Domestic Reclaimed Water for Circular Agriculture: Improving Agronomic Performance of Sweet Sorghum in a Semiarid Tropical Climate

Abstract

Water scarcity in semiarid regions represents a critical challenge for sustainable agriculture, reducing the availability of forage and affecting livestock systems. The reuse of treated wastewater offers an environmentally friendly alternative to meet water and nutrient needs, supporting the principles of the circular economy. Sweet sorghum, with its remarkable tolerance to abiotic stress, represents a resilient crop option. Evaluating its agronomic and industrial responses to different depths of irrigation using reclaimed water is essential for improving resource-efficient agricultural practices in water-limited environments. This study evaluated the effects of different irrigation regimes with treated wastewater on the growth, productivity, and water use efficiency of sweet sorghum grown in a semiarid region of Brazil. The experiment was conducted in a randomized complete block design, with five irrigation regimes ranging from 50% to 150% of crop evapotranspiration (ETc) and four replications. Irrigation was carried out with treated wastewater using a drip irrigation system. Growth parameters, fresh biomass, water use efficiency, and soluble solids content (°Brix) were analyzed in two consecutive harvests (main and ratoon crop). Deficit irrigation regimes (50% and 75% of ETc) resulted in higher water use efficiency and higher °Brix, whereas regimes above 100% of ETc reduced water use efficiency and biomass productivity. The ratoon crop showed greater sensitivity to water management, with significant productivity responses under irrigation around 100% of ETc. The first harvest was more productive in terms of fresh biomass and plant growth. Reclaimed water is a sustainable and efficient strategy for cultivating sweet sorghum in semiarid regions. Deficit irrigation regimes can be technically viable for maximizing water use efficiency and production quality, while proper irrigation management is crucial to avoiding losses associated with excessive water application.

1. Introduction

Rainfall variability in semiarid regions adversely affects circular economy systems, limiting the availability of biomass for nutrient recycling and reducing water use efficiency. This scenario calls for adaptation strategies—such as sustainable pasture management and effluent reuse—to guarantee food security and promote economic resilience [1,2,3]. In addition, water scarcity resulting from declining reservoir levels and groundwater depletion, exacerbated by historical climate change, has restricted the cultivation of forage and industrial crops, thus limiting the availability of high-quality food for livestock production [4,5,6]. In this context, improving water productivity through innovative irrigation techniques and integrated water management has been identified as a key way to strengthen the sustainability of agriculture in water-limited conditions [7].

In response to water scarcity, the reuse of domestic wastewater offers a viable source of water and nutrients for plants in semiarid regions, resulting in satisfactory crop yields and quality, even under limited irrigation [5]. In addition, the use of treated wastewater (recycled water) as an irrigation source for sweet sorghum in semiarid regions can contribute to improving the quality and quantity of silage produced in this region, reducing the need for fertilizers in agriculture, as well as contributing to more efficient nutrient uptake and water conservation in the face of climate change [8,9].

Treated domestic wastewater is a valuable yet underutilized resource in addressing water scarcity and advancing circular economy strategies in agriculture. Worldwide, it is estimated that more than 330 km³ of municipal wastewater is produced annually, of which less than 20% is reused, despite its high potential for irrigation in arid and semiarid regions [7]. Studies suggest that treated effluent could irrigate more than 40 million hectares worldwide, contributing significantly to food production without competing with freshwater resources [10]. However, long-term reuse requires careful monitoring and management to avoid the accumulation of salinity and heavy metals, which can affect soil health and crop safety [11,12,13]. When properly treated and applied, irrigation with wastewater can be a safe and productive strategy, aligned with sustainable development goals and climate resilience in agriculture.

Sweet sorghum [Sorghum bicolor (L.) Moench], a C4 grass native to Africa, is grown in tropical, subtropical and semiarid regions. Its resistance to semiarid environments makes it suitable for meeting the high demand for animal feed and ethanol production, particularly during the sugarcane industry’s off-season [14,15]. Sweet sorghum has favorable nutritional quality due to its digestible fiber and palatability, representing a significant forage option in semiarid regions. This is due to its high dry matter yield, particularly under irrigation, combined with efficient water use and drought tolerance [16,17,18].

Water scarcity in semiarid regions represents a significant challenge for circular economy agriculture, with an impact on food production and water security. In this context, the reuse of wastewater has emerged as a promising sustainable strategy. From an environmental point of view, reuse reduces the pressure on natural water resources, contributing to the conservation of aquatic ecosystems and reducing pollution caused by untreated effluents [19,20]. In terms of economic sustainability, this practice helps to reduce fertilizer costs, since wastewater contains essential nutrients for plants, while also providing an alternative source of water for irrigation, especially in water-scarce regions [21,22]. Regarding social aspects, the reuse of wastewater can increase food security by ensuring the production of forage and crops, even in adverse climatic conditions, while promoting the resilience of rural communities in the face of climate change [23,24,25]. Thus, integrating wastewater reuse into circular agriculture not only optimizes the use of resources but also aligns with the United Nations Sustainable Development Goals (SDGs), particularly those related to drinking water and sanitation (SDG 6) and sustainable agriculture (SDG 2).

