Dewatering techniques
The most common technique that has been used in Belgium since the beginning of their drain clearing issues 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 local 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.
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. 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 takes about 15 min. The de-watered 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.
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.
The implementation team 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.