Jul 08, 2011
Environmental projects at Vanderbijlpark works well under wayBack
ArcelorMittal South Africa|Viljoen & Associates|Chemical Composition|Steel|Steel Giant|Steel Manufacturing Waste Disposal Site|Steel Producer|Johan Hattingh|Karien Zantow|Soil|Wastewater|Bioremediation
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The steel giant is currently undertaking remediation projects at the former waste- water disposal dam (Dam 10), the central effluent treatment plant (CETP) sludge dams, the maturation ponds and the old steel manufacturing waste disposal site at its Vanderbijlpark works.
Mittal expects the R48-million bio- remediation of Dam 10 to be completed by October this year.
The 75 ha dam was commissioned in 1960 as part of a number of wastewater storage and evaporation dams. Two of the dams were used for organically contaminated wastewater, while the remaining dams were used for inorganic wastewater.
Organic effluent from coke ovens at the site’s coke plant was historically pumped into Dam 10 and contaminated the underlying clay sediment.
Coke oven effluent contains signiﬁcant amounts of polycyclic aromatic hydrocarbons (PAHs) along with more soluble components such as cyanide, phenols and ammonia.
Mittal environmental manager Karien Zantow explains that the insoluble PAH-containing tars settled out and built up at the bottom of the dam. Little biodegra- dation took place in the bottom sludge and the organics accumulated over the years. Thus, after extensive dewatering of the dam, tar pools appeared in some areas.
Changes in environmental policy and concern over accumulated priority pollutants resulted in the effluent-handling process being changed and the dam being decommissioned.
“Disposal of the settled sludge and contaminated soil became a priority and various methods were investigated. “Eventually 150 000 m3 of sludge and soil was identified for cleanup and on-site bioremediation was the most cost effective option,” she adds.
Use of the dam stopped in 2000 and it is currently undergoing rehabilitation using archaea microorganisms that are added to the contaminated soil to break down waste compounds.
Each species of archaea serves a specific purpose and functions optimally at varying levels of pH and temperature.
Zantow explains that the addition of high concentrations of a diverse range of archaea to a treatment system permits some species to select to the environment encountered. As the environment changes, the continuous addition of the biosystem ensures that the microorganisms continue to populate the treatment system and perform effective breakdown.
Between six and seven months are required for the soil to achieve a 97% rehabilitated status.
Between 2002 and 2004, the dam was dried out, followed by remediation of 98% of the area between 2008 and 2010. The process involved excavation of the 800-mm-thick layer of tar sludge and contaminated soil that was stacked into windrows within the existing footprint and surface of the dam.
Hay and organic compost are added to the top of the piles by 20% of volume and are mixed using the self-propelled windrow machines. Simultaneously, 25 ℓ of archaea, which consists of 8% solution with 0.05% surfactant, was sprayed for each cubic metre of soil.
The application is strictly controlled by Mittal technologist Johan Hattingh.
Zantow says testing indicated that organic breakdown had been successful in the completed section and the required soil cleanup targets for all organic pol- lutants achieved.
“Testing on the windrows showed that between 88% and 99% of all pollutants were broken down,” she notes.
Chemical composition of the prepared windrows was determined by taking core samples from each side along the length of the windrows. The cores were blended and samples submitted for analysis of PAH, which is regarded as a priority pollutant by the Environmental Protec- tion Agency and known to be present in coke oven effluent.
The rehabilitation process further entails that the windrows be turned at least every two weeks. The revitalised soil is then reworked into the lower clay layer to serve as the topsoil, after which five indigenous grass species are planted. The end result is a completely self-sustaining open field.
Similar remediation of the remaining 2% of the area is currently under way with final vegetation to be completed by October this year.
“The main objective of the project was to limit possible contamination of sur- face water and ensure clean stormwater runoff from the site after remediation,” Zantow says.
Other benefits of the rehabilitation project will be the lowering of the risk of contamination seepage into the ground and reducing dust to a large extent by planting grass. The grass also serves to transform the former dam into an aesthetically pleasing area.
Zantow adds that, when rehabilitation is complete, the space will be reserved as a storage area; however, until it is needed for that purpose, it will remain an unused vegetated field.
“This flagship project will reduce the impact of a legacy facility that has been in operation for the last 50 years,” she states.
The remediation of the CETP sludge dams comprises different phases, with the first phase, which involved the safe disposal of inorganic sludge, having been completed at a cost of R12-million last year.
“The company excavated 650 000 m3 of inorganic sludge from these dams in August last year. The remaining material is currently being windrowed for bioremediation,” says Zantow.
Further, Mittal budgeted R11-million for the second phase, during which about 45 000 m3 of sludge will undergo organic remediation. The second phase is expected to start in October and be completed by July next year.
The third phase will entail sloping and shaping of the area and the implementation of a stormwater control system. The phase is set to start in June next year and reach completion by June 2013.
Soil stabilisation and revegetation will be done during the fourth and final phase that will run from July next year to March 2014.
The maturation ponds, which comprise three dams (Dam 1, Dam 2 and Dam 3), were decommissioned in 2008.
Remediation of the dams entails soil pH stabilisation. The process at Dam 3 started in 2009 and was completed last year.
The same soil stabilisation method, which has been determined and implemented by remediation specialist Viljoen & Associates at Dam 3, will also be implemented at Dams 1 and 2.
Viljoen & Associates owner Chris Viljoen explains that the soil is characterised and polygons of contamination are drawn on areas with similar contamination levels. Specific and differential application and treatment rates are then determined for each polygon area.
This means that the entire surface is not treated or stabilised as one homo- genous soil body, but that targeted and specific treatment is applied depending on the soil quality.
At Dam 2, windrows have been packed and remediation is expected to start in October this year and be completed in December next year.
Dam 1 is currently undergoing on-site water and sludge remediation, which will be completed in 2012.
The total remediation process of the maturation ponds is expected to be complete by the end of 2015. To date, R6.5-million of the R30-million allocated for the project has been spent.
Waste Disposal Site
Zantow says about 40-million cubic metres of waste, which consists mainly of steelmaking slag, dust and filter cake, has been disposed of at the site since its inception in the early 1960s.
Disposal on the old waste site was terminated in December 2010 as dictated by water use licence requirements that prohibit disposal beyond the site reaching 40 m in height.
Remediation of the site is done in three phases, during which the area is divided into three sections that will each undergo subsequent remediation through sloping, shaping and soil capping.
Stormwater drainage canals will also be constructed to divert clean runoff into the main stormwater chute that joins the clean stormwater system.
During capping, the area surface is covered with a 1.5 mm flexible high- density polyethylene liner and then covered by a 500-mm-thick layer of clean com- pacted soil, followed by a 200-mm-thick clean topsoil layer.
The side slopes are also covered with three 150-mm-thick compacted layers of clean clay.
Fertilisers are then added to the topsoil and hydroseeding is done for the establishment of indigenous vegetation.
Sloping and shaping of the soil for the similar, but much larger, second phase began in January this year, with capping to start in July. The total environmental provision budget for phase two is R47-million.
Zantow says that packing of the liners for the second phase of the project started in June.
“According to our project programme, the second phase is to be completed in September next year,” she adds.
The third and final phase will begin in 2013 and will be completed by 2015 at the latest.
The total budget for the waste site remediation project amounts to about R100-million.
New Waste Disposal Site
Further, construction of the leachate dam, which is designed to store toxic liquids, started in April last year. The R23-million project was completed in December last year, with operation starting in January this year.
Edited by: Chanel de Bruyn© Reuse this Comment Guidelines
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