It is widely recognized that sorghum is a tolerant crop to abiotic stresses; however, its responses to different depths of irrigation—particularly deficit or excessive irrigation—on its agronomic, physiological, and industrial performance under field conditions remain insufficiently understood. This knowledge gap is critical, especially in semiarid regions where water resources are scarce and must be managed with caution to ensure sustainable agricultural practices. Therefore, this study investigates the effects of different irrigation regimes, expressed as percentages of crop evapotranspiration (ETc), on the growth, productivity and water use efficiency of sweet sorghum grown under field conditions in a semiarid tropical climate. The research focuses on two consecutive crops (main crop and ratoon crop) to assess the long-term impacts of irrigation with treated wastewater, with the aim of providing information on optimizing water use while maintaining high biomass and sugar yields. By exploring these aspects, this study advances the broader goals of sustainable agriculture and water management, aligning with circular economy principles and the United Nations Sustainable Development Goals (SDGs).

2. Materials and Methods

2.1. Localization and Experimental Conditions

A field study was conducted to evaluate the impact of irrigation regimes with treated domestic wastewater on the growth of sweet sorghum during two harvests (November 2021 to June 2022) at the Experimental Hydro-Agricultural Reuse Farm of Parnamirim—(UR-Parnamirim/INSA), in the city of Parnamirim, state of Pernambuco, northeast of Brazil (8°05′22″ S, 39°35′18″ W, 392 m) (Figure 1).

The region’s climate is classified as BSh (hot semiarid), according to the Köppen climate classification adapted for Brazil [26]. The study period was characterized by an average air temperature of 28 °C, relative air humidity of 60%, global radiation of 9.0 MJ m⁻² day⁻¹, and total rainfall of 583 mm (202 mm registered in the 1st sorghum cut and 381 mm registered in the 2nd sorghum cut, Figure 2).

The soil in the experimental area is classified as clay loam according to the Natural Resources Conservation Service/USDA [27], the detailed soil properties of which are provided in

Table 1. Physicochemical characterization of the soil in the experimental area.

Soil Characteristics	Layer (cm)
	0–20	20–40
Chemical characteristics
pH1:2.5	7.43	7.73
P (mg dm−1)	117	86
S (mg dm−1)	18.8	20.1
Soil sorption complex (cmolC dm−3)
Ca2+	14.25	15.63
Mg2+	2.45	1.97
Na+	1.87	2.33
K+	0.26	0.17
SB *	16.25	20.10
H + Al	0.27	0.46
CEC **	19.1	20.5
Al3+	0	0
V (%)	98.6	97.6
Aluminum saturation percentage
m (%)	0	0
Physical characteristics
Sand (%)	23.67	19.00
Silt (%)	45.33	47.33
Clay (%)	31.00	33.67
Soil texture	Clay loam	Clay loam
Bulk density (Mg m−3)	1.26	1.27
Soil particle density (Mg m−3)	2.45	2.43

* Sum of bases; ** cation exchange capacity.

.

2.2. Experimental Design and Treatments

The experiment followed a randomized complete block design, with five irrigation treatments based on crop evapotranspiration (ETc): IR₁ (50%), IR₂ (75%), IR₃ (100%), IR₄ (125%), and IR₅ (150%), using reclaimed water (RA) as the irrigation source. The experiment had four replicates, resulting in a total of 20 experimental plots (Figure 3). Each experimental plot covered 15 m², consisting of three single rows of sorghum, each 5.0 m long and spaced 1.0 m apart. The effective sampling area was limited to the central row, excluding 1.0 m at both ends.

2.3. Water Source, System, and Irrigation Management

Domestic wastewater was treated in a facultative lagoon (dimensions 226 × 96 m and depth 1.5 m) associated with a maturation lagoon (88.4 m × 86.0 m) operated by Pernambuco Sanitation Company (COMPESA, Parnamirim-PE, Brazil) and located 600 m from UR-Parnamirim (Figure 4). The treated effluent was stored in two 10,000 L reservoirs using a 1.0 hp open-rotor submersible pump. The effluent was pumped by a 3.0 hp horizontal centrifugal pump. The effluent was filtered using a 250 kg sand filter under a pressure of 10 kPa combined with two 2″ mesh filters at the discharge outlet. The characteristics of the treated effluent are shown in

Table 2. Chemical properties of treated domestic effluent.

