Nature-Based Solutions: Evaluation of Natural Plant-Derived Coagulants for Sustainable Water Treatment

Abstract

This study evaluates the performance of natural plant-derived coagulants as sustainable alternatives to conventional chemical coagulants in water treatment. Surface water samples were collected from the Meda Ela stream in Karadiyana, Sri Lanka, which is an urban water body impacted by leachate from the Karadiyana dumpsite, industrial discharges, and urban runoff. Grab samples were analyzed for key water quality parameters, including pH, conductivity, turbidity, dissolved oxygen (DO), chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), settleable solids, total solids (TS), total dissolved solids (TDS), total suspended solids (TSS), total nitrogen, and total phosphorus. Several parameters exceeded permissible standards established by the Central Environmental Authority (CEA) of Sri Lanka, including turbidity (35 NTU; limit: 20 NTU), COD (80 mg/L; limit: 15 mg/L), TDS (1000 mg/L; limit: 500 mg/L), and TSS (100 mg/L; limit: 40 mg/L), indicating significant pollution levels. Jar test experiments were conducted to compare the coagulation efficiency of cowpea seeds (75.8%), fenugreek seeds (69.2%), papaya seeds (72.5%), okra pods (84.6%), and Moringa oleifera (drumstick) leaves (87%) with conventional alum (94.2%) at an optimum dosage of 12 mL/L. Among the tested plant-derived coagulants, Moringa oleifera leaves demonstrated the highest turbidity removal efficiency, reducing residual turbidity to 4.54 NTU. A low-cost integrated treatment system incorporating coagulation, flocculation, sedimentation, and filtration using sawdust and cotton wool was developed, achieving average removal efficiencies of 90.13% for turbidity, 88.57% for COD, 83.46% for TDS, and 74.83% for TSS, with all effluent parameters maintained within CEA permissible limits. The results confirm that locally available plant-derived coagulants, particularly Moringa oleifera leaves, offer an effective, environmentally friendly, and economically viable approach for sustainable water treatment, highlighting the potential of nature-based solutions in strengthening climate-resilient water management strategies.

1. Introduction

The degradation of receiving water bodies due to various pollutant sources such as urban stormwater runoff, industrial effluents, and commercial discharges has become a critical environmental concern and significantly affecting urban populations [1,2,3,4]. Globally, water pollution leads to widespread diseases and approximately 505,000 diarrheal deaths annually from contaminated drinking water [5,6]. Although water quality is influenced by natural factors such as climate, vegetation cover, soil characteristics, geological conditions, and precipitation patterns, anthropogenic activities represent the most significant threat to the quality of receiving water bodies [7,8]. Point source pollution from industrial and municipal discharges, together with non-point sources such as nutrients, sediments, and hazardous substances transported through runoff, substantially deteriorate surface water quality.

Further, climate change intensifies these vulnerabilities through intensified precipitation events that amplify surface runoff carrying sediments, nutrients, and hydrocarbons and altered hydrological cycles that increase leachate mobilization from waste sites to nearby receiving water bodies [9,10]. In many developing countries, most water treatment plants abstract raw water from rivers [11] and rely primarily on conventional treatment processes due to their economic feasibility and operational simplicity [12]. However, increasing pollutant loads are placing significant stress on these treatment systems, challenging their efficiency and imposing additional pressure on regulatory authorities to maintain safe drinking water standards [13,14].

Chemical coagulants are widely employed in industrial water treatment plants for effective coagulation–flocculation to remove turbidity, colloidal particles, and organic matter from raw water. The most common include aluminum sulphate (alum, Al₂(SO₄)₃·18H₂O), which hydrolyzes to form cationic species that neutralize negatively charged particles at optimal pH 6–7 and dosages of 10–20 mg/L, achieving up to 99% turbidity removal [15]. Ferric chloride (FeCl₃·6H₂O) is effective at pH 5–6 with similar dosages yielding 92–99.4% removal and superior performance in high-turbidity waters [13,16,17]. Poly aluminum chloride (PAC) is a pre-hydrolyzed polymer offering wider pH tolerance and reduced sludge production. These salts destabilize colloids via charge neutralization and sweep flocculation, though overdosing risks restabilization, and residuals raise health concerns due to aluminum accumulation, such as impacting the cardiac, pulmonary, reproductive, gastrointestinal, and hematological systems in human bodies [18,19]. Moreover, residual aluminum in treated water has been linked to potential neurotoxic effects when present above permissible limits, while synthetic polymer coagulants may contain toxic monomers such as acrylamide, which is considered neurotoxic and potentially carcinogenic.

Despite its high efficiency in turbidity removal, chemical coagulation raises significant economic concerns [20,21] in addition to health concerns. Conventional coagulants such as alum and ferric salts produce large volumes of metal-rich sludge, which require costly treatment, handling, and disposal, thereby increasing the overall operational expenses of water treatment plants.

Continuous chemical purchasing further adds to long-term costs, particularly in developing countries, which are under pressure due to economic crises. These economic burdens and potential risks have therefore encouraged increasing research interest in safer and more sustainable natural coagulants as alternatives to chemical coagulants.

Table 1. Summary of past research on water treatment methods using natural coagulants.

