The influence of oil contaminated soil on the quality of surface waste water

In connection with this problem the task of soil purification from different pollutants including the most widespread substances, for example petroleum hydrocarbons, has to be solved (Margesin & Schinner 2001). Biological methods of soil treatment with the use of hydrocarbon oxidizing microorganisms are widely used among the well known techniques (Schussler 1986; Gerbhardt 1987; Ellis et al. 1990; Margesin et al. 2000; Beškoski et al. 2011).

Nowadays this new direction in environmental protection from chemical pollutants is called ‘Ecological biotechnologies’. The series of biopreparation-destructors of oil is created based on series of cultures. It works in a wide range of temperatures, pH, which makes it possible to use it in various climate conditions. The efficiency of purification is 80–90% of the initial pollution. The repeated treatment with biopreparation allows to achieve efficiency almost 100%. Hence biopreparations contain microorganisms which ability to utilize different kinds of organic pollutants testifies the possibility of effective solution of problem of the environment purification from oil and oil refining products.

Methods of introducing of microorganisms’ cultures are applied in cases when aboriginal microflora is absent. They can be applied when massive and emergency pollution occurs in difficult conditions with the absence of naturally developed biocenosis. The advantage of these methods is their selectivity and possibility of cultivating microorganisms strains that destruct complex toxic compounds. However their efficiency is not always equally high, because many cultures ‘work’ only in relatively narrow range of conditions. Moreover sometimes degeneration occurs before the desirable level of purification is achieved. Also there should be taken into consideration that application of such methods can offend natural biocenosis, because the change of the composition of leading microbial community population takes place in such case. Some kinds of microorganisms that utilize n-alkanes are presented in Table 1.

The cultures presented in Table 1 are the base for making preparations. These cultures need to be obtained in special conditions. Sometimes it leads to the expensiveness of such biopreparations for the soil purification from oil and causes certain difficulties in their obtaining.

These circumstances point out the necessity of search and use various waste products, for example surplus activated sludge produced during biological water treatment (Ksenofontov 2004, 2010). The researches have shown that it is reasonable to use specimens of sludge taken from purifying systems of oil refineries. Activated sludge, adapted to oxidizing of oil hydrocarbons, can be an effective biopreparation-destructor of oil hydrocarbons. Microorganisms of such sludge possess higher efficiency than cultures of sludge of municipal water treatment systems.

Comparing techniques of biological treatment of soil with well known techniques of biological treatment of water it is worth pointing out that the first problem is on the stage of creating and the second problem is on the stage of improvement. In our opinion the following complex solution is possible. Activated sludge, adapted to oil oxidizing, taken from biological treatment systems of industrial waste water, primary from oil refineries, is introduced into contaminated soil in the state of suspension. In this case the transportation of activated sludge can be implemented via pipe lines or by special cars.

In case of remote objects, which need biological soil treatment, activated sludge is delivered in the state of dried biomass of activated sludge with humidity not more than 10%. The technique of obtaining dry biomass of activated sludge with humidity not more than 10% is developed by us (Ksenofontov 2010). This technology includes the following main stages. Suspension of surplus activated sludge taken from secondary settlers with concentration of absolutely dry substance (ADS) about 0.8–1.0% is concentrated with the use of dissolved air flotation (DAF) machine to the concentration up to 2–2.5%. This concentrate is directed to further dehydration in vacuum evaporator. The concentrated suspension of activated sludge with the concentration of ADS about 8–10% is sent to spray dryer. The humidity of the obtained biomass of activated sludge is not more then 10%. The storing period of such biomass is no more than 6 months.

The disadvantage of technology described above is the death of some microorganisms in the process of dehydration. This is undoubtedly the disadvantage. However taking into account that activated sludge is waste product, the part of died biomass can be considered as substrate for the life activity of left microorganisms. There are another more gentle methods of dehydration of microorganisms including activated sludge. Firstly it refers to the drying of microorganisms suspension. In this case it is rational to use freeze dryer and other methods that allow to keep most of the microorganisms alive during drying process.

In the process of dehydration of activated sludge microorganisms suspension the following circumstance has to be taken into consideration. Obviously not all microorganisms participate in the oxidation process of oil. In this case the attention is drawn to the technological processes contributing the intensification of oxidizing oil by microorganisms. The search of new technical solutions carried out by us showed that use of flotation at preliminary stage of concentration of activated sludge allows to intensify the oxidation of oil in further process. One of possible mechanisms of this phenomenon can be the selection of physiologically active cells of activated sludge microorganisms with different flotability, as has been shown by us before.

The use of surplus activated sludge from biological water treatment plants mainly from oil refineries as the preparation for the soil purification from oil allows to decrease the price of such biopreparations. Moreover this technical solution will benefit the utilization of such large-tonnage waste product as activated sludge from biological water treatment plants of oil refineries which power has increased during last years.

The researches with different specimens of activated sludge were carried out in order to check the efficiency of use of activated sludge as a biopreparation for neutralization of oil contaminated soils. The sampling of activated sludge from waste water treatment plants of various oil refineries was carried out. (The activated sludge samples are AS-1, AS-2, AS-3, AS-4). The characteristics of the activated sludge samples are presented in Table 2.

The characteristics of activated sludge samples presented in Table 2 are quite similar, though they are different by the value.

