Evaluating the Performance of Anaerobic Digestion with Upstream Thermal Hydrolysis—What Role Does the Activated Sludge Process Play?

Abstract

The performance of anaerobic digestion of mixed sludge (MS) with upstream thermal hydrolysis of waste activated sludge (WAS) was evaluated and compared to conventional anaerobic digestion. In contrast to previous studies, this work focuses on the evaluation of the impact of the activated sludge process, which was assessed using a temperature-normalized solids retention time (SRT_ASP,T). For this purpose, data from a full-scale wastewater treatment plant related to SRT_ASP,T, primary sludge (PS) and WAS production were combined with experimental data from laboratory-scale anaerobic digestion of PS, WAS, thermally hydrolyzed WAS, and MS. The parameter SRT_ASP,T was used as a key link between the full-scale and experimental data. For WAS, SRT_ASP,T essentially influenced the efficacy of thermal hydrolysis on the performance of anaerobic digestion. The increase in methane yield was higher with increasing SRT_ASP,T. When considering MS, however, the increase was significantly lower and leveled out over the investigated range of SRT_ASP,T, mainly due to corresponding WAS/MS ratios. This study demonstrates that the knowledge of SRT_ASP,T, sludge production, and anaerobic degradability enables the assessment of the potential of thermal hydrolysis and its effect on anaerobic digestion.

1. Introduction

At larger wastewater treatment plants (WWTPs), anaerobic digestion (AD) is commonly used for stabilizing sewage sludge. Under anaerobic conditions, organic matter is degraded and transformed into biogas while the total solids of sewage sludge are reduced. Utilizing the produced biogas enables WWTPs to minimize the external energy demand by generating energy on-site. This aspect is becoming increasingly important in light of the current revision of the European Urban Wastewater Treatment Directive, which, among other things, aims to achieve energy neutrality in the wastewater sector [1]. Another critical issue for WWTPs is the reduction of disposal costs by reducing the total solids of the sewage sludge. Several upstream pre-treatment technologies have been investigated to increase the biodegradability of sewage sludge and, thus, the performance of anaerobic digestion. The thermal hydrolysis process is one such technology, which has been implemented on a full-scale for the treatment of mixed sludge (MS), a blend of primary sludge (PS) and waste activated sludge (WAS), as well as for the exclusive treatment of WAS. At pre-treatment temperatures >100 °C, thermal hydrolysis positively impacts both the biodegradability potential and the biodegradation rate of the treated sludges. Both effects result in an increased biogas production at WWTPs. It also improves the dewaterability of the digested sludge, reducing the amount of sludge that needs to be disposed of [2]. Furthermore, the thermal hydrolysis of MS with subsequent anaerobic digestion can produce biosolids that are suitable for land application, depending on national regulations [3]. When the production of biosolids for land application is not required, thermal hydrolysis of WAS offers the potential of smaller thermal hydrolysis units with lower thermal energy demand. Consequently, thermal energy demand can potentially be met on the balance sheet by recovering heat from the exhaust gases of an on-site CHP unit [2,4,5].

When it comes to sewage sludge, the impact of thermal hydrolysis in increasing methane production is more pronounced for WAS than PS, due to the composition of WAS and the mechanisms of thermal hydrolysis [6,7]. In brief, WAS consists of active biomass, inactive organic fractions like endogenous residues, inert components from the influent wastewater, and inactive mineral fractions. Of these fractions, the active biomass is mainly anaerobically degradable [8,9,10,11], while the inactive and inert fractions remain unchanged or have slow degradation rates [12,13,14]. High solids retention time (SRT_ASP) in the activated sludge process (ASP) leads to increasing degradation of the active biomass while the inactive and inert fractions accumulate. WAS is aerobically stabilized, and the biogas potential decreases with increasing SRT_ASP [8,15,16]. During the thermal treatment, microorganisms and particulate components are solubilized. Inert substances, such as endogenous residues, are partially made biodegradable for anaerobic degradation [17,18]. The improvement in anaerobic degradability, indicated by an increase in biogas or methane production, depends on the operating conditions of the ASP. Thermal hydrolysis has a greater positive impact if WAS is already aerobically stabilized in the ASP [7,19,20]. As a result, biochemical methane potential changes, in a wide range, from—6% up to ~130% at treatment temperatures of up to 175 °C [20,21,22,23]. Thermal hydrolysis at 135 to 170 °C resulted in improved methane production of 12% to 78% in semi-continuous anaerobic digestion tests with solids retention times (SRT_AD) of 15 to 20 days [6,24,25].

Considering mixed sludge (MS), the proportion of PS reduces the impact of thermal hydrolysis. Thus, the relative increase in biodegradability and biogas/methane production is lower than that of pure WAS. Literature data on semi-continuous (laboratory or pilot scale) and continuous (full-scale) experiments indicate a relative increase in biogas production from 6% to 31% at pre-treatment temperatures from 160 to 175 °C (SRT_AD 13–22 d) [5,6,26,27]. In these studies, WAS/MS ratios based on VS ranged from 44% to 55%. Under comparable operating conditions of thermal hydrolysis and anaerobic digestion, Oosterhuis et al. [28] observed a relative increase in biogas production between 30% and 40% at a WAS/MS ratio of 80%, based on TS.

As the potential positive effect on biodegradability is higher for aerobically stabilized WAS, thermal hydrolysis should be discussed mainly for WWTPs operated at high SRT_ASP, e.g., WWTPs designed for biological nutrient elimination. In the future, the SRT_ASP of these WWTPs could increase even further, mainly because of more stringent requirements for nitrogen and phosphorus removal, e.g., due to the revision of the European Urban Wastewater Treatment Directive [1]. The range of the presented results about the thermal hydrolysis of sewage sludge does not allow a satisfactory assessment of its success so far. In particular, the effects of the ASP have not yet been sufficiently analyzed in this context. New findings show that both the specific methane yield (SMY) of WAS and the SMY increase induced by thermal hydrolysis correlate with a temperature-normalized sludge age (SRT_ASP,T) [29]. SRT_ASP,T makes it possible to assess the influence of the ASP in the evaluation of anaerobic digestion with and without upstream thermal hydrolysis.

