AS	Activated sludge system
ASM	Activated sludge model
bH	Heterotrophic decay rate (1/d)
COD	Chemical oxygen demand (mg/L)
CSTR	Continuous stirred tank reactor
fS	Residual soluble metabolic fraction of endogenous biomass
fX	Residual particulate metabolic fraction of endogenous biomass
HRT	Hydrolic retention time (day)
khs	Maximum hydrolysis rate for soluble COD (1/d)
KhS	Hydrolysis half saturation constant for soluble COD (g COD/g COD)
khX	Maximum hydrolysis rate for particulate COD (1/d)
KhX	Hydrolysis half saturation constant for particulate COD (g COD/g COD)
KS	Half saturation constant for growth (g COD/g COD)
MAS	Activated sludge system with membrane separation
MASM	Model for activated sludge system with a membrane module
MBR	Membrane Bioreactor
$\widehat{{\mathit{\mu }}_H}$	Maximum heterotrophic growth rate (1/d)
MLSS	Mixed liquor suspended solids (mg/L)
PSHC	Amount of soluble hydrolysable COD captured in sludge (kg COD/d)
SH	Soluble hydrolysable COD (mg/L)
SHC	Captured soluble hydrolysable COD (mg/L)
SHE	Soluble hydrolysable COD in the effluent (mg/L)
SI	Soluble inert COD (mg/L)
SIC	Captured soluble inert COD (mg/L)
SO	Dissolved oxygen concentration (mg/L)
SP	Soluble inert microbial product (mg COD/L)
SRT	Sludge retention time (d)
SS	Soluble readily biodegradable COD (mg/L)
WWTP	Wastewater treatment plant
XH	Active biomass concentration (mg COD/L)
XI	Particulate inert COD (mg/L)
XP	Particulate inert microbial product (mg/L)
XS	Slowly biodegredable COD (mg/L)
XSS	Settlable COD (mg/L)