The industrial legacy of the United Kingdom and some current industrial activity has left large areas of land contaminated in the UK and the contamination which has been introduced into the soil poses a risk to the wider environment, human health and natural resources such as groundwater.
A very large proportion of our drinking water is pumped form aquifers beneath our feet and this water can be rendered unfit for consumption by trace quantities of some contaminants, some of these contaminants, such a chlorinated hydrocarbons can originate from apparently innocuous places as dry cleaning businesses. Other facilities such as petrol filling stations can leak contaminants into the ground for many years unnoticed.
The requirement to remediation contaminated soils and groundwater is now a standard requirement form redevelopment of most sites and in some cases is required to be undertaken at exiting sites where no alterations are being made. The process of indentifying and treating contaminated land and groundwater is now well underway in the UK and the rest of Europe but the processes and techniques that have been used in the past few decades have become outdated and unsustainable.
A good example is sending contaminated soils to landfill, this method for decontaminating sites has been shown to be highly unsustainable, not only due to the every reducing capacity in UK, but also because of the road haulage required to transport the soils over distances that often exceed several hundred miles from the contaminated site to the landfill destination. The sustainability associated with various types of contaminated soil and groundwater remediation processes is therefore a current and important topic.
The complexity and necessity of the investigation into the sustainability has been highlighted by recent work being undertaken by the Sustainable Remediation Forum (SuRF). Sustainability is becoming a benchmark for the quality of companies who offer soil and groundwater remediation solutions, the technologies that they develop and offer commercially, as well as the entire projects where they are used.
The role of organisations who are involved in developing innovative technologies, as well as improving existing processes to treat soils and groundwater have, until now been focussed on the reduction of treatment costs. This has been encouraged and aided by central government measures such as the abolition of the landfill tax exemption in October 2008. However, the increasing awareness of sustainability across a wide spectrum of industries has provided a new impetus to not only the development of new technologies, but the improvement of exiting techniques, which may be considered to be mature in terms of the design, scope of use and cost. Both the development of new technologies and the improvement of existing ones often require the use of automated monitoring and process control.
One key are in which improvement can be made is the continued development of existing techniques with the use of automation and process control to make the techniques more efficient. Recently, a technique for remediating contaminated soils that has been improved with automated control is Ex-Situ bioremediation.
Bioremediation involves the improvement of the soil ‘environment’ by adding oxygen and nutrients to speed up the natural process of bacteria degrading organic contamination such as oil. Many thousands of species of bacteria occur naturally within most soils and some of those species are capable of using the hydrocarbons found in the various downstream products of crude oil as a source of carbon for respiration.
The use of bioremediation relies upon these bacteria to degrade the hydrocarbons, such as petrol of diesel, to carbon dioxide and water, however, it is often the case that the correct conditions to enable these bacteria to thrive do not exists in the environment where the contamination is present. Any factor, such a limited oxygen or nutrient availability, which may inhibit the growth of a bacterial population in this way is often termed limiting factor and the removal of these limiting factors is the primary aim of most bioremediation processes.
“the increased awareness of sustainability has provided a new impetus to the development of exciting and new technologies and techniques”
Bioremediation is a technique with good sustainability credentials. Ex-situ bioremediation often involves the construction of Biopiles, which are engineered piles of soil that include active aeration of the soil using a network of pipework inserted into the soil pile, a vacuum blower and collection of the leachate that is created by the biological processes. The vacuum aeration system also allows the capture and abatement of Volatile Organic Compounds (VOC’s) by capturing and filtering the extracted gases. As the biological activity in the soil pile breaks down the contamination CO2 is produced and O2 is utilised by the bacteria. The exchange of these gases in crucial to the rapid remediation of the soils.
One important aspect that has been focussed upon is that the aeration the soil requires a significant amount of energy. The equipment used to aerate the soil can be electrically powered pumps and vacuum blowers which are in turn powered from the national grid or from generators on site. Some bioremediation processes also involve mechanical aeration of the soil. However the application of relatively simple process and control systems can greatly reduce the mount of energy required to aerate the soil.
