Optimization of the Solubilization of Faecal Sludge through Microwave Treatment

Abstract

This study optimized the organic matter solubilization of faecal sludge (FS). FS was treated in a microwave oven at varying microwave power and treatment times. Changes in total solids (TS), volatile solids (VS), total chemical oxygen demand (TCOD), and soluble chemical oxygen demand (sCOD) were measured. A response surface methodology (RSM) optimized organic matter solubilization during microwave treatment. A central composite design was employed, and the observed responses were used to fit a second-order response surface model. Microwave treatment at 14,000 kJ/kg. TS reduced FS volume by 58%. The VS/TS ratios remained similar before and after microwave treatment. The solubilization of organic matter (measured by the sCOD/TCOD ratio) increased after microwave treatment, showing an initial linear increase with specific energy followed by a decrease. The highest solubilization was 38%, achieved at a microwave power level of 630 W for 3 min. Organic matter solubilization was more sensitive to contact time than microwave power. RSM determined the optimized conditions to be 617.7 W and 2.4 min, within the experimental design boundaries. These findings align with similar observations from other studies using wastewater sludge. The results suggest that microwave treatment can achieve multiple FS treatment objectives. Optimal operating conditions should be identified if the aim is to solubilize organic matter in FS.

1. Introduction

The resources and efforts committed to meeting the Millennium Development Goals (MDGs) on sanitation did not achieve the desired results in sub-Saharan Africa, where 79% of the population lacks access to safely managed sanitation services [1]. The Sustainable Development Goals (SDGs) aim to reverse this trend by 2030. Goal 6.2 specifically focuses on achieving universal access to safe sanitation services, an end to open defecation, and sustainable management and treatment of human faeces.

Increasing urbanization and the rise of informal settlements in urban and peri-urban areas of low-income countries make it a challenge to expand the sewerage network. Thus, onsite sanitation systems are the primary form of safe sanitation, where the faecal sludge (FS) can be contained for up to five years [2] and needs to be safely emptied, collected, and transported to a safe treatment facility. Existing sludge treatment technologies, such as anaerobic digestion, dewatering, and solar and thermal drying, are often energy-intensive, land-consuming, and slow, making them unsuitable for densely populated areas [3,4,5,6].

Alternative FS treatment technologies that quickly offer multiple benefits, such as volume reduction and sanitization, are needed to manage large volumes of FS in informal settlements. FS volume reduction and sanitization are critical to reducing FS transportation costs, environmental contamination, and disease transmission risk. Such technologies should be energy efficient, provide quick processing times to deal with large volumes of FS, and be compact for easy deployment in hard-to-reach areas. Treatment technologies such as ultrasonication [7], thermal hydrolysis [8,9], acid and alkaline treatment [10], microwave treatment [11,12], or their combination [13,14] achieve these requirements.

Microwave (MW) heating has emerged as a promising technology that achieves rapid FS volume reduction and sanitization and improves physico-chemical characteristics that enhance downstream treatment processes like anaerobic digestion. FS has a high water content (~80%) that would efficiently couple with microwaves, leading to uniform and rapid volumetric heating. The temperature and pressure build-up during MW heating disintegrate extracellular polymeric substances (EPS) and cell walls, liberating bound organic matter [15]. MW power, specific energy, temperature, and treatment time influence the efficiency of MW heating.

Organic matter solubilization during MW treatment depends on the temperature reached and the duration of exposure to microwaves. For instance, soluble proteins and reducing sugars increased until 80 °C, beyond which they decrease due to caramelization and Milliard reactions [16]. At higher temperatures (175 °C), the temperature increase rate can impact sludge solubilization [17]. MW power and sludge concentration are critical parameters that affect sludge solubilization [18], as is the final temperature reached during MW treatment [12,19]. Varying effects of specific energy on sludge solubilization are reported. For example, waste activated sludge (WAS) solubilization increases with specific energy up to a certain limit, beyond which no further organic matter solubilization occurs [11,20].

