5.6 Measuring methods and devices for biogas plants

5.6 Measuring methods and devices for biogas plants

The purpose of conducting measurements on a biogas plant is to enable timely detection of developing problems, adjustment to optimum operating conditions, and gathering of practical 'data for comparison with those of other plants. The following variables can be measured quickly and easily:
- gas production via dry gas meter or by measuring the fill level of the gasholder
- weight of inputs via a hand-held spring scale
- temperature via an ordinary stem thermometer or electronic temperature sensor
- total-solids content by drying a sample at 104 °C and weighing the residue on a precision balance
- H₂S content of the gases via a gas test tube
- pH via litmus paper.
The contents of the substrate/slurry can only be determined by a special laboratory.
Various levels of precision are recommended, depending on the set objective and corresponding time, effort and equipment expenditure.
Observation by the user

Procedure

- measuring the gas consumption through daily checking of the calibration marks on the gasholder
- measuring the daily input quantities via defined-volume vessels
- measuring the air/slurry temperature with a thermometer.
Documentation
Daily notation of measured values.
Interpretation/results
Daily gas production as a function of substrate input and temperature.
Field testing by the extension officer

Procedure

- installation and daily reading of a dry gas meter to determine the rate of gas production
- random sampling of the CO₂ and H₂S contents of the biogas
- determination of quantities added by weighing the moist mass and water on a spring scale
- random sampling to determine the total solids content of the substrate
- measuring the digester temperature with the aid of a remote electronic thermometer
- measuring the ambient temperature with a mini-max thermometer
- determining pH levels via litmus paper
- laboratory testing to determine the C/Nratio, volatile solids content and manurial quality of digested slurry.

Fig. 5.38: Measuring instruments for biogas field tests. 1 Gas meter, 2 CO2 tester, 3 Mini-max thermometer, 4 Spring scale, 5 Stem thermometer, 6 Insertable thermometer, 7 Electric remote thermometer, 8 Litmus paper (Source: OEKOTOP)

Documentation
Daily entry of measured values in a log book.
Interpretation of results
- time history of daily gas production as a function of temperature and substrate input
- time history of specific gas yield (Gy = m³ gas/kg TS) and of specific gas production (m³ gas/m³ Vd) as a function of temperature
- time history of pH
- time history of maximum and minimum ambient temperatures, i.e. mean monthly and annual temperatures, plus extremes.

Design and construction of plant components

5.4 Design and construction of plant components

Biogas plants of simple design consist of the following main components:
- mixing pit
- inlet/outlet(feed/dischargepipes)
- digester
- gasholder
- slurry store.
Depending on the available building material and type of plant under construction, different variants of the individual components are possible.
Table 5.8: common substrate mixing ratios (Source: OEKOTOP, compiled from various sources)

Type of substrate	Substrate:	water
Fresh cattle manure	1	: 0.5 -1
Semi-dry cattle dung	1	: 1-2
Pig dung	1	: 1-2
Cattle and pig dung from a floating removal system	1	: 0
Chicken manure	1	: 4-6
Stable manure	1	: 2-4

Fig. 5.16: Mixing pit. 1 Plug, 2 Fill pipe, 3 Agitator, 4 Fibrous material, 5 Sand, 6 Drain, 7 Screen cover (Source: OEKOTOP)

5.4.1 Mixing pit
In the mixing pit, the substrate is diluted with water and agitated to yield a homogeneous slurry.
The fibrous material is raked off the surface, and any stones or sand settling to the bottom are cleaned out after the slurry is admitted to the digester.
The useful volume of the mixing pit should amount to 1.5-2 times the daily input quantity. A rock or wooden plug can be used to close off the inlet pipe during the mixing process. A sunny location can help warm the contents before they are fed into the digester in order to preclude thermal shock due to the cold mixing water. In the case of a biogas plant that is directly connected to animal housing, it is advisable to install the mixing pit deep enough to allow installation of a floating gutter leading directly into the pit. Care must also be taken to ensure that the low position of the mixing pit does not result in premature digestion and resultant slurry formation. For reasons of hygiene, toilets should have a direct connection to the inlet pipe.

Fig. 5.17: Mixing pit, gutter and toilet drain pipe. 1 Barn, 2 Toilet, 3 Biogas plant, 4 Feed gutter 2% gradient), 5 Mixing pit (Source: OEKOTOP)

5.4.2 Inlet and outlet
The inlet (feed) and outlet (discharge) pipes lead straight into the digester at a steep angle. For liquid substrate, the pipe diameter should be 10-15 cm, while fibrous substrate requires a diameter of 20 - 30 cm. Plastic or concrete pipes are preferred.
Note:
- Both the inlet pipe and the outlet pipe must be freely accessible and straight, so that a rod can be pushed through to eliminate obstructions and agitate the digester contents;
- The pipes should penetrate the digester wall at a point below the slurry level. The points of penetration should be sealed off and reinforced with mortar.
- The inlet pipe ends higher than the outlet pipe in the digester in order to promote more uniform throughflow. In a fixed-dome plant, the inlet pipe defines the bottom limit of the gasholder, thus providing overpressure relief.
- In a floating-drum plant, the end of the outlet pipe determines the digester's slurry level.

Fig. 5.18: Inlet and outlet for fixed-dome (1) and floating-drum plants (2) (Source: OEKOTOP)

Fig. 5.19: Forces acting on a spherical-dome digester (Source: OEKOTOP)

5.4.3 Digester
Design
The digester of a biogas plant must accommodate the substrate and bacterial activity, as well as fulfill the following structural functions:
- accept the given static forces
- provide impermeability to gas and liquids
- be durable and resistant to corrosion
As a rule, the digesters of simple biogas plants are made of masonry or concrete. Such materials are adequately pressure-resistant, but also susceptible to cracking as a result of tensile forces.
The following forces act on the digester:
- external active earth pressures (pE), causing compressive forces within the masonry
- internal hydrostatic and gas pressures (pW), causing tensile stress in the masonry.

