Comparative Evaluation of Chia (Salvia hispanica) and Moringa (Moringa oleifera) Seeds as Green Coagulants for Sustainable Textile Wastewater Treatment

1. Introduction

The global textile industry generates an estimated 93 billion cubic metres of wastewater annually, with dyeing and finishing operations responsible for approximately 17–20% of global industrial water pollution [1]. In Bangladesh—the world’s second-largest garment exporter, contributing approximately 12% of GDP and 84% of export earnings—this environmental burden is acutely felt [2]. Textile dyeing effluents from industrial hubs such as Narayanganj, Gazipur, and Savar exhibit persistently elevated COD (mean ~524.6 mg/L) and biochemical oxygen demand (BOD, mean ~193.7 mg/L), both exceeding Department of Environment (DoE) Bangladesh discharge limits by over 100% [3]. Untreated or inadequately treated discharge into receiving water bodies—including the Buriganga, Turag, and Shitalakhya rivers—results in dissolved oxygen (DO) depletion, eutrophication, heavy metal bioaccumulation, and severe degradation of aquatic ecosystems and human livelihoods [4,5].

Conventional coagulation employs aluminum sulphate, ferric chloride, or poly-aluminum chloride (PAC) as primary coagulants. While effective, these synthetic agents generate significant volumes of metallic sludge requiring specialized disposal, may introduce residual aluminum into treated water at concentrations associated with neurotoxicity, and impose substantial cost barriers on SMEs in developing economies [6,7]. Growing concerns about environmental sustainability, regulatory compliance, and public health have intensified interest in plant-based bio-coagulants as eco-friendly alternatives [8].

Among bio-coagulants, Moringa oleifera Lam. (moringa) seeds have been the most extensively studied, owing to their well-characterized cationic storage proteins (principally the ~6.5 kDa dimeric protein MO2.1) that act as natural polyelectrolytes capable of neutralizing anionic surface charges of colloidal particles and promoting floc formation via charge neutralization and polymer bridging [9,10,11]. Salvia hispanica L. (chia) seeds, by contrast, have attracted comparatively less attention as wastewater coagulants despite their richness in anionic mucilaginous polysaccharides and cationic proteins, which have demonstrated significant COD (up to 62.4%) and turbidity (>90%) removal from compost leachate and fertilizer industry effluents via electrostatic interactions and hydrogen bonding mechanisms [12,13].

The literature reveals a specific gap: no direct head-to-head comparison of chia and moringa seeds as coagulants applied to real textile dyeing wastewater has been reported. Most previous studies either assess moringa against synthetic coagulants [14,15,16] or investigate chia seeds in non-textile wastewater matrices. Furthermore, characterization of the influence of pH conditioning and coagulant dosage on post-treatment dissolved oxygen—a key indicator of ecological impact—has been largely neglected in the context of direct bio-coagulant comparison.

This study addresses this gap by providing a systematic, comparative assessment of chia and moringa seed powders as coagulants for textile dyeing wastewater collected from two commercial facilities in Narayanganj, Dhaka. The specific objectives are to: (i) quantify COD removal efficiency and post-treatment DO across varying pH and dosage conditions; (ii) directly compare the coagulation performance of the two bio-coagulants using one-way ANOVA; and (iii) contextualize findings relative to current literature to inform scalable, sustainable textile wastewater management in Bangladesh.

Figure 1. Experimental workflow schematic for coagulation-flocculation treatment of textile dyeing effluents. Steps proceed from wastewater collection (Effluent 1: Amjad Dyeing Ltd.; Effluent 2: Fatullah Fabrics Ltd., Narayanganj, Bangladesh) through pH adjustment (0.1 M HCl or NaOH), coagulant preparation (moringa or chia seed powder, 0.05–1.0 g/250 mL), coagulation-flocculation (gentle stirring, 5 min), sedimentation (3–4 h), filtration (laboratory-grade filter paper), and analytical measurement of COD, DO, and pH according to APHA (2017) Standard Methods.

