SWIMMING POOL FILTER SELECTION
Whilst swimming pool filters are selected according to a diverse number of pre-requisites (pool volume, bather load, pool type, turnover rate, filter rate, etc.,) the Designer’s first step is to ascertain the exact volume of water to be treated.
The next step is to establish the “recirculation rate” that is considered most appropriate for the application. In many cases, state-based health Departments will nominate a “turnover rate” that is deemed necessary for a specific type of swimming pool. In some cases, these assessments are based on pool usage and or water depth. The required turnover rate will vary, depending upon a State’s specific requirements. This variance can be significant and if in any doubt reference should be made to the applicable standards & guidelines.
In a practical sense, a final filter selection should consider the type of process treatment and the recirculation system that is to be provided. The contribution made by the selected process design has potential to affect the type and size of the required filter plant. Logically, a process treatment that includes some form of advanced oxidation (albeit a recognized ozone or UV process) may have a different filtration requirement to a system that relies purely on a traditional sanitizer. Equally a filter that operates with a conservative “filter rate” will provide a greater contribution than a filter which is operating at a fast filter rate.
Whilst general guidelines are useful, there is no substitute for the applied experience that can be obtained from other similar projects. If in any doubt, it is often best to obtain specialist advice. Ad hoc decisions or poorly conceived assumptions will have the potential to create serious problems that will affect water quality, operating costs and maintenance requirements.
SELECTION OF THE FILTER RATE
Whilst filters manufactured by CGF are hydraulically balanced for design rates of up to 42 m3/hr/m2, filtration efficiency (and dirt holding capacity) will generally increase, as the design rate decreases. In other words, slower sand filter rates will provide improved filtration efficiency.
Case history suggests that where low turnover rates are used, faster filter rates (up to 42 m3/hr/m2) are suitable for moderately loaded and seasonally used outdoor pools. Conversely, design rates as low as 20 m3/hr/m2 are often required for indoor pools that are subject year round use and regular shock loadings.
In heated applications, the exchange rate (from the body of bathers) would radically increase as the temperature rises. In such cases, additional filter area, slower filter rates, and or advanced oxidation should be considered as being almost mandatory requirements. In many cases, state based Health Standards or Guidelines do not recommend or nominate any specific filter rates.
DETERMINING THE REQUIRED FILTER AREA
When the “recirculation rate” and the “filter rate” are known, the required “filter area” can be determined by simple subdivision. For example, if the recirculation rate is 450m3/hr and the filter rate is 25m3/hr/m2, the required filter area is as follows:-
Having established the required filter area, the Designer should consider the number of filter models that would best suit the available plant space, the number of pumps that will be used, and the preferred method of backwash. Given the wide range of choice that exists within the CGF filter range, numerous options will be available. These can include a choice of vertical of horizontal filters.
OTHER IMPORTANT CONSIDERATIONS
Apart from the interpolation that is required to select the type and number of filters, the system designer has a further obligation to ensure that the selected filters are capable of being effectively backwashed. Whilst system flaws related to poor backwash conditions, may not be apparent upon start-up, failure to achieve the correct backwash flow will affect the long-term performance of any filter system.
SELECTION CHECK LIST
When selecting a swimming pool filter, consideration should be given to the following factors:-
- The product’s service history in other similar applications – be wary of over-sized domestic filters & or commercial filters that have no clear or identifiable track record.
- Place of Manufacture – be wary of imported equipment. Your filter will require future parts & service.
- The total amount of filter area – logically, this is directly related to the performance that can be expected.
- The required number of filter modules – this will be largely dictated by the system design.
- Filter construction materials and their suitability – corrosive resistant materials should be mandatory.
- Filter media requirements and their quantity – the type & quantity of media are related to filtration efficiency.
- Proposed backwash method & backwash efficiency – no matter which method you select, it must work.
- Capital and Life Cycle Costs – a low purchase cost is not always what it seems; do your sums carefully.
- Available plant space – use a proper design service & make provision for future service & maintenance.
- Pool system design and proposed process treatment – effective design can provide long-term benefits.
- Installation requirements and ease of operation – are comprehensive written Instructions available?
- Level of Operator involvement and the level of training required – is automation warranted?
- Energy consumption and maintenance costs – consider the case history of other/similar projects.
- The knowledge and experience of the installing contractor – there is no substitute for experience.
- The availability of technical support and future spare parts – Australian-made makes good sense.