5.1 Water conservation and recycling - Gujarat State fertilizer corporation, India

Introduction

Gujarat State Fertilizer Corporation (GSFC) is one of the largest integrated fertilizer and petrochemical complexes in India, producing a variety of fertilizers, intermediates and petrochemical products. GSFC has adopted an integrated approach to conserving water. This strategy has brought multiple benefits to the operations of company. By recycling their effluent streams, GSFC substitutes recycled water for raw water in their water stream, resulting not only in water conservation and cost savings, but also in the recovery of chemicals previously discharged in the process and an higher level of water pollution control compliance. Water consumption has been maintained at a low level, despite the expansion of the plant and increased production levels.

The raw water supply to GSFC is met from two sources:

· From a joint water supply scheme with Gujarat Refinery, using French-type, radial collection wells situated in the bed of the Mahi River, which provides up to 36 370 m3/d.

· From GSFC-owned French-type, radial collection wells, situated in the bed of the Mahi River at Parthampura, which supply up to 45 460 m3/d.

The actual throughput of these wells is dependent on groundwater levels. During drought periods and in the summer months, the groundwater levels drop, limiting the throughput of the wells. Nevertheless, the primary source of water supply to the GSFC operations is the jointly-operated Gujarat Refinery well and balance is met from the GSFC-Parthampur installation. The total daily water requirement of the GSFC operation prior to the installation of the water conservation and recycling practices as about 45 000 m3/d. Specifically, the water requirement of the GSFC plant, after Phase I, II and II expansions in 1977, was:

With the implementation of the integrated approach to water conservation and recycling of effluents, the present water requirement of the GSFC complex is 40 000 m3/d. Use of these technologies has helped to maintain water demands at GSFC at a low level, despite an increased level or production and an increased number of operating divisions (Table 20).

TABLE 20. Water Consumption in the Industry (GSFC).

Technical Description

GSFC opted for an integrated approach to water conservation and recycling based upon the philosophy that conserving water conserves all resources associated with the water. Conservation of steam, condensate, demineralised water, and process water leads to the conservation of water with maximum returns. For example, within a network of plants, it was possible to recycle waste stream from one plant to another plant. As a practical result of this recycling philosophy, the phosphatic group of plants achieved the total recycling of its effluent, conserving water, recovering previously lost product and controlling pollution. Similar strategies were adopted in the ammonia/urea group of plants. Some of the actions taken to conserve water are elaborated in the following sections.

· Recycling acidic effluents in chalk ponds

Chalk is a by-product produced by the ammonium sulphate plant. Chalk slurry is pumped to chalk ponds where it is mixed with highly acidic, phosphoric acid contaminated return flows. The acidic effluent is neutralized by chalk slurry and the chalk floc settles in the pond. Two chalk ponds have been sealed with polyethylene linings on their bottoms to minimise water percolation. After a period of operation, the ponds fill with chalk and must be emptied; hence, the requirement for two ponds to ensure continuous operation of the plant. During the time when the first pond is off-line and being emptied, the second, empty chalk pond is filled with water to bring it on line. Annually, 170 000 m³ to 180 000 m³ of chalk is reclaimed using this process and an equivalent amount of water is consumed in filling the chalk ponds before they are commissioned. Cooling water can be used to meet this initial water requirement. Alternatively, effluents from ammonia/urea, melamine, and caprolactum plants can be used after treatment to strip the ammoniacal nitrogen. These effluents are collected in a central collection pond, pumped to the polyethylene-lined ammoniacal effluent lagoon, and treated in an Air Stripping Tower to remove ammoniacal nitrogen, before being discharged to the chalk pond or disposed. In normal plant operations, this reclaimed water is also used as make up water for the chalk ponds.

· Recycling barometric condenser water as cooling water

In the evaporation section of the ammonium sulphate plant, there is a surface condenser followed by two barometric condensers, for vacuum generation. The gases, after coalescing in the surface condenser, are condensed in the barometric condenser through direct contact with the cooling water. The barometric condensers use cooling water at a rate of 135 m³/hr. Rather than discharge this cooling water, as was previously the case, the barometric condenser water is now segregated from the main effluent disposal grid and is pumped back to cooling tower of ammonium sulphate plant, recycling 135 m³/hr of cooling water.

