TABLE 4.

Pretreatments and effects on carcasses and swine manure biomass.

Pretreatment	Biomass	Positive aspects	Negative aspects	Conditions	Biogas and/or hydrogen yield	References
Alkaline	Manure	Easy retrieval (high volatility), not corrosive, low energy.	Implementation at a large scale, and chemical application.	Aqueous ammonia 32% w/w 20°C, 96 h.	244% increase in CH4 yield in 17 days of digestion.	Lymperatou et al., 2017
	Carcasses	No uncontrolled emission of gas, nutrients or pathogens into the environment. It is effective in eliminating pathogens.	Effluent production with toxic characteristics, high chemical and biological oxygen demand. Neutralization of digestate is necessary, i.e., mixing with other substrate.	Potassium hydroxide 2–8 M 20°C 20 days	600 mL CH4 g–1 VS in 42 days of digestion with KOH 2M.	Arias et al., 2018
Thermal	Manure	Promote the solubilization of cellulose and hemicellulose, increase in the total concentration of volatile fatty acids, and the hydrolysis of protein	High energy consumption	Continuously stirred tank reactors 70 ± 1°C, 1–4 days	281.6 mL CH4 g–1 VS in 22 days of digestion, with 3 days of pretreatment	Wu J. et al., 2017
	Carcasses	Effective pathogen inactivation. Decomposes organic matter in the solid phase.	Proteins are difficult to decompose, risking ammonium inhibition.	250 g carcasses 170°C, 30 min	236 mL CH4 g–1 VS in 25 days	Xu et al., 2018
Enzymatic	Manure	High conversion of carbohydrate and protein. Facilitates the acidogenesis step.	High mineral content and salinity (13 g L–1)	Stainless steel reactor 30–90°C Enzymes used: Delvolase; Delvozyme L; Filtrase NL; Bakezyme	36% increase in CH4 yield	Wang et al., 2015
	Carcass	Accelerate biomethane fermentation reaction	Accumulation of organic acids results in excessive acidification and slows the methane production rate	pH 6.5–9.0 Enzyme concentration (Porcine Trypsin) 0.5–2.5% 40°C, 24 h	104.59 mL CH4 L–1 of substrate in 23 days	He et al., 2019
Electrolysis cell	Manure	Electrolysis cell design is simple and can achieve high biofuel rates.	A large percentage of electrons are not successfully transferred to the current. Promising for the production of biohydrogen, but is not viable for biogas	Electrolysis cells: platinum cathode and graphite fiber anode, enriched with exoelogenic bacteria. 16–184 h 30°C. Current: 0.5 V.	14% increase in CH4 and 64% increase in H2	Wagner et al., 2009
Flocculation and Sieving	Manure	Remove the organic load and nutrients and simple to operate. Increases manure biodegradability.	Use of pretreatment chemical compounds that may affect the later stage of anaerobic digestion	Flocculation with commercial polymer (Chemifloc CV/300), and subsequent sieving (0.25 mm).	75.4% increase in CH4	González-Fernández et al., 2008; Wang D. et al., 2019; Wang L. et al., 2019
Grinding	Carcass	The digester needs to be emptied less frequently. Reduces the collection frequency and improves final product biosafety.	Not effective for pathogen control and features high power consumption.	13 mm and 4 mm double grinding	53.7% increase in CH4	Kirby et al., 2018
Ultrasound	Manure	Increases solubilization of organic matter, nitrogen and ammonia. It promotes particle disintegration, reduces bound protein and increases soluble protein.	High energy, reduces the efficiency of CH4 production, due to the formation of inhibitors	Ultrasonic probe (500 W, 20 kHz) Sonication pulses: 2 on 2 s 30°C	28% increase in CH4	Elbeshbishy et al., 2011
	Carcasses and manure (co-digestion)	Increases hydrolysis rate, methane production and inoculum methanogenic activity. Removal of volatile solids.	Release of flocculating agents and lignin compounds that decrease hydrolysis rate	Ultrasonic processor (30 kHz) 22 ± 5°C using specific energy inputs of 1000 kJ/kgTS 109 days	340 m3 CH4 t–1 VS	Luste et al., 2012