Table 3. Comparison of bioreactor configurations (1, 17, 19, 40, 46-51, 61, 65, 67, 76-79, 89, 90, 99, 100).

Bioreactor configuration	Feedstock	Advantages	Disadvantages
Conventional anaerobic bioreactor
Anaerobic sequencing batch reactor (ASBR)	wastewaters, tannery waste	easy to operate, simply constructed, low input process, low mechanical requirements, cost-effective	small volume, channelling, clogging, poor self-immobilization, poor transfer of the substrate to the microorganisms
Continuous stirred tank reactor (CSTR)	food waste, animal manure, organic industrial wastes, energy crops	complete mixing of waste and microorganisms, applicable for substrates with high TS, easy to operate, low capital and operating costs, better contact of microorganisms with the substrate	long retention time, high mixing energy consumption, difficulties to retain a high microorganism concentration
Two-stage CSTR system (TPAD, PMAD, HPAD)	animal manure, organic food and industrial wastes, energy crops	system of homogeneous bioreactors, applicable for high TS substrates, easy to operate, low operating costs, washout prevention, in situ biogas upgrading	considerable retention time, capital costs, feed of high concentrated substrate
Anaerobic plug-flow reactor (APFR)	farm liquid effluent, slurries of animal manure, cattle residues, distillery wastewater, organic fraction of municipal solid waste	simple to build and maintain, efficient in converting the substrate to biogas, stable to operate, high degree of sludge retention, stable reactor performance	no internal agitation, sedimentation of heavier parts and floatation of lighter parts
Bioreactor with sludge retention system
Anaerobic contact reactor (ACR)	wastewaters of food processing industry, pulp and paper mills, palm oil mill effluent	high concentration of active microbial biomass, rapidly achieved steady-state times due to mixing, short HRT, high effluent quality, less affected by shock loading, favourable pH, limited biomass washout and change in biogas concentration and composition	sensitive to shock loadings, VFA accumulation
Up-flow anaerobic sludge blanket (UASB)	brewery and molasses wastewater	compactness, high loading rates, low sludge production, short HRT and SRT times, low operating costs with high methane production rates	low total solids, long start-up period, significant wash-out of sludge during the initial phase, impure biogas, incomplete or insufficient removal of organic matter, pathogens and nutrients in the final effluent
Expanded granular sludge bed (EGSB)	brewery, starch, commercial laundry, domestic, municipal and pharmaceutical wastewaters, effluents from the textile industry, dyes and toxic compounds	better substrate-biomass contact, higher throughput, improved internal mixing without dead zones, higher permeability, lower footprint, low operating costs, compact design, removal efficiency up to 90 %, completely closed system with zero emission of odours	long start-up times, problems with biomass retention, granule disintegration, wash-out of hollow granules, the appearance of fluffy granules
Up-flow anaerobic solid-state (UASS)	corn silage, barley straw, wheat straw, organic solid waste	higher processing efficiency, higher volume loading rate, lower investment costs, simple operation and management	utility, scalability, operability and stability are hardly known, the system is limited by its structure, small volume
Anaerobic baffled reactors (ABR)	paper mill effluent, food waste	achieving good COD and solids removal, low sludge production, small footprint	frequent loss of microorganisms from the system, slow growth rates of methanogenesis, long start-up process, sludge washout at high hydraulic stresses, organic loading shock in the initial compartments
Internal circulation (IC)	wastewater from breweries, pulp and paper industry, distilleries, fermentation and petrochemical processes, wastewater from citric acid production	high OLR, effective stress resistance, economic space utilization, excellent operation stability, better treatment performance and faster start-up	high ammonia nitrogen content and presence of toxic substances due to high OLR, unsatisfactory COD removal efficiency, accumulation of VFAs, poor sludge retention, insufficient stability of system
Anaerobic fluidized bed reactor (AFBR) ice-cream, simulated milk, dairy, synthetic dairy wastewaters compact bioreactor size due to short hydraulic retention time, long biomass retention on the carrier, high conversion rates due to fully mixed conditions, high mass transfer rates, no channelling of flow, high organics load size limitations due to the height-to-diameter ratio, high-energy requirements due to high recycle ratios, long start-up period for biofilm formation Horizontal-flow anaerobic immobilized biomass (HAIB) domestic sewage, industrial wastewater, paper industry effluent, toxic substances (phenol, benzene, toluene, ethylbenzene, xylenes, formaldehyde, pentachlorophenol) low price, good mechanical resistance, gradual substrate degradation and microorganism growth, long cellular retention times, high biomass concentrations randomly packed-bed, channelling within the bioreactor, pressure drops Anaerobic fixed-structure bed (AFSB) sugar cane vinasse, brewery wastewater, wastewater containing sulphate, wastewater from ethanol production energy input, high sludge retention time, higher substrate conversion rates sensitivity to environmental conditions, laboratory scale only Anaerobic membrane bioreactor Conventional AnMBRs pharmaceutically active compounds, xenobiotics, residual organic matter, dyes, hormones, bactericides, municipal and domestic wastewater higher concentrations of active biomass, high OLR and HRT, short retention time, good substrate-sludge contact, sufficient mixing and compact design, lower capital costs, tolerance to toxic compounds methane recovery, product inhibition, rapid membrane fouling, membrane cost, low membrane flux, two-stage systems are often required for effective biogas production Modified AnMBRs industrial wastewaters, phenolic compounds from wastewaters, wastewater containing lipids and toxic compounds reduced membrane fouling, enhanced sludge filterability, fewer energy expenses, high nitrogen removal, overcome long start-up period, high OLR, reduced fouling strong shear stress, biogas escape from the external membrane unit, uneconomical for large-scale applications, further studies required to determine the optimal conditions