How does UV light disinfection work?

How does UV light disinfection work?

Ultraviolet irradiation

Ultraviolet (UV) irradiation is the preferred method for disinfection of small supplies except

for larger schemes in which it is necessary to maintain a residual disinfectant during storage and

distribution. UV disinfection efficiency is particularly affected by water quality and flow rate. The

water to be disinfected must be of good quality and particularly low in colour and turbidity. It is

generally necessary for the turbidity of water to be less than 5 NTU, preferably much lower, for

successful UV disinfection. Therefore pre-filtration is necessary, especially if Cryptosporidium is

likely to be present, as discussed below.

Aquada UV Disinfection for a private water supply

Aquada UV Disinfection for a private water supply

How does UV light disinfection work?

Special lamps are used to generate UV radiation, and are enclosed in a reaction chamber made

of stainless steel or, less commonly, plastics. Low pressure mercury lamps, which generate 85% of

their energy at a wavelength of 254 nm, are most commonly used; their wavelength is in the

optimum germicidal range of 250 to 265 nm. These lamps are similar in design, construction and

operation to fluorescent tubes except that they are constructed of UV-transparent quartz instead of

phosphor-coated glass. The optimum operating temperature of the lamp is around 40 °C so the lamp

is normally separated from the water by a ‘sleeve’ to prevent cooling by the water. The intensity of

UV radiation emitted decreases with lamp age; typical lamp life is about 10 to 12 months after

which the output is about 70% of that of a new lamp, and lamp replacement is required.

Quartz Sleeve

The usual UV reactor configuration is a quartz-sleeved low pressure mercury lamp in direct

contact with the water; water enters the unit and flows along the annular space between the quartz

sleeve and the wall of the chamber. Other configurations include lamps separated from the water,

for example lamps surrounded by ‘bundles’ of PTFE tubes through which the water flows.

Disinfection will only be effective provided that a sufficient dose of UV is applied. The ‘dose’ of

UV radiation is expressed as an energy flux, in units of mW.s/cm2 (milliwatt seconds per square

centimetre), which is the product of the intensity given out by the lamp and the residence time of

water in the reactor. The minimum dose required for disinfection depends on several factors, including

the susceptibility of micro-organisms but is generally taken to be in the range 16 to 40 mW.s/cm2.

It is important, to ensure effective disinfection, that both residence time and UV intensity are

adequate. UV intensity will be diminished by ageing of the lamp, fouling of the lamp by deposits,

and absorption of UV radiation by water contaminants such as natural colour. For these reasons,

lamps need to be changed at the recommended intervals and the quartz sleeve may require periodic

cleaning. Some units incorporate a manual ‘wiper’ for cleaning whilst others incorporate automatic

mechanical cleaning.

Colour and turbidity

Colour and turbidity will both affect radiation intensity in the reactor and turbidity may protect

micro-organisms from the radiation. The water to be treated should be tested for transmissivity by

the manufacturer or supplier in order to estimate worst-case transmission values and to adjust

contact time accordingly. More advanced units incorporating UV monitors have the facility to

automatically adjust the energy input to the UV lamp to achieve the required UV intensity.

Flow rate

The water flow rate affects the retention time in the reactor, which is designed for a maximum

flow rate. The maximum water flow rate should not be exceeded.


There is evidence that UV is effective in inactivating Cryptosporidium provided that a

sufficient UV dose is applied although there is a dearth of data on effectiveness under high-risk

conditions of water quality. However, where Cryptosporidium is likely to be present and cyst

removal is required then pre-filtration capable of removing particles of 1 mm diameter is

recommended prior to UV disinfection. Pre-filtration provides an additional barrier to passage of

oocysts into the treated water, removes particles that shield micro-organisms from the UV light

and helps to reduce fouling of the UV lamp.


UV irradiation equipment is compact and simple to operate. Maintenance requirements are

modest, although specific systematic maintenance is essential. Other advantages include short

contact time and the absence of any known by-products of significance to health. An ‘overdose’ of

UV presents no danger and actually adds a safety factor. The principal disadvantage is the absence

of any residual effect, necessitating careful attention to hygiene in the storage and distribution


Hard Water Scale

The build-up of scale on the sleeves of the lamps will eventually reduce their transmittance and

they must be cleaned or replaced regularly. Some units have UV intensity monitors and alarms

which provide a continuous check on performance and these are strongly recommended. These

devices may prevent the flow of water if the required intensity of UV radiation is not achieved, for

example when the lamps are warming-up or because of scale formation. UV intensity monitors

may not be available on smaller units and it is therefore essential that the manufacturer’s

instructions regarding lamp warm-up, cleaning and replacement are followed to ensure optimal


Lamp replacement

Lamp replacement is usually a simple operation but may involve a significant downtime for

reactors with many lamps. This difficulty may be overcome by use of multiple units or by having

a treated water storage tank capable of maintaining supply whilst maintenance is carried out. The

materials of construction and design of storage systems should not allow deterioration in water

quality to occur.

