Novel technological solution for oil decontamination of bottom sediments

Oil spills attract the attention of scientists and the public. According to Fingas (2011), in the past years this attention has resulted in global awareness of the risks of oil spills and the damage to the environment. At the same time, it is estimated that up to 10 or 20 million tonnes of oil enter the world's surface waters per year (Ivanov & Sidorov 2011). There have been several large spills on water since 2000, including the Prestige oil spill in the Atlantic Ocean (2002), a tanker spill near Sakhalin Island (2004), the pipe-connected spill in Ladoga Lake (2009) and the Gulf of Mexico catastrophe (2010). As a result, hundreds of thousands of tonnes of crude oil have been spilled into surface waters and the self-cleaning capacity of water in the areas affected decreased. If studies on the ecological functioning of pelagic and benthic communities in water bodies were not carried out, some of the natural resource impacts of oil spills would remain unknown (Peterson et al. 2012).

Significant ecological consequences from oil spills are typical for small bodies of water such as lakes, ponds and flooded pits in North Russia, which can completely lose their recreational and economic importance. Exploitation of the majority of Russian gas- and oil- pipelines occurs in hard climatic conditions and often activates exogenous processes like surface subsidence, frost heave, and thermokarst (Kirpotin & Polishchuk 2013). On average, 0.06 accidents happen per pipeline km per annum (Stadnikova et al. 2009); the most exposed lengths of pipelines are those at water crossings (rivers, channels, lakes and reservoirs). Other sources of oil contamination of water bodies in Russia are refueling zones, railway stations, offshore oil platforms, and river- and sea- operating oil tankers. Thus, 96 tonnes of refined petroleum products were collected from the Vladivostok port (Eastern Russia) water area and sediments in 2009–2010 (Gorda 2012).

A major problem with spilled crude oil and petroleum products is their incorporation into bottom sediments, especially in coastal environments (Alexander & Webb 1987). There is a limited amount of literature concerning the cleaning of sediments polluted with organic compounds to date (e.g., Ferdinandy-van Vlerken 1998; Lebo et al. 1999; Kruse et al. 2014, Li & Yu 2015). As for oil decontamination, pilot- and full- scale experiments have been conducted in the Russian North, and new technological solutions developed (Lushnikov et al. 2006; Lushnikov et al. 2010; Vorob'ev et al. 2010; Vorobiev et al. 2014). The lack of research in this area is connected with the technical difficulties of treating bottom sediments but that does not lessen the practical importance of the problem.

This paper aims to describe a complex airlift plant for the simultaneous cleaning of both the water and bottom sediments in water reservoirs.

The new oil decontamination plant was tested in a model of polluted freshwater bottom sediments. The experiment was conducted in triplicate. A sand from a natural lake near Tomsk (Western Siberia, Russia), with particle size 0.25 to 0.5 mm and free of oil contamination, was used as a model of bottom sediments. The sand was washed using a nylon fabric and air dried. A 10 cm layer of dry sand in the bottom of three independent glass pools was wetted thoroughly. Crude oil obtained from the Sobolinoye oil field (N 58.46 °, E 79.53 °, West Siberia, Russia) was used as the model contaminant to a final concentration (oil: dry sand) of about 16 g kg⁻¹. Oil from the Sobolinoye field is typical for the West Siberian basin and consequently for contamination of surface freshwater bodies in the region. The oil was poured slowly onto the ‘sediments’, forming a 0.5 cm layer on the surface of the sand. The contaminated sand was left for 30 days to allow oil sorption to occur, during which time all of the oil was adsorbed into the sand and remained at the bottom of the pool. After that, fresh water from the local potable supply was added carefully to bring the level up to 40 cm above the sand surface. 240 L of water was added to each of the three model pools. This manner of contamination is peculiar to West Siberia, where flooding of previously contaminated flood plains and peat wetlands is typical.

Airlift plant models, of the same design, were placed in the pools, so that their bottom edges were 3 to 5 cm above the oil-sand substrate. A water pump with an air ejector was used to create the airlift. The mixture of water and oil was collected in a transparent plastic collection tank measuring 30 × 20 × 10 cm. The water and oil mixture passed through a linked series of five floating boxes, each 20 × 10 × 10 cm, filled with natural peat as a sorbent. The plant operation process is described in more detail below, in the ‘results and discussion’ section.

Three separate sets of samples were collected: each set of samples comprising one from each of the three model pools. Samples of water (1 L each) and bottom sediment (300 g each) were collected and tested for oil content on the same day. Each bottom sediment sample was represented by a mixture of sediments from 5 different sampling points within the same model source tank. Four of the sampling points in each model pool were in the corners (10 cm from the pool walls), with the fifth in the middle of the pool. The sample bodies represent the full depth of the sand stratum (10 cm). The individual samples were mixed thoroughly to obtain an integrated sample from each of the three model pools and 300 g of the each sample was taken for chemical analysis.

