Evaporators are used extensively in the food, dairy, and distillation industries as a method of removing water from a fluid to obtain a more stable or reduced volume product, often by subsequently drying the concentrate to a powder.
The water condensed during evaporation is hot and may contain a small amount of the product itself. This ‘carry over’ in the vapour is due to splashing or boiling of the product during the evaporation process, creating a mist of tiny particles which can be distilled over into the condensate along with the water. For this reason the evaporator condensate, although relatively pure can contain enough organic material to support bacterial growth when cool. This makes the condensate unacceptable for re-use in any food related environment.
The evaporator condensate can be recovered for re-use by passing it through a reverse osmosis (RO) membrane thus providing a valuable source of hot, good quality rinse water. Additionally, a reverse osmosis plant requires a relatively small footprint when compared with conventional water treatment systems and can often be custom designed to fill a “spare” space on site, this normally avoids any need for civils on plant installation.
The advantages can be summarised as follows:
Possibly 10% of the original condensate volume as RO concentrate will be sent for disposal, this may go either to existing site effluent treatment or to drain. As much of the cost of water disposal is attributed to the water volume, rather than the loading (as in the Mogden formula), the saving, even if the concentrate is passed to drain, can be very significant. RO will not remove all the BOD from the stream, a 90-95% reduction is normally achieved. Each industry has its own requirements to get the best performance from the plant.
A few thumbnail economics are shown below, based on a 25,000 l/hour evaporator condensate flowrate, at 60°C, 360 days/year operation and with a recovery rate of 90% (i.e. the amount of permeate recovered as clean water is 90% of the feed volume).
No allowance has been made for plant installation costs, or ancillary process equipment such as buffer tanks etc. The above figures are based on a PCI-Membranes process plant which has been operating for over 5 years.