Studydehydration treatment of residual activated sludgeRefinery

Oily sludge refers to a large amount of oily solid waste generated in the process of crude oil exploitation, gathering and transportation and refining. It is generally divided into oily sludge from oil field and oily sludge from petrochemical industry (mainly refinery). Oily sludge contains a large number of pathogenic bacteria, parasites (eggs), heavy metals, radioactive elements and a large number of toxic and harmful substances that are difficult to degrade, such as benzene, phenol, anthracene and benzopyrene. If directly discharged, it will pollute the surrounding soil, water and air [1], causing harm to human body. Oily sludge has been listed in the category of oily waste in the national catalogue of hazardous wastes. The national cleaner production promotion law requires that oily sludge must be treated innocuously.

The sludge produced by the sewage treatment system of petrochemical industry mainly comes from the bottom sludge of oil separation tank, scum of flotation tank and residual activated sludge, which are collectively referred to as "three sludge". For a long time, the "three sludge" has been mixed and concentrated in the sewage treatment plant. Due to the large differences in properties and extremely complex components of "three mud", it is difficult to dehydrate, so it is difficult to treat [2,3]. In order to solve this problem, the researchers conducted research work on the treatment of "three mud" according to the characteristics of "three mud". The author dewatered the residual activated sludge, determined the compound flocculant and its dosage, investigated the influence of experimental parameters on sludge dewatering effect, and determined the best treatment conditions, which is intended to provide technical support for practical application.

Mud samples, reagents and instruments

The sludge sample is taken from the residual activated sludge concentration tank of a sewage treatment plant, which is black, has a foul smell, rich in a large number of micro flocs, and the pH value is 7 ˙ 16, moisture content 99 ˙ 68%, oil-free.

Inorganic flocculant (2000 mg) ˙ L-1), organic flocculant (1000mg) ˙ L-1), hydrochloric acid solution (1 mol) ˙ L-1), sodium hydroxide solution (1mol) ˙ L-1)。

Xqy - Ⅱ rotary coupon corrosion tester, Jiangsu Jiangdu Jianhua instrument factory; Vacuum suction filtration device; Ab204-s analytical balance, METTLER, Switzerland; 202v1 electric constant temperature drying oven, Shanghai YOUPU Scientific Instrument Co., Ltd; Ph211 pH meter, Hanna company, USA.

Method

(1) Put the beaker containing 500 ml of sludge into the rotary coupon corrosion tester for bath heat to raise the temperature of sludge to the preset value.

(2) Add polyaluminium chloride (PAC) into the beaker and stir quickly for a certain time; Then add cationic polyacrylamide (CPAM), stir slowly for a certain time, and then stop stirring.

(3) Take out the beaker, put the sludge into the Brinell funnel soaked with filter paper, and vacuum filter for 8 minutes.

(4) Put a small amount of mud cake on the surface dish, put it into the constant temperature drying oven, dry it at 105 ℃ to constant weight, cool and weigh it. First calculate the mass of the mud cake before and after drying, and then calculate the moisture content of the mud cake.

The moisture content of mud cake was determined by 105 ℃ drying weight loss method.

Screening of compound flocculant

It can be seen from table 1 that 6# the treatment effect is the best, the moisture content of mud cake is the lowest, the water is the clearest, the floc is large and dense, and the settling speed is fast. Therefore, PAC + CPAM is selected as the compound flocculant.

Effect of dosage of compound flocculant on dewatering effect of excess activated sludge

The effect of PAC dosage on the dewatering effect of residual activated sludge is 20 mg in CPAM dosage ˙ L-1. The operating temperature is 35 ℃, and the initial pH value of sludge is 7 ˙ 16. The quick stirring strength is 150 R ˙ Min-1, fast stirring time 30 s, slow stirring intensity 70 R ˙ Under the condition of min-1 and slow stirring time of 6 min, the effect of PAC dosage on the dewatering effect of residual activated sludge was investigated.

It can be seen from Figure 1 that the moisture content of mud cake initially decreases with the increase of PAC dosage; When PAC dosage is 200 mg ˙ When L-1, the moisture content of mud cake is the lowest; When the dosage exceeds 200 mg ˙ At L-1, the moisture content of mud cake increases instead. This is because: when the inorganic flocculant is added to the sludge, a series of hydrolysis and polymerization reactions will occur and a large number of hydroxyl complexes will be generated. The colloidal particles in the water can strongly adsorb various products of hydrolysis and polymerization reaction. The adsorbed positively charged polynuclear complex ions can compress the electric double layer and perform electric neutralization, so as to reduce the zeta potential of colloidal particles in the sludge, destabilize the system and improve the filtration performance; However, when the dosage of inorganic flocculant is too large, too much positive charge may make the particle surface positively charged, stabilize the system, and reduce the filtration performance [5]. Therefore, the dosage of PAC is 200 mg ˙ L-1 is appropriate.

Effect of CPAM dosage on dewatering effect of residual activated sludge

The dosage of PAC is 200 mg ˙ L-1. The operating temperature is 35 ℃, and the initial pH value of sludge is 7 ˙ 16. The quick stirring strength is 150 R ˙ Min-1, fast stirring time 30 s, slow stirring intensity 70 R ˙ Under the condition of min-1 and slow stirring time of 6 min, the influence of CPAM dosage on the dewatering effect of residual activated sludge was investigated. The results are shown in Figure 2.

