**2. Materials and methods**

In terms of reviewing basic data on morphological, thermal, and physicochemical properties of sewage sludge, a case study for the CWWTPL was conducted [10]. It is an urban one-stage mechanical-biological wastewater treatment plant aimed at conventional secondary treatment of municipal wastewater, a small amount of industrial wastewater, and providing the public service for acceptance and treatment of excess sludge from the small WWTPs and the contents of septic tanks.

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

The findings of long-term morphology, physicochemical investigations, and thermal analyses of pelletized dehydrated digestate (granules), produced at CWWTPL, which were carried out in the period from 2010 to 2020, are presented. Predominantly accredited methods for physicochemical characterization and non-accredited methods for morphological properties (bulk density, particle size distribution, and specific surface area determination) and thermal properties were applied. Chemical elements that are the basis for WtE processes, the elements that are important for the use of residues after thermal treatment (combustion or pyrolysis), and elements that are important for the use of granules as a fertilizer or as a material for soil remediation and improvement of the ecological condition were determined. Additional research of thermal properties was done with the conventional accredited methods for organic matter content, loss on ignition, and volatile matter. Enabling more extensive information in the change in weight of granules under the thermal load was derived with simultaneous thermal analysis of granules and mass spectroscopy of the released gases.

Confidence in the results of research into the properties of sewage sludge is based on an appropriate quality control system (including sampling), targeted enduse, and a list of required parameters. Only high-quality performed analysis enable credible results and the setting of optimal guidelines for the successful CE.

### **2.1 Materials**

After thickening by mechanical equipment, along with the addition of a strong cationic polyelectrolyte, the surplus sludge is anaerobically stabilized under mesophilic conditions at 37–40°C and pH of 7.4 in a digester by a hydraulic retention time of at least 20 days. The produced digestate is a thick black-gray suspension of anaerobic biomass with 3.6% m/m of dry matter. Anaerobic stabilization is followed by centrifugal dehydration (cake production) and granulation in a drum dryer at 90°C [19]. The drying process is conducted in batch mode. Simultaneously the digestate cake gets hygenized and granulated. The selection and execution of the cake drying are in accordance with Technical Standard (TS) from the group CEN/ TC 308, Characterization of sludges, CEN/TR 15473:2007, Good practice for sludge drying.

Prepared and characterized were also laboratory samples of the basic solid residues generated after controlled thermal loading of granules in order to simulate pyrolytic biochar and incineration bottom ash.

### **2.2 Methods and techniques**

Inflow and outflow of water at the CWWTPL are measured continuously with the Khafagi type venturi meter and an in-line flow meter. The wastewater samples are prepared, cooled, and stored in accordance with the standards from the group water quality—sampling: (i) ISO 5667-10:1992, Part 10: Guidance on sampling of waste waters, and (ii) ISO 5667-3: 2012, Part 3: Preservation and handling of water samples. The COD in raw and treated wastewater is monitored by measurements of daily representative 24 h time-proportional composite water samples using method from the group of standards ISO/TC 14/SC 2 "Physical, chemical and biochemical methods".

From the composite daily sub-samples, mass-proportional representative batch and monthly composite samples of dried and pelletized digestate (in granules) are made, tested, and stored at 5 1°C. Similarly, mass-proportional representative annual composite samples according to monthly delivered amount were prepared out of the refrigerated monthly samples. For the granules sampling procedure, a quality system that includes a manual time-proportionate daily sampling in

accordance to the standard ISO 5667-13:2011, water quality, sampling, Part 13: Guidance on the sampling of sludge from sewage and water treatment works, and EN 15002:2015, characterization of waste, preparation of test portions from the laboratory sample, was established, as well as a routine control of produced granules as they were shipped to the stakeholder (**Table 13**). The granules sampling plan takes into consideration the relevant technical standards (TS): (i) EN 14899:2005, Characterization of waste, sampling of waste materials, framework for the preparation and application of a sampling plan, (ii) CEN/TS 15442:2006, Solid recovered fuels, methods for sampling and (iii) CEN/TR 15310–3:2006, Characterization of waste, sampling of waste materials, Part 3: Guidance on procedures for subsampling in the field.

