Evaporation is a very old technology – think extracting salt from sea water – but has historically been considered uneconomical for wastewater treatment. However, as the costs of sewer discharge and landfill continue to rise, evaporation is starting to look more and more attractive.
Reduction of waste volume and significant cost savings
Evaporation is sustainable, environmentally friendly and can reduce the waste volume by up to 90 per cent – representing significant cost savings. Facilities that currently dispose of liquid waste off site can use evaporation to minimise operating costs and recover raw materials – including APIs, disinfectants, proteins and glycols – as well as to produce distilled water for reuse. What’s left behind in the evaporator is a concentrated liquid, usually around only 10 per cent of the original volume, reducing off-site disposal costs. For this process to work, the water has to be at its boiling point. At atmospheric pressure, this is 100 °C – hot enough to break down heat-sensitive materials, such as pharmaceuticals and cosmetic ingredients – but the latest generation of heat pump evaporators operate under vacuum to allow water to boil in the 30-40 °C range. Combined with a compressor-driven refrigerant circuit to extract the latent heat from the condensing water vapour – and re-use it to drive further evaporation – this creates a very cost-effective process that typically uses ~0.15 kWh of power per litre of distillate. This low temperature evaporation also makes it possible to recover even heat-sensitive raw materials from waste streams.
Evaporation uses less water and less energy
There are two principal technological options for evaporation: forced circulation and scraped surface. In forced circulation units, the liquid is pumped through a heat exchanger to increase the temperature to about 40 °C, then it enters the evaporator body and ‘flashes’ to a vapour under vacuum, leaving behind a concentrated liquid stream. This type of evaporation means that the liquid and vapour phases are easily separated under gravity, producing the highest distillate quality.
In contrast, scraped surface technology is about minimising the quantity of concentrate for disposal, and can be used for viscous or foaming liquids. It works in the same way as reducing a sauce in cooking. Heat is transferred rapidly by conduction through the bottom of the saucepan to the liquid. This hot liquid then moves away from the heating surface by convection. However, as the liquid viscosity increases, the rate of convection is reduced, and so stirring is required to ensure efficient liquid transfer – if you don’t keep stirring it, it’s going to burn. A scraped surface evaporator works in the same way, continuously scraping the heat transfer surface with a rotating blade prevents viscous liquids clinging to the surface and overheating.
Regardless of which technology is used, evaporation can reduce off-site disposal costs and enables the recovery of good quality distilled water and, potentially, valuable ingredients. With low capital and operating costs, using heat pump evaporators are now well within the reach of even small facilities.