Micro-nutrients make way for faster, greener remediation
A cutting edge land clean-up method that uses soil nutrients is greener and faster than traditional alternatives, argues Karim Esmail of Building Research Establishment (BRE).Two case studies - the regeneration of Cardiff harbour and a major oil spill clean-up in Teesside - illustrate his point.
Last year’s changes in legislation and landfill classification were always expected to have an impact on the re-use of contaminated land; the problem was that no one was quite sure what the real implications would be to land owners and developers. It is now clear that the re-classification of landfill sites is having a significant effect on the way in which sites are being developed or, more specifically, how they are being remediated.
Historically, the traditional method of dig and dump was usually favoured by this sector, since the sites were often too small to make in-situ remediation viable; in terms of cost of remediation technologies, long treatment cycle times and space availability. With last year’s change in the law, many contaminated soil streams may well attract hazardous waste disposal rates at landfill. Alternatives to dig and dump have become a necessity in order to ensure the economic viability of a project. One option is to bio-remediate contaminated soil in situ or ex-situ, which is gaining acceptance as a viable alternative.
BRE have been working at the cutting edge of bio-catalysis solutions for remediation of contaminated soil and cleaning of ground water. The bio-catalysis technology developed is based on plant extracted micronutrients, which increase the rate of remediation to twice or three times the speed of traditional basic nutrients (NPK) or bacteria injection methods. The micro-nutrients extracted are primary, secondary and tertiary, which are then preserved with high bio-availability. When these micronutrients are applied to contaminated soil, they are rapidly absorbed by the indigenous degrading micro-organisms present in the soil or groundwater and speed up the bio-degradation process of hydrocarbon and organic contaminants. Harnessing the abilities of indigenous organisms is of fundamental importance with respect to sustainability of the clean up process. It overcomes the the historically known failures that arise from the inability of externally cultured organisms to acclimatize to actual field conditions. Karim Esmail says, “Using laboratory cultured microorganisms in real-life clean-ups can sometimes be like placing Eskimos to work in the Sahara desert. They may not survive!”
The application of micro-nutrients is simple, easy to use and environmentally friendly and furthermore there are no leakage pathways that would cause environmental concerns. The technique, known as the Eco-Bio process, overcomes a large number of restriction of traditional bio-remediation methods using bacteria or primary nutrient based solutions.
BRE have used the Eco-Bio process on several applications to demonstrate its efficacy in different situations, where traditional NPK and bacterial inoculations do not perform as required due to high levels of contamination or where residency times are too short. The Eco-Bio process has ideal applications in soil remediation, marine clean-up and rapid groundwater de-contamination. The case studies below illustrate both in-situ and ex-situ applications.
Case study 1: Cardiff Bay International Sports Village
The goal of this Cardiff County Council development is to produce a state-of-the-art sports, leisure and entertainment complex over the next 7 years. The project aims to regenerate the harbour and transform it into a major European tourist attraction.
As part of the regeneration effort, contaminated soil at the derelict Ferry Road industrial site needed to be cleaned-up. The site was a former industrial area, which included land-filling operations, a petroleum storage yard, waste transfer stations, a scrap yard and a railway. As a result of these long-term activities, the soil and groundwater were heavily contaminated with free phase and dissolved hydrocarbons.
Main project contractors Taylor Woodrow invited BRE to demonstrate their Eco-Bio technology for treating a 2,000 m3 bio-pile of soil using Eco-Bio micronutrients. An initial soil investigation by consultants Arup showed soil TPH concentrations ranging between 10-15,000 mg/kg. However, once excavation of the soil began, TPH concentrations were found to be closer to an average of 5,000 mg/kg.
The aim of this project was to set-up 2 bio-piles of similar soil composition and strength, the first treated with Eco-Bio products, the second just NPK. The challenge was to demonstrate the ability of the Eco-Bio process to accelerate soil clean-up times, achieve treatment to a lower final soil contamination level and demonstrate the ability (of the process) to tolerate sub-optimal environmental conditions, in particular low temperatures, which can restrict conventional ex-situ bioremediation techniques during the winter.
