integrated effluent recycle-reuse and zero liquid discharge
Optimising water usage through an integrated effluent recycle-reuse and zero liquid discharge installation such as this is one method of reducing consumption of fresh water

The pressure on power station owners to minimise fresh water use has never been more acute. Can the value proposition of using treated wastewater for critical applications such as boiler feedwater be made? Deepak Kachru of Aquatech Systems (Asia) Pvt. Ltd believes so.

Many studies over the years have examined power plants’ water usage and losses. The majority of them have detailed the complete water balance of various power plant configurations, identifying losses, internal generation, discharges, etc, to provide a complete water usage picture for these plants.

There have also been studies focusing on optimising water usage within a given plant configuration through internal recycling and reuse of effluent within certain plant areas. Many subcritical and supercritical power plants now utilise wastewater for scrubbing and dust suppression applications, thereby significantly cutting their dependence on fresh water. This practice alone has nearly halved the water consumption of such plants from 5 litres per unit of power produced to less than 3 litres.

Using finalised water usage models or water balance diagrammes (WBD) from actual coal fired power plants currently under construction in India, we highlight the opportunity of using power plant effluent for more critical applications, primarily boiler feedwater, and will evaluate the benefits and value proposition of such an approach.

In water-stressed regions of the world, such as India, this option could enable coal fired power plants to cut their water footprint significantly by being smarter about how they utilise their water resource.

Water is a key component for ensuring the smooth operation of any power plant, traditionally being utilised for various cooling purposes, as well as the all-important boiler feedwater.

A coal fired power plant with a 1000–1300 MW capacity would normally require around 60–80 million litres/day of water for a COC (cycles of concentration) of five to six in the cooling tower.

For this requirement, cooling tower losses account for at least 80 per cent of the water requirement, the demineralised water requirement is around 5 per cent, with ash handling, coal dust suppression and service water requirements collectively consuming the remaining water balance, in other words, 10–15 per cent.

The majority of coal fired power plants located inland have developed a WBD model to optimise the intake of fresh water by maximising the reuse of the wastewater in various auxiliary applications within the plant, such as coal handling, ash handling, etc. Despite these best practices, however, there is always a certain amount of water that is disposed of with or without treatment; typically, this quantity is about 120 litres per MWh.

Modern coal fired power plants are now focusing on using a membrane-based effluent treatment plant (ETP) to recycle the cooling tower blowdown (CTBD) and other wastewater streams. This water can then be reused for cooling tower make-up requirements.

Current water conservation practice

Water reuse maximisation is achievable and is currently being practised in many coal fired thermal power plants. The key areas for water reuse maximisation are:

  • Ash handling
  • Coal handling
  • ETP

For example, CTBD of approximately 450 m3/h combined with boiler blowdown of 50 m3/h can be completely reused, with ash handling consuming close to 400 m3/h and coal handling around 30 m3/h, in other words, almost 95 per cent of the combined blowdown water can be reused within the plant. This has the benefit of saving an equivalent amount of the intake of fresh water for the power plant.

Most of the power plants will have a final effluent discharge to an ETP that will mostly comprise the residual CTBD and boiler blowdown. The other intermittent discharges to the ETP include regeneration wastes, transformer yard discharges, floor washings, service water, etc.

High efficiency RO
High efficiency RO lies at the heart of an integrated WTP-ETP scheme

Nowadays in India, it has become imperative for many inland-based power plants to maximise the reuse of wastewaters in order to be eligible for fresh water permits for expansions. In light of this – and also due to an increase in the awareness of power project developers to the reality of depleting fresh water resources – there has been a renewed focus on achieving near-zero liquid discharge.

This development has led to an increase in the adoption of membrane-based recycle systems in ETPs, with the treated water being recycled to augment cooling tower make up. The combined efforts at maximising the reuse of water are estimated to save close to 15 per cent in terms of a lower fresh water requirement.

Table 1 shows the average specific water consumption of various types of thermal power plants.

Table 1

We will now discuss the limited value proposition of recycling the effluent for cooling tower make up and make the case for the clear value proposition in recycling the recovered water for boiler feed instead.

Cost benefit of current practices

In a typical modern coal fired power plant being built in India today, the total make up requirement in the cooling tower stands at around 3100 m3/h, with a blowdown of 450 m3/h.

The total wastewater being sent to the ETP is 125 m3/h, which, after passing through the membrane system, is converted into high-quality cooling tower grade water. This water is then recycled back to augment the cooling tower make-up at 100 m3/h.

Table 2 illustrates this approach’s value proposition in terms of a cost benefit analysis.

