Process Description

This PTU consists of the following:

Oil and Grease Trap: The floating matters like fat and proteins presence in the effluent are removed.

Bar Screen Chamber: Major particles in the effluent are removed by the PVC/SS type rigid, non-corrosive, fixed bar screens with 5mm spacing provided.

Raw effluent Collection Tank: The screened effluent is collected in this tank before being pumped into the Overhead Sedimentation Tanks. Pre-Settlers: Two settlers of overhead type are provided for sedimentation of coarse solids in the effluent. These settlers have valves at the bottom for withdrawal of sludge to the Sludge Drying Beds/Filter Press.

Final Collection Tank: The overflow from these Sedimentation Tanks is collected in this tank before being pumped to the CETP Sewer Network through a Flow Metering System.

Sludge Drying Beds: Sludge from the sedimentation tanks is discharged into the drying beds with filter media for solar drying. The dried sludge is transported to the Secure Landfill (SLF) of the CETP. Filter Press: Wherever Filter Press is installed, the sludge from the sedimentation tanks is fed to the same for quick drying of sludge with less moisture content for disposing in to the SLF.

Flow Metering System: A skid mounted unit comprising of an Electromagnetic Flow Meter with flow Indicator and Totalizer with a pump is installed in each tannery. The effluent from the Collection Tank is pumped through the flow meter to the CETP Sewer Network. The quantity of effluent discharged is recorded in the flow meter. The flow metering system is housed in a sealed close room with opening to switch ON/OFF of the pump. The room key is in the custody of the CETP.

Chrome effluent stream is segregated at source to process in chrome recovery and reuse. Tanneries doing Chrome Tanning Process have installed Chrome Recovery Units in their premises. These tanneries segregate the chrome liquor and collect in a tank for precipitation of the chrome by adding magnesium oxide solution. The precipitated chrome slurry is added with sulfuric acid to regenerate chrome and filled in carboys. The chrome is recovered and mixed with fresh basic chromium sulphate for re-use in the tanning process.

The CETP has installed Zero Liquid Discharge (ZLD) System with partial assistance from Govt. of India and Govt. of Tamilnadu under the Up-gradation and Installation of Infrastructure for Environmental Protection in the tanning industry of the Department of Industrial Policy & Promotion (DIPP), Ministry of Commerce and Industry (MoC&I) under the ILDP scheme at a cost of Rs.460 million of 11th Five Year Plan.

The ZLD project was implemented by Chennai Environmental Management Company of Tanners (CEMCOT) as SPV. Under this scheme new equipment like Extended Diffused Aeration System, Reactivated Clarifier, Reverse Osmosis Plant, Multiple Effect Evaporator with Boiler, Lab and Administrative Building, Salt Storage Yard, Replacement of Raw Effluent Conveyance System and Recovered Water Conveyance System.

The effluent from different tanneries flow through a Mechanized Rake Bar Screen to the Receiving Sump, wherein 1No of Submersible mixers is installed for complete mixing before it is transferred to the pre-settler units for removal of easily settleable coarse solids. The overflow from this unit is passed through a rotary fine screen (which screens solids < 3 mm) before discharge into the Equalization Tank with 2 Nos of Aspirators and 3 Nos of Blowers for the coarse bubble mechanism are installed for homogenous mixing of the effluent.The effluent from the Equalization Tank is chemically treated with lime and alum in the flash mixer and polyelectrolyte is added in the flocculation to facilitate coagulation and flocculation of solids which is sedimented in the Primary Clarifier and conveyed to the Filter Press for dewatering the sludge. A pH of 8.0 to 8.2 is maintained in the Primary Clarifier to precipitate the chromium(Cr3+)present in the effluent as chromium hydroxide.

The above-mentioned primary treatment removes 65-70% and 40-45% of incoming suspended solids and BOD as well as precipitating most of the chromium. The overflow from the Clarifier is further treated in the Pre-aeration tank for removal of Hydrogen Sulphide using Liquid Oxygen with Mix flow system before entering in to an Extended Aeration Type Activated sludge process in Aeration tank I is with Diffused Aeration System and Aeration Tank -2 is with Original Hydrodynamic Reaction (OHR) a Japanese technology. Three blowers of 7000 m3/hr capacity of each have been installed to maintain a D.O between 1.5 to 2.0 mg/lit. The outflows from the Clarifiers have a BOD of around 30 mg/l. To further improve the effluent characteristics and colour, the effluent is passed through Pressure Sand Filters after the addition of Alum and polyelectrolyte in the flash mixer and flocculation in the flocculator.

The treated effluent reaching the RO system is first subjected to softening through lime-soda process in a reactivated clarifier. The softened effluent is further filtered in the dual media filters and disc filters imported from Israel and very fine filtration is obtained in an ultrafiltration system employing hollow fibre UF membranes. The ultra-filtered effluent is then polished in organic scavenger before it is desalinated in two stages Reverse Osmosis unit. The entire UF and RO skids are made in USA and is automatic, controlled through SCADA. The UF-RO skid is imported from Singapore.

The reject from the RO are again subjected to third stage High Pressure RO to recover additional permeate and thereby reduce the volume of reject fed to the seven stage Multiple Effect Evaporator with four stages of falling film and three stages of forced circulation evaporation towers (calandrias). The fuel used for producing steam is firewood and biomass briquette. The salt-laden solid residue is separated out in a pusher centrifuge.

The permeate from RO system and the condensate from evaporator are combined and distributed back to the tanneries for use in manufacturing process through a recovered water conveyance system with three Over Head Tanks at different locations in the cluster and supply lines with electronic water meters.

The Sludge from the Primary, Secondary clarifiers and Reactivated clarifier are dewatered in five number of chamber filter presses. The back-washing effluent from Sand filters is discharged back to the Receiving Sump and Back washing from Dual media filters, OS and RO are recycled back to the Secondary Effluent Collection Tank. The dried sludge is then disposed of to the Secure Land Fill (SLF) system .

The salt-laden solid residue is stored in bags and a huge salt storage yard has been constructed for the purpose.

Efforts to re-use the salt in preservation of hide / skins and purification system is underway.

The CETP meets the Zero Liquid Discharge standards of Tamil Nadu Pollution Control Board (TNPCB), and the Electro Magnetic Flow Meters connected to the CARE AIR Centre of TNPCB at Chennai for real time online monitoring.