Despite the several benefits of MW treatment, its application to treat FS is not common. Its feasibility has been evaluated on fresh fecesfaeces collected from individual donors and intensively used toilets (within 48 h). The results indicated increased hydrochar yield, improved energy and process efficiency compared to untreated samples [21], and increased volume reduction and sanitization [22]. Multiple factors influence the efficiency of MW treatment, and optimal conditions depend on specific parameters such as temperature reached, MW power, and treatment time. This research extends the application of MW treatment to aged FS (~5 years) by optimizing organic matter solubilization through varying microwave power and treatment times. By addressing the sanitation challenges in informal settlements, this study offers a compact, efficient, and rapid FS treatment solution that enhances the prospects of achieving the SDGs in sub-Saharan Africa.

2. Materials and Methods

Samples of FS were collected from ventilated improved pit (VIP) latrines at the drop hole (S1) and during delivery (S2) at a black soldier fly larvae treatment facility in Durban, South Africa. Sample S1 represented relatively fresh faeces (less than 24 h old), while S2 represented the aged FS (~5 years). The delivered FS was manually emptied from VIP latrines in informal settlements of the eThekwini Municipality and transported in buckets. Grab samples were taken from buckets and mixed into a composite sample (S2). The sample was screened to remove detritus material and homogenized onsite in an industrial grade food mincer. Both samples (S1 and S2) were taken to the Water, Sanitation, and Hygiene Research & Development (WASH R&D) Centre Specialized Sanitation Laboratory at the Howard College Campus of the University of KwaZulu Natal and stored at 4 °C till required.

2.1. Microwave Instrument

This study used a Hisense domestic MW oven. The instrument operates at a frequency of 2.45 GHz with MW power ranging from 0 to 900 W with 10% increment intervals. The MW oven was operated under a fume hood at the WASH R&D Centre laboratory.

Before each sample treatment, the MW oven was initiated by heating 200 mL of water at 100% power (900 W) for 10 min. This operation was a preparatory step to optimize experimental conditions, ensure safety, and provide a more controlled and uniform environment for subsequent sample treatment.

A total of 50 g of a well-homogenized sample was transferred into a 100 mL beaker covered with an MW safe-wrap paper and treated at different MW power levels ranging from 450 W to 810 W for 60, 120, 180, and 300 s. Each treatment was done in triplicate. All samples were weighed before and after MW treatment. The operating parameters (MW power and treatment time) were normalized to a specific energy (kJ/kg TS):

$$\mathrm{SE}={\displaystyle \frac{\mathrm{P} \times \mathrm{t}}{{\mathrm{M}}_{\mathrm{sample} \left(\mathrm{kg}\right)} \times \mathrm{TS} (\mathrm{kg}/\mathrm{kg})}}$$

(1)

In Equation (1), SE is the specific energy (kJ/kg TS), P is the MW power (kW), t is the treatment time (seconds), M is the mass (kg) of the FS sample treated, and TS is the total solid concentration in the FS (kg/kg).

The mass of the sample was recorded using a benchtop analytical balance. The samples were cooled to room temperature and weighed before further physicochemical analyses. Weight loss was determined from the difference between the initial and final weight of the sample.

2.2. Optimization of FS Solubilization

Response surface methodology (RSM) is a collection of mathematical and statistical techniques concerned with designing appropriate experiments that provide information about a response factor, selecting a suitable model to fit experimental data, and process optimization. First and second order models are commonly used in RSM. The first order model is used for exploratory experimentation, i.e., determining essential factors to be considered in the experiment. A second order model is used to determine the significance of model parameters, estimate mean response, and arrive at optimum operating conditions [23].

In this study, an RSM was used to evaluate the relationship between the solubilization of FS (sample S2 only) and the MW operating parameters using a face-centred central composite design (FCCCD). The factorial points for MW power were set at 540 and 720 W and 60 and 300 s for treatment time. The axial points were set at the same operating conditions as the factorial points, while the centre points were at 630 W and 180 s. The operating parameters were selected to be within the range of values used to study the solubilization of WAS [12,19].

