Sludge management is a major issue for wastewater utilities globally. Typically, it contributes 30-50% to the overall operational expenditure of wastewater treatment plants (WWTPs). Considering the content of sludge in terms of organics, nitrogen (N) and phosphorus (P), it is regarded as a valuable resource when applied on agricultural land as fertilizer. However, in some regions, the available area to dispose sludge is becoming limited due to the presence of toxic metals that accumulate over time. In addition to the toxic metals, the presence of pathogens forms the basis for stringent regulations as detailed in specific guidelines. For example, land application of sludge in many countries such as Germany and the Netherlands in Europe are prohibited. Indeed, in Europe and the USA, incineration and landfilling is a more common practice. While incineration can achieve complete dewater and hygienization, a major disadvantage is a need for external fuel due to the low caloric value and higher moisture content of the sludge, incurring substantial operational costs. Moreover, in recent years, there is an increasing concern regarding the presence of emerging persistent contaminants such as Poly- and perfluoroalkyl substances (PFAS). PFAS has been a major concern for the contamination of groundwater sources. These are a complex family of more than 3000 humanmade fluorinated organic chemicals with varying functional groups, which can include other elements such as oxygen, hydrogen, or sulphur. The poor capture and treatment of PFAS at the WWTPs threatens the long-term health and safety of aquatic and terrestrial environments, including soils, rivers, and estuarine waterways and can moreover contaminate drinking water supply systems [1].

In addition to the problems as mentioned earlier, the large volumes of dewatered sludge, with typically a moisture content of 70-85% even after dewatering, results in high transportation costs independent of the ultimate disposal route. To illustrate this, the typical price for transport alone is in the order of 50-60 dollars per wet tonne sludge. Considering these costs on the one hand and the enormous amounts produced annually on the other (i.e., in 2013 China alone produced some 6.25 million tons of sludge), it is evident that sludge management is a multibillion-dollar problem globally.

Considering the above-mentioned limitations of existing sludge management strategies, there is a general interest by the water industry in alternative cost-effective sludge dewatering methods. In this context, recently smouldering combustion of wastewater sludge has received increasing attention. Smouldering is a slow, low-temperature, self-sustaining flameless form of combustion that overcomes the limitations of conventional incineration by efficiently transferring the heat generated by the heterogeneous reaction to the unburned fuel (sludge), enabling self-sustaining combustion. Indeed, preliminary studies have demonstrated that this technology can be utilized for the treatment of sludge at moisture contents up to 75-80% [2, 3]. While the potential was clearly demonstrated, a systematic evaluation and rigorous assessment of smouldering as an alternative for the management of sludge have not been conducted yet. Also, there is a research gap concerning the overall emission of gases during smouldering. Detailed information and a fundamental understanding of the flue gas composition is crucial to evaluate the overall feasibility of the smouldering approach. Thus, significant research efforts are still required in order to move beyond the proof-of-concept and develop a technology that can be implemented by the water industry. Therefore, the overarching aim of this PhD thesis is to investigate the feasibility of sludge smouldering as a cost-effective and efficient sludge management strategy.

In addition to the above-described objectives and research needs, the presence of PFAS compounds, in particular, have recently become of increasing public concern. There are growing concerns amongst wastewater utilities that land application and/or other beneficial reuse options may become prohibited. Considering this concern, smouldering combustion may come with additional benefits, as PFAS compounds may be thermally destroyed during the smouldering process. Therefore, in this PhD thesis, the fate of PFAS compounds during the smouldering process will be investigated too.

Finally, the only end-product of the smouldering process is inorganic ash in which valuable resources are concentrated. A potential resource that can be recovered which is of special interest is phosphorous (P). P is a non-renewable resource and an essential component of the agricultural food supply chain. To find a long-term solution to the P supply chain, one would need to adopt a circular P management approach. In this context, various governments have set ambitious targets to achieve 100% P recycling from wastewater treatment plants by 2030. Smouldering can potentially play a vital role in the recovery of phosphorus at WWTPs and can contribute to achieving a circular economy.

The potential of smouldering has never been tested, providing this research an opportunity to explore the technical and economic potential of smouldering combustion as a novel treatment technology for sludge. This PhD thesis aims to fulfil these gaps, along with several others discussed in detail in section 3, providing a better understanding of smouldering in sludge.

