What are the process controls for treating wastewater in the pulp and paper industry?

Sept. 4, 2014

Large volumes of wastewater — with elevated physical solids and organic waste concentrations — are generated in pulp & paper making. Water plays a role …

Large volumes of wastewater — with elevated physical solids and organic waste concentrations — are generated in pulp & paper making. Water plays a role at every process stage. Wastewater treating and discharge, and sludge-waste disposal, all have their challenges.

About 3,750 gallons of water is required to produce 700 pounds of paper. That’s roughly the amount of water every American uses in a year.

The physical solids involved require primary color and total suspended solids (TSS) treatment. Organic waste requires secondary treatment of biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total organic carbon (TOC).

Rigorous, primary physical-chemical treatment removes color, solids and particulate matter. Secondary biological treatment removes biodegradable organic matter and decreases effluent toxicity for permit-compliant effluent discharge.

Wastewater treatment type is based on the volume and concentration of incoming waste load and discharge-permit requirements. A review of physical-chemical treatment controls and biological-treatment controls used in pulp and paper wastewater treatment follows.

Operations generate wastewater

There are several types of pulp mills and many grades of paper.

The amount and quality of the wastewater generated follows from general procedures related to nutrient, foam and pH control and is affected by factors such as fiber types, flow, settle-able solids, color, and odor.

Nutrient control is critical to treating paper mill wastewater. Paper-mill wastewater usually lacks nitrogen and may not have adequate phosphorus to support bacterial growth. Therefore, facilities add nutrients before the biological treatment. It’s in deciding how much or how little to add that the challenges arise. Operators should be familiar with their nutrient feeding systems and, in cold climates, the temperatures at which nutrient chemicals freeze.

Certain wastewaters only require nitrogen addition because there already is enough phosphorus as a result of boiler-water corrosion-control and cleaning operations.

Foam control is often required. An additive commonly found in pulp-mill wastewater is a de-foaming agent. Some pulp-mill effluents foam because the wood soaps are not fully captured in either the tall-oil or recovery systems.

The chemicals that cause foams are oxidized during biological treatment. However, if foaming is not controlled in the biological plant while treatment is taking place, a variety of problems may result, including frothing of mixed liquor solids which causes a loss of microorganisms from the system.

Engulfing foam

Foam can also engulf and short out motors and other electrical equipment. Additionally, foam will cause certain level recorders to misinterpret the true liquid level and potentially cause operational problems. Furthermore, foam may cause an air pollution problem, especially if foam becomes dry and wind borne. Controlling foam involves use of water sprays or anti de-foaming agents.

The pH control systems are vital for treating pulp-and-paper wastewater. The plant may be equipped with a neutralization system where acid or caustic is added prior to biological treatment. Often these chemicals are added prior to the primary clarifiers.

The paper industry manufactures fiber products made primarily of cellulose. The cellulose must be mill-treated to include cleaning, blending, refining, screening, bleaching, trimming and drying. These processes generate wastewater.

Related paper problems

Flow discharges from paper mills are fairly consistent throughout each twenty-four hour cycle. However, depending on what’s being made, high cellulose-content discharges may occur. Large quantities of extra water may also accompany these abnormally high fiber discharges. Consequently, it may be necessary to equalize using standby pumps during hydraulic surges and chemical-feed rate adjustments.

Settle-ability of solids in the final clarifier is affected by pulp-mill process discharges which contain certain chemicals separated during wood-pulping processes. These chemicals affect process controls in biological wastewater treatment and may require a flocculent to speed up secondary-clarifier settle-ability.

Color and turbidity 

Wastewater color and turbidity is the result of chemicals extracted from wood. It looks like swamp water and contains vegetative extracts. Thus, the wastewater will generally appear yellow-brown. The wastewater color will change with pH adjustments as well as varying mill processes.

Certain paper mills use dyes or pigments to color the fiber. Tissue paper, construction paper and other paper products can be colored. The wastewater associated with these grades will also be colored and affect treatment. Often, primary physical-chemical treatment process controls minimize or eliminate color in wastewater.

The odor of the water discharged from a pulp mill will reflect the chemicals used and produced in the pulping process. In most cases, these odors are associated with gasses that were captured in process water at the mill. When these waters arrive at the wastewater treatment plant and are aerated, the gases are stripped from the wastewater. The extent to which these gases are present in the wastewater depends on the pulping-process controls and how much gas-stripping control occurs before the wastewater is discharged to the sewer.

Treating wastewater in the pulp and paper industry has been described above, however if you have specific issues treating dairy waste or other wastewater queries, please submit a question.

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