Synergism of Life Cycle Assessment and Sustainable Development Goals Techniques to Evaluate Downflow Hanging Sponge System Treating Low-Carbon Wastewater

Abstract

While recent researchers have focused on optimizing the operational conditions of low-carbon wastewater treatment processes, they have not sufficiently evaluated the sustainability of such systems. This study is the first to assess the performance of a low-carbon wastewater treatment facility using an integration of techno-economic and life cycle assessment (LCA) criteria accompanied by several sustainability indicators. A downflow hanging sponge (DHS) reactor was operated at a hydraulic retention time of 3.4 h, an organic loading rate of 3.8 kg COD/m³ sponge/d, and 24–35 °C (scenario_1). Another two DHSs were operated in parallel, i.e., a 50% influent bypass (scenario_2) and 260 mg/L charcoal addition (scenario_3), providing carbonaceous matter to maintain the nitrification/denitrification pathway. Employing the DHS’s scenario_3 could fulfill most of the SDGs regarding the environmental (e.g., COD and nitrogen removals) and socio-economic (e.g., reliability, labor, and health and safety) targets. The LCA tool also confirmed the superior environmental benefits of scenario_3, concerning effluent quality, GHG emissions, and sludge generation. The synergistic interaction of LCA and SDGs approaches ranked the proposed DHS modifications as scenario_3 > scenario_1 > scenario_2. Hence, the current study provided an innovative strategy that could be employed to assess the sustainability of wastewater treatment systems worldwide.

1. Introduction

The nitrification–denitrification process is an essential step in wastewater treatment technologies, avoiding the pollution of aquatic ecosystems from nitrogenous species (NH₄⁺, NO₂⁻, and NO₃⁻) [1]. Wastewater treatment plants are also designed to remove carbonaceous pollutants, mainly via the oxidation of organic substances [2]. Recently, several strategies have been employed to maintain the objective of eliminating the dual carbonaceous and nitrogenous pollutants from wastewater [3]. One of these strategies is to rely on the availability of a carbon source in the denitrification zone, maintaining the reduction of nitrate to nitrogen gas [4]. While these studies have determined the best carbon source supplementation method based on the nitrogenous pollutant removal efficiency [5,6], the selection criterion does not cover the entire system’s techno-economic performance and life cycle impacts. As such, there is a research gap in assessing the best modification in the wastewater treatment systems to sustain nitrification–denitrification, depending on multiple technical, financial, functional, and ecological criteria. These criteria should also be chosen to fulfil the sustainable development goals (SDGs) [7], complying with the sustainability of the wastewater treatment concept.

The excess nitrogen content in water streams has negative impacts on aquaculture and marine life development [8]. These adverse consequences include the intensive growth of algae and other aquatic plants by eutrophication, disrupting the ecosystem and biodiversity [9]. Moreover, the microorganisms tend to consume high amounts of oxygen to undertake the nitrification of ammonia (i.e., 4.57 g O₂/g N oxidized), creating septic and dead zones in the water bodies [10]. Furthermore, nitrate can contaminate drinking water and cause methemoglobinemia in infants (“blue baby syndrome”) and children [11]. Nitrogen dioxide (NO₂) pollution also causes respiratory and cardiovascular problems in people [6]. Hence, environmental laws and sustainability programs have restricted N discharges into surface water bodies, requiring the establishment of reliable and sustainable wastewater treatment technologies.

Downflow hanging sponge (DHS) is a term used to describe an attached growth wastewater treatment facility that depends on an effective packing medium to remove the C- and N-related pollutants [12]. This packing material is composed of sponges with a high void ratio (greater than 95%) and a large specific surface area (more than 200 m²/m³) [13]. This property provides a promising oxygen gradient along the sponge depth, where an aerobic condition dominates the sponge surface while an anoxic environment could exist in the sponge interior zone [10]. This variation in the dissolved oxygen levels along sponge-inward depth delivers a suitable condition for maintaining the coupled nitrification–denitrification process [12].

The DHS configuration has been modified to enhance the chemical oxygen demand (COD) [14] and ammonia nitrogen [10] removal, making it a suitable approach for decentralized domestic wastewater treatment. One of these modifications is to bypass a portion of the influent feed containing high carbonaceous-based COD concentrations into the lower segments of the DHS unit. For example, Bundy et al. [3] employed a 30% v/v influent bypass to the DHS’s anoxic layer to provide an additional carbon source, maintaining COD and total nitrogen (TN) removal efficiencies >84% and ~74%, respectively. Although this scenario satisfies carbon supplementation for establishing the denitrification process, it has the disadvantage of purchasing an additional pump to control the bypass flow rate. Employing this pumping system to discharge large quantities of flow rates consumes considerable amounts of energy, further hindering the scalability and applicability of this scenario [15]. The alternative option involves the supplementation of an external carbon-based material into the feeding wastewater source (denitrification requires a COD to N ratio above 4) [3]. However, this carbon dosing scenario requires a proper synthesis of a carbon-rich material [9], which should be environmentally friendly and free of toxic chemicals to avoid pollution transfer. Hence, the best modification scheme should be appropriately selected to comply with the health and safety, cost-effectiveness, and reliability criteria, meeting the sustainability concept.

Recently, life cycle assessment (LCA) [16] and sustainable development goals (SDGs) [17] have been used to benchmark the sustainability of wastewater treatment systems. For instance, a recent review article has summarized several LCA-related studies, quantifying multiple environmental impacts and implications associated with wastewater treatment [1]. Another study demonstrated that the LCA tool could be adequately used in evaluating environmental performance and sustainability for wastewater management [18]. A previous study by Obaideen et al. [7] has demonstrated that wastewater treatment could contribute to the fulfilment of 11 out of 17 SDGs, representing an essential step to assist the decision-makers in implementing sustainable pollution reduction practices. Cossio et al. [19] used a set of environmental, social, economic, and technical indicators identified from the literature to assess the sustainability of WWTPs, where this assessment tool could be adapted according to data accessibility. These studies pave the way towards sustainable DHS practices, which should be explored further.

