Tsinghua University Professor Wang Wei: Hydrothermal Drying Technology for Sludge Treatment

At the 2007 Advanced Technology Forum on the Water Industry, Wang Wei, a professor in the Department of Environmental Science and Engineering at Tsinghua University, shared with conference delegates the current status of sludge treatment and disposal in China, as well as sludge treatment standards, the current state of technology, and existing bottlenecks. He also focused on hydrothermal drying technology for sludge treatment. This article was compiled from an audio recording and has not been verified by the speaker.

Good afternoon, distinguished leaders, experts, and guests! The topic I will be presenting today is: Hydrothermal Drying Technology for Sludge Treatment. I will begin by discussing some of the challenges in sludge treatment; these are my personal views, and I look forward to discussing them with all of you.

Current Status of Sludge Generation, Treatment, and Disposal in China

In 2004, China’s sludge generation (calculated at 1.5 metric tons of dry solids per 10,000 metric tons of wastewater) was 12.2 million metric tons; by 2010, it had risen to 27.4 million metric tons. On the one hand, sludge volumes continue to increase; on the other hand, there is a disconnect between sludge treatment and wastewater treatment in our country. First, the treatment rate is low, and processes are inadequate; second, technologies are limited, and equipment is outdated; third, the reliability of sludge disposal is relatively low, and the risk of secondary pollution is significant. Currently, the primary disposal methods for sludge from China’s wastewater treatment plants are landfilling and agricultural application. However, 80% of the sludge from these plants is rejected by properly managed landfills because it significantly disrupts landfill operations, leading to the spread of pathogenic microorganisms and posing environmental hazards; Third, sludge treatment costs are high. In large wastewater treatment plants abroad that employ digestion and mechanical dewatering, the investment ratio between wastewater treatment and sludge treatment is 1:1.7. In our country, however, during the 11th Five-Year Plan period, this ratio was 1:0.4. Of course, we are pinning our hopes on the development of more advanced and efficient treatment technologies in our country.

Given the current situation, sludge from most of our country’s wastewater treatment plants has not been properly and effectively disposed of, resulting in the transfer of significant pollution. I have highlighted here many wastewater treatment plants in the Three Gorges Reservoir area; due to the lack of a viable solution for sludge disposal, they have caused pollution issues. From another perspective, the current national tendency to “prioritize water treatment over sludge management” has resulted in wastewater treatment efforts yielding “half the results with double the effort.”

I have statistics here from the State Environmental Protection Administration showing that sludge is a byproduct of wastewater treatment; as wastewater is purified, approximately half of the pollutants are transferred into the sludge. Without properly addressing the issue of sludge treatment and disposal, the effectiveness of wastewater treatment will remain half the result for twice the effort.

From the perspective of sludge treatment, it is well understood that this is a highly capital-intensive and energy-intensive process. On the one hand, because sludge contains a large number of microbial cells and organic colloidal substances, mechanical dewatering is very difficult. The mechanical dewatering methods commonly used in China can reduce the sludge’s moisture content to about 80%. Secondly, the organic matter in sludge exists primarily in solid form, making it difficult to biodegrade.

In fact, there is widespread concern about sludge treatment, but where exactly do the challenges lie? It is more difficult than waste management. Annual waste generation amounts to 150 million metric tons, while sludge generation is only a few metric tons—in fact, only about one-tenth of the waste volume—yet people perceive it as posing many more problems. High moisture content is a major issue. The moisture content of sludge fluctuates dramatically: when it rises, the volume of sludge increases rapidly; if it drops from 95% to 80%, the volume decreases rapidly; and if it drops from 80% to 75%, the volume decreases significantly again. At the same time, the higher the moisture content of sludge, the lower its calorific value. Our current moisture content is approximately 80%, which corresponds to a calorific value of just over 100 kcal. Sludge with such a low calorific value clearly cannot be fully incinerated through direct combustion. It must either be dried or have auxiliary fuel added to complete the incineration process. If the moisture content in the sludge is reduced to 50%, the calorific value rises to approximately 1,200 kcal.

This is the current state of sludge treatment in our country, which primarily involves direct landfilling, composting, or drying and incineration. However, regarding direct landfilling, in addition to the issues mentioned by the experts earlier, there is another factor: the European Union’s newly enacted landfill regulations have already restricted landfill disposal, a development that will have significant implications for countries worldwide. Furthermore, organic matter accounts for 70% to 80% of the sludge generated during secondary treatment. As a result, such disposal will face increasing restrictions in the future. Composting, on the other hand, can lead to problems such as foul odors due to high moisture content.

Standards for Sludge Treatment

This section discusses the national standards for sludge composting for agricultural use. Are the standards for using sludge in agriculture too strict? In my personal opinion, the standards are not too strict, but rather incomplete. This is because our country only sets limits on the concentration of heavy metals in sludge.

