Treatment of Tannery Wastewater

Tannery wastewaters are highly complex and its treatment is administered by physical or chemical or biological or combination of those methods. From the review it are often concluded that physical/chemical processes combined with organic process is that the better option for the treatment of tannery wastewater.

Tanning industry is one among the oldest industries of the planet and therefore the problem of treatment and disposal of those wastes is perhaps as old because the industry itself. Tanneries wastewater effluent is treated in many various ways. There are situations during which a private tannery applies all the below-described wastewater treatment steps on site. In other situations a private tannery may apply (on site) only pre-treatment or a neighbourhood pre-treatment or no treatment in the least , sending the effluent to a centralised effluent treatment plant. Nevertheless, a treatment is important thanks to the wide selection of toxic effects on the environment caused by untreated tannery effluents and sludges. the subsequent treatment steps are necessary and can be described in additional detail afterwards:

1.      Mechanical treatment
2.      Effluent treatment
3.      Post-purification, sedimentation and sludge handling

Mechanical treatment:

Usually the primary treatment of the raw effluent is that the mechanical treatment that has screening to get rid of coarse material. Up to 30-40% of gross suspended solids within the raw waste stream.

can be removed by properly designed screens. Mechanical treatment can also include skimming of fats, grease, oils and gravity settling. After mechanical treatment, physico-chemical treatment is typically administered, which involves the chrome precipitation and sulphide treatment. Coagulation and flocculation also are a part of this treatment to get rid of a considerable percentage of the COD and SS. Effluent from tanneries after mechanical and physico-chemical treatment is usually easily biodegradable in standard aerobic biological treatment plants. the info in table 3 represent typical values for tannery wastewater treatment efficiency for conventional process liquors for production of finished leather from staple.

Effluent treatment

In order to hold out effluent treatment within the best manner, flow segregation is beneficial to permit preliminary treatment of concentrated wastewater streams, especially for sulphid- and chrome-containing liquors and although a discount of water consumption doesn't reduce the load of the many pollutants, concentrated effluents are often easier and more efficient to treat. Wherever segregation of flows is possible thorough systematic mixing of chrome-bearing effluents and other effluent streams improves the efficiency of the effluent treatment plant because the chromium tends to precipitate out with the protein during pre-treatment. The treatment of chrome-containing liquors is going to be discussed in chapter 4, so only the treatment of sulphide-containing effluent are going to be described here. it's common practice to stay sulphide-containing effluent from the beam house separate and at a high pH until the sulphide is treated, because at a pH less than 9 the concentration of toxic hydrogen sulphide (H2S) gas may occur. The sulphides within the deliming and pickle liquors can easily be oxidised within the drum by adding peroxide, sodium met bisulphite or sodium bisulphite. 

The allied emission level after treatment of sulphide is 2 mg/l during a random sample within the separate effluent. Where segregation of sulphide-bearing liquors isn't possible, the sulphides are generally removed by means of precipitation with iron (II) salts and aeration.

A disadvantage of this precipitation is that the generation of high volumes of sludge. the amount which will be achieved in treating the mixed effluent are – counting on the blending rate – 2 mg S2-/l and 1 mg Crtotal/l. (e.g. if 50% of the mixed effluent contains the sulphide-bearing effluent, emission levels for the entire effluent are going to be 1 mg S2-/l and 0.5 mg Crtotal/l). 

Post-purification, sedimentation and sludge handling

Post-purification, sedimentation and sludge handling are done in the end of wastewater treatment process. With sedimentation the sludge within the wastewater treatment plant is separated from the water phase by gravity settlement. After dewatering this sludge by means of filter presses, a sludge cake with up to 40% dry solids are often achieved, whereas belt presses produce a sludge cake with up to 20-25% dry solids. Centrifuges can be achieve up to 25-45% dry solids and thermal treatment up to 90% dry solids. Energy is a crucial think about these processes.