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The availability of freshwater to meet different water needs has raised serious concerns in the last decades all around the world. Water scarcity, deterioration of quality and increasing demand have led to the development and use of alternative sources of water. Reclamation and recycling are now considered as key components of water and wastewater management policies around the world. In this context, the adoption of soft wastewater treatments with a minimum use of chemical agents and having minimum environmental impact is widely encouraged. An experimental campaign was carried out on a pilot plant which consisted of a two-stage disinfection system for a secondary biological effluent. Disinfection consisting of a chemical step (mild chlorination) followed by a natural one (filtration through a horizontal subsurface flow (HSF) bed) was tested in order to evaluate the possibility of producing a final effluent adequate for agricultural reuse. The investigation has shown that this combined system, with low doses of NaClO (2 mg LÀ1 of disinfectant and a retention time of 30 min, corresponding to an applied dose of 2 Â 30 = 60 mg LÀ1 minÀ1) and a well designed final subsurface flow system (at least 1 m2 EIÀ1) is able to obtain an effluent complying with reuse quality limits, in particular for microbiological parameters. For HSF design parameters, neither filling material, aspect ratio nor vegetation type (Phragmites australis and turf) caused significant differences in the average levels of COD, suspended solids, NH4, total phosphorus and Escherichia coli concentrations in the effluent. Finally in order to protect HSF bed from substrate clogging and to prolong its working life, a rapid sand filter was placed before the bed and its impact analyzed. It was demonstrated that the filter was able to retain occasional but unavoidable activated sludge carry-overs from the secondary clarifier which otherwise would rapidly accumulate in the front region of the bed resulting in poor performance (overflows and medium clogging) in quite short time.

Experimental campaigns regarding disinfection of a biological secondary effluent by means of chemical and natural treatments have been carried out at the pilot plant, set up near the WWTP of the town of Ferrara, Northern Italy, whose biological secondary effluent is the pilot plant feeding. These studies took place during a period characterized by hot air temperatures (June–August average values ranging between 20–288C on most days) and continued an investigation performed during a colder period (October 2003 to April 2004). These experimental investigations have proved the soundness and effectiveness of the proposed scheme which consisted of a mild chlorination (by adding a small quantity of NaClO to the biological secondary effluent) followed by a natural disinfection by means of a HSF system. With really high air temperatures, compliance to the strict Italian legal limit for reuse is guaranteed by a mild chlorination followed by a natural disinfection through a HSF bed. 2 mg LÀ1 of NaClO and a contact time of 30 min (resulting in a dose of 60 mg LÀ1 minÀ1) must be guaranteed in the first chemical step, while the second natural stage must have a retention time of 1 d and a specific area of 1 m2 EIÀ1. This fact suggests that in hot areas (seaside places, tourist centres, etc.) a well-designed HSF system as a polishing treatment is sufficient for compliance with the legal limit for reuse and without requiring further disinfection treatment.