Introduction

The increasing rates of human water consumption and the rapidly growing world population have resulted in water preservation jumping to the forefront of global issues [1–5]. Because of the necessity of water for human survival as well as living organism, there is a need to appropriately manage the use of this valuable resource. An important aspect of responsible water management is the development of new methods of wastewater treatment [6, 7]. Large volumes of wastewater are generated daily by various industrial processes, and the implementation of efficient treatment technologies would lead to immense ecological and economic benefits [8]. The efficient purification of petrochemical wastewater has become one of the most important issues in the last years. There is evidence that the biological purification of petrochemical wastewater can be carried out with activated sludge supplied with nutrients and oxygen [9, 10]. It has also been recognized that several processes such us activated sludge and constructed wetlands methods are suitable for the treatment of wastewater incoming from oil refineries [11, 12].

For the biological aerobic treatment and the operation of activated sludge systems, the C:N:P ratio is important. A ratio of 100:15-10:1 is often recommended [13, 14].

Several new technologies in petrochemical wastewater treatment are now appearing like treatment by means of clean electrochemical technologies [15–17], enhanced biodegradation using ozonation [18], anaerobic digestion [19] and aerobic systems [20].

The constructed wetlands technique is one of the post-treatment technologies of petrochemical wastewater, but their operation is greatly influenced by environmental factors. The post-treatment of the purified wastewater is then carried out assured in a constructed wetland, which consists of a pre-settling pond unit, an oxidation pond unit and a post-settling reed pond [21]. In the post-treatment pond system, about 50 % efficiency of elimination of organic materials expressed as COD or TOC is acceptable [22, 23].

Among the various wastewater treatment technologies, degradation of organic pollutants by Photo-Catalytic Oxidation (PCO) has received much attention in recent years and is emergent as an effective treatment technique [24]. The PCO has several advantages over traditional methods, via, complete mineralization of pollutants, no requirement for disposal of sludge, and low energy consumption. In the PCO process, the most widely used catalyst is TiO2 (Titanium dioxide) because of its ability for oxidizing a wide range of organic pollutants and also because of its low cost [25].

Generally, for the textile wastewater, the biological aerobic wastewater systems are not successful for decolourization of majority of dyes. Photo-catalysis has been successfully used to oxidize this dye by many workers [26].

Moreover, the applicability of adsorption in the removal of heavy metals has been investigated by multiple researchers [27–31]. These studies confirmed the potential use of adsorption as an appropriate technique for the removal of various heavy metals.

Great quantities of water are used in the petrochemical activities, for both oil and gas process and also domestic activities such as restaurant and administration building. In the Hannibal gas treating plant within BG Tunisia, the used water is exploited locally by a deep well drilled in the plant.

Before being used in the plant, the water is treated in a specific unit called METITO Unit that provides an over-purified water eventually used in different steps of the gas process and for domestic activities.

Like the majority of Oil and Gas plants, the Hannibal gas treating plant is situated in a location where no local sewage facilities are available. Therefore, to safeguard as possible the environment, a package sewage system, called SEWAGE Unit, has been installed. The package has been designed to accept and treat all domestic sewage products from the complex. The plant is capable of treating up to 75 m3/day of raw sewage.

In this paper, our study will be focused on the sewage system procedure. The operation of the sewage system using the extended aeration method is discussed. The optimization of the sewage system, the occurring problems during operation, and the adequate corrective actions are also discussed, with suggestions of suitable processes (PCO, adsorption, constructed wetlands) for post-treatment of the petrochemical wastewater already treated by the activated sludge method.

Localization

The investigations were carried out in the Hannibal gas treating plant, the samples have been analyzed during the first 10 days of july 2011 into the HSSE department (Health, Security, Safety and Environment department) in the Hannibal plant within BG Tunisia.

BG Tunisia gas company is a part of the international oil/Gas company called BG group (the world leader in natural Gas). This company, supplying more than 60 % of domestic gas in Tunisia, was established since 1994. The gas, exploited from the Miskar field located in the Golf of Gabes, is treated in the Hannibal plant situated in the village of Nakta-Bouakkazine, 23 km to the south of Sfax, southern Tunisia (Fig. 1).

Fig.1
figure 1

Miskar platforms and Hannibal Plant Localizations

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The mixture of gas, condensate, saline water and other materials such as H2S, CO2, and waste water, incoming from the Miskar Platform, is treated during the several steps of process in several units of the Hannibal plant (Fig. 1). The water exploited from the local well is necessary for this process especially for the Glycol Unit.