· Recycling contaminated condensate for chalk repulping

Process water condensate, generated in the evaporation process employed in the ammonium sulphate plant, is contaminated and cannot be directly reused. Thus, in excess of 30 m³/hr of process condensate had been historically discharged as effluent. However, water was required elsewhere in the ammonium sulphate plant to repulp chalk after it had been used in the filtration of ammonium sulphate plant liquor. The filter cloth must also be washed with water at the same time. In order to conserve water, a system was designed to substitute process effluent, mainly process water condensate, for the non-recycled cooling water that had been previously used for this purpose. All contaminated process water condensates from different sources with GSFC are collected in a central collection pit, and pumped to the chalk filter to wash the filter cloth and to repulp the chalk. The resultant chalk slurry is pumped to the chalk pond for settling and neutralization of acidic wastewaters as described above.

It should be noted that, as this repulping and washing stage is one of the most critical in the entire operation, automated safeguards were provided to ensure that the process remained unaffected in case of any problems being experienced with the effluent recycling system. However, the recycling system is working well, with two ammonium sulphate plants being successfully operated with total recycling of the effluents to the chalk pond. This has provided significant savings in cooling water requirements, enhanced recovery of ammonium sulphate, and increased the level of water pollution control achieved.

· Recovery of pure condensate as brine-free water (BFW)

In the ammonium sulphate plant, a 40% ammonium salt solution is evaporated to produce ammonium sulphate crystals. Steam is used as the heating medium to evaporate the saline solution. Condensate from both parts of the process, previously lost as waste, is now captured in the main condensate grid as part of the process design. The purity of the condensate is analysed, with the pure condensate being directed into a separate circulation system and pumped to the steam generation plant to be used as brine-free water. Quality safeguards and process safeguards are provided so that process is not affected by any malfunction of the operational control systems.

· Recycling phosphoric acid plant effluent from the chalk ponds

In the phosphoric acid plant, water from the chalk ponds, described above, is utilized in the fume scrubbers, condensers, and flash cooling systems, and in other, miscellaneous services. The acidic return flows from the plant are pumped back to the chalk ponds for neutralization, settling and natural cooling. The cooled chalk pond water is returned to the phosphoric acid plant and remains in circulation until the build up of dissolved solids (TDS) in recirculating water begins to impair its effectiveness as a coolant, at which point, the chalk pond water is bled off to control the TDS build up. The water lost through this bleed off is subsequently made up by the addition of new cooling water. The high TDS water is discharged as effluent.

Also in the phosphoric acid plant, process water was used for gypsum repulping and washing of the cloth pan/belt filter. The cloth filter captures the gypsum cake and conveys it to a discharge point, after which the cloth is washed by a number of spray nozzles located on both sides of the belt. To conserve water, the same wash water is used for repulping the gypsum cake and conveying the gypsum slurry to the drum filters where it is further purified. As a further conservation measure, chalk pond water is substituted for process water throughout the process. Chalk pond water is mixed with condensate for use in filter cloth washing and gypsum repulping.

As a further benefit of this recycling scheme, about 2 500 metric tonnes of ammonium sulphate is recovered annually from the chalk pond. The chalk pond water contains 1.5% to 2% ammonium sulphate which is recovered in the phosphoric acid process in form of diammonium phosphate (DAP). The recovery of ammonium in the form of DAP has had a tremendous impact on profitability, and has clearly demonstrated the benefit of the integrated effluent recycling, recovery and pollution control programme.

· Recycling chalk pond water in the grinding mill dust scrubber

In the phosphoric acid plant grinding mill, there is dust scrubber to recover rock phosphate dust downstream of the product cyclones. This is a wet scrubber, which used process water at a rate of 95 m³/hr. The resultant slurry was recycled to the phosphoric acid digester. As part of the water conservation programme, chalk pond water was substituted in place of the process water.

· Recycling process water condensate in the gypsum purification section

In the gypsum purification section of the phosphoric acid plant, gypsum cake was collected on a drum filter and washed off the filter using hot water jets. Process water, heated with live steam, was used for this purpose at a rate of 5 m³/hr. As part of the integrated water conservation programme, hot condensate was used in this process.

· Demineralised water conversion in barometric condenser cooling tower

In the urea plants, cooling water is used in the crystallization section of the barometric condensers for vacuum generation. The barometric condensers are cooled in the barometric condenser cooling tower. The cooling water used in this process is contaminated with ammonia and urea due to its closed loop circulation and to process upsets which result in carry overs of ammonia and urea. Hence, it is necessary to make up the cooling water supply by bleeding the contaminated cooling water from the cooling tower to maintain water quality. This created a continuous flow of liquid ammoniacal effluent from the bleed of the cooling tower, and the subsequent consumption of cooling water supplies. As part of the water conservation programme, the cooling towers were converted to a demineralized water circulation and cooling water system in which the cooling towers were isolated. After demineralized water circulation, excess water from this cooling tower was substituted for demineralized water used in the plant.

