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The following points highlight the six main factors affecting biogas yield. The factors are: 1. Effect of Agitation on Biogas Yield 2. Effect of pH of Digester Contents on Biogas Yield 3. Effect of C: N Ratio on Biogas Production 4. Effect of Loading Rate on Biogas Yield 5. Effect of Salinity on Biogas Yield 6. Effect of Inhibitory Factors and Materials Affecting Microbial Activity.
Factor # 1. Effect of Agitation on Biogas Yield:
Mixing greatly helps to ensure intimate contact between micro-organisms which leads to improved fermentation efficiency. Mixing can be carried out in a number of ways.
For instance, if slurry is fed every day instead of feeding periodically at a certain interval, it causes more frequent contact between micro-organisms thus giving desired mixing effect. It can also be achieved by carrying out certain alterations in inlet and outlet pipes of a plant.
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Mixing can also be carried out by installing certain stirring or mixing devices in the plant. It is also possible to achieve mixing effect by incorporating a nozzle for flushing slurry as provided in the German design of Schmidt-Eggersgluss type biogas plant.
Alternatively, mixing can be achieved by installing wooden conical beams which help to break down scum following up and down movement of slurry surface at the time of filling and emptying of digester, as in case of German Weber type biogas plant.
Where no mixing device is provided, stratification can be reduced by using a horizontal displacement digester which stimulates a plug flow. In digesters which utilise municipal refuse, part of the gas produced can be recirculated to mix digester contents.
It is possible to introduce this feature in large sized biogas plants such as community type. It does not seem to be practical to introduce this feature in family sized plants mainly due to unaffordable costs and skilled supervision needed.
A number of experiments have been carried out to analyse the effect of mixing on gas yield. One of the early attempts to study the effect of gas recirculation as an aid to mixing, on biogas yield was made by G.J. Mohan Rao important findings. It is to be seen that gas recirculation helps to improve gas yield. Effect of varying degree of mixing of digester contents on per cent total solids formation at different levels in digester as reported by E.R. Coppinger.
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Factor # 2. Effect of pH of Digester Contents on Biogas Yield:
Biogas production is greatly influenced by pH of digester contents. It is essentially a measure of acidity and alkalinity of a solution. A pH value of 7 is regarded as neutral, less than 7 as acidic and greater than 7 as alkaline.
During anaerobic fermentation, micro-organisms require, a neutral or mildly alkaline environment for efficient gas production. A too acidic or too alkaline environment is viewed as detrimental for bacterial activity. A pH between 7 and 8.5 is optimum range for increased gas yield.
The pH of digester contents is mainly affected by the amount of carbon dioxide and volatile fatty acids produced in digester as intermediate products during fermentation.
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For a normal anaerobic fermentation process, concentration of volatile fatty acids comprising acetic acid in particular should be below 2000 parts per million. When acetic acid concentration exceeds this limit, bacterial activity is generally found to retard. Variation in pH value during a typical anaerobic fermentation process is shown in Fig. 6.1.
As per studies made in China, during the period when ambient temperature ranges between 22°C to 26°C, it takes about six days for pH to acquire a stable value. Similarly during the period when temperature ranges between 18 to 20°C, it takes about 14 to 18 days for pH to attain a stable value.
Normally pH stabilises spontaneously as fermentation proceeds and does not require adjustment as the system automatically tends to counter level of acidity or alkalinity of substrate by natural buffering provided by ammonia and bicarbonate ions. The buffering provided by carbon dioxide and bicarbonate during fermentation can be described by the following mathematical relationship.
When biogas plant is fed with a very improper mix of feedstock, there occurs an appreciable decline in pH following excessive formation of volatile fatty acids. As a result proportion of carbon dioxide in biogas starts increasing leading to a further drop in pH value which adversely affects system’s self- regulation. In this situation, it sometimes becomes necessary to bring the pH value to a desired range which can be done by introducing additives.
