Whitewater is water drained from the slurry in a paper-making process as it is pressed and squeezed into its final thickness before being run into the dryer. It is a very important commodity in the pulp and paper industry, because this water can be reclaimed by the process to make new paper slurry rather than using fresh water.
Pulp and paper products are made in four stages:
- Batch mixing
In the batch mixing process, the raw materials (recycled product, wood pulp, starch, clay, various chemical additives, etc) are mixed together with water to form a slurry at about 5% consistency. In the forming process, most of this water is squeezed or sucked out of the slurry to leave a thick, moist sheet that runs into the dryer, where the rest of the moisture is removed by evaporation. The evaporated water is lost, resulting in impressive white clouds streaming from the vents in the top of the factory. The water squeezed and sucked out of the forming process can be reclaimed.
This reclaimed water is called whitewater, because it inevitably brings with it some of the paper fibers, raw materials, and chemicals from the slurry, giving it a white appearance. Since this represents the majority of the water that was used to create the batch in the first place, it can be recycled back into the batch mixing process instead of using fresh water to make the next batch. This saves an enormous amount of water, and thus cost. Not only is this environmentally friendly because of the recycling angle, but it also saves the wastewater treatment facility the trouble of dealing with the sludge.
Because of the uncontrolled nature of the whitewater, it is usually processed and cleaned somewhat on site before re-use. This usually involves a water treatment machine such as a dissolved air floatation tank or a lamella system. These machines allow the light material (e.g. paper fibers) to float to the top and the heavy material (e.g. clay) to sink to the bottom, leaving cleaner water in the middle to be drawn out for re-use. The ingredients thus separated out can often be added back into the system as recycled material, otherwise it is thrown away.
Since this process isn't perfect, only being intended to remove the majority of the material, the leftover sludge in the whitewater must be accounted for when re-used for the next batch. Whitewater is typically at about 0.5% consistency after processing, but since these materials are the same ones that were originally used to make the previous batch, they can be treated like any other recycled material. It is important to account for them though, in order to accurately control the consistency of the final slurry.
This, by the way, is the difference between "recycled content" and "post-consumer recycled content". Ordinary recycled content consists of the factory's own waste products added back into the system. Post-consumer recycled content is material that had actually been sold and used, and then returned to the factory for recycling.