Anaerobic Fermentation is a biochemical process in which there is oxidation of organic compounds in absence of oxygen leading to production of a gaseous mixture of carbon dioxide and methane. Most cases it is used in domestic and industrial areas to manage waste or release energy. Through the process of anaerobic fermentation, biogas is produced. Starting and successfully operating an anaerobic treatment is an uphill task that requires a lot of care and monitoring. According to Lahav and Morgan (2004) to successfully monitor and control the treatment, a low degree of volatile fatty acids (VFA) and a pH in the range of about 6.6 to 7.4 is necessary.
In the anaerobic reactors, the carbonate systems dissipate a weak acid which help maintain pH about neutral for optimum operation. The volatile fatty acids however results in a reduction in the pH. At equilibrium operating conditions, the hydrogen gas and acetic acids produced are directly used by methanogens and then converted to methane. Consequently, this ensures that the pH remains stable given that carbonate alkalinity produced is not consumed directly. Maintenance of the fermentation system’s pH is necessary for efficient anaerobic fermentation. The accepted range values are between 6.5 and 7.6. Changes in digester operating conditions may result in the imbalance of process and accumulation of volatile fatty acids. Unless the system contains enough buffer capacity, the pH is likely to drop below optimum levels and the digester is said to be “sour”. Depending on the capacity of the pH and the drop duration, the biogas production will decrease to a point where it may completely stop. However, in an effectively operated system, a small increase of the digester’s effluent pH is foreseen. This is mainly due to production of alkalinity by organisms; as such organisms consume organic matter rich in protein (Khelifi et al, 2009).
Alkalinity refers to the total proton-accepting capacity of the carbonate weak acid subsystem together with the proton-accepting capacity of the water system (alkalinity =2[CO32−] + [HCO3−] + [OH−] − [H+]) (Venkata et al, 2008). Monitoring of Alkalinity is crucial for the operations of the reactors thus it has led to various practical methods based on titration process.
While conducting an experiment tomeasure alkalinity only by direct titration, with or without an external measurement of volatile fatty acids concentration various methods were formulated.
For instance, Hattingth et al 2000 (qt. in Falk, 2011) proposed the use of an alkalinity proton accepting capacity value that leads to a pH of 6. This therefore subjects alkalinity as the right measure of the available buffering capacity of an anaerobic digester. Rozzi and Brunetti on their part came up with a method where a sample of a digester is saturated with CO2 (to yield PCO2= 1 bar) and subsequently leading to reduction of the pH to 3.7 (Hwang et al, 2004). Such addition of carbon dioxide doesn’t interfere with the level of alkalinity. The capacity of carbon dioxide gasproduced from the solution at a pH of 3.7 is then measured by a gas meter. Given that the loss of carbon dioxide during titration is negligible and on the assumption that the original carbonate concentration at the operational pH is quite low; the measured volume of carbon dioxide gasis directly proportional to the mass of hydrogen carbonate converted. Alkalinity is thus determined. In conclusion, this method together with the parallel method is best suited for automated control of anaerobic reactors (K helifi et al. 2009).
In accordance to the research carried out by Labatut and Gooch (2001) to monitor anaerobic digestion process, the following findings are noticed. The buffering capacity of the digester is determined by the magnitude of alkalinity in the system. The bicarbonate ion is the major source of buffering capacity to maintain the system’s pH in the range between 6.5 and 7.6. The degree of concentration of bicarbonate in the solution is related to the percent of carbon dioxide in the gas. For instance, cow manure can play a significant role in co-digestion operations by raising the pH level and buffering capacity of the influent mixture when high-strength, easily degradable industrial wastes are used as co-substrates (Ribéreau-Gayon, 2006).
The optimal temperature for mesophilic anaerobic digestion is 37degrees Celsius. A negligible variation is considered normal; hence digester temperature should be always maintained between 35degree Celsius and 40 degree Celsius. Operating at temperatures outside the normal range, results in a decrease in the biogas production and stability of organic matter. Also, long time periods under such conditions may subsequently lead to a cease in biogas production, causing a digester failure. In addition, the process will be more affected at higher temperatures than at lower temperatures. Worth noting, anaerobic fermentation can also take place at psychrophilic temperatures below 20 degrees Celsius (Bouallagui et al., 2003) but most reactors operate at either mesophilic temperatures or thermophilic temperatures, with optima at 35 degrees Celsius and 55 degrees Celsius, respectively. The structures of the active microbial composition at the two temperature optima are significantly different. A change from mesophilic to ther- mophilic temperatures (or vice versa) can easily result in a sharp decrease in production of biogas until the necessary populations have increased in number. Even negligible changes in temperature from 35 degrees Celsius to 30 degree Celsius and from 30 degree Celsius to 32 degree Celsius are seen to drastically reduce the rate of biogas production (Chae et al., 2008).
