*3.2.2 Basic thermal properties of granules: determination with conventional laboratory methods*

Organic matter is the main component of granules, which releases energy during combustion, rendering steam and CO2. In general, sewage sludge is semi-biomass, its main constituents being water, organic matter, which is a mixture of residual decomposition products of excess aerobic biomass and anaerobic decomposition products, which are mostly denatured proteins (due to cationic polymer addition, dehydration, and drying of the digestate), carbohydrates, lipids, fatty acids and inorganic substances (salts and complex natural minerals) are also present. Granules behavior under thermal load with conventional laboratory methods in the oven dryer, LF, or combustion chamber (determination of NCV) is an indicator of their proximate properties: moisture, volatile and organic matter (VM, OM), carbonates, and ash content (**Table 15**). The organic fraction or biomass content that can burn in the presence of oxygen at 450°C is 66.9% m/mDM, the further fraction that can burn at 550°C is 0.7% m/mDM higher. The share of VM is 53.9% m/mDM, which means that a high volume of gaseous products is formed during incineration (**Table 15**).


*1 Temperature prescribed for biodegradable waste or biomass.*

*2 Temperature prescribed for treated waste into solid recovered fuel.*

*3 Prescribed temperature is 550°C, but the determination of the residue at 900°C also includes the thermal decomposition of carbonates.*

### **Table 15.**

*Basic thermal properties of granules.*

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

The granules ignition point is determined by a non-standardized method and apparatus. It means the temperature at which, when heated in an oxidizing atmosphere, the granules spontaneously ignite and immediately continue to burn. The granules temperature of the ignition (Tignition) is at 550°C on average (**Table 15**).

**Tables 14** and **15** provide the results, which are taken from expert assessments of granules, which were published in the period from 2010 to 2018 in expert reports prepared by an authorized waste assessment contractor. Only the results for the year 2010 differ significantly due to extensive floods on Ljubljana city area and the inflow of background water to the CWWTPL.

On average, the granules' dry matter content is at least 90% w/w. Fluctuations in the results for dry matter content are due to the drier operating conditions, the choice of analytical method, and determination temperature. It is characteristic of the granules that they lose most of their mass even at a low temperature of the thermal load, namely at a temperature of 450°C. With further heating, the weight loss is smaller.

The loss on ignition is often used as an estimate for the content of organic matter in the sample. Inorganic decomposition products (e.g. H2O, CO2, SO2) are released and some inorganic substances are volatile under the reaction conditions. Determination of LOI at 550°C and 900°C allows us to determine the proportion of organic matter (at 550°C) as well as the proportion of CO2 (at 900°C) resulting from the thermal decomposition of naturally occurring carbonates that cause CO2 emissions for which it is not necessary to purchase emission coupons (6a). In 2010, the contamination of raw wastewater with inorganic impurities led to an increase in the ash content in the granules (**Table 15**) and especially an increase in the carbonate content, which continued in 2011 (**Table 15**, **Figure 6a**). **Figure 6b** also shows the fluctuation of the calorific value on an annual basis, which is not always logically related to the fluctuation of the LOI and carbonates.

Granules production at CWWTPL is not a continuous process. Due to the specific layout of facilities and machine capacity, this process is performed in batch mode. The quality of representative samples of individual batch series for the year 2020 (**Table 13**) fluctuates markedly seasonally (**Figure 7**). The contents of the biomass (LOI at 450°C) are characterized by the lowest content over the summer. At that time, the mineral content, determined as the difference between LOI at 550°C and LOI at 450°C, is also the lowest (**Figure 7**). The latter mass loss is probably due to the thermal decomposition of MgCO3 or minerals containing magnesium, phosphorus, and carbonates.

The summer period is characterized by higher wastewater temperatures, less precipitation, lower concentration of activated sludge because of more intensive

### **Figure 6.**

*(a) Dynamics of LOI of granules at 550°C and carbonates content, and b) comparison of annual dynamics of LOI of granules at 550°C and NCV.*

**Figure 7.**

*Seasonal dynamics of OM content and LOI of the representative batch samples (Table 13) of the drying process in the year 2020.*

removal of excess sludge, and consequently higher granules production, which is consequently reflected also in their final quality.

At CWWTPL the biogas production is also the lowest during sommer and early autumn (**Figure 4a**), the same is true for NCVar (**Figure 4b**).