Table 2

Integrated ETP-WTP concept

An integrated scheme across an ETP-WTP (water treatment plant) tries to maximise the value proposition of the ETP by not only optimising throughput, but also utilising it for a boiler feed application instead of cooling tower make-up.

Essentially the wastewater from the power plant, to the tune of 125 m3/h, is treated by a recycle system to condition the water and make it suitable for feeding to a polishing membrane, and then for use as boiler feedwater. This, in other words, means that by adopting an integrated scheme with maximum recovery you can eliminate the need for a separate WTP dedicated to produce boiler feedwater.

To better understand the challenges and constraints of utilising wastewater for the critical boiler feed application requires an overview of the CTBD analysis – the major source of wastewater in a power plant.

A typical CTBD analysis indicates the following parameters as critical to the design of an integrated ETP-WTP that can operate at high efficiency:

a. Total hardness
b. Silica
c. Alkalinity
d. Trace organics
e. In some cases, oil and grease.

It is imperative to understand that the above contaminants can be addressed in a low-efficiency conventional membrane recycle process — through their physical removal.

However, this requires the involvement of high chemical dosages of lime, dolomite, and proprietary antiscalants, and having done so will only yield a recovery across the system of around 80 per cent in the best-case scenario. But our objective is to maximise recovery and then utilise the recovered water for boiler feed, thereby eliminating the need for a separate WTP.

A high efficiency process has been shown to raise the recovery rate to over 90 per cent across the membrane-based recycle system, which is achieved by operating the system at a higher pH and ensuring that most of the contaminants are removed without extensive chemical and/or precipitation requirements.

Figure 1 shows a typical WBD for the integrated scheme, and highlights the stages of treatment and the overall recovery across the system.

Figure 1: Water balance diagram (WBD) for an integrated ETP-WTP scheme, incorporating a high-efficiency membrane recycle process

Further, by maximising the recovery across membrane-based recycle systems, any downstream evaporation process – whether based on natural or thermal evaporation – will also benefit. In other words, raising the recovery across the recycle system from 80 per cent to 90 per cent halves the requirement of water to be evaporated.

In a natural or solar evaporation pond, this translates to a reduction in area requirement by 50 per cent, and in the case of a thermal evaporation system, this would also essentially reduce the sizing requirement by half. This translates into a third-value proposition for adopting an integrated ETP-WTP scheme.

Table 3 highlights the value in altering the reuse potential of the recovered wastewater and maximising the efficiency of the recycle system.

Table 3

Conclusions

It is evident that an integrated scheme combining an ETP and WTP into a single unit offers a number of advantages, including.

  • A lower fresh water off-take of approximately 40 000 m3 per year, which is a significant saving when compared to current recycling practices. It thereby reduces the specific power consumption of the facility.
  • Three times more savings in the operating cost of the water system when compared to the practice of recycling for cooling tower make-up.
  • A significant advantage in relation to the zero-liquid discharge requirements of the facility by reducing the net inflow to any required evaporation system.

    In other words, with increasing water costs and more importantly scarcity, it is essential to innovate and adopt schemes that are capable of further reducing the water footprint of a power plant.

    Using less fresh water will not only mean greater savings, but will provide a significant value proposition to the end user to adopt recycling and zero-discharge concepts into the water balance of their power plant.

    References

    1. Mukhopadhyay D., US patent 5 925 255, July 20, 1999.2. Mukhopadhyay D., Whipple S. S., ‘RO Systems that Reduce Membrane Scaling and Fouling Tendencies’, Ultrapure Water, October 1997.

    3. Boyce A., Ferrigno M., Sharma D., ‘Zero Discharge Strategy – Boiler Makeup from Cooling Tower Blowdown’, 60th International Water Conference (IWC), October 1999.

    4. BetzDearbon Handbook of Industrial Water Conditioning, Ninth edition, Copyright 1991, Betz Laboratories, Inc.

    5. Reverse Osmosis – A Practical Guide for Industrial Users, Byrne W., Tall Oak Publishing, Inc., 1995.

    6. Bradley R., ‘Pilot Testing High Efficiency Reverse Osmosis Das Well Produced Water’, 61st IWC, October 2000.

    7. Demineralization by Ion Exchange, Applebaum S. B., Academic Press, Inc.,

    8. Mcbride D., Mukhopadhyay D., 450 ppm ‘Silica sustained in Innovative Reverse Osmosis Technology’, IWC, 1996.

    9. Fritz C. H., Ranade; B., ‘HERO Process – Volume Reduction of Cooling Tower Blowdown as Pre-concentrator for ZLD Application’, The 62nd IWC, Pittsburgh, October 2001.

    10. Bulletin of Energy Efficiency, Volume 7, Issue 3, December 2006.

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