Equipments, methods and process description of sewage system

Equipments

In the sewage unit, we can discern the following equipments (Fig. 2):

Fig.2
figure 2

Sewage system and equipments

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  • Sewage Treatment Plant Blowers, C-2601 A/B,

  • Blower Control Logic,

  • Sewage Lift Station Pump,

  • Sewage Surge Tank Pumps,

  • Sewage Lift Station,

  • Sewage Surge Tank,

  • Sewage Treatment Plant.

Methods

The water used in the Hannibal plant is exploited from a local well drilled in the plant. This water is treated in the METITO unit using several techniques: chlorination, primary and secondary filtration, primary and secondary reverse osmoses, and ion’s exchanger resins. The result of this treatment is an over-purified water stored in a big tank of water process.

The pure stream sewage treatment system in the Hannibal plant is based on the extended aeration method of sewage treatment. This method consists basically of four operations: screening, aeration, settling and chlorination. Sometimes, additional operations may be required to overcome abnormal local conditions, including the disposal of any “Off spec” effluent by trucking to local authority disposal plant.

Screening

When the sewage first enters the plant, it passes through a screening device. This consists of a series of bars welded into a “via” shaped frame with approximately one inch spacing between the bars. The purpose of this device is to trap any larger particles which may have passed through the pumps, from entering the system.

Aeration

From the screening device, the sewage passes into the aeration tank. Here, the sewage is decomposed by aerobic bacteria and other organisms in the presence of air (aerobic conditions). In a properly operating plant, these micro-organisms (bugs) will form a dark brown mass called “activated sludge”. The incoming sewage mixes with this activated sludge by the introduction of air, blown through diffusers at the bottom of the tank. This mixing of the liquids through the supply of air currents allows the organisms to decompose the sewage into carbon dioxide, water and some other minor constituents: phosphates, nitrates and sulfates.

Settling

From the aeration tank, the treated sewage/activated sludge mixture passes through a port in the wall into the settling tank or clarifier. Here, the heavier activated sludge mass settles to the bottom and the clear treated liquid flows over a weir and into the discharge line. The retention time within this section is usually 4 h. The settled sludge or bacteria is then returned back to the aeration tank by the air lift sludge return system.

Chlorination

The effluent discharged by the clarifier then passes through a chlorination package that kills any disease-carrying bacteria, which might be found in the effluent. The liquids pass through a tablet-fed chlorinator with a 30-min retention time, before plant discharge.

Process description of the sewage system

The sewage unit is operating as the following manner: (1) raw sewage and domestic waste water collected from the administration building, restaurant, warehouse, and control room, combined in the underground system. This waste flows into the Sewage Lift Station and when the high level is reached, the lift station pump P-2601 A or B (Fig. 2) is activated to pump the sewage water into the sewage surge tank; (2) the raw sewage enters the surge tank after passing through a barred screen that separates any large solids from the flow; (3) the surge tank operates under level control and when the liquid reaches float switch FS-2 the lead pump starts; (4) the sewage is pumped from the bottom of surge chamber into the flow control box; (5) air for the surge tank is supplied by the Plant Air System (PAS) and regulated to 0.34 barg by a pump called PVC-26002; (6) the sewage coming from the surge tank is pumped into a flow control device by a pump named P-2602A/B (Fig. 2); (7) the aeration tank is the mainstay operation of the whole treatment unit. Here, the sewage is decomposed by aerobic bacteria and other organisms in the presence of air into carbon dioxide and water and other minor constituents; (8) from the aeration tank, the treated sewage mixed with the activated sludge passes into the settling tank or clarifier where the heavy activated sludge mass settles to the bottom and the clear treated liquid flows over a weir into the discharge line; (9) the settled sludge or bacteria is returned back to the aeration tank to decompose more incoming sewage; (10) the treated liquid (the effluent) discharged from the settling tank then passes through a chlorination unit; this is done to kill the disease-carrying (pathogenic) bacteria, which might be in the effluent. The treated effluent passes through the contact tank where a tablet feed chlorinator mixes with the solution prior to being discharged by a special pipe line in the Sea (chaffar-nakta beach), (Fig. 3).

Fig.3
figure 3

Treated water coming from Sewage Unit and rejected in the sea

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Results and discussions

The sewage daily checks

During routine daily checks, utilizing the daily check sheet (Table 1), we—area technician—will notify the shift superintendent of any anomalies according to the sewage procedure. Anomalies in the sewage system can be detected by checking the following parameters (Table 2), especially in the treated liquid (effluent) prior to being discharged.

Table 1 Sewage plant data
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Table 2 Sewage parameters and normal specifications
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During normal working hours, the shift superintendent will notify the laboratory supervisor to investigate, in case to ensure that the sewage is in specifications (Table 2) if not the area technician will divert the outfall from the field to the recycle line back to the unit.

