Biological treatment techniques rely on the natural metabolic processes of living organisms, such as bacteria and fungi, to destroy organic contaminants or convert them into less toxic forms. Carbonbearing substrates are metabolised by oxidation and other processes to generate energy for growth and reproduction. Under aerobic conditions the complete mineralisation of organics results in the production of CO2, water, biomass and inorganic salts. However, intermediate metabolites can be formed that can be more toxic than the original contaminants. Therefore, a complete understanding of the respiratory pathway and associated chemistry is essential in order to design the most appropriate system for the contaminants present. Anaerobic degradation or co-metabolism can also occur, as a result of the transformation of a non-growth substrate (the contaminant) by a microorganism using an alternative carbon substrate as an energy source.
Contaminants that can be treated include:
- petroleum, diesel, lubricating oils, kerosene, polycyclic aromatic hydrocarbons
- certain explosives
- phenols and cresols
- chlorinated hydrocarbons
- certain herbicides and pesticides
HBR carries out a variety of ex-situ techniques to accelerate natural aerobic degradation. These include pretreatment during soil homogenisation using liquid nutrients, oxygen enriched water, bacteria or fungi followed by deposition into purpose made bio-cells or formation into managed windrows, where temperature, humidity, pH, redox potential, moisture content and nutrient availability are strictly controlled to ensure optimum conditions are maintained for degradation to occur.
Windrows and landfarming
Excavated soils are stockpiled in long heaps called windrows. Mechanical turning equipment is then used periodically to enhance oxygenation and to add water and/or nutrients. We employ our own Menart 5330 tractor powered windrow turner, which is able to process windrows 2m high by 4m wide. The windrows are covered with heat retaining fleeces to ensure optimum conditions are maintained for thorough degradation of contaminants. Landfarming is probably the simplest form of ex-situ bioremediation, where soils are spread across a fairly large area in a layer of about 0.3m depth. Amendments are added and the soils are mixed to enhance aerobic degradation.
Soils are placed in a static pile incorporating a system of pipe work, through which air, which can be heated in colder conditions, is injected or extracted to maximise mass transfer of oxygen. Any emissions can be treated via off gas collection systems. Where practical, heat for the circulating air can be produced on site using purpose made mulch piles established over heat exchange systems. Bio-cells do not require as much space on site as windrows, as they can be up to 3m in height and are not mechanically aerated.
In situ bioremediation
HBR undertakes a number of techniques to stimulate the naturally occuring indigenous microbial populations within the ground, both in soils and groundwater, to increase degradation. These can either be driven by aerobic or anaerobic mechanisms depending on the contaminants and conditions present.
Bioventing and biosparging
Air is typically introduced into the unsaturated zone, by either injection (sparging) or extraction (venting) at a very low flow rate and pressure, to maximise aerobic degradation of organic contaminants, regardless of volatility. This minimises the need for the treatment of extracted vapours. However, the technique is often combined with soil vapour extraction where volatiles are present. Biosparging can also relate to the relatively slow rate of injection of air into the saturated zone, where biodegradation and chemical oxidation are enhanced, being the primary processes rather than volatilisation.
Enhanced natural attenuation: Oxygen release compound (ORC) and hydrogen release compounds (HRC):
HBR use a number of proprietary products, where applicable, to enhance natural biodegradation in saturated soils and groundwater. ORC is a mild oxidant that hydrolyses slowly into molecular oxygen when saturated with water. It is typically in jected in slurry form into the groundwater via boreholes, thus increasing concentrations of dissolved oxygen, enhancing aerobic conditions in the subsurface over relatively long periods of time. HRC promotes a slow release of lactic acid, which is gradually metabolised by bacteria into hydrogen, driving the anaerobic degradation of chlorinated hydrocarbons into less harmful constituents such as ethene and ethane, known as enhanced reductive dechlorinations (ERD). Reducing conditions are maintained within the saturated subsurface for relatively long periods of time. However, transformation products can be more hazardous than the parent compounds, therefore, its is important to have a thorough understanding of the mechanisms involved.
Enhanced reductive dechlorinations (ERD)
This technique involves the replacement of the chlorine atoms of a chlorinated compound with hydrogen by anaerobic bacteria. The chlorinated compound is acting as an electron acceptor, whereas an organic carbon substrate is acting as an electron donor. The carbon source can be organics already present, naturally occuring or other contaminants such as fuel hydrocarbons, or it can be introduced as with HRC previously mentioned. However, there are a number of other electron donors that HBR use including molasses, fatty acids, vegetable oils, sodium lactate and cheese whey. These are slurrified and introduced into the subsurface by direct push or injection methods. The reductive process can continue until all of the chlorine atoms are removed and TCE is dechlorinated completely via dichloroethene (DCE) and vinyl chloride (VC) to harmless ethane gas. However, the process can stall if all organic electron donors have been consumed, which can result in the accumulation of harmful intermediates. Thus, HBR are careful to undertake intensive monitoring over a relatively long period of time, to ensure the process reaches the required end result, with the additional periodic introduction of carbon source if required.
This involves the introduction of modified microbes and biological agents into the subsoils, where the indigenous populations are not capable of degrading the contaminants of concern. HBR use a number of preferred suppliers for commercial laboratory cultures, when and if required.