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Ammonia (NH3) is an important compound, with a significant function for the AD process. NH3 is an important nutrient, serving as a precursor to foodstuffs and fertilizers and is normally encountered as a gas, with the characteristic pungent smell. Proteins are the main source of ammonia for the AD process. Too high ammonia concentration inside the digester, especially free ammonia (the unionised form of ammonia), is considered to be responsible for process inhibition. This is common to AD of animal slurries, due to their high ammonia concentration, originating from urine. For its inhibitory effect, ammonia concentration should be kept below 80 mg/l. Methanogenic bacteria are especially sensitive to ammonia inhibition. The concentration of free ammonia is direct proportional to temperature, so there is an increased risk of ammonia inhibition of AD processes operated at thermophilic temperatures, compared to mesophilic ones. The free- ammonia concentration is calculated from the equation: where [NH ] and [T-NH ] are the free and respectively the total ammonia concentrations, and 33 ka is the dissociation parameter, with values increasing with temperature. This means that increasing pH and increasing temperature will lead to increased inhibition, as these factors will increase the fraction of free ammonia. When a process is inhibited by ammonia, an increase in the concentration of VFA will lead to a decrease in pH. This will partly counteract the effect of ammonia due to a decrease in the free ammonia concentration. 3.3.5 Macro-andmicronutrients(traceelements)andtoxiccompounds Microelements (trace elements) like iron, nickel, cobalt, selenium, molybdenum or tungsten are equally important for the growth and survival of the AD microorganisms as the macronutrients carbon, nitrogen, phosphor, and sulphur. The optimal ratio of the macronutrients carbon, nitrogen, phosphor, and sulphur (C:N:P:S) is considered 600:15:5:1. Insufficient provision of nutrients and trace elements, as well as too high digestibility of the substrate can cause inhibition and disturbances in the AD process. Another factor, influencing the activity of anaerobic microorganisms, is the presence of toxic compounds. They can be brought into the AD system together with the feedstock or are generated during the process. The application of threshold values for toxic compounds is difficult, on one hand because these kind of materials are often bound by chemical processes and on the other hand because of the capacity of anaerobic microorganisms to adapt, within some limits, to environmental conditions, herewith to the presence of toxic compounds. 3.4 Operationalparameters 3.4.1 Organicload The construction and operation of a biogas plant is a combination of economical and technical considerations. Obtaining the maximum biogas yield, by complete digestion of the biogas HANDBOOK 3.3.4 Ammonia 27PDF Image | biogas HANDBOOK
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