Wastewater Recycling Papermaking Wastewater Treatment Technology Research II

2.2 Biochemical treatment

Due to the poor biodegradability of high-molecular-weight COD substances, it is not appropriate to directly biochemically treat waste papermaking wastewater without pretreatment. The effect of biochemical removal of COD varies with the BOD/COD ratio of the wastewater. At present, several indicators have been proposed to determine whether biochemical treatment can be performed. For example, if the BOD/COD is greater than 0.6, the biochemical treatment can be performed; if the BOD/COD is less than 0.2, biochemical treatment cannot be performed. The data pointed out that for waste papermaking wastewater, BOD/COD = 0.4 to 0.7 is suitable for biochemical treatment.

Experiments show that the waste paper paper wastewater after coagulation and sedimentation treatment, the BOD/CODCr are almost within 0.4 to 0.7, suitable for biochemical treatment. At the same time, fine inorganic particles (such as pigments, fillers, etc.) that can reduce the activity of biochemical sludge can also be removed through coagulation and sedimentation treatment. Therefore, it is necessary that the waste papermaking wastewater is subjected to coagulation and sedimentation treatment (ie, the so-called primary treatment) before biochemical treatment, and the biochemical treatment can only be used as an advanced treatment (secondary treatment) process of wastewater.

In the biochemical treatment, the organic matter (BOD component) in the wastewater is removed by the following mechanism:

(1) Oxidation of organic substances; (2) Formation and growth of microbial cells in activated sludge, ie, proliferation of activated sludge; (3) Cytoplasmic oxidation of activated sludge microorganisms, that is, self-decomposition of activated sludge.

Therefore, in the biochemical treatment, the activity and oxygen supply capacity of the domesticated activated sludge cultured are important factors that determine the treatment effect.

The activated sludge used for the treatment of waste papermaking wastewater should be cultured and acclimated. The activated sludge cultured under suitable conditions can have good activity and sedimentation performance. The study showed that the COD removal rate was only 28.2% and the removal rate of COD was 68.9% when the uncontaminated sludge was used to treat the coagulation and sedimentation water. However, the BOD removal rate could be reached by treating the wastewater with domesticated sludge. 76.1%, COD removal rate of 81.2% [8].

There are many ways to provide oxygen for biochemical treatment. For example, the activated sludge method uses the method of adding air or oxygen to waste water (ie, aeration) to provide the required oxygen for the system; and the biorotary disk uses the rotation of the tray and the wastewater. In contact with air, the microorganisms and organic substances adsorbed on the biomembrane of the disk are brought into contact with the air sufficiently to obtain the required oxygen. When the nature of the treated wastewater is certain, the oxygen supply capacity of the device can be improved by increasing the contact area between the gas and liquid phases, the contact time, and the concentration difference of oxygen between the gas and liquid phases. Due to the large amount of waste water generated by the waste paper recycling papermaking, the cost of using oxygen-enriched oxidant is relatively high, so the air-through method is generally adopted. For this reason, when designing the oxygen supply device, it is important to consider the good distribution of air and the improvement of the stirring ability (or the increase of the gas-liquid contact time) [9]. The process flow of activated sludge treatment of waste paper papermaking wastewater is shown in Figure 2.

Fig. 2 Schematic diagram of the process flow of waste paper papermaking wastewater treatment with activated sludge

Improve the removal rate of COD and BOD in wastewater and make the effluent meet the emission standards. However, the biochemical method has problems such as large area and high capital investment costs, which limits the application of this method. Therefore, the development of other more simple and easy-to-handle processes is more practical for enterprises (especially SMEs).

2.3 Chemical treatment

Some of the soluble contaminants in the wastewater can be converted into a form easily separated from the water by a chemical redox process, and then removed from the water by a conventional treatment process (eg, coagulation precipitation, adsorption, etc.); Or convert it into a harmless new substance (such as CO2) to achieve the purpose of removing COD and BOD.

The common practice is to combine chemical treatment with coagulation precipitation. The chemical treatment agent (ie, oxidant) may be first added to the wastewater for pre-oxidation and then subjected to coagulation sedimentation; it may also be added simultaneously with the coagulant to produce a synergistic effect between the coagulant and the oxidant; A certain treatment stage after coagulation and sedimentation is added to perform advanced treatment of wastewater [10].

The oxidants used in the chemical treatment methods include potassium permanganate, sodium hypochlorite, chlorine dioxide, Fonton's reagent, and the like. For different types of wastewater, there are significant differences in the effects of using different oxidants [11–13]. The author has studied the effect of potassium permanganate, sodium hypochlorite, and Fonton reagents on waste paper papermaking wastewater treatment. The results show that potassium permanganate is a good pretreatment agent, and sodium hypochlorite is a good advanced treatment agent.

Using potassium permanganate preoxidation method to treat waste paper papermaking wastewater has the advantages of simple process, convenient operation, low investment and good effect. The process flow is exactly the same as that of the coagulation sedimentation method. Only by adding an appropriate amount of potassium permanganate solution to the adjustment tank, and maintaining a certain reaction time, the coagulation and sedimentation operation can be performed. The waste paper papermaking wastewater of a factory was tested and a satisfactory result was obtained. The results are shown in Table 1. The cost of potassium permanganate used for treating 1t of wastewater is about 0.08 to 0.10 yuan.

Project SS/
mg.L-1 color
/ times CODCr/
mg.L-1CODCr
Removal rate
/%BOD5/
mg.L-1BOD5
Removal rate
/% Original waste water 615400609.0-252 - Coagulation and sedimentation treatment 3.52169.272.272.971.1 KMnO4 pre-oxidation treatment 6265.389.325.989.7 NaClO
Advanced treatment 3.8472.888.0 -
Table 1 Results of Papermaking Wastewater from Chemical Waste Treatment (To Be Continued)