In the papermaking process, 99% of water is removed from paper pulp slurry to reveal the 1% of material that comprises paper. As such, the process is extremely water intensive; however, there is a large potential for water reuse. Maximizing reuse of the waste water—termed white water—serves both economical and environmental bottom lines. With only minor treatment and adjustment, white water can be reused for multiple applications within the plant, including stream dilution, machine operation, cleaning, and chemical make-down.
White water can contain as much as 30% of the incoming fiber, therefore, a good use of white water is for operations at the head of the plant where the fibers can rejoin the pulp stream. Water is needed for operation of the beaters that turn raw material into pulp and for diluting the pulp stream to create a constant flow. Consistency in the pulp stream is key for efficiency in plant operation.
To maintain consistency, it is necessary to remove filler content before returning white water to the head of the plant. Filler content, which is typically fine particle mineral content, is much smaller than the dewatering screens and enters the white water at a higher rate than pulp fibers. Instead of returning filler along with the paper fibers, filler content is removed and reclaimed using high pH precipitation, such as via lime dosing, which aggregates the mineral content. Returning a white water stream with minimal filler content avoids the need for advanced dosing control of filler material.
Due to the minimum treatment required to capture and reuse both water and fibers, return of white water to operations at the head of the plant is the first choice for reuse. In the case of excess white water, other applications for reuse exist in a plant, though with additional treatment requirements.
Applications for Clear Water Reuse
Reusing water for applications such as heating and cooling systems or chemical make-down requires a fiber-free water. Devices called a save-all are used to remove the fibers and filler, both for reclamation of raw material and to ease wastewater treatment processes further down the line. First, long fiber particles are easily caught with drum screens, then, other solids content is removed via precipitation. To achieve higher levels of solids removal than accomplished previously in the removal of filler content, the save-all process flocculates water through slow mixing after precipitation via pH adjustment. Then, the solids are removed via settling in a gravity clarifier.
In some applications, a higher degree of solids removal is required, such as for water applied through spray nozzles. After water is sent to a clear well following clarification, it is further passed through cartridge filters to catch the remaining fine content. Lastly, to use reclaimed water in contact systems such as steam boilers or vacuum pumps, the water must also be treated to prevent metal corrosion, which is again a pH controlled process.
Eventually, all water used in a papermaking plant is treated before discharge. Since the requirements for discharge are higher than the treatment requirements for the majority of white water reuse within papermaking plants, reuse reduces the total material and energy costs for water treatment. Fortunately, the processes needed for in-plant reuse are achieved fairly simply via pH adjustment and clarification.
View Sensorex Online Process pH Electrodes here.