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Original article

Studies on Polluted Soil Aeration Systems to Improve

Efficiency of Soil Remediation Using Biopile Technology

URS (Nedelcu) Alina Monica*, V. MICLE, Ioana Monica BERAR (Sur)

Technical University of Cluj - Napoca, 103-105 Muncii Bd., 400 641, Cluj-Napoca, Romania Cluj-Napoca, Romania

Received 10 June 2010; received and revised form 25 October 2010; accepted 20 November 2010Available online 1 December 2010

Abstract

Recent researches have emphasized the difficulty of biodegradation in organic compounds. To biologicallyremove these organic compounds some complex problems should be resolved: the need for direct contact betweenproducts added as stimuli in the biodegradation process and the polluted soil, as well as the thorough examination of thebiomass development and the transformation of pollutant compounds, taking into account the products resulted after thedecontamination process. In order to design aeration systems to achieve a maximum level of biodegradation throughbiopile technology simulations of this system and further experiments must be carried out to explore in detail themethod of aeration and to develop criteria for making such systems. Natural and forced aeration (blowing or extractingair through pipes) can be introduced to improve soil ventilation to assure the oxygen supply needed for the bioreactions

taking place in the pile of polluted soil.

Keywords: biodegradation, soil aeration, soil respiration, simulation

1. Introduction

Biodegradation is a natural phenomenon,subsoil and groundwater are the normal lifeenvironment for many organisms. Thebiodegradation process is developing as a chain

reaction in which carbon compounds aretransformed by successive degradation in moleculesbecoming less and less complex, until simpleproducts are obtained[3].

The gaseous phase of the soil is of great

importance in bio-geo-chemical processes occurring

in soil, as it is a crucial factor in the processes of

breathing for both plant-roots and for the

microorganisms in the soil.

* Corresponding author.Tel.: 0740571833; Fax:.+40 0264 415054e-mail: : urs_alina@yahoo.com; Alina.URS@im.utcluj.ro

The exchange of gases between the soil and

atmosphere is therefore crucial for the quality of

metabolic reactions taking place under the ground,

especially in agricultural ecosystems, where the

degree of ventilation of soil is closely related to the

state of plant vegetation, as it influences the quality

and quantity of crops.Due to the properties of the porous medium,

the gas exchange between the soil and atmosphere ispossible through a process called aeration, wherebyatmospheric oxygen penetrates the soil and soilcarbon dioxide is emitted into the atmosphere, thesetwo streams are generated due to concentrationgradients. The magnitude of the atmospheric oxygenflow is dependent on soil structure and texture andon degree of filling pores with water, the aerationprocess can be quantified both with directmeasurements and through numerical modeling [2].

The biopile technology consists inexcavating soil and forming of piles ofcontaminated soil in order to improve the conditions

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for biodegradation. These piles can be placed on amembrane or layer of impermeable clay to preventleachate contamination of the surrounding area. Foraerating the pile a perforated pipe is provided and itis installed in the pile in a permeable gravel layer

that allows regular and uniform distribution of airthroughout the whole pile [5]. A proper increase ofthe conditions during biopile process generates anincrease in the speed and the degree ofdegradation.The extracted air may be treated toremove volatile organic compounds (VOCs) using afiltration system such as activated charcoal [4].

2. Aeration systems used in the biological

treatment of soil through biopile technology

The function and performance of a biopile

method are affected by the interaction between thephysical and biological processes: the transport ofoxygen, humidity and heat caused by air flow anddiffusion, microorganism consumption of oxygenand water and the heat generated by bioreactions.When the amount of oxygen increases by increasingthe air flow, heat and water loss will be intensified.Factors affecting soil aeration are the drainage ofexcess water, the breathing rate in soil, soil profile,soil pH. Therefore, the air flow increase will notnecessarily improve biodegradation if the ambienttemperature is lower than that required for optimal

biodegradation in the pile. In order to optimize therehabilitation system it must be understood how theincrease in the airflow will change the moisturecontent and the internal temperature of the pile. Inturn, the moisture content and the temperature arealso affected by microbial activity. The natural airflow in a pile is driven primarily by wind-inducedpressure gradients.

There are two main aeration systems:

1) Horizontal tube aeration system;2) Vertical tube aeration system.

A horizontal tube aeration system usuallyincludes, blowers or fans to be attached to the vent

pipe system except for the case when soil aerationis done manually (fig. 1) [1]. Most ventilationsystems are based on horizontal perforated pipesplaced in a random manner. The testing of avertical tube aeration systemwas carried out for amonth by a group of researchers at the Center forEco-Environmental Modeling in China [2] wherethe performance of this new system made of twoperforated tubes, placed vertically with windturbines was compared with that of a pile ofstandard structure with perforated pipes placedhorizontally. Both piles were composed of similar

soil mixtures contaminated with diesel, splinters,compost and NPK fertilizer (nitrogen, phosphorus,potassium). Hydrocarbons were recovered usingsolvent extraction and were determined bothgravimetrically and by gas chromatography. Twopiles were constructed, one with a standard passiveaeration and the other one with a vertical aerationsystem. Each heap of soil was built using 3 m3 ofsoil contaminated with diesel taken from a raildepot and about 1.26 m3 of soil amendments:splinters 0.21 m3 and compost 1.05 m3. Soilhydrocarbon concentrations were between 18,000

and 25,000 ppm. Contaminated soil was stacked inpiles of about 1 m high, 2 meters wide and threemeters long. Two perforated polyethylene pipes (=140 mm) were placed in the middle of a pile("Normal" pile, N) in the manner shown in fig. 2.The position of the drain field was chosen tomaximize soil aeration. The other pile (aerated pile,A) had the new ventilation system.

a) b)Figure 1. A horizontal tube aeration system for treating of soil through biopile technology

a)desing aeration system; b)cross section

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Figure 2. The piles of soil with the two aeration systems [2]

Two perforated flexible pipes (also = 140mm) were placed vertically in the center of the pilehaving at each end turbines mounted on top (fig. 2).Because pressure gradients are relatively low, aircompressibility is neglected. The flow inside thebiopile is determined by the inducting the

difference of pressure of the wind around it. Basedon the potential flow theory, the pressure differencecan be quantified.