Figure 1. Experimental workflow schematic for coagulation-flocculation treatment of textile dyeing effluents. Steps proceed from wastewater collection (Effluent 1: Amjad Dyeing Ltd.; Effluent 2: Fatullah Fabrics Ltd., Narayanganj, Bangladesh) through pH adjustment (0.1 M HCl or NaOH), coagulant preparation (moringa or chia seed powder, 0.05–1.0 g/250 mL), coagulation-flocculation (gentle stirring, 5 min), sedimentation (3–4 h), filtration (laboratory-grade filter paper), and analytical measurement of COD, DO, and pH according to APHA (2017) Standard Methods.

2. Theoretical Background and Coagulation Mechanisms

2.1. Coagulation-Flocculation (CF) Process

Coagulation-flocculation is one of the most widely applied physicochemical unit operations in wastewater treatment, functioning by destabilizing colloidal suspensions and agglomerating fine particulates into larger settleable or filterable flocs [17]. Four principal mechanisms govern CF efficiency: (i) double-layer compression; (ii) charge neutralization; (iii) adsorption and inter-particle bridging; and (iv) sweep flocculation, wherein large amorphous precipitates enmesh and entrap colloidal particles [18]. The dominant mechanism under a given set of conditions depends on coagulant chemistry, dosage, pH, ionic strength, and the physicochemical character of the colloidal system.

2.2. Moringa oleifera Seed Coagulation Mechanism

The active coagulation principle in Moringa oleifera seeds is a cationic, water-soluble dimeric protein (~6.5 kDa) bearing net positive charges at neutral to alkaline pH. This protein interacts electrostatically with negatively charged colloidal particles in turbid and dye-laden textile effluents [19]. Additional bioactive components include rhamnosyloxy benzyl isothiocyanate, which possesses surface-active properties. Coagulation proceeds via adsorption of these proteins onto colloidal surfaces followed by polymer bridging between adjacent particles, ultimately producing large, dense flocs amenable to gravity sedimentation [20]. Maximum flocculation efficiency is typically observed at near-neutral to moderately alkaline pH (6.5–8.5), consistent with the cationic nature of the active protein and the anionic character of most textile dye colloids [21].

2.3. Salvia hispanica Seed Coagulation Mechanism

Salvia hispanica L. (chia) seeds contain both cationic proteins (~20–22% protein content by weight) and anionic mucilaginous polysaccharides (MW 0.8–2 MDa), which form a viscous hydrocolloid upon hydration [21]. The coagulation mechanism is dual in nature: (i) cationic proteins neutralize the anionic surface charges of colloidal contaminants via electrostatic interactions, while (ii) the anionic polysaccharide mucilage promotes inter-particle bridging and sweep flocculation via hydrogen bonding and physical entrapment [22,23]. Additionally, chia seeds release calcium ions upon hydration, which may promote chemical precipitation and calcium-bridging of certain contaminants. The net coagulation efficiency is consequently highly sensitive to pH, dosage, and the ionic environment of the wastewater matrix.

3. Materials and Methods

3.1. Wastewater Sampling and Characterization

Textile dyeing effluents were collected from two industrial facilities in Narayanganj, Dhaka, Bangladesh: Effluent 1 from Amjad Dyeing Ltd. (reactive dye operations, predominantly cotton processing; initial COD = 2100 mg/L) and Effluent 2 from Fatullah Fabrics Ltd. (mixed dye formulations, synthetic fabric finishing; initial COD = 2200 mg/L). Samples were collected in acid-washed high-density polyethylene (HDPE) containers and transported to the laboratory within 4 hours of collection. Samples were stored in sealed opaque PVC containers in a dark environment at temperature (~5 °C) prior to analysis. Baseline characterization included pH, COD, and DO measurements as described below.

3.2. Preparation of Coagulants

3.2.1. Moringa Seed Powder

Mature Moringa oleifera seeds were obtained from local suppliers in Dhaka, Bangladesh. The seeds were manually dehulled, cleaned with distilled water to remove adhering impurities, and dried prior to processing. The dried kernels were ground into a fine powder using a laboratory grinder and stored in airtight, light-protected containers at room temperature until use in the coagulation experiments [24]. No chemical extraction or modification was employed, consistent with maximizing simplicity and scalability for field applications.

