**1. Introduction**

The trend for continual improvement in the collection and treatment of wastewater in Europe's cities and towns is evident, but full compliance with the Urban waste water Directive (UWWTD) [1] has not been achieved yet [2]. The level and intensity of wastewater treatment depend on the sensitivity of the receiving surface waters according to the UWWTD [1]. In Europe, 3.1% of the load is treated at the primary level, 28.5% at the secondary level, and 68.4% at the tertiary level [3]. The EU Commission has also launched an impact assessment to

evaluate different policy options with the aim of modernizing the UWWTD [2]. Among others, specifically, the UWWTD requires controls of sludge disposal and reuse [4]. Article 14 of the UWWTD specifies [1]: (i) sludge arising from wastewater treatment shall be reused whenever appropriate, and (ii) disposal routes shall minimize the adverse effects on the environment. While approximately 20–25 kg of sludge dry matter (DM) are continuously produced annually per person per year all across Europe as a result of the wastewater treatment process, the wastewater operators render the valuable resources found in sewage sludge to be reusable [5].

The term"sewage sludge"does not clearly define the status of its quality. In the literature and regulations, the term"sewage sludge" refers to untreated excess sludge, partially treated sludge, digestate, and pelletized sludge. By Sewage Sludge Framework Directive (SSD) [6], the term"sludge" refers to untreated sludge (residual sludge from sewage plants treating domestic or urban waste waters and from other sewage plants treating waste waters of a composition similar to domestic and urban waste waters, and residual sludge from septic tanks and other similar installations for the treatment of sewage), while the term"treated sludge" refers to sludge which has undergone biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use. Using the appropriate term in the professional literature helps us to distinguish between the quality of sludge obtained at different levels of processing.

On the European scale, the most common recovery procedures are agriculture and incineration [3]. In the latter procedure, it is not clear whether it is incineration for the purpose of waste to energy (WtE) processes (R 1 recovery operation), or it is just a matter of reducing the amount of sewage sludge (D 10 disposal operation). According to the Waste Framework Directive (WFD) the recovery procedure R 1 takes precedence over the disposal operation D 10 [7].

Quality of municipal infrastructure of urban agglomerations and appropriate pre-treated industrial wastewater has high importance on the stable operation of municipal wastewater treatment plants, and properties of treated sludge. Continuous quality control of treated sludge is the starting point for its efficient recovery processing as a source of raw materials to follow the strategy in the new Circular Economy Action Plan [8]. Some technical solutions enable the recovery of materials from treated sludge like phosphorus, nitrogen, biopolymers, biogas, biochar, biofuel, struvite, and even recovered products from ashes [9]. A market is not always available for these innovative materials because the recovery costs are often high compared to primary raw materials. This is because fossil oil based material or extraction techniques and transportation are cheaper [9]. The chapter will address three main areas of sludge recovery operations [7], namely the use of treated sludge for: (i) energy production—waste to energy (WtE), recovery operation R 1, (ii): the organic matter recovery with pyrolysis, recovery operation R 3, and (iii) agricultural use, recovery operation R 10. All end-use utilizations have advantages and disadvantages and are suitable for recovery processes from a different points of view and engineering approaches, depending on the UWWTP organic load capacity, level of raw wastewater treatment, and excess sludge pre-treatment stage (e.g. aerobic or anaerobic stabilization, dehydration, hygienization, and pelletization). Important are also others conditions such as the UWWTP geographical site, transport infrastructure, and available facilities for sewage sludge utilization. National legislation, the national strategic program for the management program of this specific waste stream, and the national level of public awareness of the usefulness of the sludge as a raw material for the CE are also important.

*Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

A case study of the composition of treated sewage sludge, generated at Central wastewater treatment plant Ljubljana (CWWTPL) has been performed. The CWWTPL treats the municipal wastewater of the capital city Ljubljana and is the largest UWWTP in Slovenia [10]. The purpose of the analysis and the research was to evaluate the feasibility of sewage sludge pre-treatment at the site of origin, anaerobic stabilization of excess sludge and biogas production, seasonal fluctuations in the quality of treated sludge, and the comparability of quality of the annual representative samples. The purpose is also to evaluate the sewage sludge as a smart material. The obtained results are the basis for sustainable and predictable use of treated sludge in the CE [11], both in the field of WtE and material utilization. The most important legal requirements for specific purposes of sewage sludge use are given. The chapter combines specifications, guidelines, and limit values regarding WtE processes, pyrolysis, and fertilizing products with real data of treated sludge quality and its potential. Quality data of the CWWTPL sludge were collected over a ten-year period of operation, from 2010 to 2020. The last year is also the starting year for a number of forecasts on improving the environment and preventing climate change.

