Best available technology for tannery effluent treatment

Introduction
The tannery segregates effluents from beamhouse and tanyard as well as from retanning and finishing operations. The effluents are then collected in sumps and fine screened to remove course solids. The sulphide bearing liquors from liming and the subsequent washes are collected and oxidised applying Jetox Venturi aeration. In the following steps all effluents are combined and mixed in an aerated balancing tank using submerged Jetox Venturi mixing. The balancing allows for a uniform flow to the flocculator of the dissolved air floatation, where aluminium sulphate and a polymer product are added to facilitate coagulation and flocculation of the solids. The solids are then floatated in the DAF unit and removed by an automatic scraper. The sludge is pumped to a sludge holding tank and then de-watered using a high-pressure belt press.
The afore-mentioned primary treatment removes an average of 95% of suspended solids (SS) and 70–80% of COD, which is a significant improvement on the usual 80-90% SS and 50-65% COD removal achieved with sedimentation.4

Effluent segregation and screening
Tannery effluents contain many types of pollutants, which are present in all forms from large solids through colloids to dissolved salts. The flow and composition of the effluent also varies considerably during the day and from the various stages of the tanning process.1
Pre-treatment is especially important in the treatment of tannery wastewater to remove coarse solids and to equalise flow variations in order to protect and optimise the subsequent processes. Particles, which can be easily abstracted by physical and/or mechanical means, are usually removed from the liquid effluent at the earliest possible stage. This prevents problems of blockage and wear on pumps, pipework and other subsequent treatment equipment.

Sulphide oxidation
The catalytic oxidation by aeration of lime liquors is the most economical and widely used process for sulphide removal. The technique consists of aerating the spent lime liquors for 5 to 8 hours in the presence of a catalyst such as manganese sulphate. The sodium sulphide present in the spent unhairing/liming liquors is oxidised by the air oxygen into thiosulphate, and in smaller quantities into sulphate. The thiosulphate then decomposes into sulphur and sulphite.2
Depending on the length of the float used and the volume of wash and rinse waters, the lime liquors may contain between 2.5 to 8 g/l of sodium sulphide. After mixing and dilution with the rest of the tannery effluent, the sodium sulphide concentration is between 200–600 mg/l. Therefore, it is required to separately treat the unhairing-liming floats before they are mixed with other effluent streams. Common discharge limits to a municipal sewer may range from 0.5 to 10 mg/l (as S2–) for most locations, whereas typical sulphide levels in the untreated effluent is of the order of several hundreds. The sulphide oxidation system installed at Articulos de Piel los Favoritos achieves a reduction from 1,100 mg/l S2– down to 0.19 mg/l after the completion of 8 hours oxidation time. Usually 2–5 ppm of residual sulphides can be expected in similar plants.4
Consequently 0 ppm H2S was detected with a Draeger around the whole effluent treatment plant.

Balancing tank
It is necessary to balance effluents to improve the treatment efficiency and to avoid oversize treatment plants, which have to deal with peak effluent flows. In addition to flow balancing, the equalisation tank provides for neutralisation and precipitation. It is necessary to provide effective mixing and aeration to achieve equalisation, prevent anaerobic conditions and settle the suspended solids.

Primary treatment
Primary sedimentation is a physical and chemical separation, which leads to settlement of suspended solids and colloidal substances. Sedi­mentation occurs, when the velocity of effluent is reduced below the point at which it can transport the suspended solids matter. The suspended solids settle and can be removed as sludge.
The physical removal of the suspended solids and colloidal substances from the wastewater is enhanced by chemical conditioning of the wastewater. The effluent is pre-treated by dosing of a coagulant, such as alum or ferric salts, followed by polyelectrolyte flocculants, which aid the phase separation.3 The dosing of these chemical conditioners requires prior pH adjustment of the feed streams for optimum chemical dosage.

