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To produce finished leather products, the tanning industry requires a huge amount of water and chemicals. Tannery wastewater contains many toxic substances that greatly affect organisms and the environment. Therefore, the treatment of tannery wastewater is always a top concern.

Leather manufacturing process

The process of producing a finished leather piece includes many different stages and each stage is also extremely complex and important. Below is a diagram of the leather production process commonly used at facilities and factories:

Diagram of the leather manufacturing process

          Looking at the leather production process, it can be seen that most of the stages in the production process are wet processes and generate wastewater with a lot of chemicals. It is estimated that to process one ton of fresh leather into finished leather requires about 80-100m3 of water.

Composition and pollution properties of tannery wastewater

Tannery wastewater has an extremely complex composition and properties because it undergoes many stages with many different chemicals used, especially chemicals from these waste streams, when mixed, can react together.

Tannery wastewater has extremely high concentrations of BOD, COD, TSS, Chloride, Chromium, etc. It needs to be treated before being discharged into the environment. Pollution parameters of tannery wastewater are shown in the following table:

Table of parameters of pollutants in tannery wastewater

Selecting tannery wastewater treatment technology

Tannery wastewater contains many chromium ions from the tanning process. On the other hand, chemicals of different waste streams, if mixed, will react with each other. This makes treating tannery wastewater difficult. Therefore, to make the treatment process easy, we have to separate the waste streams and treat them separately from the beginning. We will separate the waste stream containing chromium ions to treat chromium in the wastewater. After that, it is brought to a general biological treatment system to process parameters of BOD, TSS, etc.

Below is the technological line of tannery wastewater treatment:

Tannery wastewater treatment

Explanation of the technological diagram of tannery wastewater treatment

Waste stream classification

Tannery wastewater is classified into two types of wastewater: chromium-containing wastewater and wastewater from other processes.

Chromium-containing wastewater 

This waste stream will pass through the coarse trash rack to be removed of large objects and then enter the collection tank. After entering the collection tank, this waste stream is passed through the fine trash rack to be removed of impurities of smaller size and then moved into the reaction tank. In the reaction tank, we add Na2S, H2SO4 to bring Chromium to valence 3, then add NaOH to precipitate chromium hydroxide. It also raises the pH of the wastewater. After that, the mixture of wastewater and precipitates is transferred to a settling tank. In the settling tank, the precipitates settle and the wastewater flows out to the conditioning tank.

Wastewater of other processes

This waste stream will pass through the coarse trash rack to be removed of large objects and then enter the collection tank. After entering the collection tank, this waste stream is passed through the fine trash rack to be removed of impurities of smaller size and then moved into the flotation tank to be removed of grease and suspended solids that are difficult to settle. The wastewater is then transferred to the conditioning tank.

Conditioning tanks

This is the place where the two waste streams are concentrated after going through the isolation pretreatment process, which helps to stabilize the concentration and flow. Here, the pH of the wastewater is adjusted with NaOH and H2SO4 to create optimal conditions for the process of coagulation-flocculation after that. In the conditioning tank, an air blower is installed to avoid sedimentation and anaerobic decomposition.

Coagulation – flocculation tank 

The wastewater is passed through the coagulation-flocculation tank. The tank is filled with chemicals to help the colloidal particles in the wastewater stick together to form larger and heavier flocs.

 Settling tank 

The mixture of water and floc from the Coagulation – flocculation tank flows into the settling tank. Here the flocs will settle to the bottom of the tank. The rest of the water after settling goes out and flows into the UASB tank.

UASB tank

Because wastewater has too high a concentration of BOD and COD, it needs to be treated by anaerobic biological methods using a UASB tank. Before conducting aerobic treatment, anaerobic digestion occurs in the UASB tank, which helps to break the chains of long-chain organic compounds, and at the same time reduces the amount of COD and BOD present in the wastewater to under 1000mg/l.

Aero tank

Wastewater after being treated in the UASB tank is transferred to the aero tank for treatment with aerobic microorganisms. Here the aeration process always takes place continuously to ensure the biodegradation of microorganisms is taking place with the most optimal conditions.

Settling tank 2

Wastewater after being processed in the aero tank is transferred to settling tank 2 to be removed of activated sludge. The clear water will be moved to the pressure filter tank. The sludge will be transferred to the sludge tank and periodically treated.

Pressure filter tank

After settling, the water is pumped to the pressure filter tank to remove residual solids. The filtered water is transferred to the disinfection tank.

Disinfection tank

The treated water will be disinfected with Chlorine to remove pathogenic bacteria and viruses and then discharged to the receiving source.

To be advised and supported on technology or construction issues related to tannery wastewater treatment, please contact Hasy Environment immediately at phone number 0972221608 or website https://xulymoitruong360.com. With a team of experienced staff and experimental machinery infrastructure, we are committed to providing you with the optimal processing model both technically and economically.