*2.1. Feedstock Characterization*

The three feedstocks used were characterized in accordance with accepted environmental practices. All measurements were conducted in triplicate and the average and standard deviation of each data is reported. The total solids content of each sludge was measured by drying in an oven at 105 ◦C for at least 8 h until constant weight was reached.

The pH of the raw and carbonized samples was measured using a Profi-Line pH 3310 (WTW, Milan, Italy) portable pH-meter by placing 1 g of solids in 20 g of deionized water, shaking for at least 90 min, allowing the mixture to settle for 15 min, and then reading the pH. The total Chemical Oxygen Demand (COD) was measured using a closed reflux titration method [43] using potassium dichromate digestion solution, sulfuric acid reagent, ferroin indicator solution, and a standard ferrous ammonium sulphate titrant according to standard procedures. The same method was used for soluble COD following filtration of the sample through a 0.45 μm filter [43]. Organic nitrogen in the trinegative state was measured via the semi-micro Kjeldahl method. Ammonium nitrogen was measured [43] by first buffering the sample at pH 9.5 with a borate buffer (to decrease hydrolysis), then distilling in a solution of boric acid, and determining the concentration via acid titration with H2SO4. To measure the total phosphorous, samples were first digested in H2SO4, forming molybdophosphoric acid, which was then reduced by stannous chloride to molybdenum blue. The concentration was measured photometrically at 690 nm and compared against a calibration curve [43].

Ultimate analysis to determine elemental composition of C, H, N, S, and O (by difference) was conducted on a LECO 628 analyzer (LECO, Moenchengladbach, Germany) equipped with Sulphur module for CHN (ASTM D-5373 standard method) and S (ASTM D-1552 standard method) determination. Proximate analyses were done on a LECO Thermogravimetric Analyzer TGA 701 (LECO Corporation, St. Joseph, MI, USA). Samples were heated at 20 ◦C min−<sup>1</sup> to 105 ◦C in air and held until constant weight (< ±0.05%) to provide a dry baseline. They were subsequently heated at 16 ◦C min−<sup>1</sup> from 105 ◦C to 900 ◦C in nitrogen with a hold time of 7 min, where the mass loss was attributed to Volatile Matter (VM). Finally, samples were held at 800 ◦C in air to oxidize the Fixed Carbon (FC) until the mass change stay within ±0.5% by weight. Mass remaining after this was considered to be ash (inorganic matter) content. The Higher Heating Value, HHV, was measured using an IKA 200C isoperibolic calorimeter (IKA-Werke GmbH, Staufen, Germany) according to the CEN/TS 14918 standard.

Inductively coupled plasma-optical emission spectroscopy (Arcos Ametek, Spectro, Germany) was used to determine the inorganic concentration of the sludge samples. Briefly, samples were oven-dried at 105 ◦C until constant weight and then acid-digested in concentrated nitric acid (650 mL<sup>−</sup>1; Carlo Erba, Milano, Italy) using a single reaction chamber microwave digestion system (UltraWAVE, Milestone Inc., Sheldon CT USA) and Teflon-lined vials to prevent interference. Elements were quantified using certified multi-element standards (CPI International).

#### *2.2. Hydrothermal Carbonization and Product Analysis*

Hydrothermal carbonization typically occurs between 180 and 250 ◦C under autogenous pressure (up to 50 bar) but below the critical point [18]. The present work utilized a 50 mL stainless steel batch HTC reactor rated to withstand 300 ◦C and 140 bar, with temperature and pressure monitoring and temperature control, as previously described [44,45]. The thickened and digested sludges were used as-received. The reactor was loaded with 35.0 mL ± 0.1 mL biomass, which maintained a biomass (dry biomass)-to-water ratio of about 0.03:1. The dewatered sludge had a solid content of 25 wt% and needed to be diluted to ensure that the biomass was fully submerged. The reactor was filled with 20.00 g ± 0.01 g of dewatered sludge and 15.00 g ± 0.01 g of deionized water to cover the sludge, resulting in a biomass-to-water ratio of 0.17:1. Prior to each run, the reactor was sealed and purged with nitrogen gas; then it was heated up to the desired reaction temperature (190 ◦C, 220 ◦C and 250 ◦C) and held at the set point for the desired reaction time (30 min and 60 min). At least three experimental runs for each of the temperature/time combinations were performed for each of the three sludge samples.

After the reaction time, the reactor was cooled by placing a cold (−25 ◦C) stainless steel disk under its bottom and by blowing compressed air into its outer walls. The reactor was cooled to ambient temperature in less than 15 min, at which point the produced gas was measured by flowing it into a graduate cylinder filled with water [45]. As reported in the literature, where the CO2 content is always greater than 90 vol.%, the produced gas was assumed to be comprised entirely of CO2 [45,46]. The gas yield was estimated according to the ideal gas law under the assumption of standard temperature and pressure as:

$$Y\_{\text{gus}} = \frac{\text{Mass}\_{\text{CO}\_2}}{\text{Mass}\_{\text{Sludgse, dry}}} \tag{1}$$

The liquid and solid HTC products were filtered through a pre-dried and weighed piece of cellulose filter paper. The filter paper was then put in the oven overnight at 105 ◦C and weighed to calculate the solids produced. The solid yield of the hydrochar, *Yhydrochar*, was calculated as:

$$Y\_{hydrochur} = \frac{Mass\_{hydrochur, dry}}{Mass\_{Sholgc, dry}} \tag{2}$$

The liquid yield was computed as the complement to 1 of the gas and solid yields.

