1、 Formation of floc flotation
1. The algae content in the raw water is relatively high
The organic matter produced by algal metabolism has an impact on flocculation and filtration, because acidic substances in the organic matter react with the hydrolysis products of coagulants (iron or aluminum salts), and the surface complexes generated adhere to the surface of floc particles, hindering particle collision. If the floc still floats up during winter or other unsuitable conditions for algae growth, this factor can be ruled out.
2. Improper sludge discharge or equipment problems
During the operation of the inclined tube sedimentation tank, if the sludge is not discharged in a timely manner or is not discharged sufficiently, it will cause the alum content in the entire sedimentation tank to exceed the acceptable limit.
At the same time, if the water plant experiences a scraper failure during actual operation and stops running, the phenomenon of alum floating up during this period is extremely obvious.
3. Difficult to control the dosage of coagulant
Generally speaking, colloidal substances contained in raw water are difficult to naturally settle. Adding coagulants to raw water is to destabilize colloidal substances and form larger flocs, which can naturally settle and facilitate subsequent treatment.
But if the on-site operators cannot adjust the dosage of coagulant in a timely manner according to the water quality of the incoming water, it will lead to insufficient coagulation reaction, and the formed flocs are difficult to sink, resulting in unsatisfactory sedimentation effect.
Mainly manifested in two aspects:
·With the addition of coagulants, the double layer on the surface of particles in water is compressed, causing effective collisions and growth of particles, which then adhere to bubbles and float up;
·When the dosage is too low, the coagulant cannot effectively compress the double layer of particulate matter and affect the growth process of flocs. The collision and adhesion efficiency between micro flocs and bubbles is low, resulting in poor adhesion with bubbles before floating up.
4. Excessive hydraulic load
When the settling velocity of particles is equal to the rising velocity of water flow, a visible clear turbid interface will appear in the inclined pipe, and the lower part of the interface is a suspended zone in a settling state.
When the flocs in the suspended area come into contact with the rising water flow, they will continuously intercept small particles in the water until they form large and heavy flocs and settle by gravity.
If the water consumption increases, the water plant often operates at overload, and the flow velocity in the inclined sedimentation tank will also increase accordingly. The flocs are difficult to settle well inside the inclined tube and are easily carried to the clear water area and deposited on the upper part of the inclined tube.
5. Influence of raw water turbidity
When the turbidity of the raw water is high, the formed flocs are coarse and dense, and the adhesion of bubbles on the surface of the flocs is limited. The required dosage of coagulant is large, making it difficult to float the flocs.
When the turbidity is low, there are fewer colloidal substances in the water, which reduces the chance of particle collision and coagulation. Therefore, the coagulation effect of low turbidity raw water is poor. In this case, the dosage of coagulant should not be too small.
It is worth mentioning that there are often a large number of microbubbles adhering to the surface and internal pores of these floating flocs. The main causes of these bubbles are as follows:
·Anaerobic fermentation of sediment at the bottom of the pool. The perforated sludge discharge pipe of the sedimentation tank does not discharge the sludge thoroughly, resulting in the accumulation and solidification of sediment in the sedimentation area. Over time, anaerobic fermentation occurs, producing gases such as methane, carbon dioxide, and a small amount of hydrogen sulfide.
·Algal action. Algae have strong respiration and photosynthesis, and gas production can be observed.
·The water pump and pipeline system are leaking air. Specifically, it manifests as air leakage in the pump body itself, air intake through the horn mouth of the water pump suction pipe, and air leakage in the water pump suction pipe.
2、 Measures to deal with the floating of flocs
1. Reasonably adjust the sludge discharge time
Add a sludge collection tank along the length of the sedimentation tank on the effluent side, with a perforated suction pipe in the tank. The perforated discharge pipe is linked to the scraper. When the scraper plate scrapes the sludge to the edge of the sludge collection tank, a large amount of sludge flows into the tank. Open the discharge valve to suction and transport the diluted sludge water to the discharge channel outside the tank. Adjust the sludge discharge time based on the quality of the raw water and the effluent from the sedimentation tank.
2. For low turbidity water, the solution is to add clay
Adding clay to raw water can increase the particle concentration in the water, increase the chance of particle collision, and thus improve the coagulation effect. This method is feasible without investing a large amount of manpower, and it is also possible to consider using a metering pump to add PAM and other coagulant aids
3. Control the dosage of coagulant
In the above analysis of reasons, it has been mentioned that controlling the dosage of coagulants can effectively suppress floc flotation. The phenomenon of floc floating usually occurs during periods of low turbidity in raw water.
Therefore, in order to prevent the phenomenon of "air resistance" caused by the direct entry of raw water containing a large amount of gas into the filter tank for filtration, the dosage of coagulant can be controlled according to the actual situation, and the treatment method of direct filtration after micro flocculation in the reaction tank can be adopted, or the treatment method of direct filtration by adding water before filtration after aeration in the reaction sedimentation tank can be adopted.
Meanwhile, SCD can also be used to control drug administration. SCD (Flow Current Detector) is an online instrument that directly measures the effect of coagulant addition and adjusts the amount of coagulant added. It can determine the amount of coagulant added by comparing the detected flow current value with the design given value. Through mathematical model calculation and analysis, the operating conditions of the dosing device can be adjusted, and the dosage of coagulant can be changed in a timely manner to achieve the desired coagulation effect. 4. Implement separate pool treatment for high hydraulic load
When operating at full capacity, open the connecting valve between the two tanks to balance the inflow of the two tanks, and try to make the two tanks work within their respective processing capacity range as much as possible to avoid overload operation; At the same time, the scheduling department coordinated and arranged the inflow, reducing significant changes in inflow and ensuring stable effluent from the sedimentation tank.
5. Abnormal flow inclined tube sedimentation tank
Affected by changes in raw water turbidity, algae and organic matter concentration. Consider transforming the original inclined tube sedimentation tank into a reverse flow inclined tube floating sedimentation tank, using inclined tube sedimentation for high turbidity and air flotation for low turbidity.