Author	Natural Coagulant Used	Type of Water Treated	Outcomes	Remark
[22]	Moringa oleifera seeds (MOS)	Detergent wastewater	95.6% turbidity, 71.3% LAS removal; better than alum	Effective, low-cost alternative
[23]	Catharanthus roseus (CR), Ocimum tenuiflorum (OT)	Aquaculture wastewater	CR roots: 99% turbidity; blend removed nutrients	Eco-friendly plant coagulants
[13]	Moringa oleifera seeds	Municipal wastewater	87% turbidity, 67% COD; biodegradable sludge	Sustainable alum alternatives
[24]	Banana peels	Domestic wastewater	52.8% turbidity; stable pH	Suitable for primary treatment
[25]	Fenugreek, Cumin	Pond water	Fenugreek ~80% efficiency; stable pH	Fenugreek effective
[26]	Papaya seed powder, Tamarind seeds, Orange peels, Neem leaves	Municipal wastewater	Neem 99.9% turbidity; orange economical	Neem most efficient
[27]	Fenugreek powder, Neem leaves powder, Custard apple seed powder	Dairy effluent	Fenugreek most effective (COD ↓ ~50%)	Promising natural options
[28]	Okra seeds	Mining wastewater	83% turbidity reduction; faster settling	Low-cost alternatives
[29]	Chicken eggshell	Domestic wastewater	Increased TDS; less effective than PAC	Better as coagulant aid
[30]	Moringa oleifera seed powder	Rural water	90–99% impurity removal; bacteria removal	Comparable to alum
[31]	Moringa oleifera with polymer	Improve coagulation/ flocculation process and reduce sedimentation time	85% turbidity; sedimentation ↓ to 15 min	Dual-system effective
[32]	Moringa peregrina, Moringa oleifera	Pre-treatment of olive mill wastewater (OMW)	35% phenol removal; comparable to alum	Eco-friendly pre-treatment
[33]	Okra (seed/leaf)	Synthetic water	92% turbidity (NaCl extract)	Leaf waste usable
[34]	Fenugreek, Papaya seeds	River water	COD & turbidity reduced; neutral pH	Both effective
[35]	Various (Review Paper) Highlighted: Moringa oleifera, Roselle seeds, Carica papaya, Orange peel, Jackfruit, Rice starch	Review study	Up to 99% turbidity; less sludge	NA
[36]	Okra Seeds	River water	Turbidity ↓ to 4.5 NTU; comparable to alum	Cost-effective alternative.

summarizes the recent research on using different natural materials in water treatment.

Moreover, ref. [37] demonstrated that plant-derived natural coagulants operate primarily through adsorption, charge neutralization, and bridging mechanisms, and highlighted that their effectiveness is significantly influenced by factors including pH, coagulant dosage, initial turbidity, and zeta potential all of which were carefully controlled in their study, thereby validating the experimental approach adopted by the researchers. Furthermore, ref. [38] comprehensively reviewed the mechanistic framework of plant-based coagulants in water treatment, emphasizing that the integration of coagulation–flocculation with downstream treatment processes substantially enhances overall pollutant removal efficiency beyond what standalone coagulation can achieve.

The observed differences in coagulation performance among the five plant-derived coagulants can be attributed to their distinct molecular interaction mechanisms with the suspended and colloidal contaminants in the raw water. Moringa oleifera leaves contain water-soluble cationic proteins that neutralize negatively charged colloidal particles through charge neutralization and polymer bridging [39], while also binding dissolved organic matter through hydrophobic interactions and electronic attraction, enabling aggregation into larger flocs conducive to gravitational settling. This dual mechanism explains its superior turbidity and COD removal. Okra pods, rich in high-molecular-weight galactomannan polysaccharides exceeding 500 kDa, operate primarily through polymer bridging and sweep flocculation, as confirmed by zeta-potential measurements [40], whereas fenugreek seeds follow a similar but less effective bridging mechanism, as their natural polysaccharide mucilage functions as a flocculating agent for removing dissolved and suspended solids with comparatively lower overall efficiency. Cowpea seeds contain globular storage proteins classifiable as cationic coagulants that achieve partial charge neutralization, yielding documented but comparatively moderate turbidity removal efficiencies [41], while papaya seeds combine polysaccharide-mediated bridging with enzymatic activity from papain, a cysteine protease with 345 amino acid residues, which may partially hydrolyse organic matter and explains the marginally elevated post-treatment BOD₅ observed for this coagulant [42,43]. Furthermore, all five plant-derived coagulants produce entirely biodegradable residuals, avoiding the persistent metal accumulation and associated neurotoxic and systemic health risks linked to residual aluminum from conventional alum coagulation, whose residues have been associated with brain accumulation and disorders including Alzheimer’s disease [44], reinforcing their suitability as sustainable alternatives for water treatment in resource-limited settings [18,19].

Although significant progress has been achieved in water treatment technologies, the development of scalable, nature-based treatment systems that use plant-derived coagulants, especially for climate-vulnerable and resource-limited areas, is still not fully investigated. Conventional chemical treatment methods are often expensive, energy-intensive, and linked to environmental and health concerns, which limit their long-term sustainability. Meanwhile, many studies on natural coagulants mainly focus on laboratory-scale performance of a particular treatment unit and do not consider integrated, practical, and replicable system designs that can address climate-driven pollution in surface water bodies. Therefore, there is a clear need for treatment approaches that combine locally available plant materials with simple and adaptable system designs suitable for real-world conditions. To address this gap, the present research develops a sustainable, nature-based water treatment system using locally sourced plant coagulants. As such, the objectives of this research are to:

• To characterize the physicochemical quality of surface water through systematic grab sampling and laboratory analysis of key water quality parameters.
• To evaluate and compare the coagulation performance of selected natural plant-derived coagulants with conventional alum, with particular emphasis on turbidity removal efficiency, floc formation characteristics, and optimum dosage determination.
• To design and assess a low-cost, integrated water treatment system incorporating coagulation, flocculation, sedimentation, and filtration using locally available materials, to promote sustainable and climate-resilient water treatment solutions for developing countries.

