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Parameters and optimization of digestion process

Vitality of methane-producing bacteria is only possible when there is no oxygen access to digester and therefore it is necessary to monitor impermeability of digester and do not allow access of oxygen in digester.


Temperature limits of the digestion process
Maintaining optimal temperature is one of the most important factors of digestion process. In natural conditions biogas is produced in temperatures from 0°C up to 97°C [19], but with regard to optimization of digestion process for producing biogas and biofertilizer differentiation is generally made between three ranges:
• The psychophilic temperature range is defined by temperatures from below 20°C up to 25°C ,
• the mesophilic temperature range between 25°C and 40°C and
• the thermophilic temperature range above 40°C.

Minimal average temperature
The rate of bacteriological methane production increases with temperature. Since, however, the amount of free ammonia also increases with temperature, the bio-digestive performance could be inhibited or even reduced as a result. In general, unheated biogas plants perform satisfactory only where mean annual temperatures are around 20°C or above or where the average daily temperature is at least 18°C.

Within the range of 20-28°C mean temperature, gas production increases over-proportionally. If the temperature of the bio-mass is below 15°C, gas production will be so low that the biogas plant is no longer economically feasible [8].

Optimal temperature
There is a different optimal temperature for digestion for every type of substrate, but empirical data collected by PF “Fluid” on biogas plants working in Kyrgyzstan on substrate of mixed manure of cattle, pigs and birds, optimal temperature for mesophilic range is 34 – 37°C, and for thermophilic – 52- 54°C. Psychophilic temperature range can be maintained in plants without heating system, where temperature control is not maintained, but the most intensive biogas production in psychophilic range is observed under temperature of 23°C.

Changes in temperature
The process of bio-methanation is very sensitive to changes in temperature. The degree of sensitivity, in turn, is dependent on the temperature range. Brief fluctuations not exceeding the following limits may be regarded as still un-inhibitory with respect to the process of fermentation:
• psychophilic range: ± 2°C/h
• mesophilic range: ± 1°C/h
• thermophilic range: ± 0,5°C/h
The temperature fluctuations between day and night are no great problem for plants built
underground, since the temperature of the earth below a depth of one meter is practically

Thermophilic or mesophilic range?
The benefits of thermophilic range of digestion include: higher speed of substrate degradation and therefore higher biogas yield as well as practically total destruction of pathogenic bacteria contained in substrate.

The shortcomings of thermophilic range include: larger energy input needed for keeping such temperatures and sensitivity of digestion process to minimal changes in temperatures well as lower quality of fertilizer produced. Mesophilic range allows for higher amino acid content of fertilizer, but disinfection of the substrate is not as complete as with use of thermophilic range.

In order to grow, bacteria need more than just a supply of organic substances as a source of carbon and energy. They also require certain mineral nutrients. In addition to carbon, oxygen and hydrogen, the generation of bio-mass requires an adequate supply of nitrogen, sulfur, phosphorous, potassium, calcium, magnesium and a number of trace elements such as iron, manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel etc. “Normal” substrates such as agricultural residues or municipal sewage usually contain adequate amounts of the mentioned elements.

Optimal digestion time depends on the daily input dose of substrate and digestion temperature. If chosen digestion time is too short, then useful bacteria lave the digester with unload of slurry faster than they can reproduce and fermentation process practically comes to a halt. Too long retention time is non-optimal from the viewpoint of maximizing biogas and biofertilizer production inside of given time period.

Digester turnover period
When determining optimal digestion time term “”digester turnover period” is often used. Digester turnover period is period during which freshly loaded substrate is digested and is unloaded from digester.
For continuous load systems average digestion time is determined by the relation of digester volume to volume of daily uploaded substrate. In practice digester turnover period is chosen according to temperature range and substrate content within following time intervals:
• psychophilic temperature range: from 30 to 40 days and longer;
• mesophilic temperature range: from 10 to 20 days;
• thermophilic temperature range: from 5 to 10 days.

Daily substrate input
Daily substrate input is determined by the digester turnover period and increases with the increase of temperature in digester. If digester turnover period is 10 days, than daily substrate load will be 1/10 from the overall volume of substrate in digester. If digester turnover period is 20 days, than daily substrate load will be 1/20 from the overall volume of substrate in digester. For biogas plants working in thermophilic range, daily substrate load can constitute up to 1/5 of overall substrate volume in digester.

Substrate digestion time
Choice of substrate digestion time depends also from the type of substrate used. For the following substrate types digested at mesophilic temperature the time over which the biogas production is maximized is approximately as follows:
• Liquid cattle manure: 10 -15 days;
• liquid pig manure: 9 -12 days;
• liquid bird droppings: 10- 15 days;
• manure mixed with vegetation waste: 40-80 days.

