Sludge incineration and the thermal hydrolysis advantage

Sludge incineration is one of several methods for sludge treatment, and arguably the most common apart from land application and landfilling. This thermal treatment method achieves the highest volume reduction for sludge. In many cases, it can be made more efficient when the biosolids it uses as feedstock is treated with the thermal hydrolysis process (THP) or THP and digestion. Discover how this is so and get to know some facilities that are strategically leveraging THP to better their incineration strategy.

Why choose incineration for sludge or biosolids?

Choosing incineration as the outlet for sludge or biosolids involves several factors, with local legislation often playing a pivotal role. Countries like the Netherlands and Germany, for example, favor sludge incineration because they have extensive limitations or prohibitions for the use of biosolids in agricultural land. Sludge containing high concentrations of heavy metals or other hazardous materials, like those found in Far East Asia, can also prevent municipalities from land-spreading biosolids.

Topography is another driver for considering thermal treatment. Hong Kong, for instance, is mountainous with a high population density, and there simply isn’t available farmland to land-apply biosolids away from residential areas.

While it may seem like a constrained choice dictated by regulations or location, sludge incineration boasts distinct advantages:

• Using incinerators is considered a final disposal route, with fly ash as the end product, unlike utilising a sludge dryer, which produces biosolids that still need another treatment step or need further management.
• In some cases, Phosphorus (P₂0₅) can be obtained from the ash of incinerated sludge, making it a nutrient recovery option
• Incineration is a decades-old proven technology that is better than landfilling

Despite these benefits, incineration still faces challenges as a sludge treatment option. Firstly, the perception of incineration is still largely negative due to the hazardous waste it generates that needs to be managed. Secondly, it’s a costly method due to both the capital investment and the fuel needed. Monitoring requirements also bump up operations costs.

These factors, however, do not seem to stifle the future of sludge incineration, especially since there is increased attention toward PFAS (per- and poly-fluoroalkyl substances), its successors, and other contaminants that may persist in treated sludge.

The sludge incineration process simplified

 Figure 1. A typical sludge incineration process. Source: Waterleau

To understand the benefits of anaerobic digestion (AD) and thermal hydrolysis for sludge incineration, it’s beneficial to first look at a typical sludge incineration process and understand its components (See Figure 1).  

Sludge cake reception and drying

Sludge incineration plants receive cake or biosolids in large silos from several wastewater treatment or sludge treatment facilities. The incineration plants typically use drag link conveying systems or even cake pumps, which can pump sludge at 20 to 30% dry solids into an incinerator.

The ideal case is for the received sludge cake to be “autothermal” or “autothermic,” which means it’s dry enough to self-combust and no auxiliary fuel will be needed to burn the material. However, the sludge cake often arriving at these plants is perhaps at 20-25% dry solids, which is typically not dry enough to be autothermal. Such feedstock needs to be pre-dried, which can be a regulatory requirement in some municipalities. The heat used for sludge dryers can come from the incineration process that follows.

Incineration

Once dry enough, the sludge is typically fed into a fluidized bed incinerator which operates at a temperature of about 850-950C.

During incineration, the energy-affecting components of sludge come into play, which are otherwise called volatile solids (VS). When sludge has a good proportion of volatile solids that provide energy, like carbon and hydrogen, then naturally more energy (heat) can be extracted from the material.

The incineration of these volatile solids and other sludge components generates heat and what is called “flue gas.”

Heat recovery

An energy recovery system receives heat which can be used to power turbines for electricity, for example, while some of the heat is diverted for pre-drying of sludge.

Flue gas treatment and fly ash recovery

Out of the incinerator comes flue gas containing ash and other particles that need to be treated. Depending on the regulatory requirements, treatment can consist of multiple stages and often consists of electrostatic precipitation.

This portion often uses gas scrubbers to remove ash or harmful components like mercury. This step has three main outputs: fly ash, which is either landfilled or used as a raw material in construction or other industries; special waste, which is further treated or disposed of; and treated gas which goes out of the smokestack.

Achieving drier, energy-rich sludge with digestion and THP

Understanding the process, it becomes clear that sludge incineration is optimized when the material to be burned is dry enough, with a good balance of the volatile solids that are sources of energy. So how do digestion and thermal hydrolysis contribute to a high-energy, sufficiently dry feedstock for incineration? The short answer is that they increase sludge dryness to provide autothermal sludge for incineration.

The sludge incineration process showed that incineration is improved when the feedstock is autothermal, i.e., dry enough so that it doesn’t need fuel to burn. For sludge to be autothermal, it needs to produce energy within a range equivalent to 1100-1500 kWh per wet tonne of material. Different types of sludge, however, have different energy compositions. They will then need distinct levels of dryness to achieve this autothermal state. Figure 2 below shows the dry solids levels needed by several types of sludge.   