2. Experimental Setup and Procedure

2.1. Selection of Sampling Locations and Sample Collection

Meda Ela, Karadiyana, Sri Lanka, was selected for sampling, as it is a stream impacted by climate variation. The presence of a nearby solid waste dumpsite contributes to leachate generation, particularly during rainfall events, leading to increased pollutant loads entering the stream. This makes Meda Ela an appropriate study site for evaluating treatment performance under conditions influenced by both climatic factors and anthropogenic activities. As shown in Figure 1, water samples were collected from three strategically selected locations along the Meda Ela stream to capture pollution gradients influenced by the upstream Karadiyana dumpsite, industrial effluents, and service station runoff. Grab sampling was employed using sterile 5 L polyethylene bottles, and water samples were collected during low-flow conditions to minimize dilution effects. Samples were collected over a one-year study period. Upon collection, samples were immediately transported to the laboratory in insulated coolers, stored at 4 °C in a refrigerator, and analyzed within 24 h to preserve integrity, adhering to APHA (American Public Health Association) standards.

2.2. Surface Water Quality Parameter Analysis

Surface water quality was evaluated using 12 key water quality parameters, including pH, turbidity, electrical conductivity, dissolved oxygen (DO), 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), settleable solids, total solids (TS), total dissolved solids (TDS), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP). Total Solids (TS) represents the entire solid content remaining after evaporation of a water sample at 105 °C, encompassing all dissolved, suspended, and settleable fractions collectively. Total Suspended Solids (TSS), in contrast, refers specifically to the fraction of solids retained on a filter membrane (Whatman No. 1) after filtration, representing only the particulate matter that remains in suspension and does not pass through the filter. The difference between TS and TSS therefore yields the Total Dissolved Solids (TDS), which accounts for the solids that pass through the filter and remain in solution. In the context of this study, TS provides an overall measure of the total pollution load in the raw water, while TSS serves as a more specific indicator of suspended particulate matter. Each parameter was analyzed in triplicate, and the average values were calculated to ensure accuracy and statistical reliability. The measured results were compared with the ambient water quality standards established by the Central Environmental Authority (CEA) of Sri Lanka to identify exceedances and determine treatment priorities. All analyses were conducted using standardized laboratory methods (APHA). The experiments were conducted in the Environmental Engineering Laboratory, General Sir John Kotelawala Defence University, Sri Lanka.

Table 2. Test procedures and apparatus used.

Parameter	Test Procedure (APHA-Based)	Apparatus	Model (Serial Number)
pH	Electrometric method using calibrated probe (APHA 4500-H+ B).	ISTEK Multimeter	CPD-65N (S/N: KI2211)
Dissolved Oxygen (DO)	Optical DO measurement after calibration (APHA 4500-O G).	HANNA Optical DO Meter	HI98198 (S/N: 0512010101)
Conductivity	Conductivity measured after calibration with KCl standard (APHA 2510 B).	ISTEK Multimeter	CPD-65N (S/N: KI2211)
Turbidity	Nephelometric method; scattered light measured at 90° (APHA 2130 B).	HACH Turbidity Meter	2100N (S/N: 14010C0331037)
BOD5	5-day incubation at 20 °C; DO difference calculated and corrected (APHA 5210 B).	DO Meter + Incubator	HI98198 (S/N: 0512010101)
COD	Closed reflux dichromate digestion followed by colorimetric reading (APHA 5220 C/D).	Lovibond COD Meter	MD 200 (S/N: 12/38053)
Settleable Solids	1 L sample settled in Imhoff cone for 1 h; volume recorded (APHA 2540 F).	Imhoff Cone (1 L)	Standard Laboratory Glassware
Total Solids (TS)	20 mL unfiltered sample evaporated at 105 °C to constant weight (APHA 2540 B).	Hot Air Oven + Analytical Balance	Laboratory Equipment
Total Dissolved Solids (TDS)	20 mL filtered sample evaporated at 105 °C to constant weight (APHA 2540 C).	Hot Air Oven + Filtration Unit	Laboratory Equipment
Total Suspended Solids (TSS)	200 mL filtered through Whatman No. 1 filter; dried at 105 °C (APHA 2540 D).	Hot Air Oven + Filtration Unit	Laboratory Equipment
Total Nitrogen (TN)	Persulfate digestion followed by spectrophotometric determination (APHA 4500-N C).	Spectrophotometer	Laboratory Equipment
Total Phosphorous (TP)	Acid persulfate digestion followed by ascorbic acid method (APHA 4500-P E).	Spectrophotometer	Laboratory Equipment

shows the test procedures and apparatus used to test the selected water quality parameters.

2.3. Jar Test Analysis and Optimization of Natural Coagulants

Jar tests were initially conducted to determine the optimum alum dosage to compare the performance of natural coagulants with conventional chemical treatment using alum. Following this optimization, as shown in Figure 2a–e, five plant-derived coagulants were systematically evaluated: cowpea (Vigna unguiculata) seeds, fenugreek (Trigonella foenum-graecum) seeds, papaya (Carica papaya) seeds, drumstick (Moringa oleifera) leaves, and okra (Abelmoschus esculentus) pods.

Coagulant solutions (5–10% w/v) were prepared from natural sources following standardized extraction protocols to ensure consistency and preservation of bioactivity. The preparation methods of the selected plant-derived coagulants are shown in

Table 3. Preparation of natural coagulants.

