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Toxic Sludge stabilisation for INAG, Portugal Originally started in 1996, this project to clean up and stabilise contaminated sludge deposits originating from one of Europe’s leading leather industries has been the largest of its type in Europe and has run in three phases so far with a fourth and final phase yet to be completed. The Rhenipal process was selected by INAG (a division of the Portuguese Ministry of Environment) because Dirk could prove that this alkaline stabilisation agent was able to immobilise the chromium content in the heavily contaminated sludge to a high extent. Typical results are shown in table 1 below. Over 100,000 M3 of sludge has been removed from unlined deposits and mixed with Rhenipal B stabilisation additive before being inserted and compacted in a 450,000 M3 Landfill specially designed and built to take the final product.
Table 1: Typical final product data showing reduction in heavy metal leachates The various stage breaks in the project have allowed Dirk to upgrade its alkaline stabilisation techniques from adjustments to the Rhenipal recipe and addition rate to optimise the process to radical changes in the mixing equipment. The Alcanena project showcases the flexibility of the Rhenipal process perfectly. From the beginning it was expected that the characteristics of the sludge would vary dramatically. This proved to be the case.
Figure 1: Ploughshare Mixer used in Phases 1 & 2 at Alcanena The deposits contained a mixture of very dry crusted sludge from the surface gradually getting wetter deeper down with some pockets heavily saturated with water. Additionally the deposits were further contaminated with stones, leather shavings, and general waste including plastic, wood and steel. The daily production from the sewage works has been treated in tandem with the sludge from the deposits and this has also changed in characteristic as the sewage works upgraded from belt pressed primary sludge to chamber pressed secondary sludge. Best mixing results were historically obtained using a specially designed ploughshare mixer (figure 1), however with a view to speeding up production and cutting down time caused by large pieces of debris jamming the mixer and breaking blades, a new mixer concept was introduced. The actual mixer design consisted of two shafts with small robust blades spinning at high revolutions. Two hoppers with moving floors fed sludge and stabilisation additives through the mixer chamber by gravity (figure 2). In ideal conditions this mixer was capable of treating 1000 M3 in a 10 hour working shift compared to a maximum of 600 M3 with the previous equipment. The equipment also proved much more resilient to the battering received from large rocks and concrete fragments which were found in one area of the deposits.
Figure 2: Twin shaft Mixing plant used in Alcanena Phase 3 Although a typical Rhenipal B recipe was selected as a benchmark for this process, it was necessary to modify the blend to cope with varying sludge conditions in order to maintain a consistent end product which conformed to set values for final dry substance, pH and mechanical strength. Specially designed variable speed powder conveyors were included in the design of the mixing plant to facilitate any alterations to the product addition. Regular samples of the treated cake were taken daily and more frequently if necessary to maintain the quality of the mix. Regular samples were also taken from the landfill to monitor the behaviour of the treated material after insertion.
Figure 3:Treated sludge being inserted to the Alcanena landfill The stabilised material in the Alcanena landfill is completely inert, produces no gas and is stable enough to support 30 tonne plus insertion vehicles (figure 3). The projected lifespan of the landfill is fifteen years after which it will be capped and blended back into the landscape |
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