The wastewater treatment process in the pulp and paper industry is a complex and systematic process, typically comprising three stages: pretreatment, biological treatment, and advanced treatment. The detailed steps of this process are as follows:
I. Pretreatment Stage
The main purpose of the pretreatment stage is to remove suspended solids, adjust pH, and reduce the concentration of organic matter in the wastewater, creating conditions for subsequent biological treatment.
1. Bar Screen: Removes large suspended solids and floating matter from the wastewater to reduce the load on subsequent treatment processes. Bar screens are generally welded from flat and twisted steel, and the spacing between the bars can be adjusted according to actual conditions.
2. Equalization Tank: Regulates wastewater flow and stabilizes water quality, providing stable water quality conditions for subsequent treatment. Equalization tanks usually have a large volume to buffer fluctuations in wastewater flow.
3. Sedimentation Tank/Flotation Tank: Further removes fine suspended solids and some organic matter from the wastewater. Sedimentation tanks utilize gravity to settle suspended solids, while flotation tanks inject microbubbles into the wastewater, causing suspended solids to adhere to the bubbles and float to the surface.
4. Neutralization Tank: Adjusts the pH of the wastewater to a suitable range for biological treatment. This is usually achieved by adding acid or alkali.
II. Biological Treatment Stage
The biological treatment stage is the core stage of papermaking wastewater treatment, mainly employing biological treatment technologies such as activated sludge and biofilm processes to remove pollutants such as organic matter, nitrogen, and phosphorus from the wastewater.
1. Aerobic Biological Treatment:
* Activated Sludge Process: Sufficient dissolved oxygen is provided to the wastewater through aeration equipment, allowing the sludge and wastewater to mix thoroughly. Microorganisms degrade organic matter. After a period of time, the mixed liquor in the aeration tank enters a sedimentation tank for solid-liquid separation. The treated water is discharged or further treated, while some of the sludge is returned to the aeration tank to maintain the microbial population.
* Biofilm Process: Utilizes a biofilm attached to the surface of a carrier to degrade organic pollutants. As wastewater passes through the biofilm layer, organic pollutants are degraded by microorganisms. Compared to the activated sludge process, the biofilm process has stronger resistance to shock loads, requires less land, and is easier to operate and manage.
2. Anaerobic Biological Treatment: Under anaerobic conditions, anaerobic microorganisms convert organic pollutants in wastewater into substances such as methane and carbon dioxide. Anaerobic biological treatment technologies mainly include upflow anaerobic sludge blanket (UASB) systems, characterized by high load capacity, low cost, and energy recovery.
3. Biological Nitrogen and Phosphorus Removal: In the biochemical treatment stage, the removal of nutrients such as nitrogen and phosphorus also needs to be considered. Biological nitrogen removal typically includes three stages: ammonification, nitrification, and denitrification, achieving nitrogen conversion and removal through the synergistic action of different types of microorganisms. Biological phosphorus removal utilizes polyphosphate-accumulating bacteria to accumulate phosphorus in the wastewater as polyphosphate within the microorganisms, which is then removed through sludge discharge.
III. Advanced Treatment Stage
The advanced treatment stage primarily targets wastewater that still contains high concentrations of recalcitrant organic matter and color pollutants after biochemical treatment. Advanced oxidation, adsorption, and membrane separation technologies are used for further treatment to ensure that the wastewater meets discharge standards.
1. Advanced oxidation technologies: such as ozone oxidation and Fenton oxidation, utilize strong oxidants to decompose recalcitrant organic matter, reducing COD and color in wastewater.
2. Adsorption technologies: Utilize adsorption materials such as activated carbon to remove organic matter, color, and other pollutants from wastewater.
3. Membrane separation technologies: Employ membrane separation technologies such as nanofiltration and reverse osmosis to achieve efficient separation of organic matter, suspended solids, and other pollutants from wastewater.
