# A Review of the Role of Critical Parameters in the Design and Operation of Biogas Production Plants
## 3 Process Parameters Involved in a Biogas Production Plant
### Feedstock
#### 3.1.3 Pretreatment
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Additive** & **Element or Compound** & **Benefit** & **Adverse Effect** \\ \hline Macro-nutrients & P, N and S & Methane production improvement and enhanced process stability & Methane or biomass inhibition by overdosing \\ Micro-nutrients, heavy metals & Cu\({}^{2+}\), Zn\({}^{2+}\), Cr\({}^{3+}\), Cd, Ni, Pb\({}^{4+}\) and Hg\({}^{2+}\) & Promoting various enzymatic reactions & Inhibition to the acetogens Restricting production of double cells (Mg\({}^{2+}\)), Inhibition of acetoclastic methanogens (Na\({}^{+}\)), Destabilizing buffering system (Ca\({}^{2+}\)), etc. \\ Iron & Zero valent iron, Clean scrap, rusty scrap and iron additives & Sulfide fixation, biomass stimulation, etc. & Methane or biomass inhibition by overdosing \\ Ash & Bottom ash and fly ash & Sulfide fixation, biomass stimulation, etc. & Precipitation and clogging risk \\ Inorganic absorbent materials & MgCl\({}_{2}\), MgCl\({}_{2}\),6H\({}_{2}\)O & Ammonia sequestration through struite formation & \\ Inorganic nitrogen & & Availability of nitrogen as nutrient & Inhibition of methane production enzymes, VFA \\ Nano particles & Ag, Au, Fe, Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), TiO, ZnO & Methane production improvement & Inhibition of hydrolytic and methanogenic biomass \\ Biological additives (bioaugmentation and enzyme) & Composti, C. proteolytic, SAA & Increased methanogenic activity, increased hydrolytic activity, increased hydrolytic activity & Cost, process control \\  & & Biochar, activated carbon, sand, zeolite, Ni-Zeo, Co-Zeo, Mg-Zeo, rockwood, membrane, molecular sieve, polyurethane foam and loofah & Biomass immobilization, buffering agents, enhanced VFA degradation \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The advantages and disadvantages of using additives in AD [90]. vibrations of the polar molecules and ion movements [95; 96]. As a result of microwave radiation, a number of pretreatment benefits can be acquired, which includes increased substrate surface area and accessibility to enzyme attack, decreased polymerization and crystallinity of cellulose and lignin depolymerization [97]. Microwave-assisted pretreatment, using microwave irradiation alone or in combination with various solvents (i.e., water, alkali, acid, ionic liquid, salt, and other organic components), has been studied. With the use of microwave pretreatment alone, a 60% methane yield increase for a continuous reactor was observed [65]. Other evidence suggests that microwave irradiation with a solvent combination is either able to rapidly hydrolyze more sugars [98] or to remove more hemicellulose and lignin from the lignocellulosic substrate [99] compared to the conventional thermal and thermochemical treatment. However, the application of this technique to date is limited to the lab-scale, as technical difficulties to expand it to the industrial scale still exist. Sonication, involving a sound frequency greater than 20 kHz (ultrasound), can be applied to physical pretreatment of biomass [100]. Brief exposure to ultrasound can cause the thinning of microbial cell walls, resulting in the release of cytoplasm (material within a living cell excluding cell nucleus) and thereby facilitating the intercellular matter to be available for further degradation to CH\({}_{4}\) and CO\({}_{2}\). Although a large variety of feedstock can be successfully pretreated using ultrasound, until now, this method is widely considered for sewage or waste activated sludge [101], as this material, compared to other substrates, requires a lower sonication time and energy. The past study found that longer sonication time and energy may lead to a lower conversion efficiency of solubilized matter into methane [65]. Based on the literature, the threshold of specific sonication energy ranges between 1000 and 16,000 kJ/kg TS (total solids) and is strongly correlated to substrate solid content, which for sludge was reported to be optimum at 20-30 g/L [65].