Guidance documents

Guide for private supply owners/users

1.1 Guide to the selection of chemical disinfection systems

What is a private supply?

In general, a private water supply is any supply that is not provided by a water company. The supply may serve a single property or several properties, commercial or public premises, be permanent or temporary (e.g. for events such as festivals and fairs) and may be sourced from a well, borehole, spring, stream, lake, or similar.

All private water supplies must meet regulations1 (PWS Regulations) which include quality standards to ensure that the water is safe to consume. The regulations include the requirement that a supply must ‘not contain any micro-organism, parasite or substance … at a concentration or value that would constitute a potential danger to human health’.

The regulations are implemented by local authorities who are responsible for monitoring private supplies2 through inspections (‘risk assessments’) and monitoring, and will advise of the actions to be taken if a supply fails to meet the required standards and/or are required to enforce where necessary in accordance with the regulations.

What is chemical disinfection?

Chemical disinfection is the dosing of a chemical to water to kill or inactivate harmful microorganisms and parasites that could otherwise cause illness if the water is consumed.

The effectiveness of chemical disinfection depends on a number of factors:

  • The chemical disinfectants used;
  • The type of micro-organism or parasite;
  • Water quality;
  • The dose of chemical disinfectant applied; and
  • The contact time.

What chemical disinfectants can be used?

Only chemicals that meet the requirements of Regulation 5 of the PWS Regulations may be used in private supplies. For chemicals, this generally requires conformity with the relevant BS EN standard. Only products specifically produced for drinking water applications should be used.


Chlorine is the most common chemical disinfectant used for drinking water treatment. It can be supplied as a liquefied gas, as a liquid (sodium hypochlorite), or as a solid (calcium hypochlorite tablets). It is also possible to produce sodium hypochlorite locally by the electrolysis of brine.

Chlorine dioxide

Chlorine dioxide is normally generated at the point of use, most commonly by mixing solutions of sodium chlorite and either hydrochloric acid or sodium hypochlorite.

How should chemicals be stored?

Any storage instructions and ‘use-by’ dates provided by suppliers must be complied with.

Specific advice for sodium hypochlorite solution

Sodium hypochlorite solutions should not be exposed to light.

High-strength solutions (14-15% active chlorine) should ideally be stored at or below 15oC. At temperatures above 15oC, such solutions should ideally be used within one month. At 15oC, such solutions should ideally be used within 2 months, and at lower temperatures should ideally be used within 3 months.

If it is necessary to store sodium hypochlorite solution during the summer for longer than 1 month in the absence of temperature control, a lower strength solution (e.g. 10% active chlorine) should be specified.

Each delivery of solution should be consumed as a batch. Fresh solution should not be mixed with older stock. It is normally necessary to dilute high-strength solution to produce a lower strength solution suitable for dosing; each batch of diluted solution should also be consumed and not mixed with a newer batch.

Solutions with a delivered bromate concentration lower than that permitted for a Type 1 product under BS EN 901:2013 should be specified. BS EN 901:2013 Type 2 solutions should not be used.

Specific advice for on-site electrolysis (OSE) generated sodium hypochlorite solution

OSE-generated solution should be consumed within 2 days of production.

For non-membrane OSE equipment, salt which meets the specifications for a Type 1 product under BS EN 14805:2008 should be used.

Is chemical disinfection effective for all micro-organisms and parasites?

Broadly, chemical disinfectants are effective for bacteria (there are exceptions, notably spore-forming bacteria such as Clostridia and Bacillus) and most viruses. Chlorine is not recommended for disinfection of Giardia (a type of parasite) and is ineffective for Cryptosporidium (another type of parasite). Chlorine dioxide is more effective for Giardia than chlorine, but ineffective for Cryptosporidium. If a source is considered to be at risk from Cryptosporidium or Giardia, it is recommended that ultraviolet (UV) disinfection3 be applied instead of, or in addition to, chemical disinfection.

Why is water quality important and why pre-treatment may be needed?

For chemical disinfection to be effective, the water must be clear and relatively free from certain dissolved substances that may react with the applied chemical, reducing its effectiveness towards micro-organisms and potentially causing aesthetic problems or problems with harmful by-products. Appropriate pre-treatment may therefore be needed. The water quality of any prospective raw water source must be tested in order to identify what treatment is necessary. The local authority will be able to advise on accredited laboratories, and may be able to provide this service itself.

The cloudiness of water is measured as turbidity. Turbidity is caused by fine particulate matter, which can shield micro-organisms from the chemical disinfectant.

