POSSIBILITIES FOR THE TREATMENT OF WASTEWATER FROM PRIMARY PETROLEUM CLARIFIERS

Wastewater from the oil industry treatment is very difficult to achieve in an uniform matter. Therefore, pre-treatment steps are often required in order to reduce the compounds’ elimination and to determine which treatment cannot be eliminated in the biological stage. Application of a pre-treatment process eliminates certain compounds with specific microorganisms that do not fit with the activated sludge. In this paper, the details of this solution were studied. It was established a reduction of the benzene concentration from 82.23 mg/L to 18.04 mg/L, toluene 3.08 mg/L to 1.15 mg/L, xylene at 4.1 mg/L to 3.1 mg/L and naphthalene 7.08 mg/L to 4.96 mg/L. Experiments have shown a decrease of the polycyclic aromatic hydrocarbons concentration below the limit of detection of the device.


INTRODUCTION
Industrial wastewater treatment is a topical issue given the need to monitor the environment continuously.The efficiency of industrial wastewater treatment plants depends significantly on the source and quality of water at the entrance, according to the regulations that in these conditions lead to inadequate treatment [1].
The composition of industrial wastewater is a decisive factor in choosing treatment processes.This is determined by laboratory tests that determine global and specific indicators of water of which the most important are: the content of suspended solids, biochemical oxygen demand (BOD 5 ), chemical oxygen demand (COD), content of chlorine, nitrates, sulphides, the pH and the residue filtered at 105 ºC etc. [2][3][4].
Greening the cleaning process is a necessity due to the relatively low degree of purification of wastewater from the oil industry and due to the emergence of harmful compounds in these which are very difficult to remove by conventional methods.
Among the most effective and economical methods for removing organic substances from wastewater, substances that are colloidal or dissolved, we may mention the biological treatment processes [1,5].
Aerobic microbial treatment technologies in the preliminary stage or tertiary treatment process may be a solution in this case where the primary clarifier effluent from oil goes to a treatment plant which applies NTPA 002/2005 entrance.
Microbiological purification technology is of particular interest because of its many advantages: easy installation of the microbiological reactor, small investment, low running costs, no risk of clogging of the reactor, high performance.The disadvantages of this process are the following: larger spaces, the production of excess sludge [10].
In this paper, we present an experimental study on the treatment of wastewater from oil in the primary settling oil tank.It contains aromatic and polycyclic aromatic hydrocarbons.

EXPERIMENTAL SETUP
The experimental study was to initially gather evidence from industrial wastewater secondary settling within a refinery.According to the current legislation [4], the experiment was set up so that the samples collected were as representative as possible for the final effluent characterized, in order to achieve the objectives of the sampling program: the determination of the loading with industrial effluents pollutants; the checking of the concentrations permissible in the limits.In this regard, samples were chosen daily over 20 days, due to wastewater origin and source of technological plant operation.For the taken water samples were analyzed specific indicators: pH, BOD 5 (biochemical oxygen demand), COD (chemical oxygen demand), filtered and the residue suspended solids content.
The second phase of the experimental study was to research the possibility for treatment of water samples with microorganisms.The microorganisms used were Candida tropicalis type.Experiments took place in a thermostatic reactor equipped with agitator blades operated in batch mode with the following features: diameter 12 cm, height 80 cm and a working volume of 800 mL [11].The amount of sample introduced into the reactor was 500 mL.At the bottom of the reactor purified water was removed.The reactor was made out of glass to allow visual observation of the formation of scale polycyclic aromatic hydrocarbons and aromatic hydrocarbons.Continuous mixing in the reactor was made after adding the sample of wastewater and microorganisms (about 5 g of Candida microorganisms is for each sample analyzed).The temperature was kept at a constant 36.5°C and was continuously measured by a temperature sensor input to the system.Maintaining the pH of the industrial wastewater with the addition of microorganisms was done by adding sodium carbonate (3 g) and values were recorded using a digital pH meter (Burette 24).Physics and chemical analyses performed according to specific standards [8] followed: -Determination of the content of aromatic hydrocarbons by means of gas chromatography (P & T-GC-MSD), the calculations were carried out by reference to a standard containing volatile aromatic hydrocarbons -benzene, toluene, xylenes, naphthalene, using the areas integrated ion's main components; -Determination of polycyclic aromatic hydrocarbons by mass spectroscopy method.The samples were extracted with iso-octane, the extract was purified by activated silica gel column, then concentrated 20 times, the calculation being carried out by reference to a standard -to 200-fold diluted; -Determinates values of COD and suspended solids by spectrophotometric method with machine HANNA HI 9143; -Determination of the BOD 5 values with VELP device.

RESULTS AND DISCUSSION
The microbiological treatment is influenced by the operating temperature, the initial concentration of polycyclic aromatic hydrocarbons and aromatic hydrocarbons in the industrial wastewater and the pH value.Among the treatment parameters of the process, the experiments carried out operating temperature, pH and agitation intensity were constant: t = 36.5°C,pH = approx.7, agitator speed n = 100 rpm.
During microbiological cleansing a crust was formed, whose mass depended on the content of aromatic hydrocarbons and polycyclic aromatic removed.
Concentrations of aromatic hydrocarbons from industrial wastewaters were determined before and after microbiological treatment.Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5 show the concentrations for the 20 samples taken from the reactor.

Initial concentrations ranged as follows:
-The benzene from 82.23 mg/L to 96.32 mg/L; -The toluene from 2.32 mg/L to 3.08 mg/L; -For xylenes from 2.17

CONCLUSIONS
Removal of polycyclic aromatic and aromatic hydrocarbons from wastewater resulted from the oil processing industry is made through microbiological treatment; process efficiency is over 85% for benzene, about 71.08 % for toluene, 70.23 % for xylenes and 76 % for naphthalene.Polycyclic aromatic hydrocarbons are removed from the system, and their concentrations are below the detection limit.
Analyzing the data presented it is observed that the variation of BOD 5 is minimum 280 mg/L and COD is minimum 480 mg/L.The filtered residue and suspended matters have minimum values of 1200 mg/L respectively 270 mg/L, thus respecting norms imposed.
The transition from laboratory scale to industrial scale is not simple, but if the following working conditions are observed -pH, temperature, content and type of microorganisms -a treatment of industrial wastewater with a high content of aromatic and aromatic polycyclic hydrocarbons may be determined.
to 2.95 mg/L; -The naphthalene between 4.28 mg/L and 4.96 mg/L.Concentrations of polycyclic aromatic hydrocarbons present in the samples analyzed were: phenanthrene -17.8 mg/L, fluorene -8.2 mg/L and anthracene -2.4 mg/L.

Fig. 1 .
Fig. 1.The benzene concentration variation during the experimental period.

Fig. 2 .
Fig. 2. The toluene concentration variation during the experimental period.

Fig. 4 .
Fig. 4. The naphthalene concentration variation during the experimental period.