Figure 2. Dryness levels (dry solids) needed for autothermal operation of various sludges in an incinerator. Source: Cambi

Raw mixed sludge and raw primary sludge need a dry solids percentage of about the same level in order to be autothermal while digested sludge needs to be at a slightly higher level at 26-30% DS (Dry Solids). Raw biological sludge or waste activated sludge (WAS) needs a dryness level similar to that of digested sludge, but due to its viscosity, raw WAS cannot achieve this dryness level. Chemically dosed sludge (for phosphorus recovery) has too much oxygen and therefore reduced energy levels and a high requirement for dryness.

Effective dewatering at wastewater treatment plants is key to achieving the necessary autothermal levels indicated above. This is where thermal hydrolysis makes a difference. As thermal hydrolysis pressure-cooks sludge, it changes the viscosity of the material to significantly improve sludge dewaterability and help achieve the needed levels of dryness.

The chart below displays the energy content of THP-treated sludge versus raw sludge and digested sludge, accounting for the effect of improved dewaterability. This shows that THP-treated sludges reach the necessary levels of dryness for incineration.

 Figure 3. Energy content of raw, digested, and THP-treated sludge considering dewaterability. Source: Cambi

In the case of the sludges treated with THP before digestion (columns 3 and 4), their calorific values about equal or surpass raw sludge when dewaterability is accounted for. Even secondary sludge or WAS, which was noted above to be unable to reach autothermal conditions because of high viscosity, improved its dryness and energy content significantly because of THP and can therefore be incinerated.  

Sludge treated with THP after digestion (column 5) is of note – dewatering efficiency can be about doubled with this configuration, notably improving energy content ahead of the rest.

Less biosolids with the same calorific value increases incinerator efficiency

Apart from the benefits of a better energy profile and dewaterability, THP has an additional advantage for incineration. Because THP reduces biosolids overall while retaining about the same energy content per unit of volume, it frees up incinerator capacity. When considering urban growth, this becomes a particularly good point to consider.

This is a major factor for some Cambi clients, as is seen in the examples below. These also demonstrate the value for incineration across various configurations and sludge types.

Case 1:  Increasing the capacity of an existing incinerator (Hengelo, the Netherlands)

Secondary solids or waste activated sludge pre-treated with THP + digested

The Hengelo plant in the Netherlands treats all the secondary sludge or WAS from 20 treatment plants owned by Waterschap Vechtstromen. By installing Cambi’s thermal hydrolysis process in Hengelo, they were able to reduce the biosolids volume leaving for incineration by around 500 trucks per year.

The incineration plant at Moerdijk thereby increased capacity and could accept more sludge cake from other facilities in the area.

Other Cambi plants treating waste activated sludge for incineration include Jurong in Singapore, the North plant in Brussels, Belgium, and the Psyttalia plant serving Athens, Greece. The Damhusåen plant in Copenhagen, Denmark will be added to the list in 2024.

Case 2: Making a flexible sludge strategy with incineration (Davyhulme, United Kingdom)

Mixed sludge pre-treated with THP + digested

Davyhulme is one of several plants owned by United Utilities funneling sludge via a pipeline to the Mersey Valley Processing Centre for incineration. By using thermal hydrolysis before digestion, the site improved the overall dryness of all solids received at the incineration plant, freeing up incinerator capacity. The biosolids produced at Davyhulme also became an enhanced cake product that gave the facility flexibility in terms of what they do with their biosolids. They could continue to send it to the incineration plant via the pipeline or land-apply when sufficient landbank was available. Not only that, but the increase in biogas has also helped the plant become energy self-sufficient.

Other plants using THP as pre-treatment for digestion prior to incineration include the Bakdal wastewater treatment plant in Anyang, Korea, which co-digests sludge with food waste, and three new Cambi contracts: one for Singapore’s Tuas plant, New Zealand’s Moa Point plant in Wellington, and the soon-to-be operational Shek Wu Hui plant in Hong Kong.

Apart from serving as pretreatment, THP can also be combined with digestion in other configurations with incineration as an endpoint. These as can be witnessed at the Południe (South) plant in Warsaw and an upcoming reference in Antwerp, Belgium.

Looking at the overall energy picture

In summary, digestion and thermal hydrolysis benefit sludge incineration because they markedly improve sludge dewaterability. Wastewater treatment facilities can therefore lessen biosolids volumes while maintaining similar energy content in the sludge. Less sludge with similar calorific value frees up incinerator capacity.

The fact that sludge is digested prior to incineration also means that energy is gained from the material in two steps: the biomethane or biogas produced during digestion, and the heat produced during incineration. When looking at the total energy picture, therefore, digesting and thermally hydrolyzing sludge prior to incineration means maximizing the renewable energy potential of the material and, at the same time, increasing incinerator capacity. In many instances, this approach lowers utilities’ expenditures and helps prolong the life of existing incinerators.

Real-world cases around the globe are proving that digestion and THP are considerable and competitive options in a sludge incineration strategy. It’s exciting to see how the next few years will see them utilized to better the thermal processing of sludge or biosolids.

Want to explore all of Cambi’s projects that use THP for incineration? Curious about plants that have shifted from incineration to land application with THP? Have a look at Cambi’s references.