Removing Tannins and Organic Colour from Water Using Organic Scavenger Resin

Yellow, brown or tea-coloured water is a common problem with private water supplies, particularly those sourced from springs, burns, rivers and boreholes in peat-rich areas.

One possible cause is naturally occurring organic matter, often referred to as tannins. These compounds can cause persistent discolouration that ordinary sediment filters struggle to remove.

Organic scavenger systems use specialist ion exchange resins to reduce dissolved organic matter and improve water colour.

However, their effectiveness depends on the type and concentration of organics present, the incoming water chemistry and the design of the treatment system.

This guide explains how organic scavenger resins work, where they are most effective and what information is needed before selecting a suitable system.

What Causes Yellow or Brown Water?

Natural organic matter (NOM) enters water when vegetation, leaves, peat and other plant materials decompose.

This process releases a complex mixture of organic compounds, including:

• Humic acids: Naturally occurring organic compounds associated with decomposing vegetation and peat.

• Fulvic acids: Generally smaller, more soluble organic compounds that can contribute to yellow or brown discolouration.

• Tannins: Plant-derived organic compounds that can also contribute to water colour.

The term tannins is commonly used in water treatment to describe brown organic discolouration, although the actual compounds present may include a wider range of natural organic matter.

These substances are particularly common in upland and peaty water sources.

Water colour may also vary seasonally, becoming more pronounced following heavy rainfall or when organic material enters the supply.

Is Brown Water Always Caused by Tannins?

No.

Discolouration can also result from:

• Dissolved or precipitated iron.

• Manganese.

• Suspended sediment.

• Organic matter associated with iron or manganese.

• A combination of several contaminants.

Identifying the cause is important because different contaminants require different treatment methods.

For example, a sediment filter may remove suspended ferric iron but will not necessarily remove dissolved organic colour.

Likewise, an organic scavenger resin should not be relied upon to remove high concentrations of iron or manganese.

What Is an Organic Scavenger?

An organic scavenger is a water treatment system containing a specialist anion exchange resin designed to remove certain dissolved organic compounds.

Water passes through a pressure vessel filled with resin beads.

The resin attracts and retains negatively charged organic substances through ion exchange and other interactions with the resin structure.

This can reduce dissolved organic matter and improve the colour of the treated water.

Once the resin has accumulated sufficient organic material, its treatment capacity decreases and regeneration is required.

How Does Organic Scavenger Resin Work?

Organic scavenger systems generally operate through four stages:

1. Service: Water passes through the resin bed, where suitable organic compounds are retained.

2. Exhaustion: As organic matter accumulates, the resin's remaining capacity decreases.

3. Regeneration: The resin is treated with a suitable regenerant, commonly sodium chloride brine, to release retained organic matter.

4. Rinsing: The resin is rinsed before returning to service.

The regeneration process allows the resin to be reused over multiple treatment cycles.

However, regeneration does not necessarily remove every accumulated organic compound.

Some organic matter can become strongly bound to the resin, eventually reducing its capacity and useful operating life.

Choosing the Correct Resin

Not all anion exchange resins perform equally when removing natural organic matter.

Resin structure is particularly important because certain organic molecules are relatively large and can be difficult to remove during regeneration.

Two established organic scavenger resins are Purolite A860S and A502PS.

Purolite A860S

Purolite A860S is a macroporous acrylic anion exchange resin commonly used for reducing natural organic matter.

Its structure is designed to provide effective organic removal while allowing a relatively high proportion of retained organic material to be released during regeneration.

This makes it particularly useful for applications involving significant dissolved organic loading.

Purolite A502PS

Purolite A502PS is a macroporous polystyrenic anion exchange resin.

It can offer different organic removal characteristics from acrylic resins and may be suitable where a lower residual concentration of certain dissolved organic compounds is required.

However, some organic compounds may be more difficult to remove during regeneration.

Which Resin Is Better?

Neither resin is automatically the best choice for every application.

Purolite notes that acrylic and polystyrenic organic scavenger resins can complement one another, and some treatment systems use combinations of both.

The appropriate selection depends on the incoming water quality, treatment objective and regeneration requirements.

GAPS Water Treatment supplies both organic scavenger systems and specialist resins.

How Effective Is Organic Scavenger Resin?

Organic scavenger resin can significantly reduce natural organic matter, but removal efficiency varies.

According to Purolite's technical information, field results for A860 resin have demonstrated typical total organic carbon (TOC) reductions of approximately 50–80%, depending on the nature of the organic matter present.

These figures are indicative manufacturer results, not guaranteed performance for every water supply.

Colour removal and TOC reduction are also not the same measurement.

A substantial improvement in visible colour does not necessarily mean that all dissolved organic compounds have been removed.

For this reason, treatment performance should be assessed against the actual incoming water quality and the required treated-water standard.

Limitations of Organic Scavenger Systems

Organic scavengers are effective in suitable applications, but they are not universal water purification systems.

Important limitations include:

Iron and manganese fouling

Elevated iron or manganese concentrations can contribute to fouling and may require separate treatment before the organic scavenger.

Suspended solids

Sediment and colloidal material can increase pressure loss and interfere with resin performance. Appropriate pretreatment may be necessary.

Organic fouling

Some large or strongly retained organic molecules are difficult to remove during regeneration. Progressive fouling can reduce resin capacity.