These are the U.S. standards for agricultural use, which Mr. Zhang (Editor’s note: Zhang Chen, Chief Engineer at the Shanghai Municipal Engineering Design Institute) just mentioned. They specify maximum limits, and many of the permitted levels for application to farmland are higher than ours; however, they also include monthly average concentration limits. When comparing our maximum concentration limits to theirs, ours are stricter. However, when comparing indicators such as monthly averages, cumulative tolerance levels, and other metrics—including annual application rates and application ratios—their standards do not result in cumulative impacts.

The situation in Japan is similar. It has a standard, along with a limit on annual application rates, designed to ensure that the soil’s natural purification capacity is maintained and aligned with fertilizer conditions. The same applies in the European Union.

Objectives, Current Status, and Technical Bottlenecks of Sludge Treatment

The objectives of sludge treatment are related to all organic matter. First, reducing moisture content to facilitate subsequent treatment, utilization, and transportation, and to minimize the volume of material requiring final disposal. Second, rendering the sludge harmless by killing or removing pathogens, organic matter, and other toxic and harmful substances. The third objective is resource recovery—improving the composition and properties of sludge to facilitate the recovery of energy and resources.

Current state of sludge treatment technology: Sludge treatment primarily involves concentration, digestion, and dewatering, resulting in a low disposal rate. A popular process is drying and incineration, which many companies are currently promoting; this involves using drying methods to reduce or increase moisture content. However, for wastewater treatment groups across the country, the biggest issue is cost—it requires significant investment, which presents a major dilemma. Another approach involves dewatering followed by semi-drying and then composting. If semi-dried sludge is used for composting, the operating costs may actually be higher than those of incineration, because there is no subsequent energy recovery, whereas incineration offers a high potential for energy recovery. If you do the math carefully, the cost of semi-drying and composting is higher.

So where does the technical bottleneck in sludge treatment lie? The biggest and most significant characteristic of sludge is that it contains a large amount of cytoplasm and colloidal matter, which makes dewatering difficult. Currently, we can only dewater it to about 80%. Due to the high moisture content, sludge volume reduction is ineffective, and energy consumption for treatment is high. If we could achieve absolute drying of the sludge, this wouldn’t be a problem—it would amount to just over 1 million metric tons, which could have been landfilled long ago. The main issue is one of efficiency.

As for the digestion process, our country currently employs the traditional CSTR (Continuous Stirred Tank Reactor) mixed fermentation method—essentially the most conventional approach. Consequently, its efficiency isn’t particularly high. Furthermore, sludge is inherently difficult to degrade, which means a large portion of the substances within it cannot be recovered efficiently. These issues result in high energy consumption, high operating costs, and poor economic returns in our sludge treatment processes. This is the reality we face, and it is a major headache for various wastewater treatment groups and water treatment plants.

The root cause of these problems lies in the long-term neglect of sludge treatment. Now, as sludge treatment capacity expands and sludge volumes increase, there is a growing recognition of the importance of treatment. We can now address our country’s sludge treatment challenges using more efficient and scientifically sound technologies.

Key Aspects of Sludge Treatment

There are three key aspects to sludge treatment. The first is reducing moisture content, which is the foundation for achieving energy savings and reduced consumption in the system, as high moisture content makes composting difficult. So how can we reduce the moisture content of sludge? This is primarily achieved through cell disruption technology, which addresses the issue at the cellular level using techniques such as ultrasonic treatment.

Second, improving digestion efficiency is central to achieving energy recovery within the system. While anaerobic digestion technologies for sludge treatment have advanced significantly—with methods such as USB and ASB—these are not suitable for sludge containing large amounts of suspended solids. Therefore, we continue to use CSTR technology. By modifying its dewatering performance, we can reduce the retention time from 30 days to 3 days, thereby increasing efficiency tenfold.

Third is the optimization and integration of processes. I believe sludge treatment does not rely on a single process, a single technology, or simply feeding in material. In reality, sludge treatment is a system that requires a systematic combination of optimized processes to improve the efficiency of the entire system.

Combined Hydrothermal Drying Technology for Sludge Treatment

The objectives of this technology are volume reduction, harmless disposal, and resource recovery.

Combined hydrothermal drying technology involves heating sludge in a concentrated, liquid state to break down the cytoplasm and colloidal substances within it, thereby significantly improving its dewatering performance. While conventional sludge can be dewatered to 80% using a standard dewatering machine, hydrothermal drying can increase this efficiency by an additional 50%.

Second, the hydrothermal treatment is conducted at temperatures of 170–180 degrees, which kills nearly all bacteria. After hydrothermal drying, the sludge decomposes at temperatures exceeding 100 degrees, improving the biodegradability of its organic matter. This allows us to replace the traditional CSTR with the USB method, increasing efficiency from 40% to 80% and thereby achieving resource recovery benefits.