It can be seen from Fig. 2 that the moisture content of mud cake is getting lower and lower with the increase of CPAM dosage at first; When the dosage of CPAM is 25 mg ˙ When L-1, the moisture content of mud cake is the lowest; When the dosage of CPAM exceeds 25 mg ˙ At L-1, the moisture content of mud cake increases instead. This is because [6 ~ 8]: when treating sludge, cationic flocculant has the dual functions of positive charge neutralization and adsorption bridging. With the increase of dosage, it promotes the charge neutralization and adsorption bridging, which is conducive to the formation and settlement of flocs, so as to improve the dewatering effect of sludge. However, the flocculant also has dispersing function. When its dosage is too large, a large number of polymers are adsorbed on the suspended particles to wrap them, forming a spatial protective layer on the surface to prevent the formation of adsorption bridging, so that the formed flocs are re dispersed and become flocs with loose structure. The water contained in the flocs is difficult to remove, resulting in the reduction of sludge dewatering effect. Therefore, the dosage of CPAM is 25 mg ˙ L-1 is appropriate.

Effect of operating temperature on dewatering effect of excess activated sludge

The dosage of PAC is 200 mg ˙ The dosage of L-1 and CPAM is 20 mg ˙ L-1. The initial pH value of sludge is 7 ˙ 16. The quick stirring strength is 150 R ˙ Min-1, fast stirring time 30 s, slow stirring intensity 70 R ˙ Under the condition of min-1 and slow stirring time of 6 min, the effect of operating temperature on the dewatering effect of residual activated sludge was investigated. The results are shown in Figure 3.

As can be seen from Fig. 3, the moisture content of mud cake initially decreases with the increase of operating temperature; When the operating temperature is 40 ℃, the moisture content of mud cake is the lowest; When the operating temperature exceeds 40 ℃, the moisture content of mud cake increases instead. This is because: when the sludge temperature increases, the chemical reaction accelerates, the viscosity of aqueous solution decreases, the diffusion of condensed molecules increases, and the growth rate of flocs accelerates. At the same time, when the water temperature increases, the Brownian motion intensifies, increasing the chance of collision, which is conducive to the agglomeration of sludge into larger particles, thus promoting flocculation and sedimentation; However, when the sludge temperature is too high, the chemical reaction is too fast, forming fine flocs, and strengthening the hydration of flocs, resulting in high water content, large volume and difficult treatment of sludge [9], which reduces the dewatering effect of sludge. Therefore, the operating temperature should be 40 ℃.

Adjust the initial pH value of sludge with sodium hydroxide solution and hydrochloric acid solution, and the dosage of PAC is 200 mg ˙ The dosage of L-1 and CPAM is 20 mg ˙ L-1. The operating temperature is 35 ℃, and the quick stirring strength is 150 R ˙ Min-1, fast stirring time 30 s, slow stirring intensity 70 R ˙ Under the condition of min-1 and slow stirring time of 6 min, the effect of initial pH value of sludge on the dewatering effect of residual activated sludge was investigated.

It can be seen from Figure 4 that the moisture content of sludge cake initially decreases with the increase of initial pH value of sludge; When the initial pH value of sludge is 8 ˙ At 0, the moisture content of mud cake is the lowest; When the initial pH value of sludge exceeds 8 ˙ At 0, the moisture content of mud cake increases instead. This is because: under the condition of low pH value, the coagulation process has relatively low coagulation speed, long reaction time and small floc particle size [10], and the dewatering effect of sludge is poor; At pH 8 ˙ Under the condition of 0, because the hydrolysate of flocculant has more positive charge, the electroneutralization of colloidal particles is larger, more flocs are formed, the filtration performance of sludge is stronger, and the dewatering effect of sludge is better; When the pH value is too high, the repulsive force between colloidal particles increases when the charge changes, which prevents aggregation and stabilizes the system, resulting in poor sludge dewatering effect. Therefore, the initial pH value of sludge is 8 ˙ 0 is appropriate.

Effect of hydraulic conditions on dewatering effect of excess activated sludge

It can be seen from table 2 that the influence order of four factors on sludge moisture content is: fast stirring intensity > fast stirring time > slow stirring time > slow stirring intensity. The best hydraulic condition for the dewatering of excess activated sludge is that the fast stirring intensity is 170 R ˙ Min-1, the fast stirring time is 60 s, and the slow stirring intensity is 50 R ˙ Min-1, slow stirring time is 8 min.

The best condition for dewatering excess activated sludge is that the dosage of PAC is 200 mg ˙ L-1, the dosage of CPAM is 25 mg ˙ L-1, the operating temperature is 40 ℃, and the initial pH value of sludge is 8 ˙ 0, the quick stirring strength is 170 R ˙ Min-1, the fast stirring time is 60 s, and the slow stirring intensity is 50 R ˙ Min-1, slow stirring time is 8 min. Under this condition, many experiments were carried out, and the moisture content of mud cake was lower than 81 ˙ 63%。

Conclusion

(5) The sludge in the excess activated sludge thickener is affected by sedimentation time, seasonal variation, heating and other factors, and its properties will change accordingly; There are some differences between the field conditions of pilot test and indoor experimental conditions. Therefore, the optimized conditions can only provide reference for the harmless treatment of residual activated sludge, and the specific application should be debugged and determined on site.