The Eco-Bio micronutrients and NPK were manually applied to each 30 tonne heap of soil as it was off-loaded by a dumper truck. The soil was turned and mixed using the long reach arm of a digger and then picked up and put through a vibratory screen to homogenise the material and remove any large rocks. The soil fell into another heap, which was finally deposited onto the bio-pile.
Each bio-pile was built over a geotextile-derived membrane designed to provide a robust impermeable layer. The overall dimensions of the bio-pile (Eco-Bio and control combined) were around 15 x 75 m in width and length respectively, and 3.5 m deep, yielding a soil volume of 4,000 m3. The bio-pile was divided into 20 cells. The two halves for the Eco-Bio test and control were separated by a 3 m air gap to prevent any cross-contamination.
The bio-piles were aerated by a network of perforated pipes arranged horizontally in layers and attached to a bank of side channel blower vacuum pumps. The off-gas was passed through a GAC filtration unit before discharge to atmosphere. As far as practically possible, leachate from the experimental bio-pile and the control bio-pile were kept separate to prevent cross-contamination. Two French Drains on either side of the bio-pile were isolated to keep the leachates separate.
When construction of the bio-piles was complete, they were covered with rain-deflecting fleeces that were solar and thermally enhancing, thus preventing water logging and helping to generate and trap heat inside (Plate 1).
Soil testing was the main method for validating the trial using a cone and quarter sampling method to give a statistically representative sample of the full depth of the bio-pile. Gas and temperature monitoring of each bio-pile was carried out using an infra-red gas detector (GAS-IR). Soil and gas data was generated over an 8-week period. Each data point per week is an average of 4 sample points.
The starting concentration of total TPH in the bio-piles ranged between 3,025 and 2,675 mg/kg. Both bio-piles showed TPH reductions in the first week, but the drop in the Eco-Bio test bio-pile was greater at 81 % compared to 60 % in the control (Figure 1). During a 6-week period, the control TPH concentrations ranged between 1,135 to 2,650 mg/kg, the Eco-Bio test bio-pile ranged between 508 to 935 mg/kg. Thus, the TPH treatment standard of 2,000 mg/kg was achieved in the Eco-Bio test bio-pile within one week and the control bio-pile took 6 weeks. The lower TPH concentrations in the Eco-Bio test also showed the benefit of using Eco-Bio micro-nutrients to comply with more stringent treatment standards, as are common place elsewhere in Europe.
Microbial Growth Count: The most striking difference in the microbial total viable count (TVC) between the control and the Eco-Bio bio-pile was found after the first week. The control had an average TVC of 48,250, the Eco-Bio bio-pile was 10 times higher at 592,000 (Figure 2). The TVC for the control peaked after week 3 and then immediately went into death phase, thus it had a very short stationary phase. The Eco-Bio curve was in a longer stationary phase between week 3 and 5 before going into death phase.
The temperature of the bio-piles can be correlated with microbial activity. Both biopiles were constructed identically, with the same amount of insulation and wind exposure. The starting temperature of both the control and Eco-Bio test bio-piles was very low, but higher in the Eco-Bio bio-pile at 5.5 ºC compared to 4.0 ºC, which is consistent with the higher microbial growth count early on in the Eco-Bio trial. This gives the Eco-Bio process a bigger window of opportunity for use during the colder months of the year. The Eco-Bio test bio-pile consistently maintained a higher temperature at the beginning and the duration of trial, on average 0.7 ºC higher and at times as much as 1.5ºC higher. This is consistent with the longer stationary phase and earlier exponential growth phase in the Eco-Bio test bio-pile.
The concentration of volatile organic compounds (VOC’s) was measured in the off-gases to determine if there was any air stripping. Both bio-piles had the same level of aeration and therefore the extent of air stripping effects should have been identical in both. However, the average and peak concentrations of VOC’s in the Eco-Bio test bio-pile were consistently lower than in the control (Figure 3 and Figure 4). Within the first week average reductions of 18.6 and 69.8 % were achieved for the control and test respectively. These reductions are consistent with the more rapid decrease in TPH levels in the Eco-Bio test bio-pile and the higher earlier microbial growth counts in this bio-pile. The VOC data differences between the 2 bio-piles in the first 2 weeks suggests that the VOC reduction in the Eco-Bio test bio-pile was more down to direct microbial activity, however, the VOC reduction in the control bio-pile was more influenced by air stripping from aeration.