### **1.1 Pyrolysis: recovery operation R 3, thermal conversion of organic matter into new products**

According to WFD [7] the recovery operation R 3 means recycling/reclamation of organic substances which are not used as solvents (including composting and other biological transformation processes), gasification and pyrolysis using the components as chemicals, waste preparing for reuse, and recovery of organic materials in the form of backfilling.

Pyrolysis is a thermochemical anaerobic decomposition of biomass into a range of useful products. The process is typically carried out in a temperature range from 300°C through 650°C [12]. During pyrolysis, large, complex hydrocarbon molecules of biomass break down into relatively smaller and simpler molecules, forming gaseous (non-condensable volatile compounds) and liquid products (bio-oil), and solid residue (biochar). Pyrolysis offers the potential of material utilization of UWWTS. The densified pyrolytic fraction of sewage sludge is suitable for energy utilization, and on the other hand, the resulting solid residue is potential for material utilization on fields or forests. Biochar is by mass the major pyrolytic product or resideous material. Its characteristics make it promising for application in agriculture due to its high immobilization affinity of heavy metals [13]. Biochar retains the main part of the carbon in stable solid form [12]. In addition, sewage sludge biochar contains significant content of phosphorus, potassium, and nitrogen, which further demonstrates the great potential of its use as a fertilizer [12].

There is no special EU legislation regarding the waste pyrolysis process. European Biochar Foundation (EBC) has launched the guidelines for specification and certification procedure for biochar evaluation [14]. As a specification for pyrolysis technology is prescribed: (i) the use of waste heat or the use of liquid and gaseous pyrolysis products, and (ii) compliance with nationally defined emission limit values. For the characterization of biochar properties, the limit values in **Table 1** must be observed. The biochar for all application classes must be analyzed at least according to the EBC Basic Analysis Package ([14], Annex 1). The latter reference also indicates which types of biomass are permissible for each application class.

As both biochar properties and the environmental footprint of its production are largely dependent on the pyrolysis parameter and the type of feedstock to be used, a secure control and assessment system for its production and analysis had to be introduced. According to reference [14], biochar is defined by its quality


### **Table 1.**

*Limit values for biochar properties.*

characteristics, the raw materials used, its sustainable production, and the end-use (**Table 1**).

The pyrolysis of non-plant biomasses such as treated sludge, livestock manure, certain digests or bones may also produce valuable raw materials and could be used in the interests of the bio-economy and climate protection, but these raw materials have not yet been included in the EBC feedstock list and are therefore not subject of the EBC guidelines [14].

According to the declaration of biochar properties, the organic carbon or TOC (Corg) must be defined. By the last version of EBC guidelines [14] the lower limit value of 50% m/mDM has been reconsidered. All solid pyrolysis products below this limit value were considered only as pyrogenic carbonaceous materials (PCM).

The EBC certificate guarantees that only climate-positive biochar production technology is used and does not release unburned pyrolysis gases into the atmosphere. Reference [14] specified the conditions for the safe and harmless operation *Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

of pyrolysis units. Regarding the environment protection and preventing climate change the most important conditions are: (i) with the exception of the preheating of the pyrolysis reactor, the use of fossil fuel for heating is prohibited, (ii) if the pyrolysis reactor is electrically heated, the use of renewable energy source or the use of surplus electricity is recommended, (iii) the non-condensable pyrolysis gas must be burned or it can be trapped and used for the chemical industry, (iv) the bio-oil can also be stored and used for other energy and material purposes, (v) syngas combustion must comply with national emission thresholds, and (vi) the heat produced by pyrolysis process must be used.

### **1.2 EU legislation regarding the energy recovery from waste**

The IED Directive [15] shall not apply to pyrolysis and gasification plants if the gases resulting from this thermal treatment of waste are purified to such an extent that they are no longer a waste prior to their incineration and they can cause emissions no higher than those resulting from the burning of natural gas.

In the EU regulation on waste incineration and co-incineration, the limit values for polluting substances, related to substances in sludge are presented. There is a possibility of linking the characteristics of the sludge with environmental pollution and harm to human health, so the energy recovery from treated sludge must be provided safely and harmlessly. Data provision and assessment must be made from a holistic point of view to consider WtE processes. These processes must be evaluated from an economic as well as from the environment point of view regarding the emissions into the air and into the surface water, and last but not least also regarding the generated residues.

