Sludge dryers: Optimize energy use with thermal hydrolysis

For numerous wastewater utilities, sludge drying is a crucial component of their sludge treatment approach. It significantly reduces the moisture content of sludge while creating a stabilized product free of harmful bacteria and pathogens, making biosolids suited for incineration, composting, or alternative applications like cement production, pyrolysis and gasification. Despite its benefits, however, sludge dryers can be notorious for their substantial fuel consumption, which can make for a poor energy case for utilities. In many scenarios, the addition of the thermal hydrolysis process (THP) offers a compelling solution to this energy dilemma, and even more so for plants dealing with waste activated sludge.

The basics of sludge drying

Sludge drying (also known as heat drying), simply put, is the removal of water from sludge via evaporation. It is not to be confused with sludge dewatering, where water is removed in liquid form by mechanical means such as belt presses or centrifuges. Sludge dryers go further than dewatering, using heat to separate water as vapour and producing biosolids in much drier forms like pellets or powders. If dewatering can get sludge to about 10-55% dry solids (DS), depending on the sludge type, drying typically produces sludge of anywhere upwards of 65% DS but more often at around 90% DS.

Many utilities dry sludge to this extent to achieve the following:

• Reduce biosolids or sludge volume more than can be achieved by dewatering. This translates to significant savings on biosolids transport, storage, or further treatment fees (though utilities must also factor for the cost of drying).
• Create a product free of pathogens with a negligible odour that can be used as material for compost or other purposes.
• Achieve biosolids with high enough dryness for sludge incineration, pyrolysis or gasification.

There is quite a broad range of equipment or tools that fit the term "sludge dryer", but they fall mainly into three categories:

• Convective dryers (also known as direct dryers) expose sludge to hot steam or air to evaporate water. Examples of convective dryers include belt dryers, fluidized bed dryers, flash dryers, and rotating or rotary drum dryers.

• Conductive dryers (also known as contact or indirect dryers) use a heated surface or medium that the sludge comes in contact with. The benefit of indirect dryers is the reduced air flow, meaning less heat is wasted as air, and less odour is generated.

  Contact drying equipment includes disc dryers, thin film dryers, and paddle dryers.

• Solar dryers are large greenhouses where the sludge is laid out to dry. These are typically used by municipalities with access to plenty of space far away from residential areas. Though it may seem like a very low-cost option in terms of energy, solar dryers may use equipment to turn or mix the sludge to facilitate faster drying, and some utilize supplementary heating. Deodorizing the air from the greenhouses, if a priority, can also be an added cost.

Figure 1. Examples of sludge dryers. From left to right: An Andritz drum dryer with ancillaries (a type of convective dryer), an LCI Corporation thin film dryer (a conductive dryer), and a Huber solar dryer with a sludge turning machine.

Depending on the characteristics of the sludge being dried, solar dryers can take up to 30 days to get sludge to its intended dryness levels.

When choosing a sludge dryer, municipalities must consider the nature of the sludge, the preferred drying capacity, energy efficiency needs, and the space available.

Anaerobic digestion and sludge drying: Is biogas enough to power sludge dryers?

As sludge dryers depend on heat to get the job done, it becomes crucial to be efficient with energy use for these systems. Dryers typically have an energy demand of 750-1100 kilowatt hours per ton of water evaporation, which is significant consumption. However, if a wastewater utility produces biogas via the anaerobic digestion of sludge, could it perhaps cover this demand? The answer is that it depends on the sludge type.

Mixed sludge and primary sludge typically require much less water evaporation than waste activated sludge (WAS) to reach the same level of dryness. Due to WAS having a higher amount of bound water, it makes for biosolids that achieve low dryness levels after mechanical dewatering. These biosolids require more water evaporation and therefore more heat per tonne of dry solids. As the proportion of activated sludge increases in a specific feedstock, so does the water content that a sludge dryer needs to turn into vapour. A dryer may, therefore, be able to run with the energy produced locally via anaerobic digestion if it's treating digested mixed sludge, but waste activated sludge may need additional power.

Thermal hydrolysis enhances sludge dryer performance: more capacity, less energy required.

Because thermal hydrolysis improves sludge breakdown and dewaterability, it results in specific benefits for digestion utilities drying their sludge:

• THP optimizes sludge dryer capacity. Because THP improves sludge breakdown, which is otherwise called volatile solids removal, there is less sludge or organic material to dewater. THP also improves dewaterability, which means that the sludge, once dewatered, has less water compared to conventionally digested sludge before it enters the dryer. 

  Having less sludge to dry and less water to evaporate positively impacts the amount and size of dryers that utilities need for operations in greenfield projects and can mean an expansion in capacity for brownfield projects.

• THP provides more energy for drying while reducing the dryer's energy demand.

