Effecting Factors

Factors Effecting Biogas Production and Anaerobic Digestion

There are some really important factors to be maintained that effect anaerobic digestion process rate and limitations that slow down the degradation rate of substrates such as temperature, hydraulic retention time (HRT), solid retention time (SRT), pH of the environment, alkalinity, C/N ratio of the substrate, organic loading rate and inhibiting parameters such as ammonia, sulfate and etc...

1-Temperature

Anaerobic processes take place in three different temperature range called psychrophilic  that is between 5-15 °C, mesophilic that is between 35-40 °C and thermophilic that is between 50-60 °C. These temperature ranges may change around -3 and +3 °C according to various sources. Most of the anaerobic process applications` temperature are either mesophilic or thermophilic. Since the anaerobic degredation of substrates is very slow compared to aerobic process, high temperatured environment increase the degredation rate. The environment in which anaerops especially methanogens live need to be at higher temperature so that in a mesophilic or thermophilic reactor, microorganisms can use their energy to grow up and degrade the substrates and not heat the environment. All anaerobic microorganisms mostly the thermophiles are sensitive to temperature changes so the temperature changes should be kept  between +/ − 2°C. Or else activity of the microorganisms would decrease so the biogas production rate. Temperature has a significant effect on growth kinetic, such as growth yield, decay rate, half-velocity constant, and maximum specific growth rate. Higher temperature values increase the solubilization rate of  the substrates and deactivation of the pathogens. At thermophilic temperatures, hardly breakable organic materials such as lignin, cellulose and fat are broken down easily compared to mesophilic temperatures. It is pointed out in many articles that thermophilic processes end up with 25-50% more biogas production compared to mesophilic processes. On the other hand thermophilic processes are tend to be inhibited by ammonia more than mesophilic processes because of the rapid decay of the proteinaceous materials. The figure below shows the growth rate of the methanogens at different temperatures.

2-HRT and SRT

In anaerobic treatment processes, HRT in continuously stirred tank reactors (CSTR) and SRT in biofilm attached on package material in reactors such as filter bed, fluidized bed, baffled r., upflow contact reactor, anaerobic sequencing batch reactor, etc. are the indicators of the time that biomass stay in the reactor. HRT should be around at least 8-12 days in CSTRs due to the slow activity of methanogens, if the reactor enviroment is thermophilic. In mesophilic CSTRs, HRT has to be maintained more than 15 days. But it should be noted that the bigger the HRT of a CSTR is kept, the higher the volume of the reactor and the construction costs of the reactors become. It is possible to decrease the HRT of the reactors by maintaining two stage reactors that separate hydrolysis-acidogenesis phases and acetogenesis-methanogenesis phases from eachother to increase the biogas production efficiency. SRT is the biomass retention time in the reactor and it may be as high as 100 days in upflow anaerobic sludge blanket reactors. The higher SRT of a reactor is the better the reactor treatment and the biogas production would be. Biomass washout from the reactors is prevented by biofilm attached packing materials or denser microorganism flocs than water.

3-pH

Anaerobic digestion process is better activating between pH values of 6.8-7.8 if the process of acidogenesis and the methanogenesis are combined together in one reactor. But acidogenic and methanogenic microorganisms are more efficient in the range of 5.5-6.5 and 7-8 pH values respectively. Especially the methanogens are very sensitive to pH variations. At pH values of 6.2 and lower, methanogenic activity can be disturbed and VFA accumulation can be observed. Also VFA accumulation may cause the pH decreases and provoke undissociated VFAs that are thought to freely permeate the cellular membrane of microorganisms. After permeating the membrane, the fatty acids internally dissociate, thus lowering the cytoplasmic pH and affecting bacterial metabolism of the  microorganisms. pH higher than 8.0 exhibits ammonia inhibition because of the conversion of ionized ammonia to un-ionized ammonia that is toxic to microorganisms. pH changes are induced by the following phenomenons:
1. ammonia consumption and release
2. volatile fatty acid production and consumption
3. sulphide release by dissimilatory reduction of sulphate or sulphite
4. conversion of neutral carbonaceous organic carbon to methane and carbon dioxide

In the case of a low pH failure in an anaerobic process, organic loading rate should be stopped or lowerer until the accumulated VFAs are used and finished in the reactor. Also to adjust the pH value to desired range, chemical additives can be used. The figure below presents the relationship between biogas generation, and pH of the reactor environment in the study conduted by Speece (1996).

4-Alkalinity

Alkalinity is the buffering capacity of an anaerobic digestion system or biogas plant in which acidification or VFA accumulation based rapid pH changes are suppressed by the strong bases such as carbonate salts, bicarbonate or carbondioxide that naturally derived in the wastewater or added from outside. There is a direct relationship between alkalinity variations and VFA accumulation. A sensitive parameter for monitoring digesters and measuring process stability is the VFA/alkalinity ratio: if this ratio is lower than 0.35–0.40 (equiv. acetic acid/equiv. CaCO3), the process is considered to be operating favorably without acidification risk. The destruction of organic matter, especially the proteins, releases ammonia–N. Each mole of organic nitrogen theoretically generates one equivalent of alkalinity. Ammonia–N reacts with carbon dioxide produced during the biochemical reaction to produce ammonium bicarbonate, which contributes to alkalinity.

Bicarbonate alkalinity

References 
1-Dublein, D., Steinhauser, A., Biogas:From Waste And Renewable Energy Resources, Wiley-Vch Verlag GmbH Co. & KGaA, Weinheim, ISBN 978-3-527-31841-4, 2008.
2-Samir K. Khanal, 2008. Anaerobic Biotechnology For Bioenergy Production, Principles and Applications. 2008 John Wiley & Sons, Inc. ISBN: 978-0-813-82346-1.
3-R. Borja, 2011. Biogas  Production. Instituto de la Grasa (CSIC), Seville, Spain.
4-DJ Batstone and PD Jensen, 2011. Anaerobic Processes. The University of Queensland, Brisbane, QLD, Australia.