Fig. 5.20: Level line, excavation and foundation. 1 Workspace, 2 Inclination of conical foundation, 3 Sloping excavation, 4 Vertical excavation, 51 Quarrystone foundation, 52 Brick foundation, 6 Packing sand, 7 Mortar screed, 8 Foot reinforcement for fixed-dome plant, 9 Level line (Source: OEKOTOP / Sasse 1984)

Thus, the external pressure applied by the surrounding earth must be greater at all points than the internal forces (pE > pW). For the procedure on how to estimate earth force and hydrostatic forces, please refer to chapter 10.1.4.
Round and spherical shapes are able to accept the highest forces—and do it uniformly. Edges and corners lead to peak stresses and, possibly, to tensile stresses and cracking. Such basic considerations suggest the use of familiar cylindrical and dome designs allowing:
- inexpensive, material-sparing construction based on modest material thicknesses
- a good volume/surface ratio and
- better (read: safe) stability despite simple construction.
The dome foundation has to contend with the highest loads. Cracks occurring around the foundation can spread out over the entire dome, but are only considered dangerous in the case of fixed-dome plants. A rated break ring can be provided to limit cracking.
Groundwork
The first step of building the plant consists of defining the plant level line with a taut string. All important heights and depths are referred to that line.
Excavation
The pit for the biogas plant is excavated by hand in the shape of a cylindrical shaft. The shaft diameter should be approx. 2 x 50 cm larger than that of the digester. If the soil is adequately compact and adhesive, the shaft wall can be vertical. Otherwise it will have to be inclined. The overburden, if reusable, is stored at the side and used for backfilling and compacting around the finished plant.
Foundation
The foundation slab must be installed on well-smoothed ground that is stable enough to minimize settling. Any muddy or loose subsoil (fill) must be removed and replaced by sand or stones. The bottom must have the shape of a shallow inverted dome to make it more stable and rigid than a flat slab. Quarrystones, bricks and mortar or concrete can be used as construction materials. Steel reinforcing rods are only necessary for large plants, and then only in the form of peripheral ties below the most heavily burdened part, i.e. the dome foundation.

Fig. 5.21: Construction of a spherical dome from masonry. 1 Dome/masonry, 2 Establishing the centerpoint, 3 Trammel, 4 Brick clamp with counterweights, 5 Backfill (Source: Sasse 1984)

Dome
The dome of the biogas plant is hemispherical with a constant radius. Consequently, the masonry work is just as simple as for a cylinder and requires no falsework. The only accessory tool needed is a trammel.
The dome masonry work consists of the following steps:
- finding and fixing the centerpoint of the dome radius in relation to the level line
- layer-by-layer setting of the dome masonry, with the bricks set in mortar, positioned and aligned with the aid of the trammel and tapped for proper seating
- in the upper part of the dome - when the trammel is standing at a steeper angle than 45°, the bricks must be held in place until each course is complete. Sticks or clamps with counterweights can be used to immobilize them.
Each closed course is inherently stable and therefore need not be held in place any longer. The mortar should be sufficiently adhesive, i.e. it should be made of finely sieved sand mixed with an adequate amount of cement.
Table 5.9: Mortar mixing ratios (Source: Sasse, 1984)

Type of mortar	Cement	Lime	Sand
Masonry mortar	2 :	1 :	10
Masonry mortar	1	:	6
Rendering mortar	1	:	4-8

Table 5.10: Suitability tests for rendering/mortar sands (Source: Sasse, 1984)

Test	Requirement
1. Visual check for coarse particles	Particle size: <7 mm
2. Determining the fines fraction by immersion in a glass of water: 1/21 sand mixed with 1 1 water and left to stand for 1 h, after which the layer of silty mud at the top is measured.	Silt fraction: < 10%
3. Check for organic matter by immersion in an aqueous solution of caustic soda: 1/2 I sand in 1 1 3 % caustic soda with occasional stirring. Notation of the water's color after 24 h.	Clear-to-light-yellow = low org. content: suitable for use Reddish brown = high org. content: unsuitable for use

Rendering
Mortar consisting of a mixture of cement, sand and water is needed for joining the bricks and rendering the finished masonry. Biogas plants should be built with cement mortar, because lime mortar is not resistant to water.
The sand for the mortar must be finely sieved and free of dust, loam and organic material. That is, it must be washed clean.
Special attention must be given to the mortar composition and proper application for rendering, since the rendering is of decisive importance with regard to the biogas plant's durability and leaktightness. Ensure that:
- trowelling is done vigorously (to ensure compact rendering)
- all edges and corners are rounded off
- each rendering course measures between 1.0 and 1.5 cm
- the rendering is allowed to set|dry slowly (keep shaded and moist, as necessary)
- the material composition is suitable and mutually compatible
- a rated break ring is provided for a fixed-dome plant
Crack-free rendering requires lots of pertinent experience and compliance with the above points. Neither the rendering nor the masonry is gaslight and therefore has to be provided with a seal coat around the gas space (cf. chapter 5.4.4).
5.4.4 Gasholder
Basically, there are three different designs/ types of construction for gasholders used in simple biogas plants:
- integrated floating drums
- fixed domes with displacement system and
- separate gasholders

Fig. 5.22: Construction of a metal gasholder with internal guide frame. 1 Lattice beam serving as cross pole, 2 Cross pole with bracing, 3 Gas pipe (2% gradient), 4 Guide frame, 5 Braces for shape retention and breaking up the scum layer, 6 Sheet steel (2-4 mm) serving as the drum shell (Source: OEKOTOP/Sasse, 1984)

Floating-drum gasholders
Most floating-drum gasholders are made of 2 - 4 mm-thick sheet steel, with the sides made somewhat thicker than the top in order to counter the higher degree of corrosive attack. Structural stability is provided by L-bar bracing that simultaneously serves to break up surface scum when the drum is rotated.
A guide frame stabilizes the gas drum and keeps it from tilting and rubbing on the masonry. The two equally suitable types used must frequently are:
- an internal rod & pipe guide with a fixed (concrete-embedded) cross pole (an advantageous configuration in connection with an internal gas outlet)
- external guide frame supported on three wooden or steel legs (cf. fig. 5.7).
For either design, it is necessary to note that substantial force can be necessary to turn the drum, especially if it is stuck in a heavy layer of floating scum. Any gasholder with a volume exceeding 5 or 6 m³ should be equipped with a double guide (internal and external).
All grades of steel normally used for making gasholders are susceptible to moisture-induced rusting both inside and out. Consequently, a long service life requires proper surface protection consisting of:
- thorough derusting and desoiling.
- primer coat of minium
- 2 or 3 cover coats of plastic/bituminous paint.
The cover coats should be reapplied annually. A well-kept metal gasholder can be expected to last between 3 and 5 years in humid, salty air or 8-12 years in a dry climate.
Materials regarded as suitable alternatives to standard grades of steel are galvanized sheet metal, plastics (glass-reinforced plastic/ GRP, plastic sheeting) and ferrocement with a gaslight lining. The gasholders of waterjacket plants have a longer average service life, particularly when a film of used oil is poured on the water seal to provide impregnation.