### *3.2.3 Basic thermal properties of granules: determination with the advanced techniques*

According to TS Combustion behavior solid fuel combustion consists of three relatively distinct but overlapping phases: (i) heating phase (time to thermal decomposition), (ii) pyrolysis, and gas phase combustion (time of gas-phase burning), and (iii) char combustion (time to char burnout). The STA is the most used technique in thermal analysis to give us sufficient information about the thermal behavior of solid fuel.

The results of two techniques of the STA method are provided on granules: (i) proximate analysis and (ii) fingerprint of thermal decomposition of granules in the oxidative and inert atmosphere provided by the non-isothermal temperature program. The proximate analysis of solid granular fuel executed with the TGA technique is a standard procedure for the determination of behavior regarding volatile release and combustion. This technique is comparable with the method as specified in TS EN 15402:2011 for volatile matter (VM) determination. The proximate analysis was performed according to the prescribed procedure (TS Combustion behavior) on the annual sample 2012/2017.

The obtained results of the performed technique (**Figure 8**), moisture (8.12% m/ m), VM (51.9% m/mDM), and ash (30.8% m/mDM), are comparable to those obtained with the conventional methods (**Figure 6**, **Table 15**). The proximate analysis performed with a TGA gives us an additional parameter of solid fuel properties, and that is a combustible carbon content or fixed carbon (Cfix = 11.8% m/mDM). When switching an inert atmosphere of thermal loading at 900°C to an oxidizing atmosphere, further mass loss can be attributed to the carbon remaining in the sample as elementary carbon, which was bound in substances that were depolymerized, and thermochemically converted to carbon. 88.2% m/mDM of granules carbon content is bound in substances which, in an inert atmosphere when heated to 900°C, are volatile or are thermo-chemically converted into volatile gaseous compounds. The content of TOC for the annual sample 2012 is 38.4% m/mDM (**Table 15**), and only 30.7% m/mDM of carbon as a part of TOC is presented as a non-volatile matter and is burnout in the third phase of combustion.

The proximate analysis was performed in six stages (**Table 16**, **Figure 8**). The moisture was determined in the first stage, when the sample was heated to 110°C with *Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

### **Figure 8.**

*The proximate analysis of granules (the annual sample 2012/2017) with TGA/DTG/DTA technique: Cfix) the fingerprint of granules decomposition in an inert atmosphere combined with an oxidative atmosphere in a sequential mode, and time program) determination of the stages of granules mass loss.*

20 K min<sup>1</sup> , keeping the temperature constant for 15 min in an inert Ar atmosphere. At the time of 4.8 min, the signal peak for DTG was 1.95% min<sup>1</sup> and the heat balance was endothermic (**Figure 8**, Cfix). VM was determined by consecutive heating to 900° C at 20 K min<sup>1</sup> . This part of proximate analysis consists of three stages: (i) second stage with two signal peaks for DTG (at 27.0 min the first signal peak is 4.34% min<sup>1</sup> and at 30.8 min the second signal peak for DTG is 3.98% min<sup>1</sup> ), (ii) third stage with one signal peak for DTG at 51.8 min (1.01% min<sup>1</sup> ), and (iii) stage four followed by the isothermal hold at 900°C in an inert atmosphere for 15 min. In the fifth stage, the purge gas was changed to the oxidizing atmosphere (80% v/v of Ar and 20% v/v of O2), and the temperature was kept constant at 900°C for 120 min. In this stage the carbonized sample was burnout (Cfix), the signal peak for DTG is 1.01 at 80 min, and the heat balance is exothermic. In the sixth stage, only insignificant mass change is recorded. The residual mass is ascribed as high-temperature ash, and the cumulative mass loss is 71.8% m/m (**Figure 8**, Time program). The mass loss followed in 8 steps of the time program is shown in **Figure 8b** and **Table 16**.

