To reduce sludge transport and landfill costs or to raise its calorific value, a greater solids content is the decisive factor. A possible starting point for increasing the solids content is lowering the pH-value. The volume of pulp fibres changes according to the pH. Raising the pH expands the fibres and lowering it causes them to shrink.
Especially in the case of sludges consisting of alkaline pulp fibres, the addition of CO2 can improve dewatering at the sludge press.
Setting the pH using carbon dioxide has many advantages:
- The process is compatible with other auxiliary agents, such as flocculants or defoamers, and supports these with an additional complimentary effect.
- In water, carbon dioxide reacts as a weak acid. Characteristic for carbon dioxide is its flat neutralisation curve which makes it easier to When using carbon dioxide there is no risk of danger to health from dangerous fumes, or of corrosion to the hardware. In addition, the storage vessel for CO2 does not require either a water-tight drip tray or other safety hardware as is the case with mineral acids. Thus handling carbon dioxide is simple, safe and economic.
- The salt load of the water found in the sludge is not increased with chloride or sulphate when neutralising with carbon dioxide. This is ecologically advantageous and can be of relevance for the discharge permit or for freight-based waste water discharge.
- In practice, the amount of carbon dioxide required is comparable to or lower than the amount of mineral acids.
To add and dissolve the carbon dioxide in highly viscose liquid/fibre mixtures, nozzles of a special construction, which minimises their tendency to block, need to be used.