Sedimentation separation, as a unit operation, has long been used in water treatment control and is a relatively practical water treatment technology. To improve the sedimentation separation effect, inclined plates or inclined tubes are often installed in the settling zone during use, so that the shallow layer principle can be applied, taking advantage of large area and high separation efficiency. However, the problems that arise during application have rarely attracted attention. This paper focuses on the analysis of key problems that appear during application and proposes corresponding solutions for reference.
1. PROBLEMS OCCURRING DURING THE APPLICATION OF INCLINED TUBE SETTLERS
If the water distribution at the inlet of the inclined tube settler is not uniform, serious turbulence will occur in the liquid movement near the inlet. This causes an extremely high local flow velocity at the inlet, making the sludge previously deposited on the inclined tubes rise again.
The internal situation of an inclined tube settler during normal operation is shown in Figure 1.
From Figure 1, it can be seen that the water flow and the sludge flow each have their own paths. If normal operation continues for a long time, the movement of the sludge flow is basically close to dynamic equilibrium:
at this time the SS removal efficiency of the wastewater is good.
However, if the flow distribution at the inlet is not uniform enough, the flow velocity at some tube inlets becomes too high, causing severe turbulence and deterioration of the effluent quality.
2. FLUID MECHANICS ANALYSIS OF THE PROBLEM
During the entire unfavourable process, the fluid will exhibit the situation shown in Figure 2.
The reasons for the above phenomenon are analysed from the following aspects.
2.1 Hydraulic analysis of the unfavourable situation
In fact, the upward and downward flow of water and sludge in the inclined tubes can be regarded as an approximate fluidisation phenomenon. Assume v is the superficial velocity of water through the tubes, u₁ is the actual velocity of water through the sludge particles, and ε is the void fraction (of the sludge particle layer). Then u₁ = v/ε, as shown in Figure 3.
If the void fraction of the suspended solids is small, the actual velocity of water through the particle layer is relatively high. If the particle settling velocity is known as a fixed value uset, according to the theory of various stages of fluidisation, when the actual velocity of water through the particle layer u₁ = uset, the fluidised state is in a fixed bed stage with a stable void fraction ε. When u₁ > uset, the original void fraction ε is broken, causing an increase in void fraction. For an inclined tube settler, this means particles rise, reducing the separation efficiency.
2.2 Analysis of different settling velocities of resuspended sludge particles in various states
The resuspended sludge particles exist in the form of ① individual particles and ② particle clusters of different sizes (with void fraction ε'). According to the settling velocity formula for individual particles in the Stokes law region:
For the settling velocity of particle clusters, according to the empirical relationship of Steinour:
where:
vt,group - settling velocity of particle clusters, m/s;
t - settling time, s;
g - gravitational acceleration, m/s²;
ρs, ρL - density of particle and liquid, kg/m³;
Dp - particle diameter, m;
μ - dynamic viscosity coefficient, kg/(m·s);
ε - void fraction.
Since 0 < ε² < 1 and 0 < 10-1.82(1-ε) < 1, it follows that ε²·10-1.82(1-ε) < 1, hence vt,group < vt.
According to sedimentation theory and the above proof, after being resuspended, particle clusters are less likely to settle, i.e., they are more easily carried away by the water flow, reducing sedimentation efficiency.
2.3 Flow characteristics in the unfavourable state
When the unfavourable situation shown in Figure 2 occurs, looking down at the water surface of the settler, the flow distribution appears as in Figure 4.
As shown in Figure 4, three zones appear on the water surface: ① a relatively calm zone, ② a transition zone, and ③ a severe turbulence zone. In the severe turbulence zone, in serious cases, the water surface may even bulge into small curved arcs. The SS concentration here is extremely high, almost the same as the influent.
2.4 On-site test results after correction based on the above analysis
In a certain wastewater treatment project, due to the above problem, the effluent was rather turbid. After hydraulic analysis and engineering correction to make the water distribution uniform, the phenomenon of flocs rising no longer appeared in the effluent. Test results are shown in Table 1.
Table 1 CODcr test results of wastewater from a certain plant
| Water sample batch | Influent CODcr/(mg·L⁻¹) | Effluent CODcr/(mg·L⁻¹) | Removal rate/% |
| 1 | 1272 | 261 | 79.5 |
| 2 | 1153 | 245 | 78.7 |
From Table 1, it can be seen that after correction and uniform water distribution, the CODcr removal rate reached a relatively high level.
3. SEVERAL POINTS TO NOTE FOR INCLINED TUBE SETTLERS
3.1 Analysis of different particles
For different particles, such as sand sedimentation, floc sedimentation of printing and dyeing wastewater, and coal slurry sedimentation, due to differences in particle density, shape, and void fraction, there are differences in sedimentation. Sand particles have a relatively high density and small size, so they settle quickly. Flocs from printing and dyeing wastewater have a relatively low density, close to water, large shape, and high void fraction, so they settle slowly. Coal slurry falls between the two. Therefore, for sand sedimentation, the influence of flow fluctuations is relatively small, while for printing and dyeing wastewater, the influence of flow fluctuations is relatively large. Thus, different particles have different requirements for flow distribution. For printing and dyeing wastewater, the flow distribution at the inlet must be relatively uniform.
3.2 Inclined tube settlers require more uniform water distribution
Because inclined tubes (typically 35 mm, 50 mm in diameter) are placed in the settler, when the hydraulic distribution under the tubes is not uniform, due to ν = Q/A, the flow velocity becomes too high in some tubes, causing flocs to rise, resulting in an effluent quality even worse than that without tubes. Therefore, the water distribution in inclined tube settlers should be more uniform than in ordinary sedimentation tanks.
3.3 The installation direction of inclined tubes should be opposite to the inflow direction
The process of water entering the settler from the inlet pipe involves a sudden enlargement of the flow cross-section. Therefore, the flow undergoes a brief change. To avoid this change, the installation direction of the inclined tubes should be opposite to the inflow direction. This can mitigate the impact of the sudden change in flow on the sedimentation efficiency of flocs.