In water treatment, substances that can make colloidal particles in water bind and coalesce with each other are called coagulants.
Basic aluminum chloride (PAC) and polyacrylamide (PAM) are two widely used coagulants.
PTA wastewater is the wastewater produced in the process of producing PTA from p-xylene (PX). It has the characteristics of complex composition and strong alkaline. Biodegradation is difficult, and advanced treatment technology is required for re-discharge. Usually, a positively charged flocculant is added to neutralize the negative charge on the surface of the particulate matter in the sewage, thereby destabilizing it. Aggregation to achieve the effect of flocculation and sedimentation. In this paper, the effects of coagulant PAC and coagulant PAM dosage on the turbidity and COD of actual wastewater effluent were studied through experiments.
Experimental materials and methods
1.1 Experimental water samples
A sewage treatment plant in a new area in Jiangsu Province mainly treats PTA industrial sewage in the new area. The long-term treatment scale is 200,000 t/d, and the first-phase project has a treatment scale of 50,000 t/d. The process flow is shown in Figure 1. The PTA wastewater used in the experiment is the effluent of the secondary sedimentation tank unit of this process, and the average water turbidity from March to April is 4.72 NTU.
1.2 Instruments and Materials
Experimental equipment: 2100Q type scattered light portable turbidimeter; ZR4-6 type coagulation test mixer.
Experimental materials: flocculant polyaluminum chloride (industrial grade Al2O3 ≥ 28%), coagulation aid polyacrylamide (anion, industrial grade molecular weight ≥ 10 million)
1.3 Experimental method
At room temperature, the concentration of the fixed coagulant PAM solution was 1 mg/L. Under this condition, the flocculation and precipitation of the effluent from the air flotation unit were investigated when the flocculant PAC concentration was 15, 10, 8, 6, 4, 2 and 1 mg/L, respectively. Effect. Then, on the basis of determining the optimal concentration of the flocculant PAC solution, the flocculation and sedimentation effects of the flotation water were investigated when the concentration of the flocculant PAM solution was 0.2, 0..4, 0.6, 0.8, and 1.2 mg/L. Take 1 L of secondary sedimentation tank effluent, add PAC solutions of different concentrations, and after stirring for 5 minutes, add 0.2 g of PAM solution with a mass concentration of 0.5% (concentration 1 mg/L), stir for 10 minutes, and settle naturally for 30 minutes. Take the supernatant to test before and after treatment Turbidity, COD changes, so as to determine the appropriate dosing concentration.
1.4 Test method
CODcr adopts “Water and Wastewater Detection and Analysis Method”; turbidity is measured by turbidimeter.
Results and discussion
2.1 The effect of PAC concentration on the flocculation effect
It is known that the turbidity of the effluent from the secondary sedimentation tank is 4.72 NTU, the COD is 61 mg/L, and the fixed PAM concentration is 1 mg/L. The influence of the PAC concentration on the flocculation effect was investigated. It can be seen from Table 1 that it is more appropriate to add PAC at a concentration of 4-6 mg/L, the turbidity removal rate is >55%, and the COD removal rate is about 11%.
2.2 The effect of PAM concentration on the flocculation effect
Table 2 shows the flocculation effect when adding different concentrations of PAM solution at a fixed concentration of PAC (4 mg/L). It can be seen from Table 2 that when the concentration of PAM is 0.6~0.8mg/L, the treatment effect is better. This may be because the long chain of PAM with large molecular weight bridges between the flocs, which makes the flocs dense and makes the flocs denser. The turbidity of the supernatant was significantly reduced. Continue to reduce the PAM concentration, the turbidity and COD will increase accordingly.
1) The dosage of PAC is 4~6mg/L, the dosage of PAM is 0.6~0.8mg/L, and the corresponding effluent turbidity is less than 2NTU.
2) The small test experiment has guiding significance for the actual production control, and no abnormality is found in the water effluent index after half a year of operation.
3) In actual production operation, the dosage of PAC and PAM needs to be adjusted appropriately according to seasonal changes.