UK drinking water regulations: prescribed values, product approval and treatment design

The design of a potable water treatment works, a private supply installation or a building water system is constrained by a compact body of UK legislation and British Standards. This note identifies the governing instruments in each part of the United Kingdom, tabulates key prescribed values as confirmed on legislation.gov.uk, and sets out how the regulations translate into treatment process requirements, including a disinfection Ct calculation.

Technical note, RB Solutions Ltd, 22 October 2026

Governing instruments in England, Wales, Scotland and Northern Ireland

Drinking water quality is a devolved matter. Each administration has its own regulations for public supplies, with closely aligned parameter sets.

Table 1. Principal UK regulations for drinking water supplied by water undertakers
Jurisdiction	Instrument	Regulator
England	The Water Supply (Water Quality) Regulations 2016, SI 2016/614 [1]	Drinking Water Inspectorate
Wales	The Water Supply (Water Quality) Regulations 2018, WSI 2018/647 (W. 121) [2]	Drinking Water Inspectorate
Scotland	The Public Water Supplies (Scotland) Regulations 2014, SSI 2014/364, as amended [3]	Drinking Water Quality Regulator for Scotland
Northern Ireland	The Water Supply (Water Quality) Regulations (Northern Ireland) 2017, SR 2017/212 [4]	Drinking Water Inspectorate for Northern Ireland

Private supplies, which are not provided by a water undertaker, are regulated separately. In England this is by the Private Water Supplies (England) Regulations 2016, SI 2016/618 [5], enforced by local authorities. Water fittings within premises connected to a public supply are regulated by the Water Supply (Water Fittings) Regulations 1999, SI 1999/1148 [6], in England and Wales.

Role of the Drinking Water Inspectorate

The Drinking Water Inspectorate (DWI) acts for the Secretary of State and the Welsh Ministers as technical assessors under section 86 of the Water Industry Act 1991 [7]. It audits water undertakers' compliance, investigates events, takes enforcement action, and administers approval of products and substances under Regulation 31 [8]. For a designer, the DWI's position is relevant at three points: the risk assessment that justifies a treatment process, the approval status of every chemical and contact material proposed, and the evidence that the treated water will be wholesome at the consumer's tap.

Wholesomeness and prescribed concentrations or values

Regulation 4 of the 2016 Regulations defines wholesome water. It must not contain any micro-organism, parasite or substance at a concentration or value that would constitute a potential danger to human health. It must not exceed (or fall below, as applicable) the prescribed concentrations or values in Tables A and B of Schedule 1. It must also satisfy the nitrate–nitrite formula [1]:

\frac{[nitrate]}{50} + \frac{[nitrite]}{3} \leq 1

where [nitrate] is the concentration as NO₃ (mg L⁻¹) and [nitrite] is the concentration as NO₂ (mg L⁻¹); each term is dimensionless. Source: regulation 4(2)(d) [1].

Table 2 lists values for parameters of direct relevance to treatment design. Each was confirmed against Schedule 1 of SI 2016/614 on legislation.gov.uk on 22 October 2026 [1]. The point of compliance matters to the designer. Most values apply at consumers' taps, so the treatment works must deliver water that remains compliant after distribution and domestic plumbing.

Table 2. Selected prescribed concentrations or values, Water Supply (Water Quality) Regulations 2016, Schedule 1 [1]
Parameter	Value	Units	Point of compliance	Schedule 1 table
Escherichia coli (E. coli)	0	number/100 ml	Consumers' taps	A, Part I
Enterococci	0	number/100 ml	Consumers' taps	A, Part I
Coliform bacteria	0	number/100 ml	Service reservoirs (95 % of samples) and water treatment works	A, Part II
E. coli	0	number/100 ml	Service reservoirs and water treatment works	A, Part II
Lead	10	µg Pb/l	Consumers' taps	B, Part I
Nitrate	50	mg NO₃/l	Consumers' taps	B, Part I
Nitrite	0.50	mg NO₂/l	Consumers' taps	B, Part I
Nitrite	0.10	mg NO₂/l	Treatment works	B, Part I
Bromate	10	µg BrO₃/l	Consumers' taps	B, Part I
Trihalomethanes, total (sum of chloroform, bromoform, dibromochloromethane, bromodichloromethane)	100	µg/l	Consumers' taps	B, Part I
Aluminium	200	µg Al/l	Consumers' taps	B, Part II
Iron	200	µg Fe/l	Consumers' taps	B, Part II
Manganese	50	µg Mn/l	Consumers' taps	B, Part II
Turbidity	4	NTU	Consumers' taps	B, Part II

The Welsh, Scottish and Northern Ireland instruments contain corresponding schedules [2][3][4]. For a project in those jurisdictions the values are confirmed against the applicable instrument, not assumed from the English Regulations. Schedule 1 is amended from time to time, so a design basis cites the version in force on a stated date.

