Water treatment in the Industrial sector
With a view to addressing the depletion of quality water resources, environmental challenges, control of total dissolved solids (TDS) and salinity, the sustainable development of a water recovery system has been developed and adopted in many tannery effluent treatment plants (ETPs) in India, China and other leather-producing countries.
The tannery wastewater is initially treated by adopting conventional physiochemical and biological effluent treatment systems to reduce hazardous chemicals, biochemical oxygen demand (BOD), chemical oxygen demand (COD) or suspended solids (SSs). Subsequently, high-pressure membrane systems associated with tertiary treatment units have been implemented for the recovery of water from tannery effluent and TDS management. A membrane bioreactor (MBR) replaces the secondary clarifier and sophisticated tertiary treatment units.
Commercial-scale treatment systems with special technical units have been implemented in many locations for capacities ranging 500-10,000m3/day with an investment of more than $200 million. Here, we detail recent developments on the environmental protection techniques in tannery wastewater treatment with a focus on water-recovery for reuse, salt recovery and marine disposal of saline reject with a proper biocontrol system.
Production and discharge estimates
Annual leather processes in Asian countries are estimated at eight to ten million tons of hides and skins, which is more than 50% of the estimated world leather production of about 16 million tons a year. Wastewater discharged from the world tannery sector is about 600 million cubic metres per annum. The tanneries in Asia discharge more than 350 million cubic metres of wastewater per annum.
The ground and surface water resources in many locations in and around tannery clusters contain high TDS and are not fit for domestic and industrial use. The conventional physiochemical and biological treatment systems are designed and implemented only to reduce BOD, COD, SSs or heavy metals, and not TDS and salinity, which are mainly contributed by chlorides, hardness and sulphates. Due to the inherent quality of wastewater from the tanning industry, the treatment plants are unable to meet the prescribed standards in terms of TDS and chlorides in salinity in the treated effluent.
There is not much scope in mixing the treated tannery effluent with domestic sewage to achieve the TDS level in many locations in Asia in the absence of organised sewage treatment plants of required capacity. Many polluting industries including tanneries are located in the landlocked areas and there are constraints to discharging the treated effluent with high TDS in the sea.
The TDS limit is being enforced in India and other parts of the world depending upon the final mode of disposal. In addition to the removal of TDS in the treated effluent, it is necessary to recover water for reuse to meet the challenge of water shortage. In many states in India, the pollution control authorities insist on water recovery integrated with a zero-liquid-discharge (ZLD) system.
Different types of units such as micro filter (MF), ultra filtration (UF), MBR, nano filtration (NF) and reverse osmosis (RO) have been developed for the recovery of water from saline groundwater, seawater and domestic/tannery wastewater with high TDS. However, the achievement of ZLD concept has got many technical challenges in addition to the application of various types of membrane systems.
Management of the concentrated saline stream treatment by adopting an energy-intensive evaporation system seems to be one of the major issues in landlocked areas. The marine disposal of saline reject from the membrane system with high TDS over and above 40,000mg/L requires special development and provisions to safeguard the aquatic life.
Technologies for treatment of salinity/TDS and water recovery
For the recovery of quality water from tannery wastewater, the required treatment steps are conventional physiochemical and biological effluent treatment systems to reduce BOD, COD and SSs, and tertiary treatment systems including, microfilter, low-pressure membrane units, such as UF, before the application of a single or multiple-stage RO system.
The number of stages and types of RO systems are based upon the TDS concentration in the feed water, estimated percentage of quality water recovery and reduction in volume of saline reject. A High-pressure seawater membrane is adopted for handling treated effluent with TDS concentration of more than 10,000mg/L. The quality water recovery rate could be achieved to the level of 70- 90% depending upon the feed water TDS level, type and stages of membrane system.
In addition to the recovery and reuse of quality water by the industry, the additional benefits are savings in chemical usage in the tanning process and reduction in pollution load in the effluent. The reject saline stream from the RO system needs to be managed by adopting the options of forced/thermal evaporation systems or disposal into the sea wherever feasible with suitable control.
Many full-scale membrane systems have been installed for the recovery of water from domestic and tannery wastewater with capacities ranging from 100 to 20,000m3 a day.
MBR integrated with RO system
MBR is a low-pressure membrane unit integrated with an aeration unit. It requires continuous recirculation through a backwash facility. An MBR system is developed using UF-type membranes with a high recirculation provision in the aeration unit along with biomass to maintain the required mixed liquor suspended solids (MLSSs). The MBR replaces the secondary clarifier and sophisticated tertiary treatment units prior to the RO system. The process flow diagram of MBR is given in the figure below.
