In the sludge treatment process of the papermaking industry, the dewatering effect directly affects the subsequent disposal cost of sludge, the difficulty of transportation and the compliance with environmental protection standards. The selection of dewatering agents is the core link that determines the dewatering effect. However, due to the complexity of the composition of papermaking sludge, the differences in the performance of chemicals, and the variability of actual working conditions, the selection of dewatering chemicals often encounters many difficulties, posing challenges to the sludge treatment work of enterprises.
First, the composition of papermaking sludge is complex and highly volatile, increasing the difficulty of reagent adaptation
The composition of papermaking sludge is influenced by multiple factors such as papermaking raw materials, production processes, and pulping methods, presenting extremely high complexity and instability. This is the primary difficulty faced in the selection of chemicals. From the perspective of raw materials, the sludge components produced by enterprises using wood pulp, grass pulp and waste paper pulp vary significantly: the cellulose content in wood pulp sludge is relatively high and its properties are relatively stable; Grass pulp sludge contains a large amount of hemicellulose and lignin, and has a high ash content. Waste paper pulp sludge, due to the complex sources of waste paper, may be mixed with impurities such as ink, adhesives, and fillers, and its composition fluctuates greatly.
Meanwhile, even within the same enterprise, the composition of sludge will change at different production cycles. For instance, when the production line switches product specifications, adjusts process parameters (such as cooking temperature, bleaching agent dosage), or changes raw material suppliers, key indicators in the sludge, such as organic matter content, pH value, and particle size distribution, will fluctuate accordingly. The effect of dewatering agents is closely related to the characteristics of sludge. For instance, cationic polyacrylamide (CPAM) is suitable for organic sludge with negative charges, while polyaluminium chloride (PAC) is more appropriate for inorganic sludge with high turbidity and high ash content. The frequent fluctuations in sludge composition may cause the originally compatible chemicals to suddenly become ineffective, requiring repeated adjustments to the types and dosages of chemicals, which increases the uncertainty of selection and operational costs.
Second, there are various types of dehydrating agents, and it is difficult to control the performance differences
There are a wide variety of dewatering agents on the current market. The mainstream types alone include inorganic flocculants (such as PAC, aluminum sulfate, ferric chloride), organic high-molecular flocculants (such as PAM series, cationic starch), and composite flocculants, etc. The action mechanisms, application scopes, and performance parameters of different types of agents vary greatly, which brings difficulties to the selection.
On the one hand, the performance of a single agent has limitations. Although inorganic flocculants are low in price and have a fast sedimentation speed, the flocs they form are small, have a high moisture content, and a large dosage will lead to an increase in the salt content of sludge. The flocs formed by organic polymer flocculants are large and compact, with good dewatering effect, but they are sensitive to the pH value of sludge and have a relatively high price. On the other hand, the formula of compound agents is complex. The proportion of components in compound agents produced by different manufacturers varies, and so do their performances. For instance, some compound agents, with PAM as the main component and a small amount of inorganic flocculants compounded, are suitable for high-organic sludge. Some others mainly use inorganic flocculants, combined with coagulant AIDS, and are suitable for high-ash sludge. It is difficult for enterprises to determine whether the reagent is suitable for their sludge merely based on the product description. They need to conduct a large number of small-scale and pilot-scale tests, which is time-consuming and labor-intensive.
Third, the actual application conditions are variable, which affects the stability of the drug's efficacy
The effect of dewatering agents not only depends on the characteristics of the sludge and the performance of the agents themselves, but is also influenced by the actual application conditions. The variability of these application conditions further increases the difficulty of selection.
The first difference lies in the processing techniques. Common processes for dewatering papermaking sludge include plate and frame filter press, belt filter press, and centrifugal dewatering, etc. Different processes have different requirements for chemicals. The plate and frame filter press has a high pressure, and the flocs formed by the chemicals need to have strong compressive resistance. The belt filter press dehydrates through the squeezing of the filter belt, and the flocs are required to have good air permeability and viscosity. Centrifugal dewatering machines rely on centrifugal force to separate water, and the flocs formed by the chemicals need to be dense and not easy to break. If the selection of chemicals does not match the dewatering process, even if the small-scale test results are good, problems such as low dewatering efficiency and high moisture content of the filter cake may occur in actual operation.
The second is the fluctuation of operating parameters. In actual production, the feed rate, concentration and temperature of sludge, as well as the dosage, dosing method and mixing intensity of chemicals and other operational parameters will fluctuate due to changes in equipment operation status and production load. For instance, if the sludge concentration suddenly increases and the dosage of chemicals is not adjusted in time, it will lead to insufficient flocculation. Excessive mixing intensity will destroy the formed flocs and affect the dewatering effect. The fluctuations of these parameters make it difficult for the efficacy of the agents to stabilize. Enterprises need to constantly adjust the selection and operation plans, which increases the difficulty of management.
Fourth, the dual constraints of environmental protection and cost requirements limit the selection space
Under the current backdrop of increasingly strict environmental protection policies, papermaking enterprises not only have to ensure that the sludge dewatering effect meets the standards, but also must comply with environmental protection discharge requirements while taking into account treatment costs. These dual constraints further compress the space for chemical agent selection.
From an environmental protection perspective, the use of some chemicals may pose a risk of secondary pollution. For instance, certain inorganic flocculants such as ferric chloride, after being added, can lead to an increase in the heavy metal content in sludge. If the sludge is subsequently landfilled or utilized for land purposes, it may cause soil pollution. Some low-quality organic flocculants may contain residual monomers (such as acrylamide), which may release harmful substances during the treatment process and affect the surrounding environment. Therefore, when enterprises make selections, they need to give priority to environmentally friendly agents, such as low-residue PAM and biological flocculants, etc. However, the prices of these agents are usually high, which increases the cost pressure.
From a cost perspective, the procurement cost, transportation cost, storage cost of dewatering agents, as well as the subsequent sludge disposal cost (such as the increase in transportation expenses and landfill disposal fees due to high moisture content) all need to be taken into account. If expensive chemicals are chosen in pursuit of dehydration effect, it may lead to excessively high treatment costs. If low-priced chemicals are chosen to cut costs, the poor dehydration effect may lead to non-compliance with environmental protection standards and the risk of fines. Enterprises need to strike a balance between environmental protection and cost, and the difficulty of selection has significantly increased.