Worked check: nitrate–nitrite formula

Inputs. A blended supply has 40 mg L⁻¹ nitrate (as NO₃) and 0.40 mg L⁻¹ nitrite (as NO₂) at the tap. Each parameter individually complies with Table 2. The formula gives 40/50 + 0.40/3 = 0.800 + 0.133 = 0.933, which is not more than 1, so the water complies. As a second route, solving for the maximum nitrate permitted at 0.40 mg L⁻¹ nitrite gives 50 × (1 − 0.40/3) = 43.3 mg L⁻¹. The 40 mg L⁻¹ present is below that limit, which is consistent. At 45 mg L⁻¹ nitrate the same water would fail the formula while meeting both individual values. This shows that the formula, not the individual values, governs blends where nitrification in distribution generates nitrite.

Risk assessment requirements

Regulation 27 requires each water undertaker to carry out a risk assessment for each treatment works and its connected supply system. Its purpose is to establish whether there is a significant risk of supplying water that could constitute a potential danger to human health or would be unwholesome. The assessment is to be kept under continuous review [1]. For private supplies in England, regulation 6 of SI 2016/618 places a corresponding risk-assessment duty on the local authority [5]. In design terms, the risk assessment is the document that links each hazard in the catchment, source, treatment and distribution to a control measure. A treatment process is justified by the hazard it controls, and its performance requirement (for example, a log reduction of a pathogen or a removal of a precursor) is derived from that assessment.

Regulation 31: approval of products and substances

Regulation 31 prohibits a water undertaker from applying or introducing any substance or product into water intended for supply unless certain conditions are met [1]. It may have been approved by the Secretary of State and be used in accordance with any conditions of approval. It may conform to an appropriate British Standard or equivalent, subject to any limits on dosing. It may be a product the Secretary of State is satisfied is unlikely to affect water quality adversely. It may be used for testing or research, after the required notice. The DWI publishes the list of approved products, including treatment chemicals and filtering materials covered by BS EN standards with national conditions of use [8]. For a designer the consequences are concrete:

Every treatment chemical (coagulant, pH adjuster, disinfectant, polyelectrolyte) is specified to its BS EN product standard and checked against the conditions of use, including any maximum dose.
Every material in contact with water in the treatment works and distribution system, such as coatings, membranes, resins, pipe linings and sealants, is checked for approval status before specification.
Products used only within a building fall under the Water Fittings Regulations rather than Regulation 31 [8].

Private supplies and water fittings

The Private Water Supplies (England) Regulations 2016 apply wholesomeness standards, monitoring and risk assessment to supplies not provided by a water undertaker, such as boreholes, springs and private distribution networks serving commercial or public premises [5]. A treatment installation for such a supply is designed against the same parameters as a public supply. Its risk assessment, monitoring frequency and enforcement are handled by the local authority.

The Water Supply (Water Fittings) Regulations 1999 restrict the installation of fittings that are not of an appropriate quality and standard, and set requirements for fittings in Schedule 2, including backflow prevention [6]. They apply to every connection from a public supply into premises, including a process plant's potable water feed, and to the fluid-category assessment that determines the backflow protection required at each draw-off point.

Treatment process implications

Coagulation and filtration

The tap values for aluminium (200 µg Al/l) and iron (200 µg Fe/l) in Table 2 [1] directly constrain coagulant selection and dosing. Residual coagulant metal passing the filters, together with any post-filtration precipitation, must remain below these values with an allowance for distribution. Turbidity at the tap is limited to 4 NTU [1]. Filtered water turbidity at the works is controlled to a much lower operational target, because turbidity is a surrogate for the removal of particles, including protozoan cysts and oocysts that resist chlorine. The design of coagulation, clarification by lamella clarifier or flotation, and rapid gravity or pressure filtration is therefore driven by the particle and pathogen log-removal required by the risk assessment, not by the tap turbidity value alone.

Disinfection and the Ct concept

Chemical disinfection is described by the Chick–Watson model, in which the log inactivation of an organism depends on the disinfectant concentration and contact time [9]:

\ln \frac{N}{N_{0}} = - k C^{n} t

where N/N₀ is the surviving fraction of organisms (dimensionless), k is the coefficient of specific lethality ((L mg⁻¹)ⁿ min⁻¹), C is the residual disinfectant concentration (mg L⁻¹), n is the coefficient of dilution (dimensionless) and t is contact time (min).

With n ≈ 1, inactivation depends on the product C × t. This is the basis of the Ct design approach. The contact time credited is not the hydraulic residence time but the time by which 10 % of a tracer has passed, T₁₀, which accounts for short-circuiting [9]:

Ct = C \times T_{10}, T_{10} = BF \times \frac{V}{Q}

where Ct is in mg min L⁻¹, C is the residual free chlorine at the contact tank outlet (mg L⁻¹), T₁₀ is the 10th-percentile residence time (min), BF is the baffle factor T₁₀/T (dimensionless), V is the effective contact volume (m³) and Q is the peak flow through it (m³ min⁻¹).