Coagulant Source	Preparation Method
Cowpea seeds, fenugreek seeds, papaya seeds (Liquid)	Fresh seeds washed; oven-dried at 40 °C for 24 h; pulverized; suspended at 5 g/100 mL distilled water; stirred mechanically (5 min); settled (1 h); vacuum-filtered through muslin cloth.
Moringa oleifera leaves (Liquid)	10 g fresh washed leaves ground into paste with minimal distilled water; diluted to 100 mL; stirred (5 min); settled (1 h); filtered through muslin cloth.
Okra pods (Liquid)	10 g washed pods sliced (1–2 cm); blended with 100 mL distilled water; settled (1 h); filtered to yield viscous solution.

.

For all plant-derived coagulants, the filtered liquid filtrate obtained after filtration through muslin cloth was used as the coagulant solution in the jar test experiments. The solid residue retained on the filter cloth was discarded and was not used in any part of the treatment process. The concentration expression of 5–10% w/v refers to the mass of raw plant material (g) either in the form of dissolved or suspended in a fixed volume of distilled water (mL) prior to filtration. This represents the preparation strength of the extract rather than the concentration of the final filtrate, as the filtration step removes insoluble plant matter while retaining the soluble bioactive compounds responsible for coagulation activity in the resulting liquid extract.

Specifically, for cowpea seeds, fenugreek seeds, and papaya seeds, 5 g of oven-dried and pulverized seed powder was suspended in 100 mL of distilled water (5% w/v), mechanically stirred for 5 min, allowed to settle for 1 h, and then vacuum-filtered through muslin cloth to yield the coagulant filtrate. For Moringa oleifera leaves, 10 g of fresh washed leaves were ground into a paste with a minimal volume of distilled water, diluted to 100 mL (10% w/v), stirred for 5 min, settled for 1 h, and filtered through muslin cloth to obtain the leaf extract filtrate. For okra pods, 10 g of washed and sliced pods (1–2 cm pieces) were blended with 100 mL of distilled water, allowed to settle for 1 h, and filtered to yield a viscous coagulant filtrate. In all cases, the resulting filtrates were freshly prepared prior to each jar test experiment to preserve bioactivity and ensure consistency across all experimental runs.

The tests were carried out using a six-paddle programmable jar test apparatus. One-litre raw water samples were treated with increasing doses of each plant extract (4–20 mL) and subjected to a controlled mixing sequence: rapid mixing at 100 rpm for 20 s to disperse the coagulant, followed by slow flocculation at 30 rpm for 20 min to allow floc formation, and then a settling period of 30 min. 30 min period of sedimentation was applied uniformly across all five plant-derived coagulants in this study, following standard jar test protocols commonly adopted in natural coagulant research. However, it is recognized that the optimal sedimentation time may vary among coagulants due to differences in their underlying coagulation mechanisms and the physical characteristics of the flocs produced. Coagulants operating primarily through charge neutralization, such as Moringa oleifera, tend to produce denser and more compact flocs that settle relatively rapidly, whereas those acting through polymer bridging, such as okra and fenugreek, produce larger but less dense floc structures whose settling behaviour may differ considerably. Similarly, the rapid mixing and slow flocculation durations applied in this study were standardized across all coagulants to ensure comparability of results under identical testing conditions. The uniform protocol adopted here was therefore intended to provide a fair and consistent comparative basis across all tested materials rather than to optimize individual coagulant performance. Turbidity was measured after settling, and the optimal dosages were identified as those achieving the highest turbidity removal (over 85%) while maintaining the lowest residual turbidity.

2.4. Development of Water Treatment Unit

As shown in Figure 3, a simple laboratory scaled two-stage treatment system was developed, consisting of (a) a coagulation, flocculation, sedimentation unit and (b) a filtration unit. Sawdust was first washed with distilled water to remove dust and soluble impurities that could affect filtration. It was then oven-dried to eliminate moisture and ensure stable and consistent properties. After cooling, it was used as the primary filter medium. Cotton wool was added as a supplementary layer to improve particle retention and provide structural support within the filter setup.

A transparent high-density polyethylene (HDPE) bucket fitted with a mid-level tap functioned as the sedimentation tank to facilitate the withdrawal of clarified water without disturbing settled flocs. The filtration unit was constructed using a polyethylene terephthalate (PET) bottle, cut and inverted to form a funnel configuration. Cotton wool was placed at the bottom to serve as a support layer, followed by a layer of prepared sawdust as the primary filtration medium.

Five one-litre surface water samples were each dosed with 12 mL of the previously optimized Moringa oleifera (drumstick) seed extract/coagulant. The samples were subjected to standard jar test procedures to ensure adequate rapid mixing and flocculation. Subsequently, the treated water was transferred into the sedimentation tank and allowed to settle undisturbed for 2 h to promote effective floc settlement. The clarified water was then withdrawn through the mid-level tap and directed through the filtration unit. The filtered effluent was collected from the neck of the inverted bottle for further analysis.

In Figure 4, the beaker on the left side shows the raw influent collected from Meda Ela, Karadiyana, which contains high turbidity and visible pollutants. The beaker in the middle represents water after coagulation, flocculation, and sedimentation, where suspended and colloidal particles have been destabilized, aggregated, and settled, resulting in reduced turbidity. The beaker on the right side shows the filtered water, where remaining fine particles have been removed through the filter media, producing clearer water with improved physical quality.

3. Results and Discussion

3.1. Analysis of Raw Water Quality

Water quality was assessed for 12 selected parameters across three samples collected from the respective sampling locations. Measurements were conducted, and average values were calculated for each parameter.