Acknowledgement: The primary source of information whilst preparing this document is Drinking Water Regulator in Scotland (DWRS).

Arsenic In Drinking water: Academic overview

Arsenic In Drinking water: Academic overview

Arsenic in Drinking Water

Arsenic is a metalloid element that is found from time to time in water sources in the UK. Its toxic properties are well known, although it has many industrial uses, including wood preservatives, glass and semiconductor manufacturing. Arsenic in drinking water is a major problem in some parts of Asia such as Bangladesh, where it is having devastating health effects on communities.

Arsenic in its common form

Arsenic is commonly found as sulphide minerals, and may also be associated with iron and manganese deposits.

Arsenic in drinking water

Arsenic in drinking water

Health Concerns

Arsenic is both acutely and chronically toxic to humans. The extent of toxicity is dependent on the chemical form of the arsenic. Prolonged exposure to arsenic can cause skin lesions, skin, bladder and lung cancers. The WHO has set 10 g/l as the health based guideline value. It acknowledges that treatment of arsenic to below this value can be difficult, but points out that increased health risks have been identified from drinking water at concentrations below 50g/l. It highlights an urgent need for further epidemiological studies to improve understanding of the risks.

Risk Assessment and Monitoring

The Private Water Supply regulations require regular monitoring for arsenic where it is present at more than 75% of the PCV. Arsenic is primarily a risk where it is naturally occurring in local mineral deposits. These can be quite localised, and studies by the British Geological Survey (BGS) into metal concentrations in stream sediments may be helpful in determining the risk from arsenic in a particular area.

Surveys in England and Wales

Surveys of 1200 groundwater sources in England and Wales Shand et al. (2007) , have shown 6% to have concentrations in excess of the 10g/l WHO guideline value and PCV. In the same study, 1% of sources had arsenic in excess of 50g/l, although the majority (68%) contained arsenic at concentrations below 1ug/l.

In groundwaters, the concentration of arsenic can vary greatly with depth. There has been shown to be an association between higher pH groundwaters and higher arsenic concentrations. Man-made sources of arsenic include pesticides and timber preservatives. Water sources where these are or could have been used should be considered at potential risk of arsenic contamination and sampled accordingly.

What should I do if a sample fails?

If a water sample fails for arsenic it would be prudent to gather additional samples to verify the failure and determine the variability of the concentration of arsenic in water. If there are multiple sources, it would be worth sampling each one to determine whether one source has greater levels of contamination than the others.

Check the following:

Is it likely that arsenic is naturally occurring, based on other sample data from supplies in the area and BGS data?
Is there any history of industrial processes that could have used arsenic (paints, pesticides, timber preservation), or large accumulations of products that could have been treated with these products? If multiple sources, are concentrations of arsenic consistent across these?
If the source is a groundwater, how much is known about the construction of the borehole or spring? Is it known at what depth water is being drawn off?

Any failure of the arsenic PCV will also exceed the WHO health-based guideline value. Health advice should be sought.

Arsenic with an industrial origin

Options for resolving at source Where there is an obvious point source of industrial origin, it may be possible to reduce the concentration of arsenic in the water by identifying and removing the contamination. If contamination is only present in one of a number of sources on the same supply, it may be possible to discontinue use of that source and rely on the others, if they provide sufficient quantity.

Naturally occurring arsenic

With naturally occurring arsenic it may similarly be possible to favour sources which have lower concentrations. Where the source is a borehole, arsenic-rich water may only be entering at certain levels or horizons. It may be possible to extend the borehole casing to screen these off, in order to only abstract water with reduced arsenic concentrations. In certain circumstances, the development of a new source of water, either to blend with the existing source or as an alternative supply, may be cost effective when compared against the initial and ongoing maintenance costs of treatment.

Treatment of arsenic

There are a number of options for arsenic removal, both at source and at point of use. The chemistry of arsenic is complex, and most removal process work optimally with arsenic in oxidation state 5 [As(V)]. As arsenic could exist in trivalent form [As(III)], especially in groundwaters, it may be necessary to introduce an oxidation stage prior to any removal process. This need not be complicated, but it is likely to involve addition of an oxidising chemical such as chlorine or potassium permanganate as simple aeration is unlikely to be sufficient. Professional technical advice should be sought concerning the oxidation state of arsenic in a particular supply and the requirement and most appropriate method of pre-oxidation. The US EPA have produced a comprehensive document on arsenic removal technologies.

