How zero liquid discharge plants work

Zero liquid discharge plants minimise the consumption of freshwater and by the reuse of wastewater it helps relieve freshwater availability limitations in places where it is scarce or expensive. Ganesh Pangarkar explains the technology and process.

Power production has an impact on the environment. Besides the fuel gas, other emissions leave the power production process, noteworthy among which is water loss in the water/steam cycle. Considerable amounts of water have to be discharged. This water contains partially chemical impurities and its discharge has an impact on the environment and also means spending on effluent treatment. Furthermore, the water needs to be replaced, and consequently additional wastewater from the water treatment plant. An important aspect to take into account that exceeding the natural available water resources also means influencing the environment in a negative way. The principle of zero discharge entails recycling of all industrial wastewater, whereby wastewater will be treated and reused in the process. Because of the water reuse, wastewater will not be released on the sewer system or surface water. The concept of zero discharge system is to ensure no discharge of pollutants into the environment, and recovery of water.

The technology

Wastewater is collected in the equalisation pond and passed through series of process equipment. Harmful organic/colloidal contents in the water are separated out in the first phase of treatment and the inorganic content in the water are separated in the second part of the process. Ultra filtration technology is used to remove harmful suspended matter in the water which can cause damage to reverse osmosis membrane. A series of reverse osmosis units are used to recover water from the wastewater. The product water is better in terms of quality than the raw water to the plant.

The water recovered in the process is 90-95 per cent of the inlet water to the plant. It means only 5-10 per cent water is wastewater which is treated further for evaporation.The remaining wastewater is treated using different technologies for converting it to solid waste which can be used for land filling activities.

Forbes Pro Water Projects uses the latest technologies such as membrane bio reactor to recycle the sewage water and industrial waste water from the plant to recycle back to process use.

Broadly the process used for this are:

1) Solar pond evaporation
2) Mechanical evaporation using solar pond
3) Thermal evaporation

Solar pond evaporation

The objective of the system is to evaporate the wastewater using solar energy for solid wastes disposal option. The high total dissolved solids (TDS) water will be pumped into the solar evaporation pond. Due to natural evaporation process, the solids concentration takes place. Once the concentration is reached, the form of salt solids gets separated out and the salt is taken out by scrapping the solar pond.

Solar pond with mechanical evaporation

The objective of the system is to concentrate the rejects or high TDS streams and make it in the solids/slurry form. The slurry/solids can be disposed off by the hazardous solid wastes disposal option. The high TDS water will be pumped into the solar evaporation pond. In the first phase of the solar ponds, the pumps re-circulate the rejects a number and increase in the solids concentration takes place.

Once the concentration is reached where the liquid cannot be pumped or re-circulated, the concentrated liquor is taken to the auxiliary ponds and allowed to dry naturally. In these ponds, no recirculation takes place.

Thermal evaporation

Normally Reverse Osmosis (RO)-rejected water contains minerals and salts in huge quantity. So, we cannot directly let this water go into the environment. It needs to be treated and made suitable as per environmental conditions. For that, we use thermal evaporation method to recycle the RO reject water.

Reject water is collected in balance tank and fed through pre-heaters (type of shell and tube heat exchangers) where it is heated up to 70-80oC with calandria chest vapours to provide the initial preheating at cheaper cost.

After evaporation, liquid is separated into vapours and concentrated liquid. Vapours are collected in vapour separators and concentrated liquid is again fed into next calandria for heating. This process is repeated until final concentrated slurry is left. This slurry could be used for land filling.

Financial advantage

Zero liquid discharge minimises the consumption of freshwater as the cost of treated water is 50 per cent less than the fresh water intake; therefore by reuse of wastewater it helps relieve freshwater availability limitations in places where it is scarce or expensive thus considerable savings are realised and that resulted in a moderate payback period. In addition, elimination of liquid discharge also helps towards the need to comply with increasingly stringent environmental restrictions.

Purchased water, wastewater treatment and disposal costs are significant; thus, savings associated with minimised new water requirement and wastewater flows can justify capital expenditures to minimise. In the case of new constructions, zero liquid discharge can save money on real estate costs, since location near a suitable water resource would not be necessary.