To the best of the author’s knowledge, there is a research gap in assessing the optimum DHS configuration operated under low-carbon wastewater conditions by sufficiently addressing life cycle impact and sustainable development criteria. To cover this research gap, the DHS unit was operated under three different scenarios to alleviate the carbon limitation issues for nitrification–denitrification in wastewater. Eight environmental and socio-economic indicators were used to evaluate the performances of these scenarios based on the achieved SDGs. These indicators were assigned by weights, and a SDGs-indicator-scenario assessment framework was involved in assessing the wastewater treatment operational scenarios. The life cycle impact of each scheme was evaluated for the environmental benefits achieved and pollution avoided. Sensitivity and uncertainty analysis using the Monte Carlo simulation was used to determine the robustness of the LCA results. The LCA and SDGs results were unified to select the best scenario, ensuring sustainable wastewater treatment strategies.

2. Materials and Methods

The study methodology started by conducting several experimental works on DHS performance in treating a low-carbon wastewater source, followed by using the experimental results along with literature review findings to establish an innovative LCA/SDGs integrated assessment tool. Three different DHS configurations were suggested, where the best scenario that complied with the socio-enviro-economic categories was proposed to ensure sustainable wastewater treatment technologies (see Supplementary Figure S1).

2.1. Characteristics of Wastewater

The feed solution was prepared to simulate the composition of low-carbon source domestic wastewater (

Table 1. Domestic low-carbon wastewater parameters (Lower C/N ratio).

Parameters	Concentration
Chemical oxygen demand, COD (mg/L)	397.25 ± 11.62
Ammonium nitrogen, NH4-N (mg/L)	46.06 ± 20.89
Total nitrogen, TN (mg/L)	79 ± 6
Total suspended solids, TSS (mg/L)	115.2 ± 6.6
Total dissolved solids, TDS (mg/L)	988.75 ± 0.53
pH	7.7 ± 0.2
Temperature (°C)	24–35

). This composition was synthesized from NaHCO₃ (200 mg/L), CaCl₂·2H₂O (10 mg/L), C₆H₁₂O₆ (260 mg/L), KH₂PO₄ (30 mg/L), K₂HPO₄·3H₂O (18 mg/L), CH₃COONa·3H₂O (260 mg/L), MgSO₄·7H₂O (10 mg/L), and NH₄Cl (190 mg/L) and trace elements [20]. The trace elements supplemented to the feeding source included H₃BO₃ (150 mg/L), CuSO₄·5H₂O (30 mg/L), MnCl₂·4H₂O (120 mg/L), and ZnSO₄·7H₂O (120 mg/L).

2.2. Downflow Hanging Sponge (DHS) Reactor and Sponge Specifications

The DHS structure (Figure 1) was similar to a tower composed of a feeding water distributor and three segments (each segment’s height = 40 cm) [4]. Each two compartments were connected vertically, with a 10 cm distance between them, and randomly filled with sponge pieces [21]. Each sponge element had a dimension of 3.3 cm in height and 3.3 cm in diameter and was wrapped with perforated plastic material. The sponge material had a density of 30 kg/m³, a specific surface area of 256 m²/m³, a porosity of 92%, and a pore size of 0.63 mm. The DHS reactor had an effective sponge volume of 6.95 L. This configuration was designed to prevent sponge voids from clogging and facilitate oxygen diffusion from the outside atmosphere into the DHS interior zones. A peristaltic pump with a variable discharge (from 0.7 to 13.0 L/h) was used to operate the DHS unit under a continuous operation mode.

2.3. Experimental Setup (Three Scenarios)

Initially, the sponge carriers were submerged in a tank containing activated sludge, promoting the bacteria culture’s adaptation to domestic wastewater. The effluent water quality was monitored during a 25-day adaptation period until reaching the steady-state condition and then evaluated for over three months. The DHS unit was operated at a hydraulic retention time (HRT) of 3.4 h, an organic loading rate (OLR) of 3.8 kg COD/m³ sponge/d, and 24–35 °C. In this study, three scenarios were used to treat the simulated domestic wastewater by the DHS unit (see Figure 1):

• Scenario_1 (control) was used to describe the conventional DHS operation regime responsible for carbonaceous substrate removal [22].
• Scenario_2 (bypass) was employed to represent the DHS modification scheme, where an influent bypass was used to provide a carbon source for the nitrification–denitrification process. This bypass was controlled by another peristaltic pump, introducing 50% v/v of the influent feed to the DHS third segment [3].
• In Scenario_3 (charcoal), the removal of carbonaceous substrates and ammonia nitrogen was supported by adding 260 mg/L of charcoal to the influent feed [10]. This greyish-black solid material was used as an alternative carbon source for nitrification–denitrification in the DHS system.

2.4. Analytical Analysis

The influent and effluent concentrations of COD, total dissolved solids (TDS), total suspended solids (TSS), and nitrogen species (NH₄-N and total N) were determined based on the “Standard Method for Wastewater and Water Examination” by the American Public Health Association (APHA) [23]. The wastewater sample was passed through a glass filter paper (0.45 μm) to measure TDS. All concentrations were measured in quadruplicate (n = 4) and reported as average ± standard deviation (S.D.). During the experimental study, three sponge samples were collected from each segment and evaluated for particulate matter entrapment and degradation. The sponge was compressed, squeezed, and dried overnight at 100 °C to estimate the amount of sludge occupying the DHS unit. Fourier transform infrared (FTIR) spectroscopy (Bruker Alpha, Bruker Optics, Ettlingen, Germany), with the KBr pellet method [24], was used to determine the surface functional groups. A scanning electron microscope (JCM-6000PLUS NeoScope Benchtop SEM, Tokyo, Japan) was applied to specimens mounted on stubs and sputter-coated with gold-palladium to detect the sponge’s surface morphology.

2.5. Data Collection

The laboratory observations derived from the treatment performance of the DHS unit were used to assess the sustainability of the three scenarios. Data collection also included results obtained from literature searches in Google Scholar and Scopus databases, where the query string used for the search was: TITLE-ABS (“downflow* hanging* sponge*”). Some qualitative data were collected using a survey questionnaire (see Supplementary Table S1). The collected data were used to define eight sustainability indicators (

Table 2. Indicators for assessing the sustainability of the DHS system and the associated SDG targets achieved.