The TGA/DTG/DTA granules analysis gives us a useful picture and data on their behavior, with which we can predict or estimate the course of incineration and


### **Table 16.**

*Stages of proximate analysis of the sample 2012/2017—overview of the time program and cumulative mass loss (Figure 8, time program).*

pyrolysis, the formation of emissions and residues on the full scale [23]. Using the TGA/DTG/DTA/EGA techniques, we can estimate the temperature range when substances characteristic of the decomposition of biological macromolecules take place, formation of the main decomposition gaseous products H2O and CO2, and determination of the temperature range when biologically inert substances react to thermal load. The latter allows us to determine the temperature range of CO2 release resulting from naturally occurring carbonates that cause CO2 emissions for which no emission coupons need be purchased. Reference [23] conducted a STA study of the representative annual sample 2010/2011 in an inert (pyrolysis) and oxidative (incineration) atmosphere. The study shows that in a comparable temperature range from room temperature to 1000°C the decomposition in an oxidative atmosphere takes place in at least in two stages, while in an inert atmosphere it takes place in three stages. Due to the continuous decrease in mass by isothermal temperature program in an inert atmosphere, the STA was prolonged to 1200°C. In the last temperature range of the inert atmosphere, the residue mass was further decreased by 4.5% m/m, which means, that the decomposition in an inert atmosphere takes place in four stages. The study [23] reveals that the first stage of granules decomposition in the temperature range from room temperature to 185°C takes place equally regardless of the atmosphere, with the evaporation of H2O. The mass loss in this temperature profile is 6.1% m/m, while the result for the mass loss for the annual sample 2010, determined with the conventional oven-dry method (non-isothermal temperature profile), is 8.7% m/ m (**Table 15**). In both atmospheres, the granules' thermal behavior in the temperature range from 185 to 420°C is very dynamic. The highest mass losses are observed: (i) in an inert atmosphere 32.5% m/m, the signal peak of DTG is 1.97% min<sup>1</sup> at 324°C, and (ii) in an oxidative atmosphere 30.8% m/m, the signal peak of DTG is 2.14% min<sup>1</sup> at 252°C. This stage in the thermal behavior of granules in both atmospheres presents the fingerprint for that type of material. It is a result of the decomposition of a major part of the granules' biomass. A comparison of the TGA curves profiles in both atmospheres shows that the TGA curves behave similarly to the temperature 486°C. At that temperature, the residue mass was 56.0% in the oxidative atmosphere and 56.4% by mass in the inert atmosphere. The further course of the TG curves was different.

Slightly different results were obtained in repeated analysis on the newer aparatous Netzsch STA 449 F3 Jupiter in the year 2012. For the sample 2010/2012 (**Figure 9a** and **b**) TGA curves behave similarly to the temperature of 346°C, whereby the residue mass was 70.1% m/m in the oxidative atmosphere and 71.2% m/m in the inert atmosphere. To the temperature of 486°C both TGA curves had a similar slope, with the residue mass being 58.0% m/m in the oxidative atmosphere and 55.2% m/m in the inert atmosphere. After that temperature TGA curves begin to behave significantly different.

This means that the presence of oxygen influences mass loss to differ significantly only above 486°C, while the dynamics of mass loss (DTG) and changes in heat balance (DTA) in different atmospheres begin to differ at temperature 185°C, immediately after the loss of free and bound water [23] (**Figure 9**). The determined temperature limit is close to Tignition of granules, which is 550°C (**Table 15**). The mass loss for the annual sample 2010 due to decomposition of carbonates determined with the combination of the two conventional methods in LF (nonisothermal temperature profile at 550°C and at 900°C) is 5.3% m/mDM (**Figure 6a**). Carbonates content evaluated on the basis of the fingerprint for thermal behavior (isothermal temperature profile) for the sample 2010/2011 is 6.5% m/m, and for the sample 2010/2012 9.2% m/m (**Figure 9a**, pyrolysis mass loss from 550 to 900°C). Similar behavior was confirmed in the sample 2012/2012 analysis (**Figure 9c** and **d**), which had at 346°C the residue mass of 67.1% m/m in the oxidative atmosphere *Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*

### **Figure 9.**

*Comparison of the fingerprint of thermal decomposition in the oxidative and the inert atmosphere for (a) mass loss for each stage for sample 2010/2012, (b) thermogram for sample 2010/2012, (c) mass loss for each stage for sample 2012/2012, and d) thermogram for sample 2012/2012.*

and 66.1% m/m in the inert atmosphere, and at 486°C the residue mass was 54.8% in the oxidative atmosphere and 49.1% m/m in the inert atmosphere.