Effluent Characteristics	Values	Reference Values
pH	7.10	6.0–8.5 (a)
Electrical conductivity (dS m−1)	1.11 (mean, Figure 5)	0–3.0 (a)
Hardness CaCO3 (mmolC L−1)	0.1996	0–3.0 (b)
SAR (mmolC L−1)1/2	5.07	0–15 (a)
Na+ (mmolC L−1)	6.61	0–40 (a)
K+ (mmolC L−1)	1.10	0–0.05 (a)
Ca2+ (mmolC L−1)	2.48	0–20 (a)
Mg2+ (mmolC L−1)	0.92	0–5 (a)
Cu2+ (mmolC L−1)	0.000157	0–0.0063 (a)
Fe3+ (mmolC L−1)	0.000250	0–0.269 (a)
Mn2+ (mmolC L−1)	0.006006	0–0.0073 (a)
Zn2+ (mmolC L−1)	0.000153	0–0.061 (a)
Helminth eggs (Eggs L−1)	<1.0	≤1 (b)
Thermotolerant coliforms (MPN/100 mL) 1	100	≤1000 (b,c)

¹ Most probable number/100 mL of sample; ^(a) [29]; ^(b) [30]; and ^(c) [31].

. The sodium concentration (Na⁺) (6.61 mmolc L⁻¹) and sodium adsorption ratio [SAR, 5.07 (mmolc L⁻¹)^1/2] of the treated domestic wastewater fell within normal ranges for irrigation (0–40 for Na+ and 0–15 for SAR) according to [28], posing no risk to the soil’s physical quality in the study area.

Monthly domestic wastewater analyses were carried out throughout the experiment. Figure 5 illustrates the time trend of the electrical conductivity of the recycled water (ECRW), measured with a portable digital conductivity meter (AKSO, model Combo 5).

Sorghum plants were irrigated via a drip irrigation system, with lateral lines equipped with in-line emitters (16 mm nominal diameter, 2.1 L h⁻¹ flow rate) spaced 0.5 m apart (Figure 6). Irrigation management was based on climatic conditions, with the calculation of crop evapotranspiration (ETc) determined from the daily reference evapotranspiration (ETo) estimated by the Penman–Monteith method, as recommended by the FAO [32] (Equation (1)). The crop coefficient (Kc) for sorghum proposed by [33] was adopted, considering the following values for each phenological stage (

Table 3. Phenological phases of forage sorghum.

Phases	Characteristics	Kc
I	initial period	0.40
II	vegetative development	0.68
III	reproductive	1.14
IV	grain filling	1.10

): I = 0.40; II = 0.68; III = 1.14; and IV = 1.10. The atmospheric data used to determine ETo was obtained from an automatic meteorological station connected to a datalogger (Campbell Scientific, model CR1000, Logan, UT, USA) installed near the experimental area, at a height of 2.0 m, recorded daily and entered into a spreadsheet.

$${ET}_O={\displaystyle \frac{0.408\left(Rn-G\right)+\gamma \frac{900}{T_2+273}{u}_2(e_s-e_a)}{\Delta +\gamma (1+0.34u_2)}}$$

(1)

where ETo—reference evapotranspiration, mm day⁻¹; Rn—net radiation at the crop surface, MJ m⁻² day⁻¹; G—soil heat flux density, MJ m⁻² day⁻¹; T₂—air temperature at 2 m height, °C; u₂—wind speed at 2 m height, m s⁻¹; e_s—saturation vapor pressure, kPa; e_a—actual vapor pressure, kPa; (e_s − e_a)—saturation vapor pressure deficit, kPa; Δ—slope of the saturation vapor pressure curve vs. temperature, kPa °C⁻¹; and γ—psychrometric constant, kPa °C⁻¹.

The crop evapotranspiration (ETc) was estimated according to Equation (2) [32]:

$${ET}_C={ET}_O{K}_C$$

(2)

where ETc—srop evapotranspiration, mm day⁻¹; ETo—reference evapotranspiration, mm day⁻¹; and Kc—crop coefficient, dimensionless, based on the crop’s phenological stage.

The irrigation depths were differentiated based on the irrigation time (T_i) established for each treatment (Equation (3)) every two days. The specific irrigation depths were determined after thinning, 20 days after emergence (DAE) [34]:

$$Ti={\displaystyle \frac{\left(F\right)\left({ET}_C\right)\left(S_{LL}{S}_e\right)}{q_e{E}_I}}60$$

(3)

where T_i—irrigation time, min; F—correction factor for the irrigation depth (0.50, 0.75, 1.00, 1.25, and 1.50 for L₁ = 50%, L₂ = 75%, L₃ = 100%, L₄ = 125%, and L₅ = 150% of ETc, respectively); ETc—crop evapotranspiration, mm day⁻¹; S_LL—spacing between lateral lines, m; S_e—spacing between emitters along the lateral line, m; q_e—emitter flow rate, L h⁻¹; E_I—irrigation system efficiency, decimal.