Dimension	Indicator	Direction	Indicator Justification	Suggested SDG Targets
Environmental	Pollution reduction (%) COD, TN, NH4-N, TDS, TSS	Positive	COD, NH4-N, TN, TDS, and TSS removal efficiencies (% Eff) were calculated using: $\%Eff=\frac{\left(C_o-C_f\right)}{C_o}\times 100$ COD accelerates oxygen depletion in water bodies, causing death to aquatic life. Excess NH4-N causes ammonia toxicity, which could lead to gill damage and respiratory hindrances. TN promotes algal blooms. TDS causes turbidity and affects water taste. TSS blocks sunlight penetration to water, affecting aquatic plant growth.	6.1, 6.3, 6.6, 14.1, 14.2, 14.a, 15.1, 15.8, 2.1, 2.2, 2.3, 3.3, 3.9, 11.6, 11.5, 11b, 12.2, 12.6, 12.4, 12.5 [22,26,27]
	GHG emissions (mg CO2,eq/m3) CO2, N2O	Negative	The scores on GHG emissions were determined based on the DHS’s ability to reduce on-site CO2 and N2O emissions during nitrification–denitrification. GHG emissions were computed as reported earlier [9,28]: $GHG=\sum \left({CO}_2+{GWP}_{N_{2}O}\times N_{2}O\right)$ where GWPN₂O is the global warming potential of nitrous oxide (taken as 298 times higher than the GHG potential of CO2). N2O was estimated from the kg NO2/kg TN fractions reported previously [28].	6.3, 13.1, 14.2, 3.9, 9.4, 11. b, 12.4 [26]
	Sludge production (g SS/g CODremoved)	Negative	Sludge is a secondary source of pollution if not properly managed. Sludge production was calculated by the following formulas [29] and represented as g SS/g CODremoved: $SRT=\frac{V\times X_{ret.}}{Q_w\times X_w+Q\times X_{eff.}}$ $Y_{obs}=\frac{∆X}{Q\left(C_o-C_f\right)}$ where SRT is the sludge retention time (day), V is the sponge medium volume in DHS (L), Xret. is the retained sludge concentration (g SS/L sponge), Xw and Xeff. are the SS concentrations (g SS/L) of excess sludge and DHS effluent, respectively, Q is the flow rate (L/d), Qw is the production volume of excess sludge (L/d), Yobs is the observed yield coefficient (g SS/g CODremoved), and ΔX is the amount of biomass produced (g/d)	6.3, 12.4, 12.5 [26,30]
	Energy consumption (kWh/m3)	Negative	For this study, energy consumption (EC) in kWh/m3 was estimated based on the related literature [26,30] and according to: EC = Electricity consumption (kWh/month) divided by treated water (m3/month)	7.1, 7.3, 12.4, 12.6 [26,30]
Socio-economic	Reliability	Positive	Reliability refers to the consistent and dependable performance of the DHS system in treating wastewater over time. The reliability of each scenario was determined based on literature reviews [3,9,26,30], expert knowledge, and questionnaires (see Supplementary Table S1)	6.3, 6.6, 6.1, 3.3, 9.4, 9.5, 12.4, 12.6 [22,31]
	System cost (USD)	Negative	The DHS system cost (in USD), encompassing the capital and operational costs, was estimated from the prices of pumps, charcoal, DHS reactor, water vessels, and energy consumption using: System cost = Capital cost + Operational cost	9.2, 9.5 [27,30]
	Labor	Positive	Labor requirement is the quantity of human resources, both skilled and unskilled, needed to operate and maintain the DHS system’s functions. In this study, the labor requirement of each scenario was determined based on literature research [3,4,10] and expert questionnaire surveys.	1.1, 1.2, 8.2, 8.3, 8.5, 8.6
	Health and Safety	Positive	The health and safety criterion represents the measures and protocols implemented to protect the well-being of workers and the surrounding environment during the operation and maintenance of the DHS treatment process. Health and safety aspects of scenarios were determined by expert questionnaire surveys and literature reviews [3,4,10]	6.1, 6.6, 14.2, 15.1, 2.1, 2.2, 3.3, 3.9, 11.5, 11.6, 11.b.

), representing the standard environmental and socio-economic criteria for wastewater treatment systems [19,25]. The environmental criterion in this study included the pollutant (COD, nitrogen, and solids) removal efficiencies, greenhouse gas (CO₂ and N₂O) emissions, energy consumption, and sludge production obtained from the DHS operation. On the other hand, the socio-economic criterion of the DHS wastewater treatment performance compressed the system cost, reliability, labor, and human health and safety aspects. These criteria were selected based on recently published articles [1,18,19] assessing the sustainability of wastewater treatment technologies in different countries worldwide. Additionally, the direction of these indicators was considered, where a positive direction represented several considerable advantages for application (and vice versa).

2.6. Methodology for Assessing SDG Achievement Integrating Scenarios and Indicators

The following steps were used to assess the three DHS scenarios in terms of SDGs, following the methodology reported earlier [32,33]:

• The SDGs targets were initially defined, depending on the correlation between the indicators and alternatives. This step would create an SDG-indicator (SDG-I) matrix (see Supplementary Table S2).
• To quantify the fulfilment of SDG, a whole point (1) was assigned to each target successfully attained. Furthermore, scores were allocated to the achieved SDGs using Equation (1). This step would develop a quantified SDG-I matrix (see Supplementary Table S3):

  $$I_{score}=100\times \left(\raisebox{1ex}{$t_a$}\!\left/ \!\raisebox{-1ex}{$T_t$}\right.\right)$$

  (1)

  where, I_score is the indicator score, t_a is the number of achieved targets of the indicator, and T_t is the total number of targets for the specific SDG.

• An Indicator–Scenario (I-Sc) matrix was constructed, capturing results from scenarios’ environmental and socio-economic indicators (see Supplementary Table S4).
• The impacts of all pollutants (e.g., NH₄ and TSS) on the environment were transformed into the COD equivalents, following the shadow prices reported earlier [34].
• Subsequently, the acquired data underwent a normalization process using Equation (2), transforming the I-Sc matrix into a standardized form.

  $$N_{score}=\frac{a}{\sqrt{a^2+b^2+c^2}}$$

  (2)

  where N_score is the normalized score and a, b, and c are the raw indicator scores of the scenarios.

• The next step involved multiplying the SDG-I (12 SDGs × 8 indicators) matrix by the normalized I-Sc (8 indicators × 3 scenarios) matrix using Equation (3), giving the final matrix. This matrix, namely SDG-Sc, defines the correlation between the three scenarios and 12 SDGs:

$${\left[\mathrm{S}\mathrm{D}\mathrm{G}-\mathrm{S}\mathrm{c}\right]}_{\left(12\times 3\right)}={\left[SDG-I\right]}_{\left(12\times 8\right)}\times {\left[I-Sc\right]}_{\left(8\times 3\right)}$$

(3)

• The results of the SDG–Sc matrix were further scaled up by multiplying each score by 10 to yield appreciable results [33].
• The culmination of the methodology was displayed by the summation of all rows in each matrix, aligning with the discernible positive and negative directions outlined in Table 2.