When pH value is low due to high acidity, lime water is preferred to be added to the digester contents. Lime is generally added in the form of lime-supernatant than as it is. Care needs to be exercised that lime supernatant is added in requisite quantity only as any excess lime can start chemically reacting with carbon dioxide content of biogas thus adversely affecting biogas yield. The following chemical reaction takes place-
On the other hand when pH of digester contents is more, it can be decreased by adding requisite quantity of hydrochloric acid to it. However use of sulfuric and nitric acids for this purpose is generally not preferred.
Factor # 3. Effect of C: N Ratio on Biogas Production:
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For efficient plant operation it is necessary to maintain proper composition of feedstock so that ratio of carbon to nitrogen in feed remains within the desired range. Both carbon and nitrogen provide requisite nutrients for efficient functioning of anaerobic bacteria. Carbon provides necessary energy to microorganisms for their sustenance whereas nitrogen helps in building their cell structures.
Depending upon relative richness in carbon or nitrogen content, feed materials can be classified as nitrogen-rich or carbon-rich materials. It is generally found that during digestion micro-organisms utilise carbon 25 to 30 times faster than nitrogen which in other words means that carbon content in feedstock should be 25 to 30 times more than nitrogen. To meet this requirement constituents of feedstock are kept in a manner so as to ensure a C : N ratio of 25 to 30 : 1, and concentration of dry matter as 7 to 10 per cent.
However, in some situations, it is possible to achieve same gas yield with a low C : N ratio and higher concentration of dry matter. As per results of experiments carried out in China for equivalent gas yield, when C : N ratio lies in the vicinity of 13:1, it is possible to keep the range of concentration of ammoniacal nitrogen between 400 to 500 ppm, when C : N ratio is 25 : 1 this range is maintained between 300 to 400 ppm, and when C : N ratio is around 30 : 1, this range is kept between 100 to 200 ppm. In plants working on animal and municipal wastes, C : N ratio lies around 30 : 1 and hence no further adjustment of this ratio is required.
However, in plant substrates which utilize food-processing wastes and agricultural crop residues as feed, this ratio may be around 100 : 1 or even higher, and in such situations additional nitrogen generally in the form of ammonia becomes necessary for bringing this ratio within the desired range. C : N ratios for commonly used waste materials is given in Table 6.6. Waste materials that are low in carbon can be combined with materials high in nitrogen and vice versa to attain desired C : N ratio of 30 : 1.
A high carbon to nitrogen ratio means that nitrogen will be exhausted earlier than carbon. Conversely a low carbon to nitrogen ratio or too much nitrogen in relation to carbon results in high ammonium concentrations which may become toxic to anaerobic bacteria. It is possible to adjust the C : N ratio by suitable additives. For instance, sawdust which has a high C : N ratio could be added to poultry manure which has a slightly low C : N ratio by the micro-organisms.
In nutshell, if C : N ratio is too high the process suffers from limited nitrogen availability, and if it is too low, ammonia may be found in quantities large enough to be toxic to the bacterial population. Even in situations where C: N ratio is close to 30:1, it will undergo efficient anaerobic fermentation only if waste materials are also biodegradable at the same time.
There is some evidence to suggest that C : N ratio varies with temperature. For instance gas yield from a plant working on cow wastes in certain colder regions of the country sometimes drops to one third of its normal value which however can be stimulated by adding easily digestible materials like powdered leaves, kitchen wastes and powdered straw that promote multiplication of micro-organisms.
According to the results of a study conducted by M.A. Idnani and O.P. Chawla, biogas production from 0.5 kg of cowdung was almost doubled from about 17.2 litres to 31.5 litres at 7°C by addition of 200 ml of urine. It was also reported that dehydrated urine-soaked materials such as powdered leaves, straw and straw dust stimulated gas production considerably.
Thus, for plant owners who own cattle shed, it is advisable that they should construct a drain for collecting urine and allowing it to soak waste materials to be used as feed for biogas plants after sun-drying. In the same way, urine available from human-beings can also be gainfully utilized by making it to drain into pits that can hold soakable waste materials. Use of urine-soaked waste materials is particularly advantageous during winter months when gas production otherwise is low.
