Conventional tannery wastewater treatment systems include physiochemical and biological treatment to reduce chromium, biochemical oxygen demand (BOD), chemical oxygen demand (COD) and suspended solids (SS). To tackle treated effluent with total dissolved solids (TDS) in the rage of 10,000–30,000mg/L, multiple-stage, high-pressure membrane units have been designed and implemented for water recovery. To reduce chemical use and sludge generation, membrane bioreactors (MBRs) have started to replace secondary clarifier and tertiary treatment units. Commercial-scale membrane systems have been implemented in many locations for capacities of 500–10,000m3 a day.
Every year, the Asian leather industry requires eight to ten million tons of hides and skins, which is more than half of the estimated global total. About 600 million cubic metres of wastewater is discharged from tanneries worldwide annually, more than half of which is generated in Asia.
Conventional physiochemical and biological treatment systems tend only to reduce BOD, COD, SS, heavy metals and so on, but not TDS and salinity, which are mainly the result of chlorides, hardness and sulphates.
The lack of organised sewage treatment plants with the requisite capacity in many locations in Asia, meanwhile, means there is limited scope for mixing treated tannery effluent with domestic sewage to achieve satisfactory TDS levels. Many polluting industries, including tanneries, are located in landlocked areas and there are constraints on discharging treated effluent containing high levels of TDS into the sea.
Meet the challenge
In addition to the removal of TDS in treated effluent, it is necessary to recover water for reuse to address water shortage. In many states in India, pollution control authorities insist that water recovery is integrated with a zero liquid discharge (ZLD) system, though this presents many technical challenges. Managing concentrated saline stream treatments by adopting energy-intensive evaporation systems, for example, is a major issue in landlocked areas.
The quality of industrial wastewater generated by tanneries means that conventional treatment plants are unable to meet the prescribed TDS level of 2,100mg/L in treated effluent. In addition to TDS management, controlling volatile solids in hazardous sludge is also becoming necessary.
In order to control sludge and recover good-quality water from waste, the required treatment steps are:
- chrome recovery and other process controls, including cleaner production
- conventional physiochemical and biological effluent treatment systems
- tertiary treatment systems including microfilter low-pressure membrane units
- single or multiple-stage reverse osmosis (RO).
How RO systems are used depends upon the TDS concentration in the feed water, the estimated percentage of quality water recovery and saline reject. A high-pressure seawater membrane is adopted for handling treated effluent with TDS concentrations higher than 10,000mg/L. The recovery rate can be as high as 70–90%, depending upon the feed-water TDS level, and the type and stages of membrane systems.
Benefits can include savings in chemical use in the tanning process and a reduction in pollution load in the effluent. The reject saline stream from RO systems needs to be managed by adopting the options of forced/thermal evaporation systems or by disposal into sea wherever feasible and under suitable control.
Membrane bioreactors (MBRs) have been adopted in many countries to remove residual BOD, suspended solids and so on from effluent. After being treated with MBR, water goes through RO in order to decrease salinity and reduce TDS levels to a drinkable 500mg/L. A common effluent treatment plant (CETP) in Spain with MBR and RO systems for water recovery was established in 2005. Recently, many CETPs in India have adopted MBR and other membrane systems for water recovery and reuse from tannery effluent.
Water is also becoming a scarce commodity in many locations in China. Expansion of industries with water consumption is only allowed where water recovery systems are present in effluent treatment plants. To recover water from the tannery wastewater, submerged MBR linked with activated biological treatment is provided in the first stage.
Then, an RO plant in a ‘Christmas tree’ configuration can produce about 70% permeate and 30% concentrate. The recovered water is drinkable.
Nanofiltration (NF) has been adopted for the removal of colour and salts such as sulphates from effluent that has already undergone ultrafiltration or MBR. NF membranes are operated under low pressure with a yield of about 90%. Adopting NF will improve the efficiency of RO in water recovery and decrease the volume of rejected saline.
Multiple-stage systems using thermal and electrical power evaporate reject saline from RO systems, but continuous operation of these creates technical issues, and capital and operational costs are high, which can make such systems unsustainable.
A novel technological development has been made to draw 30,000m3 of seawater a day at a desalination plant that is integrated with a major leather complex in southern India. About 10,000m3 of freshwater will be generated daily, and the remaining 20,000m3 will be discharged into the Bay of Bengal via a special biocontrol and dispersion system to safeguard aquatic life.
The leather complex will be using the freshwater generated by the desalination plant, and 9,000m3 of wastewater will be treated, then mixed with saline reject from the desalination plant before being stored in a watertight pond for about ten days before being discharged into the sea through a 5km pipeline in conjunction with a special sprinkling system. The combined treated saline stream will be discharged once a week under the overall control of environmental protection authorities.
With the support of many national institutions and other organisations, studies were carried out to assess the effluent’s effect on the ocean. The spreading of an effluent cloud released in a marine environment is governed by advection caused by large-scale water movements and diffusion caused by comparatively small, random and irregular movements without causing any net transport of water. Hence, the important physical properties governing the rate of dilution of an effluent cloud in coastal waters are bathymetry, tides, currents, circulation and stratification.
A five-port diffuser system with a 0.18m diameter is planned. A jet with a velocity of 2.5m a second will release effluents and reject-water from the proposed desalination plant. The environmental clearance and approval has been accorded by the Indian Government to this unique, integrated project, which is the first of its kind in India.
New horizons
Leather production activities – especially raw to semi-finishing processes – are shifting from developed nations such as the US to Asia, North Africa and Latin America. The major tanneries in leather-producing countries such as China, Italy, India and Russia must develop and adopt new environmental protection measures due to increasingly stringent regulations.
The sustainability of small-scale units is now a serious issue where environmental requirements are concerned. Investments are being made for environmental protection and resettling tanneries away from the urban areas to industrial parks that have CETPs. New regulations and restrictions, such as REACH, on the use of certain chemicals, salinity and water recovery under zero discharge guidelines, and the disposal or management of chrome-containing sludge require more research and development.
Innovative tanning processes that will reduce water and chemical use while minimising solid waste generation are needed, together with environmental planning and management.
