6.6 Alkalinity

Buffer capacity, or alkalinity in AD process, is the equilibrium of carbon dioxide and bicarbonate ions that provides resistance to significant and rapid changes in pH. Alkalinity is proportional to the concentration of bicarbonate. The imbalance of digester can be more reliably measured by a buffer capacity than a direct measurement of pH. This is because an accumulation of short-chain fatty acids will reduce the buffering capacity significantly before the pH decreases. A low buffer capacity can be improved by reducing OLR. This is because too high OLR inhibited the microorganisms due to too high fatty acid concentration. Reducing OLR therefore reduces the availability of these fatty acids. Then, the alkalinity of the system can be improved. Guwy et al. [202] and Ward et al. [189] indicated that a quicker way is to add strong bases or carbonate salts to remove carbon dioxide from the gas space and convert it to bicarbonate. Alternatively, bicarbonate (HCO3 ) can be added directly.

#### 6.7 Carbon to nitrogen (C/N) ratio

C/N ratio is a ratio of the mass of carbon to the mass of nitrogen in a substance. In AD process, a C/N ratio ranging from 20 to 30 is considered optimum [184, 185], and the value of at least 25:1 is suggested for optimal gas production [191]. If the C/N ratio is too high, methanogens will rapidly consume the nitrogen to meet their protein requirements and will no longer react with the rest of carbon in the material. As a result, gas production will be low. On the other hand, if the C/N ratio is too low, nitrogen will be liberated and accumulated in the system in the form of

6.4 Substrate composition

Biomass for Bioenergy - Recent Trends and Future Challenges

approximately 1 g-VSsubstrate/g-VSinoculum.

6.5 Organic loading rate

optimum MY.

6.6 Alkalinity

directly.

120

Biogas yield and the compositions of biogas are greatly influenced by the composition of feedstocks. AD of carbohydrates, fats, and protein yield 886, 1535, and 587 L biogas/kg-VS with methane content of approximately 50, 70, and 84%, respectively, [184]. Substrate to inoculum ratio (S/I), as well as biodegradability of the substrate, is another important factor affecting batch AD processes, especially at high solid content [192]. Too high S/I ratio may be toxic, while too low S/I ratio may prevent induction of the enzyme necessary for biodegradation [193]. Too high concentration of feedstock can cause inhibition or failure of AD [194] due to substrate inhibition. High S/I ratio can lead to overloads due to VFAs accumulation [192, 195] and long lag phase. Thus, a low S/I ratio is preferred in order to attain shorter lag phase [192, 196]. Owen et al. [197] proposed a standard S/I ratio to be

Organic loading rate (OLR) is defined as the amount of VS or COD components fed per day per unit digester volume. Higher OLR can reduce the digester's size and the capital cost as a consequence. However, enough time (HRT) should be provided to the microorganisms for breaking down the organic material and converting it into gas [198]. An increase in OLR can result in higher hydrogen production efficiency [199]. However, a further increase in OLR beyond a certain level will result in substrate inhibition, leading to a lower MY [200]. Too high OLR can shift the metabolic to solventogenic phase [201]. Hobson and Bousfield [201] and Chandra et al. [185] reported that a total solid content of 8.0–10.0% is desirable for

Buffer capacity, or alkalinity in AD process, is the equilibrium of carbon dioxide and bicarbonate ions that provides resistance to significant and rapid changes in pH. Alkalinity is proportional to the concentration of bicarbonate. The imbalance of digester can be more reliably measured by a buffer capacity than a direct measurement of pH. This is because an accumulation of short-chain fatty acids will reduce the buffering capacity significantly before the pH decreases. A low buffer capacity can be improved by reducing OLR. This is because too high OLR inhibited the microorganisms due to too high fatty acid concentration. Reducing OLR therefore reduces the availability of these fatty acids. Then, the alkalinity of the system can be improved. Guwy et al. [202] and Ward et al. [189] indicated that a quicker way is to add strong bases or carbonate salts to remove carbon dioxide from the gas space and

C/N ratio is a ratio of the mass of carbon to the mass of nitrogen in a substance. In AD process, a C/N ratio ranging from 20 to 30 is considered optimum [184, 185], and the value of at least 25:1 is suggested for optimal gas production [191]. If the C/N ratio is too high, methanogens will rapidly consume the nitrogen to meet their protein requirements and will no longer react with the rest of carbon in the material. As a result, gas production will be low. On the other hand, if the C/N ratio is too low, nitrogen will be liberated and accumulated in the system in the form of

) can be added

convert it to bicarbonate. Alternatively, bicarbonate (HCO3

6.7 Carbon to nitrogen (C/N) ratio

ammonium ion (NH4 + ). This can possibly increase the pH of the digestate to a level that is toxic to methanogens (pH 8.5) [186, 203].