Factor # 4. Effect of Loading Rate on Biogas Yield:
Loading rate normally expressed as amount of waste materials fed per unit volume of digester capacity is an important parameter that affects gas yield. Gas output is commonly expressed as m3 of gas produced per kg of volatile solids destroyed. A number of studies have been carried out to analyse the impact of varying loading rate on gas yield. It is not practically possible to describe them individually and only some representative results are included here.
Based on results of experiments carried out, usually prescribed loading rate for plants working on municipal wastes ranges from 0.03 to 0.1 lb of volatile solids per ft3 of digester capacity (or equivalently 0.48 to 1.6 kg of volatile solids per m3 of digester capacity) for a retention period ranging between 30 to 90 days.
Another set of results by J.A. Gore et al., they analysed the impact of daily and alternate day loading on biogas yield. A 50 kg daily charge and 100 kg on alternate day basis produced 2.9043 m3 and 2.9285 m3 of gas respectively. For a particular size of plant, there is an optimum feed charge rate which will produce maximum gas and beyond which further quantity of charge will not proportionately produce more gas.
Alternate day charging is particularly convenient for farmers who have their cattle sheds miles away from homes where their plants are generally situated.
According to studies made by G.J. Mohanrao, a daily loading rate of 0.42 lb of volatile solids per ft3 of digester capacity (or equivalently 16 kg of volatile solids per m3 of digester capacity) produces 0.62 to 1.19 ft3 of gas per lb of raw dung fed (or equivalently 0.04 to 0.074 m3 of gas per kg of raw dung fed). L. Ke-xin et al., carried out experimental studies in China for analysing the effect of varying batch interval in batch-fed plants on biogas yield.
As per the results of another study undertaken by G.J. Mohanrao, recommended loading rate for plants working on night-soil ranges from 0.065 to 0.139 lb of volatile solids per ft3 of digester capacity (or equivalently 1.04 to 2.23 kg of volatile solids per m3 of digester capacity). Higher loading rates are recommended only in those regions where mean ambient temperatures are higher.
Comparative volumetric loading rate varied from 1.9 to 0.89 ft3 (0.054 to 0.025 m3) per capita per day and were based on a per capita volatile solids contribution of 0.123 lb per day (0.56 kg per day). A night-soil loading rate of 0.1 lb volatile solids per ft3 (1.6 kg per m3) of digester capacity per day is recommended for temperate climates. Average gas yield was found to vary from 0.8 to 1.2 ft3 (0.023 to 0.034 m3) on per person basis.
Based on pilot-plant studies carried out on a digester of 35.31 ft3 (1 m3) capacity with a gasholder of 24.5 ft3 (0.69 m3) capacity, maximum gas yield was observed for a loading rate of 1.5 lb of dung/ft3 digester volume/day (24 kg of dung/m3 digester volume/day). At this loading rate, maximum gas yield per unit weight of volatile solids destroyed was obtained although per cent reduction of volatile solids was only two-thirds of that with smaller loading.
Other factors remaining same, retention period (which refers to the average length of time the substrate remains in the digester) has an important bearing on gas yield. Its selection depends on plant. H.M. Lapp, et al., have evolved norms for retention period selection as related to digester temperature. It is to be seen that duration of retention period decreases with rise in mean temperature at which the digester is expected to operate.
V.J. Srivastava and D.P. Chynoweth developed a mathematical model to describe gas yield as a function of organic loading rate corresponding to two different digester designs; the continuously stirred tank and the non-mixed vertical flow reactor.
Analysis of the digester operation with the help of the model indicates that optimum gas yield can be achieved by selecting a digester design and an operating technique that will increase solids conversion through longer solids and micro-organisms retention. Pretreatment of feed was identified as one of the contributing factor for increasing the biogas yield.
Factor # 5. Effect of Salinity on Biogas Yield:
B.J. Mehta, et al., studied the effect of salinity on biogas yield which was found, to have negative impact on gas output. Tap water, brackish water (1 per cent sulphur) and sea water (3.5 per cent sulphur) were used for the preparation of cowdung slurry. Biogas output was found to be inversely related to the salinity of cow manure slurries prepared with tap water, brackish water and seawater. Addition of sea mud was found to have no significant impact on biogas yield from saline slurries.
