How to handle the excess sludge from paper mill wastewater treatment

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With the development of the papermaking industry and the construction of pollution control facilities, the sludge generated as a result of the treatment of papermaking wastewater by these facilities has become a new source of pollution. It is understood that Fuyang City generates around 3,000 tons of sludge per day as a result of wastewater treatment in the treatment plants. Apart from some of this sludge being utilized in various ways, the rest is disposed of through sanitary landfilling; more than 70 mu of land is required for landfilling each year, which not only wastes land resources but also causes secondary pollution. Papermaking sludge is a type of biological solid waste; it contains large amounts of cellulose-based organic matter as well as plant nutrients such as nitrogen, phosphorus, and potassium, in addition to trace amounts of heavy metals and pathogenic bacteria. Based on this property, papermaking sludge can be turned into agricultural fertilizer. Its agricultural applications include using the sludge, after it has been air-dried, directly as a soil conditioner ； The sludge, after composting, is used as a soil conditioner ； The sludge compost is further processed into organic compound fertilizer for use. The sludge generated by papermaking wastewater treatment is an even greater problem, as papermaking wastewater treatment systems produce more sludge than other types of wastewater treatment systems. The characteristics of papermaking sludge are as follows: 1. A large amount of sludge is generated. According to the parameters of a certain paper mill, the SS in the incoming water ranges from 2100 to 4400 MG/L, while CODCR ranges from 1700 to 4800 MG/L ； The SS in the effluent ranges from 15 to 40 MG/L, while CODCR ranges from 70 to 100 MG/L. 2. High fiber content. Compared to the sludge generated from the treatment of other wastewater, it has better dewatering properties, requiring a relatively lower amount of flocculant. The sludge generated in the treatment of papermaking wastewater generally falls into two main categories: 1. Sludge remaining from the biochemical treatment system ； 2. Pulp lost in the wastewater discharged from the workshop ； The disposal options for papermaking sludge are three: Method 1 for treating papermaking sludge – the sludge can be reused in the papermaking process. Papermaking wastewater contains a large amount of fibers, and some of these fibers remain in the sludge during wastewater treatment; therefore, this sludge can be processed and reused. It should be noted, however, that biochemical sludge cannot be reused in this manner. The processing procedure is as follows: the sludge is screened and desanded, and then pumped to the paper machine for paper production. Due to the fine fibers in the sludge, its reuse may lead to a decline in paper quality and frequent breaks in the paper machine; it is best to use it in paper machines operating at lower speeds to produce low-grade corrugated paper. Method 2 for treating papermaking sludge: Papermaking sludge can be treated by filtration. The treatment process generally involves sludge conditioning first, followed by filtration. The pulp discharged from the sludge workshop has a high fiber content; adding an appropriate amount of polyacrylamide before it enters the belt filter can achieve good dewatering results ； The sludge discharged from the biochemical treatment system is, as appropriate, mixed with an appropriate amount of polyaluminum chloride, ferric chloride, etc., in combination with polyacrylamide. However, most paper mills mix the two types of sludge together for treatment. The general process for treating papermaking sludge involves: in the pre-treatment stage, the fibrous physical-chemical sludge and the excess sludge generated by the biochemical treatment system are concentrated gravitationally in a sludge thickening tank; thereafter, they are dehydrated using a belt filter before being transported for disposal. The supernatant from the sludge thickening tank and the filtrate from sludge filtration are discharged into the plant’s sewage collection system. The large amount of sludge generated by papermaking wastewater can be turned into a valuable resource if it is properly treated. First, papermaking sludge must be treated in a sludge thickening tank. Sludge concentration is the process of increasing the sludge density through gravitational settling. Once the sludge density is increased, the efficiency of flocculants can be improved as well; however, it is necessary to avoid excessively high sludge densities, as this may cause blockages in the pipelines used for transporting the sludge. Before entering mechanical dewatering, flocculants must be added to the sludge for chemical treatment, so as to form optimal flocs. Different papermaking processes generate different types of sludge, requiring the targeted addition of various types of flocculants. Common inorganic flocculants include aluminum sulfate, polyaluminum chloride, ferric chloride, and polyferric sulfate. The commonly used polymer flocculant is primarily polyacrylamide. When inorganic flocculants are used alone, the resulting flocs are small and have low mechanical strength; as a result, under the pressure exerted by the filter belt, sludge leaks away along with the filtrate, leading to poor dewatering effects and a low sludge recovery rate. Using cationic polyacrylamide alone can achieve good sludge dewatering results, but the cost is high. Therefore, combining inorganic flocculants with organic polymer flocculants can achieve better results and reduce the costs of sludge dewatering. The types and dosages of flocculants can be determined by experimental methods. After chemical treatment, the sludge must be mechanically dewatered. Since the operating costs for sludge dewatering and the costs associated with post-dewatering treatment account for a large proportion of the total costs involved in sewage treatment, it is very important to choose energy-efficient and high-performance dewatering equipment. In comparison, belt filter presses offer advantages such as lower investment costs, automatic control and continuous operation, low energy consumption and high dewatering efficiency, ease of management and low maintenance costs, low noise levels, and reduced usage of chemical agents. Therefore, a belt press is recommended for dehydration equipment. The filtration process in a belt filter press involves first passing the sludge through the gravity dewatering zone, then to the pressure dewatering zone, and finally to the high-pressure dewatering zone; after these three steps, a drier sludge cake is produced. According to data provided by a paper mill, after being pressed by a belt filter press, the moisture content of papermaking sludge is reduced to 75%~85%. At this point, the mud cake is transported out for final disposal. Method 3 for treating papermaking sludge: Papermaking sludge can be incinerated. The final disposal of the sludge can be determined based on local conditions, with a suitable method of sludge utilization being chosen; such utilization must meet strict environmental and health standards to avoid causing new environmental hazards. Combustion can be chosen, which allows for the recovery of heat while also reducing the area required to store waste. Given the large amount of sludge generated in the treatment of papermaking wastewater, choosing appropriate methods for its treatment and utilization is of great significance for both corporate development and environmental protection.