2.4. Planting Conditions

The genetic material used was sweet sorghum [Sorghum bicolor (L.) Moench] cultivar IPA 2502, a dual-purpose variety (grain and forage). The seeds were planted at a depth of 3.0 cm using a manual planter, with 3 to 5 seeds per hole. After thinning at 21 days after planting (DAP), the plant density was adjusted to 20 plants per linear meter.

2.5. Variables Analyzed

2.5.1. Gas Exchange

Leaf gas exchange rates were measured during the crop’s first cycle, at 81 days after planting (DAP), between 8:00 and 11:00 in the morning. Gas exchange rates were measured on a fully expanded leaf at the apex (the first being the most recently emerged), assessing stomatal conductance (gs, mol H₂O m⁻² s⁻¹), transpiration rate (E, mmol H₂O m⁻² s⁻¹), net CO₂ assimilation rate (A, μmol CO₂ m⁻² s⁻¹), and intercellular CO₂ concentration (Ci, μmol CO₂ mol⁻¹ air) at 130 DAE. The A, Ci, and E data were used to calculate the apparent carboxylation efficiency (CEa, μmol CO₂ m⁻² s⁻¹ ppm CO₂⁻¹) (ratio of A to Ci) and the instantaneous water use efficiency (WUEi, μmol CO₂ mmol H₂O⁻¹) (ratio of A to E). A portable infrared gas analyzer (ADC BioScientific Ltd., model LCpro+, Hoddesdon, UK) was used for the evaluations. Data was collected in an open system with the leaf chamber set to a photon flux density of 1500 ± 267 μmol m⁻² s⁻¹, since the device had no artificial light, an ambient temperature of 29 ± 1 °C, and an ambient CO₂ concentration.

2.5.2. Plant Growth

Sorghum biometric parameters were measured at 107 days after planting (DAP) in the first agricultural cycle and at 217 DAP in the second agricultural cycle. The following biometric parameters were evaluated: plant height (H, cm), measured from the plant collar to the first visible ligule/collar from the youngest leaf using a measuring tape; stem diameter (SD, mm), measured 3 cm above the collar in two perpendicular directions using a digital caliper; number of green leaves (LN), counted manually; and total leaf length (L, cm) and width (W, cm). The length of the leaves was measured from the collar (ligule) to the tip of the leaf, while the width was measured in the middle third of the leaf, and with the data of L and W, the leaf area of the plant (LA, cm² plant⁻¹) was estimated by the model proposed by [35], as follows in Equation (4), adding the leaf area of all the green leaves of the plant:

$$\sum _{i=1}^{n=LN}{LA}_{perleaf}=\left(0.7811\ast W\ast L\right)-14.964$$

(4)

2.5.3. Sweet Sorghum Harvest and Water Use Efficiency

The first sorghum harvest for fresh biomass assessment was performed at 110 days after planting (DAP), when the grains transitioned from the mushy to the hard stage. At this stage, all plants within a one-meter linear section of the effective sampling area were harvested 5.0 cm above the plant collar using sickles, then separated into stems, leaves, and panicles. The fresh mass of the stem (SFM, Mg ha⁻¹), leaves (LFM, Mg ha⁻¹), panicle (PFM, Mg ha⁻¹), and total fresh mass (TFM = SFM + LFM + PFM, Mg ha⁻¹) were then determined using a digital scale (0.5 g). The second harvest (ratoon cropping) occurred at 219 DAP, under similar grain maturity conditions as the first harvest (Figure 7). Three plants from each treatment were harvested during both the first and second cuts. Water use efficiency was calculated as the total fresh mass productivity (kg ha⁻¹) divided by the volume of water applied during irrigation (m³ ha⁻¹) [36], according to Equation (5):

$$WUE={\displaystyle \frac{TFM\left(kg{ha}^{-1}\right)}{watervolume\left(m^3{ha}^{-1}\right)}}$$

(5)

2.5.4. Juice Extraction and °Brix Determination

The leaves and panicles were separated from the stem to extract the sweet sorghum juice, which was pressed using a manual mill [37]. The soluble solids content (°Brix, %) was determined using a digital refractometer.

2.6. Data Analysis

Data were analyzed using Fisher’s analysis of variance (ANOVA) at a 0.05 significance level to evaluate the effects of irrigation treatments with reclaimed water on leaf gas exchange, growth, biomass accumulation, and sugar content in sweet sorghum. Polynomial models were fitted to the means when the irrigation treatments had a significant effect. Statistical analyses were carried out using SAS 9.0 for Windows [38]. In addition, a combined analysis was carried out to investigate the interactions between successive sweet sorghum cuts (plant and ratoon crop). Principal component analysis was used to explore the relationships between the variables measured (H, SD, LN, LA, TFM, WUE, and °Brix) and IR (50, 75, 100, 125, and 150% ETc) in both the first and second cuts (correlations between variables). The objective was to determine which variables from the initial cut might have influenced the growth and yield of the subsequent cut.

