Water Treatment

New solutions for achieving zero liquid discharge

Purnendu Dutt, President, HIRA Group

Water is nature’s most wonderful, abundant and useful resource, used as a coolant for steam condensation in thermal power generation, HVAC systems and in process plants. For the efficient use of water for power generation, solutions like reverse osmosis and chemical treatment are not sufficiently effective as their residual rejects are costly to manage and difficult to dispose off, creating environmental hazards.

Further, water contains a lot of impurities in dissolved and suspended forms. Calcium and magnesium salts present in water determine the hardness of water and these salts form hard water scale, with increase in temperature, in heat exchangers of various industries.

So far, to avoid the occurrence of the scaling phenomenon, all cooling water circulation systems are forced to make a blowdown of chemically treated water on an hourly/daily basis. This not only creates water pollution, but also makes water management very tedious, being approximately 25-40 per cent of the total makeup water quantity.

Further, the calcium and magnesium salts present in water are non-sticky in nature. However, when in salt-solution state, they become the main cause of scale formation inside heat exchanging equipment, due to a rise in the temperature of water flowing through it. At higher temperatures, dissolved calcium and magnesium salts precipitate. Similarly, water also tends to precipitate the dissolved calcium and magnesium salts at a higher pH value, that is, when water is alkaline.

Technology solutions

Non-chemical water treatment is a technology that has evolved in recent years, which is effective, simple to understand, easy to use and ensures zero liquid discharge (ZLD) with no operating hassles and additional cost. The technology has been tried, tested, proven and documented successfully by India’s premier utility, NTPC, at their 2×490 MW Dadri plant. There is no separate process involved in the treatment of water to prevent hard water scaling and technology has drastically reduced the blowdown of chemically treated water and helped in achieving ZLD.

Working principle

The non-chemical water treatment equipment prevents the deposit of hard water scales in the heat transfer area. It is a “fit and forget” mechanical equipment, installed at the inlet pipes of the heat exchangers and does not require any maintenance, resulting in zero equipment downtime and deterioration.

The working principles involved are the galvanic principle, chemical characteristics of water and fluid dynamics. This non-chemical technology uses the principle to locally enhance the pH value of circulating water, leading to precipitation of salts before entering heat exchanger, in the colloidal form suspended in water. Due to their small size and light weight, the turbulent water flowing through the system carries away these precipitated salts, leaving the exchangers scale free.

As the cooling water passes through the equipment, it acts as an electrolyte within the equipment. The whole core placed inside the equipment gets negatively charged and attracts H+ (lightest) ions from water. Thus, with the instantaneous decrease in the concentration of H+ ion, the pH value of water increases, thereby precipitating calcium salts in the colloidal form. For this technology, ideally the pH value is to be maintained between 8.3 and 8.5 in recirculation.

Capabilities

  • Use of raw/R.O. Reject/effluent treatment plants and sewage treatment plants secondary treated/sea/borewell/flue gas desulphurisation waste water directly for a cooling water circulation with 24,000 parts per million (ppm) total hardness, 58000 ppm total dissolved solids and 20,000 ppm chloride.
  • Zero scale in heat exchangers with stoppage of anti-scalant chemical, reduction in other chemicals’ usage and water savings leading to ZLD with zero maintenance cost.

NTPC’s experience

NTPC, due to heavy scaling in its 2×490 MW units, installed 53 systems of the equipment in Unit 5 in April, 2014 and in Unit 6 in May, 2015. Anti-scalant chemicals were stopped and other chemicals were reduced in dosing quantities; simultaneously the cycle of concentration (COC) was increased from 2.5 to 8. The health of different heat exchangers was physically verified (see pics.). The Ganga canal and borewell water was used to run the plant during the closure of the canal for cleaning. In December, 2015, NTPC concluded that with this technology they could save 9 million cubic metres of water annually, with zero expense on anti-scalant chemical, a substantial saving in H2SO4 and other chemicals’ dosing quantities, besides running of the plant scale-free at a higher COC. Thus, NTPC improved its operational efficiency and achieved ZLD on a standalone basis. The systems have clocked four years of operation till date at NTPC.

 Conclusion

Thenon-chemical technology” achieves zero liquid discharge with zero scaling, reduces water usage and its associated treatment cost, combined with increased water-use efficiency by 20 per cent, in line with the National Water Mission goal.

Sustainability of this technology, operating with disruptive levels of water impurities and without any adverse effect, has given the impetus to reduce the blowdown from cooling tower (higher COC operation) and to reuse all the available water resources in a plant.

Presently, amidst global industrialisation, solutions like “non-chemical technology” can help industries reduce their carbon footprint and ensure efficient water utilisation.

  

One pass of Unit 5 condenser opened on July 22, 2015 and found scale free after 15 months


 Unit 5 condenser tube condition before installation of the non-chemical technology(March 2014)           

             

Unit 5 PHE without scales: non-chemical technology installed in April 2014

                

Unit 6 PHE with scales: Without non-chemical technology installed

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