The experimental design and statistical analyses were done in R software, version 4.0.2 (https://cran.r-project.org/, accessed on 30 July 2020), using an RSM package. The total number of experimental treatments (N) required for the two control variables (the operating parameters) was calculated using Equation (2):

$$\mathrm{N}=2^{\mathrm{k}}+2\mathrm{k}+\mathrm{C}$$

(2)

In Equation (2), k is the number of control variables (2 in this study, i.e., MW power and time), and C is the centre run. An FCCCD was generated by specifying alpha (the star points) to “faces” in the ccd function of the RSM package. Hence, the coded values have α of ±1. In an FCCCD, one centre run, replicated several times, is adequate to study the second-order effect. In this study, all treatments were run in triplicate, giving 27 experimental points. Hence, using the ccd.pick function in the RSM package, the total number of experimental runs was restricted to 27. The generated experimental design was blocked and randomized with one centre point in the factorial block and two centre points in the star block.

2.3. Analytical Methods

To evaluate the effect of microwave treatment on FS characteristics, both untreated and microwave-treated samples were analysed for various parameters. The analytical methods were adapted from standard methods for the examination of water and wastewater [24] and domesticated as WASH R&D Centre laboratory SOPs [25].

2.4. Total Solids (TS) and Volatile Solids (VS)

The solid content was determined gravimetrically by measuring mass loss after 24 h oven heating at 105 °C (TS) and ignition at 550 °C for 2 h (VS). All the samples were analysed in duplicate. The VS/TS ratio was calculated to determine the degree of FS stabilization before and after microwave treatment.

2.5. Chemical Oxygen Demand (COD)

A 2 g/L solution of FS was prepared by weighing 2 g of a well-mixed FS sample. The weighed sample was blended with 500 mL of distilled water in a food grade blender for 30 s. The composite sample was transferred to a 1000 mL volumetric flask. Distilled water was added to the volumetric flask to the 1000 mL mark.

The COD of FS was determined by the dichromate method using a commercial COD test kit. The TCOD was determined on the stock solution, while sCOD was determined on the centrifuged and filtered (using a 0.45 µm filter) sample. All samples were analysed in duplicate.

The solubilization of organic matter was determined as the ratio of sCOD to TCOD (sCOD/TCOD ratio) (Equation (3)):

$$\mathrm{Solubilization} (\%)={\displaystyle \frac{{\mathrm{sCOD}}_{\mathrm{t}}}{\mathrm{TCOD}}} \times 100\%$$

(3)

2.6. Statistical Analysis

Descriptive statistics and linear regression were performed using MS Excel 2019 software, while RSM was performed in RStudio software (Version 4.0.2). A second-order polynomial was used to evaluate the influence of the control variables on organic matter solubilization (Y_i) (Equation (4)).

$${\mathrm{Y}}_{\mathrm{i}}={\mathsf{\beta }}_0+\sum {\mathsf{\beta }}_{\mathrm{i}}{\mathrm{X}}_{\mathrm{i}}+\sum {\mathsf{\beta }}_{\mathrm{ii}}{\mathrm{X}}_{\mathrm{i}}^2+\sum {\mathsf{\beta }}_{\mathrm{ij}}{\mathrm{X}}_{\mathrm{i}}{\mathrm{X}}_{\mathrm{j}}$$

(4)

In the equation, Y_i is the response variable (sCOD/TCOD), β₀ is a constant, β_i is the linear effect, β_ii is the quadratic effect, β_ij is the interactive effect, and X_i is the control variable. The coefficient of determination (R²) and statistical significance determined the quality of the model by analysis of variance (ANOVA) at 95%.

3. Results and Discussion

The characteristics of faecal sludge (FS) used in this research are presented in

Table 1. Characteristics of the untreated FS.

Parameter	Unit	S1	S2
TS	%	21.00 ± 00	31.00 ± 0.00
VS	%	13.00 ± 0.00	13.00 ± 0.00
VS/TS	%	61.90 ± 0.00	41.94 ± 0.00
TCOD	g/kg TS	1840.14 ± 327.34	711.02 ± 19.14
sCOD	g/kg TS	405.06 ± 0.00	214.18 ± 5.63
sCOD/TCOD	%	22.37 ± 3.98	30.12 ± 0.02

. Sample S1 was a composite of FS collected from the drop hole of 5 VIP latrines, while sample S2 was a composite of samples collected from different buckets as delivered to a BSFL facility. The values are averages ± standard deviation of two replicates for each parameter.

