Sludge dewatering: How does thermal hydrolysis improve dewaterability?
The dewatering of sludge coming from conventional anaerobic digestion is an important step in overall sludge treatment or sewage sludge management, as it reduces the biosolids volume to be handled. Biosolids or digestate that have undergone Cambi’s thermal hydrolysis process have been proven to have better dewaterability, as the method fundamentally changes sludge properties.
For sewage sludge treatment facilities with anaerobic digestion, final sludge dewatering is an important last step with the main aim of reducing a plant’s biosolids output. Removing as much water as one can from digested sludge or biosolids, also referred to as "digestate", ensures less biosolids to manage and an improved biosolids structure. This makes the handling of the final product easier and minimises its cost, regardless of whether the cake is meant for land application, storage, or incineration.
Sewage treatment plants often optimise this step by testing and choosing the best dewatering equipment or method. However, an equally crucial factor that determines sludge dewaterability must not be forgotten: the nature or characteristics of the sludge to be dewatered.
Different sludge characteristics, different levels of sludge dewaterability
Wastewater treatment plants that have compared the dewaterability of primary sludge and secondary or waste activated sludge (WAS) have found that there are various sludge characteristics that lend to easier dewaterability or a higher percentage of dry solids. These characteristics are listed below and show that difficulty in dewaterability increases with viscosity and the presence of organic content, bound water, and extracellular polymeric substances. Such characteristics are common in WAS, which is known to be harder to dewater than primary sludge.
Besides this difference in the dewaterability of primary and secondary sludge, it is important to note that not all substrates or feedstocks of a similar type have the same level of dewaterability either, i.e., not all primary sludges will have the same dewaterability, just as how not all WAS or mixed sludge will be as difficult to dewater.
This is because the sludges produced in various places are unique and highly influenced by the wastewater treated, the treatment process and the climate.
A municipality, for example, may have the wastewater from a dairy manufacturer entering the sewer network, resulting in a high proportion of WAS in the mixed sludge, or another municipality may use biological phosphorous removal rather than chemical removal, resulting in much higher EPS concentrations.
It becomes significant, therefore, to be mindful of this when comparing the dewaterability of sludge from different sources and to characterize the digestate when comparing its dewaterability across various methods or equipment.
Thermal hydrolysis improves sludge dewaterability by affecting water distribution
In the table above, the last characteristic listed is water distribution. By affecting this characteristic, we can make sludge easier to dewater.
Water distribution refers to water occurring in different states within sludge that determine how difficult it is to separate from solid particles. These are the following:
- Free water – flows freely between sludge flocs. This can be separated mechanically
- Interstitial water – water inside flocs that are not as tightly bound to solids
- Surface water – bound to the surface of sludge floc through adhesion
- Intracellular water – water inside cells in the sludge
Interstitial, surface, and intracellular water are known as “bound water” and cannot be removed by mechanical dewatering. This is where the thermal hydrolysis process can make a difference.
Several studies and cases have proven that the thermal hydrolysis process (THP) affects water distribution, among other properties, to improve digestate dewaterability. The thermal hydrolysis process involves treating the sludge at high temperature and pressure (160 to 180°C at about 6 bar for about 20-30 minutes) prior to releasing it in a flash tank. This disintegrates sludge particles so that interstitial water is released into the bulk liquid, becoming free water which can therefore be separated mechanically (see visual below). Freeing more water from sludge increases its solids content and has positive effects on the operational expenses of sewage sludge management facilities that must transport this final product for storage, land application, or incineration.
A 2017 study demonstrates the aforementioned effect of THP. In the study, one plant with two digesters has sludge coming from the same wastewater headworks. One digester line (denoted by the orange data points below) was switched to THP end 2015. This resulted in an improvement in dewatering in that line’s digestate by around 7-8% points, which is the typical improvement observed in full scale plants.
The dewaterability of digestate can be described in two distinct ways, namely how fast the product can be dewatered (the rate of separation) or how much dryness can be achieved in the final product (extent of separation).
The rate of separation is typically measured by Capillary Suction Time (CST), with sludge that doesn’t release water quickly having a high CST and vice-versa.
Separation rates are dependent on the type and dose of polymer used.
The extent of separation, on the other hand, tells us what % of dry solids can be achieved and is typically measured in a laboratory with mechanical or thermogravimetric analysis (TGA). Polymers may be used in some of these tests but generally don’t impact the estimate of dry solids to be achieved.
A wider laboratory study was later conducted, comparing the dewaterability of various sludges across multiple plants using conventional digestion versus those using advanced anaerobic digestion with THP as pretreatment. There needed to be a method, however, to correct for the differences in sludges, as mentioned earlier, and also the variety of mechanical dewatering equipment used. The study found that the Carbon to Nitrogen ratio and ash content (C/N●ash) of the digestates provided a good comparison point that demonstrates the range or variety of sludge types in the study. To account for the differences in dewatering equipment, it was found that using thermogravimetric analysis (TGA) following Julia Kopp’s method in the lab gave good estimations of the DS that is achieved in full-scale plants.
TGA determines how much free water there is in a specific sludge sample, hence determining how much of the water can be removed mechanically. This, by definition, gives the maximum dry solids that can be achieved with mechanical dewatering.
The results of the TGA are seen below, clearly showing improved dewaterability of THP-treated sludge over conventionally digested sludge. The range of THP plants (represented by the circles in the graph) shows a higher cake solids percentage after dewaterability compared to the conventional plants (triangles in the graph). The graph also indicates that C/N●ash is a fair predictor of the variety of digestates and gives weight to the fact that we cannot directly compare the dewaterability of inherently different types of sludge.
As thermal hydrolysis disintegrates organics and allows for interstitial water to become free water within the digestate, it greatly improves the dryness of the final biosolids cake with dewatering. As this happens, biosolids volume is significantly reduced, creating operational savings for wastewater treatment plants and food waste plants that use the technology.
11 January 2022
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