3.2.2. Chia Seed Powder

Raw mature Salvia hispanica L. (chia) seeds were air-dried under direct sunlight for approximately 7 h. The dried seeds, including the seed coat, were ground into a fine powder using a laboratory-grade electric grinder. The resulting powder was stored in airtight, light-protected containers at ambient temperature until use. Similar seed preparation and storage procedures have been reported for plant-based coagulant materials [25].

3.3. Coagulation-Flocculation Procedure

Experiments were conducted using 250 mL aliquots of textile effluent in 500 mL conical flasks. Pre-weighed quantities of coagulant powder (0.05–1.0 g per 250 mL, equivalent to 0.2–4.0 g/L) were added directly as whole-seed powders without prior solvent extraction. Where required, the initial pH was adjusted using 0.1 M HCl or 0.1 M NaOH. Following coagulant addition, the suspensions were mixed for 5 min and allowed to settle under gravity for 4 h. The clarified supernatant was decanted, filtered, and subsequently analysed for COD, DO, and pH according to Standard Methods. Coagulation–flocculation experiments were performed following the general principles of laboratory jar testing [26,27].

3.4. Analytical Methods

3.4.1. COD Determination

COD was measured using the modified open-reflux dichromate method adapted from APHA Standard Methods (23rd ed., 2017) [28]. Briefly, 1 mL sample aliquots were added to conical flasks containing 5 mL of 0.25 N K₂Cr₂O₇, 1 mg HgSO₄, and 15 mL of H₂SO₄-Ag₂SO₄ digestion solution. Flasks were heated in a water bath at 95–100 °C for 2 hours. After cooling, unreacted dichromate was back-titrated with 0.1 N ferrous ammonium sulphate (FAS) solution using ferroin indicator. COD was calculated as:

COD (mg O₂/L) = [(A − B) × M × 8000] / V

(1)

where A = volume of FAS for blank (mL), B = volume of FAS for sample (mL), M = molarity of FAS, and V = sample volume (mL). COD removal efficiency (%) was calculated as: % removal = [(COD_initial − COD_final) / COD_initial] × 100.

3.4.2. Dissolved Oxygen (DO) Determination

DO was determined using the Winkler azide modification method [29]. Manganese sulphate (MnSO₄), alkali-iodide-azide (AIA), and sodium thiosulphate (Na₂S₂O₃) were used as sequential reagents. DO was calculated as:

DO (mg/L) = [mL titrant × N(Na₂S₂O₃) × 8 × 1000 × V₁] / [V₂ × (V₁ − V)]

(2)

where V₁ = Winkler bottle volume (mL), V₂ = volume titrated (mL), V = combined volume of MnSO₄ and AIA reagents added (mL).

3.5. Statistical Analysis

One-way analysis of variance (ANOVA) was applied to compare mean COD removal efficiencies between MS (n = 11) and CS (n = 22) treatments pooled across both effluent types and all pH/dosage conditions (total N = 33). Statistical significance was accepted at α = 0.05. COD removal efficiency (%) was the primary dependent variable.

All computations were verified independently and are reported with correct degrees of freedom (df_between = 1, df_within = 31, df_total = 32). Note: descriptive statistics (means and ranges) are reported separately by effluent type (Table 4) to account for the distinct initial COD concentrations of each effluent.

4. Rseults

4.1. Moringa Seed (MS) Coagulation Performance

Table 1 summarizes the coagulation performance of moringa seed powder across both effluent types under varied pH and dosage conditions. Effluent 1 (initial COD = 2100 mg/L) showed MS COD removal ranging from 84.76% to 96.19%, with peak removal (96.19%) recorded at pH 1 and a dose of 0.10 g/250 mL (0.4 g/L), yielding a residual COD of 80 mg/L—approaching Bangladesh DoE discharge limits. Under neutral and alkaline conditions in Effluent 1, COD removal remained consistently high (84.76–92.38%), demonstrating broad pH tolerance.