( ) ( ) 22 sin412 =

Upp (1)

where:p- ambient pressure at a certain distance

form the pile

U - wind speedp - pressure on the surface of the pile- air density angle

The results obtained are: reduction ofhydrocarbons in both piles is shown in fig.3 andtable 1. The general trend of reducing hydrocarbonin both piles appears to be exponential. Thehorizontal ventilation pipe increased the velocity ofthe flow around it, but has created a stagnant area onsome distance shown above (fig. 3). Instead, theresults in of vertical aeration show a strong anduniform flow in the piles (fig. 3b).

Table 1. Temporal variations of the oil content in the pile

Pile with standard aeration Pile with the new aerationDay

Oil mg/kg Percent Oil mg/kg Percent

0 20.318 17% 17.514 4%

3 20.153 2% 15.059 1%

6 15.261 13% 19.180 36%

15 15.800 27% 15.567 8%

23 16.517 12% 11.480 2%

30 15.497 21% 12.946 6%

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Figure 3. Airflow pattern in the pile aerated conditions: a) horizontal; b) vertical

The color of the chart represents thevariations of pressure inside the pile and the arrowsindicates the pressure gradients, therefore, the speedof the airflow.

Research has shown that the water content inthe vertically aerated pile was always less than inthe horizontally aerated pile. This may be due toenhanced aeration of the former heap. The

temperature in the central part of the two piles washigher than the temperature measured at theextremities, probably because of the local speed ofthe air flow and of the stimulated metabolism. Thenew aeration system clearly stimulated the suctionpipe, leading to improved airflow in the duct. Thenew system also has the advantage of beingsensitive and responsive to wind; this is not appliedto the horizontally aerated heap.

3. Effects of aeration on the biodegradation

process

The efficiency of biodegradation based on thebreathing ratecan be calculated as the product ofCO2 accumulated throughout the entire time periodusing the following formula [5]:

EB(%) = CO2 biox 100/Ci (2)

CO2 bio= 2 x CO2 total - CO2 control (3)

where:EB Biodegradation efficiency (%

mols/mols)CO2 bio l CO2generated by microbial

activity (mols)CO2 total CO2grand total (mols)

Ci the initial equivalent in mols(theoretical value)

CO2 control accumulated control of CO2 inmols (abiotic system)

The efficiency of biodegradation can beassessed by measuring the reduction in theconcentration of hydrocarbons.

Intensification of aeration in the pileimproves the biodegradation of hydrocarbons, as itincreases soil microbial activity determining ahigher consumption of hydrocarbon bymicroorganisms.

The results obtained have confirmed thatexcessive loss of water is caused by increasingaeration, having a negative impact on thebiodegradation process. Experiments require some

detailed studies to quantify how airflow affects themoisture content of the heap.

4. Conclusions

This study was conducted in order to comparethe two systems of aeration of soil contaminatedwith hydrocarbons using biopile technology, toidentify the highest level of efficiency regarding thebiodegradation of organic pollutants. Starting fromthese results software will be created to design anaeration system which will permit to obtain a higherefficiency by adjusting the parameters.

Globally the most commonly used aerationsystems are the horizontal ones, but recent research

a) b)

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undertaken to determine a more effective ventilationmethod reveal that the vertical tube system is moreefficient. Airflow uniformity is essential in maintainingan optimum balance for soil aeration using the biopilemetho; this condition is ensured by using the vertical

ventilation system. Research conducted confirms thatincreasing aeration leads to excessive loss of watercontent, which induces a negative impact on thebiodegradation process. For a rigorous control over soilmoisture periodical motorization is needed throughcomplex studies which determine how the airflow affectsthe water content.

References

[1] Alexander M., 1994, Biodegradation andBioremediation. San Diego, CA: Academic Press

[2] Lia L., C.J. Cunninghamc, Valerie Pasd, J.C. Philpd,D.A. Barryc, P. Andersonc, 2004, Field trial of a newaeration system for enhancing biodegradation in abiopile, Waste Management 24, 127 137

[3] Micle V., G. Neag, 2009, Procedee i Echipamente dedepoluare a solurilor i a apelor subterane, EdituraU.T.Press, Cluj-Napoca

[4] Rusu T., Laura Paulette, H. Cacovean, V. Turcu,2007, Fizica, hidrofizica, chimia i respiraia solului Metode de cercetare, Editura Risoprint, Cluj- Napoca

[5] Ururahy Adriana F. P., M. D. M. Marins, R. L. Vital,Irene Therezinha Gabardo, Nei Pereira Jr., 1998, Effectof aeration on biodegradation of petroleum waste, Rev.Microbiol., vol. 29 (4) So Paulo, 254 - 258