Beyond Alum: Why Chitosan Is the Smarter Coagulant for Modern Water Treatment

Clean, resilient water systems start at the molecular level: moving beyond alum to natural chitosan unlocks safer, more sustainable treatment for the world’s most critical resource.

Aluminium sulfate (alum) has dominated global water treatment for over a century, but modern systems face pressures it cannot adequately address. Concerns about residual aluminium in drinking water, escalating sludge disposal costs, narrow operational flexibility, and the environmental footprint of bauxite mining are compelling water utilities and regulators to seek alternatives (Krupińska, 2020). Chitosan—a natural, cationic biopolymer derived from chitin—has emerged as a technically superior, safer and demonstrably more sustainable coagulant (Mohammed et al., 2016; Zaman et al., 2021).

When produced from Black Soldier Fly (BSF) exoskeletons, chitosan offers traceable, circular sourcing that transforms organic waste into high-value treatment chemicals (Surendra et al., 2021). Evidence from bench-scale and full-scale applications shows that chitosan matches or exceeds alum in turbidity and metal removal whilst dramatically reducing sludge volumes and lifecycle costs (Brown and Emelko, 2019; Zaman et al., 2021). This article compares alum and chitosan as coagulants, examines their health and environmental implications, and explains why BSF-derived chitosan positions forward-thinking water utilities at the forefront of sustainable water treatment innovation.

The Problem with Alum: Limitations of a Legacy Coagulant

Aluminium sulfate operates by releasing Al³⁺ ions that hydrolyse to form aluminium hydroxide flocs, which adsorb and trap suspended solids, colloids and organic matter (Krupińska, 2020). However, this mechanism is tightly constrained by water chemistry and creates cascading operational and health challenges.

Residual Aluminium Concerns: Alum coagulation is highly pH-dependent, requiring operation within 5.5–7.0. Outside this narrow window, residual aluminium in treated water exceeds safety limits (Krupińska, 2020). Studies show residual concentrations commonly range 0.2–0.6 mg/L, with classical alum achieving 0.47–0.57 mg/L—exceeding the WHO guideline of 0.2 mg/L (Krupińska, 2020). Expanding epidemiological evidence links chronic aluminium exposure to neurodegenerative disease, including Alzheimer's (Krupińska, 2020). The US EPA, Canada, Japan and Sweden impose stringent limits around 0.05–0.1 mg/L as a precaution.

Operational Complexity: Alum's narrow pH window demands careful dosing (20–40 mg/L) and secondary chemical addition for pH correction, increasing operational complexity and total cost. High sulphate and aluminium loadings promote infrastructure corrosion.

Massive Sludge Burden: Alum produces large volumes of non-biodegradable, mineral-rich sludge. Hu (2013) demonstrated that chitosan coagulation produces only ~20% of the sludge volume generated by alum for equivalent turbidity removal. For municipal operators, sludge handling and disposal costs represent 30–50% of total coagulation expenditure (Environmental Integrity Project, 2023).

Environmental Impact: Bauxite mining for alum production is destructive, removing forest cover and contaminating freshwater in tropical regions (UNCTAD, 2001). The Bayer Process generates ~120 million tonnes of caustic red mud annually (Georgitzikis, 2023). Primary aluminium smelting has a carbon footprint of 5–15 tonnes CO₂-equivalent per tonne (Georgitzikis, 2023), incompatible with net-zero commitments.

Chitosan: Superior Chemistry from Nature

chitin’s acetylated backbone and deacetylated chitosan chains show how exposed amino groups create the cationic sites that drive natural coagulation in water treatment

Chitosan, derived from chitin in insect exoskeletons, comprises a linear backbone with primary amino groups (−NH₂) (Mohammed et al., 2016). In mildly acidic solution, these groups protonate (−NH₃⁺), enabling electrostatic interaction with negatively charged particles.

Unlike alum's metal hydroxide precipitation at narrowly defined pH, chitosan operates via two synergistic mechanisms: charge neutralisation and polymer bridging (Mohammed et al., 2016). Protonated chitosan chains adsorb onto negatively charged colloids and particles, destabilising suspension and linking multiple solids into larger, denser flocs that settle readily.

Key advantages:

• Broader pH range (4.5–9.0 versus 5.5–7.0 for alum), eliminating the need for secondary alkali dosing

• Superior floc properties: larger, more shear-resistant flocs with improved settling and filtration

• Complete biodegradability: naturally degraded into glucose and glucosamine by microorganisms, enabling safe sludge valorisation

Performance Comparison: The Evidence

Zaman et al. (2021) compared optimised chitosan against alum and other conventional coagulants across multiple raw water matrices. At 10 mg/L, chitosan achieved turbidity removal up to 99%, outperforming alum whilst demonstrating less sensitivity to pH and dose variations. Brown and Emelko (2019) reported chitosan achieved high turbidity reduction at doses as low as 1–3 mg/L—substantially lower than alum's 20–40 mg/L requirement.

