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Nowadays sewage sludge is mainly conditioned by adding flocculation agents, i.e. by:
ferric chloride solutions and hydrated lime suspension for dewatering in filter presses
synthetic polymers in 0.05 to 0.2 % aqueous solution in centrifuges and belt filter presses, and recently often with filter presses too.
Much still remains scientifically unclear. As far as mixing coagulant and sludge is concerned, it is currently thought that this operation must be performed quickly, with relatively high energy input, to avoid unwanted side reactions. It is certainly imperative to produce a turbulent state of flow for distribution in the molecular range. The LIMUS In-line mixer was developed to provide the considerable energy density needed to process a viscous non-Newtonian liquid such as sewage sludge.
The LIMUS In-line mixer is a static mixer which uses jet turbulence to perform the mixing operation. The sludge is accelerated in a nozzle far enough for it to flow turbulently into the subsequent mixing chamber and blend with the coagulant solution. The blending process is complete on leaving the in-line mixer and the turbulent-flow energy is largely exhausted. This mixing operation requires a pressure drop of about 0.5 bar for each mixer.
The LIMUS In-line mixer is matched to the properties of sewage sludge. It is not prone to blockage, is easy to clean and resistant to corrosion and wear. It has no moving parts and takes up very little space.
The in-line mixer can be installed either horizontally or vertically in the piping. It is a simple and easy procedure, and subsequent units can be installed by sewage-works staff.
As the LIMUS In-line mixer improves the mixing process for conditioning agents, up to 25% savings in chemicals have been achieved on a number of sewage treatment plants, compared with the conventional reaction mixer with slowly-rotating stirrers. The pressing time in filter presses was also reduced.
The LIMUS In-line mixer can be used for mixing all current conditioning agents with sewage sludge.
The diagramm shows (in simplified form) a filter press plant with ferric chloride/hydrated lime suspension conditioning.
The sludge is pumped at an almost constant rate of flow from the first receiver via the in-line mixers into the second conditioned-sludge receiver. The filter press feed pump draws off the sludge and forces it into the filter press.
| Type | Flow rate* | Internal diameter | DN 1 | DN 2 | L approx. |
|---|---|---|---|---|---|
| m3/h | mm | PN 10 | PN 10 | rnm | |
| 80 F | 5 bis14 | 81.4 | 80 | 25 | 800 |
| 80 K | 99.4 | 100 | 40 | 1000 | |
| 100 F | 14 bis 25 | 99.4 | 100 | 25 | 1000 |
| 100 K | 126.6 | 125 | 50 | 1250 | |
| 125 F | 25 bis 40 | 126.6 | 125 | 25 | 1250 |
| 125 K | 144.8 | 150 | 65 | 1500 | |
| 150 F | 40 bis 80 | 144.8 | 150 | 25 | 1500 |
| 150 K | 203.4 | 200 | 80 | 2000 |
Separate in-line mixers are used for the ferric chloride solution and the hydrated lime suspension. When conditioning with polymer flocculating agents, however, only one mixer is used. The in-line mixers cope easily with a fluctuation range of 50 % to 100 % of sludge flow rate for which they have been designed.
Material:
Outer pipe: PVC, Insert: PVC, special alloy
Typ F: FeCl3 solution and similar
Typ K: Ca(OH)2-suspension and polyelektrolyt solution