1.2. Organization of Thesis

1.3. References

CHAPTER 2

Literature Review

2.1.  Sludge Production at Wastewater Treatment Plants

Sewage sludge is produced during a series of treatments involved in the wastewater treatment process, as depicted in Figure 2.1. Sludge handling has become one of the significant concerns in wastewater treatment plants (WWTPs). The total sludge production in wastewater differs greatly, ranging from 35 to 85 g dry solids per population equivalent (PE) per day [4]. It was estimated that by 2020, the annual sludge production in the European Union would exceed 13 million tons, while in China 30 million tons of wet sludge is produced annually [5, 6]. Additionally, 6.5 million metric dry tonnes of municipal sewage sludge is produced annually by the U.S. [7]. Concerning Australia, the average annual production of wastewater biosolids (treated sludge) is increasing with current levels of 1.8 million wet tonnes per year, equivalent to approximately 330,000 dry tonnes [8]. The current global sludge production rate is about 45 million dry ton sludge annually which is equivalent to roughly 2 billion PE [9]. Sludge management incurs large operational expenditure of 150-350 USD per ton in Australia or 165-550 USD per ton in Europe alone [10]. The cost varies depending on the geographical location as well as the ultimate disposal route or end-use.

Figure 2.1: Simplified schematic representation of the conventional waste activated sludge process, adapted from [11].

2.2.  Composition and Characteristics of Sludge

2.2.1.                Sludge Composition

Sludge originates from the treatment of wastewater during the primary (physical and/or chemical), secondary (biological) and tertiary treatments and comprises of diverse elements such as organic and inorganic ingredients, pollutants and carcinogens such as heavy metals, dioxins, pathogenic microbes, etc. [12]. Sludge consists of a wide range of harmful substances such as but not limited to dioxins and furans, polychlorinated biphenyls, chlorine derivatives, polyaromatic hydrocarbons, phenols, and their derivatives, etc. [13]. To determine proper management pathways for sludge, it is imperative to determine the characteristics of the sludge. Table 2.1 shows a typical chemical composition of untreated and digested sludge (different types of sludges are discussed in detail in the upcoming sections).

Table 2.1: Typical chemical composition of untreated and digested biosolids, adapted from [14].

2.2.2.                 Sludge nomenclature

Grit removal

One of the challenges of the wastewater treatment plants (WWTPs) is the removal of the grit from the wastewater. Grit is a general term applied for the heavy, predominantly inorganic solids that enter the wastewater collection system [15]. It is generally comprised of sand, gravel, cinders, and other heavy solid material with substantially higher specific gravity compared to the organic matter found in wastewater [15]. Additionally, grit is heterogeneous usually containing non-putrescent organic matter such as seeds and coffee grounds in addition to the inorganic fraction. Removal of grit is crucial to protect the moving mechanical equipment from abrasion and to reduce clogging.  Many types of grit removal systems exist such as aerated grit chamber, vortex type grit removal system, detritus tank etc. Various factors such as quantity and quality of grit and the potential effects it has on downstream processes, removal efficiency, organic content and cost must be considered before selecting the grit removal process.

Primary Sludge

The primary sludge is the result of the mechanical wastewater treatment process and comprises of settleable solids, which are usually removed in primary clarifiers. The sludge that accumulates at the primary sedimentation basin is also referred as primary sludge. The primary sludge mainly consists of organic matters but greatly varies on the source of wastewater like industrial, municipal or run-off water [16].

Waste Activated Sludge

The waste activated sludge (WAS), which is often referred to as secondary sludge, is the sludge originated from a biological process such as the activated sludge process.  The activated sludge process or the suspended-growth biological treatment process is one of the most common methods used for the treatment of secondary wastewater treatment plants. The suspended microbial culture is utilized by the activated sludge to degrade the organic material to form a biological floc under aerobic conditions. The aeration not only serves oxygen for respiration for the aerobic microorganisms but also helps to maintain the microbial flocs by assuring maximum contact between the surface of the flocs and the wastewater. The composition of WAS consists of 59-88% w/v biodegradable organic matters (OMs), 50-55% C, 25-30% N, 6-10% H and trace amount of P [17].

Mixed Sludge

Mixed sludges are a combination of primary and secondary sludges (as described above) and vary greatly from one WWTPs to the other.

Tertiary Sludge

Tertiary treatment, which generates the tertiary sludge, is regarded as the final cleaning process for the improvement of wastewater quality before it is recycled or discharged to the environment. Moreover, it is an additional process to the secondary treatment for the removal of remaining inorganic compounds, and substances mainly nitrogen and phosphorus utilizing high performance bacterial or chemical processes.

2.3.  Sludge management

Sludge produced during wastewater treatment holds a large volume in the form of liquid or semi-liquid constituting 0.25–12% solids by weight. Sludge management is of major concern taking into consideration the environmental issues associated with industrial and municipal sludge [18, 19]. The initial step for sludge management begins with reducing the water content by thickening, stabilization/conditioning, mechanical and/or thermal dewatering to save energy during the drying process [18]. Importantly, if one adopts anaerobic digestion, energy can be recovered from sludge in the form of biogas. Moreover, in this way, the sludge is stabilized and also reduced in terms of overall sludge volume (explained below).