Groundwater, especially, can contain dissolved metals such as iron and manganese, or dissolved hydrogen sulphide, which react with chemical disinfectants to produce nuisance solids. Dissolved natural organic matter reacts with chemical disinfectants to varying degrees, generating undesirable disinfection by-products. Some specific disinfection by-products are included in the water quality regulations, and there is a regulatory requirement to operate disinfection to “…keep disinfection by-products as low as possible without compromising the effectiveness of the disinfection”.

Ammonia can be present in surface or groundwater sources. It reacts with chlorine, the products of the reaction being dependent on the relative proportions of chlorine and ammonia. Chlorine consumed in reactions with ammonia will not be available for disinfection, so the concentration of ammonia must be accounted for when determining the chlorine dose. Chlorine dioxide does not react with ammonia.

The pH of the water influences the disinfection strength of chlorine. The pH for chlorine disinfection should be less than pH 8. The disinfection strength of chlorine dioxide is relatively insensitive to pH across the range pH 6-9.

Disinfection occurs more slowly at lower temperatures.

Free, Combined and Total Chlorine

After chlorine is added to water, it can exist in two forms before it is consumed:

  • unreacted or ‘free’ chlorine; or
  • ‘combined’ chlorine (particularly when ammonia is present).

The sum of ‘free chlorine’ and ‘combined chlorine’ concentrations is called ‘total chlorine’.

It is the ‘free chlorine’ concentration which is of primary significance for chlorine disinfection, because it is a much stronger disinfectant than ‘combined chlorine’4.

What dose is needed?

The effectiveness of a chemical disinfectant depends on both the concentration of the chemical and the contact time. The required exposure to disinfectant can be expressed in terms of the product of concentration (C, mg/L) and contact time (t, minutes), Ct. Under the regulations that apply in England and Wales there are no prescribed values of C, t or Ct, but guidance is provided by the World Health Organisation5 (WHO) and the United States Environmental Protection Agency6 (USEPA):

  • Chlorine
    • WHO: For disinfection of bacteria and viruses, there should be a free chlorine concentration of ≥ 0.5 mg/L after at least 30 minutes contact time (so Ct = 30 x 0.5 = 15 mg.min/L), at pH < 8.0. At the point of delivery, the minimum free chlorine concentration should be 0.2 mg/L.
  • Chlorine dioxide
    • WHO: Ct of 2 (at 10oC) – 30 (at 0oC) mg.min/L for 99% inactivation of viruses, pH 7-9.
    • USEPA: Ct of 13 (at 20oC) – 33.4 (at 5oC) mg.min/L for 99.99% inactivation of viruses, pH 6-9.
    • USEPA: Ct of 15 (at 20oC) – 26 (at 5oC) mg.min/L for 99.9% inactivation of Giardia, pH 6-9.

If using chlorine dioxide, there is a regulatory restriction (in Annex 2 of the Approved Products list7, which applies under PWS Regulation 5) on how much can be dosed: the sum of chlorine dioxide, chlorate and chlorite concentrations must not exceed 0.5 mg/L. The practical consequence is that unless there is close control of dose, it should be limited to 0.5 mg/L.

What concentration is used to calculate Ct?

The concentration of disinfectant decays over time, so the concentration used to determine Ct is that measured at the outlet of the system, i.e. after the contact time t, as indicated in Figure 1.

Figure 1 Schematic of chemical disinfection system.

Schematic of chemical disinfection system

What contact time is used to calculate Ct?

The average retention time of water in the contact tank, Tav, is determined from the volume of water in the contact tank, V, and the volumetric flow rate, Q:

In a real tank or vessel, there is likely to be some degree of short-circuiting, so some proportion of the water is retained in the contact tank for less than Tav. The contact time used to calculate Ct should make allowance for this. In the absence of any better estimate the effective contact time used to calculate Ct should be assumed to be 30% of the average retention time.

What monitoring is required?

Where a disinfection process is included as part of treatment of a private water supply, it is a regulatory requirement to:

  1. Ensure that the effectiveness of the disinfection process is maintained; and
  2. Verify the effectiveness of the disinfection process.

To comply with these requirements, the residual disinfectant concentration must be monitored. Options for measuring residual concentration are:

