Oil degradation: the natural way
Karim Esmail of the Building Research Establishment assesses biological treatment options for oily wastewater and sludge including the Eco-Bio remediation process. The EU landfill directive imposes strict controls with respect to disposal of hydrocarbon wastes in landfill sites. This presents technical and economic challenges for UK industry.
The Landfill (England and Wales) Regulations 2002, which came into force on June 15, 2002 and implement the Landfill Directive (Council Directive 1999/31/EC) aim to prevent, or to reduce as far as possible, the negative environmental effects of landfill.
In summary, the directive requires existing and new landfill sites:
- to be classified into one of three categories: hazardous, non-hazardous or inert, according to the type of waste they will receive,
- to submit site conditioning plans and demonstrate that operators and their staff are technically competent to manage the site and have made adequate financial provisions to cover the maintenance and aftercare requirements of the site,
- to prohibit liquid waste to hazardous landfills (all List 1 category materials including oils and oily sludge),
- to use waste acceptance criteria at landfill sites,
- to pre-treat waste entering hazardous landfill sites by July 16, 2004 on existing sites,
- to implement the European waste acceptance procedure by July 15, 2005, but use the interim procedures set by the regulations.
The objectives of the directive are to improve and regulate the waste management procedures. However, the directive will put constraints on the admittance of oily sludge at hazardous landfill sites. Considering there are likely to be increasing amounts of waste generated from industrial sources as economies grow, the future costs of oily waste disposal are also set for a sharp increase, unless better methods of waste disposal or pre-treatment are identified.
The pre-treatment of
oil sludge can to be an attractive option for the landfill operators, pre-treatment operators or industries with on-site treatment facilities, if alternative, simple and cheaper technologies and methods are identified.
Safe sludge disposal
To date, chemical precipitation processes have been widely used for the treatment of oil-contaminated wastewaters. Whilst such processes have served to partially decontaminate the water, they tend to produce significant quantities of sludge, which is difficult to dewater and handle. Legislative restrictions will make it difficult for waste generators to dispose of sludge in landfill sites. Therefore, the industry has to resort to alternative treatment methods for decontaminated oily wastewater and sludge.
Bioremediation processes have been used in the environmental industry for the past 60 years for the decomposition of oils and hydrocarbons in soil and water. The standard approach in conventional bioremediation is to inoculate the contaminated soil or water with externally cultured microorganisms, which are then fed with primary nutrients such as nitrogen, phosphorous and potassium, to enhance their ability to break down the oil and hydrocarbons. Whilst bioremediation processes have been successfully applied in oily wastewater treatment, there have been certain important limitations, which include:
- relatively long residence time requirements for the break down of the oil, resulting in very large bioreactor systems, and hence, high capital costs,
- population loss of the externally introduced microorganisms due to their inability to acclimatise themselves to the oil toxicity and ionic environment in the wastewater. Such population losses can be catastrophic, because it can often take up to 4-6 weeks before the microbial populations can be restored. Apart from production down-time, the new inoculations can also be very costly,
- inability of the externally inoculated microorganisms to handle fluctuations in oil and chemical concentrations in the wastewater. This results in violation of the consent standards and in certain extreme cases, loss of the viable microorganism populations,
- challenges with respect to maintaining process control.
Another biological approach that has been used is the introduction of externally cultured enzymes into biological reactors. Such enzymes are prepared under controlled laboratory or plant conditions through microorganism secretions, packaged in plastic containers and shipped to the WwTW. Once injected into the oily wastewater, the enzymes hydrolyse the oil and break it down into smaller polymeric chains, and finally into carbon dioxide and water. The externally cultured enzyme treatment approach was developed in an attempt to overcome the limitations of the conventional bioremediation processes. Enzyme processes also suffer from limitations, which include:
- instability of enzymes during storage and handling, particularly with respect to temperature. Enzymes are easily inactivated by fluctuations in temperature and pH,
- enzyme processes are inflexible with respect to responding to fluctuations in the oil content of the wastewater as well as chemical inhibitors, which are often present in wastewater,
- enzymes are specific with respect to the contaminants that are to be treated. Therefore, variations in hydrocarbon species often result in poor degradation kinetics,
- enzyme dosage rates increase significantly at higher oil concentrations, making such processes prohibitive from a cost point of view.
Eco-Bio is a simple concept which overcomes the
limitations of the conventional bioremediation and enzyme processes.
The earth has been endowed with a very effective self-cleansing mechanism through the action of a multitude of different types of microorganisms, which fully degrade contaminants down to their primary components. However, with the rapid pace at which our industries evolve, and with the sharp increase in environmental contamination, this self-cleansing mechanism is no longer capable of eliminating pollutants within
the short time frames demanded. Therefore, the Eco-Bio process was developed with the view to providing natural catalysts that can be easily applied in order to accelerate the performance of these natural environmental cleansing microorganisms. The process has excellent applications in the oil and hydrocarbon wastewater treatment industry.