This study evaluates for the first time the influence of the ASP on the performance of anaerobic digestion of MS with upstream thermal hydrolysis (WAS only) compared to anaerobic digestion of MS without upstream thermal hydrolysis. For this evaluation, data from a full-scale WWTP are analyzed, laboratory-scale experiments are conducted, and the results are aggregated in a potential assessment for the investigated WWTP. Consequently, this study is divided into three parts. First, the ASP of a full-scale WWTP is examined in terms of the SRT_ASP, SRT_ASP,T, sewage sludge production (WAS, PS), and WAS/MS ratio. Second, the anaerobic digestion of PS, WAS, and MS is investigated in semi-continuous experiments on a laboratory scale. It is also examined whether the anaerobic digestion of MS can be estimated based on the results for the individual sludge digestion of WAS and PS. In the third part of this study, the performance of the anaerobic digestion of MS with upstream thermal hydrolysis (WAS only) depending on the operation of the ASP is evaluated and discussed in comparison to the conventional anaerobic digestion of MS. The focus here is on considering the SMY, the reduction of the digestate solids, the sludge liquor quality, and the net electricity generation due to utilizing the produced methane. For this purpose, the results of Rühl et al. [29] regarding the influence of SRT_ASP,T on the anaerobic degradability of WAS and the relative degradability increase by upstream thermal hydrolysis are used and extended.

2. Materials and Methods

2.1. Full-Scale Wastewater Treatment Plant

The investigated WWTP, with a size of 240,000 PE, is located in Germany, and is designed for biological nitrogen removal in a simultaneous denitrification/nitrification process with primary clarifier. Phosphorus is removed by simultaneous precipitation. Enhanced biological phosphorus removal is not intended (there is no anaerobic tank). Sewage sludge, which consists of WAS and PS, is treated in anaerobic digesters on-site. The return liquor resulting from the dewatering of digested sludge is returned to the ASP.

2.2. Anaerobic Digestion Experiments on a Laboratory Scale

Semi-continuous digestion experiments were performed on a laboratory scale using four digesters with a working volume of 16 L each. The reactors were operated in a climate chamber at 37 °C with intermittent mixing by an overhead stirrer (15 min run time, 15 min pause). Digested sludge from the full-scale digester of the investigated WWTP was used as inoculum. The laboratory digesters were fed with PS and WAS from the full-scale WWTP, as well as thermally hydrolyzed WAS (WAS_TH) and MS. MS was produced manually as a mixture of PS and WAS. Each sludge was fed to the reactors over feeding periods of 11 d to 33 d. Feeding was carried out between 5 and 7days per week. The reactors were operated at a calculative SRT_AD of 18 to 20 d. For proper manual feeding over each period, feed samples were frozen batch-wise at −18 to −24 °C and thawed at room temperature one day before usage. Storage had no relevant effect on the sludge properties and, thus, negligible effects on the digestion experiments. The performance of the digesters was evaluated for each feeding period. Only consecutive days without operational disturbances (e.g., malfunction of the gas volume measurement) were considered. For this reason, the evaluation period is, in some cases, shorter than the feeding period. As shown in Figure 1, the experiments are grouped into two phases to distinguish two research objectives. The relevant data for both phases and for each evaluation period are summarized in

Table A1. Results of the anaerobic digestion experiments (extension to [29]).

Sample		SRTASP,T	Feeding/Evaluation Period	WAS			WASTH			PS			MS
				OLR	SMY	ΔCOD	OLR	SMY	ΔCOD	OLR	SMY	ΔCOD	OLR	SMY	ΔCOD	WAS/MS
		d	d	kg COD/(m³∙d)	NL CH4/ kg CODin	%	kg COD/(m³∙d)	NL CH4/ kg CODin	%	kg COD/(m³∙d)	NL CH4/ kg CODin	%	kg COD/(m³∙d)	NL CH4/kg CODin	%	kg COD/kg COD
PI_1		29	32/19	3.1 ± 0.05	120.2	−5.5	3.1 ± 0.06	156.7	−11.9
PI_2		26	30/19	3.2 ± 0.02	127.3	4.4	3.2 ± 0.01	162.6	−1.7
PI_3		26	29/6	2.3 ± 0.02	121.3	4.6	2.2 ± 0.01	159.5	−6.0
PI_4		37	27/21	2.8 ± 0.02	113.2	1.4	3.0 ± 0.02	148.8	−5.9
PI_5		38	11/11	2.7 ± 0.01	117.4	9.3	2.7 ± 0.01	166.3	2.0
PI_6		39	24/24	2.8 ± 0.02	110.3	−10.3	2.8 ± 0.01	159.9	−6.9
PI_7		53	18/18	3.1 ± 0.11	102.9	4.4	3.1 ± 0.09	149.1	−7.7
PI_8		46	17/17	3.0 ± 0.02	111.6	−16.9	3.0 ± 0.13	158.0	−6.5
PI_9		48	14/14	3.0 ± 0.02	108.9	−3.0	2.9 ± 0.02	157.6	−1.0
PI_10		48	14/14	4.1 ± 0.07 (2.9) a	106.5	3.5	4.1 ± 0.02 (2.9) a	159.3	9.6
PI_11		50	14/14	4.5 ± 0.03 (3.2) a	99.3	−4.1	4.5 ± 0.02 (3.2) a	149.4	−11.4
PI_12		49	14/14	4.1 ± 0.02 (3.0) a	101.4	−11.8	4.2 ± 0.02 (3.0) a	144.7	−15.6
PI_13	PII_1	51	14/14	3.7 ± 0.03 (2.7) a	103.2	−0.5	38 ± 0.02 (2.7) a	153.7	−10.7	4.2 ± 0.07 (3.0) a	225.8	−1.7	4.1 ± 0.02 (2.9) a	173.5	−6.2	0.47
PI_14	PII_2	60	14/14	1.9 ± 0.02	107.1	−3.5	1.9 ± 0.01	154.6	−11.6	3.4 ± 0.02	217.4	−3.7	2.5 ± 0.02	181.4	−7.5	0.44
PI_15	PII_3	46	14/14	2.9 ± 0.02	92.6	2.2	2.9 ± 0.01	145.1	−1.2	2.9 ± 0.02	240.2	+7.0	3.0 ± 0.02	194.4	+3.1	0.41
PI_16	PII_4	31	14/14	2.7 ± 0.02	116.4	−10.7	2.7 ± 0.01	152.6	−7.8	2.2 ± 0.01	267.4	+4.6	2.5 ± 0.01	201.4	+6.2	0.43

^a Feeding on a 5 d/week regime; values in parentheses represent the calculated OLR for feeding on a 7 d per week regime.

.

2.2.1. Phase I

The results of Phase I have been published in Rühl et al. [29], and are extended in this study as described in Section 2.5. The objective was to examine the impact of thermal hydrolysis on the SMY of WAS. WAS were treated for 30 min at an average temperature of 158.0 °C ± 2.5 °C (n = 74) and a corresponding pressure of 6.0 bar ± 0.4 bar (n = 74). For more information on the thermal hydrolysis process, see Appendix A. The study period of each digester consisted of 300 days and 16 evaluation periods. The sludge characteristics during Phase I are summarized in

Table 1. Summary of sludge characteristics after storing (arithmetic mean ± standard deviation) in Phase I.

