Zeroing in on discharge

As a rapidly developing country, India is struggling with the growing challenge of managing its hazardous waste. In 2019, the Central Pollution Control Board (CPCB) released the National Inventory Report of industries producing hazardous waste, as well as a review of hazardous waste management in India based on the information from state pollution control boards (SPCB) and pollution control committees (PCC), for the year 2016–17.

The report states that there are 56,350 industrial units generating hazardous waste in India, producing a total of 7.17 million tonnes of hazardous waste annually (during 2016–17). 

Proper disposal of hazardous waste is a challenge. For this purpose, it is imperative to understand the characteristics of this waste stream. A majority of the production processes in industries happen in a liquid medium, implying that the waste is primarily in the form of liquid effluent. On World Water Day today, We emphasise on the theme of ‘valuing water’ which is more than it economic value.

As per the waste management hierarchy, effluent is supposed to be treated, and water and minerals recovered and reused. The end product — which cannot be recovered — is treated and made chemically inert before being safely disposed in a secured landfill. In common parlance, this process of recycling and reuse of waste in water-intensive industries is known as Zero Liquid Discharge (ZLD).

ZLD, as the name suggests, is a technology where no liquid is discharged as waste liquid, instead it is treated and reused in operations. Ideal as it may sound, and despite widespread awareness and National Green Tribunal (NGT) guidelines for adoption of ZLD, its use has been limited to units of major multi-national companies like Unilever and Procter & Gamble.

One of the major impediments in wider adoption of ZLD is its operating cost in terms of fuel expenses and one-time high capital cost (during installation).

In essence, ZLD is an improvement on the traditional processes of effluent treatment that have been mandatory in India for almost a decade now. ZLD mainly incorporates effluent treatment, reverse osmosis (RO) and multi-effect evaporators (MEE). Among the three systems that combine to form a ZLD, MEEs are the most expensive to operate and are the greatest challenge in the water treatment industry.

The initial process in ZLD system is a treatment plant in which biological matter is oxidised by agitation using a mechanical pump. Through agitation, oxygen from the air is used by bacteria to break down organic matter, thereby reducing the biological oxygen demand (BOD) and chemical impurities.

Some other tertiary processes such as addition of chemical agents are also employed, depending on the nature of the effluents. By the end of the effluent treatment, the dissolved impurities become suspended and are settled in secondary tanks. After effluent treatment, water is passed through RO membranes. This is a fairly straightforward process, wherein clean water (potable standard) is produced.

At most plants, except in the food and pharmaceutical industries, water is reused in process. In the last 20 years, RO technology has matured to a point wherein installation, operation and maintenance (O&M) are not challenges any longer for even semi-skilled technicians. Most plants are available in a plug-n-play model and can be easily installed.

The cost of installation and spares has been declining significantly over the last 10 year. Generally, running an RO plant is not an energy-guzzling process and is fairly inexpensive in terms of energy use. Since it is a pressure-driven process, energy is used in the form of electricity. 

The major challenge of a ZLD process is the rejected water (from the RO process). This water has high total dissolved solids (TDS — represents concentration of total substances dissolved in water). Once TDS reaches 3 per cent, the RO plant no longer remains suitable for operation. At that point, the only option is MEE.

These are thermally driven processes in which steam is passed through heat exchangers in a chamber containing effluent. At each stage, the pressure is reduced, reducing the boiling point, thereby yielding treated water. This enables recycling of energy that is put in the first stage three–four times, depending on the number of stages.

However, MEEs require a high amount of energy in the form of coal or natural gas to produce steam. On an average, it takes 300 kilograms of steam to treat 1,000 litres of effluent water. A kg of steam from piped natural gas (PNG) costs Rs 3.60. Hence, just the fuel cost required to operate MEE is about Rs 1.20 per litre of effluent water.

After adjusting for other O&M costs, the cost of treating a litre of effluent adds up to Rs 1.50. According to CPCB data, a medium-sized chemical manufacturing plant in Gujarat uses about 80,000–100,000 litres of water a day. If 40 per cent effluent water is subject to MEE process, it would roughly translate into a cost of Rs 60,000 per day. This prohibitively high cost significantly disincentivises adoption of ZLD. 

Besides high operating cost, the adoption of ZLD (particularly MEE) is also hampered by the fact that they emit significant quantities of carbon dioxide and are powered by fossil fuels.  

New hope 

Thanks to new technological developments in the membrane distillation technology, possibilities have emerged of significant reductions in operating costs as well as carbon emissions of ZLDs. Membrane distillation (MD) is an advanced form of membrane-assisted MEE.

An MD system is powered by hot water instead of steam, so it can be powered by commonly used solar thermal collectors, thereby reducing the need of boilers (that will only be used during monsoon, when the solar option is limited). 

MD technology comprises a multistage process (generally five stages). The basic operating principle of MD is that by reducing pressure in a closed chamber, the boiling point of water is reduced. In each successive chamber, the boiling point is reduced and hence the rejected heat from the previous stage is used as input heat in the next stage. Thus, the same input heat is recycled five times (in the five-stage process).

Currently available MD systems are all modular and generally come in 40 feet shipping containers in a plug-n-play model. Besides running on solar energy, these systems are also more resistant to corrosion than MEEs, and increase efficiency. Once membrane distillation has been performed, the remnants are discarded to a TSDF or they can be used to extract minerals which may be sent to cement kilns as part of cradle-to-cradle economy.

In India, an MD system has been developed by the Indian Institute of TEchnology-Guwahati, which is operating a pilot plant within the campus. As the technology is nascent, even by global standards, there is a need for more demonstration plants with significant amounts of operational data.

The Union Ministry of Environment, Forest and Climate Change and the Ministry of New and Renewable can subsidize the setting up of pilot plants in different industries where MEE systems are currently operational such as pharmaceuticals and metal finishing.

The water and energy crisis our world is facing are intertwined. They need innovative synergetic solutions and new business models such as “water as a service” wherein a company installs the system and charges clients on the basis of the amount of water that has been treated.

Doing so will expand the user base of ZLDs as the capital investment will be carried out by a third party and all O&M risks of new technologies will be transferred to technology providers. There is a need to push for such innovative models to proliferate ZLD and make renewable-based ZLDs as common a technology as RO has become. 

WATER RECYCLING Waste India