In addition to the daily checks, the parameters are coherently followed at regular periods, and a meeting of the concerned department is organized in the end of each period to discuss the evolution of the system and the problems that occurred during the last period. The aim required from this regular following is to study the parameter’s evolution and identify the most frequent problems occurred during the sewage system operating. An example of this consistent following in the first 10 days of July month 2009 is illustrated in the Table 3. We can note the High level of BOD5 in the last 4 days from 07/07 to 10/07; such anomalies show that a problem of aeration occurs in the treatment system.

Table 3 Sewage parameter’s followed during 10-day period
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Occurring problems in the sewage system and preferred alternatives

During normal operation, the system is totally automatic and is properly adjusted, thus the plant will have the following appearance:

  • Aeration tank: air bubbles rising, medium brown color

  • Settling tank: surface clear and liquid clear to depth of 18 or more inches

  • Effluent: clear, sparkling. No solids, more than 1 ppm D.O.

While the sewage system is in operation, poor performance symptoms could appear and corrective actions must be done immediately. Some of the problems that could occur are listed below:

  • Diffuser or airline blockage;

  • Over aeration;

  • Sludge not returning;

  • Skimming not returning;

  • Excessive foaming.

We can list examples of such problems and appropriate corrective actions that should be taken, in the following Table 4.

Table 4 Poor performance symptoms and appropriate corrective actions
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The outlet from the sewage treatment plant normally flows to the leaching field irrigation for dispersion; however, there are other routes which may be used if problems occur; if the laboratory analysis of the effluent shows the discharge to be out of specifications, and then the flow to the leaching field must be stopped.

The preferred alternative dispersal route now becomes the contactors vacuum truck to the local municipality sewage plant which we call “ONAS”.

Other dispersal routes available for the outlet effluent include returning it back to the lift station for recycling or routing it to the open ditch where it mixes with excess outflow from the well water. The two valves in the pit adjacent to the tank are used to divert the effluent either to the leaching field irrigation or back to the open ditch which takes the excess well water alongside the administration building—this is not an approved route.

At no time, unfortunately, the effluent shall be discharged to the sea (Fig. 3) from the sewage treatment plant and the sewage disposal becomes a serious problem when it is knowingly out of the acceptable specification limits stipulated by Tunisian Law.

The extended aeration method should be carried on with combination of other processes. The constructed wetland technique is one of the post-treatment technologies of petrochemical wastewater, but their operation is greatly influenced by environmental factors. The post-treatment of the purified wastewater is then carried out assured in a constructed wetland, which consists of a pre-settling pond unit, an oxidation pond unit and a post-settling reed pond. In the post-treatment pond system, about 50 % efficiency of elimination of organic materials expressed as COD or TOC is acceptable.

We suggest the treatment using the Photo-Catalytic Oxidation method. In fact, various researchers have reported the photo-catalytic degradation of large variety of organic pollutants using TiO2. The heavy metal removal can be carried on by the adsorption technique using prepared adsorbents.

Conclusion

The sewage treatment system in the Hannibal gas plant is based on the extended aeration method (Biological Aerobic Treatment and Activated Sludge System). The main important equipment in this system is the Sewage Treatment Plant that consists of four separate compartments contained in one rectangular structure (sludge holding tank, aeration tank, clarifier and chlorination contact tank). This system is running since 1994, and despite this continuous operation over several years, the ecological structure and the functioning of the system is still acceptable. However, we have to note that the daily measurements of several parameters (especially pH, COD, BOD5, TOC…) of the discharged treated liquid (effluent) show serious problems related to the environment protection, in fact the effluent incoming from the discharge line is not perfectly compatible with the required specifications (high level of BOD5 and COD) and so that it will be dangerous on the life of micro and macro-organisms in this part of the sea.

During any period when the outlet water from the clarifier fails to meet the required specifications, the liquids can be routed back into the Sludge Holding tank and this is situated at the very front end of the unit. The problem in this case is that the capacity of this tank less than 16 m3, and the excess sludge removed from the clarifier should not be stored if the aeration tank is full of sludge, thus a problem of aeration could occur in the aeration tank. The liquids or sludge transferred here (into the Sludge Holding tank) have to be removed by truck and it is not the approval solution. It is one among many problems that could occur during the operating of the system.

Moreover, an emergency shutdown has no effect on the sewage system and this is a serious problem that should be resolved as soon as possible. The wastewater quality could be improved by the installation of post-treatment processes using effective technologies. The implementation of constructed wetlands is suitable; however, the operation of such process is greatly influenced by environmental factors. We suggest the treatment using Photo-Catalytic Oxidation (PCO) technique; the efficiency of this technology in the removal of organic pollutants has been approved.