2.1 Sewage reclamation using conventional wastewater treatment

Technical description

In India, treated municipal sewage is being used by industry for cooling water and in firefighting. The Hindustan Petroleum Company Limited (HPCL), located in Bombay, has used seawater for these purposes, but is in the process of converting to the use of reclaimed water (see the Indian Case Studies in Part C of this Source Book). The use of reclaimed water better meets the pollution control regulations established by the Central Pollution Control Board's MINAS (Minimal National Standard) regulations, and minimizes the operation and maintenance problems inherent in the use of the seawater for cooling and firefighting purposes. Use of reclaimed sewage also enables the refinery to reduce the number of blow downs from the cooling towers and thereby reduce the volume of cooling water effluent required to be treated to MINAS.

The components of the sewage reclamation, treatment and reuse process are a sewage pump housed in a dry well for easy access, a rising main to convey wastewater from the pump house to the factory premises, a water reclamation plant within the factory, a storage reservoir for the reclaimed water, and a distribution system to channel the reclaimed water to the cooling towers. A wet well is likely to be required to retain the incoming sewage from the sewer and balance flows to the treatment plant, and a mechanical screen should be installed in the system upstream of the pump to remove particulates prior to the wastewater entering the pump house.

The reclamation plant typically consists of a flash mixer for mixing of chemicals with the incoming wastewater. An alum solution, used to flocculate particulates that have passed through the mechanical screen, is dosed in a chamber upstream of the flash mixer. Following dosing. The wastewater passes through a clari-flocculator to remove fine suspended matter and colloidal turbidity. The resultant clear liquid flows over a weir and is collected in the launder. The resultant sludge is collected in the bottom of the clarifier tanks and discharged via the excess sludge sump. The clarified wastewater is then filtered through a rapid sand filtration unit, using a layer of quartz sand and a layer of graded gravel, and separated into two streams. One stream is passed through an ion-exchange softener unit, prior to being recombined with effluent stream in proportions calculated to produce the desired degree of hardness. The blended, reclaimed water is then chlorinated using a vacuum type chlorinator.

Extent of Use

This technology can be used in industries where a large volume of cooling water is required and an adequate source of wastewater is readily available.

Operation and Maintenance

Maintenance is related to the operation of the water reclamation plant and pumping system. Operations are generally conducted over a 24 hour period, requiring adequate trained human resources in at least three shifts to operate the treatment plant.

Level of Involvement

This technology may be implemented at the individual industry level or incorporated into a local government wastewater treatment scheme.

Costs

The total capital cost of a 15 million litre per day (MLD) reclamation system is about $ 4 million. The annual operation and maintenance costs are about $410 000, or about $ 0.02 per m³.

Effectiveness of the Technology

Use of reclaimed water 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).

Suitability

This technology is suitable where large quantities of wastewater are available nearby. Advantages

Use of this technology reduces the problem of high TDS in the cooling water which occurs when sea water is used as cooling water. Where municipal water is used for cooling purposes, use of reclaimed wastewater also results in a net savings in the drinking water supply of a municipality since industrial demands on this source are reduced.

Disadvantages

Domestic wastewater is best suited for reclamation as industrial wastes may contain contaminants that make such wastes unsuitable for reclamation. This technology has an high capital cost, especially if the sewage line is far away from the industry, and may have relatively high operation and maintenance costs, depending on the reclamation technology used.

Cultural Acceptability

No problems have been noted since the reclaimed water is not for human consumption.

Further Development of the Technology

The technology is readily transferable and can be used by other industries. New industries should consider the use of reclaimed water in their overall plan, which will make it cost effective to implement.

Information Sources

AIC Watson,
The Sewage Renovation Project at Hindustan Petroleum Corporation Limited (HPCL), Bombay, India. AIC Watson.