In Effluent 2, adding sub-threshold amounts of 0.1 M HCl failed to lower the pH below 7, showing that the mixed-dye industrial effluent possessed a strong natural buffering capacity. Furthermore, duplicate testing under identical alkaline conditions (pH 10) showed high variability (52.73% vs 96.36), which was verified against raw laboratory records and retained to accurately represent real-world effluent batch heterogeneity.

In Effluent 2 (initial COD = 2200 mg/L), MS performance was more variable. Under alkaline conditions (pH 8–10) at 0.75 g/250 mL, COD removal ranged from 52.73% to 96.36%. The lowest removal in Effluent 2 (20.00%) was observed under acidic pre-treatment conditions (pH 7 with HCl reagent; note: the reported pH₀ = 7 for this condition indicates the post-adjustment pH was neutral, consistent with HCl being used at sub-threshold amounts). Two runs under apparently identical conditions (NaOH, 0.75 g, pH₀ = 10, Effluent 2) yielded divergent results (52.73% vs. 96.36%), suggesting experimental variability or possible batch-to-batch effluent heterogeneity; authors are requested to verify and clarify this data point. Post-treatment DO levels under MS treatment ranged from 5.71 to 14.28 mg/L across both effluents, consistently exceeding the ecologically critical threshold of 5.0 mg/L.

4.2. Chia Seed (CS) Coagulation Performance

Table 2 presents the coagulation performance data for chia seed powder. In Effluent 1, CS performance was markedly more variable than MS, with COD removal ranging from 8.57% to 88.57%. The lowest removal (8.57%) was observed under strongly acidic conditions (pH 1, 0.05 g dose), indicating that chia seed coagulation is highly sensitive to extreme acid conditions—likely due to disruption of the polysaccharide mucilage network and denaturation of bridging polymers at very low pH. Under near-neutral to alkaline conditions, CS achieved competitive removal of 54.29–88.57% in Effluent 1.

In Effluent 2, CS performance improved substantially under neutral to mildly acidic conditions, with 81.82–85.45% removal at pH 2–6 across dosages of 0.10–1.0 g/250 mL. The maximum CS removal (96.36%) was achieved in Effluent 2 at pH 10 (alkaline), matching the peak moringa performance. Notably, CS removal in Effluent 2 at pH 6 was identical across dosages of 0.10, 0.50, and 1.00 g (all yielding 81.82%), demonstrating dose-insensitivity within the neutral pH range—a practically important observation. Post-treatment DO values ranged from 3.80 to 9.31 mg/L in Effluent 1 and 6.60 to 10.47 mg/L in Effluent 2. Importantly, two CS experiments on Effluent 1 under NaOH conditions yielded DO values below 5.0 mg/L (3.80 and 4.76 mg/L), suggesting potential biochemical oxygen consumption by mucilage degradation.

4.3. Statistical Comparison: One-Way ANOVA

To assess whether the difference in mean COD removal efficiency between MS and CS treatments was statistically significant, a one-way ANOVA was conducted pooling all experimental runs (MS: n = 11; CS: n = 22; total N = 33). Table 3 presents the ANOVA summary. The group means were: MS = 80.86% and CS = 67.85% (grand mean = 72.18%). The between-group sum of squares was SS_between = 1241.6, with SS_within = 18070.9 and SS_total = 19312.5.

The analysis revealed no statistically significant difference between MS and CS coagulation performance [F(1,31) = 2.13, p = 0.155 > 0.05]. The F-critical value at α = 0.05 with df₁ = 1 and df₂ = 31 is 4.16, which the observed F of 2.13 did not exceed. The numerical difference in group means (~13 percentage points), while practically noteworthy, was not confirmed as statistically significant given the high within-group variance across pH and dosage conditions.

4.4. Comparative Summary of Key Performance Metrics

Table 4 provides a consolidated summary of corrected performance metrics for both coagulants across both effluent types. All means have been recalculated from the raw data in Table 1 and Table 2. Note that mean COD removal differs substantially between effluents for the same coagulant, reflecting differences in initial COD concentration, dye matrix complexity, and pH conditioning protocols.

