A year ago I was in a commercial presentation of Tencate, where they showed one of their geo textile fabrics for dewatering purposes, Geotube. I was impressed by these products, a kind of stackable "canelonni".

There are more similar commercial geo textile products available easy to find just asking Mr Google. You can download their brochures to get a previous idea.

•Bury it.

•Mix with fly ash from nearby coal-fired power plant.

•Mix with excavator.

Then, see step 1.

I will review it. It’s very interesting. It’s a good idea to mix the sludge with fly ashes to bury it. My concern and main doubt is how to dewater the sludges and dry it, easily and cheap.

I would appreciate your help.

We performed cost analysis to evaluated 7 separate methods of drying AMD sludge generated from hydrated lime process.

We toured the Globe smelter site in Denver where they used Geotubes to consolidate the sludge and transfer the de-watered sludge to trenches where alcohol is added to feed sulphur reducing bacteria.

I am very interested in your sludge drying study. Would you be willing to share a copy? We rarely look at anything beyond filter presses for dewatering but I am interested to see what other solutions could be considered.

I agree with the posters above suggesting geotubes. They're quite useful as a punctual solution for dewatering slimes with very low solids content.

I got a question about the geotubes. Is that a permanent facility? Or we have to replace when it is fully loaded with solid? How the solid inside the geotube can be removed?

There are several methods for dewatering ferric based AMD sludges.

•Centrifuges - labour intensive - giving sludge cakes between 25-34% solids. To achieve best results, additional polymer may be needed.

•Plate press - labour intensive - sludge cake - +40% solids, may need polymer too.

•Geotubes - no information for solids content, control of poly dosing regimen needed.

•Sludge Treatment Reedbeds – under drained reedbed - sludge cakes circa 50% solids.

•Under drained sand bed - sludge cakes estimated at 40-50% solids.

•Tailings pond - filled and then left to dry out - variable but quite acceptable results.

Limed sludges should theoretically dewater quite well as the flocc size ought to be quite large and should be less inclined to entrain water within the cake structure.

This based on personal experience of AMD, let me know if you need more info etc.

As you will note in the list, many solutions are available but the selection will depend on sludge quantities you have to manage and also the initial solid content.

Can you share this information?

Solids inside the Geotube were removed by Bobcat after cutting them open so the drainage pad can be reused like an on-off leach pad. We are also evaluating leaving them in place and stacking the new ones on top, this will require geotechnical review.

How about the efficiency of dewatering of this geotube at wet area? I mean if the location for applying this technology is in a tropical area where heavy rainfall is everyday part of life. The water separated from slurry needs to be collected for another treatment for meeting the regulation requirement before discharging?

Of course an alternative is not to add lime at all. Hydrotalcite-based precipitation that can incorporate a range of contaminants, generally forms substantially less solids, and de-waters very rapidly should be considered. Costs vary but are comparable over the life cycle if you don't have to de-water or transport lots of low solids sludge to pits etc. In general an Na-SO4 water is produced minus many of the foulants so that RO performance is improved if you need to go this way.

Treatment of the Geotube filtrate will depend on the local jurisdiction requirements and sludge stability. We have been using the Kosson method to determine sludge stability as a function of pH, we are planning to try the up flow column version of the Kosson, USEPA 1314 for sludge characterization to determine the effect of solid-liquid separation. The filtrate should be a small proportion of the treated water and can be returned to the feed of the water treatment plant, if necessary.

Many thanks for your comments; all your comments are very useful for me.