Spent caustic is a waste from oil refineries which is difficult to treat and dispose of due to its
noxious properties such as high content in paraffins and asphaltenes hydrocarbons which form
all sorts of emulsions and solutions, from 5 to 18% of free NaOH, up to 35 g of Cl-/L, phenol
compounds, and organic and inorganic sulfur compounds. Its composition depends on the source
of the fuel being desulfurated. This mixture is very reactive and therefore its characterization is
difficult and dependent on the way the sample is handled. Samples have to be collected cold
from the storage container with no more than 6 h of having been rejected from the process. The
sampling container must be sealed and pressure-proof. Part of the sample has to be acidified in
situ for certain analysis. The spent caustic from Tula refinery in Mexico has 320 g of COD/L
from which 114 come from sulfur organic compounds (3.6 g COD/g of S), 153 from phenolic
compounds (phenol 78 g of COD/L) and the rest from other petroleum derivatives such as
asphaltenes, paraffins, and mercaptans. Present treatment processes involve incineration of spent
caustics, and an effort is needed for biological treatment in order to cut maintenance costs.