**3. POME polishing technologies**

In recent years, many studies conducted to investigate alternative POME treatment technologies, especially in secondary and tertiary treatment. The technologies that are widely investigated are adsorption, coagulation or flocculation, membrane filtration and advanced oxidation processes. Most of these investigations are in laboratory scale, but they show potential to overcome the drawback of conventional ponding system [2]. **Figure 7** shows an overview of recent POME polishing technologies.

**Figure 7.** *Overview of recent POME polishing technologies [2].*

## **3.1 Advanced oxidation process (AOP)**

AOPs are the processes which degrade the organic pollutant by the powerful and reactive hydroxyl radical (OH∙). Hydroxyl radical (OH∙) generated would have an oxidation potential of 2.8 eV OH∙ can generate through either one or a combined of chemical oxidation by using H2O2, ozone, ultrasound and radiation assisted source (ultraviolet) [15]. During the treatment of wastewater, OH∙ will attack the organic pollutants and convert them to CO2, H2O and inorganic salt [16]. AOPs can effectively degrade the pollutants and have its advantages of non-selectively, mineralization of pollutants and ease of operation compared to the conventional methods. The most popular AOPs are Fenton oxidation, photocatalysis, ultrasound cavitation and ozonation. Fenton oxidation uses the reaction between Fe2+ and H2O2 to produce OH∙ [17]. Photocatalysis applies metal oxide (such as TiO2) in the presence of irradiation (UV and Vis) to produce OH∙ [18]. Ozonation uses the ozone, which is a powerful oxidant with high thermodynamic oxidation potential [19]. Ultrasound (US) cavitation uses ultrasound to oxidize the pollutants. AOPs are more effective when combined two or more AOPs due to more OH∙ is generated, lower catalyst consumption and shorter process time [20]. AOPs have successfully adopted as tertiary treatment of wastewater such as olive oil mill wastewater (OOWW), agrochemicals, pulp and paper, textile wastewater and pharmaceutical [2].

#### **3.2 Membrane technologies**

Besides advance oxidation processes (AOPs), membrane technology is also one of the popular polishing methods of POME. The advantages of membrane technologies are high removal rate, modularity, and ability to integrate with other water treatment method. However, the main drawback of membrane technologies is that the membrane fouling will cause significant reduction in permeate flow due to the surface and pore-blocking of the membrane. The high initial capital and maintenance costs have also limited the application of the membrane. The most commonly used membrane in membrane technologies are microfiltration (MF), ultrafiltration (UF), nano-filtration (NF) and reverse osmosis (RO) [2]. There is an argument about membrane technologies in removing COD from POME compared to other technology. Higher pressure might have provided higher treatment efficiency but at the same time also contributes to the increasing rate of membrane fouling. The effectiveness of membrane technologies in POME treatment depends on the properties of the membrane. Nano-filtration performs better than ultrafiltration, but it has a higher level of fouling compared to ultrafiltration. Membrane technologies can be combined with other technology such as coagulation and flocculation to increase their treatment effectiveness [2]. **Table 3** shows some of the application of membrane technologies used in POME polishing.

#### **3.3 Adsorption technologies**

For adsorption technologies, it is a physicochemical separation process involving inter-phase transfer between an adsorbent and a solution. The pollutant in the solution (adsorbate) absorb onto the surface of the adsorbent. Adsorption can be a reversible process which offers the possibility of adsorbent regeneration through desorption [28]. Adsorption mechanism mainly influenced by the physical forces (physisorption) or chemical interactions (chemisorption) between the adsorbent and adsorbate. The adsorption is also influenced by characteristics of the adsorbent such as specific surface area, porosity and surface charge. Chemical structure of

*Performance of Chitosan as Natural Coagulant in Oil Palm Mill Effluent Treatment DOI: http://dx.doi.org/10.5772/intechopen.94330*


#### **Table 3.**

*The application of membrane technologies for POME polishing.*

the adsorbate and environmental condition such as temperature, pH, solubility and ionic strength will influence the adsorption performance [29]. **Table 4** shows some of the application of adsorption process in POME polishing.