Water chemistry

pH, dissolved salts, competing ions and the nature of the organic compounds can all influence performance.

Regeneration requirements

Organic scavenger resin requires sufficient contact with the regenerant and appropriate rinsing. Regeneration settings should follow the resin and equipment manufacturer's specifications.

Limited contaminant coverage

An organic scavenger is not a general-purpose treatment for pesticides, hydrocarbons, PFAS, industrial chemicals or every dissolved organic contaminant.

Where such contaminants are suspected, specific water testing and appropriate treatment selection are required.

No microbiological disinfection

Organic scavenger resin does not provide a substitute for UV disinfection or another suitable microbiological treatment process.

Understanding Flow Rates and Resin Sizing

Correct sizing is important because water must remain in contact with the resin for sufficient time.

Organic scavenger systems are generally sized using resin bed volume and an appropriate service flow rate.

For example, Purolite's engineering guidance for A860 describes typical service flow rates of approximately 8–16 bed volumes per hour for certain co-flow applications.

A system containing 50 litres of resin would therefore have an indicative service flow range of:

• 8 bed volumes/hour: 400 litres/hour.

• 16 bed volumes/hour: 800 litres/hour.

This is approximately 6.7–13.3 litres per minute.

These figures illustrate the sizing method only. They are not universal operating limits, and actual design flow depends on resin type, vessel configuration, incoming water quality and treatment requirements.

Both peak water demand and daily organic loading need to be considered.

A system may be hydraulically capable of passing a higher flow than is appropriate for achieving the required treatment performance.

Backwash and regeneration requirements must also be considered when selecting the control valve and vessel.

What Water Testing Is Required?

Before selecting an organic scavenger, we recommend obtaining a suitable water analysis.

Useful parameters include:

Parameter	Why It Matters
Colour (Hazen/Pt-Co)	Indicates the extent of water discolouration
Total Organic Carbon (TOC) or Dissolved Organic Carbon (DOC)	Helps assess organic loading
Turbidity (NTU)	Identifies suspended material that may require pretreatment
Iron and manganese	Helps determine whether additional filtration is required
pH	Influences water chemistry and resin selection
General mineral analysis	Helps assess conditions affecting resin operation
UVT at 254 nm	Particularly useful when UV disinfection will follow

Testing both filtered and unfiltered samples may help distinguish dissolved colour from colour associated with suspended material.

For peaty or surface water supplies, it is also worth considering seasonal variation.

A sample collected during a dry period may not represent the water quality following heavy rainfall.

Why Is Testing After Installation Important?

Testing should not stop once the system has been installed.

Where performance is critical, treated-water samples should be taken after commissioning and periodically during operation.

Comparing inlet and outlet colour or TOC results can help identify declining resin performance or organic breakthrough.

This information can also be used to review regeneration frequency.

Can Organic Scavengers Improve UV Disinfection?

Yes, where the resin successfully reduces organic compounds that absorb ultraviolet light.

Naturally occurring organic matter can lower a water supply's ultraviolet transmittance, commonly called UVT.

Low UVT reduces the amount of UV light passing through the water and can affect the dose delivered by a UV disinfection system.

Reducing suitable dissolved organics upstream may therefore improve UVT and make downstream UV disinfection more effective.

However, visually clear water does not necessarily have acceptable UVT.

Where UV treatment is required for drinking water safety, UVT should be measured and the UV system selected for the actual treated-water conditions.

Read our guide to UV water disinfection and system selection.

Organic Scavenger Resin vs Activated Carbon

Both technologies can reduce certain organic compounds, but they operate differently.

Feature	Organic Scavenger Resin	Activated Carbon
Main process	Ion exchange and adsorption	Adsorption
Common application	Dissolved natural organic matter and colour	Taste, odour and selected organic compounds
Regeneration	Generally regenerated using brine	Usually replaced or professionally reactivated
Performance	Depends on resin chemistry and organic loading	Depends on carbon type, contaminant and contact time
Main limitation	Fouling and incomplete regeneration	Finite adsorption capacity and breakthrough

Activated carbon may be useful as a polishing stage, but it should not automatically be assumed to provide the same tannin-removal performance as a specialist organic scavenger.

The correct choice depends on the contaminants and treatment objective.

Can You Test Organic Scavenger Resin Before Buying a Large System?

For uncertain applications, small-scale testing can be useful.

GAPS Water Treatment supplies a Pallas 10-inch Organic Scavenger Colour Reduction Cartridge containing Purolite A860S resin.

This can provide an initial indication of whether the resin reduces colour in a particular water supply.

However, cartridge testing does not directly establish the required resin volume, full-scale service flow or regeneration frequency.

A successful cartridge trial should therefore be treated as preliminary evidence rather than a guarantee of full-scale system performance.

Organic Scavenger Systems from GAPS Water Treatment

GAPS Water Treatment supplies automatic organic scavenger systems suitable for a range of domestic and commercial applications.

Available options include:

• Organic scavenger systems with Clack control valves.

• Systems using Autotrol Logix control valves.

• Purolite A860S and A502PS organic scavenger resins.

• Small organic scavenger cartridges for preliminary testing.

System selection depends on the water analysis, required flow rate, organic loading and available backwash and regeneration capacity.

Browse our Organic Scavenger Systems and Resins

For assistance selecting equipment, email sales@gapswater.co.uk with your water analysis, required flow rate and details of the water source.