Fig. 5.23: Construction of a fixed-dome gasholder. 1 Slurry level for an empty gasholder (zero line), 2 Slurry level for a full gasholder, 3 Overflow, 4 Inlet = overpressure relief, 5 Earth cover (at least 60 cm), 6 Reinforcing ring at foot of dome, 7 Max. gas pressure. A Detail: wall construction: .1 Outer rendering,.2 Masonry, .3 Twolayer inner rendering, .4 Seal coat. B Detail: rated break point: .1 Masonry bricks (laid at right angles), .2 Joint reinforced with chicken wire, .3 Seal rendering - inside and out (Source: OEKOTOP)

Fixed domes
In a fixed-dome plant the gas collecting in the upper part of the dome displaces a corresponding volume of digested slurry. The following aspects must be considered with regard to design and operation:
- An overflow must be provided to keep the plant from becoming overfilled.
- The gas outlet must be located about 10 cm higher than the overflow in order to keep the pipe from plugging up.
- A gas pressure of 1 mWG or more can develop in the gas space, Consequently, the plant must be covered with enough earth to provide an adequate counterpressure; special care must be taken to properly secure the entry hatch, which may require weighing it down with 100 kg or more.
The following structural measures are recommended for avoiding or at least limiting the occurrence of cracks in the dome (cf. fig. 5.23):
- For reasons of static stability, the centerpoint of the dome radius should be lowered by 0.25 R (corresponding to bottom center of the foundation). This changes the geometry of the digester, turning it into a spherical segment, i.e. flatter and wider, which can be of advantage for the plant as a whole.
- The foot of the dome should be made more stable and secure by letting the foundation slab project out enough to accept an outer ring of mortar.
- A rated break/pivot ring should be provided at a point located between 1/2 and 2/3 of the minimum slurry level. This in order to limit the occurrence or propagation of cracks in the vicinity of the dome foot and to displace forces through its stiffening/ articulating effect such that tensile forces are reduced around the gas space.

Fig. 5.24: Entry hatch of a fixed-dome biogas plant. 1 Concrete cover, 2 Gas pipe, 21 Flexible connection (hose), 3 Cover wedging, 31 Length of pipe anchored in the masonry, 32 Retaining rod, 33 Wooden/metal wedges, 4 Edge seal made of loam/mastic compound, 5 Handles, 6 Weights, 7 Water (Source: OEKOTOP)

In principle, however, masonry, mortar and concrete are not gaslight, with or without mortar additives. Gastightness can only be achieved through good, careful workmanship and special-purpose coatings. The main precondition is that the masonry and rendering be strong and free of cracks. Cracked and sandy rendering must be removed. In most cases, a plant with cracked masonry must be torn down, because not even the best seal coating can render cracks permanently gaslight.
Some tried and proven seal coats:
- multilayer bitumen, applied cold (hot application poses the-danger of injury by burns and smoke nuisance); solvents cause dangerous/explosive vapors. Two to four thick coats required.
- bitumen with aluminum foil: thin sheets of overlapping aluminum foil applied to the still-sticky bitumen, followed by the next coat of bitumen.
- plastics, as a rule epoxy resin or acrylic paint; very good but expensive.
- paraffin, diluted with 2 - 5% kerosene heated to 100 °C and applied to the preheated masonry. The paraffin penetrates deep into the masonry, thus providing an effective (deep) seal. Use kerosene/gas torch to heat masonry.
In any case, a pressure test must be performed before the plant is put in service (cf. chapter 7.1).
Table 5.11: Quality ratings for various dome-sealing materials (Source: OEKOTOP)

Material	Processing	Seal	Durability	Costs
Cold bitumen	++	o	o	++
Bitumen with alu-foil	+	++	+	+
Epoxy resin	++	+	++	-
Paraffin	+	o	o	++
++ very good	+ good	o satisfactory	- problematic

Fig. 5.25: Sealing the masonry with paraffin. 1 Heat wall to 60 - 80 °C with soldering torch, 2 Apply hot (100 °C) paraffin (Source: OEKOTOP/ BEP Tanzania)

Plastic gasholders
Gasholders made of plastic sheeting serve as integrated gasholders (cf. chapter 5.3.3: earth pits), as separate balloon/bag-type gasholders and as integrated gas-transport/ storage elements.
For plastic (sheet) gasholders, the structural details are of less immediate interest than the question of which materials can be used. Table 5.12 (p. 74) surveys the relative suitability of various commercial grades of plastic sheeting.

Fig. 5.26: Separate, mobile, plastic-sheet gasholder. 1 Cart for gasholder volumes of 1 m³ and more, 2 Stabilizing weights and frame, 3 Reinforced plastic gasholder (Source: Wesenberg 1985)

Separate gasholders
Differentiation is made between:
- low-pressure, wet and dry gasholders (10 - 50 mbar) Basically, these gasholders are identical to integrated and/or plastic (sheet) gasholders. Separate gasholders cost more and are only worthwhile in case of substantial distances (at least 50-100 m) or to allow repair of a leaky fixed-dome plant.
- medium- or high-pressure gasholders (8 - 10 bar/200 bar)
Neither system can be considered for use in small-scale biogas plants. Even for large-scale plants, they cannot be recommended under the conditions anticipated in most developing countries. High-pressure gas storage in steel cylinders (as fuel for vehicles) is presently under discussion. While that approach is possible in theory, it would be complicated and, except in a few special cases, prohibitively expensive. It would also require the establishment of stringent safety regulations.