2.7. Life-Cycle Assessment (LCA) Methodology

2.7.1. Goal, Scope Definition, and System Boundaries

The objective of the LCA was to compare the three scenarios regarding the minimum environmental impacts and the maximum resource efficiency. This assessment focused on the operation of the DHS system, excluding materials for constructing the reactor, sponge media, and tank vessels that are common in the three scenarios. Figure 2 shows the system boundaries of the three scenarios under investigation, following the conceptual framework of ISO14040–14044 [35]. The system boundary included pump operation, DHS pollutant removal efficiencies, sludge generation, charcoal supplementation, and energy consumption. LCA calculations were performed over a functional unit (FU) of 1 m³ of treated domestic wastewater by the DHS, and all data were normalized to this unit.

2.7.2. Life Cycle Inventory

Foreground data, such as domestic wastewater quality, sludge generation, and pollutant reduction, were obtained through laboratory-scale experiments. Background data, e.g., electricity for pumps, charcoal price, and CO₂, and N₂O emissions, were accessed through the Ecoinvent version 3.9.1 database, focusing on the African continent (

Table 3. Life cycle inventory for the three scenarios.

	Item	Unit	Scenario_1	Scenario_2	Scenario_3
Input	Domestic wastewater	m3	1	1	1
	Charcoal	mg/L	-	-	260
	Energy (Pump)	kWh/m3	0.07	0.105	0.07
Output	Treated effluent
	COD	mg/L	79.25	58.38	15.1
	NH4-N	mg/L	11.9	8.96	3.48
	TN	mg/L	38.1	17.53	6.14
	TDS	mg/L	883.67	849.94	832.24
	TSS	mg/L	20.26	12.17	5.83
	Sludge generation	g SS/g CODremoved	0.092	0.075	0.06
	GHG emissions	mg CO2,eq/m3	805.53	539.46	360.42

). Further, the database was integrated and analyzed through the OpenLCA software 2.0.3 version.

2.7.3. Life Cycle Impact Assessment Method

The ReCiPe method was applied at the endpoint and midpoint levels in a hybrid approach, adopting a Hierarchist (H) perspective [16]. The impact at midpoint was classified into six categories: (i) Climate change—global warming potential GWP (CC, kg CO₂-Eq), (ii) Ecotoxicity potential (ETP) as Freshwater ecotoxicity potential (FETP, kg 1,4-DCB-Eq), Marine ecotoxicity potential (METP, kg 1,4-DCB-Eq), and Terrestrial ecotoxicity potential (TETP, kg 1,4-DCB-Eq), (iii) Eutrophication potential (EP) as Freshwater eutrophication potential (FEP, kg P-Eq) and Marine eutrophication potential (MEP, kg N-Eq), (iv) Human toxicity potential (HTP) as Carcinogenic (HTPc, kg 1,4-DCB-Eq) and Non-carcinogenic human toxicity potential (HTPnc, kg 1,4-DCB-Eq), (v) Terrestrial acidification potential (TAP, kg SO₂-Eq), and (vi) Fossil fuel potential (FFP, kg oil-Eq). At the endpoint level, Ecosystem quality (EQ, Species·yr), Human health (HH, DALYs), and Natural resources (NR, USD 2013) impact categories were considered to make a holistic assessment of the environmental impacts of the scenarios. The definition and justification of these midpoint and endpoint impact categories are summarized in Supplementary Tables S5 and S6.

2.8. Integration of SDG-Sc Framework and LCA Results

The life cycle impact category results, both at midpoint and endpoint levels, underwent a normalization step to establish a standardized scale for assessment. The normalized outcomes for each scenario were then aggregated, offering a comprehensive overview of each scenario’s performance at both midpoint and endpoint levels. These cumulative midpoint and endpoint results were subsequently integrated with the SDG–Sc normalized results to holistically evaluate the overall performance of the scenarios.

3. Results and Discussion

3.1. Variation of Treatment Performances among the Three Segments

The COD concentrations decreased from 397 mg/L (influent) to 40–113 mg/L (effluent of three scenarios) along the reactor height (Figure 3a). This finding shows a statistical significance variation (p-value < 0.05) across the three segments. This occurrence could be attributed to the abundance of organic matter and oxygen at the top segment, promoting a higher aerobic microbial growth for COD removal [15]. However, this organic matter degradation pathway reduced the amount of available carbonaceous substrates (as COD concentrations) in the subsequent segments [21]. As such, heterotrophic bacteria in the first segment’s packing materials utilized the accessible substrate as an electron donor and natural oxygen as an electron acceptor for aerobic growth [10]. Some microbial species responsible for COD elimination, such as Selenomonas, Clostridiales, and Xanthomonas axonopodis, could be dominant at the top segment during sewage wastewater treatment [31]. It has also been reported that the microbial degradation and physical entrapment of organic compounds were the main mechanisms responsible for decreasing the COD concentrations along the DHS profile [30].

The NH₄-N and TN removal efficiencies were limited to about 8–24% at the DHS’ top segments (Figure 3b,c) probably due to the lower activity of ammonia-assimilating bacteria responsible for N transformation. The Tukey’s post hoc test indicated that the N concentrations varied significantly (ANOVA: p-value < 0.05) across the three segments. In the upper part, with a higher organic loading rate (3.8 g COD/L/d), the existing ammonium oxidizers could not compete with heterotrophs for the space and oxygen required to maintain efficient nitrification. This organic load dropped by about 10.2% as the wastewater trickled downwards, providing a favourable condition for the dominance of the ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) responsible for nitrification–denitrification (i.e., nitrification: NH₄ → NO₂⁻ → NO₃⁻; denitrification: NO₃⁻ → NO₂⁻ → NO → N₂O → N₂). A study by Kubota et al. [14] also showed that the sponges beyond the first DHS segment were occupied mainly by Nitrosomonas (AOB) and Nitrospira and Nitrospina (NOB), supporting the N removal capabilities.

