Sludge Dewatering Technologies
In nearly all wastewater treatment technologies a solid waste or secondary raw material is produced that contains the original pollutant and therefore requires further treatment – disposal, recovery, etc.
It can be stated with confidence that whatever the fate of these materials – generally called sludge – their water content must be reduced, if only to lower transport costs. This is the aim of sludge dewatering operations: reducing the water content and, with it, the mass. Today these are almost exclusively mechanical operations; the most common dewatering machines are the belt filter press, the plastic chamber filter press, the centrifuge and the screw press.
Belt filter presses have become widespread in municipal wastewater treatment plants. Continuous operation and relatively high capacity have made these machines almost exclusive, even though their wash-water demand is high and their complex belt guidance practically requires continuous operator supervision.
The centrifuge gradually entered this field; the reason is its high investment and service cost, offset by higher dry-solids content and large capacity.
For mainly industrial sludges, where increasing the dry-solids content – reducing the mass – is especially important because of disposal as hazardous waste, the use of plastic chamber filter presses has become widespread. Examples include galvanic, paint and machinery industry sludges. For sludges typically containing metal hydroxides, the plastic chamber filter press achieves the highest dry-solids content, up to 35–40%. However, the machine is batch-operated and its capacity is limited by the chamber volume.
A recent development is the screw sludge press, which can essentially replace belt filters in the municipal market and, for lower capacity needs, centrifuges. Its operation is simple, it has no noise emission, and service costs are also low.
A simplified comparison of the dewatering machines
| Feature | Screw press | Centrifuge | Belt filter press | Plastic chamber filter press |
| dewatering efficiency | same as centrifuge | same as screw press | 4–6% lower than the screw press | high |
| rotational speed | 0.8–1.4 rpm | 2,800–3,500 rpm | not applicable | not applicable |
| noise level | low | high | low | only the sludge pump |
| wear part cost | low | high (drum and screw wear) | high (belt wear) | low |
| lifespan | 15–20 years | 10–15 years | 10–15 years | 15–20 years |
| filtrate quality | average | depends on disc setting | average | suspended-solids free |
| polymer consumption | average | average | good | good |
| rinse water demand | minimal | at every start and stop | continuously high | none |
| reaching operating state | instant | time-consuming | time-consuming | instant |
| daily maintenance | short | at every start and stop | constant belt cleaning | short |
From the brief descriptions and comparison above it is clear that there is no such thing as "the best" process. There is an optimal process for a given task – but this must be selected through a detailed design procedure.