The methane-producing bacteria live best under neutral to slightly alkaline conditions. In the methane fermentation process the second step of biogas production is the time of activity of acid bacteria. During this time pH value is lower, which means that the environment is more acid.
However under normal progress different bacteria in digester work effectively and acids are used by the methane-producing bacteria. Optimal pH value will normally be depending on substrate between 6,5 and 8.5 [18, 19].
PH value can be measured with litmus paper. PH value will correspond to color of the paper after placing it in substrate.

One of the most important factors that influence methane fermentation is ration of carbon to nitrogen in digested substrate. If C/N ratio is too big than lack of nitrogen will be the limiting factor of methane fermentation. If this ratio is too small then the quantity of ammonia produced is so big that the environment inside the digester becomes toxic for bacteria.

Microorganisms need both nitrogen and carbon for assimilation into cellular construction. Different experiments have shown that the biggest biogas yield is observed when the C/N ratio takes on values between 10 and 20, wit optimal value depending on the type of substrate. For maximizing biogas production different substrates are normally mixed to reach optimal C/N ratio.

Table 3. Nitrogen content and C/N ratio for organic substances. [8,18]

Fermented material Nitrogen N [%] Ratio Carbon to Nitrogen C/N
Cattle 1,7 – 1,8 16,6 – 25
Chicken 3.7 – 6,3 7,3 – 9.65
Horses 2,3 25
Pigs 3,8 6,2 – 12,5
Sheep 3,8 33

Unimpeded metabolism in the substrate is a prerequisite for high bacteria activity. It is only possible when substrate viscosity allows for free movement of bacteria and gas bubbles between the liquid and contained solids. There are different solids contained in agricultural waste.

Solid and dry materials in substrate
Solid particles like sand, clay and so on cause formation of sediment. Lighter materials float up on the surface of substrate and form scum which leads to decrease in gas production. Therefore it is recommended to thoroughly fragmentize the vegetative waste – straw and food leftovers, before loading them into digester and to try excluding all solid particles.

Dry material content is determined by the moisture contained in the manure. With moisture of substrate equal to 70% there is 30% of dry matter. Approximate values of moisture content of manure and excrement (manure and urine) for different animal types are contained in Table 4.

Table 4. Quantity and moisture content of manure and excrement for one animal [18]

Animal type Average manure yield, kg/day Manure moisture, [%] Average excrements, kg/day Exrements moisture, [%]
Cattle 36 65 55 86
Pigs 4 65 5,1 86
Poultry 0,16 75 0,16 75

Moisture content of the substrate loaded into digester has to be no less than 85% in winter time and 92% in summer time. For obtaining the correct moisture content substrate is normally diluted with warm water in volumes, determined according to following formula: W = S×( (M2 – M1):(100 – M2)), where S – quantity of loaded manure, M1 – moisture content of original substrate, M2 – moisture content desired, W – water quantity in liters. Table 5 provides necessary water volumes needed to dilute 100 kilogram of manure up to 85% and 92% moisture content.

Table 5. Quantity of water needed to reach necessary moisture content for 100 of manure

Initial moisture level of substrate
60% 65% 70% 75% 80% 85% 90%
85% 166 liters 133 liters 100 liters 67 liters 33,5 liters
92% 400 liters 337 liters 275 liters 213 liters 150 liters 87,5 liters 25 liters

For effective operation of a biogas plant and stable digestion process it is necessary to regularly agitate substrate in the digester. The main aims of agitation are:
• freeing of produced biogas;
• mixing of fresh substrate with bacteria population (inoculation);
• preventing of scum and sediment;
• preventing differentiation in temperature between different portions of substrate inside digester;
• providing homogenous distribution of bacteria population;
• preventing formulation of voids and accumulation of substrate that decrease usable volume of digester.
When choosing appropriate agitation method it has to be taken into account that the process of fermentation represents symbiosis of different bacteria and that when bacteria association is broken fermentation process will be unproductive until new association is formed.
Therefore too frequent, long and intensive agitation is harmful to digestion process. It is recommended to slowly agitate substrate every 4 – 6 hours.

Fermented organic mass should not contain any substances that can negatively influence microorganisms involved in digestion process (antibiotics, detergents and so on). The presence of some inorganic substances can also have inhibitory effect on the process of biomethanation and it is not advisable to use for dilution of substrate water leftover after washing clothes with synthetic detergents.