The total solids (TS), volatile solids (VS), total chemical oxygen demand (TCOD), and soluble chemical oxygen demand (sCOD) were within the range of values for FS collected from VIP latrines in the same study location [26,27]. The TCOD of sample S1 was higher than the values reported by Zuma et al. (2015), possibly due to its freshness, as it was collected within 24 h of defecation. Hence, it still contained high amounts of readily degradable organic matter [26]. The high sCOD/TCOD ratio (30.12 ± 0.02%) of sample S2 indicated that it degraded during storage in the pit latrine. Comparing the changes in the physicochemical characteristics before and after MW treatment provided the means for evaluating the influence of the treatment process. The results are discussed in the following sections.

3.1. Effects of Microwave Treatment on FS Solids

MW treatment increased the TS and VS concentrations in both S1 and S2 samples. TS and VS increased linearly with the treatment time for each MW power applied (

Table 2. Effect of microwave treatment of solids.

Microwave Treatment		S1 Sample			S2 Sample
Power (W)	Time (s)	TS	VS	TS/VS	TS	VS	TS/VS
0	0	21 ± 0.00	13 ± 0.00	62 ± 0.00	31 ± 0.00	13 ± 0.00	42 ± 0.00
450	60	23 ± 0.711	13 ± 0.00	58 ± 1.82	32 ± 0.00	13 ± 0.00	41 ± 0.00
450	120	24 ± 0.71	15 ± 0.35	63 ± 3.39	40 ± 0.71	17 ± 0.71	42 ± 2.54
450	180	28 ± 0.71	16 ± 0.00	58 ± 1.50	51 ± 2.83	22 ± 1.41	43 ± 0.38
630	60	24 ± 0.00	14 ± 0.00	58 ± 0.00	33 ± 0.71	14 ± 0.00	43 ± 0.94
630	120	27 ± 0.71	16 ± 0.00	60 ± 1.61	43 ± 0.71	18 ± 0.00	42 ± 0.70
630	180	32 ± 0.00	19 ± 0.00	59 ± 0.00	63 ± 0.71	271.41	43 ± 0.1.77
810	60	24 ± 0.00	14 ± 0.00	58 ± 0.00	33 ± 1.41	14 ± 0.00	42 ± 1.82
810	120	32 ± 0.00	19 ± 0.00	59 ± 0.00	47 ± 0.71	20 ± 0.71	42 ± 0.88
810	180	46 ± 1.41	26.5 ± 0.71	58 ± 3.31	74 ± 2.12	32 ± 0.71	43 ± 2.20

).

For the S1 sample, the highest TS and VS were 46.00 ± 1.41% and 26.50 ± 0.71%, respectively, recorded when the sample was treated at 810 W for 180 s. The highest TS and VS were 73.50 ± 2.12% and 31.50 ± 0.71% for the S2 sample, respectively, after MW treatment at 720 W for 300 s. An increase in solid content is beneficial for downstream biological treatment processes and handling of FS [22].

To further understand the effect of MW treatment on FS solids, a VS/TS ratio was calculated. The VS/TS ratio remained similar before and after treatment (Table 2). After MW treatment, the VS/TS ratios ranged between 58 and 63% and 41 and 43% for S1 and S2 samples, respectively, within the range of stable sludge. MW treatment did not lead to the loss of VS or the accumulation of unwanted materials in the FS. Indeed, the highest temperatures recorded (97 °C for S1 and 92 °C for S2 samples) were lower than the VS ignition temperatures (~550 °C) [24].

MW treatment is known to cause friction of polar molecules in a sample, leading to heat generation and subsequent temperature rise. Temperature rise causes cell lysis and evaporation of free water from the FS. Friction also causes the disintegration of complex FS structures. It is cell lysis and disintegration that liberate intra and extracellular biopolymers, which could have caused an observed increase in VS content, while the TS increased due to the evaporation of free water [12].

