• **Aeration system:**

This system is responsible for supplying oxygen into a biological reactor, which gives the bacteria the ability to digest and oxidize wastes (oil and all other undesirable materials) [9–11].

Oxygen concentration can be estimated in a theoretical way as in equation no (1) [6, 12]

**Figure 1.** *Activated sludge system.*

*Study of Change Surface Aerator to Submerged Nonporous Aerator in Biological Pond… DOI: http://dx.doi.org/10.5772/intechopen.104860*

$$\ln \frac{\mathbf{C}\_S - \mathbf{C}\_o}{\mathbf{C}\_S - \mathbf{C}\_t} = \mathbf{K}\_{la} t \tag{1}$$

Where:

Cs: Oxygen concentration at saturation mg.l−1.

Co: Oxygen at time 0 mg.l−1.

Ct: Oxygen at time minute.

Kla: Transfer coefficient.

t: Time in minutes.

There are many types of aeration systems to supply oxygen to the biological pool as follows:

	- Fine bubble aerator (nonporous diffusers): This type of aeration supplies oxygen to the bioreactor with a small bubble size (fine bubble), with a high rate of oxygen transfer in terms of efficiency, low consumption power [11]; there are many types of these diffusers (such as membrane diffuser, coarse diffuser) [14, 16].
	- Porous diffusers: Classified into four classes: disc diffuser, dome diffuser, tube flexible sheath diffuser, and plate diffuser [11, 17]. These types of diffusers are always made of membrane, ceramic, and plastic [11, 14]; this type of diffusers supply oxygen to the biological reactor at high rate and efficiency [6, 18].

The oxygen transfer rate for each aeration system can be estimated in terms of horsepower as in Eq. (2) [6].

$$hp = \frac{\mathbf{Q} \ast \mathbf{d} \ast \mathbf{L}}{\mathbf{24} \ast \mathbf{q}}\tag{2}$$

Where:

hp.: Horsepower required.

Q: Liquid flow rate million gallons per day (mgd).

d: Density of liquid 8.314 lb.gal−1 for water.

L: BOD – biological oxygen demand (PPM).

q: Oxygen transfer rate in lbO2.hp-h.−1

**Sludge treatment system**: sludge treatment needs a train of processes with the aim of treating sludge as follows:

**Figure 2.** *Wastewater treatment system in general [5].*

Sludge decanting separates water from sludge, sludge from decanting system is sent to the incineration system, ash and other nonhydrated sludge send to the rotary drum vacuum filters with the aim to separate the maximum amount of water from sludge, water is separated and sent to the API separator again (**Figure 2**) [5].

## **1.1 Wastewater treatment in Daura Refinery (DR)**

Wastewater treatment system in DR, designed with a capacity of 850 m3 .h−1 and operating capacity of 750 m3 .h−1, and 1450 m3 .h−1 in stormy weather for 2 hours only. Polluted water is received from many sources as follows: sewer water, drainages, foul water out of desalters, saline water from reverse osmosis units (RO), blowdown of boilers, cooling towers, condensate, equipment washing or the hydrostatic test, oil spills, and stormy weather [19].

The wastewater treatment system consists of the following operations:

1.API separator: all the wastewater in the refinery is collected in the header and entered the API separator to separate hydrocarbons from water by stormy water basin, and precipitated sludge at the bottom is removed by gravity separation.

Oil removed from the surface of water is sent to a slop tank in the DR, and collected sludge at the bottom of the API separator is sent to the sludge treatment unit (thickener).

2.Flocculation and flotation: at this stage, de-oiled water out of the API separator passes to the flocculation basin; at this stage, pH is controlled from 7 to 8 by adding sulfuric acid, or adding caustic soda, aluminum, or ferrous basin also contain mixer to homogenize the mixture.

*Study of Change Surface Aerator to Submerged Nonporous Aerator in Biological Pond… DOI: http://dx.doi.org/10.5772/intechopen.104860*

### **Figure 3.**

*Wastewater treatment system in Daura Refinery.*

### 3.Bioreactor unit


### **1.2 Problem**

Aeration system installed in biological pond in wastewater treatment in Daura refinery with dimensions of (16,000 X 32,000) mm two pools, type of aeration is surface aeration of mechanical fan aerator fixed at the concrete supports.

Four fans were fixed at the top of the pond, these fans were installed in the middle of each quarter of the pond, Fins of the fan were fiberglass type, fins of each fan were corrupted and replaced with stainless steel fins.

These fins are heavier than fiberglass, and the vibration generated is more than that generated by the original fan.

Stainless steel fan installed in 2004, due to the continuous operation of fans, cracks in concrete foundations of each fan appear; cracks in the foundation as a result of excess vibration, cracks in the foundation as in **Figure 4**.

Cracks in the bearer foundations make run of these fans' type of imagination, due to the risk of failure of the concrete foundations.

### **Suggested solution:**

Maintenance measures to solve vibration problems or fix the bearer foundation, this type of solution does not pass away, and the problem is just raised to the surface. Replace the surface aeration system with another type, such as a submerged aeration system, this system will be a suitable type of aeration in terms of solving the problem and avoiding vibration and foundation failure.

### **Figure 4.**

*Surface with corrupted foundation due to the vibration.*