· Recycling of effluent in the diammonium phosphate plant

In the diammonium phosphate plant, pre-neutralizer temperature control is accomplished with the addition of process water. Temperature control is quite critical for plant operation. As part of the water conservation programme, water from washings and leakages, etc., was collected in a pit and substituted for process water in the temperature control function. As in other critical systems, an automated temperature control system was retained in the process so that the process is not affected by problems with the effluent circulation system.

· Renovating water at the sewage treatment plant

GSFC Township houses about 1 700 families in a setting that has vast areas of open land with lawns, plants and recreational facilities. Sewage water from the Township was pumped into the main effluent grid and discharged with other plant effluents at a rate of 230 m³/hr. As part of the water conservation programme, an activated sludge sewage treatment plant has been installed and commissioned recently, which reclaims about 135 m³/hr of treated effluent as cooling water and irrigation water for the Corporation's experimental farm.

· Recycling of clean water in the cooling towers

In the industrial complex, there are number of applications in which cooling water is used for open cooling. The spent cooling water was typically released as waste through the sewers. To minimize this loss of water, many of the open cooling water applications were converted to jacketed cooling water operations. Cooling jackets were installed on transfer lines, secondary transformers, high tension shift converters, and other equipment, with the return flows of spent cooling water sources being diverted to cooling towers. Similarly, water from air conditioner package units was diverted to suitably modified cooling towers. All of these measures resulted in a substantial savings of cooling water.

Operation and Maintenance

In every water conservation project which recycles cooling water or substitutes recycled water for process water, continuous monitoring is needed to ensure that the recycling practices do not result in any disruptions to the process, or to contamination or problems of corrosion in the equipment. In the case of GSFC, special attention is given to:

· monitoring of the chalk pond effluent, as well as monitoring of the ponds for any seepage into the ground

· monitoring of cooling water quality at critical points in the cooling water stream

· monitoring pump performance, especially of those pumps handling effluent

· maintaining and testing the automatic safeguards installed to ensure continued plant operations in the case of problems with the effluent recycling system, especially at critical points in the process

· analysing the purity of the condensates during the recovery of ammonium sulphate crystals, and providing the necessary quality safeguards and process safeguards so that process is not affected by any malfunction in the control systems

· controlling the continuous circulation of recycled water to maintain optimal cooling effectiveness in the plant, and replacing the recycled water with fresh water as necessary to minimize TDS build up

· maintaining and testing the automatic temperature control systems to ensure continued plant operations in the case of problems with effluent circulation system, especially at critical points in the process.

Level of Involvement

The project was implemented at the individual industry level, with major involvement of the senior middle level management of the company. The cooperation of the staff and the support of the top management were the additional factors in its success.

Effectiveness of the Technology

Various benefits were achieved as a result of the water conservation projects implemented at GSFC. A summary of the quantifiable benefits associated with this integrated programme of water conservation and recycling is presented in Table 21 for each component activity of this project. In addition, there were numerous unquantifiable benefits derived from the project, which are not listed.

Advantages

The advantages of undertaking water conservation projects were several. First, GSFC conserved water resources while increasing their productivity and profitability. Second, water conservation led to a significant reduction in the cost of water purchased. Third, water conservation reduced the volume of effluent generated in the production process and reduced the cost of effluent handling and treatment. Fourth, the energy required for plant operations was also greatly reduced. The programme also enhanced the ability of the Corporation to achieve its water quality goals.

TABLE 21. Water Conservation Benefits Achieved through Integrated Water Management in Industry.

Disadvantages

In all of the water recycling projects, and especially in those related to cooling water within the GSFC plants, there was an increase cost associated with monitoring water quality to optimize both the cooling benefits and level of water conservation. Additional control systems were also required to ensure the proper functioning of the plants which use recycled flows.

Further Development of the Technologies

Water conservation is an attractive option for large and complex industries like GSFC to reduce the water costs, increase production and decrease the consumption of energy. The. experience in a public sector organization like GSFC has shown that water conservation is possible and profitable on a large scale. The experience at GSFC can be easily transferred to other fertilizer or other complex industrial units for getting similar benefits.

Information Sources

Contacts

S.M. Singh and C. M. Patel, Gujarat State Fertilizer Corporation, Baroda, India.

Bibliography

The Fertilizer Association of India s.d. Water Conservation in Fertilizer Industry; A Workshop Report, The Fertilizer Association of India.