Table 4. Analysis of water quality.

Parameter	Analysis Result (Average Value)	Unit	Maximum Allowable Limit (CEA Guidelines)	Notes
pH	7.8	-	6.0–8.5	Satisfactory
Turbidity	35	NTU	20	Unsatisfactory
Conductivity	407	µs/cm	500	Satisfactory
DO	7.1	mg/L	5 (minimum limit)	Satisfactory
BOD5	3.46	mg/L	4	Satisfactory
COD	80	mg/L	15	Unsatisfactory
Settleable Solids	0.9 (45 min) 1.1 (1 h)	mg/L	-	-
Total Solids (TS)	1000	mg/L	-	-
Total Dissolved Solids (TDS)	1000	mg/L	500	Unsatisfactory
Total Suspended Solids (TSS)	100	mg/L	40	Unsatisfactory
Total Nitrogen	12.4	mg/L	-	-
Total Phosphorous	0.1	mg/L	-	-

presents these average values with the corresponding tolerance limits for comparison.

Analysis of the water quality parameters revealed that pH, electrical conductivity, dissolved oxygen (DO) and 5-day biochemical oxygen demand (BOD₅) levels were within the permissible tolerance limits stipulated by the Central Environmental Authority (CEA), Sri Lanka. In contrast, settleable solids, total solids, total nitrogen, and total phosphorus lack established CEA limits. However, turbidity, chemical oxygen demand (COD), total dissolved solids (TDS), and total suspended solids (TSS) exceeded the allowable thresholds, indicating the requirement for remedial treatment prior to discharge or reuse.

3.2. Jar Test Using Alum

Jar tests using a 5 g/L alum stock solution were conducted to establish benchmark coagulation performance for subsequent comparison with natural coagulants. Five 1 L water samples were prepared, with pH adjusted using diluted nitric acid (HNO₃). Each sample was then dosed with 2, 4, 6, 8, and 12 mL of the alum solution, corresponding to incremental dosages. The standard jar test procedure was followed, including rapid mixing, slow mixing, and settling phases. Residual turbidity was measured as a function of pH, with results plotted to determine the optimum pH at the minimum turbidity value (refer to Figure 5a).

The turbidity vs. pH relationship showcased a U-shaped trend. This indicates a strong relationship between pH and alum coagulation efficiency. The lowest turbidity levels were found around 6.2–6.4 pH values. Therefore, this is the optimum pH level for effective coagulation.

As shown in Figure 5b, jar tests with alum demonstrated optimal performance at a dosage of 8 mL (from the 5 g/L alum solution), reducing raw water turbidity from 35 NTU to a minimum of 2.04 NTU. This corresponded to a turbidity removal efficiency of 94.2%, establishing a reference benchmark for natural coagulant evaluation.

3.3. Jar Tests Using Natural Coagulants

Jar tests were performed to evaluate the coagulation efficiency of selected natural coagulants at dosages of 4, 8, 12, 16, and 20 mL added to one-litre raw water samples with an initial turbidity of 35 NTU. The optimum dosage for all coagulants was found to be 12 mL, at which minimum residual turbidity was observed. In all cases, turbidity decreased with increasing dosage up to this optimum and increased thereafter due to overdosing effects such as particle restabilization and floc breakup.

Figure 6a illustrates that cowpea seed coagulant (50 g/L) achieved a minimum residual turbidity of 8.47 NTU, corresponding to a removal efficiency of 75.8%. Figure 6b shows that fenugreek seed coagulant (50 g/L) reduced turbidity to 10.76 NTU, achieving 69.2% removal. According to Figure 6c, papaya seed coagulant (50 g/L) attained a removal efficiency of 72.5% at the optimum dosage. Figure 6d demonstrates that drumstick leaf coagulant (100 g/L) exhibited the highest performance, reducing turbidity to 4.54 NTU with an efficiency of 87%. Similarly, Figure 6e indicates that ladies finger coagulant (100 g/L) achieved a minimum residual turbidity of 5.39 NTU, corresponding to 84.6% removal. Overall, drumstick leaf and lady’s finger coagulants showed comparatively superior performance among the tested natural coagulants.

The coagulation performance observed in this study is mechanistically governed by two fundamental processes: adsorption with charge neutralization and adsorption with polymer bridging. Cationic proteins and polysaccharides in the plant-derived extracts, particularly those from Moringa oleifera leaves, adsorb onto negatively charged colloidal particles in raw water. This adsorption reduces electrostatic repulsion, destabilizes the particles, and initiates floc formation [45]. Beyond the optimum dosage of 12 mL, the increase in residual turbidity across all coagulants is consistent with the charge reversal phenomenon. At this stage, excess cationic coagulant saturates particle surfaces, reverses their net charge, and restabilizes the colloidal system, thereby inhibiting effective floc formation [46]. The superior performance of Moringa oleifera leaf extracts compared to seed-based coagulants such as fenugreek and papaya is attributable to their higher active protein content per unit volume. This enhances both charge neutralization efficiency and bridging capacity across the diverse particle size range present in leachate-impacted Meda Ela surface water. Consequently, leaf extracts demonstrate stronger and more consistent coagulation efficacy than seed-derived alternatives.

