Re‑thinking Coagulation: Where Natural Chitosan Coagulants Can Cut Chemical Use in Water Treatment

A visual comparison of conventional metal-salt coagulation (left) versus natural chitosan-based coagulation (right).

Across municipal and industrial plants, alum, ferric chloride and related inorganic coagulants are still the workhorses of clarification and colour removal. Yet rising sludge costs, residual‑metal constraints and ESG pressures are forcing operators to ask a pragmatic question: where can natural chitosan‑based coagulants and coagulant aids genuinely help reduce metal‑salt usage without compromising performance or compliance?

This article takes a plant‑focused view of the current evidence and outlines where chitosan makes sense today – and where it does not.

Why rethink coagulation now?

Conventional coagulation with alum, ferric chloride or polyaluminium chloride (PAC) remains attractive because these chemicals are cheap, robust and well understood in design standards. However, they present several growing challenges:

• Residual metals in finished water. EU and UK drinking‑water frameworks set parametric values for aluminium and iron at 200 µg/L at consumers’ taps, requiring tight control of coagulant dose, pH and particle removal (Directive (EU) 2020/2184; DWI, 2024).

• Sludge volume and handling. Metal hydroxide sludges are often voluminous and difficult to dewater, increasing haulage and disposal costs and complicating beneficial use routes (Xu et al., 2016; Zhang et al., 2019).

• ESG and chemical scrutiny. Utilities and industrial sites are under pressure to reduce the embedded environmental footprint of treatment chemicals and sludges, particularly as they also manage micro‑pollutants, microplastics and PFAS, and strive for more circular approaches (Matilainen and Sillanpää, 2010).

Policy drivers such as stricter micro‑pollutant control, tighter disinfection by‑product expectations and circular‑economy strategies further encourage exploration of bio‑based coagulants that can complement or partially replace metal salts (Sillanpää et al., 2018; Sharma et al., 2022). Within this landscape, chitosan has emerged as the benchmark natural coagulant and coagulant aid (Abu Hassan et al., 2009; Badawi et al., 2022).

How chitosan works as a coagulant and coagulant aid

Chitosan is a cationic biopolymer derived by deacetylating chitin – the structural polysaccharide found in crustacean shells, fungal cell walls and insect exoskeletons. Its backbone carries amino groups that become positively charged under acidic to mildly neutral conditions, giving chitosan a high cationic charge density (Rinaudo, 2006; Ashraf et al., 2024).

In coagulation–flocculation, chitosan acts through three main mechanisms:

• Charge neutralisation of negatively charged colloids such as clays, natural organic matter (NOM) and microorganisms.

• Bridging flocculation, where long polymer chains connect destabilised microflocs into larger, denser aggregates.

• Adsorption/complexation of some dissolved species, including dyes and selected metal ions, especially when chemically modified (Wei et al., 2018; Ashraf et al., 2024).

Depending on the application, chitosan is used as:

• A coagulant aid alongside metal salts. Here, low doses of chitosan (often 0.5–3 mg/L) are added with alum, ferric or PAC to achieve the same or better turbidity and colour removal with lower metal‑salt doses (Khairul Zaman et al., 2021; Bina et al., 2008).

• A primary coagulant in niche cases. At suitable pH and turbidity, chitosan alone at 5–20 mg/L can act as the main coagulant, especially in low‑turbidity or specific industrial effluents (Altaher, 2012; Khairul Zaman et al., 2021).

A recent optimisation study on Malaysian surface waters (turbidity 20–826 NTU; pH 5.2–6.8) showed that 10 mg/L chitosan achieved up to 99% turbidity removal while meeting national standards for residual aluminium, iron and manganese (Khairul Zaman et al., 2021).

Comparative work on eco‑friendly coagulants also notes chitosan’s relatively wide operational window in acidic conditions, making it less sensitive to modest pH and dose deviations than some aluminium salts (Khairul Zaman et al., 2021; Soros et al., 2026).

Evidence from real and realistic systems

Drinking‑water and centralised plants

Several jar‑test and pilot‑scale studies show how chitosan behaves in realistic drinking‑water treatment contexts:

• In Iranian surface water, using 0.5 mg/L chitosan as a coagulant aid allowed ferric chloride dose to be halved from 10 to 5 mg/L while still achieving around 95% turbidity removal and residual turbidity below 5 NTU (Jafarpour et al., 2014). Residual iron in treated water decreased accordingly.

• In bentonite suspensions representing raw waters of 50–200 NTU, a combination of 20 mg/L PAC and 2.5 mg/L chitosan removed ~96% turbidity, compared with 87% for 30 mg/L PAC alone – a ~35% reduction in PAC demand (Babaei et al., 2014).

