Feb 16, 2009

Oil Refinery Improves Efficiency of Wastewater Plant

For years, the Chennai metropolitan area in India’s Tamil Nadu state has suffered from severe water shortages because of below-average annual rainfall. In lieu of adequate rainwater, 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.

Making matters worse, the limited water that Chennai can access is polluted by sand mining and nearby textile and leather tanning plants.

Reclaiming Municipal Wastewater

During the late 1980’s, Chennai Petroleum Co., Ltd. (CPCL) was forced to curtail its Manali Refinery operations because of the city’s severe water shortage. Located outside the city, CPCL depends on water for many of its processing applications, so the water shortage was a challenge for the company’s business.

To increase the amount of available water in Chennai, the company’s management team 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 as much as 40% of the refinery’s raw water needs, resulting in a first-of-its-kind solution for the Asian refinery industry. The new system operated successfully for more than 10 years, and CPCL was able to increase productivity without taking water from the people of Chennai.

However, the company found the system to be 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 very small molecular weight organic compounds from the water.

In the late 1990s, CPCL investigated new technologies that would improve the reclamation plant, reduce the number of steps and processes used for effluent pretreatment and extend 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.

With the UF system, CPCL is operating more efficiently and effectively.

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, Ltd. supplied the entire UF system, while Koch supplied the hollow-fiber UF membrane cartridges.

One hundred and eight 10-in.-diameter TARGA PMC UF membrane cartridges are mounted on six cartridge racks, with 18 cartridges per rack. 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. Additionally, UF membranes consistently deliver water with very low turbidity, less than 0.1 NTU and an SDI less than two.

“The TARGA 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, and RO cleaning cycles have been reduced along with floor space concerns. Also, because of a high level of automation, the UF system requires minimal operator attention. Operating at 90% water recovery, the plant production capacity of treated water is 430 m3 per hour, making it the largest wastewater reclamation plant in India.

The wastewater treatment plant at CPCL has also, indirectly, made more water available to Chennai’s 4.5 million inhabitants. And the new reclamation process has prevented any untreated sewage from damaging the environment. In December, CPCL certified that the membrane system has met performance expectations.

CPCL also used Koch membranes for a recent upgrade to its RO plant. Despite CPCL’s harsh feedwater conditions and temperatures, the new RO membranes are expected to last four to five years. The previous membranes lasted for 16 to 18 months.

Better permeate water quality was an important factor in selecting the membranes because CPCL has an ion exchange system downstream from the RO plant. Any additional dissolved solids in the RO system permeate would have required more frequent chemical regeneration and higher chemical usage in the downstream ion exchange system. If permeate quality was low, the company would have needed to increase the capacity of the ion exchange system, an impossibility given space limitations.

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