How to use polyaluminum chloride and polyacrylamide in sewage treatment

 

 

Polyaluminum chloride and polyacrylamide in sewage treatment are a lot of applications, in sewage treatment occupies a very important position. But have you thought about mixing and matching them? Today we will show you how to correctly combine polyaluminum chloride and polyacrylamide in wastewater treatment applications. Firstly, I will analyze their relationship, and then I will introduce the steps, precautions and relevant knowledge of mixing. The wastewater from detergent production has the characteristics of complex components, high content of CODcr and LAS components in the wastewater, which are difficult to biodegrade directly, and low pH value of the wastewater. At the same time, when the detergent components in the wastewater reach a certain concentration, it will affect the process of wastewater treatment, such as aeration, precipitation and sludge digestion. In the actual wastewater treatment process, flocculants are often used to solve the problem that high concentration LAS is difficult to biodegrade. Therefore, it is of great practical significance to study the selection, dosage and influencing factors of flocculants in the flocculation treatment.

 

In order to determine the operation parameters of flocculation treatment in the wastewater treatment test system, the flocculation experiment was carried out on the production wastewater of the detergent factory, and the optimal operating conditions of flocculation treatment were determined, such as the type of flocculant, the dosage of flocculant, pH value and stirring conditions.

 

The waste water in this experiment comes from a detergent factory in Yishui County, Shandong Province. The waste water mainly includes washing water for detergent production raw materials, production process waste water, washing water of various containers and some domestic sewage. The products of the detergent factory mainly include kitchen tile detergent, dishwashing agent, dishwashing agent, bleach agent, cleaning agent and so on, among which kitchen tile detergent contains irritant acidic active agent; Dishwashing agent, dishwashing detergent is acidic or strongly acidic, and has strong corrosion; Bleach is a white powdery organic bleaching agent; Clean is oily liquid, the main ingredients are surfactants and spices. Pilot tests were carried out on the wastewater by using flocculation and biological treatment system.

 

The experiment equipment

 

The beaker for the flocculation experiment was carried out on a multi-linkage variable speed mixer, and the wastewater used in the experiment was directly taken from the above detergent factory. The experimental instruments include JJ-3 six-link electric mixer, pHS - 3C precision pH meter, MS-1 microwave digestion COD tester, 721B spectrophotometer and beaker, etc. Experimental drugs polyacrylamide (PAM), polyaluminum chloride (PAC), caustic soda (NaOH) solution, dilute sulfuric acid solution, ferric chloride, ferrous sulfate and alum, etc. Experimental methods and conditions in 6 ~ 8 beakers, add 200ml water samples, add a certain amount of flocculant, coagulant aid, respectively, with caustic soda solution or dilute sulfuric acid solution to adjust the pH value, stirring for a certain time, standing precipitation, take the supernatant to measure the CODcr value. In the experiment, the stirring speed was 60r/min, the stirring time was 30rain, and the precipitation time was 30min. The water quality was determined according to the standard method stipulated in "Water and Wastewater Monitoring and Analysis Method", and the CODc was determined by microwave digestion potassium dichromate method.

 

Experimental results and discussion

 

Selection of flocculant Type The type of flocculant is related to the production cost and treatment efficiency, so it is necessary to determine the flocculant with low price, easy to obtain and convenient to use. Here, several commonly used flocculants, such as ferrous sulfate, polyaluminum chloride (PAC), ferric chloride, alum and so on, are selected to conduct flocculation experiments on wastewater from detergent production. According to the experimental method, quantitative agents were added to the water sample, and the pH value of the fixed water sample was 7.0. The experimental results are shown in the table below. CODcr of raw water is 4480 mg/L. The experimental results show that under the same experimental conditions, the effect of polyaluminum chloride is the best, followed by ferric chloride. The formation of alum flower of both of them is rapid and the stratification is obvious, but ferric chloride can not solve the chroma problem well. After ferric chloride treatment, the supernatant obtained after precipitation has chroma. In addition, from the economic analysis, the price of poly aluminum chloride for industrial use on the market is about L200-1400 yuan per ton, while ferric chloride is generally 2400 ~ 3900 yuan, so it can be seen that the use of poly aluminum chloride is more economic and effective. According to the analysis of usage, the cost of poly aluminum chloride for each ton of waste water is 1.5 ~ 1.8 yuan. Therefore, it is feasible to choose polyaluminum chloride as flocculant in the wastewater treatment test system.