Parameter	WAS (n = 16)	WASTH (n = 16)
TS [% solids]	4.7 ± 0.6	4.6 ± 0.5
VS [% TS]	76.1 ± 2.0	75.7 ± 2.1
COD/VS [g/g]	1.50 ± 0.03	1.55 ± 0.03
CODs/COD [%]	6.5 ± 1.8 (n = 15)	39.4 ± 1.6 (n = 11)
pH range a [-]	6.1–6.7 (n = 10)	5.8–6.2 (n = 10)

^a Before storing; n: number of analyses.

.

2.2.2. Phase II

The objective was twofold: (1) to investigate the anaerobic digestion of PS; and (2) to compare the performance of the anaerobic digestion of WAS and PS with the anaerobic digestion of MS. Throughout 154 d, one digester was fed with MS and throughout 98 d, one digester was fed with PS. Phase II includes four samples of WAS, PS, and the corresponding MS, and thus four evaluation periods. The adaption time of the digesters fed with WAS, PS, and MS was 277 d, 42 d, and 98 d, respectively. The sludge characteristics are summarized in

Table 2. Summary of sludge characteristics after storing (arithmetic mean ± standard deviation) in Phase II.

Parameters	WAS (n = 4)	PS (n = 4)	MS (n = 4)
TS [% solids]	4.3 ± 0.7	4.5 ± 1.1	4.4 ± 0.6
VS [% TS]	74.9 ± 0.9	84.8 ± 3.6	80.1 ± 1.8
COD/VS [g/g]	1.51 ± 0.02	1.52 ± 0.10	1.56 ± 0.04
CODs/COD [%]	5.9 ± 2.0	6.5 ± 2.6	7.2 ± 2.1
pH range a [-]	6.3–6.7	5.2–6.1	5.8–6.2

^a Before storing.

.

2.2.3. Analytical Methods of the Laboratory Experiments

Total solids (TS) were measured according to DIN EN 12880:2001-02 [30], total volatile solids (VS) according to DIN EN 15935:2012-11 [31], and total COD in analogy to DIN 38414-9:1986-09 [32]. pH was measured using a pH meter Type 197 WTW GmbH, Germany. After filtering the samples with 0.45 µm syringe filters (polyethersulfone, VWR international), soluble COD (COD_s), ammonium (NH₄-N), ortho-phosphate (PO₄-P), and volatile fatty acids (VFA) were measured using cuvette tests LCK 514, LCK 303, LCK 350, and LCK 365 (Hach GmbH, Germany). Feed analyses were conducted right after sampling (TS, VS, soluble parameters) and after storing (TS, VS, COD, COD_s, NH₄-N, PO₄-P). Analyses of the effluent were conducted at least once a week (COD, TS, VS, COD_s, NH₄-N, PO₄-P, and VFA).

Biogas flow rates were measured continuously using drum-type gas meters (Type TG 0.5, Dr. Ing. Ritter Apparatebau GmbH & Co. KG, Bochum, Germany). Methane and carbon dioxide concentrations of biogas were measured constantly in the moist biogas every two weeks for one week using gas analyzers (BCP-series, BlueSens gas sensor GmbH, Herten, Germany). Ambient pressure was determined daily by using a handheld device. Methane concentration was calculated for dry gas according to VDI 4630 [33]. Biogas production was normalized to standard conditions according to VDI 4630 [33].

2.3. Calculation Methods

2.3.1. Full-Scale SRT_ASP,T and Sludge Production

The operating data of the full-scale WWTP regarding SRT_ASP and the sludge production of PS and WAS were evaluated over a one-year period.

WWTPs rarely work in steady state conditions due to varying influent and operating conditions. Therefore, a moving average of SRT_ASP was employed to account for variations in the SRT_ASP gradually [34]. Based on the yearly average SRT_ASP of 25 ± 6 d, a moving average of 25 d was used for determining SRT_ASP. To consider seasonal fluctuations of the mixed liquor temperature in the ASP (T_ASP) and its effect on microbial activity, SRT_ASP was referred to a reference temperature (T_ref) as stated by Clara et al. [35]:

$${\mathrm{SRT}}_{\mathrm{ASP},\mathrm{T}}={\mathrm{SRT}}_{\mathrm{ASP}}·{1.072}^{(\mathrm{T}-{\mathrm{T}}_{\mathrm{ref}})}$$

(1)

For determining SRT_ASP,T, a moving average of T_ASP was used for the same interval as for SRT_ASP. As T_ref, the design temperature of 12 °C was used as defined for WWTP in Germany, according to [36].

Consistently, the production of WAS (before thickening) and PS were determined using a moving average of 25 d. The volume of all sludges was recorded daily. For WAS, total suspended solids (TSS) were recorded daily by an online device and were regularly validated with manual measurements in the laboratory. Volatile suspended solids (VSS) were usually measured once a week. The yearly average of VSS was used to determine the organic sludge production. This mean value resulted in a deviation of only <10% when determining the organic sludge production for the minimum and maximum values of VSS, which is considered sufficiently accurate in the context of this study. PS production was determined using a yearly average for TS and VS since TS (n = 38) and VS (n = 37) were measured randomly for PS. This procedure is discussed in more detail in Section 3.1. All measurements were manually conducted in an on-site laboratory using standard methods.

Table 3. Sludge characteristics of the investigated WTTP (full-scale data).

Parameters	WAS			PS
	$\bar{x}$ ± SD	RSD	n	$\bar{x}$ ± SD	RSD	n
TSS [g/L]	7.4 ± 0.7	9%	365	-	-	-
VSS [%]	71.8 ± 2.3	3%	43	-	-	-
TS [%]	-	-	-	4.2 ± 1.1	27%	38
VS [%]	-	-	-	83.8 ± 3.1	4%	37
COD/VS [g/g] a	1.50 ± 0.03	2%	16	1.58 ± 0.09	5%	12

^a COD/VS was measured in the samples used for the anaerobic digestion experiments. For WAS, the COD/VS ratio was measured in the thickened WAS.

shows the characteristics of the full-scale sludges.

2.3.2. SMY and COD Balance of the Laboratory Anaerobic Digestion Experiments

SMY for each evaluation period was calculated as follows:

$$\mathrm{SMY}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{gas}} · \frac{\left({\mathrm{CH}}_4\right)}{100}}{{\mathrm{B}}_{\mathrm{COD},\mathrm{in}}}}$$

(2)

where SMY is the specific methane yield (NL CH₄/kg COD_in), Q_gas is the biogas produced (NL/evaluation period), CH₄ is the weighted daily average concentration of methane (%), and B_COD,in is the total influent COD (kg/evaluation period). Since the CH₄ was not measured daily, the average of the measurement days was used in Equation (2).