Most viable treatments

Listed below are the most viable treatment solutions for arsenic removal on small supplies:

Activated Alumina

Activated Alumina is an adsorptive filtration process. The media consists of small granules of aluminium oxide which have been specially treated at high temperature. Arsenic ions adsorb onto the surface of the grains and are removed from the water stream. When the alumina media is exhausted it is usually discarded and replaced with fresh alumina, although regeneration on or off-site is possible. In order to remove arsenic effectively, it needs to be in pentavalent form [As(V)] and may therefore require pre-oxidation from As(III), especially if the source is a groundwater. The process is also very pH sensitive and operates best at a pH between 5.5 and 6.0, although in practice anything at pH less than 7 should prove satisfactory.

pH Range

This pH range clearly suits upland waters with high humic acid content, provided dissolved organic carbon (DOC) concentrations are not too high. This process is not especially selective of the arsenic ion, therefore if the water has significant concentrations of other minerals, these can competitively adsorb onto the media and reduce the efficiency of the process. Fluoride in excess of 2mg/l (which is unlikely in the UK) and dissolved organic carbon (DOC) in excess of 4mg/l can have this effect and may require pre-treatment. High concentrations of iron and manganese can also have a detrimental effect on the process by fouling the media. It is feasible to operate activated alumina as a point of use system, but the need for pre-oxidation should be remembered. If this occurs too far upstream, it is possible for reduction of arsenic to take place in the plumbing system prior to the POU treatment, impeding the effectiveness of arsenic removal.

Iron Based Adsorbants

Arsenic has a strong chemical affinity to iron, and a number of iron based adsorbents have been developed to exploit this. There are a range of media on the market, including Bayoxide ® which is an iron oxide based product which has been used to remove arsenic on a number of public water supplies in the English midlands. The principle of the process is similar to activated alumina, although iron adsorbents may operate effectively at slightly higher pH values. Pre-oxidation of As(III) to As (V) will still be required where the arsenic is predominately present in the reduced form. Competing ions that may also be removed at the expense of arsenic removal are antimony, phosphate and silica. Also, where pre-oxidation is required this may also oxidise iron and manganese to insoluble forms which may foul the filter media. Some iron adsorbent filters are designed with backwashing facilities, with backwash frequencies of the order of a few months depending on loadings. As with alumina, exhausted media tends to be discarded. The used media does not usually require special disposal arrangements. Point of use arsenic removal systems are available. Some of these include prefiltration and oxidation stages.


Membrane treatment involves forcing water at high pressures through fine porous sheets (the membrane). Membranes come in differing pore sizes, with increasing pressures (and therefore pumping costs) required as pore sizes decrease. For arsenic removal, microfiltration may be feasible if iron coagulation is used upstream, however the complexity and cost of this probably makes it unsuitable for most private water supplies. Reverse Osmosis (RO) membranes are the most viable membrane technology for arsenic removal as pore sizes are sufficiently small that arsenic ions are retained.

Reverse Osmosis

RO systems have the added benefit of removing a number of other contaminants, but operating costs can be quite high and a percentage of water is lost in the waste stream. Disposal of the concentrate (waste stream) may be problematic as chemicals removed from the water are concentrated here. RO membranes are well established as point of use treatment on private water supplies. Some pre-treatment (coarse filtration) may be required to remove suspended solids prior to the membrane. Even then, membrane fouling can be a problem, especially where the water contains a large amount of natural organic matter or salts such as calcium, magnesium, sulphate or chloride. Cleaning an RO membrane is possible, but costly and requires chemicals and expertise that may be beyond the scope of most private water supply users.

Other Treatment

The following processes may also be suitable for arsenic removal in small water supplies, depending upon individual circumstances:

Coagulation and Filtration

Lime softening

Oxidation and Filtration (with iron and manganese removal)


Macdonald, A M, Fordyce, Fm, Shand, P And Ó Dochartaigh, B É. 2005. Using geological and geochemical information to estimate the potential distribution of trace elements in Scottish groundwater BGS / SEPA Groundwater Programme Commissioned Report CR/05/238N

Shand, P, Edmunds, W M, Lawrence, A R, Smedley, P L, and Burke, S. 2007. The natural (baseline) quality of groundwater in England and Wales. British Geological Survey & Environment Agency, RR/07/06 & NC/99/74/24 USEPA 2003

Arsenic Treatment Technology Design Manual for Small Systems EPA 816-R-02-011 USEPA Office of Water

Acknowledgement: The primary source of information whilst preparing this document is Drinking Water Regulator in Scotland (DWRS).

Boggy Bit o land

True Springs are rare

True spring supplies are rare in the UK. Even if shown on a map as a spring, the chances are that the water is not coming from deep underground.

Surface derived source

Most ‘springs’ in the UK can best be described as surfaced derived sources. This is because the water probably never passes more than a meter below the surface. The top layers of soil acting a sponge that slowly releases water throughout the year. The water having passed through the top layers of soil, hit shale or rock and then run horizontally. Culverts, land drains, ditches and a under ground pipe channel the water to a collection chamber. Water from the chamber is then piped down to a property, or several properties.