Sludge thickening is a process in which the solid concentration or content is increased by curtailing the volume of free water in the sludge, thereby decreasing the load on the downstream process such as digestion and dewatering. There are several methods for sludge thickening such as dissolved air floatation (DAF), centrifugal thickening, gravity belt thickening and gravity thickening. Gravity-thickening is the most commonly used method to accomplish thickening and can be classified as plain settling and mechanical thickening. In gravity thickening the higher-density solids settles out of the liquid forming concentrated solids due to gravity. In general,  a solids concentration (TS) of about 2-7 % concentration is usually attained using gravity thickeners [14] with typical solid loading rates ranging between 25-80 kg/m².d for primary and waste activated sludge.

One of the most commonly used biological processes for the stabilization of sludge from wastewater treatment plants (WWTPs) is anaerobic digestion. Stabilization is crucial for the destruction of pathogens and reduction in volatile solids and odours. Digestion can be carried out either aerobically or anaerobically. Studies have demonstrated that both aerobic [20, 21] and anaerobic digestion [22] causes more inferior dewatering properties, however, very few researchers have investigated the reasons for the modifications in the dewatering properties [23] and differences have been found in the degree to which dewatering deteriorates following digestion.

Aerobic digestion

Aerobic sludge digestion is a biological process that occurs in the presence of oxygen. With oxygen, the bacteria present in the activated sludge consume organic matter and transforms it into carbon dioxide. Aerobic digestion has been broadly utilized in WWTPs globally. Aerobic digestion aims to stabilize raw sludge and generate biosolids for further treatment and disposal. However, long aerobic digestion time can result in a considerable reduction of sludge dewaterability and an elevation of the solid concentration to 4-6 percent.

Anaerobic digestion

Anaerobic digestion (AD) is a biological process where the organic materials/solids are transformed to biogas via a biochemical reaction in the absence of molecular oxygen. Biogas usually comprises of methane (60-70%), carbon dioxide (30-40%) and traces of other gases such as hydrogen, hydrogen sulphide and nitrogen constituting of a relative density of 0.85 and a calorific value of 13–21 MJ kg^-3 similar to that of lignite (12-16 MJ kg^-3) [24]. Operating temperature play a crucial role in determining the quantity and quality of the biogas and digestion rate, different temperature range such as 12-25 ^oC (psychrophilic), 35–38 ^oC (mesophilic) and 50-60 ^oC are adopted based on the design of the AD process [25, 26]. However, the mesophilic process remains dominant over other methods mainly because of the degree of benefit it provides with energy consumption, reliable process operation, and favourable process performances such as sludge reduction and biogas generation [26, 27]. Currently, increasing amount of work/research has been carried out to enhance the production and quality (high ratio of methane to carbon dioxide) of biogas, for instance, optimizing the process conditions (sludge retention time (SRT) and sludge loading rate) [28, 29],  implementing multi-stage process (temperature phased and micro-organism community-phased) [30, 31]. Likewise, aerobic digestion also yields digestate comprising of an elevated amount of nutrients such as phosphorus, potassium, and nitrogen which holds benefits as fertilizers or compost. Additionally, biogas can also be utilized onsite through using combined heat and power systems (CHP) or further upgraded to biomethane and fed into the natural gas grid.

2.3.1.                Dewatering

Moisture Distribution in sludge

Understanding the fundamentals of moisture distribution in sludge is beneficial for providing a better approach for the sludge dewatering process. Vesilind and Hsu [32] defines the water present in the sludge as (Figure 2.2) (i) free water which is not adhered to the solid particles and can be separated by gravitational settling; (ii) interstitial water is trapped within the floc structure or within a cell and can be separated only by breaking the floc or disrupting the cell. Mechanical dewatering system such as centrifugation or vacuum filtration can be used to remove small amount of the interstitial water; (iii) vicinal water are those water molecules which are physically bound to the surface of the solid particles and cannot be removed by mechanical means; (iv) the water molecules which are chemically bound to the solid particles and are released by thermochemical destruction at temperatures above 105 ^oC [33, 34]  and are termed as the water of hydration. The bound water, which constitutes interstitial, vicinal and hydration water, is one of the major challenges for dewatering efficacy as it demands more energy.

Another challenging factor responsible for the dewatering of sludge is the presence of extracellular polymeric substances (EPS) [35]. Disrupting the EPS by various means, such as but not limited to enzymatic [36, 37] ultrasonic  [38, 39], or thermal pre-treatment  [40], [41] can enhance the dewaterability of the sludge.

