Towards Zero Discharge

The current technological trend in the treatment of water and wastewater, whose maximum objective is to obtain the highest level of purity in the outlet water and the maximum level of drying of the extracted sludge, is giving rise, more and more frequently, to the appearance of concentrated liquid rejects that are often difficult to treat, being destined in most cases to be managed as waste, even though their water content may even be up to 95%.

This trend is totally predictable if we think of water purification as a system for the extraction of suspended or dissolved solids found in it, the classic concept of purification treatments seen from a generalist point of view, providing concatenated processes that use all possible physical, chemical and biological means to precipitate, coagulate, decant, float or filter these solids and separate them from water.

However, it is when technology thinks about extracting water from the wastewater flow that we can begin to talk about the long-awaited “zero discharge”, that is, the total closure of the cycle and the recovery of all or almost all of the water flow contaminated by a given process.

To this end, the technology has high-performance thermal processes, which allow the evaporation of large quantities of water with low energy consumption, allowing in turn to recover most of what escapes with these waste flows usually and even bring it to reuse.

In this field, Vacuum Evaporators have been considerably developed, mainly from the food sector, as a mature technology that is currently being applied to the environmental sector (water treatment) with more than satisfactory results.

The principle is quite simple and is applicable to any evaporation process. The boiling temperature of water drops considerably depending on the pressure to which it is subjected, so the lower the pressure, the lower the temperature is necessary to evaporate the water, with current evaporators being able to evaporate at temperatures of 30ºC to 35ºC (with depressions that reach 33 mbar).

These low temperatures lead to high energy efficiency and to the fact that heat can be supplied from waste flows, heat pumps or through mechanical steam compression.

In either case, the final result is obtaining a concentrated flow, the result of bringing the waste flow practically to the point of saturation, and in some cases it can even reach dry waste (5% of water), using special concentration equipment called crystallizers, although this usually exceeds the cost-effectiveness limit.

In this way, vacuum evaporators can reduce incoming waste flows by up to 95%, with the cost savings that this may entail for the industry in terms of management expenses, and thanks to these savings, this equipment is currently under two years of amortization.

Beyond vacuum evaporation, there are Spray Drying processes, also called Spray Dryer or spray drying, which give water evaporation systems a further twist.

These processes, which have been widely implemented in industries such as pharmaceuticals or food, where they have been used for decades to manufacture powdered or encapsulated products, are now being considered as a viable alternative for use in the final treatment of problematic flows.

The principle of the process is to increase the free surface of the liquid to be treated as much as possible, while being placed in intimate contact with hot air, allowing the amount of water subject to evaporation at the time to be as high as possible, thus optimizing its drying.

To achieve this purpose, the heart of spray dryers lies in the flow atomization system, which can be centrifugal (rotating disc), pressure nozzle, or ultrasonic, and whose objective is to generate homogeneous mists of microdroplets ranging from 9 µm to 250 µm.

The dryers are designed following different flow patterns, with the ultimate goal of ensuring that the generated hot air/fog mixture is intimate, and the flow pattern of both provides sufficient contact time for all the liquid to evaporate.

This evaporation causes a transfer of heat from the incoming air (which is cooled) to achieve rapid evaporation, while the core of the generated particle, where the solids are concentrated, remains at low temperatures thanks to the effect of the evaporation of the water itself.

In the end, what is achieved is a completely dry particle (about 4% humidity) with a controlled granulometry, which can be handled perfectly thanks to its small volume and solid state without creating the risks of contamination of a liquid, and having a much lower management cost due to its smaller volume. All this without counting the possibilities opened up by the potential for recovering compounds or recovering water flow, which can also be condensed for use.

These technologies, together with the new possibilities that are opening up in the field of reengineering processes for the use of waste heat streams, even at low temperatures, as well as the possibility of applying the latest advances in renewable energy to these processes (as is the case with solar power concentration technologies for the generation of process heat and/or cold), open up an interesting future in the treatment of waste flows, closing the cycle and allowing the full use of water used in our processes.