As shown in the hydrothermal drying performance graph, the black curve represents slow growth; at 180 degrees, the effect is actually worse. At 120°C, and even at 150°C and 170°C, this interface grows very rapidly. This graph illustrates the improvement in viscosity: at 170°C, for different treatment durations, we can see that within approximately 10 to 15 minutes, the sludge viscosity decreases from 6,000 to below 1,000. After treatment at 170°C, the sludge transitions to a continuous fluid state, demonstrating the effect of its structural transformation.

Dewatering involves mechanically removing free water from untreated sludge. However, much of the water within the sludge—including capillary water, interstitial water, adsorbed water, and internal water—is difficult to remove by mechanical means alone. Therefore, we use hydrothermal treatment to break down this colloidal structure. This is the underlying principle. In our experiments at Beixiaohe, the dewatering rate of the treated sludge was approximately 50% to 55%.

Our anaerobic technology has made significant technical strides in recent years, providing an excellent foundation for efficient sludge treatment. However, this highly efficient anaerobic technology cannot be directly applied to the sludge sector. On the one hand, anaerobic technology is advancing rapidly; on the other hand, our sludge anaerobic digestion has continued to rely on traditional digestion methods. Since these two approaches are incompatible, we can use hydrothermal drying technology to separate the SRT and HRT, thereby improving the ASBR process.

This is the hydrothermal treatment of ASBR conducted by one of our doctoral students, which can enhance the conversion efficiency of solid organic matter. By retaining a large amount of solid material in the reactor and allowing it to react for a longer period, we can achieve higher efficiency. We achieved a sludge reduction of approximately 70% within 20 days, which is about twice as efficient as the average efficiency of direct sludge digestion in our country.

Through our research, we have proposed a conceptual framework encompassing concentration, hydrothermal drying, dewatering, and digestion. There are two approaches: one involves hydrothermal drying, dewatering, removal of the sludge cake, and subsequent disposal via incineration; the other involves ASBR digestion followed by disposal.

The process flow diagram shown for Xiamen is the one we used during our pilot tests at Beixiao River. Once the concentrate enters the system, it cools the reactor and then flows into a secondary thickening tank, where it can be concentrated to about 10%, significantly reducing the subsequent hydraulic load. The supernatant undergoes USB treatment, and the resulting sludge cake contains 8,000 kJ of thermal energy. Finally, the sludge is incinerated, and the heat generated provides the thermal energy required for direct sludge treatment. We reduce the energy consumption of the main system through heat recovery, and the secondary thickening tank is primarily used to reduce the hydraulic load. Any surplus energy can also be supplied to external users.

If digestion is carried out—excluding incineration—the system’s sludge reduction rate can reach over 95%. Analysis of the final sample from our experiment showed a moisture content of 43%; under these conditions, its energy content was 8,021 kcal.

The so-called hydrothermal drying technology improves the overall treatment efficiency of the system through mechanical concentration, thereby reducing the volume of the hydrothermal treatment reactor. After concentration, the sludge solids content reaches over 5%. Through the hydrothermal reaction, under specific temperature and pressure conditions, approximately 30% of the suspended solids in the sludge dissolve and transfer to the liquid phase, resulting in an increase in the COD concentration of the supernatant. Through secondary concentration, leveraging the sludge’s good settling properties, the sludge can be concentrated to a solids content of approximately 10% within a short period, thereby significantly reducing the hydraulic load on subsequent dewatering processes. Finally, dewatering improves the dewaterability of the hydrothermal sludge; direct dewatering yields a sludge cake with a solids content greater than 50%. Compared to direct dewatering processes, this method achieves a sludge volume reduction of over 80%;

Finally, regarding heat exchange between cold and hot sludge, we can use a heat exchanger to bring both the hot and cold sludge to a temperature of approximately 60 degrees. Below is the material balance for our daily treatment of 200 metric tons of sludge. Thank you, everyone!

Question: Hello, Professor Wang! Since you are raising the moisture content of sludge from over 90% to over 70%, what is the energy consumption like?

Wang Wei: Actually, I touched on this issue earlier. We’ve incorporated a heat exchanger into this system, which allows cold sludge to exchange heat with hot sludge before entering the reactor. The hot sludge is at 120 degrees, and the cold sludge is at 20 degrees, but the heat exchange occurs at 40 degrees, so the sludge only needs to be heated by 40 degrees. Furthermore, under non-pressurized conditions, raising the temperature of one metric ton of sludge by 40 degrees requires 40 kcal. If we were to use a drying method, removing this water would require a significant energy input. With the mechanical removal method I’ll describe below, however, the water can be removed with very little energy. If we were to use a drying method to reduce the moisture content from 80% to 50%, we would need to evaporate 600 kilograms of water per metric ton of sludge to achieve a 50% moisture content. That amounts to 36 kcal—roughly equivalent to our energy consumption. Compared to traditional evaporation drying processes, which also reduce moisture content from 80% to 50%, this method reduces energy consumption to one-tenth. This is the key to this technology’s energy savings: one involves latent heat of evaporation, while the other does not. (China Water Network)