### *1.2.1 Technical provisions to waste incineration plants and waste co-incineration plants*

Technical provisions to the new waste incineration plants and waste coincineration plants, according to IED Directive [15] are considered. In **Table 2** presented emission limit values of polluting substances into the air according to the standardized oxygen atmosphere. Limit values for waste incineration plants in **Table 2**, footnote a, apply for facilities: (i) that were in operation and have a permit in accordance with applicable Union law before 28 December 2002, and (ii) that were authorized or registered for waste incineration and have a permit granted before 28 December 2002 in accordance with applicable Union law, provided that the plant was put into operation no later than 28 December 2003. In the view of the competent authority, these facilities were the subject of a full request for authorization before 28 December 2002.

Limit value for NOx in **Table 2**, footnote b, applies for existing waste incineration plants with a nominal capacity exceeding 6 tonnes per hour or new waste incineration plants. The limit value for NOx in **Table 2**, footnote c, applies for existing waste incineration plants with a nominal capacity of 6 tonnes per hour or less. A new waste incineration plant means any waste incineration plant not covered by the existing plant.

**Table 3** presents the limits values for polluting substances causing the emissions into the air according to transitional provisions of the IED Directive. The limit values shall be applied as follows: (i) in **Table 3**, footnote a, to combustion plants that co-incinerate waste from 1 January 2016 referred to the reference ([15], Article 30 (2)) and (ii) in **Table 3**, footnote b, to combustion plants that co-incinerate waste from 7 January 2013 referred to the reference ([15], Article 30 (3)).


### **Table 2.**

*Emission limit values for the following polluting substances in mg/Nm<sup>3</sup> for heavy metals and in ng/Nm<sup>3</sup> in combination with toxic equivalence factor (TEF) for dioxins and furans in their gaseous and vapor forms.*

## *1.2.2 Technical provisions to technological wastewater from incineration plants and waste co-incineration plants*

In flue gas cleaning technological wastewater is generated, which captures dust of fly ash and condensed volatile compounds of heavy metals. The generated process water must be treated in accordance with the limit values for pollutants in the generated wastewater, which are uniformly determined for all waste incineration and co-incineration plants [15]. Limit values for total suspended solids (TSS) in pre-treated technological wastewater as defined in Annex I of UWWTD [1] are: (i) 30 mg L<sup>1</sup> by 95% of all results or (ii) 40 mg L<sup>1</sup> by 100% of all results.

In terms of sustainable energy utilization of sewage sludge, in order to prevent the circulation of already removed polluting substances back to nature, it is necessary to assess the quality of treated technological wastewater and prevent deterioration of the chemical and ecological status of the receiving surface water. **Table 4** shows a comparison of the limit values for the discharge of treated technological wastewater from the incineration and co-incineration plant [15] with the limit values for good chemical status and very good and good ecological status of receiving surface water. The chemical status of waters is assessed by the content and concentration of Environmental quality standards (EQS), expressed as priority substances (PS) or priority hazardous substances (PHS) [16, 17]. The ecological status is assessed, among other conditions, by the presence and concentration of non-synthetic special pollutants (SP) [16]. Since the dilution factor of some polluting substances in the treated wastewater from the incineration and co-incineration


*Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

### **Table 3.**

*Emission limit values for combustion plants co-incinerating solid fuels with the exception of biomass for the following polluting substances in mg/Nm<sup>3</sup> and in ng/Nm<sup>3</sup> in combination with toxic equivalence factor (TEF) for dioxins and furans in their gaseous and vapor forms ([15], Articles 30(2) and 30(3)).*


**Table 4.**

*Emission limit values for discharges of wastewater from the cleaning of waste gases.*

plant into the environment must be around 1000, it is necessary to pay full attention to this sustainability aspect of the WtE process.

EU Member States may, when assessing the monitoring results against the relevant EQS, take into account: (i) natural background (NB) concentrations for metals and their compounds where such concentrations prevent compliance with the relevant EQS, (ii) hardness, pH, dissolved organic carbon or (iii) other water quality parameters that affect the bioavailability of metals, the bioavailable concentrations being determined using appropriate bioavailability modeling.

Mercury is an atmospheric deposit and is behaving like a ubiquitous persistent, bioaccumulative and toxic substance (PBT). Due to its volatility mercury is often present in the environment. Limiting the circulation of mercury in the atmosphere as much as possible is an important task in establishing the sustainability of sewage sludge utilization in the CE.

Establishing the sustainable utilization of sewage sludge with WtE processes is based on a holistic assessment of the environment in which the facility is located. Sewage sludge is a specific waste stream with a complex composition and only a good knowledge of all aspects of environmental impact and properties of sewage sludge allows the design of sustainable WtE technologies from treated sludge.

