Study of Change Surface Aerator to Submerged Nonporous Aerator in Biological Pond in an Industrial Wastewater Treatment in Daura Refinery

Abstract

Daura refinery, with a capacity of 140,000 barrel per stream day as a refining capacity, wastewater discharged from refining and treatment processing units, polluted water as foul water, drainages, oil spills, blowdown of boilers and cooling towers, and many other polluted water sources, aims to remove pollutants and reject clean water to the river; wastewater treatment system takes place in this treatment process. Wastewater treatment system suffers from many problems and specifically biological stage; at this stage, activated sludge with bacteria, should be supplied with oxygen, aeration system done by surface aerators with four surface fans; these fans suffer from high vibration, loss support, and in consequence, lack in oxygen supply to aerobic bacteria less than 4 ppm. The nonporous aerator is suggested as an oxygen source for the biological pool. The pilot plant builds the aim to study the ability to apply the new aeration system at the biological pool, pilot plant build with 1 cubic meter capacity tank and continuous overflow of wastewater of 10 liters.min−1, air injected with the pressure of (0.5–0.75) bar(g), and airflow of (7.6–9.7) liter.min−1 respectively. Oxygen concentration was recorded as (3.4–6.0) ppm; in terms of consumption power, changing the aeration system reduces it to less than 20%.

1. Introduction

Industrial wastewater treatment in refineries just receives water infected with a large number of pollutants; these pollutants are hydrocarbons (light oil products – heavy oil products), phenols [1, 2, 3, 4], solvents, high turbidity water, saline water, foul water, drainages, and water may be discharged from equipment under maintenance or test [2]. Water with all of these pollutants is treated with a train of processes as follows:

1. API separator: Hydrocarbon cuts, free oil, and greases can be removed by skimming from the surface of the water [2, 5], oil skimmed from the surface is sent to the tank, settled, and dehydrated, the water is sent back to the API separator; it may contain a low percentage of oil suspension, separated oil pumped to the slop tanks in refinery [6].

2. Clarifiers and flocculation: Emulsified oil can be removed by using clarifier and flocculation (air flotation), which are attached to a dissolved air floatation system (DAF) [5]; emulsified oil is separated by flocculation and sludge sent to the sludge dehydration and loading system [5, 6].

3. Biological reactor: Dissolved oil can be removed by using a biological tank reactor (biological digestive pool) [5]; this stage uses the activated sludge [1, 7], which uses aerobic bacteria aiming to break down organic material into carbon dioxide (CO₂) and biomass with aid of oxygen (O₂).

Oxygen supply can be done by surface aeration or direct-contact air injection inside the water (porous or nonporous aeration system). Water overflows from the biological tank to another stage, which contains a secondary clarifier, overflowed water from biological reactor is treated in this clarifier (called secondary clarifier), water is separated as treated water and sludge divided into two portions, portion disposal as sludge and another portion of sludge circulated back to the bioreactor [7, 8]. The aeration tank (Biological Reactor) in an industrial wastewater treatment system needs urea and phosphoric acid, which are added to the biological reactor as a feed to bacteria, but in the municipal system, it is not required due to the high content of urea and phosphoric acid in the feed. Treated water is discharged to the river as clean treated water (Figure 1) [6].

Figure 1.

Activated sludge system.

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, 10, 11].

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

$$ln\frac{C_S-C_O}{C_S-C_t}=K_{la}t$$E1

Where:

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

C_o: Oxygen at time 0 mg.l⁻¹.

C_t: Oxygen at time minute.

K_la: Transfer coefficient.

t: Time in minutes.

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

1. Surface aeration: This type of aeration is a mechanical-type aerator, which supplies oxygen by introducing air into the water in the (biological reactor), by making turbulence at the surface of the liquid inside the pool with a depth of not more than 3.5 m, types of rotor (blades type, brush type) vertical or horizontal position [11, 13, 14]. A mechanical aerator supplies the biological reactor with a sufficient amount of air (oxygen) and mixes the content, oxygen supply will promote the biological activities and digest wastes, remove carbon dioxide and other undesirable gases released due to the biological activities [15].

