New 50-ton-per-day Sludge Drying and Advanced Dewatering System

Currently, many wastewater treatment plants in China simply subject sludge to basic thickening and dewatering before transporting it off-site, without providing any information regarding its final destination. Even when wastewater treatment plants do perform sludge concentration and dewatering, the results often fall short of requirements, leaving the sludge with a high moisture content. Inadequate sludge treatment not only complicates transportation but also poses a threat to the environment along transport routes. More seriously, it creates significant difficulties for subsequent sludge disposal. For composting, the moisture content requirements cannot be met; for landfilling, the sludge fails to meet landfill acceptance criteria, leading to rejection by landfills; and for incineration, the sludge lacks the necessary calorific value, while simultaneously consuming excessive amounts of heat, resulting in severe air pollution.

Surveys have found that the primary method of sludge disposal at most wastewater treatment plants is landfilling, followed by land application. Landfilling accounts for a significant proportion of sludge disposal; however, since most landfills are open-air facilities, sludge that has not been stabilized or rendered harmless quickly reverts to its original state after leaching by rainwater, posing a serious threat to the safety of the landfill site. Improperly treated sludge also causes severe blockages in landfill leachate systems, seriously contaminating nearby groundwater. In particular, when sludge is co-landfilled with municipal solid waste, it significantly shortens the lifespan of many landfills, creating major challenges for urban waste management.

Therefore, moisture content is the key factor limiting sludge disposal and utilization.

The core element of this process technology is mechanical deep dewatering. It organically combines mechanical deep drying and flash drying technologies, forming the latest technology for the comprehensive resource utilization of sludge in China.

① Sludge Stabilization and Solidification Technology

Sludge solidification treatment has gained widespread attention and become a commonly used method in industrial sludge treatment in recent years. It refers to a process in which sludge particles are cemented, blended, and encapsulated within a dense, inert matrix using physicochemical methods to form a solidified body with good structural integrity. The inert material used in solidification is called a solidifier, and the product formed after sludge undergoes solidification treatment is referred to as a solidified body.

Stabilization is the process of converting toxic and harmful pollutants into substances with low solubility, low mobility, and low toxicity. Stabilization can generally be divided into chemical stabilization and physical stabilization. Chemical stabilization involves using chemical reactions to transform toxic substances into insoluble compounds, trapping them within a stable crystal lattice; physical stabilization involves mixing sludge with a loose material (such as fly ash) to form a coarse-grained solid.

Sludge solidification and stabilization are generally carried out simultaneously. The mechanism involves adding a solidifier to the sludge, which, through a series of complex physicochemical reactions (such as hydration reactions), traps toxic and harmful substances within the resulting network (crystal lattice), transforming the sludge into a soil-like material or a solid with high binding strength. This treated material can be landfilled in situ or used as construction material. Sludge solidification treatment technology can be applied to the solidification of hazardous waste, such as special industrial sludges (e.g., sludge containing heavy metals, oil-contaminated sludge, electroplating sludge, and dyeing sludge), as well as to the solidification of ordinary sludge generated by municipal wastewater treatment plants.

② Type A1 Solidifier

(1) Design Principles for Sludge Solidification Treatment

Ø All sludge generated during the wastewater treatment process at the treatment plant must undergo stabilization and harmless treatment to ensure the plant fully complies with national standards and facilitates resource recovery.

Ø The design of the sludge treatment project fully accounts for variations in the operating conditions of the wastewater treatment process. Sludge treatment takes into consideration the current status of the wastewater treatment facility and the differing properties of various sludge types, ensuring that the treatment process can be flexibly combined, separated, or switched as needed.

Ø The solidified product formed after hazardous waste undergoes solidification treatment should possess good resistance to permeation, leaching, moisture, and freeze-thaw cycles, as well as sufficient mechanical strength;

Ø It should control the volatilization of odors from the sludge to meet the exhaust gas emission standards specified in the “Emission Standard for Pollutants from Urban Sewage Treatment Plants” (GB18918-2002).

Ø Material and energy consumption during the solidification process should be low, and the volume expansion ratio should be low;

Ø The solidification process should be simple and easy to operate; solidification agents should be readily available and inexpensive; treatment costs should be low.

Ø Various types of waste residues should be utilized as much as possible to prepare solidification agents, thereby achieving the goal of treating waste with waste.

Ø The solidified sludge should be environmentally friendly and, once stabilized, capable of being reused for applications such as land reclamation, embankment construction, and road building.