• For low‑temperature, low‑turbidity waters, integrating high‑basicity PAC with high‑viscosity chitosan improved turbidity and NOM removal (about 87% turbidity, 63% DOC and 82% UV₂₅₄ removal), with better control of residual aluminium compared with lower‑basicity PAC alone (Ma et al., 2019).

Household and point‑of‑use systems

Chitosan has been evaluated extensively as a pre‑coagulant for household and small‑system treatment:

• In ceramic water filters, chitosan doses of 5–30 mg/L upstream of filtration improved turbidity removal and delivered 2–4 log₁₀ reductions of E. coli and MS2 coliphage compared with filtration alone (Chiew et al., 2016; Amburgey et al., 2022).

• Long‑term column tests dosing chitosan ahead of slow and rapid sand filters similarly improved microbial and turbidity performance, with optimal doses around 10 mg/L for combined bacteria and virus removal (Amburgey et al., 2022).

A recent systematic review of chitosan for microbial removal concluded that chitosan can deliver substantial reductions of bacteria, protozoa and some viruses when applied as a coagulant or coagulant aid, especially when combined with filtration or disinfection steps (Tavares et al., 2024).

Industrial effluents

In industrial settings, chitosan sees two main roles: primary coagulant/flocculant for colour and solids, and polymeric flocculant following metal‑salt coagulation. Examples include:

• Model and real textile effluents: chitosan doses of tens of mg/L achieved very high colour and turbidity removal (>90–99%) for reactive and acid dyes through combined charge neutralisation, hydrogen bonding and hydrophobic interactions (Abdullah and Jaeel, 2019; Wei et al., 2018; Waliullah et al., 2023).

• Fish‑processing wastewater: a liquid chitosan biocoagulant at around 5.5 mL/0.5 L effluent (optimised pH 10.5) achieved ~98% turbidity removal and ~53% BOD₅ removal (Elmidaoui et al., 2021).

• Sea‑water pre‑treatment: in desalination feed water with high turbidity, chitosan at 18 mg/L reached 97.5% turbidity removal at pH 8.1, comparable to alum but at significantly lower dose, thus reducing residual aluminium risk (Altaher, 2012).

These results confirm that chitosan can perform as a primary coagulant in some industrial effluents and as an effective aid in more complex systems, though metal salts often remain necessary for extreme loadings or challenging chemistries.

Operational and design implications

pH window and mixing

Chitosan’s cationic nature is strongest in acidic to mildly neutral conditions, typically pH 4–6.5, where amine groups are fully protonated (Rinaudo, 2006; Tavares et al., 2024). Many studies therefore report best turbidity and colour removal in this range. However, there are notable exceptions:

• Altaher (2012) observed excellent turbidity removal from sea water at pH ~8.1 with 18 mg/L chitosan.

• Elmidaoui et al. (2021) optimised chitosan performance for fish‑processing effluent at alkaline pH 10.5.

In practice, engineers should treat pH as a key design variable rather than assuming a fixed “acidic only” requirement. For coagulant‑aid duty with alum or ferric, integration at the existing plant pH (often 6–7.5) may be sufficient, but jar testing is essential to check whether slight pH optimisation could unlock dose reductions.

Mixing regimes resemble those used for synthetic organic flocculants:

• Rapid dispersion (e.g. ~100–200 rpm for 0.5–1 minute) to distribute chitosan.

• Gentle flocculation (e.g. 20–40 rpm for 15–30 minutes) to grow settleable flocs (Abu Hassan et al., 2009; Khairul Zaman et al., 2021).

Sludge quantity, dewaterability and valorisation

Switching part of the coagulant demand from metal salts to chitosan alters sludge composition and behaviour. Studies on both drinking‑water and wastewater sludges show that chitosan can:

• Increase floc size and compactness, improving settling velocity (Xu et al., 2016; Zhang et al., 2019).

• Enhance dewaterability – for example, chitosan conditioning, alone or in dual systems with metal cations, reduced capillary suction time and improved cake solids, enabling lower moisture contents and smaller volumes (Zhang et al., 2019; Liu et al., 2019; Xu et al., 2016).

• Reduce metal content in sludge cakes and filtrates, improving the prospects for agricultural use or digestion where metal limits apply (Xu et al., 2016).

Reviews of natural conditioners emphasise that chitosan‑conditioned sludges often contain more extracellular polymeric substances but still dewater better, as the polymer promotes aggregation rather than dispersion of EPS (Ma et al., 2017; Zhang et al., 2019). For plants where sludge haulage dominates OPEX, this can be as valuable as coagulant savings.