3. Results

3.1. Gas Exchange

Leaf gas exchange was not significantly affected by the irrigation regimes with reclaimed water (Figure 8), although a trend of increasing values with greater soil water availability was observed. The averages for stomatal conductance to water vapor, leaf transpiration rate, net CO₂ assimilation rate, and intercellular CO₂ concentration were 0.16 mol of H₂O m⁻² s⁻¹, 5.60 mmol of H₂O m⁻² s⁻¹, 21.0 µmol of CO₂ m⁻² s⁻¹, and 446.0 µmol of CO₂ mol of air⁻¹, respectively. The accumulated rainfall up to the period of the gas exchange measurements was 152 mm (Figure 8), which may have contributed to the lack of significant effects of the irrigation regimes due to the dilution effect of the treatments applied.

3.2. Plant Growth

No significant differences were observed in sorghum growth during the first cycle (○) (Figure 4), with plants averaging 225 cm in height, 25 mm in stem diameter, 9 leaves, and a leaf area of 6023 cm² per plant. However, the sorghum ratoon (●) was influenced by the irrigation regimes with reuse water, except for the number of leaves (Figure 9C), responding positively to the effects of increased water replenishment (greater irrigation depth) (Figure 9A,B,D).

Lower averages were observed for sorghum grown on ratoon (●), with plants showing a lower height (Figure 9A), stem diameter (Figure 9B), and leaf area (Figure 9D). The number of leaves was 7.65 per plant in the ratoon (Figure 9C). Irrigation regimes corresponding to 150% of sweet sorghum ETc provided maximum values of 233 cm, 18 mm, and 3048 cm² per plant for height, stem diameter, and leaf area, respectively.

3.3. Sweet Sorghum Harvest and Water Use Efficiency

The accumulation of fresh sorghum biomass was similar to the growth responses obtained. In the first cycle (○), there were no significant differences between the irrigation regimes for the fresh mass of the stem (Figure 10A), leaves (Figure 10B), panicle (Figure 10C), and total (Figure 10D), with averages of 25.87, 7.98, 1.36, and 35.22 Mg ha⁻¹, respectively.

Stem fresh mass, leaf fresh mass, and total fresh mass production of sorghum ratoon crop (●) increased with the application of the irrigation regimes, peaking at 106.25%, 112.5%, and 100% of crop evapotranspiration (ETc), respectively. Further increases in applied water resulted in reduced yields. The modeling indicated maximum yields of 10.70, 3.65, and 13.03 Mg ha⁻¹ for stem fresh mass (Figure 10A), leaf fresh mass (Figure 10B), and total fresh mass (Figure 10D), respectively. In addition, the fresh mass of the panicle (Figure 10D) of sweet sorghum was not affected by the irrigation regime with recycled water, producing an average of 0.18 Mg ha⁻¹, which represents 13% of the productivity obtained with the main crop (○) (1.36 Mg ha⁻¹).

Higher irrigation volumes reduced water use efficiency (WUE) in the first harvest, whereas in the second harvest (ratoon crop), WUE followed a quadratic response pattern. The highest WUE obtained in the first harvest was around 32.26 kg of the total fresh mass produced per m³ of recovered water applied at IR = 50% ETc, while at IR = 150% of ETc, WUE corresponded to 15.26 kg m⁻³, a value 52% lower than the deficit regime (Figure 11).

In the second harvest (ratoon crop), the maximum WUE was observed with the irrigation regime corresponding to 95% of ETc, resulting in an efficiency of 12.15 kg m⁻³, while the irrigation regime of 50% of ETc promoted a water use efficiency of 10.1 kg m⁻³, 17% lower than the irrigation regime corresponding to 95% of ETc (Figure 11).

3.4. Soluble Solids Content (°Brix)

Soluble solids content (°Brix) decreased with increasing irrigation depths in both harvests (Figure 12). The highest °Brix values were recorded under the deficit irrigation regime (50% ETc), reaching 14.23% in the first harvest and 12.77% in the second. The excessive irrigation regime (150% of ETc) reduced the soluble solids content to 11.23% and 9.77% in the first and second harvests, respectively (Figure 12).

3.5. Sorghum Cross-Crop Harvest Analysis

A comparison between sorghum from the first (main) harvest and the ratoon crop (second harvest) revealed significant differences in growth characteristics, biomass production, water use efficiency, and soluble solids content (Table 3). Plant height and water use efficiency (WUE) were significantly influenced (p < 0.05) by the interaction between irrigation regimes and sorghum harvests. However, stem diameter, number of leaves, leaf area, total productivity, and soluble solids content were not affected by this interaction. Only plant height and soluble solids content were directly influenced by irrigation regimes (

Table 4. Analysis of variance between crop harvest (sorghum plant and sorghum ratoon-cropping) of sweet sorghum under irrigation regimes with reclaimed water.