Most of the results for the annual representative samples are obtained from official evaluation prepared for the purpose of conducting a public tender for the transport and final treatment of granules from the place of origin. The evaluations were prepared by an authorized contractor [22] and are part of the tender documentation for the implementation of large value public contracts. The methods are selected according to the legal requirements depending on the sample matrix and the scope of the sample under consideration. Slovenian national legal requirements prescribe the quality of the sewage sludge used as a bioresource or as a material for the WtE, and the range of methods and techniques for quality parameters determination. Standard analytical methods for characterization of granules as a potential fertilizer (PF) and as a solid recovered fuel (SRF) were applied as specified by TSs: (i) CEN/TC 223 "Soil improvers and growing media", (ii) CEN/TC 292 "Characterization of waste", (iii) CEN/TC 308 "Characteritation of sludge", and (iv) CEN/TS 343 "Solid recovered fuels". In accordance with the CEN/TS 343, TS EN 15402:2011, Determination of the content of volatile matter (VM), the proportion of VM that causes the granules mass loss in a covered crucible at a thermal load of 900°C over a period of 7 min is determined.

### *2.2.1 Sampling pattern for representative samples of granules*

Periodicaly, every 3 h, 200 mL samples of granules were manually collected from the conveyor belt. Preparation of representative composite daily sample for each shipment is provided by homogenizing and quartering the three-hour subsamples. Daily samples are stored in plastic bottles of 1 L at 5°C 1°C. The sampling period runs from 1 January to 31 December. At the end of each batch of drying process preparation of the composite batch sample (sample increment of 400 mL in volume from each daily sample) is executed by homogenizing and quartering of sub-samples. Representative batch samples are stored in glass bottles of 0.75 L at 5°C 1°C. At the end of the calendar month preparation of a composite monthly sample (sample increment of 400 mL) from each daily sample is provided. Prepared are four composite samples (4 1 L) by homogenizing and quartering the daily sub-samples. The monthly samples are stored at 5°C 1°C in brown glass bottles. At the end of the calendar year preparation of a composite massproportional annual sample is prepared from each monthly sample. Four annual composite samples (4 1 L) are obtained by homogenizing and quartering the monthly sub-samples. Storage is done at 5°C 1°C in brown glass bottles.

All annual representative samples are still kept properly and are available for further investigation.

### *2.2.2 Granules characterization*

At CWWTPL the quality control of granules is provided by performing regular quality control at all levels of sludge pre-treatment. To ensure the stable granules

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

quality, dry matter is regularly checked on an hourly and daily basis with moisture analyzers with a halogen heater. In addition, the quality of representative samples of batch, monthly and annual samples are checked in the laboratory using standard analytical methods for characterization of granules: (i) the oven dry method for moisture content at 103°C (CEN/TC 223, EN 13040:2007, Sample preparation for chemical and physical tests, determination of dry matter content, moisture content and laboratory compacted bulk density) and at 105°C (CEN/TC 444, Environmental characterization, Sludge, treated biowaste, soil and waste, EN 15934:2012, Calculation of dry matter fraction after determination of dry residue or water content), (ii) laboratory furnace (LF) for organic matter content (OM) at 450°C (LF) (CEN/TC 223, EN 13039:2011, Determination of organic matter content and ash), (iii) LF for loss on ignition (LOI) at 550°C (CEN/TC 292, EN 15169:2017, Determination of loss on ignition in waste, sludge and sediments), (iv) LF for ash content at 900°C in LF (CEN/TS 343, EN 15403:2011, Determination of ash content, modified), and (v) standard laboratory equipment for bulk density (as received) (CEN/TS 343, CEN/TS 15401:2010, Determination of bulk density, modified). The content of carbonates (inorganic source of carbon and CO2) is determined as a mass difference between LOI at 550°C and LOI at 900°C due to the thermal decomposition of magnesium- and calcium carbonates, predominately caused by the latter.

Measurements of the specific surface area of granules by gas adsorption are performed using a Micromeritics ASAP 2020 analyzer according to ISO 9277:2010. The adsorptive gas was nitrogen, and the specific surface area (SSA) was calculated using the Brunauer-Emmett-Teller method (BET). A simple method for determining the ignition temperature in the granules layer was provided by a non-standardized method in an open LF with flues and with the isothermal temperature program.

To determine the chemical properties of granules the annual composite granular sample was milled to <1 mm using the Retsch SK1 hammer mill and, when necessary, down to <0.5 mm in the Retsch ZM 200 mill. Microwave-assisted digestion procedure for preparation of the PF to determine the PTMs content was done with aqua regia, while microwave-assisted digestion of granules as a SRF was done with a more invasive acid mixture of concentrated hydrofluoric (HF), nitric (HNO3) and hydrochloric acid (HCl), and the results obtained are shown in separate tables.

The majority of the methods used for wastewater and granules characterization are accredited according to the Technical Standard SIST EN ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories and performed by authorized contractors.