### **1.3 EU legislation regarding material recovery from treated waste in the agriculture**

The framework European Sewage sludge Directive (SSD) [6] regulates the use of sewage sludge in agriculture for the EU Member States. According to reported quantities on recovery procedures for sewage sludge for the period 2013–2015 in Europe, 48.5% m/m sewage sludge is used for agriculture [3]. The data in Ref. [3] show that in many EU Member States this process is still the most topical.

The selection of polluting substances, for which a limit value is set, include heavy metals, which are potentially toxic and are essential in a small amount for various biochemical and physiological functions within the plants, animals, and human [18] (**Table 5**). A pH limit is additionally prescribed for the quality of the soil. In adapting its national legislation EU Member States may permit the limit values they fix to be exceeded in the case of the use of soil and sludge for cultivation of commercial food crops exclusively for animal consumption (**Table 5**, footnote a).


### **Table 5.**

*Limit values for use of sewage sludge in agriculture according to SSD [6].*

*Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

EU Member States also may permit the limit values they fix to be exceeded in respect of these parameters on soil with a pH consistently higher than 7 (**Table 5**, footnote b). The maximum authorized concentrations of PTMs may not exceed those values by more than 50%. The EU Member States must also seek to ensure that there is no resulting hazard to human health or the environment and in particular to ground water. The amount of PTMs annually spread on soil is calculated on a 10-year average (**Table 5**, footnote c).

National legislation of the EU Member States regarding the use of sludge in agriculture is not uniform, there are differences in the limit values for the key parameters, and there is also a difference in the legislation demands between USA and EU [19].

Given the strategic objectives of the CE philosophy and the reduction of waste, it is necessary to give priority to the use of recovered waste over the depletion of raw materials. In order to include as much recovered waste as possible in the raw material cycle, the EU has adopted a Fertilizing Resolution (FR) [20] for the use of recovered waste in fertilizer, which aims to unify the European fertilizer market. According to the vision of FR, the 'EU fertilizing product' means a fertilizing product which is labeled with the logo CE ("conformité européenne") when made available on the market. It includes fertilizers produced from recovered or organic materials. The FR, which will not enter full force until 16 July 2022, will not affect the implementation of SSD [6]. The possibility of using sewage sludge as a fertilizer product is still ambiguous. From the conditions for the selection of possible treated waste for use as a fertilizer, it can be understood that this can be all wastes that are composted or anaerobically stabilized [20].

FR does not prescribe the extent of plant response when applying a particular fertilizer. Slovenian legislation is more restrictive in this area and prescribes the quantity and quality of the digestate used for the purpose of a fertilizer (**Table 6**) [21].

### *1.3.1 Designation of component material categories for the potential EU solid fertilizer*

According to FR sewage sludge could be used as input material for the component materials categories (CMC) for fertilizer as a blending compost (CMC 3) and treated digestate (CMC 5), if it does not contain more than 6 mg PAH kgDM<sup>1</sup> . **Tables 7** and **8** provide the basic starting conditions for the quality of the recovered waste that can be used as input material for the production of a fertilizer.


### **Table 6.**

*Determination of the plant response. Digestate with >20% m/mDM: Characterization of the effect of digestate granules as soil improvers and growth substrates on plant germination and growth [21].*


### **Table 7.**

*Designation of component material categories (CMCs) for the potential EU solid fertilizer incorporating sewage sludge [20, 21].*


### **Table 8.**

*Designation of component material categories (CMCs) for the potential EU solid fertilizer incorporating sewage sludge [20, 21].*

Conditions for use include the content of total MI, polycyclic aromatic hydrocarbons (PAHs) as persistent organic pollutant (POPs), adequate stability and content of pathogens as evidence of hygienic integrity.

According to FR stricter limit values will apply in the future for the presence of plastics above 2 mm (**Table 7**, footnote a): (i) from 16 July 2026 the maximum limit value point shall be no more than 2.5 g/kg dry matter, and (ii) by 16 July 2029 this limit value shall be re-assessed in order to take into account the progress made with regards to separate collection of bio-waste.

According to FR, sixteen PAHs have been identified as polluting substances indicative of POPs contamination (**Table 7**, footnote b): naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a] anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[ghi]perylene.

*Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

Residual biogas potential of digestate (**Table 8**, footnote a) is an indicator of mineralization efficiency of excess sludge into the digestate and is expressed as a produced biogas per unit of material that is lost on ignition of the dry solids at 550° C. Loss on ignition is expressed as volatile solids (VS). The treatment process of the digestate must include the hygenization process. It must be free of *Salmonella* spp. and the maximum number of *Escherichia coli* or *Enterococcaceae* expressed in colonyforming units (CFU) is 1000 (**Table 8**, footnote b).