The Table 3 presents the utilization of the most widely spread hydrocarbons in dynamics of consumption. As we can see from the table, microorganisms of activated sludge (samples AS-1, AS-2, AS-3, AS-4) actively utilize n-alkanes contained in oil and diesel oil, accumulating microbial biomass.

Considering data presented in Table 3 we can see that after 36 hours the most part of oil is oxidized, moreover paraffin is oxidized quicker than crude oil. The practical technique has been worked out for the practical use of the approach described above.

The technique of oil contaminated area treatment with the use of biopreparations contains following stages:

1. Determination of necessary dose of introduced bio preparation according to initial concentration values of oil.

2. Preparation of biopreparation mixing biomass with water.

3. Soil loosening and introducing loosening components and mineral nutritious substances and also wetting of the soil.

4. Introducing of biopreparation to the soil in a form of water suspension.

The sequence of these operations has to be strictly kept and planned arrangements have to be carried out in sunny days with no rainfall. Rainfall has negative meaning especially during the introduction of biopreparation, when its most part can be washed away, so that the efficiency of soil treatment using biopreparation can become minimal.

The necessary dose of biopreparation, needed for treatment of oil contaminated soil is adjusted after defining area of contaminated sectors. Further the calculation was carried out according to the norm of consumption 10–20 kg/ha depending on the degree of contamination. In this case according to the results of primary observations the norm of consumption 5 kg/ha was suggested. This quantity of biopreparation was diluted with 180 l of water in special tank. Biopreparation was mixed with NPK fertilizer during the delution according to following proportion: 30 g to 1 m² of the area. After two hours the mixture was introduced into the soil by manual spraying.

During this process the special mineral fertilizing was added according to following proportion: 100 g to 1 ha of the area. Preparation and mineral salts were mixed periodically. Working solution was introduced with the use of pump hose, so that intensive mixing of all suspension for steady distribution of biopreparation was performed. Humidity of the area treated with biopreparation has to be about 60–80%. In dry weather watering of treated area is required. In case of strong pollution of the soil the secondary treatment with biopreparation without fertilizers is possible.

The researches carried out by us previously have shown that the addition of peat to the oil-contaminated soil as former of structure and carrier for immobilization of microorganisms-destructors of oil pollutants facilitates the increase of soil purification efficiency.

The data presented in the Figure 3 shows that the peat carrier that includes the mixture of high-moor peat and valley peat mostly influences on the efficiency of soil purification from oil with the use of activated sludge as biopreparation. The duration of the soil treatment is about several weeks (Figure 3). The use of peat carrier allows to immobilize of microorganisms and fulfill the lack of elements of nutrient medium.

Table 4 presents the conditions of introducing the peat carrier into the oil-contaminated soil.

Analysis of the data, presented in Figure 4, has shown that efficiency of soil purification from oil with use of peat carrier depends on the degree of contamination. In case of low degree of contamination (not more than 1 kg/m³) the effect of the peat presence does not appear. The strongest effect appears when the concentration of oil pollutant is about 10 kg/m³. With the further increase of concentration the effect decreases.

The optimal dose of peat carrier introduced into soil was defined at the oil contaminated area. This area was divided into three parts so that the certain dose of peat carrier was introduced into each part according to Table 5.

According to data presented in Table 5, the efficiency of purification of oil-contaminated soil does not change after dose of 5 kg/m². So this value can be recommended as a dose for field test.

Visual definition of the preparation efficiency and destruction of oil at the surface of soil for the primary estimation were defined according to change of colour from grey to dark-reddish brown. Aggregative state of oil changed from viscous liquid to easily decomposing solid participles with smell of putridity in wet state. Pieces of soil with remains of decayed oil are not combustible.

Biopreparation added into the soil as well as peat carrier was used after concentration of activated sludge suspension in the DAF machine. Concentration of the biomass in concentrated product was 3–4 times more than in initial suspension. Further oil-contaminated area was treated with concentrated product. Control of oil content was carried out once a week. The results of influence of soil treatment with such biopreparation as concentrated product of microorganisms biomass with peat carrier are presented in Table 6.

Results presented in Table 6, show that contents of oil-pollutants decreases much quicker when contaminated soil is treated with biopreparation (concentrated product of microorganisms biomass with peat carrier) in comparison with untreated with biopreparation soil.

Experience of microbiological soil treatment allows to make following conclusion and give recommendations:

1. Neutralization of oil contaminated soil with the use of activated sludge increases degradation of petroleum hydrocarbon with optimization of microorganisms life activity (nutrient medium, humidity, temperature etc.). The most favorable for microorganisms conditions are: humidity within 60–80%, temperature about 20–25 °С, presence of potassium, phosphor, microelements sources along with oil hydrocarbons. The degradation process of oil hydrocarbons increases in optimal conditions due to the life activity of microorganisms presenting in soil.

2. In order to keep high speed of degradation process of oil it is necessary to provide the presence of microorganisms in soil in optimal concentration (with approximate consumption of activated sludge biomass: 80–120 kg to 1 ha).

The use of activated sludge leads to the decrease of soil and surface waste water contamination. This fact has been proved is by experimental data. It allows us to make a persuasive conclusion that activated sludge produced in waste water treatment plants can be used as a destructor of oil contaminated soil. The considered way of surplus activated sludge utilization is also the additional source of valuable elements that increase fertility of soil. Such aspect increase the value of considered method.