank. The gas shall then be supplied to the 
kitchen through the pipe line.  
11 Operational  requirements of the plant  
11.1 Feeding  of biodigester  
11.1.1  Before operation is started, clean interior digester and after that recheck the installed system to ensure  
airtight or leakage proof. In addition,  all components of the system shall be checked to ensure gastightness.  
11.1.2  To ensure durable and sustainable operation of biodigester, the diges ter shall be fed daily with raw 
substrate. Fresh material shall be fe d daily in the digester to sustain the cooking gas need of the family.  
11.1.3  The operator shall ensure that the materials listed below are not fed in the digester as they 
interruptreaction of bacteria in anaerobic condition. Presence of those materials ma y cause less biogas 
production oreven stop production as a whole:  
a) soil, sand and stones;  
b) branches; and  
c) soaps and detergent solution, cleaning chemicals, antiseptic products, paint and rain water.  
11.2 Operation  of biodigester  
11.2.1  At the beginning, i f gas cannot be ignited due to the reason that it is not o f high quality as it contains a 
lot of carbonic dioxide resulting from digestion of substrate with oxygen remaining before closing the tank, 
firstgas should be released from the reservoir, and wait until the next gas comes in which the balloon is 
refilled.  
11.2.2  If biogas doesn’t supply sufficiently for cooking, rubber should be emplo yed to tie across the reservoir 
to increase pressure inside the gas holder. After cooking, attention should be paid t o remove rubber from 
thereservoir so that it can receive new biogas from the plant.11.2.3 If gas holder is seen loose, check the RS 306: 2016  
17                ©RSB 2023 - All rights reserved  
 pipelines and gas holder system itself, immediately for fear thatthey are bent or clogged, or control water in 
the bottle safet y valve or at siphon inlets that it could be empty. Ifso, repair shall be done urgently.  
11.3 Pumping  of substrate  
11.3.1  If the amount of substrate requires fast movement, to mix the substr ate and when the gravity cannot be 
used for reasons of topography or su bstrate characteristics, pumps should be used.  
11.3.2  Centrifugal pumps for liquid substrate or rotary pumps for substrate of  less than 8 % solid content or 
positive displacement pumps for substrate with higher solid content may be used.  
11.3.3  Pump delivery lines should be made of steel, PVC (rigid), PE (rigid or flexible), or flexible 
pressure tubing made of reinforced plastic or rubber . 
12 Safety considerations  
12.1 Visual  check  
Cracks, gap and tightness of duct in the digester inner wall should be ca refully checked using wooden 
hammer. If vacant sound was heard in a certain area, there is some warping in plastering. Leak trace for wall 
should be checked by spreading some cement powder over the surface, the wet spot or wet line proves that 
there is lea k hole or leak crack.  
12.2 Water -holding  mark  
12.2.1  There shall be no visible cracks in the digester dome; there shall be no hollow space in the plaster 
layer.  
12.2.2  The digester should be filled with water up to the inlet and outlet pipes level. Allow it to set for 3 h - 5 h 
until the walls are saturated  with water and mark the water level. Set it for overnight, if there is significant drop 
in water level, it indicates that there are leaks or cracks.  
12.3 Air tight method  
12.3.1  Fill the digester with water or slurry and observe the drop in level after 24 h. T he level shall not drop by 
more than 10 cm.  
12.3.2  After water tightness test, gas test should be followed. Manhole and gas valves should be closed and 
sealed. Add water through the inlet to increase the air pressure inside the digester up to 0.4 m of wate r 
column. Or air may be blown into the digester up to the same pressure. Leave it for 24 h, if the pressure drop 
is about 10 mm - 20 mm, the digester is gas tight. But if the pressure drop is about 50 mm, the dome is not 
gas tight.  
13 Sludge management  
13.1.1  The digested slurry should be either spread on the fields before the beginning of the vegetation period 
or further conditioned.   
©RSB 2023 - All rights reserved                  18 
 13.1.2  The sludge should be channelled to a sludge conditioning facilities where the solid component is 
separated and sun -dried  while the liquid part is aerated to oxidize the toxic component.  
 RS 306: 2016  
19                ©RSB 2023 - All rights reserved  
 Annex  A 
(informative ) 
 
Layout of biogas plant  
 
 
Figure A.1 — Domestic biogas plant design  
 
 
 
©RSB 2023 - All rights reserved                  20 
 Table A.1 — Measurement of various  sizes of biogas plants  
Components  
of the plant in  
cm    Plant sizes in m3   
4  6  8  10  15  20  
A  140  150  170  180  248  264  
B  120  120  130  125  125  176  
C  135  151  170  183  205  233  
D  50  60  65  68  84  86  
E  154  155  172  168  180  203  
F  102  122  135  154  175  199  
G  195  211  230  243  265  293  
H  86  92  105  94  115  115  
I  112  116  127  124  132  137  
J  151  110  175  171  193  203  
A is the Length of outlet;  
B width of outlet;  
C radius of pit;  
D height of outlet;  
E depth of excavation;  
F radius of digester;  
H height of digester wall;  
I height of outlet passage;  
J inner height of digester and dome . RS 306: 2016  
21                ©RSB 2023 - All rights reserved  
 Annex  B 
(informative ) 
 
Lay-out of pipeline  
 
 
 Pipe size: 10 mm to 20 mm inside diameter  
Figure B.1 — Schematic for condensate drain Valve in gas line  
 
 
 
 
 
 
 
 
 
 
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 Table B.1 — Pipe diameter for different pipe lengths and flow -rate (maximum pressure loss < 5 mbar)  
 Galvanised steel pipe  
(mm)  PVC 
(m) 
Length(m)  20  60  100  20  60  100  
Flow rate 
(m3/h)        
0.1  12.7  12.7  12.7  12.7  12.7  12.7  
0.2  12.7  12.7  12.7  12.7  12.7  12.7  
0.4  12.7  12.7  12.7  12.7  12.7  12.7  
0.5  12.7  12.7  12.7  12.7  19.0  12.7  
1.0  19.0  19.0  19.0  19.0  19.0  19.0  
1.5  19.0  19.0  19.0  25.4  19.0  19.0  
2.0  19.0  19.0  25.4  19.0  19.0  25.4  
 
 
 
 
 
 
 
 
 
 
 RS 306: 2016  
23                ©RSB 2023 - All rights reserved  
 Bibliography  
[1] PNS/PAES 411:2003 Agricultural structure –‒ Biogas plant  
[2] Modified GGC Model Biogas plant for Pakistan  
[3] Biogas plant construction manual, Fixed -dome Digester: 4 to 20 Cubic Meters  
[4] Biogas handbook : 2008, ISBN 978 -87-992962 -0-0 
 DRS 306: 2016 
 
Price based on  22pages  
©RSB2023 - All right s reserved