5.3 Use of reclaimed water - Hindustan petroleum corporation limited, India

Introduction

The Hindustan Petroleum Corporation Limited (HPCL) uses seawater for cooling and fire protection purposes at their refinery. The seawater is pumped, by means of pumps located in a salt water pump house situated on a jetty, through a 36-inch diameter cement-lined pipe, to the refinery. The refinery is switching over, in phases, from once through cooling systems to circulating cooling systems to minimize dredging frequency, effluent generation and interruptions to their operations due to pump failures. Concurrent with this change in cooling system operations, the Corporation plans to switch their cooling systems from seawater to reclaimed water from nearby sewage plants. Once this is achieved, the jetty pumps will be exclusively for fire protection service. The reasons for switching to reclaimed water for cooling purposes were principally due to changes in the pollution control regulations, and to operation and maintenance problems related to the use of the seawater.

Blow down from cooling towers, when using salt water for cooling purposes, is very high in total dissolved solids (TDS is more than 15%), and more frequent blow downs are required if the effluent from this process is to contain TDS at the level required to meet the Central Pollution Control Board's Minimal National Standards (the MINAS regulations). Use of reclaimed sewage for cooling enables the refinery to reduce the frequency of blow downs from the cooling towers and thereby reduce the volume of effluent requiring treatment to MINAS to manageable quantities.

Likewise, seawater places an undue strain on heat exchangers due to its high salt content. Switching to treated wastewater may be expected to improve the life of the heat exchangers. However, it is important that the wastewater be domestic in nature to ensure that it does not contain any industrial chemicals. Using reclaimed wastewater is expected to reduce the cooling system make-up water requirement from 4 500 m3/hour (or 108 000 m³/day) to about 625 m³/hour (or 15 000 m³/day).

Technical Description

The design of the project entailed extensive engineering evaluations to identify the most appropriate source from which to obtain the treated wastewater. These investigations considered not only the location and infra structural requirements for conveying the treated wastewater to the refinery, but also the quality of the effluents. Four different sites from which to obtain treated sewage were studied before deciding on a site near the Ghatkopar-Chembur fly-over. This site was chosen because the source of the wastewater was purely domestic in nature (whereas other sources had some industrial loading to the treatment works), of adequate volume (not less than 15 MLD throughout the year, with an average flow of 50 MLD), and had available space in which to site a pump house without causing any outside disturbance. Sewage obtained from this source would be pumped 5.4 km to a 15 MLD sewage reclamation plant at the HPCL factory (expandable to 23 MLD, if required). Of the three possible routes between the fly-over and the plant, the route along Chembur-Govandi Road, Maharishi Dayanand Saraswati Marg Road, Golf Club Road, and Koliwada-Kurla Road, through the RCF colony to the Ashish Theatre, and along Corridor Road to the HPCL refinery site was chosen for the rising main. This route was selected because it passed along wide roads with relatively little traffic where pipe-laying work will be easier and faster. This route also facilitates future maintenance. Cast iron pipes were selected for conveyance of the sewage because of their ability to withstand heavy external pressure and resist corrosion.

Tertiary treatment of the wastewater will be provided by chemical treatment of secondary treated sewage. An alum solution will be dosed in a chamber upstream of the flash mixers and conveyed to clari-flocculators for the removal of fine suspended matter and colloidal turbidity. Clear liquid overflows the weir at the top of the clari-flocculator and flows into the launder, while the particulate sludge is collected on the bottom of the clari-flocculator and conveyed to sludge sump. The clarified effluent from the launder is then routed to four twin bed rapid sand filters containing a layer of quartz sand and a layer of graded gravel. The under drainage system includes air blowers and water wash lines for back washing the filters. The filtered water will be pumped from a storage reservoir, routed through four identical ion-exchange softener units, and chlorinated prior to use as cooling water. Each softener unit will have adequate cation exchange capacity so that regeneration will not be required more frequently than once in 12 hours. The chlorination system will consist of two vacuum type chlorinators.

Extent of Use

The project is being implemented by HPCL.

Operation and Maintenance

The wastewater reclamation plant uses proven technologies for treating water and wastewater. No extraordinary requirements are anticipated. The primary operation and maintenance requirements include replenishment of chemicals, maintenance of pumps and dosing equipment, monitoring of inflow and outflow effluent quality, and general plant supervision. Skilled staff are required.

Level of Involvement

This technology is being implemented at the industry level.

Costs

The operation and maintenance costs are broken down into the costs of human resources, chemicals, energy, and related costs, including the repair and replacement of equipment. These costs are summarized in Tables 27 through 29.

TABLE 27. Annual Human Resources Costs.

TABLE 28. Annual Chemical Costs.

^aFirst stage = 15 MLD,
Second stage = 30 MLD.

TABLE 29. Annual Operation and Maintenance Costs.

TABLE 30. Total Operating Cost of the Treatment Plant.

From the Table 30, the annual operating cost per unit of flow of the 15 MLD plant will be $0.08/m³. The comparative cost of municipal freshwater supplied to industries in Bombay is $0.45/m³ in 1996. Hence, HPCL can achieve a substantial savings in operational expenditure in addition to the savings in freshwater achieved.

A breakdown of the capital and engineering costs of the various components of the project is given in Table 31.

TABLE 31. Capital and Engineering Costs of the Reclamation Project.

Effectiveness of the Technology

The use of reclaimed wastewater is expected to reduce the cooling makeup water requirement from 4 500 m³/hr or 108 000 m³/day to about 625 m³/hr or 15 000 m³/day, with a concommitant cost savings over acquiring this makeup water from municipal sources.

Advantages

Reclamation of sewage for cooling water use has three advantages for HPCL; namely:

· the problem of high TDS that arises from the use of seawater is eliminated

· the demand for freshwater from the municipal supply is reduced, making more water from municipal sources available for use as drinking water in Bombay.

· the cost of municipal water to HPCL is reduced, providing a financial advantage to the Corporation in light of an increasing tariff structure for freshwater supplied to industrial users in Bombay.

Disadvantages

This technology requires a significant capital expenditure and is technology intensive, requiring skilled staff to install, operate and maintain.

Future Development of the Technology

The technology of sewage reclamation is an established technology that may be attractive to industries like HPCL who have large water requirements and who depend on (diminishing) water supplies from municipal corporations. The technology is transferable and can be used by other industries.

Information Sources

AIC Watson, Consulting Engineers, Bombay, India.

Hindustan Petroleum Corporation Limited (HPCL), India.