2 PVC (polyvinyl chloride)	7 Short-term/continuous load
PE (polyethylene)	11 Permeability coefficient, P, for new material
CPE (chlorinated polyethylene)	12 HF = high-frequency seam welding
IIR (isobutylene-isoprene rubber)	HW = hot-wedge seam welding
EPDM (ethylene-propylene diene monomer)	HA = hot-air seam welding
4 Inflatable gasholder, approx. 2.5 m³,	C = cementing
3-fold protection against rupture	HV = hot vulcanizing
6/8/9 - poor, o satisfactory,	FF = fusion firing
10/13 + good, ++ very good	HT = heat-solvent tape sealing

5.4.5 Gas pipe, valves and fittings
Gas pipe
The following types of gas pipes are in use:
- PVC pipes with adhesive joints
- steel pipes (water supply pipes) with screw couplings
- plastic hoses.
Galvanized steel water supply pipes are used most frequently, because the entire piping system (gas pipe, valves and fittings) can be made of universally applicable English/U.S. Customary system components, i.e. with all dimensions in inches. Pipes with nominal dimensions of 1/2" or 3/4" are adequate for small-to-midsize plants of simple design and pipe lengths of less than 30 m. For larger plants, longer gas pipes or low system pressure, a detailed pressure-loss (pipe-sizing) calculation must be performed (cf. chapter 10.2).
Table 5.13: Gas-pipe pressure losses (Source: OEKOTOP)

Volum	Pipe (galv. steel pipe)
flow, Q	1/2“		¾”		1”
(m³ /h	v1	dp/l2	v1	dp/l2	v1	dp/l2
	m/s	cmWG/10m	m/s	cmWG/10m	m/s	cm WG/10 m
0.1	0.35	0.03	0.16	0.004	0.09	0.001
0.2	0.71	0.12	0.32	0.02	0.18	0.004
0.4	1.4	0.47	0.64	0.06	0.36	0.016
0.6	2.1	1.06	0.94	0.15	0.53	0.034
0.8	2.8	1.9	1.3	0.27	0.72	0.06
1.0	3.5	2.9	1.6	0.41	0.88	0.09
1.5	5.3	6.7	2.3	0.85	1.33	0.2
2.0	7.0	11.8	3.2	1.6	1.8	0.4

¹ Velocity of flow in the pipe
² Differential pressure (pipe only) stated in cm WG per 10 m pipe
When installing a gas pipe, special attention must be paid to:
- gastight, friction-type joints
- line drainage, i.e. with a water trap at the lowest point of the sloping pipe in order to rule out water pockets
- protection against mechanical impact.
Some 60% of all system outages are attributable to defective gas pipes. For the sake of standardization, it is advisable to select a single size for all pipes, valves and fittings.
Valves and fittings
To the extent possible, ball valves or cock valves suitable for gas installations should be used as shutoff and isolating elements. Gate valves of the type normally used for water pipes are conditionally suitable. Any water valves used must first be checked for gastightness.

Fig. 5.27: Gas pipe, valves and fittings of a biogas plant. 1 Plant shutoff valve, 2 Water trap, 3 Pressure gauge, 4 House shutoff valve, 5 Cookstove, 6 Lamp, 7 Appliance shutoff valve, 8 Gasmeter (Source: OEKOTOP)

Gas manometer
A U-tube pressure gauge is quick and easy to make and can normally be expected to meet the requirements also of a fixed-dome system.

Fig. 5.28: Gas valves and fittings: U-tube pressure gauge (a), water trap with drain valve (b), U-tube water separator (c), "gravel-pot" flashback arrestor (d). 1 Gas pipe, 2 Condensate collector, 3 Shutoff valve, 4 Manometer valve, 5 U-tube pressure gauge made of transparent hose, 6 Wooden balls, 7 Antievaporation cap, 8 U-tube, 9 "Gravel-pot" flashback arrestor (approx. 51) filled with 20 mm gravel (Source: OEKOTOP)

Pressure relief
The task of running a fixed-dome system can be made easier by installing a spring-loaded pressure reducing valve that guarantees a constant (adjustable) supply pressure.
Water separation
If at all possible, the water trap should operate automatically. However since fixed-dome systems need a high water seal, often amounting to more than 1 m WG, the use of condensate collector with a manually operated drain valve is advisable.
Backflow prevention
As a rule, the water trap also functions as a flashback chamber. If deemed necessary, a gravel trap can be installed for added safety.

Biogas plants of simple design

Biogas plants of simple design

There are two basic types of tested biogas plants that have gained widespread acceptance in agricultural practice:
- floating-drum plants in which the metal gasholder floats on the digester, and
- fixed-dome plants in which gas storage is effected according to the displacement principle.

5.3.1 Floating-drum plants

A floating-drum biogas plant essentially consists of a cylindrical or dome-shaped digester and a movable, floating gasholder, or drum. The drum in which the biogas collects has an internal or external guide frame that provides stability and keeps the drum upright. Braces can be welded into the drum as a means of breaking up the scum layer when the drum is rotated. The digester is usually made of brick, concrete or quarrystone masonry with rendering, while the gasholder is normally made of metal.
Floating-drum plants are used chiefly for digesting animal and human excrements on a continuous-feed mode of operation, i.e. with daily input. They are used most frequently by:
- small-to-midsize family farms (digester size: 5 - 15 m³)
- institutions and large agroindustrial estates (digester size: 20-100 m³).
Advantages: Floating-drum plants are easy to understand and operate. They provide gas at a constant pressure, and the stored volume is immediately recognizable.
Drawbacks: The steel drum is relatively expensive and maintenance-intensive due to the necessity of periodic painting and rust removal. If fibrous substrates are used, the gasholder shows a tendency to get "stuck" in the resultant floating scum.
Floating-drum plants can be recommended as a mature, easy-to-operate, functionally capable means of producing biogas, particularly when reliability is deemed more important than inexpensiveness.
Floating-drum plants with gasholder in the digester (cf. fig. 5.6)
The dome shape is inherently sturdy, compact and material-sparing. The digester is easy to build, and the techniques can be learned by local craftsmen in a short time (cf. fig. 5.21).

Water-jacket plant (cf. fig. 5.7)

Water-jacket biogas plants are characterized by a long useful life and a more aesthetic appearance (no dirty gasholder). Due to their superior hygiene, they are recommended for use in the fermentation of night soil and for cases involving pronounced scumming, e.g. due to rapid evaporation, since the gasholder cannot get stuck in the scum. The extra cost of the masonry water jacket is relatively modest.
Cylindrical plant for quarrystone masonry and concrete (cf. fig. 5.8)
It is anything but easy to make a dome-shaped digester out of quarrystone masonry; it is much easier to build a concrete cylinder. In such cases, the classical (Indian) version with a cylindrical digester is quite practical Note: Quarrystone masonry consumes a lot of mortar.

