various treatments

There are other conditioning processes but they are rarely used either because of their investment or operating costs or because they are not very effective or not enough reliable.

freezing-thawing

Freezing, a process to completely solidify sludge, separating water into sheets of ice and concentrating the electrolytes in the sludge, is a very effective method for reducing the quantity of water bound in the matter. This property remains stable after the ice has melted and produces a sludge that is much more filterable.

To date, freezing-thawing has been used in a few applications on predominantly mineral sludge that is difficult to dewater: this particularly applies to aluminium hydroxide sludge generated by drinking water production or the preparation of industrial water. This energy-hungry treatment can be combined with vacuum filtration in order to obtain sludge that has a dry solids content of more than 30 %.

«conditioning» using inert additives

Adding dry matters that are usually inert will improve sludge cohesion and, mainly, its compressibility factor.

When used to condition liquid sludge, adding dry matters (CaCO₃, gypsum, sawdust, fly ash, coal….) allows us to reduce, but not eliminate, the amounts of reagents normally used. The main result is an improved cake texture either to facilitate its handling or for easier running of dewatering equipment: e.g. by adding carbonates to biological sludge or sawdust or other fibrous matter to oily sludge before it goes to the belt filter.

Also, it is always interesting to combine the dewatering of problematic sludge (hydroxide, biological sludge) with dense mineral sludge (carbonate removal, gas scrubbing, calcium sulphate, paper mill sludge ...).For it to be effective, this loading has to be added at the rate of approximately 20 to 40% of the sludge’s initial suspended solids. Adding loading to pre-dewatered cakes can also:

• «artificially» increase dry solids content before the sludge is sent to a landfill site or recycled (e.g. adding CaO or sawdust);

• reduce the sludge compressibility factor prior to the second stage of dewatering ;e.g. adding fly ash to a filter belt cake before it goes through a very high pressure filter (10 or even 40 bar).

treatment using electro-osmosis

This process produces a synergy between:

• a pre-flocculation that is always required for sludge;
• an electro-osmosis which, through the effect of a continuous electrical field, draws water up out of the capillaries towards the cathodic surface.

This type of conditioning has especially been used to process sludge that resists conventional mechanical dewatering (e.g. very hydrophilic greasy sludge) and to acquire a further 5 to 10 points of dry solids content. However, its industrial development remains very limited because investment and operating costs are still high.

Improvements to belt filter performance using an electro-osmosis phenomenon are still at the prototype stage.

toxic sludge stabilising agents

Various reagents are used, either individually or in combination: Portland cement, slag cement, sodium silicate, plaster, fly ash, slurries, organic resins …

These treatments are not conditioning processes in the true sense of the term: applied to thickened liquid sludge, these treatments are not followed by mechanical dewatering but cause the sludge to solidify.

Depending on the amounts used, the end result will be a pasty, easy-to-shovel or completely solid product. This product can be disposed of at a landfill site, most metals having been chemically fixed (will not leach).However, this process is subject to major limitations: increased sludge liquid volume, high amounts of stabilising agents (15 to 35% of the liquid sludge weight) and, therefore, these expensive processes are only used in specialised toxic sludge processing centres.