Effects	DF	Mean Square
		H	SD	LN	LA	TFM	WUE	Brix
IR	4	3171 *	10.02 NS	0.65 NS	906,923 NS	47.97 NS	184.97 *	13.06 **
error	15	651	4.06	1.61	710,550	54.19	40.238	1.71
CH	1	8873 **	814.82 **	36.10 **	130,064,567 **	6008.22 **	2155.83 **	25.33 **
CH × IR	4	918 *	11.95 NS	1.41 NS	850,117 NS	28.91 NS	66.49 *	0.06 NS
error	15	201	4.01	1.43	816,150	36.00	21.36	1.84

DF—degrees of freedom; IR—irrigation regime; CH—crop harvest; H—height; SD—stem diameter; LN—leaf number; LA—leaf area; TFM—total fresh mass; WUE—water use efficiency; **,*—significant at 1 and 5% probability, respectively. ^NS—not significant.

).

Sweet sorghum plants from the first harvest had thicker stems (Figure 13A), a greater number of leaves (Figure 13B), and, consequently, a larger leaf area (Figure 13C) and higher total fresh mass (Figure 13D). The first harvest (sorghum plant) resulted in average values of 25.80 mm, 9.55 leaves, 6074 cm² plant⁻¹, and 35.23 Mg ha⁻¹ for SD, LN, LA, and TFM, respectively. These averages were 35%, 20%, 59%, and 70% higher, respectively, than the corresponding values in the second crop.

A 12.5% increase in soluble solids content (°Brix) was observed in the second harvest, reaching 12.7%. In contrast, the first harvest produced a °Brix value of 11.07% for sweet sorghum (Figure 14).

In the first harvest, the sorghum plants were taller than those in the second harvest by 19.85%, 24.03%, 13.0%, 2.17%, and 5.36% under irrigation regimes of 50%, 75%, 100%, 125%, and 150% of crop evapotranspiration (ETc), respectively. However, the irrigation regimes did not significantly affect the sorghum height in the first harvest (

Table 5. Height measurements and water use efficiency of sugar sorghum in the first and second harvests under irrigation regimes with reclaimed water.

IR (%)	First Crop Harvest		Second Crop Harvest
	H (cm) ± SD	WUE (kg m−3) ± SD	H (cm) ± SD	WUE (kg m−3) ± SD
50	207.44 ± 13.32	33.55 ± 12.06	166.25 ± 8.88	8.76 ± 2.36
75	241.88 ± 20.38	27.00 ± 10.05	183.75 ± 21.55	13.07 ± 4.24
100	243.38 ± 0.25	21.18 ± 4.62	211.75 ± 11.98	9.10 ± 1.28
125	218.25 ± 13.32	17.17 ± 1.98	213.50 ± 5.00	6.46 ± 1.14
150	247.00 ± 51.04	16.31 ± 2.56	233.75 ± 16.58	4.41 ± 1.81

IR—irrigation regime; SD—standard deviation; H—height; WUE—water use efficiency.

). Water use efficiency was also significantly higher in the first harvest, with averages of 74, 52, 57, 62, and 73% higher than in the second harvest for irrigation regimes of 50, 75, 100, 125, and 150% of ETc, respectively (Table 5).

Figure 15 presents the principal component analysis (PCA) of the sugar sorghum cultivated under the different irrigation regimes with reclaimed water from treated domestic wastewater. PCA accounted for approximately 92% of the total variability (PC1 = 62.09 and PC2 = 29.83%) (Figure 15). The growth variables (H, SD, LN, and LA) in the second season were strongly influenced by the different irrigation regimes, while sorghum growth in the first season showed lower correlations.

Total fresh mass was positively correlated with the irrigation regimes evaluated; however, this correlation was weaker in the second season. On the other hand, water use efficiency (WUE) in the first harvest was negatively correlated with increased irrigation. In sorghum–soybean (second crop), the influence of irrigation regimes on WUE was insignificant. Similar to the WUE in the first harvest, the soluble solids content (°Brix) of the sugar sorghum showed a strong positive correlation with increased irrigation using domestic reclaimed water in both harvests (Figure 15).

4. Discussion

4.1. Gas Exchange

Irrigation with reclaimed water had no significant effect on stomatal conductance (Figure 8A), leaf transpiration rate (Figure 8B), net CO₂ assimilation (Figure 8C), or intercellular CO₂ concentration (Figure 8D). Our results corroborate those of [39], who evaluated different irrigation depths and electrical conductivities of water and observed no changes in leaf gas exchange (VPD, gs, E, and A) in the BRS 502 sorghum cultivar.

