OUTLINES OF THE CLARIFLOCCULATION PROCESS
AWe wrote about the theory of sedimentation in the section of the “Scraper Bridges for Sedimentation Tanks“.
In the Sedimentation with flocculation the particles do not keep their individuality, but tend to agglomerate helped by some coagulants. The sludge floc size increases and, as a consequence, the sedimentation speed increases too.
SEDIMENTATION WITH FLOCCULATION
A complete process of clarification mainly consists of three subsequent stages:
• Mixing of the inflow (waste water) with the reagents
The waste water mixing with the reagents must be made very quickly and with a strong stirring.
On the contrary, the flocculation must be slow in order to allow the flocs agglomeration and any turbulent motion must be avoided.
The clarified water is then collected in the upper part through circular or radial ditches, the sludge sediments on the tank bottom and is conveyed into a drain well by some scrapers.
CLARIFLOCCULATOR WITH SLUDGE RECYCLE
The clariflocculator with sludge recycle is equipped with a turbine to recycle and mix the already settled sludge with the new water to be treated which has already been conditioned with reagents.
The “old” sludge flocs, usually of medium-large size, help the production of new sludge flocs.
The conditioned water, when entering the clariflocculator, is mixed with the settled sludge in the reaction chamber as it is lifted by a radial-axial flow turbine.
It is fundamental that this operation occurs at the minimum speed as possible for not breaking the sludge flocs.
The sludge recycle flow rate is variable and is adjusted by means of a variable-speed geared motor.
The lifted sludge and the water to be treated, after having been mixed in the reaction zone, are conveyed to the flocculation zone where the flocs gather and become larger and the settling phase begins.
The clarified water can be collected through radial or peripheral ditches.
The radial ditches are more efficient because the water flow towards them is more homogeneous and the flow speed does not become too fast in certain zones.
According to the already mentioned Stokes’ law, the settling speed is higher for larger suspended solids.
Conclusions For the same flow rate of water to be treated, the clariflocculators require smaller tanks.
OUR RANGE OF PRODUCTION INCLUDES THE FOLLOWING TYPES OF CLARIFLOCCULATORS
- Accelator type clariflocculator with turbine for sludge recycle mod. EM11
- Central drive clariflocculator with turbine for sludge recycle mod. EM20
- Peripheral drive clariflocculator with gate mixers mod. EM21A
- Peripheral drive clariflocculator with turbine for sludge recycle mod. EM21B
- Clariflocculator with torsion trestle and turbine for sludge recycle mod. EM22
TECHNICAL COMPARISON BETWEEN CLARIFLOCCULATORS AND ACCELATORS
This is a qualitative comparative study of the realization, process requirements and degree of simplicity of construction and running between CLARIFLOCCULATORS and ACCELATORS.
This comparative study was carried out on the basis of the general configuration of the two machines (placed in the same tank to make the comparison).
The Clariflocculator and Accelator must accomplish:
1. a process of contact between the pre-existing sludge and the incoming waste water to obtain larger flocs which tend to sediment more easily;
2. a process of sedimentation based on Stokes’ law.
The clarified water re-surfacing speed remains, according to the experimental results achieved in machines with a process of sludge contact, between 2 and 2.5 m/h. with light sludge (such as Al (OH)3).
Here is a description of how the processes 1 and 2 are accomplished.
1A: There is a small, but well stirred, contact zone between the sludge and the incoming waste water where the intimate contact is ensured.
After the intimate mixing, the period of contact is kept in a “calm” volume, there is no narrowing so the flocs integrity is respected and they can gather and become larger in the next calm zone.
2A: The “calm” volume allows to convey the water to be treated to the sedimentation zone, at a very low elevation, without any turbulence and with a gradual slowing down due to the section widening along the waste water flowing upwards.
1B: There is no zone of intimate mixing, the volume (and the time of contact) is almost the same as in the clariflocculator and coincides with the mixing time. There is a narrow passage between the mixing zone outlet and the gate to regulate the outflow opening, therefore the newly formed floc risks to break just before entering the sedimentation zone.
2B: The flowing from the contact volume to the sedimentation zone is faster than in the clariflocculator, so more whirling, moreover it occurs at a very high elevation, near the zone where water is already clear. Therefore there is a high risk of sludge overflow into the clarified water.
As for the simplicity of 3) construction and 4) running, we point out that:
3A: The clariflocculator turbine is smaller, so the manufacturing of the rotary part is simpler in terms of centering and vibrations; the only adjustment of the rotation speed allows to eliminate complicated mechanisms.
4A: The automatic extraction of sludge, interlocked with a turbidimeter, allows to avoid any manual control and eliminates the risk of producing a not completely clear water, due to a lack of sludge discharge, in case of a suddenly turbid feed of waste water.
3B: The manufacturing and centering of the big turbine are particularly difficult; the mechanism (manual) to regulate the turbine outflow opening often gets blocked.
4B: The extraction of sludge is manual so in case of a suddenly turbid feed of waste water there is a risk of discharging turbid water.