Fig. 5.6: Floating-drum plant with internal guide frame. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 41 Discharge pipe, 5 Gas pipe, 51 Water trap (Source: Sasse 1984)

Fig. 5.7: Water-jacket plant with external guide frame. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas pipe (Source: Sasse 1984)

Fig. 5.8: Cylindrical plant design for quarrystone masonry construction. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas pipe (Source: KVIC)

Fig. 5.9: Basic function of a fixed dome biogas plant. 1 Mixing pit, 2 Digester, 3 Gasholder, 4 Displaceinent pit, 5 Gas pipe -(Source: OEKOTOP)

5.3.2 Fixed-dome plants
A fixed-dome plant comprises a closed, dome-shaped digester with an immovable, rigid gasholder and a displacement pit. The gas collects in the upper part of the digester. Gas production increases the pressure in the digester and pushes slurry into the displacement pit. When gas is extracted, a proportional amount of slurry flows back into the digester.
The gas pressure does not remain constant in a fixed-dome plant, but increases with the amount of stored gas. Consequently, a special-purpose pressure controller or a separate floating gasholder is needed to achieve a constant supply pressure. The digesters of such plants are usually made of masonry, with paraffin or bituminous paint applied to the gas-flled area in order to make it gastight.
Fixed-dome plants can handle fibrous substances in combination with animal excrements, since the motion of the substrate breaks up the scum each day. The plant is a continous-feed type, but can accept several days' worth of substrate at a time, if the displacement pit is large enough.
Fixed-dome plants must be covered with earth up to the top of the gas-filled space as a precautionary measure (internal pressure up to 0.1-0.15 bar). As a rule, the size of the digester does not go beyond 20 m³, corresponding to a gasholder volume of 3-4 m³. The earth cover makes them suitable for colder climates, and they can be heated as necessary.
Advantages: Fixed-dome plants are characterized by low initial cost and a long useful life, since no moving or rusting parts are involved. The basic design is compact and well-insulated.
Drawbacks: Masonry is not normally gaslight (porosity and cracks) and therefore requires the use of special sealants. Cracking often causes irreparable leaks. Fluctuating gas pressure complicates gas utilization, and plant operation is not readily understandable.
Fixed-dome plants are only recommended in cases where experienced biogas technicians are available for building them, and when the user is amply familiar with how the plant operates.
Fixed-dome plant with central entry hatch (cf. fig. 5.10)
The digester has the form of a hemispherical dome which is easy to build. Floating scum can be removed from the full digester through the central entry hatch.
Fixed-dome plant with suspended dome (cf. fig. 5.11)
Providing a separate foundation for the gas dome yields a statically advantageous, material-saving configuration that is very well suited for fixed-dome plants of ample size. The dome's foundation helps prevent cracking due to tensile stress, and the digesting space is made less expensive, since it can be built of thinner masonry, ferrocement rendering or - in the case of impervious soil - even left unlined.

Fig. 5.10: Fixed-dome plant with central entry hatch. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gas holder, 31 Entry hatch, 32 Gas cover, 33 Seal coating, 34 Rated break ring, 4 Displacement pit, 41 Outlet pipe, 42 Overflow, 43 Cover, 5 Gas pipe, 51 Water trap, 52 Cover (Source: Sasse 1984 / BEP Tanzania 1987 / OEKOTOP)

Fig. 5.11: Fixed-dome plant with suspended dome. 1 Mixing pit, 11 Fill pipe, 2 Digester, 21 Digester rendering, 3 Gas holder, 31 Entry hatch, 32 Cas cover, 33 Seal coating, 34 Dome foundation, 35 Dome masonry, 4 Displacement pit, 41 Outlet pipe, 42 Overflow, 43 Cover, 5 Gas pipe (Source: BEP Tanzania 1987/ OEKOTOP)

5.3.3 Other types of construction
In addition to the two most familiar types of biogas plant, as described above, a selection of special-purpose and otherwise promising designs are briefly presented below.

Fig. 5.12: Horizontal balloon-type biogas plant. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 4 Slurry store, 41 Outlet pipe, 5 Gas pipe, 51 Water trap, 6 Burden, 61 Guide frame (Source: OEKOTOP)

Inflatable balloon plants (cf. fig. 5.12)
Inflatable biogas plants consist of a heatsealed plastic or rubber bag (balloon), the top and bottom parts of which serve as the gasholder and digester, respectively. The requisite gas pressure is achieved by weighting down the bag. Since the material has to be weather-resistant, specially stabilized, reinforced plastic or synthetic caoutchouc is given preference. The useful life amounts to 2 - 5 years.
Advantages: Standardized prefabrication at low cost; shallow installation suitable for use in areas with a high groundwater table.
Drawbacks: Low gas pressure requires extra weight burden, scum cannot be removed. The plastic balloon has a relatively short useful life, is susceptible to damage by mechanical means, and usually not available locally. In addition, local craftsmen are rarely in a position to repair a damaged balloon.
Inflatable biogas plants are recommended, if local repair is or can be made possible and the cost advantage is substantial.

Fig. 5.13: Earth-pit plant with plastic-sheet gasholder. 1 Mixing pit, ll Fill pipe, 2 Digester, 21 Rendering, 22 Peripheral masonry, 3 Plastic-sheet gasholder, 31 Cuide frame, 32 Wooden frame, 33 Weight, 34 Frame anchorage, 35 Plastic sheeting, 4 Slurry store, 41 Overflow, 5 Gas pipe (Source: OEKOTOP)

Earth-pit plants (cf. fig. 5.13)
Masonry digesters are not necessary in stable soil (e.g. Iaterite). It is sufficient to line the pit with a thin layer of cement (netting wire fixed to the pit wall and rendered) in order to prevent seepage. The edge of the pit is reinforced with a ring of masonry that also serves as anchorage for the gasholder. The gasholder can be made of metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden frame that extends down into the slurry and is anchored in place to counter its buoyancy. The requisite gas pressure is achieved by placing weights on the gasholder. An overflow point in the peripheral wall serves as the slurry outlet.
Advantages: Low cost of installation (as little as 1/5th as much as a floating-drum plant), including high potential for self help.
Drawbacks: Short useful life, serviceable only in suitable, impermeable types of soil.
Earth-pit plants can only be recommended for installation in impermeable soil located above the groundwater table. Their construction is particularly inexpensive in connection with plastic sheet gasholders.

Fig 5.14: Ferrocement biogas plant. 1 Mixing pit, 11 Fill pipe, 2 Digester, 21 Backfill soil, 22 Ferrocement, i.e. rendered lathing on surrounding soil, 3 Ferrocement gasholder, 31 Guide frame, 41 Outlet pipe, 5 Cas pipe, 51 Water trap (Source: OEKOTOP/BEP Caribbean 1986)

Ferrocement plants (cf. fig. 5.14)
The ferrocement type of construction can be executed as either a self-supporting shell or an earth-pit lining. The vessel is usually cylindrical. Very small plants (Vd <6 m³) can be prefabricated. As in the case of a fixed-dome plant, the ferrocement gasholder requires special sealing measures (provenly reliable: cemented-on aluminium foil).
Advantages: Low cost of construction, especially in comparison with potentially high cost of masonry for alternative plants.
Drawbacks: Substantial consumption of necessarily good-quality cement; participating craftsmen must meet high standards; uses substantial amounts of steel; construction technique not yet adequately timetested; special sealing measures for the gasholder.
Ferrocement biogas plants are only recommended in cases where special ferrocement know-how is available.

