Sand filter (outdoor)

Standard process combination for public pools: multi-layer sand filter + flocculation + chlorine-dioxide or chlorine-gas disinfection. Alternatives use activated carbon (treatment) or UV (disinfection). Sand-filter layering: fine sand 0.4-0.6 mm on top, gravel support layer below. Filter velocity max 30 m/h. Backwash at 50-60 m/h up-flow for 5-10 min, backwash volume approx. 5× the filter-bed volume.

Basis: the applicable national standard

The applicable standard prescribes a maximum residence time of the pool water, depending on pool type: swimmer pools 4 h turnover, non-swimmer/paddling 1-2 h. This sets the minimum filter flow rate. For a 242 m³ lap pool at 4 h turnover that is 60.5 m³/h of filter capacity. Filter backwash once per day or above 0.5 bar differential pressure. Backwash water is warm (pool temperature − 3-5 K) and therefore a valuable heat-pump source.

Basis: the applicable national standard

Evaporation from the water surface is the dominant heat loss in an indoor pool (60-80 %). Smith/Löf variant of the Carrier formula: m_evap [kg/h] = β × A [m²] × (p_w_pool − p_w_hall) [kPa]. β-coefficient per the pool energy guideline (evaporation and ventilation) Tab. 4: 0.013 unused pool (covered), 0.040 normal use, 0.080 wave/diving pool. Evaporation enthalpy at 30 °C: 0.694 kWh/kg. Pool saturation vapour pressure via the Magnus formula: 611.2 × exp(17.62 × t / (243.12 + t)) Pa.

Basis: the applicable national standard

Outdoor pools have higher convection and radiation losses. Convection: h_conv = 4 + 11 × v_wind [W/(m²·K)] with v in m/s. At 1 m/s wind: h ≈ 15 W/(m²·K). Night radiation (clear sky): ~60 W/m² over a 10 h night. Solar gain during the day (global radiation × absorptivity 0.82 × 12 h) is a credit — in high summer it can drop the heat demand close to zero. A night cover reduces both evaporation (−85 %) and radiation loss (−90 %).

Basis: the applicable national standard

the national pool-operators association guideline R 60.03 on heating public outdoor pools: heat-up phase in April over 30 days, raising the set-point from 10 to 22 °C; slow heat-up (45 days) or solar absorbers as a pre-heat stage save energy. For 2,508 m³ of water × 12 K × 1.16 kWh/(m³·K) = 34,928 kWh — spread over 30 days that is 1,164 kWh/d of extra load. Recommendation: heat up via a pool heat pump on a warm source (backwash water already at 15 °C before season start) instead of a gas peak boiler. A pool cover shortens the time (solar gain by day, loss reduction by night).

Basis: the applicable national standard.03