A COD balance gap for each evaluation period was calculated according to Equation (3):

$${∆}_{\mathrm{COD}}={\displaystyle \frac{{\mathrm{B}}_{\mathrm{COD},\mathrm{gas}}+{\mathrm{B}}_{\mathrm{COD},\mathrm{out}}+\left({\mathrm{c}}_{\mathrm{COD},\mathrm{DS},\mathrm{n}}-{\mathrm{c}}_{\mathrm{COD},\mathrm{DS},0}\right)· {\mathrm{V}}_{\mathrm{D}}}{{\mathrm{B}}_{\mathrm{COD},\mathrm{in}}}}·100-100$$

(3)

where Δ_COD is the COD balance gap (%), B_COD,gas is the COD of the produced methane (g COD/evaluation period) calculated using SMY_max = 350 NL CH₄/kg COD, B_COD,out is the effluent total COD (g COD/evaluation period), c_COD,DS,n is the concentration of total COD (g COD/L) in the digested sludge on the last day (n) of the evaluation period, c_COD,DS,0 is the concentration of total COD (g COD/L) in the digested sludge on the first day (0) of the evaluation period, and V_D is the volume of the digester (L).

2.3.3. Specific Release of COD_s, NH₄-N, and PO₄-P of the Laboratory Anaerobic Digestion Experiments

The specific release of COD_s, NH₄-N, and PO₄-P was used to compare the liquor qualities of the digesters. For this purpose, the load of the respective substance was calculated based on the average concentration and the total effluent over all evaluation periods (study period) according to Equation (4):

$${\mathrm{SR}}_{\mathrm{x}}={\displaystyle \frac{{\mathrm{B}}_{\mathrm{x},\mathrm{out}}}{{\mathrm{B}}_{\mathrm{COD},\mathrm{gas}}}}$$

(4)

where SR_x is the specific release of x (COD_s, NH₄-N, PO₄-P) per COD degraded (mg x/g COD_deg) and B_x,out is the load of x in the effluent (mg/study period).

2.3.4. Calculation of SMY and SR for MS

Using the WAS/MS ratio (in%, based on COD) as well as the SMY and SR_x of WAS and PS, SMY_MS,calc and SR_x,MS,calc were calculated using Equations (5) and (6):

$${\mathrm{SMY}}_{\mathrm{MS},\mathrm{calc}}={\mathrm{SMY}}_{\mathrm{WAS}} · {\displaystyle \frac{\mathrm{WAS}/\mathrm{MS}\mathrm{ratio}}{100}}+{\mathrm{SMY}}_{\mathrm{PS}} · {\displaystyle \frac{\left(100-\mathrm{WAS}/\mathrm{MS}\mathrm{ratio}\right)}{100}}$$

(5)

$${\mathrm{SR}}_{\mathrm{x},\mathrm{MS},\mathrm{calc}}={\mathrm{SR}}_{\mathrm{x},\mathrm{WAS}} · {\displaystyle \frac{\mathrm{WAS}/\mathrm{MS}\mathrm{ratio}}{100}} · {\displaystyle \frac{{\mathrm{SMY}}_{\mathrm{WAS}}}{{\mathrm{SMY}}_{\max }}}+{\mathrm{SR}}_{\mathrm{x},\mathrm{PS}} · {\displaystyle \frac{\left(100-\mathrm{WAS}/\mathrm{MS}\mathrm{ratio}\right)}{100}} · {\displaystyle \frac{{\mathrm{SMY}}_{\mathrm{PS}}}{{\mathrm{SMY}}_{\max }}}$$

(6)

For MS_TH,calc, the values for WAS_TH were used instead of the values for WAS. As the evaluation for the respective sludges is based on calculations, the sludges are labeled with the index ‘calc’.

2.4. Data Analysis of the Laboratory Experiments

The aim of the laboratory experiments was to obtain meaningful results for each sludge sample, rather than results relating to daily variations. For this reason, evaluation periods were defined for the determination of SMY, during which the same substrate was fed to the reactors. The results of Phase II in terms of SMY (Section 3.2) allow the assessment of mean values and standard deviations over four evaluation periods. The balance gap of each evaluation period is discussed to assess the validity of the results.

To compensate for variations in COD and nutrient loads of the feed, SR_x was determined over all evaluation periods.

2.5. Evaluation of the Influence of the ASP on the Performance of Anaerobic Digestion with Upstream Thermal Hydrolysis and Energy Generation

2.5.1. Determining the Performance of Anaerobic Digestion

The anaerobic digestion of MS with upstream thermal hydrolysis of WAS was examined and compared to conventional anaerobic digestion without thermal hydrolysis. The performance of the anaerobic digestion was evaluated using SMY, sludge liquor quality (SR_x), and digested sludge COD and TS. The SMY and sludge liquor quality were determined according to Equations (5) and (6). As input data, the daily WAS/MS ratios of the full-scale WWTP, as well as the SMY and SR_x of the individual sludges (WAS, WAS_TH, PS) were used. The SMY of WAS and WAS_TH depends on the operating conditions of the ASP. The anaerobically degradable fraction of the WAS decreases with increasing SRT_ASP,T, and the SMY of WAS also declines as a result. At the same time, the non-degradable fraction, such as endogenous residues, increases. These components are made partially accessible for anaerobic degradation by thermal hydrolysis [17,18], which leads to a rise in SMY. For the WWTP investigated, Rühl et al. [29] showed that the influence of SRT_ASP,T on the SMY of WAS and the increase in SMY of WAS_TH relative to WAS can be described by linear correlations, as shown in Figure 2. The linear regression analysis showed significant correlations (p < 0.01); more information can be found in Rühl et al. [29]. In the current study, both correlations were used to determine the SMY of WAS and WAS_TH including the daily SRT_ASP,T of the full-scale WWTP, according to

Table 4. Basics for the calculation of SMY and SR of COD_s, NH₄-N, and PO₄-P for MS_cal and MS_TH,cal.

Parameter	Unit	PS	WAS	WASTH
SMY	NL/kg CODin	237.7 a	−0.7∙SRTASP,T + 139.5 b	(−0.7∙SRTASP,T + 139.5) b (1 + (0.6∙SRTASP,T + 14.1)/100) c)
Degree of degradation	[-]	SMY/SMYmax	SMY/SMYmax	SMY/SMYmax
SRCODs	[mg/g CODdeg]	16.5 a	40.8 d	129.6 d
SRNH4-N	[mg/g CODdeg]	12.4 a	68.1 d	68.3 d
SRPO4-P	[mg/g CODdeg]	0.6 a	13.2 d	11.4 d
COD/VS of the digestate	[-]	1.65	1.51	1.66

^a Results based on Section 3.2; ^b correlation for SMY of WAS depending on SRT_ASP,T (Figure 2); ^c correlation for the increase in SMY for WAS_TH relative to WAS depending on SRT_ASP,T (Figure 2); ^d taken from Rühl et al. [29].

. The application of the correlations was transferred from the SRT_ASP,T range of 26 d to 60 d to the range of 19 d to 60 d in the context of this study. SR_x for WAS and WAS_TH can also be found in Table 4. SMY and SR of PS are part of the results in Section 3.2.