Floatation
Modern tannery effluent treatment plants apply mainly dissolved air flotation (DAF), which is the most efficient method of removing suspended solids. The main advantages are less space requirements and better performance compared with settlement techniques. Dissolved air floatation works on the reverse principle to sedimentation, employing fine air bubbles to lift suspended solids to the surface. This leads to the formation of a floating sludge layer, which is removed with a scraper. The main advantage is that fine solids such as hair and fibres or fats and proteins can also be very efficiently removed by flotation. Air is dissolved under pressure in a saturator with part of the recycled treated/effluent. When the pressure is subsequently relieved in the floatation vessel, small air bubbles rise to the surface, carrying the suspended solids. A scraping device periodically removes the surface sludge ‘blanket’.
The flotation process relies on coagulant and flocculent chemical conditioning of the feed stream, in order to enhance the solids separation process. A suitable coagulant is in-line dosed to the effluent, followed by pH adjustment and polymer addition. The suitable polymer is required for optimum phase separation, especially of colloidal solids. After chemical dosing the effluent flows into the flotation tank and is mixed with the rising air bubbles stream. The air-saturated stream is formed by pumping of the treated effluent into a pressurisation chamber along with air, which under pressure, dissolves in the water. The sudden release of pressure in the flotation tank causes the dissolved air to form ‘clouds’ of tiny air bubbles which come up to the surface carrying the suspended and colloidal solids with them to form a surface scum or sludge layer which is regularly scraped off.
The installed DAF system, achieves a significant reduction of COD and suspended solids with a removal of up to 90% of the COD, achieving an average of 1,004 mg/l (daily sampling in the first quarter 2009) in the effluent. A further advantage is that the floated sludge has a higher solids content, up to 10% dry material, which is much easier and cost effective to handle.

Sludge handling
The primary sludge after floatation has a dry matter content of 8-10 %, which is a considerable increase compared to sludge dryness of 2-3% achieved after settlement. The primary sludge is thickened in a sludge thickener and then de-watered using a belt-press to reduce volumes and disposal costs to landfill.

Results
The new primary effluent treatment plant at Articulos de Piel los Favoritos was started up in March 2008. The performance has been improved by testing various primary treatment chemicals and selection of the most suitable polymer. In the following, the addition of the selected coagulants and polymers has been adjusted and a reduction of the overall treatment chemicals has been achieved.
The implementation of the new primary treatment has significantly improved the effluent quality and has also reduced the sludge disposal volumes and costs. The plant is fully automatic and requires minimum supervision.

Conclusions
The installation of a self-cleaning screen with a slot size of 1 mm has been shown to efficiently remove coarse solids and to protect pumps and Jetox Venturi used during the subsequent treatment steps from wear and blockages.
The sulphide oxidation using blower assisted Jetox Venturi aeration has been shown to efficiently remove sulphides with an average concentration of 0.19 ppm (data from 2008/2009) achieved with the completion of the full 8-hour oxidation cycle.
The Jetox Venturi aeration and mixing in the balancing tank has shown to completely homogenise the varying pH and to prevent solids settlement.
The dissolved air floatation removes 70–80% of COD, discharging a clear and transparent effluent of average 1004 mg/l COD. According to benchmarking with similar primary plants a COD reduction of 55–75% can be achieved. The outstanding results of the new primary treatment plant at Articulos de Piel los Favoritos places this plant as one of the best performing primary treatment plants in the world.
The DAF treatment achieves a high primary sludge dryness of up to 10% and, therefore, considerably reduces the volumes of sludge for de-watering.

References:

1 ATV Regelwerk, Merkblatt ATV-DVWK-M 774 Abwasser aus lederherstellenden Betrieben, July 2001
2 M Bosnic et al. Pollutants in Tannery Effluent UNIDO Report, 20-21, 2003
3 Integrated Pollution Prevention and Control (IPPC), Reference document on Best Available Techniques for the Tanning of Hides and Skins, February 2003
4 Jakov Buljan, Benchmarking: Part 4 – Typical pollution values and performance in conventional tannery processing, World Leather, Vol 22, No 1, 39–40, Feb/March 2009