Sludge management is sometimes overlooked at the WWTP. It is however a crucial component. Sludge management consists of all processes that are needed to dispose the sludge that is removed from the various treatment stages:
- primary treatment: grit, fiber, suspended solids, heavy metals
- secondary treatment: biosolids
- tertiary treatment: suspended solids, chemical sludge
These processes include:
- transportation: pumping, conveying
- thickening: volume reduction to decrease size of dewatering equipment and decrease chemical consumption for conditioning
- conditioning: to improve dewaterability
- de-watering: final volume reduction before disposal
Thickening of sludge can be done with several types of equipment. Gravity thickeners and rotary drums are often used. A gravity thickener works like a clarifier, solids settle to the bottom and supernatant flows over the top. A rotary drum works like a fine screen that is rotating. It retains the solids inside the screen while the water decants through the screens.
Conditioning of sludge is usually done by adding chemicals. Chemicals include several coagulants and polymers. Required dosages vary widely and need to be evaluated for each type of sludge.
Dewatering of the sludge is the final stage before disposal. If sufficient land is available non-mechanical methods such as sludge drying beds and sludge lagoons can be used. Otherwise mechanical dewatering equipment is used:
- Belt filter press
- Filter press
- Decanter centrifuge
Belt filter press
The belt filter press uses a few different processes. First there is a gravity thickening
section to reduce the volume. Then there is a low pressure section where the sludge is
squeezed in between two belts. Lastly the sludge passes through a high pressure
section where the belts pass through rollers.
Achievable dryness: 15-25% (depending on sludge mixture)
In a filter press high pressure is applied to sludge to remove as much water as
possible. The most common filter presses are the fixed and variable volume recessed plate filter presses.
The fixed volume recessed-plate filter press consists of a number of rectangular plates
with filter clothes hung over each plate. The plates are held together in between a fixed
and movable head. The movable head is forcing the plates together, to enable the
plates to withstand the pressure that is applied during the filtration process.
The variable volume recessed-plate filter press differs because a rubber diaphragm is
placed behind the filter media. This rubber diaphragm expands to achieve the final
squeezing pressure. During this step the final cake volume is reduced further.
Achievable dryness: 25-40% (depending on sludge mixture)
The screw press consists of a rotating bowl. The sludge is fed at a constant flowrate and
is separated in a cake and the centrate.
Achievable dryness: 20-40% (depending on sludge mixture)