In order to determine the impact on the stability of annual representative samples due to longer storage and in order to determine the repeatability of the TGA/ DTA/DTG techniques, repeated thermogravimetric analysis in an inert atmosphere were performed on annual samples 2010 and 2012 (pyrolysis) (**Figure 10**). Compared to the analysis in the year 2011 [23] when apparatus Netzsch STA 409 was used with the temperature range from room temperature to 1200°C, aparatous Netzsch STA 449 F3 Jupiter was used, which allows the temperature range from room temperature to 1500°C, and the heating rate was the same. Repeated analysis were performed in the years (**Figure 10**): (i) 2012 (sample 2010/2012 and sample 2012/2012), (ii) 2016 (2012/2016), and 2018 (2010/2018 and 2012/2018). The partial mass loss of samples at individual characteristic temperatures is repeated in the same way in all samples (**Figure 11a**), with only a slight upward trend at 400°C. The same is true for the peaks of the DTG signals (**Figure 11b**). There were no significant biological-chemical transformations or a significant change observed in the quality of granules after the storage period. Some substances became slightly more thermally stable, suggesting a smaller temperature lag for the DTG signal peaks toward higher temperatures (**Table 17**).

Pyrolysis is an anaerobic thermo-chemical decomposition process that enables the generation of various useful groups of substances from the treated sludge, which can then be utilized separately in various energy and material recovery

### **Figure 10.**

*Fingerprint of thermal decomposition in an inert atmosphere for annual samples 2010 and 2012 - analyzed in 2011, 2012, 2016, and 2018.*

### **Figure 11.**

*Comparison of the fingerprint of thermal decomposition in the inert atmosphere (5 stages of mass loss) for the annual samples 2010 and 2012 (analyzed in the years 2011, 2012, 2016 in 2018) at the temperature of the DTGpeak for the sample 2012/2016 (Figure 10, Table 17): (a) Δmass loss (% m/m) and (b) DTG (% min<sup>1</sup> ).*


### **Table 17.**

*Comparison of Tpeak(DTG) of the samples 2010 and 2012 analyzed in 2011 (only sample 2010), 2012, 2016 (only sample 2012), and 2018.*

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

processes. Due to carbonization, part of the carbon remains in the residue (PCM) as bound carbon, so pyrolysis is useful as a technology that reduces the carbon footprint as opposed to the incineration procedure that generates greenhouse gases. The use of PCM in agriculture actually means carbon sequestration and thus a double bonus for the environment.

Further pyrolytic experiments were performed on the sample 2012 [13, 24]. Under pyrolytic conditions two main processes were taken place—volatilization and carbonization. Despite the high temperature (up to 1.500°C) that was reached in the apparatus Netzsch STA 449 F3 Jupiter (**Figure 9d** and **10**) the sample during pyrolysis remained solid. Melting of ashes occurs during granules incineration above temperature 1100°C. The pyrolytic products are [24]: (i) solid residue (pyrogenic material), (ii) aqueous light oily fraction (water condensate), (iii) pyrolysis oil as a heavy fraction (bio-oil), and (iv) non-condensable gas (syngas). The latter product was continuously withdrawn from the reactor and was co-fired (potential recovery procedure R 1). The semi-pilot scale experiment yielded [24]: 15.1% m/m syngas, 17.8% m/m water condensate, 16.8% m/m bio-oil and 50.3% m/m of PCM. Most of the water condensate originating from moisture, chemically or crystalline bound water, and water produced during thermal decomposition of the sludge was collected in the light oily fraction and contains a high concentration of condensed water-soluble substances. The yield of the most valuable product bio-oil was low due to the low content of macromolecules and their poor quality (oxidized and degraded organic matter). The bio-oil must be further refined and completely free of water to make it useful as a fuel. The produced PCM contained 32.4% of carbon by weight and had a calorific value of 11.1 MJ kg<sup>1</sup> .

### *3.2.4 Characterization of granules according to energy and material recovery operations*

Despite anaerobic stabilization and biogas production (**Figure 4a**), there is still enough organic matter left in the granules that can be used for WtE process (**Figure 6**, **Tables 15** and **18**). **Table 18** provides the chemical properties of representative annual samples for the period from 2010 to 2018. In addition to energy recovery, the WtE processes enable also the material recovery of many inorganic compounds or elements present in the granules. They also contain some interesting elements that remain in the ash. These are primarily elements of the lithosphere (silicon, calcium, sodium, magnesium, aluminum, potassium, and iron, all in the form of oxides). They are in addition to carbon, nitrogen, hydrogen, sulfur and phosphorus the major elements in treated sludge (**Table 18**). Some other elements presented in **Table 18** are also on the list of critical raw materials [25, 26].

