Pulp & Paper

A pulp and paper mill has a wide variety of needs for water treatment, both upstream and downstream of the plant. Its uses for clean water are multiple, with different levels of quality depending of the need:

• Influent water, that typically requires chemical treatment, clarification and filtration of the raw water.
• Boiler feedwater, which needs to be further treated for higher purity, through: microfiltration, reverse osmosis, CEDI, mixed beds and/or ion exchange resins.
• Cooling water, usually treated by clarification, softening and filtration (media or membranes).
• Debarking wash water is often also treated to help reduce raw water consumption by recycling.

Pulp and paper mills produce various effluents needing treatment:

• The effluent undergoes primary mechanical treatment, secondary aerobic or anaerobic biological treatment and, in some cases, tertiary/quaternary treatment.
• Black liquor is treated by evaporation systems, condensate stripping and segregation.
• Chlorides can also be evaporated by a specific removal process (CRP).
   

Mill effluent characteristics are defined by the raw materials used and products produced at the mill: type of wood, pulping process, paper grade, etc. In general, the effluent coming from the mill is warm and has a high organic load (COD and BOD) and a high solids content, with varying pH levels and low nutrient levels.

Solids and settleable particles are removed from the effluent in primary treatment, typically consisting of screening and primary clarification / dissolved air flotation (DAF). The effluent is then cooled and conditioned for secondary biological  treatment, where dissolved compounds are converted into biomass and then separated in secondary clarification. Traditionally the most common method used in secondary treatment is activated sludge process, but biofilm technologies like BAS and MBBR are gaining ground due to their high performance, robustness with peak loads and flows, and lower costs induced by a reduction in footprint. For the effluents with the highest concentrations of COD/BOD, high-rate granular anaerobic treatment is a process that will effectively remove the pollutants while generating biogas, which is a green renewable energy. In most demanding circumstances biological secondary treatment is further complemented by tertiary treatment, where residual COD, phosphorus and colour are removed chemically and mechanically.

Black liquor is the by-product of the Kraft process used in the industry to convert wood into pulp and then into paper. It is a toxic liquid that typically contains about 15 to 20% solids by weight of residues and inorganic chemicals.
Specialized processes are used to optimize the treatment chain, leading to heat and power generation and resource recovery with high efficiencies and making a quality product. Thermal treatment by evaporation first increases the solids content and reduces the flows ahead of recovery boilers. Condensate stripping is also an important part of the recovery cycle. 
 

The pulp and paper industry is known to be one of the largest consumers of water globally, which in result generates large volumes of wastewater. It is therefore important to use optimal and state-of-the-art water treatment technologies to:

• Guarantee the best ingredient water for a quality product.
• Ensure that effluents comply with the most stringent regulations.
• Maximize water reuse and product recovery to reduce and control costs of water management.