Fig. 5.15: Horizontal biogas plant (KVIC shallow design). 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 41 Outlet pipe, 5 Gas pipe, 51 Water trap (Source: OEKOTOP / KVIC 1978)

Horizontal plants (cf. fig. 5.15)
Horizontal biogas plants are usually chosen when shallow installation is called for (groundwater, rock). They are made of masonry or concrete.
Advantages: Shallow construction despite large slurry space.
Drawbacks: Problems with gas-space leakage, difficult elimination of scum.
Plants with separate gasholders
Masonry dome plants are sometimes equipped with separate gasholders. That approach always involves substantial extra cost and therefore is rarely recommended. Plants with separate gasholders are justifiable, when the points of gas consumption are a considerable distance away from the digester (at least 1 00 m).
Altematively, a separate gasholder could be useful for restoring the utility value of, say, a fixed-dome plant that has been found to leak at an elevated pressure level.

Design principles of simple biogas plants

Design principles of simple biogas plants

The technical conception of biogas plants is determined by the aim of achieving optimal parameters for the biological process (cf. chapter 5.1).
That being so, the following operating requirements/limitations must be given due consideration:
- type and composition of organic material, which determines the choice of process
- given demand for biogas and fertilizer, in addition to the available substrate quantities, which determines the size of the biogas plant
- economy of labor input for building and operating the plants, including consideration of the necessary mechanical equipment.

Fig. 5.3: The batch-feed principle (1) vs the continuous feed principle (2) (Source: OEKOTOP)

Fig. 5.4: The fermentation channel (1) vs the complete-mixed digester (2) (Source: OEKOTOP)

Fig. 5.5: Slurry flow for various configuration of feed, discharge and stirring. 1 Low inlet, outlet at top (beside the gasholder); 2 High inlet, low outlet (normal); 3 Low inlet, low outlet (with partition wall); 4 Vertical agitator; 5 Fixed-dome plant; F: Quality factor for thorough mixing and favorable throughflow conditions, normal situation = 100% (Source: OEKOTOP)

The range of simple biogas plants includes the following basic types:
Batch-type plants are thus referred to because they are charged with successive batches of organic material and a certain amount of seeding slurry to serve as starter. The digestion process is interrupted as soon as the rate of biogas production has slowed down to the point that continued digestion would be uneconomical. Then, the plant is cleaned out and refilled. To achieve a more or less uniform rate of biogas production, several digesters must be operated in parallel, i.e. filled at staggered intervals. Differentiation is made between semi-dry plants (operating on a total-solids content of more than 15%) and liquid plants.
Batch plants are suitable for digesting strawy, fibrous material with a high solids content, usually in areas with low annual precipitation, and for use as simple demonstration plants.
Continuous-feed plants are those in which there is a continuous throughflow of biomass, resulting in a near-constant volume of slurry in the digester. In practice, such plants are fed once or twice each day. There are three main sub-versions:
- complete-mixed digesters
- fermentation channels and
- combinations of the two.
The advantage of continuous-feed plants is that the bacteria receive a regular supply of substrate and are therefore able to generate a more constant supply of biogas. The problem is that buoyant constituents tend to form a stiff layer of scum that impedes biogas production and may even plug up the plant. That drawback can be countered by installing suitable agitators and lengthening the retention time.
The digester inlet, outlet and, to the extent applicable, the agitator must be designed to work together in ensuring the proper retention time, i.e. to avoid short-circuit flow, because the gas production rate would otherwise stay well below the optimum level.
Continuous-feed biogas plants are sized on the basis of the desired retention time for the organic material, in combination with the digester load, which in turn is a function of the prevailing temperature and type of substrate (cf. chapter 4.3).

Sample calculations for biogas plant

Sample calculations

Energy demand (D)
Basic data
8-person family, 2 meals per day. Present rate of energy consumption: 1.8 1 kerosene per day for cooking and fueling 1 lamp (0.6 1 kerosene = 1 m³ biogas).
Desired degree of coverage with biogas
Cooking: all
Lighting: 2 lamps, 3 hours each
Cooling: 60 I refrigerator
Daily gas demand (D)
Cooking
1. Present fuel demand for cooking:
1.21 kerosene = 2 m³ gas
2. Gas demand per person and meal:
0.15 m³ biogas
Gas demand per meal: 1.2 m³ biogas
Cooking-energy demand: 2.4 m³ biogas
3. Consumption rate of gas burner:
175 l/h per flame (2-flame cooker)
Operating time: 2 x 3 h + 1 h for tea
Biogas demand: 7 h x 3501 = 2.5 m³
Defined cooking-energy demand:
2.5 m³ biogas/d
Lighting
Gas consumption of lamp: 120 1/h
Operating time: 2 x 3 h = 6 h
Biogas demand: 0.7 m³/day
Cooling (60 l refrigerator)
Specific gas demand: 30 1/h
Biogas demand: 0.7 m³/day
Total biogas demand: 3.9 m³/d

1st priority: cooking	2.5 m³
2nd priority: 1 lamp	0.35 m³
3rd priority: 1 lamp/refigerator	1.05 m³