Comprehensive insight of both mesophilic and thermophilic digesters have conflicting results as pointed out by:(Gannoun et al., 2007) who examined the anaerobic digestion of combined olive mill and abattoir waste water at 37 degree Celsius and 55 degrees Celsius. The findings were that the thermophilic reactor produced a higher COD removal and biogas yield than the mesophilic reactor, and could as well sustain it at a high organic loading rate.
During an experiment involving batch digestion of vegetable waste and wood chips, more rapid degradation of fatty acids was found at 55 degrees Celsius than at 38 degrees Celsius, and also 95% of the methane yield was seen after 11 days under thermophilic conditions compared to 27 days under mesophilic conditions (Hedge and Pullammanappallil, 2007). It is quite evident as well that the total net energy output from 18 l thermophilic digesters was 427 kJ per day higher than that produced by thermophilic digesters( Fezzani and Behn Cheik, 2007). However, Parawira et al(2007) compared two-stage digesters of mesophilic mesophilic, mesophilic–thermophilic, and thermophilic–thermophilic configurations treating potato waste and observed that the methane yield was higher in the mesophilic second stage than the thermophilic second stage, but the thermophilic second-stage reactors could successfully sustain a shorter retention time. On comparing the mesopholic and thermopholic digesters it is evident that mesophilic temperature digesters have improved rates of degradation as compared to thermophilic digesters. For instance, experiments with proteinaceous wastewater using 2.8 l UASB laboratory scale reactors under mesophilic (37 _C) and thermophilic (55 _C) conditions showed that the mesophilic reactor removed ca. 84% of COD whereas the thermophilic reactor dissipated only 69–83 (Ribéreau-Gayon, 2006).
COD is a good indicator of the degree of completeness of the degradation process, as any undigested material will require oxygen (in an aerobic environment) to complete degradation.
Worth noting is that an increase in methane yield or production rate from a thermophilic process has to be balanced against the increased energy requirement for maintaining the reactor at the higher temperature. Often, this has not been an important consideration when the biogas produced is used for the generation of electricity, since heating the reactor is accomplished by routing the waste heat from the gas engines to heat exchangers within the reactor, and the engines generally produce more heat than the reactor requires. The optimum temperature for methanogenesis may not necessarily be the optimum for other processes in anaerobic digestion, such as hydrolysis or acidification. Thus, the research shows that multi-stage digesters could be used for temperature optimization of the separate processes occurring place in the different respective tanks (Venkata et al, 2008).
Hydraulic retention time is the mean residence time of the waste suspension in any given bioreactor. It is calculated by comparing the liquid volume of the reactor and the effluent withdrawal. While carrying out an experiment, a modified three-stage methane fermentation system was made to digest food waste efficiently. This system was made up of three stages: semi anaerobic hydrolysis, anaerobic acidogenesis and the anaerobic methanogenesis. From the study, examination of the effects of temperature and hydraulic retention time on the methanogenesis was carried out. Temperatures were adjusted from 30 degrees Celsius to 55 degrees Celsius, and the HRTs ranged from 8- 12 d. The rate of soluble chemical oxygen demand removal, in correlation with digestion time and in accordance to the first-order kinetic model developed by Grau et al is taken into account. It was observed from the experiment that thermophilic digesters had a higher rate of sCOD removal than mesophilic digesters. The rates of biogas and methane produced by thermophilic digesters were quite higher compared to those by mesophilic digesters regardless of HRT. Even though maximum biogas production was significantly observed when an HRT of 10 d was used, the yield of methane was the highest in the reactor when an HRT of 12 d was employed (223 l CH4/kg sCODdegraded). However, fermentation stability declined when an HRT of 8 d was used. It is observed from the experiment that the concentration of NH3-N generated did not inhibit anaerobic fermentation (Ward et al, 2008).
The control of biological processes is significant. It has been well established that only combined methods as opposed to a single set ups are able to successfully handle control objectives. With a proper monitoring and control, very high values of organic load with excellent process functioning is expected. With a better system operation, careful process control and monitoring enables it necessary to ensure efficient organic waste stabilization and constant and stable production of biogas. The control of such biological process helps to prevent digester upsets and potential system failure (Nackano and Alan, 2004).
To conclude Anaerobic fermentation is an effective waste treatment method that leads to natural decomposition and reduction of waste as well as generation of biogas.
Its application has been widely exhibited in the treatment of waste from industrial and agricultural operations. Depending on the origin, the waste material may constitute inhibitory or toxic substances such as ammonia, sulfide, heavy metals, and organics. Continual accumulation of the substances may result into reactor upset, as shown by reduced production in biogas and/or biogas methane content (Mudhoo, 2012). Owing to the difference in anaerobic microorganisms, waste composition, and experimental procedures and conditions, the results from previous investigations on inhibition of anaerobic processes vary significantly. Getting information on waste components is important as it successfully helps in application of anaerobic digestion. Many a times, it is suggested that co-digestion with other waste materials, adaptation of microorganisms to inhibitory substances, and incorporation of methods to do away with or counteract toxicants before anaerobic digestion can substantially improve the waste treatment efficiency.
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