Relationship between Total Solids Concentration, Loading Rate, Hydraulic Retention Time:
Total Solids Concentration:
All waste materials fed to a plant consist of solid matter and water. Solid matter is made of volatile organic matter and non-volatiles (fixed solids). During anaerobic fermentation process volatile solids undergo digestion and non-volatiles remain unaffected.
Fresh cattle wastes, for instance, consist of around 20 per cent total solids and 80 per cent water. Total solids in turn consist of 70 per cent volatile solids and 30 per cent fixed solids. For optimum gas yield through anaerobic fermentation normally 8 to 10 per cent TS in feed is desired. This is achieved by making slurry of fresh cattle-dung in water in ratio of 1 : 1.
Loading Rate:
This is usually expressed as kilograms of volatile solids (kg VS) fed to the digester per day per cubic metre of digester volume. Thus a 5 m3 biogas plant with 50 kg of dung fed per day should have loading rate of around 1.4 kg VS/day m3. Temperature – controlled – mechanically stirred large biogas plants can have loading rates as high as 5 kg VS/day m3. It is to be mentioned that if loading rate is too high, pH of the digester content tends to fall due to its becoming acidic following inability of micro-organisms to biodegrade all feed materials.
Hydraulic Retention Time (HRT):
HRT is defined as the average time spent by the input slurry inside the digester before it comes out. Rate of gas generation is initially high and then gradually declines as the digestion approaches towards completion. Thus the time required for 70-80 per cent digestion is considerably less than that needed to achieve cent per cent or complete digestion.
HRT is chosen so as to achieve at least 70-80 per cent digestion. HRT varies between 20 to 120 days depending upon the design and operating temperature of the digester. HRT for digesters operating in countries of tropical region like India is usually taken as 40-50 days. In countries of colder climates like China, digesters are designed for HRT of about 100 days.
The total solids concentration, loading rate (LR) and HRT are related by the following equation:
For cattledung, LR is expressed in units of kg. VS/day. M3, TS in percentage and HRT in days, and k is found to have an approximate value of 7.
Factor # 6. Effect of Inhibitory Factors and Materials Affecting Microbial Activity:
There are several factors and class of materials which act as inhibitor to bacterial activity during anaerobic fermentation. When these chemicals and factors reach certain level, bacterial activity almost stops thereby severely affecting gas yield. For example, when volatile acid concentration reaches a value of 200 ppm, or ammoniacal nitrogen concentration exceeds a value of 1500 ppm, microbial activity is greatly retarded.
A retarded microbial activity ultimately results in low quality gas with low methane content in relation to other constituents. There are several factors which contribute to excessive acid formation such as higher proportion of acid forming over methanogenic bacteria in substrate, high acid contents in feed, delays in addition of fresh feed and delayed withdrawal of sludge and improper feed composition.
Problem of excessive fatty acid formation can be minimised by having a proper composition of feedstock along with adequate mix of some sludge seeding bacteria. A variety of materials act as inhibitory to the fermentation process.
For instance, presence of certain metals such as copper in waste materials act toxic during anaerobic digestion. Some of the commonly recognised toxic materials include common alkali and alkaline-earth cations such as sodium, potassium, calcium and magnesium.
Traces of pesticides such as DDT and chlorinated hydrocarbons which usually accompany crop residues as feed tend to produce toxic effect during anaerobic fermentation. For minimising toxicity, dilution with water or addition of certain non-toxic materials to substrate is recommended.
Normally when a material is present in low concentration, its presence may be stimulatory but when this concentration exceeds certain value, its effect becomes toxic. When poultry manure is used as feed, toxicity on account of excessive ammonia formed can be minimised by adjusting C : N ratio by adding shredded straw or bagasse which have a high C : N ratio, or simply by diluting the slurry with water.
Stimulatory, moderately inhibitory and strongly inhibitory concentrations of elements like sodium, potassium, calcium and magnesium; when these materials are present in excess, dilution of digester contents by water is generally found helpful in limiting their toxic effect.