Untreated ash may be used in the construction industry. All procedures of material recovery must be used sensibly because during combustion granules substances are thermo-chemically converted, most of the elements are converted into oxides that are insoluble in water and their bioavailability is highly questionable.

Recovery procedures regarding ash utilization on the full scale, which purpose is to obtain only a certain element, e.g. extraction of phosphorus or other critical raw materials, are economically still unprofitable. WtE processes have economic benefits, but the facilities are also the source of emission of produced volatile compounds and particles into the air (**Tables 2**–**4**). **Table 18** presents the parameters, which are arranged in different groups: (i) elements relevant to the economic benefits, but are also the source of emission into the air (C, H, N, and S), (ii) halogens, which can cause corrosion of mechanical equipment (F and Cl), (iii) the major elements, which are important for material recovery of the ash (P), and (iv) volatile metals and their compounds, that are harmful to human health (Cd, Hg). Based on the given values for all the listed elements (**Table 18**), it is not possible to accurately


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


### **Table 18.**

*Basic physicochemical properties of granules, interesting for WtE processes.*

predict whether the WtE process will exceed the permitted emissions into the environment. The latter will certainly occur if the flue gas cleaning system is inadequate. For the sustainable energy use of granules, in addition to reducing their amount and economic benefits offered by this recovery procedure, it is very important that advanced flue gas cleaning and volatile metal removal techniques prevent the polluting substances their circulation in nature as much as possible. The granules produced at CWWTPL are a sustainable material for SRF is revealed by reference [27]. The latter is a case study of the establishment of an integral quality system for wastewater treatment and sludge management at CWWTPL according to TS CEN/TS 343, EN 15358:2011, Quality management system, Particular requirements for their application to the production of SRFs. For the period from 2012 to 2020, a classification of the granules defines their conformity as an SRF with Class code NCV 3–4; Cl 1; Hg 3. In Ref. [27] is also pointed out that mercury is one of the most problematic volatile metals regarding the quality of cleaned flue gases. It is transmitted to the environment through flue gas emissions [28]. By effectively cleaning the flue gases generated by WtE processes, we achieve the interruption of the circulation of substances that cause emissions into the air.

### *3.2.5 Granules and their solid residues after thermally loading evaluation as a fertilizer*

In the EU the use of sewage sludge in agriculture is regulated by the framework directive SSD [6], which is summarized differently in each EU Member State. The limit values for soil and sewage sludge quality prescribed by SSD are presented in **Table 5**. The set of parameters for which the SSD prescribes limit values is short. It also does not prescribe a declaration for the content of Corg, primary nutrients, secondary nutrients, macronutrients, and micronutrients as does the Fertilizing Resolution (FR) (**Tables 9**, **10** and **12**). In general, the existing European market for fertilizers, which are produced from treated waste, is non-uniform. The latter will be addressed by (FR) [20], which will take full effect from 16 July 2022 (**Tables 7**–**12**). The use of granules in agriculture should be viewed in terms of soil quality (heavy metal content, pH, organic content) and in terms of granules quality. For their application, it is necessary to take into account the seasonal time, the type of plantation, the purpose of the crop usage, and the period of the plant growth.

Metals in the environment are usually in the form of compounds. Soil pH plays an important role in the bioavailability of heavy metals into the plant. If the soil is alkaline, soluble metal ions precipitate into less soluble compounds, e.g. hydroxides, which are less bioavailable and therefore less harmful. Elements in granules can have the function of nutrients if they are water-soluble or if they are bioavailable. This is especially important for phosphorus and potassium.

**Table 19** presents the biological properties of representative annual samples for the years 2016 and 2018. Granules are anaerobically stabilized and hygienized waste, which therefore has limited biological properties and, depending on the degree of stabilization, still has the possibility of further self-degradation. In terms


### **Table 19.**

*Biological parameters/stability criteria of CWWTPL granules [21].*

of the "stability" parameter, the limit value for short-chain fatty acids for 2018 was slightly exceeded (**Tables 8** and **19**). There is no limit value for parameter AT4 for granules as a digestate. The biological properties of the granules are suitable. By preparing an appropriate substrate, they enable germination and plant growth and do not introduce weeds into the plantation (**Table 19**).