  • Test strips. A test strip is dipped in a sample of the water, and a reagent coating on the test strip reacts with the disinfectant causing a colour to develop, the intensity of which is related to disinfectant concentration. This is the simplest option, but has the lowest resolution – the colour scale graduations are typically in steps of 0.5 mg/L. Test strips are not recommended for verification monitoring because of the low resolution. They may be useful for determining presence/absence of a residual at a customer’s tap.
  • Comparators. These are manual instruments, whereby a reagent (in tablet, powder or liquid form) is added to a sample of the water in a glass tube or cuvette, causing a colour to develop, the intensity of which is related to disinfectant concentration. The reagent is usually N,N-diethyl-p-phenylenediamine (DPD), which imparts a red colour on reaction with the disinfectant. The colour is then visually compared to a coloured scale, typically graduated in steps of 0.2 mg/L. There is some subjectivity in the result when interpolating between graduations, and further variability might be introduced if using such a device under different light conditions. The scale should be chosen such that the target residual concentration being measured is towards the middle of the scale range; hence if the target residual is 0.3 mg/L, a scale range of 0 – 1.0 mg/L is suitable.
  • Colorimeters/Photometers. These are electronic instruments, whereby a reagent (in tablet, powder or liquid form) is added to a sample of water in a glass tube or cuvette, causing a colour to develop, the intensity of which is related to disinfectant concentration. The reagent is usually DPD, which imparts a red colour on reaction with the disinfectant. Concentration is determined automatically by passing a beam of light of fixed wavelength through the sample and measuring how much of the light is transmitted relative to a blank sample. Accuracy is about ±0.05 mg/L. This type of device removes the subjectivity of visually comparison, and avoids possible variability of different light conditions.
  • Amperometric sensors. The disinfectant concentration is measured using an electrode. Sensors of this type are more commonly used for continuous monitoring, but some portable meters are available. They are not suitable for users without appropriate technical experience because of the care and maintenance requirements of the sensors.

How frequently the residual concentration should be monitored depends on how variable it is. Daily monitoring is the default. If experience shows little day-to-day change, then less frequent monitoring may be justifiable; the minimum frequency is weekly.

Turbidity is measured by determining the extent of light scatter. Portable instruments are available, but are relatively expensive (c. £1,000). On cost grounds such an instrument is difficult to justify for the smallest supplies (single domestic dwellings). For larger supplies, a judgement must be made taking account of the nature of the water source, the adequacy of treatment, and the nature of the supply (domestic or commercial), as to whether the additional assurance of water quality provided by routine monitoring of turbidity justifies the cost.

The disinfection pH should be checked. Test strips are the simplest means, but strips with a broad pH range (0-14 or 1-13) should be avoided because resolution will be low. Simple electrodes are available but may need routine maintenance such as periodic calibration to retain accuracy. Variability of pH will normally be lower in groundwater sources than surface water, and this will influence frequency of measurement.

How should the treatment system be operated and maintained?

All treatment units must be operated and maintained according to manufacturers’/suppliers’ instructions. In particular, cartridges and filters must be replaced at recommended intervals. Service contracts with specialist companies should be considered, particularly for more complex systems.

When any maintenance is performed on the treatment system or infrastructure, precautions must be taken to avoid contamination. It is recommended that dedicated tools are kept for the purpose of working on any part of the system that is in contact with the water.

Planning for loss of disinfection

It is particularly important for supplies which have multiple customers to have plans in place for responding to events which might cause loss of disinfection. Owners should consider the following questions:

  • What might happen that will result in loss of disinfection?
  • What operational or procedural steps can be taken to minimise risk of loss of disinfection?
  • How will loss of disinfection be detected?
  • Can the distribution to customers of water which hasn’t been disinfected be prevented?
  • How will customers be contacted, and what advice will they be given?
  • What contingency arrangements are there to provide an alternative supply?

These plans should be documented, made available to and be understood by all involved in operating the supply. They should be discussed with the LA, as they are likely to inform the risk assessment of the supply that the LA undertakes.

Can disinfected water be stored?

Disinfection with chlorine or chlorine dioxide provides a disinfectant residual. In principle, it is safe to store chemically disinfected water for as long as a measurable residual remains. It is, however, critical that any water storage tanks are secure from contamination and are hygienically maintained.

Where can I obtain further information?

Further information can be obtained from:

  1. Your local authority.
  2. The Drinking Water Inspectorate (DWI) (
  3. Manual on Treatment for Small Water Supply Systems (updated report) (
  4. Principles and Practices of Drinking-water Chlorination (WHO)

1 For England, The Private Water Supplies Regulations 2016 and The Private Water Supplies
(England) (Amendment) Regulations 2018. For Wales, The Private Water Supplies (Wales)
Regulations 2017. Available to download from:
2 In England, a supply to a private single dwelling, where there is no commercial or public activity, is
excluded from monitoring unless requested by the supply owner. In Wales, this exclusion applies
except where the dwelling is part of a domestic tenancy.
3 Guidance on UV disinfection can be found at:
4 Ammonia is sometimes added deliberately to form combined chlorine, which, although a weaker
disinfectant than free chlorine, provides a longer-lasting residual.
5 WHO (2017). Guidelines for Drinking-water Quality (4th edition incorporating the First Addendum).

6 USEPA (2003). LT1ESWTR Disinfection Profiling and Benchmarking Technical Guidance Manual.
EPA 816-R-03-004.