Simultaneous thermal analysis (STA) with thermogravimetric analysis (TGA), derivative thermogravimetric analysis (DTG), and differential thermal analysis (DTA) of granules and mass spectroscopy of released gases are performed in an oxidative and in an inert atmosphere. STA apparatus (Netzsch's products were used) allowed the simultaneous acquisition of mass loss (TGA) and thermal effects (DTA) during thermal analysis. The DGT curve is obtained by calculating the derivative of the TG curve. The DTA measurement gives combustion curve or thermal change curve as a function of thermal load and information on the heat balance for each stage of thermal decomposition of granules. The DTG analysis enables a "fingerprint" of the thermal behavior and a thermal decomposition profile curve (Δm/Δt). Data manipulation and transformation are performed by Netzsch Proteus 6.1.0 software.

Fingerprint of thermal decomposition of granules for determination of their behavior at thermal load is carried out on the two representative annual samples (**Table 13**): (i) for the year 2010 and (ii) for the year 2012. In 2011 the annual sample 2010 (labeled as 2010/2011) was characterized with the TGA/DTG/DTA


### **Table 13.**

*Sampling procedure for granules.*

analysis in both atmospheres: (i) the inert atmosphere of Ar (purity 99.999%) and in (ii) the oxidizing atmosphere (80% v/v of Ar and 20% v/v of O2, purity 99.999%). The findings are presented as a short review of the study in Ref. [23]. In 2012 TGA analysis for the annual sample 2012 is performed in both atmospheres (labeled as 2012/2012), and additional the TGA analysis in both atmospheres for the sample 2010 (labeled as 2010/2012) for the purpose of comparison was repeated. In 2016 the TGA analysis (inert atmosphere) was repeated on the sample 2012 (labeled as 2012/2016), and again in 2018 on both samples: (i) the sample 2010 (labeled as 2010/2018), and (b) the sample 2012 (labeled as 2012/2018). Both representative samples, 2010 and 2012, are still properly stored (closed packaging, in a dark and cool room). Each time, 100 mg of finelly ground granules were weighed into the TG/DTA crucible (0.3 mL, Al2O3) and exposed to heating from Troom to 1500°C at the heating rate of 10 K min<sup>1</sup> . Protective and purge gas flows were set to 30 and 50 mL min<sup>1</sup> , respectively.

In the year 2017, using a method described by CEN/TS 343, CEN/TR 15716:2008, Determination of combustion behavior (Combustion behavior), the proximate analysis was performed on the representative annual sample 2012 (labeled as 2012/2017) with the TGA technique in the combination with: (i) isothermal and non-isothermal temperature program in the temperature range from room temperature (Troom) to 900°C in the inert atmosphere, and (ii) the isothermal temperature program in the oxidative atmosphere at 900°C. The proximate analysis was performed in Netzsch STA 449 F3 Jupiter. 50.0 0.5 mg of the milled granules were used to provide the analysis. The sample was weighted into the TG/DTA crucible (0.3 mL, Al2O3). The contents of moisture, volatile matter (VM), fixed carbon (Cfix), and ash were determined with this single analysis of the granules consisting of several stages. In the first stage, moisture is determined by sample heating to 110°C with 20 K min<sup>1</sup> , keeping the temperature constant for 15 min in an inert Ar atmosphere (purity 99.999%). VM is determined by consecutive heating to 900°C at 20 K min<sup>1</sup> , followed by an isothermal hold for 15 min. In the last stage, the purge gas is changed to the oxidizing atmosphere (80% v/v of Ar and 20% v/v of O2, purity 99.999%), and the temperature is kept constant at 900°C for 120 min. The mass loss in this stage is attributed to Cfix, and the residual mass is ascribed as ash.

The year 2012 is chosen also as the reference year for further investigations of representative annual samples of CWWTPL and comparisons of results obtained in future research.

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

### *2.2.3 Characterization of solid residues after thermally treated granules*

Two types of techniques for solid residue preparation were used: (i) pyrogenic residue, prepared with TGA technique using inert atmosphere (pyrolysis, Ar, 99.999 vol%)—2000 mg of granules were isothermally heated for 60 min at 450°C, in TG crucible 3.4 mL, Al2O3; the procedure was repeated until enough amount of sample has been achieved, and (ii) simulation of four type of bottom ash - the ignition (incineration) of granules in the laboratory heating furnace (LF) at 450°C (240 min), 550°C (180 min), 700°C (120 min) and 900°C (60 min). In the resulting residues were determined the content of nutrients (TOC, N, P, K, and Mg), the proportion of volatile part of nutrients due to the thermal treatment, and the proportion of water-soluble nutrients with the extraction method in accordance with the TS CEN/TC 223, EN 13652:2002, Extraction of water-soluble nutrients.