4.5. Effect of pH on COD Removal

pH exerted a significant modulatory effect on both coagulants (Figure 2). For MS in Effluent 1, peak COD removal (96.19%) was achieved at pH 1 under HCl pre-treatment at 0.10 g dose. This may reflect enhanced protonation of amine groups on seed proteins, augmenting cationic character and improving charge neutralization under the specific colloidal chemistry of Effluent 1. However, this peak was not reproduced at the lower dose (0.05 g, pH 1: 88.57–92.38%), suggesting a dose-pH interaction. Under alkaline conditions (pH 7–10), MS maintained 84.76–92.38%, demonstrating broad pH tolerance.

For CS, strongly acidic conditions were consistently detrimental, with Effluent 1 at pH 1 yielding only 8.57% removal at 0.05 g dose. Neutral to mildly alkaline conditions (pH 6–10) provided optimal CS performance, with removal of 54.29–96.36%. Importantly, the pH was stable before and after filtration in most experimental conditions, indicating that the coagulation process itself did not substantially alter solution pH—an important consideration for post-treatment discharge compliance.

Figure 3. Effect of coagulant dose (g/250 mL) on COD removal efficiency (%) for Moringa seed (MS, circles) and Chia seed (CS, squares) in (a) Effluent 1 and (b) Effluent 2. Marker edge colour indicates pH regime: orange = acidic (pH ≤2), green = neutral (pH 3–8), purple = alkaline (pH >8). Dotted line = 80% removal threshold.

Figure 3. Effect of coagulant dose (g/250 mL) on COD removal efficiency (%) for Moringa seed (MS, circles) and Chia seed (CS, squares) in (a) Effluent 1 and (b) Effluent 2. Marker edge colour indicates pH regime: orange = acidic (pH ≤2), green = neutral (pH 3–8), purple = alkaline (pH >8). Dotted line = 80% removal threshold.

Figure 4. Post-treatment dissolved oxygen (DO, mg/L) versus coagulant dose (g/250 mL) for Moringa seed (MS, circles) and Chia seed (CS, squares) coagulants in (a) Effluent 1 (nᴹᴸ = 7, nᶜᴸ = 10) and (b) Effluent 2 (nᴹᴸ = 4, nᶜᴸ = 12). Dashed red line = ecological threshold of 5.0 mg/L DO. Dash-dot lines show group means. Shaded green region = ecologically safe zone (DO > 5.0 mg/L).

Figure 4. Post-treatment dissolved oxygen (DO, mg/L) versus coagulant dose (g/250 mL) for Moringa seed (MS, circles) and Chia seed (CS, squares) coagulants in (a) Effluent 1 (nᴹᴸ = 7, nᶜᴸ = 10) and (b) Effluent 2 (nᴹᴸ = 4, nᶜᴸ = 12). Dashed red line = ecological threshold of 5.0 mg/L DO. Dash-dot lines show group means. Shaded green region = ecologically safe zone (DO > 5.0 mg/L).

Figure 5. Distribution of COD removal efficiency (%) by coagulant type and effluent source. (a) Box-and-whisker plots grouped by coagulant-effluent combination; (b) Pooled one-way ANOVA comparison of Moringa (MS, n = 11) vs. Chia seeds (CS, n = 22). Box = interquartile range; horizontal line = median; ◆ = group mean; whiskers = full data range; points = individual observations (jittered). ANOVA: F(1,31) = 2.13, p = 0.155 (not significant, n.s.); 80% threshold indicated by dotted line.

Figure 5. Distribution of COD removal efficiency (%) by coagulant type and effluent source. (a) Box-and-whisker plots grouped by coagulant-effluent combination; (b) Pooled one-way ANOVA comparison of Moringa (MS, n = 11) vs. Chia seeds (CS, n = 22). Box = interquartile range; horizontal line = median; ◆ = group mean; whiskers = full data range; points = individual observations (jittered). ANOVA: F(1,31) = 2.13, p = 0.155 (not significant, n.s.); 80% threshold indicated by dotted line.