The DHS reactor eliminated a small portion of TDS (Figure 3d), equivalent to 5.6 ± 0.3% in the first segment. This value slightly declined to 2.8 ± 0.1% in the subsequent compartments, with statistically insignificant variation (p > 0.05). The sponge media have the potential to act as an adsorbent material, providing an ion exchange pathway to remove portions of dissolved ions [10]. The first segment exhibits a greater concentration gradient between the dissolved ions in wastewater and those on the sponge surface than the subsequent sections. This concentration gradient tends to decline due to the decrease in TDS concentration and the creation of a thick biofilm due to the accumulation of biodegradation products and cell debris [36]. This pattern hinders the boundary layer and intra-particle diffusion routes, making the driving force insufficient to overcome the mass transfer resistance between the sponge and dissolved ions [8]. A substantial TSS removal efficiency (46–65%) was observed in the first segment (Figure 3e), probably due to the physical entrapment and settlement of suspended solids within the sponge matrix [29]. However, the tendency of the sponge dimension to capture TSS became limited as the wastewater passed downward, owing to the reduction of the available vacant sites. One-way ANOVA with Tukey’s post hoc test showed an insignificant difference (p > 0.05) in TSS removal between the second and third segments.

3.2. Effect of DHS Modifications

The results in Figure 3a show that the COD removal for scenario_2 was greater than that for scenario_1 by ≈10%. However, scenario_3 showed the highest COD removal efficiency of 96.20 ± 5.41%, better than scenarios 1 and 2 by 22% and 11%, respectively. As such, there was a statistically significant (p-value < 0.05) difference between scenario_3 and other scenarios regarding the DHS’s ability to eliminate the COD load. Introducing a bypass to the third segment (scenario_2) enhanced the distribution of organic load and oxygen levels along the DHS column profile, promoting aerobic microbial growth for efficient COD removal [3]. The remarkable enhancement in COD reduction for scenario_3 could be explained by the effective adsorption affinity of charcoal toward the organic contaminants [37]. The charcoal particles have a large surface area with numerous pores to attract and capture these organic contaminants, further reducing the COD levels.

Because most carbonaceous compounds were removed in the top aerobic layers, wastewater bypass (scenario_2) was used to supply extra carbon to the subsequent layers to promote denitrification, converting NO⁻ to N₂. Introducing an influent bypass by scenario_2 improved NH₄-N removal by 6.4% more than scenario_1 (Figure 3b). This feed bypass technique has also been reported by Jong et al. [9] to reduce N concentration due to C-source availability, showing the dominance of some species, such as Nitrosomonas, Nitrobacter, and Pseudomonas [14]. A similar observation was noticed for the total N removal efficiencies (Figure 3c). This influent bypass (scenario_2) provided a carbon source for improving nitrifying and denitrifying bacterial growth in the anoxic segments. A study by Bundy et al. [3] also emphasized that introducing an influent bypass to the lower anoxic layer provided supplemental COD necessary for promoting nitrification–denitrification. However, scenario_3 improved the N removal efficiencies by 41% and 15% better than scenarios 1 and 2, respectively. This finding could be because charcoal (i) provides a carbon source for anoxic condition completion [38], (ii) offers adsorption sites (large surface area with ample pores and microstructures) to capture more N molecules [39], and (iii) facilitates the attachment and growth of nitrifying bacteria, such as Nitrosomonas, Nitrospira, and Nitrospina [14,38].

The TDS removal efficiencies among the three scenarios are illustrated in Figure 3d, exhibiting an enhancement order of Scenario_3 > Scenario_2 > Scenario_1. This order of scenarios complied with the TSS reduction patterns for the three DHSs’ configurations (Figure 3e). The feed bypass allowed an extended contact time between the wastewater components and the sponge media to adsorb more solid constituents. Prolonging this contact period for improving the entrapment and settlement of suspended solids by the media carriers has also been reported [3]. During scenario_3 operation, the enhanced DHS performance toward solid pollutants removal could be ascribed to the additional adsorption sites in charcoal. However, this removal performance depends on the charcoal’s adsorption capacity, which could be enhanced using some surface modification techniques.

3.3. Characterization of Sponge Material

3.3.1. Sponge Surface Morphology

Figure 4 shows the SEM images of a sponge piece before charcoal acclimatization (in the pristine state) and its transformed condition after acclimatization. The SEM image in Figure 4a reveals an immaculate polyurethane sponge with well-defined pore structures ranging from 131 to 414 μm. A profound alteration is evident upon introducing charcoal, which infiltrated the vacant hexagonal sponge pores during DHS operation (over 100 days). This transformation highlights the sponge’s unique ability to entrap microorganisms, which could be the reason for enhanced nitrification–denitrification by scenario_3. The sponge voids could be partially filled by organic molecules, as well as dead-end products and debris sloughing. This charcoal-occupied sponge enjoys an expansive surface area that could promote the colonization of nitrifying and denitrifying bacteria, such as Nitrosomonas, Nitrobacter, and Pseudomonas, as previously noticed [14]. A previous article also justified the development of microbial attachment and colonization on biochars, further immobilizing denitrifiers responsible for improving nitrate removal efficiency [40].

3.3.2. Surface Functional Groups

Figure 4b shows the FTIR spectroscopy results used to evaluate the chemical bonds and functional groups of sponge samples collected under the three scenarios. In scenarios 1 and 2, some characteristic FTIR peaks were observed at 3442 cm⁻¹ (O–H stretching) [41], 2923 cm⁻¹ (C–H stretching) [42], 2055 cm⁻¹ (C≡C triple bond vibrations), 1640 cm⁻¹ (C=O stretching in proteins or amides) [30], 1367 cm⁻¹ (CH₃ deformation), and 1215 cm⁻¹ (C–N stretching vibrations). These functional groups could justify the role of bacterial biomass in forming extracellular polymeric substances (EPS) that could protect the microbial biofilm against unfavourable environmental conditions, as previously reported [13]. The secretion of EPS by the bacterial cells is an essential strategy to enhance the strength, structure, and granulation of microbial biofilms attached to the sponge surface. In scenario_3, introducing charcoal exhibited distinctive FTIR peaks at 3275 cm⁻¹ (N–H stretching in secondary amines), 1537 cm⁻¹ (N–H bending in primary amines), 1218 cm⁻¹ (C–O stretching in alcohols or phenols), 1083 cm⁻¹ (C–O stretching in carboxylic acids), and 750 cm⁻¹ (C–H out-of-plane bending). These consistent functional groups could justify the better COD removal efficiency by scenario_3, where EPS occupies the adsorption sites and then captures organic contaminants from wastewater. These functional groups could also emphasize the better biological activity and microbial health of sludge attached to the sponge pieces for scenario_3, as previously demonstrated [43]. The observed peaks could signify the presence of nitrogen-related functional groups associated with an improved nitrification–denitrification process [40,44].

