Consorzio Aquarno S.p.A. was established in 1983 to manage the centralised wastewater treatment plant of Santa Croce sull’Arno (whose first tanks were built as early as 1974). The facility is dedicated to the treatment of civil and industrial wastewater from the municipalities of Santa Croce sull’Arno, Fucecchio, Castelfranco di Sotto, and Santa Maria a Monte.
Over time, Aquarno has expanded its activities, also managing other facilities connected to water treatment and sludge treatment, in addition to operating the separate industrial sewer network. The plant in fact manages the two types of wastewater on separate treatment lines, which merge before the final discharge into the Usciana Canal. Small quantities of liquid waste are accepted into the industrial line.
Wastewater treatment plays a strategic role in the economy and environmental sustainability of the Tuscan Leather District, an area with a high concentration of industry. With a catchment area of 129 km², 52,000 inhabitants, and more than 300 factories, Aquarno operates in a complex and challenging context, especially due to the strong presence of mineral chrome tanning.
The Consortium deals daily with environmental and production-related challenges, ensuring quality and efficiency with no margin for error. The dynamic management of the treatment plant, which has operated under an IPPC Permit (Integrated Environmental Authorisation) since 2020, responds to continuous regulatory updates and to changes in the district’s production flows. Over more than 40 years, the plant has expanded its volume from the initial 10,000 cubic metres to more than 150,000 cubic metres dedicated to treatment processes, reaching a treatment capacity equivalent to 2,050,000 inhabitants.
Future developments, which see the Consortium at the forefront of the “Supplementary Agreement for the Protection of Water Resources in the Lower Valdarno and the Fucecchio Marsh through the reorganisation of wastewater treatment in the leather district and the Empolese area, Valdera, Valdelsa and Val di Nievole,” envisage a further increase in treatment capacity in order to receive urban wastewater from Val di Nievole and Valdera, consolidating Aquarno as one of the leading wastewater treatment plants in Europe.
4M
cubic metres of (industrial) wastewater treated in one year
7M
cubic metres of water returned to the environment in one year
27.5K
tonnes of COD removed in one year (97% removal rate)
Consorzio Aquarno manages the treatment of civil wastewater from the municipalities of Santa Croce sull’Arno, Fucecchio, Castelfranco di Sotto, and Santa Maria a Monte. This wastewater differs significantly from industrial wastewater both in qualitative and quantitative terms; the main incoming parameters are considerably lower than those of industrial wastewater, which consequently affects the type and scale of the treatment stages. Civil flows account on average for 40–45% of the total annual volume, and their management requires treatment solutions that are different and more specific than those used for the industrial component. The treatment of civil wastewater is divided into three main phases:
Incoming civil wastewater undergoes an initial coarse screening through a moving screen, followed by a screening stage and a grit removal phase. These processes are essential to reduce cleaning and maintenance activities in the subsequent sections.
The wastewater leaving the pre-treatment stages is conveyed into a single circular biological oxidation tank with a volume of 9,000 m³, inside which pure oxygen is diffused. The combination of the tank’s geometric configuration, oxygen supply, and optimised management of internal flows makes it possible to integrate the denitrification and biological oxidation phases within a single volume.
After passing through a degassing tank, the wastewater containing suspended biological sludge is divided into five rectangular tanks of 900 m³ each, where the two phases are separated: the clarified liquid overflows, while the sludge settles on the bottom. The clarified wastewater then continues towards the final clarification-flocculation stage, which is common to both lines. Along the way, if necessary, a disinfectant product may be dosed to reduce the bacterial load. The sludge, on the other hand, is partly recirculated to biological oxidation, while the remainder is sent for thickening.
The centralised plant in Santa Croce sull’Arno mainly treats industrial wastewater: the incoming flow to the industrial line consists of approximately 95% tannery wastewater, while the remaining 5% comes from secondary processing activities or deliveries by tanker trucks. To manage the complexity of this wastewater, an “All Biological” system has been in use since 2002 which, together with the construction of a second biological stage, has made it possible to limit sludge production. At present, the plant has a total volume of approximately 150,000 m³, distributed across 34 tanks with capacities ranging from 100 to 15,000 m³, ensuring a wastewater residence time of around one week and compliance with discharge limits.
The wastewater is pumped and subjected to screening and grit removal in order to remove coarse and fine materials. In the subsequent tanks, desulphurisation and pre-oxidation take place. The first process is carried out using pure oxygen, in order to prevent the formation of hydrogen sulphide and therefore unpleasant odours, and to eliminate the toxicity caused by the high concentrations of incoming sulphides. The second consists of an equalisation tank which also serves to induce an initial biodegradation of organic pollutants through the dosing of pure oxygen.
The first-stage biological oxidation process, built in 2001 with the adoption of the “All Biological” system, is based on a conventional activated sludge treatment that uses acclimatised biomass (activated sludge) to degrade pollutants. The process does not require chemical additives, but it does require careful control of aeration and other key parameters such as pH and temperature to ensure the efficiency of the microorganisms. All of this takes place in two twin rectangular tanks, each with a volume of 15,000 m³. At the end of the treatment, the water is separated from the sludge by gravity in rectangular settling tanks.
In the two 4,500 m³ tanks, configured as carousels, anoxic biomass converts nitrate nitrogen into gaseous nitrogen, removing nutrients and additional pollutants. The process does not require aeration, as the microorganisms use the oxygen contained in the nitrates formed from the oxidation of ammonia in the previous stage.
The flow enters the second stage of biological oxidation, distributed across three aerated tanks of 10,500 m³ each, with carousel geometry. Automated control and analytical monitoring by the in-house laboratory ensure the efficiency of the process, which removes a fraction of pollutants that are more recalcitrant and resistant to biological treatment.
The aerated mixture leaving the second biological stage is treated in two circular settling tanks of 4,000 m³ each. The clarified wastewater overflows towards tertiary treatment. The sludge is partly recirculated to the secondary oxidation stage and partly sent for thickening.
It consists of the combination of a Fenton process (an advanced oxidation process using iron salts, hydrogen peroxide, hydrochloric acid, and lime milk) followed by clarification-flocculation of the treated wastewater. At this stage, the industrial waters are combined with the civil wastewater and sent to a coagulation unit, where, through the dosing of polyelectrolyte, separation takes place between the liquid phase and the chemical-physical sludge. The treated water, now purified, is discharged into the receiving water body, the Usciana Canal.
The treated water is discharged in compliance with regulations, with constant checks carried out by the in-house laboratory and by the competent authorities.
Excess sludge is thickened in three circular tanks of 2,800 m³ each in order to reduce its volume. The supernatants are returned to the denitrification stage, while the concentrated sludge flows into a storage tank, from which it is pumped to the sludge treatment unit through a sludge pipeline.
Santa Croce sull’Arno (Pisa)
Via del Bosco, 283
Transparent Administration
Consorzio Aquarno spa Registro Imprese PISA P.IVA e Cod.fisc. 00857500508 R.E.A. PISA PI026-8385 Capitale sociale € 800.000 i.v.