The COD of the digested sludge was determined by subtracting the anaerobically degradable COD from the COD of the raw sludge. The anaerobically degradable COD was calculated using the respective degree of degradation. Considering the COD/VS ratios of the digested sludges, the VS, and the fact that the inorganic solids of the raw sludge were known, the TS of the digested sludge was determined. The digested TS was used to determine the mass reduction due to mechanical dewatering.

2.5.2. Energy Assessment for Full-Scale Implementation of Thermal Hydrolysis

For the energy assessment, the produced biogas was assumed to be completely used in on-site CHP units. First, the electricity generation for a WWTP with and without upstream thermal hydrolysis was calculated according to Equation (7):

$${\mathrm{EG}}_{\mathrm{y}}={\mathrm{SMY}}_{\mathrm{y}} · {\mathrm{B}}_{\mathrm{COD},\mathrm{MS}} ·\mathrm{EC}·{\displaystyle \frac{{\eta }_{CHP}}{100}}$$

(7)

where EG is the electricity generation (kWh/d), B_CODMS is the COD load of MS (kg/d) according to Figure A1, EC is the energy equivalent of methane (9.97 kWh/m³ CH4), η_CHP is the electrical efficiency of the CHP unit (%), and y stands for MS_calc or MS_TH,calc.

The additional methane production induced by thermal hydrolysis, alongside its conversion to electricity, enhances on-site power generation. This extra electricity generated is referred to as gross electricity generation EG_gross (kWh/d), as defined by Equation (8):

$${\mathrm{EG}}_{\mathrm{gross}}={\mathrm{EG}}_{\mathrm{MSTH},\mathrm{calc}}-{\mathrm{EG}}_{\mathrm{MScal}\mathrm{c}}$$

(8)

At the same time, the electrical energy demand of thermal hydrolysis and the electricity required to treat the additional NH₄-N load of the return liquor in the ASP must be considered. This extra power demand must be deducted from the gross electricity generation, resulting in the so-called net electricity generation EG_net (kWh/d), following Equation (9):

$${\mathrm{EG}}_{\mathrm{n}\mathrm{et}}={\mathrm{EG}}_{\mathrm{g}\mathrm{ross}}-{\displaystyle \frac{{\mathrm{B}}_{\mathrm{TSS},\mathrm{WAS}}}{{\mathrm{TS}}_{\mathrm{WAS}} ·10\mathrm{k}\mathrm{g}/\mathrm{m}³}} · {\mathrm{ED}}_{\mathrm{TH}}-\left(B_{N,MSTH,calc}-B_{N,MScalc}\right)· {ED}_N$$

(9)

where B_TSS,WAS is the TSS load of WAS (kg/d), TS_WAS is the TS content (%) of the thickened WAS, ED_TH is the electricity demand (kWh/m³ WAS) of the thermal hydrolysis process, B_N,MScalc and B_N,MSTH,calc is the load of NH₄-N in the return liquor (kg NH₄-N/d), and ED_N is the electricity demand for treating the additional NH₄-N load in the ASP (kWh/kg N_elim). The volume of the return liquor produced during the mechanical dewatering step was determined based on the TS content of the digested sludge and the dewatered sludge. All necessary assumptions can be found in

Table 5. Assumptions for the energy assessment.

Assumption		Reference/Comment
TS content of PS	4.2%	Average TS content for the investigated WWTP
TS content of WAS after mechanical thickening	4.8%	Average TS content for the investigated WWTP
Production of WAS (kg TSS/d) and MS (kg TS/d)	See Section 3.1	Sludge production of the investigated WWTP
Production of MS (kg COD/d)	See Figure A1	Sludge production of the investigated WWTP
TS content of the digested sludge after dewatering (without upstream thermal hydrolysis)	25%	Average TS content for the investigated WWTP
TS content of the digested sludge after dewatering (with upstream thermal hydrolysis)	25%	No improvement in dewaterability (worst case)
Electricity demand for the thermal hydrolysis process via heat exchanger (EDTH, related to WAS with 4.8% TS)	7 kWh/m3 WAS	Assumption based on the data from Hüer et al. [37]
Release of NH4-N for MScalc and MSTH,calc (mg/g CODin)	See Section 3.3.2
Electricity demand for treating the NH4-N load of the return liquor in the ASP (related to eliminated nitrogen)	1.95 kWh/kg Nelim	[38]
Electrical efficiency of the CHP unit	38%	[2,38]

. According to Table 1, TS content of WAS and WAS_TH is the same (thermal hydrolysis process via heat exchanger).

It is assumed that the thermal energy demand of the thermal hydrolysis process is covered by the waste heat of the CHP unit [2,4,5].

In addition to the sludge volume to be treated and the improvement in anaerobic biodegradability of the treated sludge, the electricity demand of the thermal hydrolysis process has a decisive impact on the net electricity generation. As part of a sensitivity analysis, the electricity demand for thermal hydrolysis and the increase in SMY were varied.

3. Results and Discussion

3.1. Evaluation of the Full-Scale WWTP

In a first step to evaluate the influence of the ASP on the performance of MS anaerobic digestion with upstream thermal hydrolysis, the ASP and WAS were characterized, and the sludge production of WAS and PS was determined. For this purpose, the full-scale WWTP data were analyzed for the investigation period of one year (Figure 3a). Here, the operation of the ASP was characterized by the two parameters SRT_ASP and SRT_ASP,T (Equation (1)). SRT_ASP,T was used to show the influence of the temperature on aerobic degradation. The temperature of the mixed liquor varied significantly, from 12 °C in winter to 22 °C in summer. The deviation between SRT_ASP and SRT_ASP,T is the temperature difference related to the reference temperature of 12 °C and, thus illustrates the seasonal temperature influence. In October and November, a comparable high SRT_ASP was reached due to a malfunction of the mechanical thickening unit at the full-scale WWTP. In addition, Figure 3a shows the corresponding WAS production and the share of WAS relating to the total sludge production of MS (WAS/MS ratio) of the WWTP within one year.

The relation of the WAS production and WAS/MS ratio to the SRT_ASP,T is shown in Figure 3b. The WAS production decreased from around 8600 kg TSS/d to 4400 kg TSS/d with increasing SRT_ASP,T from 20 d to 60 d. The PS production fluctuated mainly between 6000 and 8000 kg TS/d without dependency on the influent flow rate or the wastewater temperature. The PS production was determined based on a yearly average TS content. Due to the small number of measured values and the heterogeneous PS composition, the TS content had a high relative standard deviation of 27%, corresponding to the normal operating variation. Nevertheless, the uncertainty of the PS production determined might have influenced the WAS/MS ratio and its trend in relation to the SRT_ASP,T. Despite this uncertainty, the course of the WAS/MS ratio was mainly characterized by the decreasing trend of WAS production (Figure 3b). In general, Figure 3 shows the operational variation of the sludge production and WAS/MS ratio of the investigated WWTP. The results indicate that WAS is already subject to further aerobic degradation, especially in summer (high SRT_ASP,T). The WAS/MS ratio fluctuated between 38 and 63%, based on TS (34–59% based on VS), and is within the usual range of WWTPs [39,40]. SRT_ASP,T also influenced the anaerobic biodegradability of WAS, which is analyzed in more detail in Section 3.3. Furthermore, the results of the sludge production and SRT_ASP,T were used to evaluate the influence of the ASP on the performance of anaerobic digestion with upstream thermal hydrolysis.

