For years, the Chennai metropolitan area in India’s Tamil Nadu state has suffered from severe water shortages because of below-average annual rainfall. Because of this, the city has depended on a combination of alternative sources for water, such as surface water runoff, groundwater from aquifers surrounding the city and water from the Krishna River in Andhra Pradesh.
To make matters worse, the limited water that Chennai can access is polluted by sand mining and nearby textile and leather tanning plants.
During the late 1980’s, the city’s water shortage forced the Chennai Petroleum Company Limited (CPCL) to curtail its Manali Refinery operation. Located outside the city, CPCL depends on water for its processing applications. To increase the amount of available water in Chennai, the company invested in a new wastewater reclamation plant that would take municipal sewage from the Chennai Metro Water District and render it clean enough for industrial operations in the refinery.
The new plant met nearly 40% of the refinery’s raw water needs. The new system operated successfully for more than 10 years, and CPCL was able to increase productivity without taking water from Chennai’s people.
However, the system was a time-consuming process. First, the secondary-treated sewage from the city’s sewage treatment plant had to be pumped through 3 km of pipeline and into a 3-million-gal reservoir. The reservoir, located at the CPCL campus, also collected sewage from the refinery.
From the reservoir, the treated sewage passed through a biological treatment system and was stored in a 2.5-million-gal intermediate storage pond. Next, lime was added to the wastewater, and the sewage underwent another multi-step process of pressure sand filtration, ammonia stripping, break-point chlorination, multi-media gravity filtration and cartridge filtration. Finally, reverse osmosis (RO) removed salts and small organic compounds from the water.
In the late 1990s, CPCL investigated new technologies that would improve the reclamation plant by reducing the number of steps and processes used for effluent pretreatment and extending the expected life of the RO membranes. The company also wanted the RO system to operate at a higher efficiency with an increased flux rate.
After a thorough evaluation of possible solutions, CPCL selected ultrafiltration (UF) hollow-fiber membranes from Koch Membrane Systems, Inc., to pretreat the feedwater.
In November 2004, CPCL commissioned its UF membrane system, which was designed to remove very small contaminants in the feedwater, including nearly all suspended solids, colloidal particles and microorganisms. The goal was to reduce both the turbidity and the silt density index (SDI) and provide a good quality feed to the RO.
Doshi Ion Exchange Limited supplied the entire UF system, and Koch supplied the hollow-fiber UF membrane cartridges.
The system contains 108 UF membrane cartridges, mounted on six cartridge racks. The cartridges contain polymeric, hollow-fiber membranes that have a nominal molecular weight cutoff of 100,000 Dalton, operate over a pH range of 1.5 to 13 and tolerate oxidizing agents like chlorine and peroxide. The UF membranes deliver water with very low turbidity, less than 0.1 NTU and a less than 2 SDI.
“The modules were selected because they offered tighter membrane porosity than competitive products,” said Samir Chaubal, regional manager of the Indian subcontinent for the Koch Membrane Systems Division. “They also had a higher membrane area per module, resulting in a smaller system with lower pump capacities. The smaller size helped CPCL minimize the system floor space area, easing floor space constraints in the existing plant.”
With the UF system pretreating the feedwater before it enters the RO system, CPCL is operating more efficiently and effectively. RO cleaning cycles and concerns about floor space have been reduced. And because of a high level of automation, the UF system requires minimal operator attention.
Operating at 90% water recovery, the plant’s production capacity of treated water is 430 m3 per hour, making it the largest wastewater reclamation plant in India.