Storage, make‑up and dosing logistics

Chitosan is supplied as dry powders/granules or as liquid formulations. Key practical points include:

• Dry forms require controlled make‑up (wetting, hydration, ageing) in polymer preparation systems analogous to those used for synthetic flocculants (Badawi et al., 2022).

• Liquid biocoagulants can be fed via existing polymer dosing skids, though viscosity and shear sensitivity vary with molecular weight and concentration (Elmidaoui et al., 2021).

• Performance is sensitive to molecular weight, degree of deacetylation and purity; consistent supplier QA and specification are critical (Rinaudo, 2006; Badawi et al., 2022).

Entoplast focuses on controlling these parameters to deliver chitosan grades optimised for coagulation duty rather than generic industrial use, and can support utilities and industrial users in matching product characteristics to process needs.

Cost, ESG and supply‑chain considerations

On a per‑kilogram basis, high‑grade chitosan is typically more expensive than bulk alum or ferric salts (Badawi et al., 2022). However, cost‑effectiveness hinges on the whole system:

• Coagulant‑aid studies show 30–50% reductions in alum or ferric doses while maintaining target turbidity and colour, directly lowering metal‑salt spend (Jafarpour et al., 2014; Babaei et al., 2014; Ma et al., 2019).

• Improved sludge dewaterability and lower metal content can reduce transport and disposal costs and make beneficial use routes more accessible (Zhang et al., 2019; Xu et al., 2016).

• Reduced risk of breaching residual‑metal limits, or of future tightening of those limits, has reputational and regulatory value (DWI, 2024; Directive (EU) 2020/2184).

From an ESG standpoint, chitosan is biodegradable and exhibits low acute toxicity at water‑treatment doses, with minimal ecotoxicity reported in relevant studies (Badawi et al., 2022; Tavares et al., 2024). Sludges from chitosan‑based treatment may be more compatible with composting or digestion, subject to local regulations and the nature of co‑contaminants (Scholes et al., 2019).

Supply‑chain robustness is a legitimate concern. Traditional chitosan production depends on crustacean waste, raising questions around seasonality, fisheries impacts and processing footprints (Schäfer et al., 2025). Insect‑derived chitosan, for example from black soldier fly residue streams, offers an emerging alternative with lower greenhouse‑gas emissions, reduced land and water demand, and no marine ecosystem pressure, while delivering physicochemical properties comparable to crustacean‑derived products (Schäfer et al., 2025; Triunfo et al., 2022). Entoplast is actively developing such insect‑derived chitosan within strict quality specifications for water treatment.

Regulatory and compliance perspective

Chitosan’s natural origin does not exempt it from regulatory scrutiny. In practice, it can help answer some existing regulatory challenges while introducing new aspects that must be managed.

• Drinking‑water regulations. EU and UK rules set stringent limits for aluminium and iron (200 µg/L), and emphasise minimisation of disinfection by‑products and residual treatment chemicals (Directive (EU) 2020/2184; DWI, 2024). Reducing alum/ferric doses through chitosan coagulant aids can directly support these goals. However, regulators also expect evidence that chitosan does not introduce problematic organic by‑products or create new toxicological risks (Rizzo et al., 2008; Frisbie et al., 2015).

• Wastewater and sludge regulations. Discharge consents limit solids, colour, COD and metals; sludge destined for land must meet metal limits and biosolids codes of practice (Scholes et al., 2019; UKWIR, 2017). Lower metal input via coagulant substitution can improve compliance headroom for biosolids, while improved dewaterability supports sludge minimisation strategies.

Guidance on natural polymer coagulants is still evolving. Recent EU and WHO documents on treatment chemicals and materials in contact with drinking water adopt a risk‑based approach, requiring full characterisation, toxicological assessment and performance validation for new products, regardless of whether they are “natural” (WHO, 2017; JRC, 2021). Early adopters should therefore engage regulators and competent authorities early, sharing pilot data and risk assessments as part of change‑control processes.

Where chitosan coagulants make sense today – and where they do not

High‑potential near‑term applications

Based on the current evidence, the most realistic and attractive use‑cases are:

• Municipal plants using coagulant aids

  • Raw waters with low–moderate, relatively stable turbidity.

  • Plants where sludge haulage and disposal costs are significant.

  • Sites operating close to residual aluminium or iron limits, or anticipating tighter standards.

    
    

    In these cases, adding 0.5–3 mg/L chitosan alongside optimised alum or ferric/PAC can reduce metal‑salt doses by roughly one‑third to one‑half (Jafarpour et al., 2014; Babaei et al., 2014; Ma et al., 2019).