The turbidity removal efficiencies of the five selected natural coagulants were comparatively analyzed to determine the most effective material for the proposed water treatment system, as presented in Figure 7. The results reveal clear differences in coagulation performance under identical testing conditions. Among the tested coagulants, drumstick leaves exhibited the highest turbidity removal efficiency of 87%, indicating superior effectiveness in destabilizing suspended and colloidal particles and promoting efficient floc formation. Lady’s fingers also showed relatively high removal efficiency, while cowpea, papaya, and fenugreek demonstrated moderate performance. Based on this comparative evaluation, drumstick leaves were identified as the most suitable natural coagulant for the proposed treatment system due to their consistently high turbidity removal efficiency and overall effectiveness.

The convergence of the optimum dosage at 12 mL across all five natural coagulants is primarily attributed to the consistent physicochemical characteristics of the raw water sourced from Meda Ela, Karadiyana, which was used uniformly across all jar test experiments. Since all tests were conducted on the same raw water matrix with a fixed initial turbidity of 35 NTU, pH of 7.8, and TSS of 100 mg/L, the colloidal particle concentration and surface charge density remained constant throughout, meaning the threshold quantity of coagulant required to achieve sufficient charge neutralization and initiate effective floc formation converged toward a similar point for all coagulants tested. This is consistent with established coagulation theory, wherein coagulant demand is fundamentally governed by the total colloidal surface area and particle charge characteristics of the raw water rather than exclusively by the nature of the coagulant itself. However, it is important to note that although the optimum dosage volume was identical at 12 mL for all five coagulants, the actual mass of active coagulant material delivered per litre of treated water was not equivalent across all coagulants, since cowpea, fenugreek, and papaya seed extracts were prepared at 50 g/L, delivering an effective dose of 0.6 g/L, whereas Moringa oleifera leaf and okra pod extracts were prepared at 100 g/L, delivering 1.2 g/L at the same volume, reflecting that higher concentration extracts were necessary for Moringa oleifera and okra to achieve their respective optimum performance levels, which itself indicates differences in active coagulant content per unit mass between plant sources rather than a uniform intrinsic coagulant demand.

Post-coagulation BOD₅ analysis was conducted to evaluate residual organic matter. Although the raw Meda Ela water BOD₅ (3.46 mg/L) was below the CEA limit of 4 mg/L, variations were monitored, as natural coagulants may elevate BOD₅ levels. As shown in above Figure 8, papaya seed coagulant yielded the highest post-treatment BOD₅ (3.57 mg/L), while ladies finger coagulant produced the lowest (3.41 mg/L); cowpea seeds, fenugreek seeds, and drumstick leaves exhibited intermediate values. All treated samples remained below the CEA threshold of 4 mg/L. Although ladies finger coagulant excelled in BOD₅ reduction, drumstick leaves provided the greatest turbidity removal (lowest residual value), positioning it as the prioritized choice for the treatment system despite acceptable BOD₅ levels across all options.

The marginal increases in BOD₅ observed in certain treated samples are attributed to the biodegradable organic constituents, primarily proteins and polysaccharides, which are inherently present in the plant-derived coagulant extracts. Upon dosing, these natural macromolecules introduce additional organic load into the treated water [16]. This outcome reflects a well-documented characteristic of bio-based coagulation systems where in the use of crude plant extracts can release dissolved and suspended organic matter, including biodegradable carbon fractions, into the treated water. Such additions represent an inherent trade-off of natural coagulant application, where improved turbidity and particle removal may be accompanied by slight increases in biodegradable organic content. In the context of Meda Ela surface water, the observed BOD₅ increases were marginal and remained within ranges.

3.4. Sample Analysis of the Treatment System for Different Influents

The preliminary water quality analysis showed that turbidity, chemical oxygen demand (COD), total dissolved solids (TDS), and total suspended solids (TSS) were the main parameters exceeding acceptable levels and therefore required treatment. Based on this, the performance of the sustainable treatment system was evaluated by analyzing these parameters, along with 5-day biochemical oxygen demand (BOD₅), in both influent and effluent samples.

The system was operated in batch mode to clearly observe treatment performance over time. For each experimental run, fresh influent water was introduced into the system and allowed to undergo treatment for a defined period. In the first run, Sample 1 was treated for one day before the effluent was collected and analyzed, after which the system was completely drained. The procedure was repeated with fresh influent for Sample 2, which was treated for two days before drainage and analysis. This process continued similarly up to Sample 5, where the water was treated for five days. Following laboratory analysis, removal efficiencies for each parameter were calculated to assess the treatment effectiveness over increasing retention periods.

Figure 9a–e collectively illustrates the performance of the treatment system in improving the quality of Meda Ela surface water over five consecutive days. As shown in Figure 9a, influent turbidity ranged from 33 to 49 NTU, indicating variable pollution loads, while all effluent samples remained well below the permissible limit of 20 NTU, with the highest recorded value being 4.8 NTU. The system achieved a high average turbidity removal efficiency of 90.13%. According to Figure 9b, total dissolved solids (TDS) decreased from influent levels of 1000–2000 mg/L to effluent values of 0–450 mg/L, remaining within the 500 mg/L standard and achieving an average removal efficiency of 83.46%. Furthermore, Figure 9c shows that total suspended solids (TSS) were reduced from 100–200 mg/L in influent samples to 0–40 mg/L in effluents, with most values complying with the 40 mg/L limit and an average removal efficiency of 74.83%.