The MBR system is commonly adopted in many countries to remove the residual BOD and suspended solids/coliform from the effluent. After treatment with MBR, the water is applied through an RO system for the removal of TDS and salinity to get drinkable-quality water with TDS less than 500mg/L.
A common effluent treatment plant (CETP) in Spain with MBR and an RO system for water recovery was established in 2005 and, recently, many CETPs in India have adopted MBR and other membrane systems for water recovery and reuse from the tannery effluent. After MBR/UF treatment, the SSs and BOD values in the effluent are below detectable level and taken for treatment with RO system for recovery of water after the removal of TDS and salinity.
In China, water is also becoming a scarce commodity in many locations. The expansion of high-water-consuming industries is allowed only if they are provided with a water recovery system in the effluent treatment plants. To recover water from the tannery wastewater, a submerged MBR linked with activated biological treatment is provided in the first stage. Following the MBR system, an RO plant in a ‘Christmas tree’ configuration has been installed and operated at 12-16 bars. The RO plant produces about 70% permeate and 30% concentrate, and the quality of the recovered water meets the drinking water standards. The saline water concentrate stream is further treated with Fenton process before disposal.
Nanofiltration (NF) is adopted for the removal of colour and salts such as sulphates from the treated effluent after the UF or MBR stage. NF membranes are operated under low pressure with a high yield of about 90%. Adopting NF will improve the efficiency of RO in water recovery and decrease the volume of saline reject.
Techno-economical aspects of residual saline streams
The disposal of the concentrated saline stream from the membrane system is one of the important issues to be addressed in landlocked areas. The conventional method for disposal of the saline stream is to adopt open solar evaporation pans. The average evaporation rate in solar pans is 4-6mm depending on the location. These evaporation pans occupy a large land area.
The sprinkling system, linked with solar warming developed under the UNIDO-assisted programme, improves the evaporation rate by 300%. However, the spreading of aero salt from the sprinklers is one of the limiting factors. To overcome the problems, a forced evaporation system in a closed unit is designed, developed and implemented as a demonstration unit. This system improves the evaporation rate of the saline stream by about 800%. A condensed saline stream is discharged in the solar pan and the dried salt is collected periodically.
Multistage evaporators using thermal and electrical power have been installed for the evaporation of the reject saline stream from the RO system. However, there are many technical issues such as constrains on continuous operation of the system, meeting the required quality of the condensate water from the evaporator for reuse, management/use of the recovered salt with impurities. The capital and operational costs are also high. Further techno-economical review and modified options are required on the sustainability of the system – particularly in landlocked areas.
Novel marine disposal of treated saline streams
A novel technological development has been made for 30,000m3 a day from the nearby sea for the desalination plant integrated with a major leather complex in South India. Out of the total water quantity, freshwater of about 10,000m3 a day will be generated and the remaining 20,000m3 a day will be discharged into the Bay of Bengal with a special biocontrol and dispersion system to safeguard the aquatic life.
The leather complex will be using the freshwater generated by the desalination plant for its process requirements and 9,000m3 a day wastewater will be treated, mixed with saline reject of the desalination plant, stored in a water tight pond for a capacity of about ten days, and discharged into the sea by laying 5km of pipeline using high-pressure HDPE pipe and a special sprinkling system. The combined treated saline stream, with a quantity of about 29,000m3 a day, will be discharged once a week under the overall control of environmental protection authorities.
With the support of many national institutes and other organisations, model studies were carried out in finalising the novel marine outfall. 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-scale 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 systems with a 0.18m diameter is planned with a jet velocity of 2.5m a second, for the release of treated effluents and reject water from the proposed desalination plant.
The environmental clearance (EC) has been accorded to this unique integrated project with water recovery using a desalination process, tannery wastewater treatment, and novel and safe saline reject disposal into the sea without affecting the marine life, which is first of its kind in India.
The leather production activities, especially raw to semi-finishing processes, are being shifted from developed nations such as the US and western European countries to Asian, North African and Latin American countries. The major tanneries in leather producing countries such as China, Italy and India have to develop and adopt new environmental protection measures such as an adoption membrane system and water recovery due to the enforcement of stringent environmental regulations.
The sustainability of the small-scale units is becoming a serious issue to meet the environmental requirements, and major investments are being made for environmental protection and resettlement of tanneries from the urban areas to the industrial parks with CETPs. New regulations and restrictions, such as REACH, on the use of certain chemicals, salinity and water recovery under the zero-discharge concept, disposal/management of chrome containing sludge envisage continued research and development. Innovative tanning processes that will greatly reduce the water and chemical use, and minimise solid waste generation are needed together with overall environmental planning and management.