This is significant as VOC release via air stripping is an important environmental factor from a health perspective during bioremediation programmes. The growth of VOC degrading micro-organisms is generally very slow. The Eco-Bio test showed the ability of the micro-nutrients to accelerate their natural selection and growth.
Case Study 2: The Conoco Phillips Oil Terminal
This site receives crude oil from the North Sea offshore fields. The terminal imports approximately 80,000 barrels of oil per day and processes 67,000 barrels per day.
A mixer valve on one of the large 100,000 Te crude oil storage tanks failed, resulting in a large oil spill. A substantial portion of the oil was recovered by suction pumps; however, the site remained heavily contaminated. It was estimated that approximately 40,000 litres of oil had contaminated the ground over an area of approximately 13,700 m2.
The Challenge: The oil spill contaminated the soil just above the groundwater table. Immediate remediation work was needed in order to prevent contamination of the aquifer and river from crude oil seepages. The problem was being further compounded by heavy rainfall, strong gusts of wind and cold weather starting to set in.
BRE was asked by the client to investigate the incident and recommend remedial measures. It was calculated that if the contaminated soil were to be excavated and sent to landfill as a hazardous waste, the minimum cost of disposal would be around £250,000. In addition, the Client would have to wait until weather conditions improved to undertake this work, thus exposing the groundwater to a high contamination risk.
The Solution: BRE proposed in-situ bioremediation of the site by treating the contaminated soil with natural Eco-Bio micronutrients, which accelerate the natural biological degradation of the oil. Whilst natural soil remediation was expected to slowdown in the cold weather, the application of Eco-Bio products would ensure maximum oil decomposition even in the cold weather and improve as the weather warmed up.
The Eco-Bio products were supplied in liquid and powder form and mixed and sprayed using manual equipment over the surface of the contaminated soil over a three day period. A single application was sufficient for the majority of the site; however, for heavily contaminated hot spots it was recommended that a further application should be made.
12 sample points were set-up on site to monitor the progress of the trial. The samples were independently tested by a UKAS accredited soil analysis laboratory. The results of this analysis is shown below.
Results: The Eco-Bioprocess successfully managed to treat the oil contamination. Despite low ambient temperatures of less than 10 ºC during the day, heavy rainfall, snow and winds of up to 80 knots which were adding significantly to the wind chill factor, rapid reduction of the oil contamination was recorded.
The initial soil TPH contamination averaged 6,612 mg/kg, with hot spots measuring as high as 43,000 mg/kg. Before the Eco-Bio products were applied, the soil was heavily coated with a thick black layer of crude oil. Twenty-four hours after application, the crude oil had reduced to a thin, brown fluid film on the surface of the soil. There were visible air bubbles in areas holding stagnant water indicating microbial conversion of the oil to carbon dioxide and water. After 20 days there was just a light sheen of oil left on the surface water, with a lot of trapped air bubbles from the intense microbial activity. Analysis showed that there had been an 85 % reduction in the TPH concentration, down to 975 mg/kg after 20 days. When the site was next sampled about 3 months later, there was virtually no visible sign of oil contamination. There had been a 99 % reduction of the TPH to a concentration of just 67 mg/kg.
The application of Eco-Bio products at this site was found to be an extremely efficient and fast method of oil spill clean-up. Despite poor weather conditions which would have hampered other remediation techniques, the Eco-Bio process was found to be versatile. The Eco-Bio process is completely environmentally friendly because it brings about enhancement of indigenous oil degrading microbial populations through accelerated microbial selection. External inoculations of enzymes or chemical agents are therefore not required.
From a legislative perspective, the Landfill Directive of 2004 is set to ban the land-filling of oil contaminated waste. This means efficient, robust and flexible bioremediation techniques such as the Eco-Bio process have an important role to play in oil spill clean-ups and the bioremediation of contaminated land. As well as being extremely efficient, the Eco-Bio process is also very cost-efficient. In this project, the clean-up cost was approximately 10 % of the conventional dig and dump cost, thus providing the client with a 90 % saving.
BRE are now actively looking for industrial applications of this new method of treatment, which overcomes some of the disadvantages of bio-remediation methods that do not always perform in conditions of high levels of hydrocarbon contamination in soils.