5. Discussion

5.1. Moringa Seed Performance and Mechanism

The broadly consistent and high COD removal performance of Moringa oleifera seeds across pH 1–10 in this study is consistent with the extensive literature documenting moringa’s effectiveness across diverse wastewater matrices [30]. The cationic dimeric protein (MO2.1, ~6.5 kDa) functions primarily through charge neutralization of anionic colloidal dye particles—a mechanism particularly effective in the pH range of 6.5–10, where the protein retains cationic character and textile dye colloids maintain negative zeta potentials [31]. The observed peak performance under acidic conditions (96.19% at pH 1 for Effluent 1) may reflect enhanced protonation of terminal amine groups at very low pH, transiently amplifying cationic character; however, this interpretation requires verification through zeta-potential measurements, which were not conducted in the present study and are recommended for future work.

The markedly lower MS performance in Effluent 2 under the HCl pre-treatment condition (20.00% at pH 7 post-adjustment) warrants specific discussion. The reported pH₀ = 7 for this condition (rather than pH 1–2) indicates that the HCl pre-treatment did not strongly acidify this effluent—possibly due to its higher buffering capacity—meaning this data point represents near-neutral rather than acidic performance. This is consistent with the complex mixed dye formulation of Effluent 2 (Fatullah Fabrics Ltd.) and underscores the importance of effluent buffering capacity as a variable in coagulation performance.

The elevated mean post-treatment DO under MS treatment (10.45 mg/L for Effluent 1; 9.92 mg/L for Effluent 2) relative to pre-treatment textile dyeing wastewater DO (typically <2 mg/L due to high organic loading) indicates that effective COD removal substantially reduces the biochemical oxygen demand on the water column, permitting DO recovery toward saturation. This has important implications for discharge into receiving water bodies: treated effluent with DO > 5 mg/L is far less likely to cause acute ecological harm to aquatic biota in the Buriganga and Shitalakhya rivers, which currently suffer critical hypoxia from untreated industrial discharge [32].

5.2. Chia Seed Performance: Strengths and Limitations

The performance of Salvia hispanica seeds as coagulants was more variable overall but reached equivalently high removal efficiencies (96.36%) under optimised alkaline conditions in Effluent 2. The dual mechanism of CS—combining cationic protein charge neutralisation with anionic mucilage bridging—renders its activity more sensitive to ionic environment than the more narrowly targeted protein-based mechanism of moringa [33]. Under strongly acidic conditions (pH 1), the hydrocolloid network collapses, effectively eliminating the bridging component and leaving only partial charge neutralization insufficient for adequate coagulation, particularly at lower dosages.

The coagulation matrix of CS showed distinct threshold dependencies under extreme conditions. At pH 1, a low dose (0.05 g) was entirely insufficient to overcome acid-induced mucilage collapse, yielding a negligible 8.57% COD removal. However, doubling the dose to 0.10 g triggered an interactive polymer-bridging effect that restored removal efficiency to 84.76%. Furthermore, the introduction of organic polysaccharides through raw seed addition caused minor post-treatment dissolved oxygen depletion (3.80 mg/L) at neutral pH matrices. This indicates a localized oxygen demand caused by remaining dissolved mucilage fractions, representing a process limitation. This limitation was completely resolved under optimized alkaline conditions (pH 10), where CS achieved peak efficiencies of 96.36% without affecting dissolved oxygen stability.

A practically significant finding is CS dose-insensitivity at neutral pH in Effluent 2: equivalent COD removal (81.82%) was achieved across 0.10, 0.50, and 1.00 g doses at pH 6, suggesting that within the economically and ecologically relevant neutral pH range, CS performance is robust to significant under- or over-dosing. This is consistent with Tawakkoly et al. (2019) [13], who reported RSM-optimised removal of 62.4% using chia mucilage from compost leachate, with pH and contact time as dominant optimisation variables. The recent study by Silva RMP Da et al. (2026) [12] reported 85.7% COD removal using chia mucilage from fertilizer industry wastewater at pH 7 and 0.05 g/100 mL—comparable to the 80.95–88.57% achieved in the present study under similar neutral-pH conditions—lending cross-study confidence to the observed performance envelope.

