Generally, the design rules of biological reactors are all based on high removal efficiency of degradable organic matter. Consequently, if the substrate composition and strength of a wastewater are known, a basic design of a high-rate anaerobic system can be established. According to [52], the main design criteria of UASB reactors are, among others: applicable organic load, upflow velocity, three-phase separator, and influent distribution system.

The UASB reactors are generally designed based on the organic volumetric load (OVL) (kgCOD/m3-day) that is defined as follows:

OVL = QSo / V (21)

where Q: influent flow rate (m3/day), So: influent COO (kgCOD/m3) , and V: volume of reactor (m3), From Equation (21) the volume of the reactor, V, can be obtained:

V = QSo / OVL (22)

For most industrial wastewaters, the OVL (based on degradable COD) is the critical factor for the reactor volume. Its value depends on the quantity and quality of the granular sludge; the nature, type, and concentration of the pollutants; the temperature; the required treatment efficiency and the desired safety regarding peak loads [16, 53].