This response in sweet sorghum may be attributed to the 152 mm of accumulated rainfall during the study period. The rainfall, combined with the soil texture (clay), may have promoted excess water, masking the effect of the irrigation regimes studied in this research. These results indicate that deficit irrigation regimes (50 and 75% of ETc) with treated domestic effluent can be used for irrigation with recycled water in sugar sorghum without altering the physiological responses.

4.2. Plant Growth, Fresh Biomass, and Water Use Efficiency

Sorghum growth and fresh biomass in the first harvest were not significantly affected by irrigation with reclaimed wastewater. However, in the second harvest (ratoon crop), sorghum growth increased linearly with irrigation (Figure 9), while fresh biomass production showed a quadratic response, peaking at 106.25, 112.5, and 100% of ETc for SFM, LFM, and TFM, respectively (Figure 10). For the second cut of sweet sorghum, fresh biomass production under the deficit irrigation regimes was 7.53 Mg ha⁻¹ at 50% of ETc and 1.66 Mg ha⁻¹ at 75% of ETc, corresponding to water applications of 155 mm and 217 mm, respectively. Despite the lower volume of water, the regime of 75% of ETc showed greater efficiency in the use of water, resulting in greater productivity per unit of water applied, thus highlighting its potential as a more sustainable irrigation strategy in conditions of limited water availability.

Our results agree with those of [40], who reported no impact of treated wastewater irrigation on Sudan sorghum growth and dry matter in the first harvest but observed significant effects in subsequent harvests.

The effects of irrigation treatments on sorghum may have been masked by accumulated rainfall, reduced solar radiation, and cooler temperatures in the first harvest, which kept soil moisture high in all plots, especially due to the clay soil texture.

Normally, growing sorghum plants promotes better growth and fresh mass production (ideal for silage production); however, the effects of excess water from reclaimed sources can lead to a decrease in crop yield. This may be associated with root development in the second sorghum crop under conditions of excessive water stress (greater irrigation depths).

Water use efficiency (WUE) in the sorghum declined with increased soil water replenishment (higher irrigation depths), showing a negative correlation in the first harvest. In the second harvest, WUE decreased at irrigation depths above 95% of ETc. This observation corroborates the fact that excess water can reduce the amount of fresh mass produced, suggesting that deficit irrigation regimes with reclaimed water may be economically viable for growing sweet sorghum in semiarid conditions [18,41].

4.3. Soluble Solids Content (°Brix)

The soluble solids content decreased as the irrigation depth increased with domestic reclaimed water in both the first and second sweet sorghum harvests (Figure 12). Water stress can lead to an increase in the soluble solids content of sweet sorghum. Under osmotic stress conditions, such as water scarcity, the plant accumulates soluble sugars and amino acids, which function as osmolytes to mitigate the effects of stress and maintain cellular homeostasis. This fact is corroborated by [42], who found significant accumulations of soluble sugars such as raffinose, trehalose, glucose, sucrose, and melibiose in sweet sorghum under induced water stress.

4.4. Cross-Crop Harvest Analysis

In the first harvest, the sweet sorghum plants developed thicker stems, a higher number of leaves, a larger leaf area, and greater fresh biomass (Figure 13). In a study on sorghum yields in a semi-arid region Brazil, [40] found that sorghum yields varied with different depths of irrigation from treated domestic wastewater, with greater tillering in the second crop.

An increase in tillering can intensify intraspecific competition for resources such as light, water, and nutrients, potentially reducing overall growth and biomass production. This occurs because additional tillers may not fully develop or may compete with the main stem, decreasing resource use efficiency, as reported by [43,44].

In general, forage sorghum requires between 380 and 600 mm of water per growth cycle, with optimal development occurring around 26 °C [33]. In the present study, the first crop cycle recorded an average temperature of 29 °C and a total precipitation of 202 mm. Despite the limited rainfall, no significant effects of the irrigation treatments were observed on plant development. However, in the second cycle, environmental conditions were more favorable, with an average temperature of 26 °C and precipitation totaling 381 mm—closer to the crop’s ideal water demand range.

These findings highlight the importance of the interaction between the applied irrigation regimes, environmental conditions, and water quality in shaping the physiological and agronomic performance of sorghum. The lack of clear symptoms of water stress—except in the treatments using irrigation depths below 100% of crop evapotranspiration (ETc)—suggests that the species exhibits considerable phenotypic plasticity and tolerance to moderate water scarcity. This reinforces its role as a sustainable alternative for cultivation in water-limited regions.

Regarding water quality, the physical–chemical and microbiological parameters of the effluent used for irrigation remained within acceptable safety limits (Table 2), posing no constraint to plant growth. Furthermore, sorghum’s inherent resilience supports its viability in wastewater reuse systems.