5.3. Statistical Interpretation and Revised Conclusions

The one-way ANOVA [F(1,31) = 2.13, p = 0.155] demonstrates that the mean COD removal difference between MS (80.9%) and CS (67.8%) across all pooled conditions is not statistically significant at the α = 0.05 level. The absence of statistical significance does not indicate equivalent performance under all conditions; rather, it reflects that (i) the within-group variance is high due to the wide range of pH and dosage conditions deliberately studied; and (ii) both coagulants exhibit considerable condition-dependent variability. Qualitatively, MS demonstrates more consistent high performance across the full pH range tested, particularly in Effluent 1, while CS exhibits greater pH sensitivity but comparable peak performance under optimized conditions.

5.4. Contextualization with Literature

Table 5 situates the present findings within the recent literature. The COD removal achieved by moringa seeds in this study (up to 96.36%) substantially exceeds that reported by Rendana et al. (2025) [34] using moringa combined with anionic polyacrylamide (PAM) in textile wastewater (~29% COD), and is consistent with the upper range reported by Worku and Abate (2025) [35] for moringa versus cactus pads (up to 88%) under optimized jar-test conditions. The higher removal observed in the present study relative to Worku and Abate likely reflects the very high initial COD (2100–2200 mg/L) and specific colloidal composition of Bangladesh reactive-dye effluents, which may be more amenable to charge-neutralization coagulation than the Ethiopian textile effluents studied by Worku and Abate.

Figure 6. Pearson correlation heatmaps for the four primary study variables—initial pH, coagulant dose (g/L), COD removal (%), and post-treatment DO (mg/L)—for (a) Moringa seeds (MS, n = 11), (b) Chia seeds (CS, n = 22), and (c) all data pooled (n = 33). Colour scale: blue = negative, red = positive Pearson r. Values in cells are Pearson r coefficients; * indicates p < 0.05.

Figure 6. Pearson correlation heatmaps for the four primary study variables—initial pH, coagulant dose (g/L), COD removal (%), and post-treatment DO (mg/L)—for (a) Moringa seeds (MS, n = 11), (b) Chia seeds (CS, n = 22), and (c) all data pooled (n = 33). Colour scale: blue = negative, red = positive Pearson r. Values in cells are Pearson r coefficients; * indicates p < 0.05.

Figure 7. Multi-metric performance summary comparing Moringa (MS) and Chia (CS) seed coagulants. (a) Radar chart of normalized performance metrics (0–100 scale): maximum COD removal, minimum COD removal inverse (resilience), mean post-treatment DO, pH tolerance breadth, and dosage robustness. (b) Mean COD removal ± SD by coagulant-effluent group. (c) Mean post-treatment DO ± SD by group; dashed red line = 5.0 mg/L ecological threshold.

Figure 7. Multi-metric performance summary comparing Moringa (MS) and Chia (CS) seed coagulants. (a) Radar chart of normalized performance metrics (0–100 scale): maximum COD removal, minimum COD removal inverse (resilience), mean post-treatment DO, pH tolerance breadth, and dosage robustness. (b) Mean COD removal ± SD by coagulant-effluent group. (c) Mean post-treatment DO ± SD by group; dashed red line = 5.0 mg/L ecological threshold.

5.5. Environmental and Practical Implications for Bangladesh

Bangladesh’s garment and dyeing sector operates under significant and growing environmental pressure. The government introduced stricter wastewater discharge regulations in 2023 mandating installation and operation of effluent treatment plants (ETPs) in all textile facilities, yet compliance remains incomplete, particularly among SMEs . Natural coagulants derived from locally sourced or readily importable plant seeds offer a scalable, cost-effective first-stage treatment option that could be integrated ahead of biological treatment in ETP systems.