3.2. Anaerobic Digestion of PS and Evaluation of Single vs. Mixed Sludges

The Phase II laboratory-scale experiments were used to evaluate the PS digestibility and to determine whether the MS digestibility could be calculated from the results of the individual sludges. The latter is the fundamental prerequisite for evaluating the anaerobic digestion of MS with and without upstream thermal hydrolysis of WAS in the next section.

The SMY of PS was more than twice as high as the SMY of WAS (Figure 4a). WAS consists of a complex matrix of microbial and exopolymeric substances, making the sludge more difficult to degrade than PS. The different composition also affected the specific release of COD_s, NH₄-N, and PO₄-P, which was significantly lower for PS than for WAS (Figure 4b). The rationale is that the degradation of proteins, for example, leads to the release of NH₄-N. PS consists of a higher proportion of carbohydrates and lipids and a lower proportion of proteins than WAS [41,42], and consequently, the release of NH₄-N is significantly lower in PS.

For MS_calc, SMY and the specific release of COD_s, NH₄-N, and PO₄-P were calculated according to Equations (5) and (6). As shown in Figure 4a, the average SMY of MS_calc over the four evaluation periods was comparable to the experimental average SMY of MS, and differed only by −4.2%. Considering each evaluation period, the deviation between the results for SMY ranged from −7.3% to 0.4%. Concerning the specific release of COD_s, NH₄-N, and PO₄-P for MS and MS_calc, shown in Figure 4b, the results were comparable and deviated by 5.2%, 0.1%, and 10.6%, respectively. The results of SMY for the sludges investigated were in a typical range compared to other studies [6,43]. The same applied to the specific release of NH₄-N for MS [44]. To assess the validity of the conducted experiments and, therefore, the validity of the results, the COD concentrations of MS based on the COD concentrations of the individual sludges and the respective mixing ratios were calculated. The deviation of the calculated from the measured COD concentration of MS ranged from −4.7% to −0.2%. In addition, the Δ_COD (COD balance gap) of the digesters ranged from −10.7% to 7.0%, as presented in Figure 4a. The deviation between the results for MS_calc and the experimental results for MS was within the uncertainty of the experiments performed. For further validation, the data of Haug et al. [6] were used. They investigated the individual digestion of WAS, WAS_TH, PS, and PS_TH, as well as the digestion of MS and MS_TH, on a laboratory scale. With their results, we calculated the SMY of the mixed sludges according to Equation (5), based on the SMY of the individual sludges. The calculated SMY deviates by approx. −5% and −4% from the measured SMY, which is comparable to the results of this study.

Consequently, the results of the single sludge digestion (Figure 4) could be used to determine the performance of the anaerobic digestion of MS in terms of SMY and sludge liquor quality with sufficient accuracy. This approach was also applied to determine the performance of the anaerobic digestion of MS_TH. The prerequisite for this procedure is that neither inhibiting nor favoring effects occur in the digesters investigated. Stable operation was achieved for each digester with low average concentrations of VFAs of 121.5 ± 43.6 mg HA_c/L for PS, 165.4 ± 67.7 mg HA_c/L for WAS, and 138.7 ± 64.4 mg HA_c/L for MS.

Regarding this prerequisite, the results of Phase I should also be discussed. The digestion of WAS and WAS_TH led to comparably high concentrations of NH₄-N. Over all evaluation periods, the average concentrations were 1230 ± 191 mg NH₄-N/L (maximum 1690 mg NH₄-N/L) and 1750 ± 214 mg NH₄-N/L (maximum 2210 mg NH₄-N/L) for WAS and WAS_TH, respectively. The pH range was 7.1–7.6 for WAS and 7.2–7.7 for WAS_TH. NH₃-N was also present, even though NH₄-N dominated at this pH range, according to the ammonium–ammonia equilibrium. Calculated according to Anthonisen et al. [45], the average concentrations of NH₃-N were 34.5 ± 9.1 mg NH₃-N/L (maximum 67.2 mg NH₃-N/L) for WAS and 65.6 ± 11.7 mg NH₃-N/L (maximum 107.0 mg NH₃-N/L) for WAS_TH. At these concentrations, only a low inhibition level is expected [46,47,48]. It has also to be considered that high free ammonia concentrations lead to a shift towards hydrogenotrophic methanogenesis, which is less sensitive to ammonia inhibition compared to acetoclastic methanogenesis [48,49]. The average concentration of VFA was 165.4 ± 67.7 mg HAc/L for WAS and 667.7 ± 200.0 mg HA_c/L for WAS_TH. There was no significant loss of methane production (≤3%), although VFA concentrations for WAS_TH were elevated.

3.3. Influence of the ASP on the Performance of Anaerobic Digestion with Upstream Thermal Hydrolysis and Energy Generation

The evaluation included the following three steps: (1) impact on SMY and digested sludge COD and TS; (2) impact on the sludge liquor quality; and (3) impact on the electrical energy generation.

3.3.1. Influence of SRT_ASP,T on SMY and Digested Sludge COD and TS

The further the WAS is aerobically stabilized in the ASP, the lower its SMY. Accordingly, the SMY of WAS decreased with increasing SRT_ASP,T. It was expected that a similar, albeit weaker, trend would result for the SMY of MS_calc. However, as Figure 5a shows, an influence of SRT_ASP,T on the SMY of MS_calc could not be identified. The reason for this is that, in addition to the SMY of WAS, the amount of WAS and, thus, the WAS/MS ratio, decreased with increasing SRT_ASP,T for the investigated WWTP. In combination with the high SMY of the PS, this resulted in a balancing effect. An influence of SRT_ASP,T existed, but could not be shown on the basis of the SMY of MS_calc. The influence of SRT_ASP,T was also not identifiable when analyzing the SMY of MS_TH,calc, which was expected when looking at the course of the SMY of WAS_TH,calc.