  The graph below models the energy needed to run a mechanical sludge dryer in four different scenarios (from left to right):

  1. Drying raw sludge with no digestion and, therefore, no biogas
  2. Drying digested mixed sludge
  3. Drying digested mixed sludge with the waste activated sludge portion pretreated with thermal hydrolysis
  4. Drying digested mixed sludge pretreated with thermal hydrolysis

  

   

  Figure 2. Energy required (in Metric Million British Thermal Units per day or MMBTU/d) for sludge drying versus energy gained from biogas during digestion and digestion with thermal hydrolysis. Source: Cambi Webinar by Dr. Bill Barber, 2021.

  The “digested” column set shows that the biogas produced during anaerobic digestion may be sufficient to power a sludge dryer. However, the energy available from the process is much higher in the last two column sets, which show the effect of THP.

  Thermal hydrolysis is associated with up to a 50% increase in biogas production compared to conventional digestion. Though it can consume a portion of the extra biogas it generates for steam production, THP still increases the overall biogas produced on-site, allowing for more energy for the dryer. This becomes important, especially for sites with waste activated sludge as dryers processing WAS will typically need additional fuel.  

  Though not shown in the graph above, it is important to consider that thermal hydrolysis systems need energy in the form of steam to operate, and this should be factored in apart from the energy benefits it provides.  

Sludge drying and thermal hydrolysis have proven to be a good mix in various plants in Cambi's portfolio. About half of the Cambi THP plants that use sludge drying have land application as a biosolids endpoint, while the rest either incinerate the dried product or use it for other purposes. One such plant in the latter group is the Psyttalia plant servicing Athens, Greece.

A successful scheme for drying waste activated sludge: The case of Psyttalia

Figure 3. The Psyttalia wastewater treatment plant, owned and operated by EYDAP, sits on an island off the coast of Athens, Greece.

The Psyttalia facility owned by EYDAP sits on an island just outside of Athens, catering to a population equivalent of about 3.5 million. Prior to utilizing the thermal hydrolysis process, the site's mixed sludge was sent to mesophilic anaerobic digesters, dewatering and then drying. The biogas generated from digestion was used to power four rotary drum dryers, with excess biogas going to cogeneration. The dried sludge or biosolids were then sent to cement kilns for incineration.

In 2014, EYDAP set out to improve their energy use at the plant, and in 2015, project contractor AKTOR and Cambi used THP in a specific configuration to deliver on the target. Half of the plant's waste activated sludge would be thermally hydrolyzed and mixed with a portion of the site's primary sludge before digestion. The remaining 50% of the waste activated sludge would then be combined with the remaining portion of primary sludge prior to being sent to a separate set of digesters. The two digested loads then go on to separate dewatering systems and are then dried together.

The results of this scheme for Psyttalia were presented by AKTOR and Cambi in 2017. They include:

• A 16% increase in biogas produced on site from the digesters that were receiving THP-treated activated sludge and non-hydrolyzed primary sludge
• Increased dry solids percentage of biosolids. The dryness achieved surpassed what was expected by pilot studies, as shown below:

  	Before THP	After THP Expectation based on pilot studies	After THP Actual
  Dryness of dewatered cake dry solids (DS)	20-22%	28%	29-31%

• A 40% reduction in the energy demand for the sludge dryer due to increased volatile solids removal and the increased dryness of the dewatered cake, which contributes to the next benefit below.
• More biogas was made available for cogeneration, an increase of 260% compared to before thermal hydrolysis was utilized.

The impact of thermal hydrolysis can be more visually appreciated in the following diagram showing the energy use on-site before and after the Cambi installation:

Figure 4. Biogas use at the Psyttalia wastewater treatment plant before and after the Cambi thermal hydrolysis process was installed in 2015. Source: Cambi Webinar by Dr. Bill Barber, 2021.

Note that the energy needed to run the thermal hydrolysis system in Psyttalia was provided by the high-grade heat coming from its combined heat and power (CHP) or cogeneration system, which is the recipient of the green energy stream in the sankey diagram above.

The unique configuration in Psyttalia, where only 50% of the site's secondary solids was treated, allowed the plant to make a smaller capital investment in their thermal hydrolysis system while still resulting in substantial benefits. In 2023, the facility upgraded its THP plant with an additional train to treat the remaining 50% of WAS produced at the site, augmenting the positive effects on the site's energy use. Cambi's thermal hydrolysis process now treats 100% of the site's waste activated sludge.

Figure 5. The two trains of Cambi THP now processing 100% of Psyttalia's waste activated sludge (2023)

Plants using THP to improve sludge dryer performance include the Anyang-Bakdal plant in South Korea, the Jurong plant in Singapore, the Ringsend facility in Scotland, the Vigo plant in Spain, the Vilnius plant in Lithuania, and several others.

Potential to change the sludge drying landscape

Considering the volatility of today's fossil fuel market and the increasing pressure on utilities to save on costs, wastewater treatment plants dependent on thermal drying stand to gain from looking at the potential synergy that can be had by using thermal hydrolysis to improve sludge dryer capacity. THP has the ability to paint a better energy picture for such utilities. 

Want to learn more about other sites whose energy use has benefitted from THP? Check our references page.