It is also noteworthy that increased rainfall during the second crop cycle may have contributed to greater dilution of salts and organic compounds present in the effluent. This natural dilution effect reduces the risk of salinity build-up and enhances soil and environmental conditions—an especially relevant mechanism in the context of agricultural water reuse, where sustainability and ecological balance are critical.

Another important aspect concerns the root development observed during the second crop cycle. At this stage, the plants had a more established root system compared to the initial cycle (sowing phase), which led to increased tillering. However, this morphological response did not result in improved agronomic performance. In fact, higher tillering may have increased intraspecific competition for resources such as water and nutrients, thereby compromising the expression of desirable productive traits, such as high-quality biomass yield. This outcome highlights the need for appropriate management of water and nutrient inputs—even in resilient crops like sorghum—to avoid compensatory responses that could ultimately reduce productivity.

Moreover, the absence of soil fertilization in the second crop contributed to reduced nutrient availability, further exacerbating the stress conditions for sorghum growth. These findings highlight the greater stress experienced by the second sweet sorghum crop, leading to increased osmolyte accumulation and a higher soluble solids content, as shown in Figure 14. The principal component analysis (PCA) (Figure 15) further corroborates these results, demonstrating the impact of both excess and deficit irrigation on sorghum growth across the first and second harvests.

4.5. Risks of Wastewater Reuse in Agriculture: Heavy Metal Accumulation, Pathogen Load, and Future Perspectives

The reuse of wastewater in agriculture is a critical strategy for water-scarce regions, but risks linked to heavy metal contamination and pathogens require rigorous scientific and regulatory attention. Studies show that metals like cadmium (Cd), lead (Pb), and copper (Cu) can accumulate in soils and plant tissues, threatening food safety and human health [45,46]. For instance, long-term irrigation (8–12 years) with wastewater increased Cd and Pb levels in soils, exceeding safe thresholds and rendering the practice unsustainable [47]. Additionally, the uptake of pharmaceuticals such as diclofenac by crops [48] heightens toxicological risks, necessitating continuous monitoring.

The pathogen load (bacteria, viruses, helminth eggs) in untreated wastewater poses another challenge, particularly in low-income countries where intestinal infections correlate with contaminated irrigation [49]. The World Health Organization [30] recommends ≤1 viable helminth egg/L for non-food crops, but compliance is inconsistent, as seen in Uganda and Vietnam [50].

Given the potential risks associated with wastewater reuse in agriculture, it is important to emphasize that, in the present study, water quality data were monitored throughout the entire experimental period, as shown in Table 2. Heavy metal concentrations and thermotolerant coliform levels consistently remained within the safety thresholds established by national and international guidelines (FAO, WHO, and CONAMA), thereby minimizing agronomic, environmental, and public health risks. Furthermore, the effluent used originated exclusively from domestic sources, reducing the likelihood of emerging contaminants such as industrial pharmaceuticals. Therefore, despite theoretical concerns regarding long-term risks, the findings of this study demonstrate that, under controlled conditions and proper monitoring, the use of domestic reclaimed water poses low risk and presents high feasibility for agricultural applications.

In this context, sustainable wastewater reuse hinges on multidisciplinary approaches that balance agricultural benefits with risk mitigation, ensuring food and water security under climate change scenarios. Additionally, future perspectives should prioritize strategies such as remediation technologies, advanced treatments, and integrated policies.

5. Conclusions

This study demonstrated that irrigation with treated wastewater under different regimes can enhance the agronomic performance of sugar sorghum in semiarid regions, reinforcing its potential as a key strategy within the circular economy framework. Deficit irrigation regimes (50% and 75% of ETc) demonstrated higher water use efficiency and increased soluble solids content (°Brix), indicating their economic viability and technical suitability for producing high-quality biomass and silage. On the other hand, excessive irrigation reduced water use efficiency and fresh biomass accumulation, highlighting the need to optimize water management. The second crop (ratoon) showed greater sensitivity to water management, with significant responses in growth and productivity in regimes close to 100% of ETc.

This study highlights the potential of using reclaimed water as a sustainable strategy to address the challenges of water scarcity in semiarid regions. In addition to providing a viable alternative to irrigated agriculture, the use of deficit irrigation regimes can maximize the efficiency of available water resources, promoting more resilient and affordable agricultural production. With its resilience to adverse conditions and capacity to produce high-quality biomass, sweet sorghum can be effectively integrated into agricultural systems to enhance forage availability, reduce dependence on scarce natural resources, and strengthen both food security and environmental sustainability in semiarid regions.

Despite the limitations imposed by the absence of a control treatment using freshwater, the differentiated responses among the irrigation regimes and the compliance of the reclaimed water with quality standards support the validity of the obtained results. Nonetheless, future studies are encouraged to include an absolute control irrigated with freshwater to enhance the experimental design’s comparative strength and deepen the understanding of the agronomic and environmental effects associated with treated wastewater reuse.