In addition to Corg, the granules contain a wide range of other elements, which are declared as either primary nutrients, secondary nutrients, or macro-and microelements. **Table 20** presents the chemical composition of representative annual samples for the period from 2010 to 2018. The values of the elements are expressed in units, as prescribed for the declaration and limit values of the elements or their relevant compounds according to FR [20] (**Tables 9**–**12**). Among the polluting substances in the granules, the most critical is mercury, the value of which exceeds the limit values for fertilizers (**Tables 9, 10** and **12**). Mercury is an environmental pollutant from atmospheric deposition, which enters with precipitations to a municipal treatment plant. The effect of biological removal of mercury from wastewater at CWWTPL is 84.6% on average with standard deviation of 17.0% [19, 27], so municipal wastewater treatment plants play an important role in preventing the circulation of mercury in the environment and protecting human health. The phenomenon of Hg content in Slovenia is historically linked to the operation of a mercury mine in Idrija, where cinnabarite ore mining was supposed to begin in the last decade of the 15th century, and ceased in 1977. It was the secondlargest cinnabarite mine in the world. In the period from 2010 to 2018, a decrease in Hg content in granules produced at CWWTPL is observed (**Tables 18** and **20**). It is likely that the Hg content will continue to decline.

The general finding regarding the possible use of granules as compost or digestate is that these two treated forms of treated sludge are useful primarily as an additive to other processed waste, which then forms a mixture corresponding to the quality declaration and limit values set by FR.


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

*\* Microwave-assisted digestion with hydrofluoric (HF), nitric (HNO3), and hydrochloric (HCl) acid mixture for subsequent determination of elements.*

### **Table 20.**

*Basic physicochemical properties of granules, relevant for fertilizing.*

Solid residues generated at thermally treated sludge are an interesting substitute for phosphorus-potassium fertilizers and, in addition, a good conditioner for acidic, mineral-poor soils. At the laboratory level, the study was carried out regarding the water solubility of nutrients in resulting residues from the sample 2012 after its

exposure to inert and oxidative thermal treatment (**Table 21**). PCM is by mass the major pyrolytic product with the organic carbon content of 29.4% m/mDM, BET surface area of 6.82 m<sup>2</sup> g1, and bulk density of 820 kg m<sup>3</sup> . The study on the water solubility of generated PCM shows that pyrolysis offers a much greater opportunity for material recovery from sewage sludge than from incineration ashes. It was found that the water solubility of phosphorus in treated sludge's PCM produced at 450°C is higher compared to the residues from the oxidizing atmosphere; in contrast, the water solubility of K and Mg is lower (**Table 21**). According to ashes properties, it was found that the highest water solubility and the highest BET


### **Table 21.**

*Fraction characterization of granules and their solid residues after thermal treatment.*

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

surface area has the ash generated at 450°C. At higher temperatures of oxidative thermal treatment, the nutrients are chemically converted to a form that is no longer water-soluble, reducing the possibility of nutrient recovery of the residue (**Table 21**) [29]. The advantage of extracting phosphorus from PCM is also that pyrolysis does not cause sintering or melting, as happens with incineration at temperatures above 1000°C. According to the guidelines for sustainable production of biochar [14], it is potentially useful as a fertilizer for non-agricultural land, for reclamation of degraded land, and for the preparation of artificial soils. It could be used to amend depleted soil to increase fertility, porosity, and water retention. However, the concentration of copper and nickel increases due to the volatile and decomposed fraction of the pellets, while the concentration of mercury decreases [13, 19].

To increase the useful bioavailable components new economically viable technologies need to be developed to liberate the nutrients from sewage sludge ash. Some materials recovered from the UWWTS are still considered waste even if they are of good quality and have a market value [9]. At the moment, European legislation does not favor the recycling of these materials in the economy. The example of the FR, explicitly defining the conditions or processes under which struvite or ashes produced from UWWTS could cease to be a waste and become a product, is a good starting point [9]. Due to the impoverishment of natural sources of phosphorus, it is especially important to find opportunities for recycling it.
