Overview

Tanning is one of the oldest industries in the world. During ancient times, tanning activities were organized to meet the local demands of leather footwear, drums and musical instruments. With the growth of population, the increasing requirement of leather and its products led to the establishment of large commercial tanneries. Two methods are adopted for tanning of raw hide/skin, vegetable tanning, and chrome tanning.

Tanneries are typically characterized as pollution intensive industrial complexes which generate widely varying, high-strength wastewaters. Nearly 30 m³ of wastewater is generated during processing of one tonne of raw skin/hide[1]. Variability of tannery wastewaters are not only from the fill and draw type operation associated with tanning processes, but also from the different procedures used for hide preparation, tanning and finishing. These procedures are dictated by the kind of raw hides employed and the required characteristics of the finished product. The tanning industry also has one of the highest toxic intensity per unit of output. During tanning processes, at least about 300 kg chemicals are added per ton of hides[2]. Tannery effluent is among one of the most hazardous pollutants of industry. Major problems are caused by wastewater containing heavy metals, toxic chemicals, chloride, lime with high dissolved and suspended salts, and other pollutants.

Tannery wastewater characteristics

Tannery wastewater is characterized mainly by measurements of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), suspended solids (SS) and Total Dissolved Solids (TDS), chromium and sulfides, etc. Typical characteristics of tannery wastewater are given in Table 1 and they vary considerably from tannery to tannery depending upon the size of the tannery, chemicals used for a specific process, amount of water used, and type of final product produced by a tannery.

Table 1. Physicochemical characteristics of tannery wastewater

Wastewater Treatment

Screening, equalization, and coagulation/flocculation followed by an extended aeration treatment process is the general treatment scheme for tannery wastewater. The conventional biological treatment systems in use for tannery wastewater are either inadequate or less cost-effective due to the large variations in tanning practices and the types of chemicals used in the process. Sequencing batch reactor (SBR) technology is advantageous compared to the extended aeration process due to the higher COD and nitrogen removal rates at comparatively shorter HRT[3].

Mechanical treatment

Usually the first treatment of the raw effluent is the mechanical treatment that includes screening to remove coarse material. Up to 30-40% of gross suspended solids in the raw waste stream can be removed by properly designed screens. Mechanical treatment may also include skimming of fats, grease, oils, and gravity settling.

Coagulation/Flocculation

Due to the inherent characteristics of tannery effluents, various physicochemical techniques have been studied for the applicability to the treatment of tannery wastewater. Tannery effluents are generally treated biologically for the removal of biodegradable organics before discharge to receiving waters. Coagulation–flocculation is one of the most important physicochemical treatment steps in industrial wastewater treatment to reduce the suspended and colloidal materials responsible for turbidity of the wastewater and also for the reduction of organic matters which contributes to the BOD and COD content of the wastewater. The application of coagulation includes removal of dissolved chemical species and turbidity from water via addition of conventional chemical-based coagulants, namely, alum (AlCl₃), ferric chloride (FeCl₃), and poly aluminium chloride (PAC).

Post-purification, sedimentation and sludge handling

Post-purification, sedimentation and sludge handling are the last steps in wastewater treatment. With sedimentation, the sludge in 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 can be achieved, whereas belt presses produce a sludge cake with up to 20-25% dry solids. Centrifuges achieve up to 25-45% dry solids and thermal treatment up to 90% dry solids. Energy is an important factor in these processes.

Sequencing Batch Reactor (SBR) Technology for the treatment of tannery wastewater

BIO-BATCH SBR Tank- Morley, Alberta – Design Flow 375 m3 per day

Tannery productive cycle includes a series of chemical treatments using a large number of chemicals to transform animal hide/skin into an unalterable and imputrescible product. The inclusion of such a wide range of chemicals has rendered the biological treatment of tannery wastewater difficult and more complicated due to their low biodegradability. The conventional biological treatment systems in use for tannery wastewater are either inadequate or less cost-effective due to the large variations in tanning practices and the kinds of chemicals used in the process. It has been well documented that sequencing batch reactor (SBR) system is an attractive option due to its greater flexibility and cost effectiveness[4]. SBR can obtain higher nitrogen and organic carbon removal efficiencies than conventional systems. Cyclic concentration gradients, to which biomass is exposed in a SBR reactor permit selection and enrichment of particular microbial species more capable of carrying out biological processes such as nitrification and denitrification in the presence of inhibiting substances.

A study conducted by the Environmental Engineering Department at Istanbul Technical University, concluded that SBR technology is well suited to tannery wastewater for effective COD and N removal. It lowers soluble COD to a level essentially consisting of initial soluble inert COD and additional residual COD generated as metabolic products. It offers the flexibility of adjusting the degree of N removal by appropriate manipulation of the operating parameters. Compared to continuous-flow activated sludge, the V₀/V_F ratio is the essential additional parameter for this purpose.

Napier-Reid is a leading water and wastewater treatment solutions company with decades of experience in the design and installation of SBR technology plants. Its BIO-BATCH™ SBR technology has been implemented in several water treatment plants around the world. In one of their latest applications, a BIO-BATCH™ SBR plant was designed and manufactured for a tannery in Louisiana, United States. You can review the case study at: http://napier-reid.com/?p=1142

Recovery and reuse of water from treated tannery wastewater

Due to the high conductivity of the global wastewater from a tannery industry, wastewater reuse is only possible if a reverse osmosis (RO) process is implemented in the wastewater treatment. The supernatant of a physicochemical treatment is still very polluted, containing high COD values between 3000 mg/l and 4000 mg/l, and conductivities of nearly 20 ms/cm. Studies have shown that membrane filtration systems can successfully be applied for recovery of water from secondary treated tannery effluent, provided a suitable and effective pre-treatment system prior to membrane system is employed. Combining nano and RO membranes improved the life of the membranes and permeate recovery rate. Total dissolved solids (TDS) removal efficiency of more than 98% and permeate recovery of about 78% have been obtained with the use of polyamide RO membrane[1]. The water recovered from the membrane system, with very low TDS concentration, can be reused for wet finishing process in the tanneries.

 Reverse Osmosis Filtration System by Napier-Reid Ltd.

References

[1] Suthanthararajan, E. Ravindranath, K. Chits, B. Umamaheswari, T. Ramesh, S. Rajamam. 2004. Membrane application for recovery and reuse of water from treated tannery wastewater.  p. 151-156 R.

[2] Verheijen, L.A.H.M., D. Weirsema, L.W. Hwshoffpol and J. Dewit, 1996. Live Stock and the Environment: Finding a Balance Management of Waste from Animal Product Processing. International Agriculture Centre, Wageningen, The Netherlands.

[3] Ganesh, R., Balaji, G., and Ramanujam, RA., (2006), Biodegradation of tannery wastewater using sequencing batch reactor—respirometric assessment, Bio resource technology, 97, pp 1815–21.

[4] Ketchum Jr., L.H., 1997. Design and physical features of sequencing batch reactors. Water Sci. Technol. 35 (1), 11–18.