2. Subsurface aeration: This type of aeration injects air inside the biological reactor pool directly; there are many types of methods 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].

$$\mathit{hp}=\frac{\mathrm{Q}\ast \mathrm{d}\ast \mathrm{L}}{24\ast \mathrm{q}}$$E2

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:

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].

Figure 2.

Wastewater treatment system in general [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.

3. Bioreactor unit

4. Clarification and filtration

5. Final check basin (filtrated water basin)

6. Sludge treatment unit (sludge thickener, incinerator, ash thickener basin, rotary drum filter) (Figure 3).

Figure 3.

Wastewater treatment system in Daura Refinery.

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.

Figure 4.

Surface with corrupted foundation due to the vibration.

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.

1.3 Methodology

The pilot plant was just built to study the performance of air injection nonporous diffuser (aeration system) in an activated sludge tank with a continuous overflow system of wastewater out of a flocculation system with the aim to simulate a biological tank.

The apparatus is just built as in Figure 5 from the following items:

1. Isocontainer with dimensions of 1X1X1 m with a capacity of 1 m³.

2. ¾” high-pressure flex rubber hoses ended with connection adapters, six hoses.

3. Foul water flow measurement 0–50 liter.min⁻¹ (Nippon – Japan)

4. Pressure gauge 0–6 bar (Wika – Germany).

5. Airflow measurement 0–50 liter.min⁻¹ (Yokogawa – Japan).

6. Air distributor with a diameter of 200 mm, 89 holes, 5 concentric circles with a hole diameter of 0.8 mm, ½” female threaded end connection.

7. ½” Polyvinyl chloride pipe SCH-80 (Al Amal Al Sharief)

8. ½” ball valve

9. Air source 0–4 bar (g).

10. Oxygen concentration indicator

Figure 5.

Test apparatus arrangement: (a) sludge tank. (b) oxygen indicator device, (c) air diffuser.

The procedure of the experiment:

1. All the previous parts are connected as in Figure 6.

2. The air valve opened and the air passed to the air distributor inside the sludge tank, the pressure was just set at 0.5 bar(g) with a flow rate measured as 7.5 liter.min⁻¹ at 30°C.

3. Oxygen concentration was recorded for each minute.

4. The second experiment with an air pressure of 0.65 bar(g) and 30°C, i.e., air pressure increased by 0.1 bar(g), and airflow of 8.5 liter.min⁻¹ and oxygen concentration was recorded for each minute.

5. The third experiment with an air pressure of 0.75 bar(g) and 30°C, i.e., the air pressure increased by 0.1 bar(g), and airflow of 9.7 liter.min⁻¹ and oxygen concentration was recorded for each minute.

6. Foul water inlet to sludge tank just set to 10 liter.min⁻¹, constant at all three experiments.

Figure 6.

Apparatus diagram.

2. Result and discussion

Air was injected inside the tank through the air diffuser, and the feed of wastewater to be treated, which was pumped into the tank, activated sludge placed in the experiment tank.

The oxygen transfer rate was recorded through the experiment and the oxygen concentration was measured with an oxygen detector.

Oxygen concentration was measured through the first experiment and recorded with 0.45 PPM at the beginning and 3.2 PPM after 33 minutes as in Figure 7.

Figure 7.

Concentration of oxygen and temperature of liquid with time at 0.5 bar(g) and air flow of 7.5 L.min⁻¹.

The temperature of the system raised from 26.7°C at the beginning of the experiment to 27.7°C after 33 minutes as in Figure 7.

Oxygen concentration was measured through the second experiment, oxygen concentration was recorded at 3.23 PPM 34 minutes from the beginning and 4.46 PPM after 18 minutes from the beginning of the second experiment as in Figure 8.

Figure 8.

Concentration of oxygen and temperature of liquid with time at (0.5–0.65) bar (g) and air flow of 8.7 L.min⁻¹.

The temperature of the system raised from 27.8°C at the beginning of the experiment to 28°C after 18 minutes as in Figure 8.

Oxygen concentration at the last experiment was recorded as 4.8 PPM after 41 minutes from the beginning of the experiment, concentration of oxygen was recorded at the end of the experiment at 6.01 PPM, Temperature increased from 28 to 28.2°C at this experiment.