(2) Development and Performance of the A1 Solidifier

Based on the aforementioned design principles, our company collaborated with relevant domestic research institutes to formulate a new type of magnesium-based cementing solidifier (A1 Solidifier) by blending several magnesium salts and adding coagulants and water-repellent agents. Compared to other solidifiers, this product offers the following advantages for the solidification treatment of sludge from wastewater treatment plants:

Ø Short curing time: It can solidify sludge within a short period (48 hours), meeting landfill requirements;

Ø Low dosage (5%–10%): It causes minimal changes to the sludge pH while suppressing odor generation;

Ø An eco-friendly sludge conditioner: It does not cause secondary pollution to the sludge, improves sludge properties, and promotes sludge stabilization.

Ø Simplified solidification process, making it easy to produce and apply;

Ø After a 2–3-year stabilization period in the landfill, the treated sludge forms a soil-like substance that can be excavated and utilized, enabling the sustainable use of the sludge landfill.

Comparative experiments were conducted on sludge solidification using different solidifiers at varying addition ratios. When the cement solidifier was added at 20%, the compressive strength reached 58.17 kPa; however, the volume increased significantly after solidification, with a volume expansion ratio of 1.52; Lime-solidified sludge exhibited poor strength (<20 kPa), and at higher addition rates, it caused the sludge pH to shift toward alkalinity, resulting in increased malodorous emissions; the A1 solidifier demonstrated superior performance, achieving a compressive strength of 52 kPa at a 5% addition rate, while reducing the landfill volume of the sludge and having minimal impact on sludge pH.

③ Mechanical Drying Equipment and Advanced Dewatering Technology

Its performance, operational efficiency, and degree of dewatering are unmatched by comparable technologies and products. It features proprietary intellectual property and, when combined with solidification technology, forms a unique sludge solidification and filter press process. Sludge solidification and filtration essentially involves modifying the sludge by adding a solidifier, followed by rapid filtration and dewatering using mechanical drying equipment to reduce the sludge’s moisture content and odor. Key indicators of sludge treated by this process include: odor reduced to below Level 3, moisture contents of 60%, 40%, and 20%, compressive strength ≥100 kPa, and shear strength ≥50 kPa. Mechanical drying and deep dewatering technology has completely transformed the disadvantages of other technologies—such as long processing cycles, large land requirements, and air pollution—enabling industrial-scale and large-scale sludge treatment.

④ High-Temperature Thermal Drying Technology (inlet air temperature up to 500°C)

After mechanical drying, the sludge moisture content is approximately 60%. At this stage, the sludge is in a semi-wet state, which is suitable only for landfilling. If incineration is required, this moisture content is too high, necessitating further mechanical drying to reduce the moisture content to approximately 40%. Thermal drying rapidly evaporates moisture from the sludge within a short period to achieve the goal of moisture removal. The dried sludge resembles power plant fly ash in appearance, and sludge with a moisture content of 20% can be recycled. Drying equipment utilizing high-temperature drying technology achieves a comprehensive thermal efficiency of over 82%, with a coal consumption of only 80 kg per ton of sludge processed.

5.1 Technical Features

Ø Sludge volume reduction is significant. By treating sludge with our proprietary “mechanical drying and deep dewatering technology,” we can achieve immediate dewatering, reducing the moisture content from 80% to below 60%, thereby significantly reducing sludge volume.

Ø Secondary pollution is avoided during the treatment process. The dewatering filtrate generated during deep dewatering contains no biohazardous components; after pretreatment, it can be directly returned to the wastewater treatment plant for processing. The dewatered sludge has no noticeable odor and is convenient to load, unload, store, and transport.

Ø Mechanical drying equipment can be flexibly selected to handle raw sludge volumes ranging from tens to thousands of tons; the equipment is easy to maintain, operates stably and reliably, and currently allows for maintenance without production downtime. The technology is mature and the equipment is reliable, making it suitable for direct introduction and adaptation.

5.2 Technical Advantages

Currently, the most common sludge treatment technologies in China include high-temperature aerobic fermentation, thermal drying, incineration, and mechanical drying with advanced dewatering. A detailed comparison of these four technologies is provided in the table below:

As shown in the table above, the mechanical deep dewatering process for sludge offers significant advantages over other processes in terms of capital investment, operating costs, air pollution, product utilization, and energy consumption.

  Sludge moisture content is a key factor limiting sludge disposal and recycling; 60% is the threshold for landfilling and composting, while 50% is the threshold for incineration. The drying stage is the primary energy-consuming component of sludge treatment and disposal systems. Currently, the development of new technologies for the drying stage is the focal point for achieving energy conservation and reduced consumption in sludge treatment systems, while minimizing moisture content as much as possible. In this regard, mechanical drying and deep dewatering of sludge can effectively meet these requirements; the dried sludge possesses a certain calorific value, making subsequent resource recovery possible.