Determine the optimal dosage of chemical agents
When using flocculation to treat any sewage, there are optimal flocculants and optimal dosages. The optimal dosage of flocculants refers to the minimum dosage of flocculants to achieve the established water quality target, which has important technical and economic significance for sewage flocculation treatment. Generally, the optimal dosage is determined by combining laboratory simulation and on-site methods. There are four main laboratory simulation methods: beaker stirring test, potentiometric analysis, colloid titration and filtration.
Due to the large number of influencing factors, data is generally obtained through flocculation beaker stirring experiments. The test includes three steps: rapid stirring, slow stirring and static sedimentation: the added flocculant is quickly dispersed after rapid stirring and contacts the colloid particles in the water sample, and the colloid particles begin to flocculate and produce micro-flocs; through slow stirring, the micro-flocs further contact each other and grow into larger particles; after stopping stirring, the formed colloid aggregates naturally settle to the bottom of the beaker by gravity. Through a comprehensive evaluation of the flocculation effect, such as flocculent sedimentation, supernatant turbidity, CODcr, chromaticity, pH value, etc., the appropriate flocculant variety and its optimal dosage are determined. During the test, the raw water and actual water quality must be exactly the same, and the type of flocculant, dosage, addition sequence and hydraulic conditions (mainly the degree of stirring during coagulation and the flocculation reaction time (excluding mixing time)) must be the same or close.
The following is the specific operation method for determining the optimal dosage of coagulant in the beaker coagulation sedimentation experiment:
Equipment used: beaker, coagulation test agitator, measuring cylinder, raw water sample, turbidity meter, pH value, thermometer;
Determine the raw water characteristics (such as turbidity, pH value, temperature, etc. of the water sample) and record them;
Take 6 1000ML beakers and fill them with equal amounts of raw water samples;
Place the 6 beakers in fixed positions of the coagulation test agitator, and set different speeds and times for the six mixing cups;
Use a pipette to add the same amount of the coagulant prepared in advance to the dosing tube in turn, and start the agitator;
After the mixing is completed, turn off the agitator and observe the formation of alum flowers in the static sedimentation in different mixing cups;
After 10 minutes, 30-50ML of the supernatant in the 6 cups can be drawn out with a 50ML syringe (pipette) and put into a 500ML mixing cup;
Immediately measure the different turbidities in the 6 cups in the turbidity meter, record and compare;
Take the beaker with the smallest turbidity as the best dosage choice.
Of course, there are many ways to determine the dosage of coagulant. Friends can also discuss in the comment area and write down the simplest and most limited method you think for everyone to learn.
Regarding the calculation of dosage, the most convenient way is to refer to the data of the built sewage treatment plant with similar water quality, and make appropriate adjustments based on the former.
Calculation of coagulant dosage: T=aQ/1000
T--Daily coagulant dosage (kg/d)
a--Maximum dosage of unit coagulant (mg/L)
Q--Daily treated water volume (m3/d)
The difference between solid and liquid coagulants
According to the solid and liquid state of the reagent, the dosing method can be divided into dry dosing and wet dosing.
Taking PAC as an example, the use of solid PAC is convenient for reagent storage and reduces the area occupied by the storage tank. However, there are also the following disadvantages:
Increases the number of workers in the sewage treatment plant, and the labor intensity is relatively high;
During the unpacking of solid reagents, it is difficult to avoid the debris of the woven bag falling into the dissolving tank, causing pipe blockage;
The reagent and water may be mixed unevenly, and the coagulation effect is affected;
The storage and handling process has a great impact on the sanitary environment of the dosing room.
The addition of liquid PAC can overcome some of the above disadvantages, save manpower, have good sanitary conditions and lower costs. However, the addition of liquid coagulants also has problems such as complex dosing system, large area, and easy corrosion of equipment.
Therefore, it is also very necessary to take certain measures to improve it.
Use an integrated dosing system. That is, optimize the combination of dissolving, stirring, quantitative control, dosing and other parts in space to reduce the floor space. For example, the liquid storage tank is placed at the bottom, the dissolving agent and stirring device are placed on the top, and a platform is set up in the empty space above the liquid storage tank. The quantitative control and dosing device are placed on the platform, which can greatly reduce the floor space of the equipment.
The dissolving agent stirring equipment uses a paddle stirrer, and is equipped with a baffle and a guide tube. It is used to eliminate the overall rotation of the stirred solution, strengthen the axial and radial flow, increase the turbulence, improve the stirring effect, and improve the dissolution efficiency.
Take certain anti-corrosion measures. For the stirring parts, epoxy primer and epoxy topcoat can be applied. If the agent is very corrosive, anti-corrosion materials such as fiberglass, rubber and enamel can be used. For the dosing pipe, plastic pipe or rubber pipe is used depending on the amount of dosing.
Selection of dosing method
Similarly, the dosing method is also divided into dry dosing and wet dosing.
Dry dosing is difficult to measure and control, and has poor regulation performance. It mainly uses a dry dosing machine to directly put the solid agent into the water or into a dissolving container, and then into the water.
Wet dosing can be divided into gravity dosing in a high-level solution pool, dosing before the pump, dosing by a water ejector, dosing by a water pump, and many other forms.
(1) Gravity dosing
The liquid medicine flows out of the high-level solution pool and is put into the water by gravity through a constant liquid level water tank. The more common adjustment method is to adjust the valve on the dosing pipeline, observe the indication of the rotor flowmeter, and change the dosage. This dosing method is suitable for those whose water intake pump room is far away from the water plant. It is safe and reliable, but the location of the solution pool is high.
