ECO water treatment from Devin – Targets, standards and guidelines

Targets’ is not exactly a word that is commonly used in association with pool water treatment systems sustainability. And some of the targets that are in place do not address key system energy loads, while others could in effect be much more challenging.  So, this article looks at where we currently are and where there is scope for improvement.

The latest guidance publication on sustainability is the UK Net Zero Carbon Buildings Standard.  This publication has been collaboratively developed by a wide range of stakeholders in the United Kingdom’s (UK’s) built environment industry including CIBSE, the RIBA, the RICS and the Carbon Trust. While the standard provides guidance on operational  energy intensity limits for leisure centres – 350 kWh/m²GIA/yr for ‘wet’ leisure– this value relates to the ‘wet’ building as a whole, and does not make any specific reference to the pool water treatment.  And nor does it make any specific reference to the type of pool, which can be a significant factor in terms of the treatment required and associated energy use.

For sustainable pool water treatment design, there are two reference documents that should be considered and referred to for further detail and understanding as required:

• Passive House concept for indoor swimming pools: Guidelines, Passive House Institute.
• Guidance note 1 – Net zero carbon pools, Water treatment issues, Pool Water Treatment Advisory Group.

A key aspect of sustainable pool water treatment design is that it must take into consideration the water quality standards to be maintained; for the UK these are in PWTAG guidelines and are set out as follows:

Water treatment

• circulation rate 1.7 m³/hr/bather
• dilution 30 ltrs/bather (in practice this is often circa 15 ltrs/bather)

Water quality targets

• free chlorine > 0.5mg/l, < 2.0mg/l
• combined chlorine < 0.5mg/l (preferably < 0.2mg/l, from German DIN 19634 )
• turbidity < 0.5 NTU
• TDS < 1,500mg/l

Identifying and getting client commitment to sustainability targets is an essential first step. Passive House identify reasonable and achievable energy targets (though currently UK pools rarely achieve these).

The first target in the Passive House document is the amount of electrical power required to generate unit flow rate: 25-40W per m³/h of circulating water. This target reflects the hydraulic efficiency of the circulation system. The aim should be less than 40W. (Passive House also identify an alternative approach based on pool type).

The second target in the Passive House document is for a total head loss sum of static and friction, of 5-10m.

The amount of power needed to circulate 1m³/h of flow depends on many factors, that in essence fall into three categories:

• anywhere in the circulation that water drops by gravity – so minimise static head loss
• energy required to overcome frictional resistance in the system – so minimise frictional resistance
• the efficiency of the conversion of electrical into hydraulic power by the pump (including the efficiency of the motor and any variable speed drive) – so select for maximum efficiency.

A third target, not addressed by Passive House, is the total electrical power per unit flow rate. This would include the power for ultra violet (UV) treatment, booster pumps, backwashing and air scouring of sand filters, backwashing and cleaning of ceramic membrane filters, chemical systems and controls. Typical electrical power for UV is 15 – 25W per m³/h of circulating water; a huge range with a key influence being the means of UV dose control. A target of total electrical power per unit flow rate is therefore proposed 45-80W per m³/h of circulating water.

A key initial action is that energy targets are agreed with the client and the design team. Following this, there are many design measures that can be addressed to positively influence the pool water treatment system sustainability characteristics, and as such to help reduce the energy and resource expenditure on a project.

The next article looks in detail at the many engineering elements that can be addressed to positively challenge the energy targets – our ECO water treatment system from Devin – and also questions some of the sustainability information being generated!

Image credit: Raffles London at the OWO & Eastbourne College