3.4. Assessment of the Three Scenarios Using SDGs

Figure 5a,b displays the achievement of SDGs and their targets due to implementing the three DHS scenarios (see Supplementary Table S7). The robustness of the SDG-Sc outcome was further tested using sensitivity analysis (see Supplementary Figure S2).

3.4.1. SDG 1: No Poverty

Because the DHS modification scenarios showed better pollution reduction patterns than the control, the final effluent could be reused in various industrialization facilities. Moreover, the DHS effluent could be subjected to a post-treatment process and utilized in agricultural and cultivation projects, further increasing the percentage of people with secure rights to water resources (Target 1.1 “Eradicate extreme poverty”). The local workers can be involved in these water-related projects and infrastructures, offering additional income and raising the living standards of people experiencing poverty. Job planning becomes more streamlined with establishing scenario_3 because it is associated with less sludge production and disposal, no requirement of intensive post-treatment, and no need to employ highly skilful and knowledgeable maintenance labour [26]. This benefit would support the achievement of Target 1.2 by reducing the number of impoverished residents.

3.4.2. SDG 2: Zero Hunger

The treated effluent for the DHS’s scenario_3 (with enhanced C and N removal performances) could be introduced to filtration and disinfection post-treatment processes, giving a final discharge applicable for several agricultural and grazing activities. This action would guarantee food security and increase the proportion of the population above the minimum level of dietary energy intake (Target 2.1 “End hunger and ensure access”). This final effluent could support the water–food nexus by irrigating nutritious fresh vegetables and healthy crops, avoiding stunting and wasting in children (Target 2.2 “End malnutrition”). Farmers are encouraged to increase their agricultural productivity and incomes, especially in recycling agricultural wastes for charcoal (scenario_3) preparation, distribution, and selling (Target 2.3 “Double agricultural productivity”).

3.4.3. SDG 3: Good Health and Well-Being

Applying DHS for domestic wastewater treatment maintains good water quality with acceptable carbonaceous and nitrogenous compounds. High pollution reduction by scenario_3 is essential to reduce mortality or deaths due to water-borne infectious diseases, such as cholera, typhoid fever, and hepatitis A. This advantage meets Target 3.3 by recycling agricultural residues (e.g., coconut shells, bamboo, and wood) to prepare charcoal used in wastewater treatment facilities, reducing the amount of input chemicals. This benefit associated with the DHS application also supports Target 3.9 by reducing hazardous chemicals and water pollution.

3.4.4. SDG 6: Clean Water and Sanitation

Water security in remote areas can be achieved by implementing DHS because it is simple and cheap and requires less knowledge for operation. Using this water (i.e., after proper post-treatment to remove faecal and priority chemical contamination) for irrigation would potentially reduce water withdrawal for agriculture. Consequentially, the water stress index in the relevant rural areas would be reduced (Target 6.1 “Safe and affordable drinking water”). Moreover, reusing the treated effluent for irrigation and food production positively contributes to the crop–water productivity index (Target 6.4 “Increase water-use efficiency”). Increasing the proportion of treated (or partially treated) water and supporting water resource management are the main objectives of using charcoal (scenario_3) to enhance DHS treatment performance (Target 6.3). This treated effluent would avoid wastewater disposal into the aquatic environment, further minimizing eutrophication, water toxicity, and bioaccumulation (Target 6.6 “Protect water-related ecosystems”).

3.4.5. SDG 7: Affordable and Clean Energy

The DHS technology is considered cheap because the oxidation process is facilitated by air diffusion from the surrounding atmosphere to the sponge voids (excluding external aeration). This benefit would reduce energy consumption, fulfilling the domestic electricity demand, i.e., electricity can be saved and used in other avenues (Target 7.1 “Ensure access to affordable energy”). Scenario_2 is operated using an additional pump for influent bypass, consuming more energy for operation (i.e., pump electrical power consumption). Moreover, the effluent of the control DHS with unsatisfactory water quality should be subjected to a post-treatment process that utilizes greater amounts of electrical energy attributes. As such, the amount of energy used in wastewater treatment by DHS scenario_3 could further guarantee sustainable energy services (Target 7.3 “Improvement in energy efficiency”).

3.4.6. SDG 8: Decent Work and Economic Growth

Because the DHS system is simple and cost-effective, it can support direct investment in wastewater treatment projects and infrastructure (Target 8.6 “Reduce youth proportion not in employment”). Moreover, the reuse of treated wastewater in the agricultural sector encourages the farmers to implement medium-sized irrigation farms (Target 8.5 “Achieve full and productive employment”). Using charcoal to enhance the DHS performance captures more biomass and saves the energy required to operate an additional pump that diverts feed to the anoxic segments. Moreover, this charcoal application endorses the innovation and improvement of wastewater treatment technologies, describing the research and development (R&D) approach (Target 8.2 “Achieve productivity of economies”). The current study also demonstrated that using DHS, with reduced electricity requirement, provided an effluent that could be reused to irrigate different non-edible crops (or irrigating golf courses), supporting the water–food–energy nexus. Moreover, the treated effluent could be used for industrial development, green landscapes, and public parks and grounds. Job satisfaction tends to be more significant when using an approach that yields better results, such as scenario_3 with charcoal, boosting overall productivity and morale. These facilities and financial benefits would offer many job opportunities, viz, Target 8.3 “Promote new jobs and micro-sized businesses”.

3.4.7. SDG 9: Industry, Innovation, and Infrastructure

Applying the DHS-treated effluent for non-potable industrial activities offers the double advantage of (i) limiting the related water stress (and overall environmental impacts) and (ii) supporting the agro-industrial sector and food manufacturing companies. This approach fulfils Target 9.2 by promoting inclusive and sustainable industrialization. The current study also demonstrated the preparation of charcoal as an innovative C-source to enhance the nitrification–denitrification process and an adsorbent to capture aqueous contaminants, meeting Target 9.5 “Encouraging innovation”. Because scenario_3 showed a good score on the cost criterion, it could be used to establish a cost-effective decentralized wastewater and wastewater reuse infrastructure. Additionally, this scenario showed the best score on GHG emissions, representing a clean and environmentally sound technology (Target 9.4).