A recent project undertaken by Ecologia used an automated gas analyser to measure the CO2 and O2 concentrations within the soil pile, which was approximately 3,500 m3. A Gas Data LMSp multi gas analyser was used to sample and analyse the gases within the biopile and provide a 4-20 mA output for the remediation system process logic controller (PLC). The PLC was then able to switch the aeration blowers on and off line as required by the gas concentrations within the biopile.
This relatively simple use of automated process control made a substantial energy saving of 80% and therefore a significant cost saving for the project and the technique as a whole. In addition to the energy saving the automated analysis of the gases provided data which was logged and enabled the rate of CO2 production and O2 utilisation to be studied, a key factor in monitoring the overall ‘health’ of a bioremediation process as well as an important source of data to demonstrate that the project was successful.
Contaminated soils and groundwater
Contaminated groundwater is often a more tenacious problem than contaminated soils and traditionally contaminated groundwater has been, and still is, pumped to the surface and treated before being discharged to sewer, a watercourse, or back to the aquifer form where it was pumped. This technique is commonly referred to as ‘Pump and Treat’. A recent project undertaken in Yorkshire has highlighted the practical and economic benefits of using automated process monitoring and control within a pump and treat system to reduce costs and energy and therefore make the project itself and overall technology more environmentally and economically sustainable.
The project utilised twin 7.5 kw submersible pumps to drain a counter-fork drainage system and pump the contaminated groundwater to the surface where it underwent air stripping by violently agitating the groundwater, remove volatile contaminants for the groundwater for more efficient filtering in the vapour phase. The vapour phase filter comprised granular activated carbon. Following the first phase of treatment the contaminated water was treated with granular activated carbon in the liquid phase prior to be discharged into a local brook with the appropriate consent from the Environment Agency. Projects such as this may require daily site attendance to measure filter pressures and monitor the discharge of treated water. Both these items were automated for the project in Yorkshire to reduce daily visits to site to once per fortnight, with the agreement of the Environment Agency.
The discharge monitoring was undertaken with a Total Organic Carbon (TOC) analyser with a measurement range of 0 to 5 parts per million. The analyser works by using Ultra-Violet promoted persulphate oxidation to continuously oxidise the dissolved organic carbon (petrol in this case) in the sample feed to determine the contamination of organic chemicals present in the sample. A carrier gas then continuously sparges the reaction vessel liberating the resultant CO2 gas which is delivered to an Infra-Red detection system. Inorganic carbonates were automatically removed by sample pre-treatment using an acid sparge. The system was equipped with a 4-20 Ma output which was connected to the main system control panel to shut the system down in the event of the filter breakthrough. The system also altered the project manager by SMS of such an event.
In addition to the automated continuous monitoring of the discharge the pump and treated system incorporated an automatic ‘sleep mode’. The groundwater requiring extraction was not present in sufficient quantities all the time and therefore the pumps were not running continuously. This aspect was particularly important given that the system was being powered by a generator located within the equipment compound. When the generator was off line the limited amount of power required byt eh process controller was provided by batteries. The system was ‘woken up’ by an increase in the water level in the groundwater pump sump to allow the process to operate as required. the automation of the power supply reduced the operational time that the generator was running by in excess of 60%.
The use of Radio Frequency
In contrast to using process and control to improve existing methods for the remediation of contaminated soils and groundwater the introduction of technology into the contaminated land sector can make new technologies viable. An example of this improvement is the use of Radio Frequency (RF) energy to heat soils. The benefits of this technique have been demonstrated recently with two commercial projects which used (RF) energy to heat soils while they remained in-situ. The use of RF energy for the remediation of contaminated land and groundwater represents a technological quantum leap from the low-tech world of bioremediation and disposal to landfill.
Heating materials using radio frequency to recover petroleum produces was initially explored during the 1970’s oil crisis to remove oil from tar sands. The process has since been adapted and improved to enhance the remediation of soils. RF heating occurs when an oscillating electromagnetic field interacts with polar molecules in the subsurface, causing them to rotate and generate heat within the soil. Water is the primary molecule involved in the generation of heat but other molecules, including the contaminants themselves, are affected by the introduction of RF energy into the sub-surface. The rate of the rotation of each molecule that is affected depends upon the dipole moment of the molecule, in addition the frequency of reversal of the RF energy emitted into the soil will create optimal results when it matches the rate at which the molecules can rotate.
