Mobile dredging sludge treatment installation at Hourpes (Belgium): example of an innovative treatment combining mechanical dewatering and lime Sandra Vandorpe, CARMEUSE Benelux Dredging sludge in Belgium Since 2 decennia Belgium, and especially Wallonia, is well behind the curing of its water ways causing navigation and economical problems. This situation is partly due to the fact that since 1995, dredging sludge (mixture of mainly clay and silt and in a lesser proportion of sand and gravels) is considered as a waste which is subject to a specific waste policy. Previously dredging sludge was simply recovered from the water ways and dumped along the river banks. This new dredging sludge policy reviewed in 1999, introduced a classification of the sludge depending on its origin and on its composition: type A stands for unpolluted or slightly polluted sludge while type B for the other categories. Type A sludge can be recycled on site or via “grouping centres” whereas type B sludge is oriented to pre-treatment centres to re-classify them as type A or are oriented to a “grouping centres” for treatment and further elimination (ex: cement, landfill sites,…). The Walloon MET (Ministry Equipment and Transport), which is in charge of the curing estimates that the current maintenance of the 450km water ways implies the annual evacuation of some 600.000m³ dredging material while the built up sludge from previous years still to be dredged is estimated to more than 5 million m³. Whatever type A or B, and whatever the orientation of the sludge, landfill site, specific valorisation, or treatment, the sludge needs to be dewatered to become “shovable”. Dewatering techniques The most common technique that has been used since the beginning is “lagooning”. The technique is quite easy and is based on natural de-watering by drainage. As the surface needed for this kind of treatment is important, the exploitation permits are not easy to obtain with NIMBY effects playing an important role. The more, the technique is time consuming , even if it can be accelerated in some cases from 12 to 3 months by optimising the drainage properties of the soil. Interesting to mention is that sludge of type A, treated by “lagooning”, is sometimes further mixed with lime (a few percent CaO) to be recycled as backfill material. ENVISAN International (group Jan de Nul specialised in the dredging) developed with CARMEUSE an innovative alternative treatment using the well-known and simple dewatering technique of the filter- press. The technique was optimised for the dewatering of dredging sludge by using fine ground lime with a specific reactivity for the pre-treatment of the sludge. This technique enabled a decrease of the filter- press cycles and an increase of the final Dry Solids (DS) content. Mobile dredging sludge treatment at Hourpes The installation was placed on 3 pontoons on the Haute Sambre (upriver from Charleroi). The requirements by the MET implied the dredging of 30.000m³ sludge to bring the draught from 1,8m to 2,2m. The work had to be realised on a period of 200 days, including 50 days of building up, preliminary analysis, demand of exploitation permit and 150 days of production at 250m³/day. The treatment The 30.000m³ dredging sludge that was dredged on the Haute Sambre was directly treated on the mobile dewatering installation. The sludge was of type A and could thus be valorised as backfill material. The installation worked as followed: 1. Discharging liquid dredging sludge from the barge 2. Charging of the sludge on the vibrating sieve 3. Sieving of the sludge at 0/8mm 4. Elimination of the exogenous material (for 7. high pressure pumps 8. filter-press further recycling or landfill site) 5. Pumping of the sludge to the mixing tank 6. Addition of fine ground quicklime at a ratio 1 to 2% depending of sludge quality 7. Pumping of the lime treated sludge at high pressure (8 bars) to the chambers of the filter-press 8. Pneumatic jacks are used to automatically separate the plates of the 7. water treatment filter-press 9. Dewatering of the sludge from 35% to 70% DS 10. The filtrate is treated before getting back to river 11. Filter cakes are stored in barges of 250T and transported further to their final destination (backfill material) The addition of 1-2% lime to the sludge (pp of the DS content of the sludge) enables to decrease the filter cycles to less than 10 minutes; a complete cycle 8. de-watered sludge at 70% DS takes about 15 min. The dewatered filter cakes have also a high DS content of 70% which makes them an excellent recycling product. Tests without lime all showed very poor dewatering results and very long filter cycles, above 30 min. Nevertheless, the use of lime is an extra cost that has to be taken into account. The follow-up This first mobile treatment work was a success. In November last year, after six and a half month of functioning, about 26.500m³ sludge were dredged and treated for a total of about 2 million euros. The installation was easily dismantled. The MET will now compare this mechanical dewatering technique with lagooning aiming to point out the most efficient treatment method. We are confident that the mechanical dewatering technique using lime has important advantages such as cost, treatment cycle of the sludge, the mobile aspect of the work and high DS content of the filter cake that will make it an interesting alternative to lagooning and thus give lime producers some new future perspectives for lime sales.
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