3.4.8. SDG 11: Sustainable Cities and Communities

The DHS wastewater treatment system coupled with sand filtration and chlorination can be installed in different cities for the transition toward the adoption of decentralized water reuse projects. This action tends to meet escalating water demand accompanied by the insufficiency and irregularity of municipal water supply in densely populated cities. Recycling agricultural residue for preparing charcoal used in wastewater treatment could mitigate pollution generated by solid wastes in rural farming areas. This pattern would meet Target 11.6 by paying particular attention to agricultural waste management. The proper management of the DHS effluent could promote people’s settlement in safe and affordable residential buildings because raw wastewater is usually responsible for transmitting water-borne illness (Target 11.5 “Reduce the number of deaths from water-related disasters”). Policymakers, planners, and administrators can share their responsibilities for implementing appropriate DHS configuration modification to remove persistent organic contaminants (Target 11.b “Integrated policies and plans towards disaster risk management”).

3.4.9. SDG 12: Responsible Consumption and Production

Water is a finite natural resource, and it is utilized in most human activities, e.g., agriculture practices share about 70% of freshwater consumption. Employing the DHS system for wastewater treatment encourages industry leaders and businesses to reduce the amount of sludge generated (Target 12.4 “Reduce release of pollutants to air, water, and soil”) and recycle/reuse the effluent after the required treatment (Target 12.5 “Recycling and reuse of waste generation”). Developed nations and various non-governmental organizations are responsible for supporting low-income countries in installing the DHS unit and reusing the final effluent (Target 12.6 “Encourage companies to adopt sustainable practices”). The DHS technology could also be modified by using more rigid and porous sponge media, allowing the development of effective nitrifiers for N removal. Using natural ventilation from the surrounding atmosphere slows down the depletion of natural resources (e.g., natural gas and fossil fuel), adding benefits to Target 12.2 “Efficient use of natural resources”.

3.4.10. SDG 13: Climate Action

Wastewater treatment is closely linked to climate change causes, where the biological processes generate gasses, such as CO₂, CH₄, N₂O, and H₂S, accompanied by organic degradation processes. The climate change policy developed by the government obligates companies to use more renewable resources in operating wastewater treatment facilities. Using charcoal to modify the DHS configuration has several environmental merits on human health (pollution minimization) and climate change (N₂O emission reduction), positively contributing to Target 13.1 “Strengthen resilience to climate-related hazards”. Using scenario_3 as an alternative N removal scheme to extra pump (bypass) installation tends to reduce electricity consumption. Large amounts of CO₂ can be emitted during pump and automatic control operation, where this GHG is considered a primary contributor to global warming and other environmental hazards. Moreover, many countries have shifted their wastewater treatment strategy by avoiding intensive energy inputs for supplying air into the bioreactors to maintain aerobic conditions.

3.4.11. SDG 14: Life below Water

Improving the DHS configuration to have a better final quality positively contributes to life underwater (Target 14.1 “Reduce marine pollution”), preserving the aquatic ecosystem (animals, plants, and other organisms). The DHS system treats water contaminated by human activities, which could be released into rivers and transfer chemical substances to the adjacent seas and oceans (Target 14.2 “Achieve healthy and productive oceans”). As such, the DHS wastewater treatment facility is essential to the health of fish and other underwater animals consumed by humans as food in several regions worldwide. Further scientific and applied knowledge of charcoal modification methods is required to develop concrete strategies for involving a higher carbon source (as electron donors) in denitrification (Target 14. a “Develop research capacities to enhance the contribution of marine biodiversity”).

3.4.12. SDG 15: Life on Land

Water pollution is highly correlated with land degradation, reducing the quality of soil that affects the growth of forests and plants (Target 15.1 “Conservation of terrestrial ecosystems”). The treated wastewater by DHS could be used to plan and implement agro-reclamation activities, assisting in restoring farmlands influenced by critical desertification status. Measuring the water quality parameters of the DHS effluents is essential to determine the most suitable post-treatment process, giving a final product that complies with waste disposal regulations (Target 15.8 “Prevent the introduction of contaminants on land and water ecosystems”).

3.5. Environmental Evaluation of Three Scenarios Using LCA

Figure 6 shows the environmental evaluation of the three DHS operational scenarios using the midpoint and endpoint impact categories. At the midpoint level (Figure 6a), the major impacts were related to the categories of ecotoxicity, human toxicity, and climate change.

Scenario_3 demonstrated superior pollution reduction compared with other scenarios (see Figure 3), where its effluent could have lower impacts on the marine life and freshwater ecosystem (FETP and METP midpoint levels). The disposal of insufficiently treated wastewater (e.g., scenario_1) into the aquatic environment could be accompanied by human health concerns (HTPc and HTPnc midpoint impacts). Insufficient denitrification in scenario_1 due to C-limitation affects the HH endpoint level in diverse ways upon disposal. For instance, higher nitrogen concentrations in the water streams could harm native aquatic species and dominate harmful algal blooms that deplete oxygen when they die and decompose (FEP and MEP midpoint categories). Consequently, dead zones are created where marine life cannot thrive. Thyroid dysfunction, kidney disease, and various chronic health effects have been accompanied by the ammonia poisoning of fish. Furthermore, N₂O released into the atmosphere due to incomplete nitrification–denitrification (scenario_1) might contribute to air pollution, potentially resulting in respiratory problems and exacerbating pre-existing conditions.

Scenario_2 exhibited higher GHG emissions (CC midpoint level) associated with N₂O release compared with other scenarios. In the DHS technology, N₂O gas could be emitted from the incomplete nitrification or denitrification processes driven by AOB and heterotrophic denitrifiers [37]. Importantly, N₂O carries a GWP 298 times greater than that of CO₂, making it a potent GHG responsible for global warming and climate change. A lower effect of scenario_3’s operation on GHG emission could be ascribed to the efficient denitrification process associated with charcoal addition. Scenario_1 exhibited a beneficial contribution to CC (midpoint category) probably because of the avoided CO₂ emissions from charcoal manufacturing and pump operation.