Increasing the temperature can relate to the biological activities of bacteria due to the digesting of wastes in wastewater [20].

The rate of oxygen (air), supplied with an air diffuser in the experiment tank was high compared with the calculated air required by Eq. (2) [6] and as in Table 1.

Pressure bar (g)	Air flow rate L.min^−1 (required)	Air flow rate L.min^−1 (Measured) (Maximum)	Oxygen concentration PPM	Temperature °C of tank
0.5	9.4	7	1.46	27.1
0.55	8.642	7.5	3.23	27.7
0.65	8.428	8.5	4.46	28
0.75	7.940	9.7	6.01	28.2

Table 1.

Final results and estimated amount of air required.

The amount of air supplied was more than that estimated in the third experiment, because the short path of air from the injection point to the surface leads to air bubbles escaping out of the liquid tank, [21], the pressure of injected air increased with the aim to increase the flow rate of air and reach the required concentration of 6.01 PPM.

All three experiments were continuous and pressure increased with the aim to increase the flow rate of air, this was when the oxygen concentration did not increase and stopped increasing and sometimes decreased when the temperature of the liquid increased [22], and this was observed very clearly through the experiments data (Figures 7, 8 and 9).

Figure 9.

Concentration of oxygen and temperature of liquid with time at 0.75 bar (g) and air flow of 9.7 L.min⁻¹.

New Aeration System Proposed for Biological Reactor of Waste Water Treatment Complex in DR:

The biological reactor in wastewater treatment complex in DR will be changed from mechanical aeration to air direct injection by nonporous diffusers, air distributor applied at 1cubic meter capacity can be applied at the biological reactor.

The power duty and oxygen transfer rate (air flow rate) required can be estimated as in Eq. (2) [6]. power duty with 10% over design is 200 hp. at (BOD 250 PPM as a design condition), the amount of air required to realize 6 PPM oxygen concentration is not less than 587 kg.h⁻¹ (454 Nm³.h⁻¹) at a liquid flow rate of 500 m³.h⁻¹ as a charge to the biological reactor.

The total number of nonporous diffusers required to cover the required amount of air is 604 with a diffuser capacity of 0.9735 kg.h⁻¹ (12.532 NL.min⁻¹), which was used in the experiments.

The number of diffusers will be used more than that calculated according to the capacity of the diffuser, in terms of providing the required amount of oxygen required.

The number of diffusers will be 1024, distributed for two sides of the biological reactor of dimensions 16,000 X 32,000 mm, the distance between each diffuser and the other is 1000 mm, and the distance from the wall is 500 mm as in Figure 10.

Figure 10.

Arrangement of air diffusers in biological pool.

Installing excess numbers of diffusers can provide well mixing and at the same time avoid the dead zones in the biological reactor; dead zones in the biological reactor activate anaerobic bacteria and in consequence reduce the efficiency of waste digesting [22].

The consumption power through the new proposed system of fine bubbles (diffuser nonporous type) will be reduced from 241.4 hp. in surface aeration to 200 hp, and this is due to the reduction in the amount of air required in the aeration process.

3. Conclusions

1. Oxygen concentration achieved is 6.01 PPM, which is close to the optimum required concentration when 0.9735 kg.h⁻¹of air is injected through a fine bubble diffuser (nonporous type) in a 1 m³ liquid tank filled with sludge.

2. Biological reactor (the liquid tank) temperature increased with increasing oxygen concentration due to bacteria’s biological activities. Even if the temperature rise affects the oxygen concentration in an aqueous solution, airflow must be increased.

3. The type of diffuser used in the experiments can be applied at the biological reactor with an excess aim to supply the oxygen demand, which realizes well mixing, avoids dead zones, and avoids the growth of anaerobic bacteria.

4. Replace the surface aeration system with a direct injection fine bubble diffuser (nonporous diffuser) will reduce consumption power from 241.4 hp. to 200 hp. due to the reduction in air supply, which can be considered more economic and power saving.

Acknowledgments

I would like to thank Mr. Waleed H. Mhaseb Head, division of wastewater treatment for his support and cooperation, and also I would like to thank Miss Aza A. Fiadh, senior instrument engineer for her cooperation.