The proportion of WAS_TH,calc increased the SMY for MS_TH,calc relative to MS_calc in the range from 8.1% to 13.0% and, on average, by 9.8% ± 0.9% (Figure 5b). The increase in SMY for WAS_TH,calc relative to WAS_calc was more than two-to-five times higher than the increase for MS_TH,calc. SRT_ASP,T had a decisive influence on the efficacy of thermal hydrolysis in increasing the SMY of WAS_calc relative to WAS_calc: The higher the SRT_ASP,T, the higher the relative increase in SMY [29]. This relationship can be explained by increasing proportions of non-biodegradable or slowly degradable compounds at higher SRT_ASP,T, which are made more bioavailable by thermal hydrolysis [17,18]. The influence of SRT_ASP,T was also present when considering the increase in SMY of MS_calc vs. MS_TH,calc, but could not be shown in the results of Figure 5b, due to the balancing effect described above.

These results need to be compared with other studies focusing on the anaerobic digestion of MS. Görlich [26] investigated the thermal hydrolysis of WAS and MS at 160 °C and its influence on the anaerobic digestion of MS for three different WWTPs. The methane production (SRT_AD = 20 to 22 d) for MS increased by 9% to 18% at WAS/MS ratios (VS) between 44% to 55%. Due to the thermal hydrolysis of MS at 175 °C, Haug et al. [6] reported a relative increase in SMY (SRT_AD = 15 d) by 14% at a WAS/MS ratio (VS) of 50%. The relative increase in biogas production even ranged between 21 and 31% (SRT_AD = 15 to 20 d) at WAS/MS ratios (VS) of 50 to 54% [5,27]. Although our results for the relative increase in SMY are similar to those of previous studies, they are in the lower range of the reported values.

During anaerobic digestion, organic matter is degraded, which leads to a reduction in the treated sludge solids. By implementing upstream thermal hydrolysis, the degradation of organic matter is increased, and consequently, the COD load and TS load of the digestate are reduced. For MS_TH,calc compared to MS_calc, the reduction of the COD load and TS load in the digestate was improved by 9.5% to 12.1% and 9.0% to 11.3%, respectively (Figure A2). In contrast, the COD load reduction for WAS_TH,calc was enhanced in the range of 14.4% to 19.4%, compared to WAS_calc. The influence of SRT_ASP,T on the efficacy of the thermal hydrolysis could not be demonstrated when looking at the results for MS_TH,calc. For all sludges investigated, the higher the SRT_ASP,T, the lower the COD in the digestate.

After the anaerobic digestion process, the digested sludge is mechanically dewatered to decrease the sludge volume to be disposed of. Assuming an equal and constant TS for MS_calc and MS_TH,calc, the enhanced reduction of the TS load in the digestate for MS_TH,calc compared to MS_calc directly corresponds to savings in sludge mass to be disposed of. Several studies show that due to thermal hydrolysis (WAS only), higher TS values after the dewatering step can be attained by 1.8-up-to-10.3% points [5,26,28]. The influence of SRT_ASP,T on this effect cannot be assessed at present. Over the period of one year, reduction in sludge mass for the digestate could be enhanced by 24.9% for MS_TH,calc compared to MS_calc when TS content after dewatering was 30% for MS_TH,calc and 25% for MS_calc. Accordingly, savings in sludge disposal were additionally increased, due to the positive influence of thermal hydrolysis on the dewaterability of MS.

3.3.2. Impact on the Sludge Liquor Quality

The return liquor produced during the mechanical dewatering step needs further treatment, due to its contamination with NH₄-N, PO₄-P, and COD_s. It is treated in a sidestream process (e.g., deammonification) or in the mainstream ASP. In the following, the sludge liquor quality of the digested sludges is discussed.

As summarized in Table 4, the specific release of NH₄-N and PO₄-P are in a comparable range for WAS and WAS_TH. Consequently, thermal hydrolysis does not lead to a significant additional release of ammonium and phosphate. The specific release of NH₄-N and PO₄-P depends only on anaerobic degradation. Regarding COD_s, the specific release for WAS_TH is significantly higher than for WAS (Table 4). During the thermal hydrolysis process, the formation of dissolved refractory organic compounds takes place. These compounds are related to the Maillard reaction [50,51,52]. This reaction is very complex, and occurs increasingly at elevated temperatures when a carbonyl compound (reducing sugar) and an amino compound (amino acid, peptide, or protein) come into contact [53]. In the context of this work, it was assumed that these compounds originate mainly from the degradable portion of the organics. Accordingly, the specific release with respect to the degradable COD was used to determine COD_s, NH₄-N, and PO₄-P in MS_calc and MS_TH,calc. For comparison and discussion of changes in sludge liquor quality, the specific release results for the sludges considered are related to the influent COD. This relationship allows the results to be discussed independently of the sludge liquor concentrations, which are influenced by the degree of thickening of each sludge.

The proportion of PS resulted in a lower release of NH₄-N for MS_calc compared to WAS_calc (Figure 6a), due to the lower protein content of PS. At the same time, the release of NH₄-N for MS_calc decreased with increasing SRT_ASP,T. Firstly, this was due to decreasing WAS/MS ratio with increasing SRT_ASP,T. Secondly, this was due to the decreasing anaerobic degradability of WAS_calc with increasing SRT_ASP,T. The same findings can be transferred to the release of NH₄-N for MS_TH,calc and WAS_TH,calc. In both cases, thermal hydrolysis of WAS increased the release of NH₄-N for MS_TH,calc and WAS_TH,calc. Figure 6b shows that the increase in the release of NH₄-N was greater at higher SRT_ASP,T for MS_TH,calc relative to MS_calc and for WAS_TH,calc relative to WAS_calc. The influence of SRT_ASP,T on the relative increase was more pronounced for WAS_calc than for MS_calc. These observations also apply to the release of PO₄-P; see Figure A3. The release of COD_s for MS_calc and WAS_calc were in a comparable range (Figure 6c). Thermal hydrolysis of WAS strongly increased the release of COD_s for MS_TH,calc and WAS_TH,calc. In all cases, a slight decrease in the release of COD_s with increasing SRT_ASP,T was found. Due to the share of PS, the release of COD_s for MS_TH,calc was significantly lower compared to WAS_TH,calc. Figure 6d shows the corresponding increase in the release of COD_s for MS_TH,calc, relative to MS_calc, and for WAS_TH,calc, relative to WAS_calc.

Increased release of NH₄-N, PO₄-P, and COD_s for MS_TH,calc was detected at lower SRT_ASP,T or lower WAS/MS ratios. Overall, the release of NH₄-N, PO₄-P, and COD_s depended on SRT_ASP,T and increased by 20% to 29% for NH₄-N, 8% to 25% for PO₄-P, and 126% to 201% for COD_s. When treating the return liquor in the ASP or a sidestream treatment, the additional NH₄-N-load increases the oxygen demand and, thus, the electricity demand of the WWTP. This additional electricity demand is considered in the next section. In particular, the extra load of COD_s has to be considered, since a part of it consists of refractory organic compounds [51,54,55]. These refractory compounds are known to inhibit sidestream treatment of return liquor, like deammonifcation [56,57]. Negative impacts on downstream processes of the ASP, such as UV disinfection, have also been reported [51]. Recent studies show that, depending on the production of WAS, thermal hydrolysis at 160–165 °C can increase the COD concentration in the effluent of WWTPs by 3 to 13 mg/l [50,58]. Higher shares of WAS result in higher concentrations of COD_s in the effluent of WWTPs, which is also confirmed by the results of this study.