1 Dissolution tank 2 Solution tank 3 Lifting pump
4 Water seal box 5 Float valve 6 Flow Meter
7 Control valve 8 Pressure pipe
(2) Dosing before the pump
The liquid medicine is added to the water pump suction pipe or the head of the suction pipe. This dosing method is safe and reliable, and is generally suitable for those whose water intake pump room is close to the water plant.
1 Dissolution tank 2 Lifting pump 3 Solution tank
4 level water tank 5 Float valve 6 Dosage nozzle
7 Water seal box 8 Suction pipe 9 pump 10 Pressure pipe
(3) Water ejector injection
The high-pressure water is used to suck the liquid medicine through the vacuum suction between the nozzle and the throat, and at the same time, it is injected into the raw water pipe with the residual pressure of the water. The water ejector has low efficiency and is easy to wear, but the equipment is simple and easy to use, and the height of the solution pool is not too limited.
1 Solution tank 2 Dosage box 3 Funnel
4 Water ejector 5 Pressure pipe 6 High pressure water pipe
(4) Dosing with metering pumps
The traditional method is to use a centrifugal dosing pump to deliver the drug solution into the water. The adjustment method is to adjust the valve of the dosing pipeline and the rotor flow meter indication. Later, the application of dosing with metering pumps began to appear. Dosing with metering pumps does not require additional metering equipment, is suitable for coagulant automatic control systems, and is conducive to the mixing of drugs and water.
1 Solution tank 2 Metering pump 3 Pressure pipe
For small and medium-sized sewage treatment plants, gravity dosing before the pump should generally be used.
Its biggest advantage is that it eliminates the need for mixing equipment, but there are two points that must be noted: First, a water seal box should be set to prevent the introduction of air; second, the pipeline from the water pump to the reaction tank should not be too long, otherwise the reagent will react in the pipeline, forming alum flowers that will easily break into the reaction tank, and it will be difficult to re-agglomerate after breaking, affecting the coagulation effect.
For large water supply or sewage treatment plants, pressure dosing after the pump can be used. Its core equipment is an acid-resistant metering pump, which is particularly suitable for situations where the pump station is far away from the reaction tank.
Coagulant dilution factor and concentration
For polymers, especially organic polymer coagulants, if they are linear, in order to play the role of the functional groups on their long carbon chains, they must be fully stretched, so dilute solutions should be prepared.
In general, the concentration of organic polymer coagulants is about 0.1%, and inorganic polymer coagulants can be higher, but the mixing effect after addition must be guaranteed.
Taking the inorganic polymer coagulant PAC (Polybasic aluminum chloride)as an example, if the quality of the agent cannot be guaranteed, the concentration should not be too low.
This is because when the concentration of the PAC solution is too low, the pH value of the solution will rise to a certain extent, causing some metal ions with low solubility products to precipitate in the form of oxides, thereby forming impurities that block the pipeline.
It is understood that most PAC agents supplied on the market are made of mineral raw materials, which contain other metal elements in addition to aluminum. In order to control costs, some manufacturers have simplified the process of impurity removal, which ultimately leads to the presence of considerable amounts of other metal ions in the PAC finished product.
Selection of dosing point
(1) You can add the chemical agents at multiple points or at one point. However, the dosing point must be set in the laminar flow area. At the same time, in order to extend the residence time and enhance the coagulation reaction effect, the reaction tank should be built into a baffle or equipped with a partition.
(2) From the perspective of simplifying the operation, you can use a one-time addition, which is the traditional method currently used. If you want to save the amount of reagents and improve the coagulation effect, you can use batch addition, that is, add one part before the mixing equipment and one part after the mixing equipment. The reason why this method is better than one-time addition is that the contact collision coagulation speed between particles of different particle sizes is greater than the collision coagulation speed between uniform particles.
(3) All the required coagulants can be added to a part of the treated water, and after sufficient mixing, it can be mixed with another part of the water without drug addition. This method can also increase the collision coagulation speed between different particles, but it also has the disadvantage of complicated process.
There are generally three points for adding coagulants:
Pretreatment stage: The purpose of adding coagulants in the pretreatment stage is to strengthen the pretreatment effect, increase the removal efficiency of pretreatment, and remove toxic substances in wastewater that hinder the growth of microorganisms, so as to reduce the burden of subsequent biochemical treatment structures and ensure the effect of biochemical treatment.
Biochemical treatment stage: The addition of coagulants in the biochemical treatment stage is generally to increase the flocculation of microorganisms, so that the activated sludge can be separated more thoroughly in the subsequent mud-water separation facilities.
Deep treatment stage: The addition of coagulants after biochemical treatment is mainly to remove the remaining, non-biodegradable pollutants in the wastewater, which is a guarantee measure for the effluent to meet the standards after treatment.
Key points in designing dosing system
(1) Consider building the dosing room, drug storage and disinfection chlorination room together. This design is to facilitate operation and management, reduce labor intensity and save investment.
(2) The volume of the liquid storage tank should be optimized to ensure liquid supply for 2 hours. If the time is too long, the volume of the liquid storage tank will be large and occupy more space. If the time is too short, the operation will be troublesome. If the amount of dosing is small, it can be combined with the dissolving tank into one tank or box.
(3) The storage capacity of the drug storage should be determined according to the specific situation. If the supply of drugs is sufficient and transportation is convenient, it can be considered to store 20 to 30 days at the maximum dosage. If the drug supply conditions are poor, it can be considered to store 60 to 80 days at the maximum dosage.
(4) If pre-pump dosing is used, the dosing system should be as close to the pump room as possible.
(5) The depth of the dosing pipe inserted into the water pipe is generally 1/3 to 1/4 of the diameter of the water inlet pipe. To ensure smooth dosing, the direction of the dosing pipe outlet should be consistent with the water flow direction of the water inlet pipe.
The pipe fittings of the dosing system should be made of corrosion-resistant materials.