• Industrial effluents with strong colour and moderate solids

  
  

  Textile, paper, some food‑processing and aquaculture effluents have shown high colour, turbidity and COD removal with chitosan, either as primary coagulant or as a flocculant after metal‑salt coagulation (Wei et al., 2018; Abdullah and Jaeel, 2019; Elmidaoui et al., 2021; Badawi et al., 2022).

• Decentralised and household systems

  
  

  Point‑of‑use and small community systems benefit from chitosan’s low toxicity, combined turbidity and microbial removal, and straightforward dosing. Coagulation–filtration configurations with chitosan pre‑treatment have demonstrated substantial improvements in microbial safety at manageable doses (Chiew et al., 2016; Amburgey et al., 2022; Tavares et al., 2024).

Less suitable or higher‑risk cases

Chitosan is currently less convincing as a wholesale replacement for metal salts in:

• Very high‑turbidity or highly variable waters, including some storm‑driven surface waters and industrial streams, where ferric or alum remain more robust across load spikes and difficult chemistries (Rizzo et al., 2008; Bina et al., 2008).

• Streams with tighter NOM‑control requirements, where metal salts may still outperform chitosan in DOC and DBP‑precursor removal (Rizzo et al., 2008; Matilainen and Sillanpää, 2010).

• Plants with little scope to adjust pH, such as those running at high pH for corrosion control, where moving into the ideal chitosan window could conflict with other constraints.

• Situations where chitosan supply, cost or regulatory acceptance remain uncertain, making it difficult to justify a high degree of dependence on a bio‑polymer feedstock.

In these contexts, chitosan may still add value in specific steps – for example, as a sludge conditioner or as a polishing/coagulant aid – but not as a full substitute for alum or ferric.

How to design pilots and evaluate success

To move beyond bench‑scale promise, plants need carefully designed jar‑tests and pilots that produce credible, decision‑ready data. A robust approach typically involves:

1. Jar‑testing matrix

   • Establish a baseline with the existing coagulant (e.g. alum or ferric) across typical dose and pH ranges.

   • Compare: (i) metal‑only, (ii) metal + chitosan, and (iii) chitosan‑only (where feasible) at realistic temperature and turbidity conditions (Abu Hassan et al., 2009; Khairul Zaman et al., 2021).

   • Systematically vary mixing conditions and dose sequences (metal first vs simultaneous dosing).

2. Metrics to monitor

   • Clarified water: turbidity, colour, UV₂₅₄, DOC, residual aluminium and iron, microbial indicators where relevant (Khairul Zaman et al., 2021; Tavares et al., 2024).

   • Sludge: settleability, sludge volume index, cake solids, capillary suction time, metals in cake and filtrate (Zhang et al., 2019; Liu et al., 2019; Xu et al., 2016).

   • Operations: coagulant and polymer consumption, any impacts on downstream biological stages or membranes (Ma et al., 2019; Ang et al., 2015).

3. Pilot‑scale trials

   • Side‑stream or dedicated‑train pilots over weeks to months to capture seasonal variations and upset events.

   • Inclusion of worst‑case scenarios (e.g. high turbidity, low temperature) to define the safe operating envelope.

4. Scale‑up decisions

   • Move to full‑scale adoption only where pilots demonstrate robust performance, net cost or ESG benefits, acceptable operational complexity and regulatory comfort.

Working with a technically capable chitosan partner greatly increases the likelihood of a successful pilot. Entoplast supports utilities, industrial plants and technology providers from initial screening and specification of chitosan grade (molecular weight, deacetylation degree, purity) through pilot design, monitoring and full‑scale implementation, ensuring that chosen formulations are optimised for coagulation/flocculation rather than generic bulk use.

Conclusion – re‑thinking coagulation with Entoplast

Re‑thinking coagulation is less about abandoning alum or ferric and more about using bio‑based polymers intelligently to cut metal‑salt dependence, improve sludge behaviour and strengthen ESG performance where the evidence supports it. Current studies show that chitosan:

• Can reduce metal‑salt doses by around one‑third to one‑half in coagulant‑aid roles while maintaining required turbidity and colour removal.

• Often improves sludge dewaterability and reduces metal content in sludge cakes, supporting more sustainable and economical sludge management.

• Aligns with natural‑polymer and circular‑economy narratives, particularly when sourced from sustainable feedstocks such as black soldier fly residues, provided that quality and regulatory requirements are met.

Entoplast offers high‑quality chitin and chitosan, including emerging insect‑derived streams, and technical support to design, pilot and scale chitosan‑based coagulation strategies that work in real plants, under real constraints.