Biomass supply/Biogas production (P)
Basic data
9 head of cattle, 230 kg each, 24-h stabling,
green matter from garden as supplement.
Daily biomass incidence
Animal dung, calculated as % liveweight (as per 1.) or as daily yield per head (as per 2.) as listed in pertinent literature..
1. Dung as % liveweight
Daily yield per head of cattle: 10% of
230 kg = 23 kg/d
Volatile solids/d: 1.8 kg VS per day and animal
Total yield: 207 kg/d (16 kg VS/d)
2. Manure yield on per-head basis
Dung yield per head of cattle: 15 kg/d
Urine: 9 1/d
Volatile solids: 9% = 2.1 kg VS/d
Total yield: 216 kg/d (19 kg VS/d)
Useful percentage: 75%
The lowest values are used as the basis of calculation.
Green matter: 20 kg agricultural waste with 30% VS.
Total biomass incidence 170 kg/d (18 kg VS/d)
Category 1: cattle 150 kg (12 kg)
Category 2: green matter 20 kg ( 6 kg)
Sizing the plant
Basic data (calculation for category 1)
Daily biomass: 150 kg/d
VS: 12kg/d
TS-content: 12%
Soil temperature: max 31 °C, min. 22 °C, average 25 °C
Digester volume (Vd)
Retention time (chosen): RT = 60 d (at
25 °C, i.e. f = 1.0)
Substrate input: Sd = biomass + water
Digester TS-content: = 7% (chosen)
Daily water input: Wd = 100 kg
Sd= 100+ 150=250 l
Digester volume: Vd = 250 1 x 60 d = 15 000 = 15 m³
Daily biogas yield
G = kg/d VS x Gy,vs .
= 12 kg/d x 0.25 = 3.0 m³/d
G = kgid biomass x Gy (moist)
= 150 x 0.02 = 3.0 m³/d
G = number of animals x Gy per animal x d
= 9 x 0.35 = 3.2 m³/d
Anticipated daily biogas yield = 3.0 m³/d
Balancing the biogas production and demand
Demand: 3.9 m³/d
Production: 3.0 m³/d
Changes/accommodations
On the demand side: 1 less lamp, reducing the demand to 3.55 m³
Production side: increasing the digester volume to 18 m³, resulting in a retention time of 75 days (f = 1.2) and a daily gas yield of 3.6 m³
Plant parameters
Digester volume: Vd = 18m³
Daily gas production: G = 3.6 m³
Daily substrate input: Sd = 2501
Specific gas production:
Gp = G : Vd
Gp = 3.6 (m³/d): 18 m³ = 0.2 m³/m³ Vd x d
Digester loading:
Ld = TS/VS-input: Vd
Ld = 18: 18 = 1.0 kgTS/m³ Vd
Ld = 12: 18 = 0.7 kg VS/m³ Vd
Gasholder volume:
Vg = 1.6 m³, as calculated on the basis of:
consumption volume:
Vg1 = 0.175 m³/h x 2 flames x 3 h = 1.05 m3
Storage volume:
Vg2 = 10 h x 0.15 m³ gas/h = 1.5 m³
Vd:Vg=18: 1.6=11 :1

Integral agriculture

Integral agriculture

Integral agriculture, also referred to as biological or ecological farming, aims to achieve effective, low-cost production within a system of integrated cycles. Here, biogas technology can provide the link between animal husbandry and crop farming.

Fig. 3.8: Flow diagram for integral farming with a biogas plant (Source: GTZ 1985)

Fig. 3.9: Site plan of the Bouaké Ecofarm in Côte d'lvoire. 1 Impounding reservoir for rainwater, 2 Fallow land, 3 Manioc (1st year), 4 Yams and Manioc (2nd year), 5 Farmhouse, 6 Stables, 7 Biogas plant, 8 Sugar cane, 9 Water reservoir, 10 Fishpond, 11 Vegetable garden, 12 Various food plants (Source: GTZ 1985)

Consider, for example, the planning of a GTZ project in Cote d'Ivoire. The project included the development of a model farm intended to exploit as efficiently as possible the natural resources soil, water, solar energy and airborne nitrogen.
The integral agricultural system "Eco-ferme" (ecofarm) comprises the production elements gardening, crop farming (for food and animal fodder), stock farming (for meat and milk) and a fishpond. A central component of such closed-loop agricultural production is the biogas plant, which produces both household energy and digested slurry for use in the fishpond and as a fertilizer.
The average family-size "eco-ferme" has 3 ha of farmland with the following crops:

Fodder plants
Panicum (for the rainy season)	0.15 ha
Sugar cane (for the dry season)	0.50 ha
Leucaena and brachiaria (mixed culture)	0.50 ha
Panicum, brachiaria and centrosema (mixed culture)	0.50 ha
Food plants
Manioc	0.20 ha
Corn	0.40 ha
Yams	0.10 ha
Potatoes - beans	0.10 ha
Vegetables	0.20 ha
Rice and miscellaneous crops	0.17 ha

Four milk cows and three calves are kept year-round in stables. The cattle dung flows via collecting channels directly into a 13 m³ biogas plant. The biogas plant produces 3.5-4 m³ biogas daily for cooking and lighting. Part of the digested slurry is allowed to flow down the natural gradient into an 800 m² fishpond in order to promote the growth of algae, which serves as fish food. The remaining digested slurry is used as crop fertilizer.

Fertilizing with digested slurry

Fertilizing with digested slurry

The practice of regular organic fertilizing is still extensively unknown in most tropical and subtropical countries. Due, however, to steady intensification of agricultural methods, e.g. abbreviated fallow intervals, some form of purposeful organic fertilizing, naturally including the use of digested slurry as fertilizer, would be particularly useful as a means of maintaining tropical soil fertility. Since Third World farmers have little knowledge of or experience in organic fertilizing methods, particularly with regard to the use of digested slurry, the scope of the following discussion is limited to the general plantgrowth efficiency factors of digested slurry.

Fermentation-induced modification of substrate

- Anaerobic digestion draws carbon, hydrogen and oxygen out of the substrate. The essential plant nutrients (N, P, K) remain, at least in principle, in place. The composition of fertilizing agents in digested slurry depends on the source material and therefore can be manipulated within certain limits.

- For all practical purposes, the volume of the source material remains unchanged, since only some 35 - 50% of the organic substances (corresponding to 5 - 10% of the total volume) is converted to gas.

- Fermentation reduces the C/N-ratio by removing some of the carbon, which has the advantage of increasing the fertilizing effect. Another favorable effect is that organically fixed nitrogen and other plant nutrients become mineralized and, hence, more readily available to the plants.

- Well-digested slurry is practically oderless and does not attract flies.

- Anaerobic digestion kills off or at least deactivates pathogens and worm ova, though the effect cannot necessarily be referred to as hygienization (cf. Table 3.8). Ninety-five percent of the ova and pathogens accumulate in the scum and sediment. Plant seeds normally remain more or less unaffected.

- Compared to the source material, digested slurry has a finer, more homogeneous structure, which makes it easier to spread.