A boggy bit o land

Listen to this short humorous account of what most springs actually are – A bogyy bit o land.

How can a private water supply affect health? Literature review 2008

How can a private water supply affect health? Literature review 2008

Mains water vs Private Water Supply

The quality of drinking water supplied to those people on the mains in the United Kingdom is regularly analysed to make sure that it meets the standards set by the European Commission. The responsibility for independently monitoring the water companies rests with the Drinking Water Inspectorate (DWI) and they advise the government of their performance. According to the report submitted by the Chief Inspector of the Drinking Water Inspectorate (DWI, 2005), English water companies carried out just over 1.8 million tests and the mean compliance rate was 99.94%. On a local level, Yorkshire Water carried out 447,822 tests with a compliance rate of 99.97%. The DWI (2005) report these statistics as showing that the water provided by this company is generally of a very high quality and the health risks relating to drinking mains water remains low. However,  Medema et al. (2003) believes the microbiological quality of water should remain a cause of concern for all water suppliers, regulators and public health authorities as the potential for drinking water to transport microbial pathogens to great numbers of people causing subsequent illness, is well documented in all countries at all levels of economic development.

Untreated water from a private water supply can be harmful to health

Private water supplies are not regulated by the Drinking Water Inspectorate, but are the responsibility of local authority environmental health departments who must register the supplies and approve them based on a regime of chemical and microbiological analysis of water samples (Smith et al., 2006).

The report on the outbreaks of waterborne infectious intestinal disease in England and Wales (Smith et al., 2006) reports 89 outbreaks of waterborne diseases affecting 4321 people in England and Wales for the period 1992 to 2003. Public water supplies were implicated in 24 outbreaks (27%); private water supplies in 25 (28%); swimming pools in 35 (39%) and other sources in five outbreaks (6%). There are 56 potentially pathogenic organisms that can be found in drinking-water (Clapham, 2004) with the majority of waterborne outbreaks in private water supplies caused by Campylobacter, E.coli 0157, Cryptosporidium and Giardia (PWS Technical Manual, 2007). Supporting this view, Said et al. (2003) identified the pathogens infecting private water supplies in the period 1970 to 2000, based on 25 reported outbreaks, as being Campylobacter 13 (52%), Cryptosporidium and Giardia 4 (16%), Cryptosporidium and Campylobacter 2 (8%), E.coli 0157 1 (4%). No pathogens were identified in 4 (16%) despite microbiological investigation. One of the outbreaks (4%) implicated Giardia and/or Campylobacter, but no actual causation agent was identified. Other pathogens of potential concern are Streptobacillus, Enteric viruses and Paratyphoid, however, these pathogens are rarely found in United Kingdom private water supplies (DWI, 2001).

Illness from a private water supply is 35% higher than from mains

The report on outbreaks of infectious disease associated with private drinking water supplies in England and Wales for the period 1970 to 2000 (Said et al., 2003) reports the incidence rate of outbreaks for recipients of private water supplies may be as high as 35 times the rate of those receiving public water supplies. The figure of 35 was based on 53 outbreaks per million population on mains supplies compared to 1830 outbreaks per million population for private water supplies. Clapham (2008) believes the figure for outbreaks for private water supplies is closer to 50 times higher than that of a mains supply if only the figures for the last 10 years are considered.

The most common symptoms

The Said et al. (2003) report highlights 25 outbreaks with private water supplies and subsequent investigations identified 1584 cases and at least 5190 people at risk. The most common symptoms were gastrointestinal and there were no recorded secondary cases of death, although there were several hospital admissions. Clapham (2004) believes these figures underestimate the true number of outbreaks for private water supplies because of general under-reporting of gastrointestinal illness. Supporting evidence for this view is contained in a paper by Wheeler et al. (1999) which concluded that for every case of infectious intestinal disease identified by the national surveillance system, another 1.4 were identified by laboratories. This study concluded that infectious intestinal disease is common, with 9.4 million estimated cases each year in England. However, only 1.5 million were presented to a general practitioner and only a fraction of cases involving Campylobacter being reported to national surveillance.

Drinking water from a private water supply can cause diarrhoea and vomiting

Drinking water from a private water supply can cause diarrhoea and vomiting

When the rain comes?

clean and dirty filters

Research shows that Spring supplies are most likely to contain Faecal Coliforms (E.coli) within 3 days of heavy rainfall. The level of contamination is much higher after a long dry spell as the rainfall collects layers of detritus matter that has built up over the summer months. This means that clean filters can become blocked very quickly. The guiding principle is, if the inside of your filter is discoloured, then it is time to change the filter. This will avoid ‘breakthrough’ whereby contaminants overwhelm the filter leading to discolouration of water and the blinding of quartz sleeves.