Mixing speed and time of polyacrylamide solution
Usually polyacrylamide solution is prepared by means of mechanical mixing. The mixing speed has great influences on the preparing time of the solution. However, too fast a speed shall cause degradation of polyacrylamide and part of polyacrylamide long chains shall break and influence sediment effect, so the mechanical mixing speed must be controlled strictly. The rotating speed in a 1m-diameter mixing barrel should not be more than 800rpm, and in a 1.5~2m-diameter barrel, it should not be more than 600 rpm. To increase the temperature of mixed solution may reduce the time of solution. But if the water temperature should not exceed 55℃, otherwise it shall also lead to degradation and influence the using effect.
Special equipment must be used for preparation and addition of polyacrylamide solution
Special equipment must be used for preparation and dosing of polyacrylamide solution. The equipment must not be used commonly for other coagulants. Dosing must not be done commonly in the same dosing pool. Otherwise two types of chemicals shall have copolymerization precipitation, which does not only influence the effect, but also block the dosing equipment.
The measuring device must be calibrated with the solution.
Since polyacrylamide solution is a drag reducer, its calibration value has relatively great differences from the value of clear water. Therefore, the measuring device for polyacrylamide solution must be calibrated with the solution instead of clear water. Otherwise the dosing quantity of polyacrylamide shall be increase, which shall result in increased treatment cost as well as unnecessary consequence.
Dosing concentration
The thinner the added polyacrylamide solution is, the better the effect is. Thinner concentration can make the solution expanded quickly and mixed thoroughly, which shall prevent colloid protection phenomenon due to excessively high concentration and influence the using effect. But if the solution is too thin, the dosing equipment has to be too huge. Generally, the dosing concentration should be 0.5%-1% and the preparation concentration should be 2%.
Dosing in batches
Polyacrylamide flocculant used for treatment of highly turbid water shall have better flocculation effect when it is dosed in batches than at one time. If the precipitation rate of turbid liquid level is used as the comparison value, the former shall be over 3 times that of the latter. The so-called dosing in batches means the dosage is divided into two parts and dosed into the water separately, i.e. a part of flocculant is dosed first, which is mixed with water rapidly, then in 1 ~ 2 minutes, the other part of flocculant shall be dosed and mixed with water. Dosing in batches shall result in better effect because it may prevent excessively concentrated flocculant from combining with sediment, which may close the active group. Generally, the ratio of dosing in batches should be 60% first, then followed by 40%. In design of water supply project, dosing in batches should be adopted as much as possible if the condition permits.
Dosing sequence
When polyacrylamide is used as coagulant aid, generally it is dosed after addition of coagulant. In case that it is used as flocculant for treatment of highly turbid water independently, polyacrylamide flocculant must be dosed first, otherwise the using effect could be affected. Alternative measures for dosing sequence must be taken into account in design of the dosing room.