The VS/TS is an essential parameter in biological treatment because it indicates the amount of organic matter potentially available to microorganisms during the biological degradation of organic matter. MW treatment achieved comparable VS/TS ratios in both samples, meaning that it did not lead to the loss of organic matter. Therefore, the treated samples could be potential substrates for downstream biological treatment. Monitoring and optimizing changes in solids during MW treatment is essential for evaluating the efficiency and effectiveness of the process. These parameters help achieve a stabilized, sanitized, and reduced-volume FS that is safe to handle, dispose of, or reuse.

3.2. Effects of Microwave Treatment on FS Volume

It was assumed that the density of the samples before and after MW treatment remained constant. Therefore, the observed weight loss was proportional to volume reduction. The observed volume reduction values were plotted against the normalized operating conditions (MW power, time, and TS) of specific energy (SE, kJ/kg TS) (Figure 1).

In both samples, volume reduction was linear, with a maximum volume reduction at the highest specific energy. Effectively, there may be further changes once all the water is removed. However, in this research, the water remained, so the boiling temperature was maintained.

The energy applied at the start of MW treatment raised the water temperature in the FS sample to the boiling point, hence low volume reduction. The biggest jump in volume reduction was observed when the samples reached the boiling temperature (i.e., 87 °C and 93 °C).

Treating the sample at high specific energy increased the rate of moisture loss. At high specific energies, the remaining sample mass was low. The highest volume reductions achieved were 50 and 58% for the S1 and S2 samples, respectively. Volume reduction could lower FS collection and transportation costs, the highest cost borne by the household in a faecal sludge management-based sanitation service [28].

3.3. Effect of Microwave Treatment on Organic Matter Solubilization

Only S2 FS samples were used for solubilization experiments and RSM because they represented the material most likely to be received at a municipal FS treatment facility. The sCOD/TCOD ratio indicates the degree of solubilization of the organic matter in the substrate. It is a commonly used metric to describe the extent of hydrolysis, a rate-limiting step in anaerobic digestion [29]. It was hypothesized that MW treatment would transfer COD from the solid phase to the liquid (soluble) phase. Therefore, the sCOD/TCOD ratio and sCOD in the treated FS will be higher than in the untreated FS.

Generally, MW treatment increased sCOD in the sample (

Table 3. Effect of microwave treatment on sCOD (gCOD/kg TS).

Time (min)	540 W	630 W	720 W
0	214.18 ± 5.63	214.18 ± 5.63	214.18 ± 5.63
1	227.42 ± 1.10	224.48 ± 1.10	238 ± 3.32
3	236.60 ± 4.37	274.01 ± 3.31	232.92 ± 4.36
5	211.12 ± 1.14	183.75 ± 5.44	129.95 ± 2.24

).

At 540 W, 630 W, and 720 W, sCOD increased by 11%, 39%, and 11%, respectively. MW treatment disrupted cell walls, disintegrated sludge flocs, and released the intracellular and extracellular biopolymers into the liquid phase, increasing sCOD [16,30,31].

MW treatment operating parameters were normalized to specific energy (SE, kJ/kg TS) and used to evaluate the effect of microwave treatment on organic matter solubilization. Two solubilization profiles were observed, i.e., an initial increase in sCOD up to ~7300 kJ/kg TS, beyond which solubilization decreased linearly (R² = 0.94) (Figure 2).

Data on organic matter solubilization in FS is lacking. However, the value reported in this research (30.12 ± 0.02%) was relatively high compared to wastewater sludge [32,33]. The high initial sCOD/TCOD value suggested that the sample underwent considerable solubilization in the pit latrine. Indeed, the age of VIP latrine sludge in the study area can be as high as five years [2]. Also, solubilization could have occurred during storage in the laboratory [34].