3.3.3. Energy Assessment of the Electricity Generation

Figure 7 shows the additional gross and net electricity generation calculated according to Equations (8) and (9). The net power generation tends to be higher at higher SRT_ASP,T, due to lower amounts of WAS to be treated and lower additional NH₄-N loads in the return liquor. With an electricity demand of 7 kWh/m³ WAS for the thermal hydrolysis process, the net electricity generation was even negative at low SRT_ASP,T. In this case, additional energy would be required to operate the thermal hydrolysis process. The development of net electricity generation with lower electricity demand for thermal hydrolysis is interesting from an energy point of view. For this reason, a sensitivity analysis was performed with reduced power requirements of 3 kWh/m³ WAS and 5 kWh/m³ WAS for the thermal hydrolysis process. Even with a theoretical electricity demand of 3 kWh/m³ WAS, the net power generation for the considered full-scale plant was only between 475 kWh/d and 714 kWh/d. The influence of the SRT_ASP,T decreased as the electricity demand of the thermal hydrolysis process decreased (Figure 7). In comparison, the total electricity generation of MS (without thermal hydrolysis) ranged from 8000 kWh/d to 13,600 kWh/d. The relative increase in electricity generation over the period of one year was 0.3% to 5.1% (net) for the highest and lowest electricity demand for the thermal hydrolysis process. Figure 7 indicates that high SRT_ASP,T resulted in more favorable net electricity generation. However, at the same time, higher SRT_ASP,T resulted in increased aerobic stabilization of WAS in the ASP. Aerobic stabilization causes a higher energy demand for the ASP and negatively affects methane production during anaerobic digestion. For the full-scale plant, WAS production decreased from approximately 9200 kg COD/d to approximately 4800 kg COD/d when SRT_ASP,T increased from 19 d to 60 d (see Figure A1). As a result, the total methane production decreased, although this was not apparent from the results presented for SMY of MS_calc and MS_TH,calc. Methane production from WAS_calc and WAS_TH,calc as a percentage of total methane production ranged from 18 to 41% and 24 to 47%, respectively. Higher proportions were achieved at lower SRT_ASP,T.

As discussed above, the results of increase in SMY from 8.1% to 13.0% for MS_TH,calc relative to MS_calc in this study are in the lower range compared to published values. As part of a further sensitivity analysis, the increase in SMY of WAS_TH,calc relative to WAS_calc was raised by 25 percentage points, which resulted in an increase in SMY for MS_TH,calc relative to MS_calc, ranging from 12.5% to 23.1%. The results of this sensitivity analysis are shown in Figure A4. The gross electricity generation tends to decrease with increasing SRT_ASP,T, and ranged from 1325 kWh/d to 2260 kWh/d. The net electricity generation is positive in all cases. However, the net power generation is only between 558 kWh/d and 851 kWh/d with a power requirement of 7 kWh/m³ WAS for thermal hydrolysis. Considering a reduced electricity demand of the thermal hydrolysis process of 3 kWh/m³ WAS, the net electricity generation can be increased in the range of 924 kWh/d to 1530 kWh/d (Figure A4). Over a one-year period, the electricity generation can be increased by 6.5 to 11.3% (net) for the highest and lowest power requirements of the thermal hydrolysis process. These considerations also show that at low rates of increase in SMY, high SRT_ASP,T results in more favorable net electricity generation, while at high rates of increase, this trend slowly reverses.

In addition to electrical energy, thermal hydrolysis requires thermal energy. If only WAS is treated, the waste heat (especially high-grade heat) from the CHP unit can cover the thermal energy demand. This coverage is possible for a thermal hydrolysis process in which the heat is provided by steam [2,4] or by a heat exchanger system [5]. If not, biogas must be used directly in a boiler to generate the required heat. Consequently, less biogas can be converted into electricity, and thus, the possible net electricity generation is further reduced.

The results indicate that the contribution of thermal hydrolysis for boosting the energy self-sufficiency of a WWTP was comparably low, even though the results for SMY were promising for WAS_TH, especially at high SRT_ASP,T. Thermal hydrolysis had a greater effect on reducing the sludge mass, especially when the dewaterability of the digested sludge was improved. It can be concluded that an advantageous integration of the thermal hydrolysis process into an existing sludge treatment line in terms of reducing the operation costs of WWTPs is mainly driven by savings in disposal costs. These findings are consistent with those reported by other authors [26,59,60,61].

4. Conclusions

In this study, the SRT_ASP,T was used for the first time to assess the role of the ASP on the performance of anaerobic digestion of MS with upstream thermal hydrolysis (WAS only). SRT_ASP,T played a decisive role in both anaerobic degradability and the efficacy of thermal hydrolysis. This role was particularly evident when analyzing the anaerobic digestion of WAS and WAS_TH.

As SRT_ASP,T increased, SMY of WAS_calc declined, while SMY of WAS_TH,calc relative to WAS_calc improved more significantly. Although SRT_ASP,T also influenced the digestion of MS_calc and MS_TH,calc, its influence was less pronounced. This reduced influence was attributed to the higher anaerobic degradability of PS and the decreasing WAS/MS ratio with increasing SRT_ASP,T. The latter had a balancing effect on the results of the anaerobic digestion with and without upstream thermal hydrolysis. In order to accurately evaluate the influence of SRT_ASP,T on anaerobic digestion with upstream thermal hydrolysis, the study of WAS and WAS_TH is recommended. This recommendation also applies to studying the influence of SRT_ASP,T on anaerobic digestion without upstream thermal hydrolysis. Moreover, future research should consider the variability of SRT_ASP,T and the WAS/MS ratio when evaluating the impact of thermal hydrolysis on the anaerobic digestion of WAS and MS.

If the impact of thermal hydrolysis on the degradability of WAS is known or reasonably estimated, an assessment of the potential of thermal hydrolysis based on the sludge production, SRT_ASP,T, and PS degradability becomes possible. This allows us to estimate the methane production, the reduction of the digestate COD, the sludge liquor quality, and the additional net electricity generation. For the WWTP investigated, additional electricity could be generated. The contribution to increasing the energy efficiency was comparably low. However, a worthwhile implementation depends on the specific boundary conditions and the objective to be achieved (e.g., minimization of disposal costs, producing biosolids for land application), so that thermal hydrolysis can also be used, preferably for WWTPs operating at lower SRT_ASP,T.