**3.5 Biological**

Some bilge water treatment units include biological treatment, which is called as a bioreactor. In this method, microorganisms are used to eliminate or reduce the organic and inorganic compounds before the treated wastewater being discharged to the sea or to a collection system [29]. The microorganisms convert the dissolved and particulate carbonaceous organic matter, including oil, in the bilge water into simple end products through the oxidation process. The equation below is representing the aerobic biological oxidation of organic matter [29].

$$\text{Organic material} + \text{O}\_2 + \text{NH}\_3 + \text{PO}\_4^{3-} \rightarrow \text{new cells} + \text{CO}\_2 + \text{H}\_2\text{O} \tag{1}$$

As can be seen from Eq. 1 above, the oxidation process needs oxygen (O2) and nutrients, which are nitrate (NH3) and phosphate (PO4 <sup>3</sup>�), in order to convert the organic matter to carbon dioxide (CO2) and water (H2O). Other than that, the new cells generated are referring to the biomass produced after the oxidation of organic matter takes place.

There are two principals of biological processes used to treat wastewater, which are suspended growth and attached growth, or also known as biofilm, processes.