The heating effect is similar to that within a microwave oven. Typically a microwave over will transmit electromagnetic radiation with a frequency of 2450MHz, because water molecules are strongly influenced by this frequency. However the depth of penetration is inversely proportional to the frequency and therefore a much lower frequency of between 5 to 45MHz is commonly used for heating soil. This frequency allows penetration into soils of several metres rather than a few centimetres at 2450MHz.
The use of Radio Frequency to heat soil is designed to enhance existing techniques such as Soil vapour Extraction (SVE), commonly SVE is sued to extract volatile organic compounds, such as petrol contamination, from relatively porous soils such as gravels or sands. The technique involves applying a vacuum to the soils via vertical wells. The application of heat to the soil, usually to approximately 100OC means that semi volatile contamination such as diesel spills can be treated and in addition soils with lower porosity such as clays and chalk can be treated since the increase in vapour pressure caused by the increase in temperature appears to be more effective method for removing contaminants molecules from pore spaces within soils than the application of a vacuum alone. In combination, the increase in temperature and the application of a vacuum is capable of greatly reducing treatment timescales.
“the use of process control, automation and telemetry in remediation systems has been demonstrated to be useful for the development of new and existing technologies”
The use of RF soil heating on a contaminated site relies heavily upon the use of automated and electronic monitoring and control of systems and would not be possible to operate on a manual basis die to the complexity of the process. An array of monitoring and process equipment is employed to enable various processes to proceed unhindered. One principal aspect is that a certain proportion of the power emitted into the ground is refracted back to the antennae depending upon factors such as the moisture content, soil type and temperature and the amount of power must be continually matched against the reflected power, this is only possible with the implementation of automated control.
One particular axially system was designed during the implementation of the first project following health and safety concerns that arose from a concern that fencing was not sufficient to stop determined trespass on an inactive construction site at, for example, the weekend period. Although the power at ground level is well within safe limits an infra red fence was erected around the active area undergoing radio frequency soil heating that would switch the system off line of the infra red beam was broken. The beam was emitted by small devices and was invisible and proved to be an effective way of addressing concerns.
Part of the process of ensuring that the soil has been effectively remediation is to closely monitor the amount and type of contamination that is removed from the soil by the remediation technique. The process equipment used to undertake the project included an automated hydrocarbon vapour analyser fitted with a datalogger.
The equipment was capable of removing a continuous sample from the vacuum lines used to extract the contaminant vapour from the heated ground, multiple sample point were employed by using a sampling manifold consisting of solenoid valves linked to a dedicated process logic controller. the use of a process logic controller within the sampling device allowed a single analyser to be used on multiple sampling locations on a predetermined timing arrangement. The extracted sample was passed through a drying stage and then through a photoionisation detector.
Within the detector the hydrocarbons are ionised with high energy photons, typically from an ultra violet light source, the charged ions then release an electric current which is proportional to the amount of hydrocarbon in the sample gas stream. The photoionisation detector fitted into the analyser was capable of providing a 4-20 Ma output to the datalogger, which was in turn able to record a data set spanning two weeks of operation.
The use of the automated system fro measuring the quantity of vapour that the remediation system as able to remove from the ground played a vital role in demonstrating the effectiveness of the overall process and the suitability of the land for re-use.
The use of process control, automation and telemetry in remediation systems for contaminated land has been demonstrated to be useful for the improvement of existing techniques and technologies as well as allowing new technologies to be utilised within the industry. The overall effect is to improve the economic and environmental sustainability of these technologies and make more contaminated land available or potential re-development.
Tom Hayes began work in the field of contaminated land in 1997 and he has since undertaken the management of many remediation projects as well as site investigations, quantitative risk assessment (QRA), project estimation and client/regulator liaison. Tom works as a Director at Ecologia, a consultancy led remediation contractor who undertake site investigations and remediation of contaminated soils and groundwater. Tom was the lead author of the CL:AIRE report TDP12 covering the ex-situ bioremediation of 22,000 cubic metres of hydrocarbon contaminated soils at Askern colliery Nr Doncaster and is a Chartered Water and Environmental Manager.
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Published: 10th Mar 2010 in AWE International