Typically, oceans, rivers, watercourses and soils have 20-25 families of microorganisms that break down long and short polymeric chains in hydrocarbons, reducing them to carbon dioxide and water. Many inter-relationships exist between these microorganisms. Certain families break down complex polymeric chains to smaller chains. Other families reduce the smaller chains to even smaller chains until complete degradation of the hydrocarbons is achieved. In nature, the key limitation with respect to these processes is one of time. For example, complete natural degradation of crude oil in the ocean takes between 4-6 months. Microorganisms in rivers could take even longer.
The Eco-Bio process introduces an active natural ingredient called Dakarin, together with nutrients, vitamins and bio-stimulants, herein referred to as micronutrients, which achieve the following:
- a sharp increase in the desired indigenous microorganism populations, which are already fully acclimatised to the existing environment,
- an exponential increase in the metabolic rates of the indigenous oil and hydrocarbon degrading microorganisms,
- minimal residence time requirements for complete contaminant degradation, resulting in smaller, compact and low capital cost treatment systems,
- application of protective coatings on the cell walls of the microorganisms to prevent cellular destruction (lysing) by toxins. This prevents loss of the microbial populations under extreme process conditions,
- process flexibility with respect to catalysis of microorganisms in an aerobic and anaerobic environment,
- the process can handle sharp variations in contaminant levels in the feed, without loss of efficiency.
The process is capable of completely degrading crude oil to carbon dioxide and water within less than 24 hours in an aerobic bioreactor. It also destroys lighter fuels such as gas oil and solvents in 4-12 hours. Eco-Bio has also demonstrated sharp reductions in the time required for biological decontamination of oil polluted soils, bringing the total oil degradation time down from several months to only a few days, depending upon the nature of the soils and site conditions.
Another unique aspect of the Eco-Bio process is its ability to accelerate local, indigenous microorganisms in biological de-pollution programs, without the need for importing external microorganisms into the ecosystem. The only exception is in cases where the environment is completely sterile with respect to microbial growth, in which case, external inoculations may be necessary.
The key reagents of the Eco-Bio process are available in powder and liquid form. These can be easily dosed into process streams with minimal cost and complexity. The products are made from all-natural ingredients and are certified as being non-toxic to aquatic life.
The process has been successfully used for full-scale treatment of oily wastewater and sludge at various locations in the UK and internationally.
A 250m3/day oily WwTW in Southampton treats wastewater and slops from ships that dock at the port of Southampton. The first stage in the treatment process is the recovery of oil using high temperature and flocculation. The oil is skimmed from the separation columns and the wastewater, containing emulsions and interfaces are charged on a continuous basis to a two-stage bioreactor, having a residence time of 24 hours. The bioreactor is dosed with Eco-Bio products daily, which create and maintain an active, robust microbiological environment in the reactors.
The chemical oxygen demand (COD) in the inlet stream of the bioreactors is typically 20,000-30,000ppm, with a total petroleum hydrocarbon (TPH) level ranging between 1,000-3,000ppm.
The biocatalysis that is achieved from the Eco-Bio process permits continuous discharge of treated effluent containing less than 1,500ppm COD and 10ppm TPH.
Prior to using Eco-Bio this plant used chemical precipitation to remove the oil from the water, delivering a feed to the bioreactors of approximately 10,000ppm COD. The bioreactors discharged treated effluent containing between 3,000-4,000ppm COD. As Eco-Bio products were introduced to the bioreactors, the COD in the treated effluent dropped to 1,000ppm and went to levels as low as 500ppm. The Eco-Bio product dosage was then reduced to maintain a treated effluent COD level of 1,500ppm COD. The chemical precipitation process was shut down and 100 per cent of the oily wastewater was then treated in the bioreactors. The net saving achieved was substantial. The treatment plant produces little-to-no sludge as most of the biodegradable organic contaminants are fully digested in the biocatalytic reactors.
A recent set of tests were conducted by Shanks Waste Solutions to evaluate the impact of the Eco-Bio process with respect to accelerating the treatment of hydrocarbon solvent-bearing chemical wastewater. The strategy was to assess the percentage increase in throughput that could be achieved from an existing sequence batch reactor (SBR) as a result of the biocatalysis provided by Eco-Bio products. Parallel test runs were conducted using broth from the existing SBR. Test one, which served as a control, involved the introduction of a continuously increasing throughput rate of wastewater having a COD of 27,000ppm. Test two was based on treating the SBR broth with Eco-Bio products.
Results showed Test one failed when the throughput was increased by 100 per cent, causing a drop in mixed liquor suspended solids (MLSS) from 6,688-3,000ppm. The dissolved oxygen level in the reactor also fell from 8.45-0.56ppm. Test two maintained successful treatment of the wastewater at a throughput rate that went as high as 250 per cent, at which time the test was stopped. The Test two reactor consistently yielded COD ranging between 160-260ppm at the peak throughput rates, whilst maintaining an MLSS of between 6,000-7,000ppm and a dissolved oxygen level of over 3ppm. See Figure one.
Microscopic studies showed the presence of a robust microbiological environment in Test two, through all phases of rising throughput. The bioreactor in this test displayed rotifiers, stalked ciliates (Vorticella and Suctorian), free swimming ciliates (Paramecium) and bacteria (spirals and spherical bacteria), with a well-defined, mature floc. The microscopic studies reflected a complete change in the microbiological environment when Eco-Bio products were added.