URUGUAY

The treatment processes of spent caustic must guarantee the elimination of the  pollutants in order to reach the authorized limit for discharge. The elimination of  pollutants in aqueous solution may need one or variou s basic treatment techniques depending on the type of compounds and concentration in solution [25]. It is necessary to choose the most adequate method according to the characteristic of the effluent.

 Numerous efforts have been made to develop and to enhance the treatment of spent caustic. Treatment methods for spent caustic can be classified as biological, chemical and thermal processes [22]. According to Andreozzia,R. Caprioa, V.

Insolab, and A. Marottac,R. (1999) the treatment process could be selected depending on COD concentration [26]. Figure 8 shows the relation between COD value and the appropriate treatment method [26].

Advanced oxidation processes (AOPs) is selected for COD less than 20 g/l while wet air oxidation (WAO) is implemented for COD values between 20 and 200 g/l higher than this value incineration is considered to be the best method [26].

Figure 8 :Treatment technologies according to COD contents [26].

1.4.1. Biological Treatment of Spent Caustic :

As mentioned before spent caustic stream needs excessive treatment before discharge. Biological treatment is preferred due to the low cost and the low environmental impact [27]. The treatment should be done by two steps:

 pretreatment followed by biological treatment. The biological treatment can be an inexpensive disposal option; however there are several drawbacks if it is applied directly to without pretreatment. These drawbacks are:

1-  Noxious odors: Sulfides and mercaptans are highly odors even at the ppb level. These compounds are considered very toxic and hazardous [28].

2- It is not readily biodegradable: Often spent caustic contains a mixture of compounds that would limit the biodegradation in the biological treatment  processes [29, 5].

3- Foaming: Spent caustic has compounds that have foaming characteristics when aerated or agitated during the treatment [29, 5].

0 20 200 300

AOPs

WAO

Incineration

COD g/l

4- High chemical oxygen demand (COD): This can cause high load to the  biological process [29, 5].

5- PH swings: spent caustic is highly alkaline solution with a pH value that can reach up to 14 [28].

1.4.2. Thermal Treatment:

1) Wet Air Oxidation (WAO)

Another conventional method is the wet air oxidation (WAO), which is a high  pressure treatment at elevated temperature. Here the oxidation agent is the oxygen  present in the air, which is introduced into the spent caustic as steam. This reaction can accomplish either mineralization of organics into CO2  and H2O or destroy complex molecules into simpler molecules that is easier to degrade [30, 31]. The process is very expensive, and due to severe reaction conditions, safety is a main concern. Although several tests with low pressure has been conducted without remarkable success [32]. WAO can be classified into three types based on the temperature implemented to achieve the oxidation. Table 3 shows the three types of WAO that can be applied to the different kinds of spent caustic.

Using appropriate catalysts for WAO process minimize the severity of reaction conditions and simply destroy refractory pollutants resulting in reducing capital and operational cost [33].

The operating cost of catalytic wet air oxidation (CWAO) is around half the non-catalytic WAO. However, an additional step is required to remove the metal ions from the treated effluent that would result in increasing operational costs [34].

Catalysts allow overcoming the drawbacks of the WAO; however the discovery of

low cost and stable catalysts remains the major weaknesses of CWAO for wide applications [35].

Table 3:WAO operational conditions [5]

Type of WAO Temperature (°C)

Pressure (psig)

Kind of spent caustic

Low

temperature

110-120 25 to 100 sulfides in spent caustic

Mid

temperature

200-220 300 to 600 complete treatment of sulfides and mercaptans , cresylic acids and naphthenic acids

High

temperature

240-260 700 to 1100 complete treatment of sulfides and mercaptans, cresylic acids and naphthenic acids

2) Incineration:

Incineration is a process used to convert solid, liquid or gas at elevated concentration of pollutants into more stable states at higher temperatures [36]. The economic aspect presents the disadvantage of this process because it requires high energy cost. In addition toxic emissions that results from this process are high [22].

1.4.3. Chemical Treatment:

1) Neutralization Followed by Air Stripping:

Aeration depends on two fundamental principles: equilibrium conditions and mass transfer considerations [36]. Equilibrium conditions will identify the limits of the gas transfer process. Aeration is an efficient method for H2S gas removal. The function of aeration is not particularly to oxygenate the water, but it is to strip the dissolved gas (H₂S) out of the water by changing the equilibrium conditions of the water and thus drive the dissolved gas out [36].

 Neutralization converts the spent caustic components into their original elements, such as hydrogen sulfide (H2S), mercaptan sulfur (RSH), phenol and naphthenic acid. But it requires stripping and additional managing of volatile gases [22]. This technology is a widely understood method and the simplest and cheapest for the removal of volatile compounds. However, the effluent stream has elevated COD concentrations because a major part of the organic component is unaffected by the stripping process [32].

2) Chemical Oxidation

Chemical oxidation is a method that involves the transfer of one or more electrons from an electron donor (reductant) to an electron acceptor (oxidant), which has a higher affinity for electrons. The result of electron transfer is a chemical change of the oxidant and the reductant [37]. Oxidation technologies are established to decompose refractory molecules into simpler molecules that can be further treated by other methods. The mechanism of oxidation works by

oxidized and the oxidant that accepts the electron is reduced [38]. In natural waters, chemical oxidation processes also take place due to the presence of microorganisms that work as natural oxidants [37]. Oxidation reactions generate chemical species with an odd number of valence electrons known as radicals.

These tend to be highly unstable therefore, highly reactive because one of their electrons is unpaired. The reactions that produce radicals tend to be followed by chain reactions between the radical, oxidants and other reactants until stable oxidation products are formed. The ability of an oxidant to initiate chemical reactions is measured in terms of oxidative power of an oxidant [39]. The oxidation potentials are presented in terms of the potential. The electro- potential based on half-cell reactions [38]. Figure 9 shows the potentials of the

most commonly used oxidizers [40, 41].

Figure 9 : Oxidation potential [1, 2]

Hydroxyl Radical Sulfate Radical  persulfate Ozone Hydrogen Peroxide

Permanganate Chlorine Dioxide Chlorine Oxygen Bromine

0 0.5 1 1.5 2 2.5 3

   C   h  e  m    i  c  a    l  o  x    i   d  a  n    t

Oxidation potential (volts)