The solid hydrochar was characterized according to the same methods described in Section 2.1. for Higher Heating Value (HHV), proximate, and ultimate analyses. The hydrochar's relative solid reactivity was measured using a Mettler-Toledo Thermogravimetric Analyzer–Differential Scanning Calorimeter (TGA-DSC-1, Mettler-Toledo LLC, Columbus, Ohio, USA) in an oxidative atmosphere. The TGA-DSC was calibrated with NIST-traceable gold, indium, and aluminum and the mass was measured to ±0.1 μg and temperature to ±0.1 ◦C. Approximately 10 mg of sample was loaded into a 70 μL alumina crucible. Samples were heated at 20 ◦C min−<sup>1</sup> up to 110 ◦C in air flowing at 50 mL min−<sup>1</sup> and held for 30 min to drive off any residual moisture. They were subsequently heated at 20 ◦C min−<sup>1</sup> up to 950 ◦C and held for 30 min to oxidize all material. The mass fraction of sample converted (*X*) at any time, *t*, was calculated as:

$$X = \frac{m\_i - m\_t}{m\_i - m\_f} \tag{3}$$

where *mi* is the initial mass, *mt* is the mass at any time, *t*, and *mf* is the final mass after the hold at 950 ◦C. Derivative thermogravimetric (DTG) curves were plotted as *dX*/*dt* (s−1) versus temperature. Differential scanning calorimeter (DSC) data was normalized as heat flow per sample mass at any given instant (*mt*). DTG curves are compared to those from an in-house sample of Illinois No. 6 coal, a high volatile bituminous coal from the Illinois #6 (Herrin) seam from the Argonne Premium Coal Bank [47]. The coal sample is well characterized in the literature and is often used as a standard on which to compare solid fuel oxidation [47–49].

Dewaterability—and the improvement due to HTC was determined by measuring the Capillary Suction Time (CST) required for water to be separated from sludge across a filter paper (Whatman 17 CHR, VWR International, Milan, Italy) using a Triton Electronics Ltd. capillary suction timer type 304B according to standard methods [43]. CST provides a quantitative assessment of how readily sludge releases water.

The liquid phase remaining after hydrothermal carbonization was characterized by measuring pH, COD, organic nitrogen, ammonia nitrogen, and phosphorous as described above. Measurements of Readily Biodegradable COD (RBCOD) were performed following the procedure described in literature [50]. To measure NH4 <sup>+</sup> nitrogen and soluble COD, samples were screened through a 0.45 μm filter [43].

#### **3. Results**

To assess the optimal point to withdraw sludge from the wastewater treatment process for hydrothermal carbonization in terms of resulting hydrochar properties, three samples were pulled from various points along the process: Thickened, Digested, and Dewatered (Figure 1). The feedstocks characteristics are presented in Tables 1 and 2. These three samples were subjected to hydrothermal carbonization at three temperatures (190 ◦C, 220 ◦C, 250 ◦C) and two residence times (30 min, 60 min) each, producing a total of 18 hydrochar samples for analysis.

Looking at the data of the different raw sludges, it is clear that they differ substantially. Thickened sludge contains about 46 wt% elemental carbon and 15 wt% ash. In digested sludge, the carbon decreases to about 26 wt%, and the ash content increases to 45 wt%, due to stabilization during anaerobic digestion. Dewatered sludge contains 36 wt% carbon and about 28 wt% ash. Thus, even if dewatering is a mechanical process, it greatly modifies the sludge characteristics. The supernatant from sludge dewatering has high concentrations of inorganic compounds such as N-NH4 <sup>+</sup>, P compounds, CaCO3, Mg, K, Na, and other minerals that contribute to the ash content [51]. The dewatering unit washes away these inorganics strongly decreasing the ash content of the dewatered sludge: this reflects on an increase in elemental C, H, N, and O, and also in FC and VM. VM variation is extremely significant, passing from a value of 50 wt% in the digested sludge to about 66 wt% in the dewatered sludge. These differences are also due to the fact that the digested sludge, immediately upstream of the dewatering operation, is chemically conditioned with organic polyelectrolyte (1–10 g/kg dry solids [52]) that is quickly adsorbed on the sludge particles.


ofrawthickenedsludge,rawdigestedsludge,andrawdewateredsludgeandproductsofhydrothermalcarbonizationofthickened,


#### *Energies* **2020** , *13*, 2890


**Table 2.** *Cont*.