Sludge incineration and landfilling cause pollution and degradation in terrestrial soil, showing negative impacts on the TETP and TAP midpoint indicators of LCA. The amount of sludge generated from scenario_3 (0.06 g SS/g COD_removed) was lower than those from scenarios 1 and 2 by 35% and 20%, respectively. For instance, charcoal could immobilize the bacterial biomass inside the DHS’s sponge, representing ≈22 g VS/L sponge. The accumulation of microbial communities on biochar material was also observed [29], reducing the amount of sludge produced. The study found that the packing material could provide a larger surface area to attract and accommodate more microorganisms responsible for organic pollutant reduction [29].

The total energy consumption for each of scenarios 1 and 3 was estimated as 0.07 kWh/m³ of sewage load, with a power cost of 0.07 USD/kWh [30]. The energy consumption cost (NR endpoint level) for scenario_2 was approximately greater than for the other scenarios by 50%, owing to the use of additional equipment (pump) for wastewater bypass. With a market price range of 1500–2000 USD, this peristaltic pump diverts carbon-laden wastewater to the desired segment of the DHS unit.

Scenario_2 exhibited a notably higher system cost primarily attributed to the purchase of an additional pump for operational needs. The used charcoal in scenario_3 was locally acquired at a market price of 0.11 USD/kg. A quick survey, involving physical and online market sources, was conducted to confirm this price. Based on this survey, charcoal prices could range from 0.1 to 2.5 USD/kg, depending on material type and mode of delivery. The NR (endpoint level; Figure 6b) exhibited a lower value in scenario_1 because it is operated with no need for charcoal or a bypass pumping system.

With the unique advantages of lower-cost resource utilization and improved system performance, scenario_3 exhibited good reliability in wastewater treatment technology. Determining the optimum scenario using the midpoint and endpoint indicators is also influenced by having a final effluent quality that complies with discharge regulations. The total N effluent levels from other scenarios were still higher than the desired values, indicating that carbon source adaptation is required to complete denitrification. Higher capital expenditures required for extra pump installation mean that scenario_2 is not the best option for implementation. In scenario_1, poor denitrification for N removal could occur due to C-source limitation; however, there is an insignificant (p > 0.05) difference between scenarios 1 and 2 toward COD removal (see Figure 3).

Based on the LCA observations, scenario_2 was less preferable for implementation because it exhibited relatively lower pollution reduction with higher energy consumption by pumping (FFP midpoint impact). Scenario_2 was responsible for moderate treatment efficiency because the bottom DHS segment may receive a high solids load and an unbalanced C:N ratio influent, impacting the overall treatment process [3]. Charcoal supplementation as a C source (scenario_3) maintained environmental-related advantages (e.g., a better-quality effluent with lower sludge production than other scenarios), alleviating the adverse impacts on EQ and HH endpoints. Scenario_3 also reduces the need for chemical-based carbon sources, such as methanol, acetic acid, and sodium acetate, further minimizing operators’ chemical exposure.

Sensitivity analysis of LCA computations was performed using Monte Carlo simulation to assess the uncertainties associated with impact categories [45] (see Supplementary Table S8). Scenario_3′s lower uncertainty values compared to the other scenarios confirmed its feasibility in wastewater treatment, considering the variation in environmental impacts (human toxicity, climate change, and ecotoxicity potentials).

3.6. Selection of Best Operation Regime

Based on the aforementioned environmental and socio-economic pillars, scenario_3 enjoyed the highest scores in the SDG-Sc framework. This option was followed by scenarios 2 and 1, respectively. Using the LCA tool, the net results indicated that environmental benefits followed the order of scenario_3 > scenario_1 > scenario_2. Hence, both evaluation tools (SDG-Sc and LCA) were integrated for final selection and ranking (Figure 7). Scenario_1 demonstrated moderate performance due to its increased GHG emissions accompanied by greater sludge production and inefficient pollutant removal. Scenario_2, on the other hand, garnered the least performance in terms of sustainability and environmental impacts owing to increased energy consumption, moderate pollutant removal efficiencies, and GHG emissions. Interestingly, scenario_3 exhibited commendable performance due to the availability of C-source for nitrification–denitrification within the DHS reactor leading to overall sustainability and environmental feasibility. Charcoal’s superior performance in reducing pollutants was particularly compelling, as evidenced by exceptional COD, TN, TDS, and TSS removal efficiencies (see Figure 3) and fewer environmental concerns. In particular, scenario_3 would entail a promising choice for sustainable wastewater treatment by DHS reactors using the data obtained in this study.

3.7. Study Limitation and Future Investigation

This research has specific barriers that open avenues for future investigations (Figure 8):

• The proposed evaluation method should consider the DHS’s performance in removing various heavy metals, such as lead, mercury, cadmium, and arsenic, disposed of by the industries. This criterion would offer a promising area for further research, as heavy metals pose significant environmental and health risks.
• The optimization of the charcoal dosage and bypass flow rates entails further exploration to enhance the accuracy and effectiveness of this treatment process.
• Future studies are required to test the changes in the voids, porosity, and effective surface area of the sponge material during operation because its durability and resilience affect the long-term performance of DHS.
• Incorporating environmental indicators, such as odour and faecal coliform reduction efficiency, is advisable for conducting a comprehensive sensitivity analysis, further improving the robustness of the proposed assessment tool.
• Using mathematical models to optimize the C-source implementation scenarios by increasing the analysis to include salts, metal ions, volatile organic compounds (e.g., benzene, toluene, and xylene), pharmaceuticals, chlorine, and chloramine compounds present in wastewater.
• Because the study outcomes were based on assessing the performance of the treatment scenarios using 12 SDGs, further investigation would be required to acquire more data for addressing the remaining five SDGs.

4. Conclusions

The study succeeded in introducing an innovative approach to evaluate wastewater treatment technologies, particularly the Downflow Hanging Sponge (DHS) system, through means of life cycle assessment (LCA) and the sustainable development goals (SDGs). By integrating LCA results with SDG achievements, operating the DHS under scenario_3 (charcoal supplementation to enhance the nitrification/denitrification strategy) could align with broader sustainable development objectives. This scenario enjoyed higher scores in the environmental criteria (COD and N removal efficiencies, GHG emissions, and sludge production) and socio-economic dimensions (energy consumption, human health, and reliability), as compared with operating the DHS under a 50% v/v bypass scenario. This approach not only advances the field of environmental engineering but also provides valuable insights for policymakers and practitioners, aiming at improving sustainability in wastewater management. The study outputs could also be used as a foundation for future studies by involving more indicators related to the microbial community in each DHS configuration and the best charcoal supplementation dose and influent bypass percentage.