Regardless of the high initial sCOD/TCOD ratio, MW treatment influenced organic matter solubilization in FS, similar to wastewater sludge solubilization [35]. Applying MW energy could cause rapid heating, disrupting the cell walls and membranes of microorganisms and other organic matter. This releases intracellular organic compounds into the soluble phase, leading to an increase in sCOD. Also, intense microwave heating could solubilize complex organics, such as proteins, carbohydrates, and lipids, increasing the sCOD. Therefore, the initial increase in solubilization (phase 1) was attributed to the disintegration of sludge flocs, disruption of cell walls, and solubilization of complex organics that release organic biopolymers [12]. In contrast, as microwave treatment continues, some solubilized organic compounds are thermally degraded into simpler molecules. These simpler molecules may be volatile and can be lost as gases (e.g., CO₂, NH₃), decreasing sCOD. In addition, thermal processes can lead to the formation of more stable, less soluble, and refractory organic compounds that do not contribute to sCOD. These compounds may precipitate or form insoluble aggregates, reducing the measured sCOD and leading to the degradation phase (phase 2) [36]. Understanding the solubilization profile is crucial for optimizing the treatment process. It implies that the operation can stop at the first peak. Continued operation wastes energy and does not result in additional sCOD.

3.4. Response Surface Modeling Results

The results of response surface modeling (using S2 sample only) are presented in

Table 4. Design matrix of a 2² full factorial experimental design (FCCCD) with a response factor.

		Microwave Power (MP)	Contact Time	Coded Values			Response (Y)
run.order	std.order	Watts	min	x1	x2	Block	sCOD/TCOD (%)
1	2	540	1	−1	−1	1	32.0
2	13	630	3	0	0	1	36.2
3	7	540	5	−1	1	1	29.5
4	4	720	1	1	−1	1	34.5
5	10	720	5	1	1	1	18.1
6	6	720	1	1	−1	1	33.8
7	12	720	5	1	1	1	17.9
8	1	540	1	−1	−1	1	32.7
9	8	540	5	−1	1	1	29.1
10	5	720	1	1	−1	1	34.2
11	9	540	5	−1	1	1	29.3
12	3	540	1	−1	−1	1	32.9
13	11	720	5	1	1	1	18.6
1	12	630	5	0	1	2	26.2
2	6	720	3	1	0	2	32.4
3	4	630	5	0	1	2	27.3
4	1	540	3	−1	0	2	32.9
5	8	630	5	0	1	2	26.4
6	2	720	3	1	0	2	34.2
7	9	540	3	−1	0	2	34.7
8	14	630	3	0	0	2	39.6
9	11	630	1	0	−1	2	33.6
10	5	540	3	−1	0	2	33.8
11	13	630	3	0	0	2	38.9
12	7	630	1	0	−1	2	32.4
13	3	630	1	0	−1	2	32.7
14	10	720	3	1	0	2	33.3

. Operating factors were coded as x1 (MW power) and x2 (treatment time).

The highest solubilization was obtained when the FS was treated at 630 W for 3 min. MW treatment at the highest power (720 W) and longest contact time (5 min) yielded the lowest solubilization. The influence of each factor was not analysed independently. However, a statistical analysis was done to understand which factors and interactions influenced the observed solubilization (

Table 5. Model parameters and their estimated values.

	Estimate	Std Error	t Value	pr (>|t|)
Intercept	−1.143 × 102	7.1953 × 101	−1.5886	0.18735
x1	4.185 × 10−1	2.2931 × 10−1	1.8250	0.14204
x2	1.8786 × 101	4.5223	4.1540	0.01421 *
x1:x2	−1.7886 × 10−2	6.2280 × 10−3	−2.8719	0.04538 *
x21	−3.0382 × 10−4	1.8121 × 10−4	−1.6766	0.16892
x22	−1.5951	3.6694 × 10−1	−4.3469	0.01219 *

Notes: The * helps to quickly identify model parameters influencing the observed values.

and

Table 6. Analysis of variance (ANOVA) table.