Moringa oleifera seeds, while not cultivated at large scale within Bangladesh, are increasingly available through regional agricultural channels from South and Southeast Asia. Their superior and more consistent pH-independent performance makes them the preferred primary recommendation. Chia seeds, predominantly imported and relatively expensive, may be economically less competitive at scale, but their comparable peak performance under neutral pH and dose-insensitivity in Effluent 2 suggest a complementary role, particularly in effluent streams with naturally neutral pH that do not require chemical pre-adjustment. Both bio-coagulants generate biodegradable, heavy-metal-free sludge—a significant advantage over aluminum- or iron-based coagulant sludges—and the spent biomass potentially has value as soil amendment or compost input.

5.6. Limitations and Future Directions

This study was conducted at laboratory scale (250 mL batch experiments) without pilot-scale or continuous-flow validation. Several important limitations should be noted: (i) no turbidity, total suspended solids (TSS), colour/absorbance, heavy metal, or BOD measurements were performed, limiting the comprehensiveness of treatment characterization; (ii) zeta-potential measurements were not made, precluding direct mechanistic verification of charge-neutralization claims; (iii) coagulant was applied as whole seed powder rather than purified active fraction, introducing variability in active compound concentration; (iv) jar test mixing energy (G-value and Camp number) was not quantified, limiting reproducibility; and (v) replication at identical conditions was limited, making it difficult to distinguish experimental variability from true treatment differences (see Table 1 and Table 2 entries).

Future studies should: (i) characterize initial effluent quality comprehensively (turbidity, colour, TSS, TOC, heavy metals, zeta potential, and BOD₅); (ii) employ response surface methodology (RSM) or central composite design for systematic optimization of pH, dosage, mixing energy, and contact time; (iii) quantify protein and mucilage content of both seeds and characterize active coagulation fractions; (iv) standardize jar-test mixing (rapid/slow phases, G-value); (v) assess sludge volume, dewatering properties, and valorization potential; and (vi) conduct pilot-scale and cost-effectiveness analyses relative to PAC.

6. Conclusions

This study provides the direct, statistically evaluated comparison of chia (Salvia hispanica) and moringa (Moringa oleifera) seeds as natural coagulants for textile dyeing wastewater treatment under systematic pH and dosage variation. The principal conclusions are:

• Moringa seeds (MS) demonstrated broadly consistent and high COD removal efficiencies across pH 1–10 (84.76–96.19% in Effluent 1; 20.00–96.36% in Effluent 2), while chia seeds (CS) showed greater pH sensitivity (8.57–88.57% in Effluent 1; 23.64–96.36% in Effluent 2), particularly under strongly acidic conditions.
• One-way ANOVA across all pooled conditions (MS n = 11, CS n = 22, N = 33) revealed no statistically significant difference in mean COD removal [F(1,31) = 2.13, p = 0.155]; the observed mean difference (~13 percentage points) reflects within-group variance arising from the deliberately wide pH and dosage ranges tested rather than a consistent performance advantage.
• Post-treatment dissolved oxygen was consistently higher under MS treatment (mean 10.45 mg/L, Effluent 1; 9.92 mg/L, Effluent 2) compared to CS treatment (6.91 mg/L; 8.30 mg/L), indicating superior post-treatment ecological compatibility of moringa-treated effluent. Two CS experiments yielded DO below the 5.0 mg/L ecological threshold.
• Both coagulants achieved equivalent peak COD removal (96.36%) under optimized alkaline conditions in Effluent 2, confirming that chia seeds are capable of moringa-equivalent performance under appropriate pH control.
• This study was conducted at laboratory scale. Batch experiments were used instead of a continuous system. Wastewater composition may vary across sources. Only short-term treatment performance was evaluated. Long-term floc stability was not assessed. Sludge characterization was not included. Mechanistic analysis was based on literature support.
• Chia seeds demonstrated dose-independent performance at neutral pH in Effluent 2 (81.82% across 0.10–1.0 g doses), which is practically advantageous where precise dosage control is difficult.
• Both natural coagulants represent environmentally sustainable alternatives to synthetic chemical coagulants, generating biodegradable, heavy-metal-free sludge, with applicability at SME scale in the Bangladesh textile sector, subject to further optimization and pilot-scale validation.