Huwa-San: The Constant Dose Chemical

Constant dosing is when a small amount of chemical is added proportionally to the incoming mains water or tank water to keep the supply free from bacteria. In our experience it is normally used when the traditional method of temperature control for preventing legionella bacteria is failing or insufficient. However  it is worth looking at constant dosing, as an alternative to temperature control. It provides benefits in its own right such as energy savings and pipework being constantly disinfected.

Huwa-San constant dosing can be used to keep your water bacteria and biofilm free. At dose rate concentrations it will not corrode pipework and it continually cleans and disinfects. It is dosed in several properties throughout the UK using a simple set up.

Huwa-San: Constant Dosing How it works

The Equipment Required:

The set up for a simple Huwa-San dosing system is uncomplicated. When dosing directly into the mains supply it requires:

  • a plastic bypass where the water meter and injection point should be fitted
  • a pump with a degas head
  • a flying lead to attach the pump to the water meter
  • a dosing drum and bund

It is important that the water meter and the injection points are fitted into the plastic bypass for two reasons. One it makes maintenance of the system easy and two it prevents the neat Huwa-San TR-50 product coming into direct contact with metal. There must be at least four inches of plastic on either side of the injection point to prevent copper corrosion.

The flying lead connects the pump to the water meter which allows them to communicate. The pump is then set to dose depending on the rate of water.

To automatically monitor the level of Huwa-San in the system Prominent have a Dulcotest sensor that can measure between 0.2mg/l and 2000mg/l of hydrogen peroxide without any interference from chlorine.

The Chemical Levels Required:

Initially, when the Huwa-San dosing is set up it is advisable to do an in-depth clean on the system at 100ppm peroxide/ 200ppm Huwa-San for a week. This will clean the system pipework, removing biofilm. After this peroxide levels will be reduced to 10-20ppm peroxide/20-40ppm Huwa-San. This will keep the water clean and legionella free.

Huwa-San: Constant Dosing the legislation

HSG 274: Part 2

There has been Huwa-San dosing in several buildings since 2004. This includes airports, schools, activity centres and nursing homes. In 2013 the HSE recognised the fact that Huwa-San has an important part to play in legionella control.

HSG 274 Part 2 it states

2.116 Silver stabilised hydrogen peroxide has a history of use in the control of legionella in water systems. A silver hydrogen peroxide solution is injected directly into the water system and if applied and maintained according to the manufacturers’ instructions, can be an effective means of control. As with any water treatment programme it should be validated to ensure it is effective in controlling legionella. Silver hydrogen peroxide should not be used in water systems supplying dialysis units.

Therefore, the HSE advise that Huwa-San can be used as a constant dose chemical to combat a legionella risk.

Where a biocide is used as the primary method of control, temperatures of the hot water can be reduced except in health care premises and where water is required for laundries or kitchens.

Reducing the temperature of the hot water will reduce the amount of energy required to heat the water system leading to significant energy savings.

Huwa-San: Constant Dosing and the Drinking Water Inspectorate

Huwa-San and the Drinking Water Inspectorate.

Currently the DWI have jurisdiction on drinking water up to the stop valve on an owner’s premises. However, after this point the owner can evaluate the best course of action for dosing their water. We have several constant dosing installations that dose into the mains supply as the water enters the building.

The DWI currently do not approve silver stabilise hydrogen peroxides for use in mains water because they are awaiting the approvals from the biocidal product regulation. Huwa-San is registered for this use under the BPR for product type 2-5 and full approval is expected in 2021. The registration for constant dosing would fall under product type 5 the disinfection of animal and human drinking water.

Huwa-San: The Constant Dose Chemical: Energy Savings

As energy costs rise, people are looking at alternative methods of legionella control rather than temperature. Some systems for practical purposes will have to be run at 50°C- laundries and kitchens etc but other hot water systems can certainly be reduced in temperature. In an earlier blog we looked at how an activity centre improved the operation of its building while reducing temperatures. Read the case study here.

Teeside University have done theoretical work on the cost savings that can be achieved utilising constant dosing. They found that reducing the flow temperature of a calorifier from 60°C to 50°C can reduce the energy costs for heating water in a building by 10%. As a rule of thumb energy costs will reduce by 1% for every 1 degree centigrade that the temperature is reduced.

Teesside University based this information on a circulating system in normal operation where the return temperature would be 50°C as a minimum.

The return temperature is significantly reduced when biocidal control is then put in place. For example, to 40°C

Reducing the return temperature means that the water added to the system, after drawing off, will have to be heated less to achieve system temperature. At lower temperatures, less energy is required to heat the water to temperature and maintain the temperatures.

Huwa-San: The Constant Dose Chemical: the advantages over chlorine dioxide

The benefits of Huwa-San dosing over chlorine dioxide are the following:

  • There are reduced capital equipment costs and maintenance costs.
  • Huwa-San dosing is flexible.
  • There is has been no corrosion associated with Huwa-San dosing
Capital Costs

Traditionally most water systems use chlorine dioxide for constant dosing.

Chlorine Dioxide can now be made from preformed tablets, or dosed as a dilute solution but generally Chlorine Dioxide is made and dosed on site by reacting sodium chlorite with hydrochloric acid to form the gas which is then dissolved in water in a mix tank. It is imperative that Chlorine Dioxide is dosed accurately and safely. This means that a suitable Chlorine Dioxide system may cost £15,000 to £20,000. Huwa-San is dosed using a conventional dosing set which costs around 25% of this capital cost.

Huwa-San dosing is flexible.

When dosing chlorine dioxide the combined concentration of chlorine dioxide, chlorite and chlorate in the drinking water must not exceed 0.5 mg/l as chlorine dioxide. To achieve biofilm control, levels of chlorine dioxide may have to be increased. Increasing the chlorine dioxide levels will also increase the chlorite and chlorate in the system and these can have an adverse effect on red blood cells. In addition, chlorate can adversely affect thyroid.

Hydrogen Peroxide in water does not make water non-potable. The no adverse observed effect level is 728ppm (National Sanitisation Foundation). The level of Huwa-San used for constant dose in water systems is 10-20ppm peroxide, and this normally gives excellent microbiological control. On the occasions when positive legionella results are recorded the Huwa-San level can be increased to give 50 ppm or even 100 ppm peroxide for a few days to remove any biofilm. The water is safe to drink throughout the process.

Huwa-San is considerably more flexible than Chlorine Dioxide.


Chlorine Dioxide is corrosive because it is generally made in an acidic environment and is a strong oxidant. This corrosion can be countered by inhibiting the acid activator but when the acid activator is not inhibited, system metals will be exposed to acid. Many hospital systems have witnessed corrosion and have had to replace parts of the system.

Huwa-San The Constant Dose Chemical: The Benefits

As discussed  at present Huwa-San constant dosing is used in challenging water systems where there have be issues with microbiological control. However it also has the potential to reduce energy costs whilst disinfecting water systems in systems that operate free from legionella bacteria.

Of course, like all methods of controlling legionella there has to be monitoring of the system to ensure that the control measures are working.

The benefits over chlorine dioxide are clear with reduced capital costs and more flexibility in dosing. There are also several by-product’s created by dosing with chlorine dioxide while the breakdown products of Huwa-San are oxygen and water.

Please listen to our webinar to find out more. Its number 3 on the list.

SafeSol have worked with Huwa-San technology for over 20 years and have more than 75 years experience in water treatment. We are happy to help with any queries and provide technical support .

Call 0191 4478008 or