2.2 Sewage reclamation using reverse osmosis

Technical description

Industries in growing metropolitan areas may face production losses as a result of excess demand for municipal water. Madras Fertilizers Limited (MFL), Madras City, Tamil Nadu, India, has faced such a situation in 1983 and 1987 (see the Indian Case Study in Part C of this Source Book). As a result, MFL has explored alternatives including the use of desalinated sea water and treated wastewater to supply process and cooling water to its operations. After detailed review of these alternatives, the Company decided to reclaim water from city sewage using advanced waste water treatment followed by Reverse Osmosis (RO) as an additional purification step. The Company has a daily water requirement of 20.25 MLD, 68% of which is required for cooling purposes.

Wastewater used by the plant is treated to tertiary standards using an activated sludge process, with the treated water being further reclaimed through excess lime addition, ammonia stripping, recarbonation, chlorination, multimedia filtration, activated carbon filtration, cartridge filtration, and reverse osmosis using thin film polyamide membrane.

Ammonia stripping is carried out in first-stage and second-stage counter current flow ammonia strippers, which are similar to cooling tower cells. Treated wastewater is sprayed from the top while air is sucked in from the bottom of the tower by an induced draft (ID) fan located at top of the tower. Free ammonia is blown out of water into the air. The ammonia-stripped water is pumped to a first stage carbonation tower and calcium carbonate clarifier, where the pH is brought drown to 7.0, and chlorinated before being sent to storage. The excess sludge from the clarifier is disposed of in sludge beds, and water drained from sludge is recirculated into the inlet lagoon.

Although most undesirable constituents like BOD, hardness, and ammonia are removed by tertiary treatment, the total dissolved solids (TDS) content is generally higher than well water. This would increase overall water consumption by making it necessary to add make up water regularly to dilute the salinity, increase the corrosiveness of the recirculating water, and increase chemical dosing needed to keep corrosion and sealing problems in check; all of which result in increased operating costs. To reduce these undesirable salinity-related costs, MFL selected Reverse Osmosis (RO) treatment of the treated effluent as a convenient and viable method.

Extent of Use

Industries requiring large volumes of cooling water could use this technology.

Operation and Maintenance

A qualified chemical engineer is required to supervise the treatment process. Other operation and maintenance requirements include the maintenance of the physical facilities, routine monitoring of the plant operation, and oversight of the supply and circulation system.

Level of Involvement

This technology is typically implemented at the individual industry level.

Costs

The total capital cost of a 20 MLD reclamation and reuse facility is estimated to be about $ 18 million. Annual operation and maintenance costs are about 10% of the capital cost.

Effectiveness of the Technology

Reclaiming sewage releases an equivalent amount of potable water in the municipal water system for domestic and other uses in the city.

Suitability

This technology is suitable for use in areas where a large quantity of sewage water is available nearby.

Advantages

Use of reverse osmosis proved to be a less expensive alternative than other alternatives such as sea water desalination, and resulted in a savings in the drinking water supply.

Disadvantages

The initial capital cost of an RO system may be high, especially if the sewerage line is far away from the industry. This system is also expensive to operate due to high power consumption requirements.

Cultural Acceptability

No problems are known as the reclaimed water is not for human consumption,

Further Development of the Technology

The technology is transferable and can be used by other industries. New industries should consider integrating this technology into their overall plant design to make it cost effective.

Information Sources

Rajappa, M.S. 1990. Reclaimed City Sewage as Industrial Water. Journal of Indian Water Works Association, Jan-March, 95-100,