|Sourcebook of Alternative Technologies for Freshwater Augmentation in some Asian Countries (UNEP-IETC, 1998)|
|Part C - Case studies|
The Madras Metropolitan area, with scant rainfall, no river sources, and no major nearby watersheds, has always been chronically short of freshwater. The demand on the existing scarce sources has increased substantially due to increases in population and industrial development. Lately, the problem of water availability has reached crisis levels and has resulted in several stoppages of water supplies to industries, leading to heavy losses in production and related financial losses to the industries located in this area. Madras Fertilizers Limited (MFL) faced such situations in 1983 and, again, in 1987, prompting MFL to explore alternative sources of water such as desalinated seawater, seawater and treated sewage for meeting its process cooling water supply needs.
The Company's total 20.25 MLD water requirement had been supplied by the Madras Metropolitan Water Supply and Sewerage Board (MMWSSB) from two aquifer sources located 20 km away from MFL at Panjetti and Tamarapakkam. Of the total water consumed, 13.7 MLD were required as makeup cooling water, with the balance used in the production process and for other general uses. Of the options available to the Company for supplementing or replacing the municipal water sources, the Company decided to reclaim water from city sewage using advanced wastewater treatment technologies, followed by reverse osmosis (RO) as an additional purification step. Installation of the facility was completed by 1991. This treatment plant, one of the few units in the world treating city sewage for industrial use, has freed about 13.7 MLD of potable water for domestic use in the City.
The MFL reclamation plant processes include wastewater treatment through tertiary treatment using an activated sludge process, followed by excess lime addition, ammonia stripping, recarbonation, chlorination, multimedia filtration, activated carbon filtration, cartridge filtration, and reverse osmosis filtration using thin film polyamide membranes. In the primary treatment phase, the bulk of the suspended solids is removed by coarse mesh screens and in primary clarifiers. At this point, the BOD has been reduced from 350 ppm to 50 ppm. In the secondary treatment phase, the effluent is passed through a trickling filter (biofilter) into an aerated lagoon. The biomass formed in the aerated lagoon is removed in the secondary clarifier as sludge. The sludge is recirculated back to primary clarifiers to maintain bacterial population in the biofilter. The waste sludge from the plant is removed as underflow from the primary clarifier, digested biologically, and disposed of. Analysis of the water after primary and secondary treatment showed that impurities such as ammonia-nitrogen, dissolved phosphorus, silica and BOD remain too high for use in the cooling system. Therefore, the overflow from the secondary clarifier is to be sent to the tertiary treatment phase for further physciochemical treatment.
The secondarily treated effluent is transferred to a receiving lagoon, from which it is pumped to two aeration tanks equipped with four 25 HP fixed surface aerators. The aerated sewage effluent then flows into a biomass clarifier where the biological sludge formed during aeration is settled out. The bulk of the biological sludge is pumped back to aeration tank by sludge transfer pumps to maintain the Mixed Liquor Suspended Solids (MLSS) of 3 000 mg/l. The overflow from the bioclarifier is pumped to the lime softener, where ferric coagulant and lime are added along with a polyelectrolyte solution. The Milk of Lime, or calcium hydroxide solution, is added in sufficient quantities to maintain the pH between 10.8 and 11.5 to facilitate ammonia stripping. The lime softener overflow is passed through five gravity sand filter beds to trap and reduce the physical carryover of lime and calcium carbonate from softener, so that they do not foul the ammonia strippers. These sand filters are periodically backwashed using water recycled to the lime softener. The filtered water is pumped to first-stage and second-stage, counter-current flow ammonia strippers consisting of two cells in each stage. These are similar to cooling tower cells. Effluent is sprayed from the top of the towers while air is sucked into the bottom of the towers by induced draft fans located at the tops of the towers. In the process of air-liquid contact, free ammonia is removed from the liquid into the air. The atmospheric carbon dioxide also partially neutralizes the Ca(OH)2 in the water, resulting in a slight drop in the pH during this process. A caustic soda solution is added after first stage ammonia stripping to restore the pH level. From the second stage ammonia strippers, the effluent is pumped to first stage carbonation tower in which CO2 is injected into the effluent to reduce the pH to 9.3. The precipitated CaCO3 is removed in the calcium carbonate clarifier, and the overflow from the clarifier flows to the second stage carbonation tower, in which the pH is further reduced 8.3 with sulphuric acid. Following the second stage carbonation, the treated effluent is acidified to pH 7 and chlorinated before it is sent to the tertiary treated water storage.
The excess sludge from the bioclarifier underflow is disposed off in sludge drying beds operated on a 7 day drying cycle. Water from the sludge drying beds is recirculated into inlet lagoon. The sludge from the underflow of calcium carbonate clarifier and lime softener is thickened to between 7% and 10% solids, and dewatered on vacuum belt filters prior to disposal. The water separated from the sludge by this system is also recycled to the plant.