In terms of oxygen-related parameters, Figure 9d demonstrates significant chemical oxygen demand (COD) reduction, with influent values reaching up to 85 mg/L and effluent concentrations consistently maintained between 5 and 12 mg/L, below the 15 mg/L limit, resulting in an average removal efficiency of 88.57%. As presented in Figure 9e, 5-day biochemical oxygen demand (BOD₅) values for both influent (2.9–3.5 mg/L) and effluent (2.4–3.8 mg/L) remained below the 4 mg/L permissible limit, despite slight increases in certain samples potentially due to the use of natural coagulants. The overall average BOD₅ removal efficiency was 2.72%. Collectively, these results confirm that the treatment system effectively reduced key physicochemical parameters and consistently maintained effluent quality within acceptable standards.

3.5. Sample Analysis of the Treatment System for the Same Influent

To evaluate the time-dependent performance of the developed treatment system, a single influent sample collected from Meda Ela was introduced into the system and monitored continuously over five days. Effluent samples were collected at 24 h intervals consecutively for five days and analyzed for all key water quality parameters, enabling the assessment of how the removal efficiency evolved with increasing hydraulic retention time. This approach was designed to simulate real-world batch treatment conditions and to determine whether treatment performance remained stable, improved, or declined over extended retention periods.

Table 5. Time-dependent removal efficiencies (%) of key water quality parameters over five days of hydraulic retention (same influent).

Parameter	Influent Value	Day 1	Day 2	Day 3	Day 4	Day 5	CEA Limit	Average Removal (%)
		Effluent Value (mg/L or NTU)—Removal Efficiency (%)
Turbidity (NTU)	41	4 (90.2%)	4.5 (89.0%)	3 (92.7%)	3 (92.7%)	4.5 (89.0%)	≤20 NTU	90.7%
TDS (mg/L)	1500	500 (66.7%)	450 (70.0%)	500 (66.7%)	450 (70.0%)	400 (73.3%)	≤500 mg/L	69.3%
TSS (mg/L)	150	30 (80.0%)	30 (80.0%)	40 (73.3%)	30 (80.0%)	20 (86.7%)	≤40 mg/L	80.0%
COD (mg/L)	76	10 (86.8%)	12 (84.2%)	10 (86.8%)	11 (85.5%)	12 (84.2%)	≤15 mg/L	85.5%
BOD5 (mg/L)	3.8	3.2 (15.8%)	3.4 (10.5%)	3.4 (10.5%)	3.5 (7.9%)	3.5 (7.9%)	≤4 mg/L	10.5%

summarizes the removal efficiencies (%) achieved at each day of retention, and Figure 10a–e illustrates the corresponding variation in influent and effluent concentrations over time.

As shown in Figure 10a, influent turbidity of 41 NTU was reduced to 3–4.5 NTU in effluent, well below the 20 NTU limit, achieving an average removal efficiency of 90.73%, with turbidity remaining compliant even after five days of storage. Similarly, Figure 10b indicates that total dissolved solids (TDS) decreased from 1500 mg/L influent to 400–500 mg/L in effluent, generally below the 500 mg/L limit, achieving 69.33% average removal efficiency, with compliance maintained after storage. Figure 10c shows total suspended solids (TSS) reduced from 150 mg/L influent to 20–40 mg/L in effluent, with most values below the 40 mg/L limit and an 80% average removal efficiency, and compliance persisted after five days.

For oxygen-related parameters Figure 10d shows that chemical oxygen demand (COD) dropped from 76 mg/L influent to 10–12 mg/L in effluent, consistently below the 15 mg/L limit, achieving 85.53% average removal efficiency. Finally, Figure 10e indicates that 5-day biochemical oxygen demand (BOD₅) decreased from 3.8 mg/L influent to 3.2–3.5 mg/L in effluent, all below the 4 mg/L limit, with an average removal efficiency of 10.53%, and both influent and effluent remained compliant throughout. Collectively, these results confirm that the treatment system effectively improved effluent quality while maintaining compliance over five days of storage.

While direct measurement of Total Organic Carbon (TOC) was not conducted in this study due to the unavailability of a TOC analyzer within the accessible laboratory facilities, the concurrent measurement of BOD₅, COD, and Soluble Chemical Oxygen Demand (SCOD) across both influent and effluent samples collectively provides a comprehensive indirect characterization of the organic matter present in the treated water matrix. COD quantifies the total oxygen demand required to oxidize all oxidizable organic constituents encompassing both biodegradable and non-biodegradable fractions, while BOD₅ specifically captures the biodegradable organic fraction, and SCOD, representing the oxygen demand of dissolved organic compounds obtained after filtration of particulate matter, directly approximates the dissolved organic carbon fraction that TOC would otherwise measure [47]. Together, these parameters collectively represent the principal organic fractions constituting TOC [48]. Importantly, TOC is considered a complementary analysis to COD and BOD rather than a replacement, as these parameters measure different aspects of organic matter and are mutually informative in water treatment assessment [49].

In order to further validate the performance of the developed treatment system, the removal efficiencies achieved in this study were compared with recently published literature reporting the use of natural plant-derived coagulants, with particular focus on Moringa oleifera, which demonstrated the highest coagulation performance among the five coagulants evaluated in this research.

Table 6. Comparative analysis of Moringa oleifera-based coagulation performance: removal efficiencies of key water quality parameters reported in the recent literature against the present study.