### **3. Results and discussion**

### **3.1 Sewage sludge treatment at the CWWTPL**

Because of the mixed sewer system, the amount of treated wastewater over a 10 year period varies between 1.5 million and 4.045 million m<sup>3</sup> per month and 32.6 million m<sup>3</sup> in the year 2010 to 24.6 million m<sup>3</sup> in 2020. The removed COD varies between 550 t of O2 to 1770 t of O2 per month and is lower at the higher quantity of inflow in the period of wet weather.

The average daily amount of excess sludge over a 10-year period is about 17.5 tDM or 51 g PE<sup>1</sup> COD (18.6 kg PE<sup>1</sup> COD year<sup>1</sup> ). The produced amount of excess sludge is lower than that reported in the literature (20–25 kg of sludge per one PE year<sup>1</sup> ) [5]. Due to the fluctuations of the raw municipal wastewater inflow and related removed COD (**Figure 1**), the amount of generated granules, and also their useful matter content delivered to the stakeholder, varied accordingly (**Figure 2**). On average, the daily amount of granules production over a 10-year period is 10.6 tDM or 31 g PE<sup>1</sup> COD (11.3 kg PE<sup>1</sup> COD year<sup>1</sup> ). Over the year, these values fluctuate more markedly due to the influence of wastewater temperature on the properties of activated sludge and consequently on the final quantity and quality of granules.

Dynamics at CWWTPL is very important for the granules stakeholder to be able to make a risk assessment and adjust his procedures to the quantities and energy

**Figure 1.** *Dynamics of the amount of treated wastewater and COD removal rate.*

### **Figure 2.**

*Dynamics of granules production in their net calorific value (NCV) for the consecutive months in the 10 years time period.*

content of granules (**Figure 2**). The problem for the sustainable use of granules in agriculture is the seasonal demand for fertilizer, while the granules are generated over the whole year with the highest production rate in the summer [19]. On average, 6.380 tDM of excess sludge is produced annually (**Figure 3a**) with content of organic matter of 73.0% m/mDM. The anaerobic stabilization reduces the amount of sludge for 39% m/mDM, yielding about 3880 tDM to be transformed into granules having average organic matter content of 67.4% m/mDM. By taking over the granules, the stakeholder gained an average of 55,300 GJ of energy per year in the ten years period (**Figure 3b**).

Biogas production in digestor contributes to a double economic benefit. The amount of sludge is reduced and fuel is obtained for site heat supply (heating the digester and drying the cake). Biogas production has its seasonal fingerprint as well. Due to summer conditions at the biological stage of wastewater treatment, the biogas production (**Figure 4a**) and the granules NCV (**Figure 4b**) are lowest. On average, 17.2 L of biogas per one PECOD was produced at CWWTPL over the 10 years period.

### **Figure 3.**

*Dynamics of 10 years period: (a) excess sludge production and an organic load of inlet wastewater and of excess sludge, and (b) annual dynamics of granules production, their organic matter content, and calorific value.*

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

**Figure 4.**

### **3.2 Granules characterization**

### *3.2.1 Morphologic properties*

**Table 14** presents data for the morphological characteristics of annual representative granules samples. Granules are a dry, homogeneous non-hazardous hygienized fine-grained waste with a grain size distribution of 2–4 mm (**Figure 5a**). No foreign solid particles are observed with the eye. Granules do not contain macroscopic impurities or solid particles of glass, plastic, and metals exceeding 2 mm in diameter. Also, no other mineral particles exceeding 5 mm in diameter are present. Granules' response to mechanical stress expressed as mechanical durability is 95.6%. Mechanical durability is a measure of the resistance of compressed fuels toward shocks and/or abrasion during handling and transportation.

Seasonal fluctuations in bulk density in the period from 2017 to 2020 are not significant, the bulk density of granules is highest in early spring. There is also no significant fluctuation in bulk density at the annual level. Only the value for the annual sample 2010 differs significantly (**Table 14**), and that is also determined for other quality parameters of that sample, which will be presented below. The volume of 1m<sup>3</sup> granules for the period 2017–2020 has a mass of about 640 kg (**Figure 5b**).

Granules are slightly basic. Their electrical conductivity is 2.38 mS cm<sup>1</sup> , which means that they contain a small proportion of salts. The granules are compacted, difficult to break, which is evident in their low value for the specific surface area (BET) from 0.9 m<sup>2</sup> g<sup>1</sup> to 1.4 m<sup>2</sup> g<sup>1</sup> (**Table 14**).


### **Table 14.**

*Basic morphologic and physical properties of granules.*

**Figure 5.**

*(a) Granules size distribution, and (b) dynamics of granules bulk density.*