 

Dosage of flocculant polyaluminum chloride: whether the dosage of flocculant is reasonable is related to whether the precipitation is thorough, whether the treatment is more effective and whether the production cost is lower, so it needs to conduct experiments to determine the dosage accurately. The CODc of the wastewater used in the experiment was 4480 mg/L. According to the experimental method, the amount of fixed polyacrylamide was 10mg/L and the pH value was 7.0. Only the dosage of polyaluminum chloride was changed. The effect of PAC dosage on CODcr removal rate was investigated. The experimental results are shown in Figure 1. Figure 1 shows that the removal rate of CODcr in wastewater first increases with the increase of PAC. When the dosage of polyaluminum chloride is about 1.5 g/L, the removal rate of CODcr in wastewater is the highest, reaching over 81%. However, when the dosage of flocculant exceeds 1.5 g/L, the removal rate of CODcr begins to decrease. Because the amount of flocculant polyaluminum chloride is insufficient, the flocculation treatment is not sufficient, and the flocculant polyaluminum chloride hydrolyze produces more H, resulting in the pH value of wastewater reduction, change the flocculation reaction conditions, inhibit the combination and reaction of flocculant polyaluminum chloride with wastewater, reduce the efficiency of wastewater treatment; However, when the amount of polyaluminum chloride is too large, the particles in the wastewater are surrounded by too many flocculants and lose the chance to combine with other particles, so it is difficult to condense. At the same time, more OH- is produced by hydrolysis, which leads to the increase of the pH value of the wastewater, changes the conditions of flocculation reaction and reduces the treatment efficiency.

 

Dosage of coagulant assistant polyacrylamide: according to the experimental method, the dosage of fixed flocculant PAC is unchanged, while the pH value is 7.0, only the dosage of polyacrylamide is changed, and the effect of polyacrylamide dosage on CODcr removal rate is studied. The experimental results are shown in Figure 2. Figure 2 shows that polyacrylamide dosage is beneficial to the removal of CODcr to a certain extent, which can promote the formation of flocs, improve the sedimentation rate and change the sedimentation performance. It was found that the CODcr removal rate of wastewater was significantly increased after adding polyacrylamide as coagulant aid. This was because the flocculation reaction environment was improved after adding coagulant aid, and the reaction between polyaluminum chloride and colloidal particles in wastewater was promoted, so that the CODcr removal rate was improved. With the increase of polyacrylamide dosage, the particles in the supernatant decreased obviously, and the precipitation time shortened obviously. When the dosage was 10mg/L, the removal rate of CODcr reached the highest, which was above 81%. But polyacrylamide dosage can not be too large. Because polyacrylamide is soluble and organic, the CODcr value of wastewater increases after excess.

 

pH

 

Keep the dosage of flocculant polyaluminum chloride and coagulant polyacrylamide unchanged, adjust pH value with caustic soda solution or dilute sulfuric acid according to the experimental method, and observe the influence of pH value on CODcr removal rate. The experimental results are shown in Figure 3. Figure 3 shows that pH value has a great influence on the removal of CODcr. With the increase of pH value, the removal rate of CODcr increases. When pH value is between 6 and 8, the flocculation effect is better. When pH value is equal to 7, the removal rate of CODcr reaches the highest value, which is above 81%. This is because when the pH value is low, polyaluminum chloride is easy to form a complex with H or aluminum sulfate and other substances. When the pH value is high, polyaluminum chloride is easy to form aluminum hydroxide and other substances with OH, which makes PAC difficult to combine with pollutants, hinds the smooth flocculation reaction and greatly reduces the treatment efficiency.

 

The effect of mixing time

 

According to the experimental method, the best experimental conditions, namely the dosage of flocculant is 1.5g/L, coagulant aid; 1.5 g/L. The dosage was 10mg/L, the pH value was fixed at 7.0, and the stirring speed was 60r/min. Only the stirring time of flocculation reaction was changed to observe the influence of stirring time on the removal rate of CODcr. The experimental results were shown in Figure 5. Figure 5 shows that CODcr removal rate increases gradually with the extension of stirring time, and the effect is the best when stirring time is 30min. After stirring time exceeds 30min, CODcr removal rate decreases gradually. This is because the mixing time is too short, flocculating agent and wastewater can not be fully mixed, the growth time of flocs is not enough, the contact time of flocs and wastewater pollutants is not enough, the amount of pollutants absorbed by flocs is less, so the flocculation is not sufficient, the reaction speed is slow; And mixing time is too long, it will make the flocs that have been formed and broken, can not form alum flower and settlement.

 

Effect of precipitation time

 

According to the experimental method, take the best experimental conditions, namely the amount of flocculant is 1.5 g/L, the addition of coagulant aid amount to 10 mg/L, fixed pH value of 7.0, stirring speed of 60 r/min, the mixing time for 30 min, just change the flocculation reaction, precipitation time, to observe the influence of precipitation time of CODcr removal rate, the results are shown in figure 6. As can be seen from FIG. 6, with the increase of precipitation time, the higher the removal rate of CODcr, the better the flocculation treatment effect. The slope of the curve of the flocculation treatment effect in the period of 15-30min precipitation is obviously smaller than that in the period of 0-15min precipitation. The effect of the flocculation treatment in the period of 35min precipitation is slightly improved than that in the period of 30min precipitation, that is, when the precipitation time reaches 30min, the extension of the precipitation time has little contribution to the effect of the flocculation treatment. Therefore, the precipitation time of flocculation reaction was selected as 30min in this test system.