The determinations were carried out using infrared (IR) spectrophotometry in the hydro chemical laboratory of Tomskgeomonitoring Ltd. (Tomsk, Russia). The method is based on extraction of the oil with CCl₄ and chromatographic fractionation with aluminum oxide, followed by IR quantitative determination using a Specord M 80 (Carl-Zeiss Jena, Germany) with spectral range 4,000–400 cm⁻¹. The oil concentration in the sample was calculated by comparing the absorbance determined from the sample extract with that of prepared samples containing known concentrations. Arithmetical means and standard deviations were calculated from analytical triplicate results.

The air is discharged to the atmosphere through the nozzle (Figure 1 (8)). Hydrophobic oil fractions accumulate in the oil collection tank (Figure 1 (7)). When this is full, it is changed or emptied, e.g., by vacuum pump, so that the oil can be used.

It is well known that oil spill operations can sometimes cause more harm to fragile coastal environments than the oil itself (Onwurah et al. 2007). Flotation technology has been successfully tested for bottom sediment decontamination in a full-scale experiment in Lake Schuchye (Komi Republic, North Russia) (Lushnikov et al. 2006). The main disadvantage of flotation was a sharp increase in the oil hydrocarbon content of the water, which is why its application is limited. The new airlift plant was developed to enhance water and bottom sediment decontamination productivity while minimizing harmful secondary effects on the ecosystem. The water collected with the oil requires further treatment, with an oil-sorbing material held in cartridges (Figure 1 (17)), in the decontamination tanks before recycling. Several floating decontamination tanks (Figure 1 (16)) can be used in sequence to enhance the treatment rate. The cartridges need to be changed as their sorption ability decreases and the waste material can be treated ex situ. A wide range of synthetic or natural substances can be used as oil sorbants (Fingas & Charles 2001).

When the airlift plant (Figure 1) was switched on, it drew water in from the pool and aerated it. It was noted that a covering of air bubbles formed on the oil-contaminated sediment. After some time the oil moved away from the sediment in clumps, carrying some of sand with it. This mix of materials was transferred via the airlift channel (Figure 1, 5) to the oil-collection tank. The collection hood (Figure 1, 6) leading into the channel was a truncated cone 12 cm in diameter, to confine the spread of oil in the model. It was effective and no visible oil was seen on the water surface other than the slight, iridescent film arising from the original ‘spill’. It needs to be pointed that the process was modelled and could not account for all of the effects of a real-world oil spill. It is considered very likely that a recent oil spill would leave visible oil on the water surface, particulary in areas of peculiar shore configuration.

The mix of oil and water that was collected from the pool was transferred to the decontamination tanks via the hydraulic lock. A series of five inter-linked rectangular boxes (decontamination tanks) filled with natural peat, as a sorbent, was used in the model. Natural, peat-based sorbents are comparatively cheap and effective (Cojocaru et al. 2011), and can be inoculated with oil-oxidizing microorganisms. An effective approach to this is the use of indigenous microbial cultures. For example, cultures isolated from Arabian Sea sediments in the vicinity of an oil field degraded diesel oil both aerobically and anaerobically (by nitrate reduction) (Mukherji et al. 2004).

Chemical analysis of the bottom sediments from the model pools after cleaning reported a total oil content of 0.431 ± 0.229 g kg⁻¹ (Table 1), while the estimated initial oil content of the sand after ‘contamination’ was 16 g kg⁻¹. This suggests a decrease in oil concentration of about 97%. The oil may be distributed through the body of sand irregularly, but oil/sand stratification was not evaluated in this study. The total oil concentration through the full depth of the sand stratum (10 cm) in each model pool was measured and averaged.

Russian National Standards for maximum permissible total oil concentration in fisheries and recreation/drinking water bodies are 0.05 mg L⁻¹ and 0.3 mg L⁻¹, respectively. Internationally, OSPAR (the Convention for the Protection of the Marine Environment of the North-East Atlantic) includes recommendations for the management of water produced from offshore installations (OSPAR 2001). These say that water discharged to the sea should contain no more than 30 mg L⁻¹ of dispersed oil.

The water recovered with the oil and contaminated sand in the model was treated in the series of decontamination tanks. Analyses showed that the final treated waters from all three model pools contained 0.077 ± 0.033 mg of dispersed oil per L on the average (see Table 1).

The work was performed within the development of the scientific-practical direction in the Institute of Biology, Tomsk State University.