The DWI guidance on private supply treatment cites the World Health Organization criterion of a free chlorine concentration of at least 0.5 mg L⁻¹ after at least 30 minutes of contact at pH below 8.0, which corresponds to a Ct of 15 mg min L⁻¹ [10][11]. The efficacy of free chlorine falls as pH rises because the hypochlorous acid fraction falls. The pH condition is part of the criterion and is not optional.

Worked example: Ct for a chlorine contact tank

Inputs. A contact tank has an effective volume of 400 m³ and a peak flow of 800 m³ h⁻¹ (13.33 m³ min⁻¹). A tracer test at peak flow has measured T₁₀ = 18 min. The free chlorine residual at the outlet is 0.9 mg L⁻¹ at pH 7.4. The criterion is Ct ≥ 15 mg min L⁻¹ at pH < 8.0 [10][11].

Route 1, from the measured T₁₀. Ct = 0.9 × 18 = 16.2 mg min L⁻¹, which meets the criterion with a ratio of 16.2/15 = 1.08.

Route 2, from hydraulic residence time and baffle factor. The theoretical residence time is T = V/Q = 400/13.33 = 30.0 min. The tracer result implies BF = 18/30.0 = 0.60. Hence Ct = 0.9 × 0.60 × 30.0 = 16.2 mg min L⁻¹, which agrees.

Sensitivity. The minimum outlet residual that achieves the criterion is 15/18 = 0.83 mg L⁻¹. If flow rose to 960 m³ h⁻¹ and BF stayed at 0.60, T₁₀ would fall to 15 min. The residual would then need to be at least 1.0 mg L⁻¹. Ct is therefore evaluated at the maximum flow through the tank and the minimum outlet residual, not at average conditions.

Table 3. Ct worked example results
Case	Q (m³ h⁻¹)	T₁₀ (min)	C (mg L⁻¹)	Ct (mg min L⁻¹)	Criterion 15
Design peak	800	18.0	0.90	16.2	Met
Minimum residual at design peak	800	18.0	0.83	15.0	Limit
Flow 20 % above design	960	15.0	0.90	13.5	Not met

Disinfection design must also respect the tap values for disinfection by-products. Total trihalomethanes are limited to 100 µg/l and bromate to 10 µg BrO₃/l [1]. This favours removal of natural organic matter before chlorination, and control of ozone dose where bromide is present, over increases in disinfectant concentration.

British Standards for supply systems and installations

BS EN 805:2000 sets requirements for water supply systems and components outside buildings, including design, pressure testing and commissioning of mains [12].
BS EN 1508:1998 sets requirements for systems and components for the storage of water, including service reservoirs and their disinfection and testing [13].
BS EN 1717:2000 classifies fluid categories and specifies protection against pollution of potable water by backflow. It is the technical basis for selecting air gaps, check valves and reduced-pressure zone devices at each connection [14].
BS 8558:2015 gives UK guidance on the design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages, complementing BS EN 806 [15].

References

The Water Supply (Water Quality) Regulations 2016, SI 2016/614. legislation.gov.uk/uksi/2016/614 (Schedule 1: /schedule/1; regulation 4: /regulation/4; regulation 27: /regulation/27; regulation 31: /regulation/31)
The Water Supply (Water Quality) Regulations 2018, WSI 2018/647 (W. 121). legislation.gov.uk/wsi/2018/647
The Public Water Supplies (Scotland) Regulations 2014, SSI 2014/364. legislation.gov.uk/ssi/2014/364
The Water Supply (Water Quality) Regulations (Northern Ireland) 2017, SR 2017/212. legislation.gov.uk/nisr/2017/212
The Private Water Supplies (England) Regulations 2016, SI 2016/618. legislation.gov.uk/uksi/2016/618
The Water Supply (Water Fittings) Regulations 1999, SI 1999/1148. legislation.gov.uk/uksi/1999/1148
Water Industry Act 1991, section 86. legislation.gov.uk/ukpga/1991/56/section/86
Drinking Water Inspectorate. Regulation 31 approval, and List of Approved Products. dwi.gov.uk/drinking-water-products/reg-31-approval/; approved and considered products
Crittenden, J.C., Trussell, R.R., Hand, D.W., Howe, K.J. and Tchobanoglous, G. (2012) MWH's Water Treatment: Principles and Design, 3rd ed. Hoboken: Wiley.
Drinking Water Inspectorate. How to ensure sufficient disinfection (private water supplies treatment guide). dwi.gov.uk
World Health Organization (2022) Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda. Geneva: WHO.
BS EN 805:2000 Water supply. Requirements for systems and components outside buildings. London: BSI.
BS EN 1508:1998 Water supply. Requirements for systems and components for the storage of water. London: BSI.
BS EN 1717:2000 Protection against pollution of potable water in water installations and general requirements of devices to prevent pollution by backflow. London: BSI.
BS 8558:2015 Guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages. Complementary guidance to BS EN 806. London: BSI.

Related technical notes

RB Solutions Ltd provides regulatory basis-of-design reviews, treatment process selection, Ct and contact tank design, and product approval checks for public and private drinking water supplies.

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