Reference	Coagulant Used	Water Type	Turbidity Removal (%)	COD Removal (%)	BOD5 Removal (%)	TDS Removal (%)	TSS Removal (%)	Cost Assessment
[50]	Moringa oleifera seeds/extract	Sewage & grey water	61%	65%	55%	68%	69%	Very low; seeds locally and freely available in most developing regions
[51]	Moringa oleifera seeds—HCl activated powder	Municipal sewage wastewater (India)	~90%	~30%	27%	Reduced (% Not reported)	Reduced (% Not reported)	Low; minor cost increase from HCl activation vs. native powder
[52]	Moringa oleifera seeds—seed powder	Raw drinking water (MahmoudiaCanal, Egypt)	83.6%	77.4%	51.1%	Not Reported	Not Reported	Very low; seeds widely available in Egypt & Africa; cheaper than alum
[53]	Moringa oleifera, (leaves & seeds)	Sewage water (India)	92%	88%	Not Reported	96% (Total Solids TS reported)	Not Reported	Very low; no chemical pre-treatment; minimal equipment
[54]	Moringa oleifera seeds—powder (5 g/L, pH 4)	Textile industrial wastewater (Ethiopia)	82.33%	74.63%	Not Reported	Not Reported	43.67%	Low; no chemical coagulant purchase required; seeds locally sourced
[13]	Moringa oleifera seeds	Municipal wastewater (Pakistan)	87%	67%	Not Reported	Not Reported	Not Reported	40–70% cost saving; biodegradable sludge reduces disposal costs; no metal residuals
This Study	Moringa oleifera leaves (liquid extract, 100 g/L) + Sawdust & Cotton wool filtration	Urban polluted surface water (leachate + urban runoff impacted stream, Sri Lanka)	90.13–90.73% (sequential batch &continuous trials)	85.53–88.57%	2.72–10.53% (influent BOD5 already within CEA limit of 4 mg/L)	69.33–83.46%	74.83–80%	Very low; Moringa oleifera leaves, sawdust & cotton wool all locally sourced; zero chemical input; no sludge disposal cost

presents a comparative summary of turbidity, COD, BOD₅, TDS and TSS removal efficiencies, alongside treatment time and cost assessment, drawn from studies conducted on various water and wastewater types. The results confirm that the integrated treatment system developed in this study, incorporating Moringa oleifera leaf extract with sawdust and cotton wool filtration, achieves removal efficiencies comparable to or exceeding those reported in the literature, while simultaneously addressing a broader range of water quality parameters under real-world polluted stream conditions.

3.6. Sustainability Aspects of the Treatment System

The developed treatment system demonstrates strong sustainability potential by integrating locally available, plant-based coagulants with a simple, low-cost treatment design. The use of Moringa oleifera leaves as the primary coagulant significantly reduces dependence on conventional chemical coagulants such as alum, thereby minimizing chemical costs and associated health risks from residual metals. In addition, the natural coagulants are biodegradable and produce comparatively lower volumes of sludge, which simplifies handling and disposal while reducing environmental impacts. The filtration unit, constructed using readily available materials such as sawdust and cotton wool, further enhances the system’s affordability and accessibility, particularly for rural and resource-limited communities.

From an operational perspective, the system requires minimal energy input and technical expertise, making it highly suitable for decentralized water treatment applications. The batch-mode operation and simple construction allow for easy replication and maintenance without the need for sophisticated infrastructure. Furthermore, the system demonstrated consistent performance in reducing key water quality parameters such as turbidity, COD, TDS, and TSS to within acceptable limits, even under varying influent conditions and retention times. These characteristics highlight the system’s resilience to climate-induced water quality fluctuations and its potential as a practical nature-based solution for sustainable water management in developing regions.

4. Conclusions

This study comprehensively evaluated the potential use of natural plant-derived coagulants as sustainable, nature-based alternatives to conventional chemical coagulants for treating climate-impacted urban surface water in Sri Lanka. Surface water collected from the Meda Ela stream exhibited dramatic pollution, with turbidity (35 NTU), COD (80 mg/L), TDS (1000 mg/L), and TSS (100 mg/L) exceeding CEA permissible limits, underscoring the urgent need for resilient and low-cost treatment strategies in developing regions. Among the five plant-derived coagulants evaluated, Moringa oleifera leaves demonstrated the most promising coagulation performance, achieving 87% turbidity removal at an optimum dosage of 12 mL/L, closely approaching the 94.2% efficiency of conventional alum, while eliminating the associated health risks of residual metal accumulation. The remaining coagulants exhibited moderate to good performance, with okra pods achieving 84.6%, cowpea seeds 75.8%, papaya seeds 72.5%, and fenugreek seeds 69.2% turbidity removal, respectively.

Based on these findings, an integrated low-cost prototype treatment system incorporating Moringa oleifera leaf extract with sawdust and cotton wool filtration was developed and validated using two experimental approaches. Sequential batch trials with five independent influent samples achieved average removal efficiencies of 90.13% for turbidity, 88.57% for COD, 83.46% for TDS, 74.83% for TSS, and 2.72% for BOD₅, with all effluent parameters consistently maintained within CEA permissible limits. Continuous retention trials using a single influent monitored over five days demonstrated similarly stable performance, with average removal efficiencies of 90.73% for turbidity, 85.53% for COD, 69.33% for TDS, 80% for TSS, and 10.53% for BOD₅, confirming system reliability and treatment stability over extended retention periods. The low BOD₅ removal efficiencies recorded in both trials reflect the fact that influent BOD₅ values were already within the CEA permissible limit of 4 mg/L, leaving minimal room for measurable percentage reduction rather than indicating any deficiency in treatment performance. The treated effluent is recommended for non-potable applications such as irrigation, sanitation flushing, and surface cleaning, where its quality is demonstrably adequate.

Overall, this study postulates that locally sourced, plant-based coagulant systems offer a practical, environmentally responsible, and economically viable pathway for improving water resilience in climate-vulnerable and resource-constrained communities, highlighting the broader significance of nature-based solutions in sustainable water management.