	DF	Sum sq.	Mean sq.	F Value	pr (>F)
FO (x1, x2)	2	116.854	58.427	11.6230	0.022155
TWI (x1, x2)	1	41.460	41.460	8.2478	0.04538
PQ (x1, x2)	2	124.796	62.398	12.4130	0.01926
Residuals	4	20.107	5.027
Lack of fit	3	15.649	5.216	1.1699	0.57660 *
Pure error	1	4.459	4.459

Note(s): FO = First order terms; TWI = Two-way interaction terms; PQ = Pure quadratic terms. The * helps to quickly identify model parameters influencing the observed values

).

It was observed that one linear term (x2), the interaction terms (x1:x2), and one quadratic term (x²2) had the most influence on the observed values (p < 0.05) (Table 5). The model multiple R² and adjusted R² were 0.93 and 0.85, respectively. The intercept and significant terms (Table 5) yielded the model in Equation (5):

$$\mathrm{Y}=-1.143\times {10}^2+{1.8786\times 10}^1\mathrm{x}2-1.7886\mathrm{x}1\mathrm{x}2-{1.5951\mathrm{x}}^{2}2$$

(5)

Results of the analysis of variance indicated that the model’s lack of fit was not significant (p > 0.05) (Table 6). Therefore, this model adequately fits the current data and could be used for predictions, analysis, and decision making.

3.5. Characterization of the Response Surface and Contour Plot

The analysis of the response surface and contour plot (Figure 3) showed that FS solubilization increased at low MW power while slowly increasing the treatment time.

A concave response surface graph (Figure 3a) indicated that changing MW power and treatment time resulted in an initial increase in organic matter solubilization (phase 1) in FS, which levelled off and eventually decreased (phase 2). An optimal combination of MW power and treatment time maximized organic matter solubilization as indicated by the response surface’s stationary points (617.3 W, 2.4 min). The stationary points were within the region of the experimental design (540–720 W, 1–5 min). These were the points of maximum response, i.e., the operating conditions that can yield the maximum sCOD/TCOD.

The contour plot (Figure 3b) confirmed this observation as the response increased towards the centre, representing the maximum solubilization. Solubilization decreased when moving away from the centre. The area within the innermost contour line represented the region with the highest organic matter solubilization, i.e., the optimal MW power and treatment time combination for maximum organic matter solubilization. Examining the contour plot revealed that the solubilization of organic matter in FS increased as the MW power decreased and that the process was slightly sensitive to changes in contact time [19]. MW power and heating time were crucial in solubilizing WAS, such that solubilization was high when the sludge was treated at low MW power for a longer time [12].

The plot of SE versus percentage solubilization (sCOD/TCOD) indicated a maximum sCOD percentage that could be achieved; however, as this is due to a combination of MW power and treatment time, the surface response rather pinpoints the optimal treatment settings for both as individual factors. So, while different combinations of MW power and treatment time can result in the same SE, the model fit to the two data sets can optimize both MW power and treatment time. Both plots indicate that there are maximum MW power and treatment times for optimal sCOD solubilization, with increases of sCOD solubilization up to an optimum, after which increasing MW power will reduce solubilization for a given treatment time, and increasing treatment time would decrease solubilization at a given MW power setting. It also indicates that the optimum MW power for COD solubilization is as given, i.e., Around 630 W; a higher range of power settings still results in higher solubilizations over short times.

4. Conclusions

This research evaluated the effect of MW treatment on the volume reduction of FS and sCOD release and applied a design of experiment and RSM to optimize the solubilization of organic matter.

MW treatment evaporated free water and reduced the volume of sludge, which increased with increasing SE. The highest volume reduction was 50 and 58% for the S1 and S2 samples. MW treatment initially increased the sCOD and solubilization of the FS, followed by a slight decrease. The highest solubilization was recorded at 630 W and 3 min.

Characterization of the response surface and contour plot revealed that solubilization increased with a decrease in the MW power. FS solubilization was slightly more sensitive to treatment time than MW power. Treatment time (x2), its quadratic term (x²2), and the interaction of MW power and contact time (x1:x2) significantly influenced organic matter solubilization in FS (p < 0.05). The optimum operating conditions for FS solubilization were 617.7 W and 2.4 min. Thus, the process should be operated close to these points to optimize organic matter solubilization in this FS.