Though undesirable constituents like BOD, hardness, ammonia, etc., are removed by tertiary treatment, the total dissolved solids (TDS) content of the treated water remains higher than that of well water. High TDS concentrations result in higher rates of water consumption, increased corrosion in the recirculating water system, and higher chemical dosing costs to keep corrosion and sealing problems in check. Thus, TDS has to be reduced to lower levels to ensure the smooth, uninterrupted operation of the cooling water system. The removal of TDS will also reduce the frequency of blow downs and result in both a saving of water and costly cooling water treatment chemicals lost in the blow down process. Two alternative technologies were considered viable for reducing the salinity of the treated water; namely, conventional demineralization by ion exchange (IEDM) resins which must be regenerated periodically by acid/alkali treatments, and reverse osmosis (RO). The latter was considered to be the more convenient alternative.
The MFL cooling water system is designed to operate over four to five cycles between blow downs using a makeup water containing up to 300 mg/l chloride. Because the RO permeate (water that has passed through the filter membrane) has a lower salinity, part of the feed water to the RO is bypassed and blended with the permeate to achieve the operating salinity of 300 mg/l chloride. The portion of the treated water which is to be subjected to RO filtration is further treated in the chemical pretreatment section to lower the pH. Sodium hexametaphosphate and sodium bisulphate are added to inhibit scaling by CaS04 and for chlorine removal, respectively. This pre treated water is filtered through polypropylene micron cartridge filters, and pumped to the RO unit by high pressure pumps. The RO unit is laid out in multiple streams and each of these streams has three brine stages. The brine, or rejected water, from one stage passes, through a common header, to the next stage to recover the maximum quantity of permeate from the feed water. Rejected water from all RO streams is collected and sent for effluent disposal. The RO unit has been designed for a recovery of 75% of the feed water as permeate.
Extent of Use
As noted, MFL considered the following alternatives in detail to meet its process cooling requirements: desalination of seawater, direct cooling with seawater, indirect cooling with seawater, and use of sewage effluent after suitable treatment. The seawater desalination alternative was considered to be too power/energy intensive as well as capital intensive. The cost of water produced by this technology was estimated to be very high. Direct cooling with seawater was considered to involve the complete replacement of existing admiralty brass fittings used within process water heat exchangers with titanium or cupronickel fittings. This would have required heavy investment and amounted to revamping the entire plant, involving considerable time loss as well as production losses. In indirect cooling, process water heat is transferred to a treated intermediary freshwater which, in turn, is cooled by seawater in a large sized titanium plate exchanger. The disadvantages of this system are its high capital investment cost and anticipated delay in implementation of the project due to long delivery periods for items such as the plate exchanges. (For both indirect and direct seawater cooling alternatives, seawater must be filtered and chlorinated before transfer to MFL from the nearby Ennore Estuary through large diameter pipelines.) In contrast, the sewage treatment project was estimated to be able to be completed within 14 months with a lower level of capital investment as well as lower operating costs. Hence, the use of reclaimed water was considered to result in the least level of disruption to plant operations and the lowest cost.
Operation and Maintenance
Qualified chemical engineers are required to operate and maintain a reverse osmosis unit. Skilled operators are also required to operate and maintain a tertiary wastewater treatment plant.
In spite of the elaborate pretreatment of the reclaimed water, the membranes in the RO unit are likely to become fouled after a period of operation, resulting in a reduction in permeate yield. At this stage, the membrane must be cleaned to remove the fouling using proprietary chemicals supplied by the manufacturers. However, over about a three year period, there is slow and progressive loss of membrane efficiency due to a certain amount of irreversible fouling and degradation from various causes, and the membranes must be replaced.
Level of Involvement
The predominant involvement in this project was by the industry. However, there was significant participation by the MMWSSB and government in specific phases of the project.
The capital cost of the entire project is estimated at $18 million.
Effectiveness of the Technology
Due to the sewage reclamation scheme, 13.7 million litres of potable water, previously supplied each day to MFL from the Metro water treatment plant, has been redirected to domestic use in the City.
The advantage of reclaiming water from treated sewage for industrial use is that it is more economical than the other options suggested. Similarly, the use of RO as the final polishing stage has an edge over the Ion Exchange Demineralization (IEDM) technology because the RO unit is geographically compact, with its components situated close to each other, making operational control of the plant much easier. The RO unit also will run over longer periods without interruption, minimizing the need for constant care and reducing operational errors. The operation of the RO unit is more convenient, less complex and cumbersome, and less prone to the possibility of errors than IEDM techniques. Maintenance is also easy, and the replacement of filter cartridges and membranes can be quickly carried out without much effort.
In terms of operating costs, the RO unit requires only small quantities of acid and sodium bisulphite, etc., and the RO unit can cope with increases in salinity without any significant adverse effect on the product quality or cost. Annual operating costs of the RO unit may be as much as 40% lower than those of alternative technologies which produce water of the same quality and quantity.
The major disadvantages of RO are the high installation cost of the system as well as its high energy costs, since its rate of power consumption is very high.
Further Development of the Technology
It often requires a crisis to provoke the reuse of such a valuable water resource as sewage for industrial activities. One important aspect of the sophisticated tertiary treatment plant that remains to be addressed is the high level of nitrates in the effluent (10 to 12 mg/l as nitrate). This is due to nitrification occurring within the activated sludge process, and may affect the longevity of the RO plant. It is therefore suggested that a combined carbon oxidation and nitrification step be developed for use in the activated sludge treatment process.
Rajappa, M.S. 1990. Reclaimed City Sewage as Industrial Water, Journal of Indian Water Works Association, Jan-March, pp 95-100.