Waste Water Treatment Solutions in Pulp and Paper Industry
Papermaking is a highly sophisticated process involving several processing steps to turn wood into paper products. In addition to the actual papermaking process, there are also supporting areas such as waste water treatment, recovery boiler operation, steam production, and cooling towers that benefit from on-line analytical measurements.
In the Pulp and Paper industry a lot of effort is used to water saving and closing water circuits, and to reduce substantially the environmental impact, by process modeling and Kidney technologies as internal process water treatment.
Effluents from pulp and paper mills contain solids and dissolved matter. Principal methods used to remove solids from pulp and pape r mills effluents are screening, settling/clarification and flotation. The method chosen depends on the characteristics of the solid matter to be removed and the requirements placed on the purity of the treated water.
The separation of solids from the effluents is accomplished with help of screens, grid chambers and settling tanks. Screens are units which operate according to the sieving/filtration process. The function of the screens is to remove coarse, bulky and fibrous components from the effluents. The grit separating systems currently in use are subdivided into longitudinal grit traps, circular grit traps and vortex grit traps, depending on their design and process layout.
The paper industry uses a variety of effluent treatment systems. The preferred process combination for each individual case depends on the grade -specific quality of the effluent that is going to be treated.
Sedimentation technology is the simplest and most economical method of separating solid substances from the liquid phase. High efficiency is achieved in subsequent effluent treatment processes when the solid substances suspended in the effluents settle in a sedimentation tank as completely as possible, and settled sludge is removed from the sedimentation tank. Sedimentation equipment with sets of lamella-shaped passages, are employed in the paper industry, especially for effluents with high fibre concentrations.
Biological waste water treatment is designed to degrade pollutants disso lved in effluents by the action of micro-organisms. The micro-organisms utilize these substances to live and reproduce.
Pollutants are used as nutrients. Prerequisite for such degradation activity, however, is that the pollutants are soluble in water and non-toxic.
Since the early 1980s, anaerobic treatment of industrial effluents has found widespread application in the pulp and paper industry. Several hundreds of installations are treating a large variety of different pulp and paper mill effluents.
Anaerobic treat ment is most commonly used for effluents originating from recycle paper mills, especially during production of containerboard. Moreover wastewater of mechanical pulping (peroxide bleached), semi-chemical pulping and sulphite and kraft evaporator condensates may
Aerobic micro-organisms require oxygen to support their metabolic activity. In effluent treatment, oxygen is supplied to the effluent in the form of air by special aeration equipment. Aerobic treatment allows fully biological degradation of paper mill effluents. Aerobically operated plants exhibit higher plant stability and are less sensitive to fluctuations in effluent and plant parameters.
Secondary clarification is intended to separate the biomass (activated sludge) formed in biological reactors and is therefore a key element in all processes employed in the final stage of a treat ment plant. The quality of the separation process is just as crucial for the final effluent quality as is biological treatment itself.
Advanced and tertiary treatment:
Tertiary and advanced waste water treatment is used to remove specific waste water constituents that cannot be removed by secondary treat ment. Different treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics or dissolved solids. Sometimes it is referred to as tertiary treatment because advanced treatment usually follows high-rate secondary treat ment. However, advanced treatment processes are sometimes combined with primary or secondary treat ment (e.g., chemical addition to primary clarifiers or aeration basins to remove phosphorus) or used in place of secondary treatment (e.g. overland flow treat ment of primary effluent).
Advanced waste water treatment in the pulp and paper industry is focused mainly on additional biological membrane reactors, ozone treat ment and membrane filtration techniques such as micro-, ultra- or nanofiltration and reverse osmosis.
Chemical flocculation process is a crucial process as it promotes the aggregation of particles after being destabilized by a chemical agent.
In pulp and paper industry, flocculation is involved in different parts of the process: it is essential to form the paper sheet in the forming wire, determining retention, drainage rate and the formation, and it is also used in the wastewater treat ment to separate the colloidal material and in the sludge thickening.