Two types of water leave swimming pools, both public and private: water from emptying and renewing the pool and water from filter backwashing. Before discharge they must comply with the limits of Italian Legislative Decree 152/2006 (D.Lgs. 152/2006): chlorine, kept at around 1.0 mg/l in the pool for disinfection, must be reduced to within 0.2 or 0.3 mg/l depending on the receiving body.
Water from emptying the pool and from periodic renewal may be treated as domestic wastewater: for small and medium-sized enterprises this is provided for by Italian Presidential Decree 227/2011 (DPR 227/2011, table 2, point 19), in the absence of regional regulations. After dechlorination, it is discharged in accordance with the regulations in force.
Backwash water is excluded from this treatment if it has not been treated beforehand. The Italian Court of Cassation (judgment no. 10373/2020) classified it as industrial wastewater: without treatment, its discharge requires authorisation and must comply with the limits of Table 3.
The emission limit values are those of Annex 5 to Part Three of Italian Legislative Decree 152/2006 (D.Lgs. 152/2006), unless more restrictive limits are set by the Regions, the competent authorities or the sewer network operators.
Table 3: free active chlorine not exceeding 0.2 mg/l.
Table 3: free active chlorine not exceeding 0.3 mg/l, in the absence of different limits set by the competent authority or by the operator of the final treatment plant.
Table 4: active chlorine not exceeding 0.2 mg/l, in addition to the other parameters specified for discharge on soil.
With free chlorine in the pool at 1.0 mg/l, it must therefore be reduced to 0.2 – 0.3 mg/l before discharge. Backwash water also contains the solids retained by the filters and may exceed other parameters of the table, such as chlorides.
A dosing pump injects a dechlorinating agent, such as sodium metabisulphite or hydrogen peroxide, into the discharge pipe, with constant dosing activated by a water presence sensor or proportional to the volume measured by a water meter.
Dechlorination systems →Some local regulations allow, as an alternative to dechlorination, emptying water to be discharged after a waiting period following the last chlorination (for example 15 days), during which the chlorine is depleted.
For backwash water, a settling chamber is provided to retain the sludge and solids removed from the filters, followed by inspection chambers for checking the discharge.
Where permitted, dechlorinated water can be collected in a storage tank and reused, for example for irrigation, avoiding waste of water.
A capacitive sensor, installed on the outside of the pipe without contact with the water, detects water during the discharge phases and activates the pump, which doses a constant quantity of dechlorinating agent (0 – 800 or 0 – 8,000 g/h). The quantity is determined on the basis of the total chlorine in the pool, the flow rate of the recirculation pump and the quantity of product required to reduce the chlorine by 1 mg/l, stated on the label.
A turbine or Woltmann water meter on the discharge pipe sends a pulse for each set volume of water and the pump doses the dechlorinating agent in proportion to the volume discharged. The settings are the pulses per litre (turbine meters) or the litres per pulse (Woltmann meters), the total chlorine, the concentration of product to be dosed and the strength of the product.
The discharge of water from a private swimming pool must comply with the same discharge limits: here too, the water must be dechlorinated before emptying and backwashing.
Systems for private swimming pools →Treatment as domestic wastewater and the discharge methods (sewer, soil, water body, recovery) are governed by Regions and Municipalities, which may also prohibit the discharge of emptying water into the wastewater network.
This page is an informative summary and does not replace legal texts and local regulations: before creating a discharge, always check the requirements of the competent authority.
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