Table 3.8: Survival time of pathogens in biogas plants (Source: Anaerobic Digestion 1985)

Bacteria	Thermophilic fermentation	Mesophilic fermentation		Psycrophilic fermentation
	53-55 °C		35-37 °C		8-25 °C
	Fatality	Fatality		Fatality
	Days	Rate	Days	Rate	Days		Rate
		(%)		(%)			(%)
Salmonella	1-2	100.0	7	100.0	44		100.0
Shigella	1	100.0	5	100.0	30		100.0
Poliviruses			9	100.0
Schistosoma ova	hours	100.0	7	100.0	7-22		100.0
Hookworm ova	1	100.0	10	100.0	30		90.0
Ascaris ova	2	100.0	36	98.8	100		53.0
Colititre	2	10-1 - 10-2	21	10-4	40-60		10-5 -10-4

Table 3.9: Concentration of nutrients in the digested slurry of various substrates!
(Source: OEKOTOP, compiled from various sources)

Type of substrate	N	P2O5	K2O	CaO	MgO
	—% TS—
Cattle dung	2.3 - 4.7	0.9 - 2.1	4.2 - 7.6	1.0 - 4.2	0.6 - 1.1
Pig dung	4.1 - 8.4	2.6 - 6.9	1.6 - 5.1	2.5 - 5.7	0.8 - 1.1
Chicken manure	4.3 - 9.5	2.8 - 8.1	2.1 - 5.3	7.3 - 13.2	1.1 - 1.6

Fertilizing properties

The fertilizing properties of digested slurry are determined by how much mineral substances and trace elements it contains; in tropical soil, the nitrogen content is not necessarily of prime importance—lateritic soils, for example, are more likely to suffer from a lack of phosphorus. The organic content of digested slurry improves the soil's texture, stabilizes its humic content, intensifies its rate of nutrient-depot formation and increases its water-holding capacity. It should be noted that a good water balance is very important in organically fertilized soil, i.e. a shortage of water can wipe out the fertilizing effect.

Very few data on yields and doses are presently available with regard to fertilizing with digested slurry, mainly because sound scientific knowledge and information on practical experience are lacking in this very broad domain. Table 3.10 lists some yield data on digested-slurry fertilizing in the People's Republic of China.

For a practician faced with the task of putting digested slurry to good use, the following tendential observations may be helpful:

- While the nitrogen content of digested slurry is made more readily available to the plants through the mineralization process, the yield effect of digested slurry differs only slightly from that of fresh substrate (liquid manure). This is chiefly attributable to nitrogen losses occurring at the time of distribution.

- Digested slurry is most effective when it is spread on the fields just prior to the beginning of the vegetation period. Additional doses can be given periodically during the growth phase, with the amounts and timing depending on the crop in question. For reasons of hygiene, however, lettuce and vegetables should not be top-dressed.

- The recommended quantities of application are roughly equal for digested slurry and stored liquid manure.

- The requisite amount of digested-slurry fertilizer per unit area can be determined as a mineral equivalent, e.g. N-equivalent fertilization. The N, P and K doses depend on specific crop requirements as listed in the appropriate regional fertilizing tables.

With a view to improving the overall effect of slurry fertilizer under the prevailing local boundary conditions, the implementation of a biogas project should include demonstration trials aimed at developing a regionally appropriate mode of digested-slurry application. For information on experimental systems, please refer to chapter 10.6 - Selected Literature.

Proceeding on the assumption that the soil should receive as much fertilizer as needed to replace the nutrients that were extracted at harvesting time, each hectare will require an average dose of about 33 kg N, 11 kg P₂O₅ and 48 kg K₂O to compensate for an annual yield of 1 - 1.2 tons of, say, sorghum or peanuts. Depending on the nutritive content of the digested slurry, 3-6 t of solid substance per hectare will be required to cover the deficit. For slurry with a moisture content of 90%, the required quantity comes to 30-60 t per hectare and year. That roughly corresponds to the annual capacity of a 6-8 m³ biogas plant.

Like all other forms of organic fertilizing, digested slurry increases the humic content of the soil, and that is especially important in low-humus tropical soils. Humus improves the soil's physical properties, e.g. its aeration, water retention capacity, permeability, cation-exchange capacity, etc. Moreover, digested slurry is a source of energy and nutrients for soil-inhabiting microorganisms, which in turn make essential nutrients more available to the plants. Organic fertilizers are indispensable for maintaining soil fertility, most particularly in tropical areas.

Table 3.10: Effects of digested slurry on crop yields (Source: Chengdu 1980)

Plants tested	Quantity of digested slurry	Yield		Increase
		with digested slurry	with liquid manure
	(m³ /ha)	(kg/ha)		(kg/ha)	(%)
Sweet potatoes	17	24000	21500	21500	12
Rice	15	6500	6000	500	8
corn (maize)	22.5	5000	4600	400	9
Cotton	22.5	1300	1200	100	8

The importance of digested slurry as a fertilizer is underlined by the answers to the following questions:

- How much chemical fertilizer cap be saved with no drop in yield?

- Which yield levels can be achieved by fertilizing with digested slurry, as compared to the same amount of undigested material, e.g. stored or fermented liquid manure?

- By how much can yields be increased over those from previously unfertilized soil? Depending on those answers, a certain monetary value can be attached to digested slurry, whereas the labor involved in preparing and applying the fertilizer must be given due consideration.

Storing and application of digested slurry

With a view to retaining the fertilizing quality of digested slurry, it should be stored only briefly in liquid form in a closed pit or tank and then applied to the fields. Liquid storage involves a certain loss of nitrogen due to the evaporation of ammonia. For that reason, and in order to limit the size of the required storage vessels (a 30-day supply corresponds to about 50% of the biogas plant volume), the storage period should be limited to 2-4 weeks. The resultant quasi-continuous mode of field fertilization (each 2-4 weeks), however, is in opposition to the standing criteria of optimum application, according to which fertilizer should only be applied 2-4 times per year, and then only during the plants' growth phase, when they are able to best exploit the additional nutrient supply.

Fig. 3.7: Slurry storage and composting. 1 Biogas plant, 2 Slurry composting pit with green cover 3 Masonry storage pit (V = 10 Sd), 31 Sturdy wooden cover, 32 Overflow (Source: OEKOTOP)

The practice of spreading liquid digested slurry also presents problems in that not only storage tanks are needed, but transport vessels as well, and the amount of work involved depends in part on how far the digested slurry has to be transported. For example, transporting 1 ton of dung a distance of 500 m in an oxcart takes about 5 hours (200 kg per trip). Distributing the dung over the fields requires another 3 hours or so.
If, for reasons of economy and efficiency, liquid fertilizing should appear impractical' the digested slurry can be mixed with green material and composted. This would involve nitrogen losses amounting to 30 - 70%. On the other hand, the finished compost would be soil-moist, compact (spade able) and much easier to transport.
If irrigated fields are located nearby, the digested slurry could be introduced into the irrigating system so that it is distributed periodically along with the irrigating water.