Sludge Reduction: An 80% reduction in sludge volume directly translates to lower dewatering energy, transport costs and disposal fees—typically saving municipal utilities £50,000–£100,000 annually (Environmental Integrity Project, 2023).

Beyond Turbidity: Chitosan removes dissolved metals (zinc, copper, chromium, iron) at 80–90% efficiency (Althiery, 2012). In algae-rich waters, a hybrid chitosan-aluminium approach achieved 98.32% algae removal and 66.25% dissolved organic carbon removal, reducing downstream membrane fouling by 58.80% (Zhou et al., 2020).

Metric	Alum	Chitosan
Optimum pH	5.0–7.0	4.5–9.0
Typical dosage	20–40 mg/L	1–10 mg/L
Sludge volume	High	~20% of alum
Biodegradability	No	Yes (fully)
Residual toxicity	Possible (Al)	None
Turbidity removal	85–97%	95–99%
Lifecycle cost	Higher	15–25% lower

Environmental and Health Advantage

Black Soldier Fly (Hermetia illucens) transforming organic waste into chitin-rich biomass

From Waste to Resource: BSF larvae are cultivated on organic waste—food scraps, agricultural residues, animal manures—that would otherwise be landfilled (Surendra et al., 2021). The larvae convert waste into high-value products; exoskeletons become chitosan feedstock. Green extraction processes (superheated water hydrolysis, biological demineralisation) minimise hazardous waste (Quigg et al., 2025). This circular pathway is traceable and verifiable, enabling utilities to certify sustainable sourcing for ESG reporting (Surendra et al., 2021).

Zero Health Risk: Chitosan carries no aluminium risk. The US FDA grants it "Generally Recognised as Safe" (GRAS) status (Coja et al., 2025). Chitosan sludge is completely biodegradable and can be safely applied as compost or biosolid—unavailable for alum sludge due to persistent aluminium concerns. Biodegraded chitosan becomes a soil conditioner, improving moisture retention and microbial activity (Dambuza et al., 2025).

Operational and Economic Reality

Lifecycle cost analysis reveals significant advantages. A textile facility in Southeast Asia reported 22% annual water treatment cost reduction after switching from alum to chitosan, driven by lower sludge disposal and reduced pH correction chemical costs (Tianya Chemical Ltd., 2025).

Key benefits:

• pH management: Broader operating window eliminates secondary alkali dosing

• Lower chemical inventory: 75–95% dosage reduction decreases procurement, storage and transportation costs

• Sludge savings: 80% volume reduction directly lowers dewatering, energy and disposal expenses

• Process robustness: Less sensitive to source water variability; fewer operator adjustments

• Infrastructure longevity: Non-corrosive sludge extends asset life and reduces maintenance

Regulatory Momentum

The EU's revised Water Supply Directive (2020/2184/EU) mandates stringent aluminium limits (0.2 mg/L) and encourages process optimisation to minimise residual contamination (European Council, 2020). The UK's Drinking Water Inspectorate increasingly acknowledges utilities' responsibility to assess alternatives where residual aluminium poses compliance challenges.

The European Parliament's 2024 vote to tighten phosphorus discharge limits explicitly favours chitosan and biodegradable alternatives, recognising that inorganic coagulants create intractable sludge burdens (European Parliament, 2024). Chitosan adoption directly supports UN Sustainable Development Goals: SDG 6 (Clean Water), SDG 12 (Responsible Production) and SDG 13 (Climate Action).

Conclusion: The Strategic Choice

Aluminium sulfate's century-long dominance reflects historical cost advantages within narrow constraints. Modern water systems face challenges alum cannot adequately address: tightening regulations driven by neurotoxicity evidence, escalating sludge costs, operational inflexibility and growing pressure to decarbonise.

Chitosan represents a convergence of technical excellence and environmental sustainability. Derived from renewable, waste-valorised feedstock, it achieves equal or superior coagulation performance across a broader pH range with 80% less sludge and completely biodegradable solids. Its biocompatibility and GRAS status eliminate toxicity concerns.

Entoplast's BSF-derived chitosan is engineered specifically to meet modern performance standards whilst supporting circular economy principles. Water utilities and industrial operators are invited to explore pilot programmes and large-scale deployments. Early transition positions organisations as environmental leaders whilst securing regulatory compliance and building resilience into treatment infrastructure for decades to come.

For partnership inquiries and technical support, please contact Entoplast at hello@entoplast.com.