Figure 2.2: Schematic model for various forms of water, adapted from [42].

Dewatering of sludge

Before the dewatering of the sludge, the digested sludge is usually conditioned to generate flocs, which are easy to filtrate. One of the most primary choices is chemical conditioning using polyelectrolytes [43, 44]. However, good control of polyelectrolyte dose is critical as an overdose will elevate the cost and decrease sludge dewaterability. The efficacy of sludge treatment for sludge dewatering can be measured in terms of an increase in total solids (TS). The most efficient dewatering treatments include centrifuges and filter presses, which may improve the concentration of TS up to 35%. Table 2.2 provides an overview of typically obtained sludge moisture contents after dewatering using state-of-the-art dewatering methods. Note that these values should be taken as a guideline and expected range as the sludge dewaterability can substantially differ per wastewater treatment plant (WWTP) and process configuration and geographical location.

Table 2.2. Typical attainable sludge moisture contents after sludge dewatering using conventional methods.

2.3.2.                Drying

Literature shows the behaviour of sludge during the drying process and has categorized it into four major parts as shown in Figure 2.3 representing a typical drying curve [47, 48]. AB (constant rate) is the free water and is removed first, followed by two falling rates BC (first falling rate) and CD (second falling rate) which is the removal of interstitial and surface water, respectively. Finally, the bound water is removed, and the equilibrium of moisture is achieved.  On the contrary, Leonard et al. [49, 50] found that the behaviour of the sludge during drying is relative to the origin of the sludge. Furthermore, various studies have established two phenomena: shrinkage and cracks that occur during the sludge drying process [51, 52].

Figure 2.3: Sludge drying curve, adapted from [47].

Thermal drying process utilizes thermal energy to evaporate the water present in sludge. It is considered an energy and cost intensive process. However, it can reduce the mass and volume of sludge up to 85%, thereby substantially decreasing the transportation cost.  Equally important, it also increases the calorific value of the sludge making incineration more economic and practically feasible.

Solar drying, on the other hand, can be one of the alternative solutions for drying of the sludge. However, the major drawback of solar drying is the continuous change in operating conditions with time. Moreover, several studies have been tested and modelled for solar drying of the wastewater sludge, taking into consideration the various parameters such as solar radiations, inlet, and outlet temperatures [53, 54] along with other combined materials such as fans, ventilation [53], and auxiliary sources. Although solar drying is a cost-effective method it has practical limitations, such as, it depends on the degree of sunlight and temperature which varies throughout the year, and is a slow process demanding larger area. Another drawback is the odour emissions linked to additional investment in filters and the use of additional heat source to dry the sludge.

2.4.  State-of-the-Art: Sludge Minimization and Reduction Technologies

The conventional municipal sewage treatment plants commonly utilize mechanical and biological processes. Additionally, the activated sludge process is extensively practiced for biological wastewater treatment plants globally resulting in a significant amount of sludge production. Due to the presence of high fractions of volatile solids (VS) and greater water retention, these sludges contain high volumes of residual solid and higher disposal costs. Moreover, the treatment and disposal of the excess sludge accounts for 25-65% of the total plant operational cost [55]. The quintessential objective of sludge reduction is the zero discharge of the excess sludge. For municipal wastewater treatment plants (WWTPs) achieving the zero discharge can be a challenge because of the presence of a considerable amount of inorganic and biodegradable solids, which are complicated to destroy even after the additional specific treatments. Therefore, it is crucial to initiate strategies for the reduction and minimization of excess sludge production.

A variety of chemical, physical, and biological approaches are taken into consideration for sludge reduction or minimization. For instance, the biological methods for sludge reduction incorporate the thermophilic aerobic digestion stage intending to solubilize excess sludge with the help of thermophilic bacteria growing under aerobic or microaerobic conditions [56].  Similarly, the physiochemical methods for the minimization of sludge include: (i) chemical oxidation with strong oxidants such as ozone [57, 58]; (ii) thermal hydrolysis above 150 ^oC [59]; (iii) acid or alkaline thermolysis at comparatively lower temperature (<100 ^oC); (iv) mechanical disintegration [60] and, (v) ultrasonic cavitation. Three main strategies that have been identified can be incorporated in any existing processes; (i) in the wastewater line, (ii) in the sludge line, and (iii) in the final waste line, which is summarized in Table 2.3 [55]. Furthermore, few operational parameters that must be taken into consideration to evaluate sludge reduction are sludge retention time, observed sludge yield, sludge reduction in terms of solids, treatment frequency and physical properties of sludge that influence sludge dewaterability.