Since 1960, the most diffused vegetable tanning process has started its conversion to the chrome tanning process, leading to a change and a worsening in wastewater quality. Furthermore  in Santa Croce sull'Arno the number of industries was increased from 90  in 1948 with about 800 employees, to 180 industries in 1959 with 1400 employees. With its less than 10.000 inhabitants, Santa Croce sull'Arno had a water pollution rate as high as a city of about 1.000.000 inhabitants. Although the solution of this issue was urgent, the absence of any previous experience in Italy, neither in the world, about efficient methods to solve the problem of tannery wastewater depuration,  created relevant difficulties.

In the meanwhile, in 1964 tanneries were about 270, so the purification of tannery wastewater became a necessity impossible to overcome without the construction of centralized wastewater plants.

Nevertheless, the depuration processes would led to other side-problems like sludge disposal and unpleasant/toxic emissions. Consequently, other dedicated plants were necessary, and tanneries in Santa Croce sull'Arno were called to answer to all these issues in order to solve them.

Municipal administration, with the resolution of October, 7th 1966, approved the project for the "Construction of the wastewater treatment plant and sewer system of the county town".

The project's budget was 747.000.000 Lire and it was adopted on September, 6th  1967 by the superintendent of public works of Regione Toscana.

On 28th December 1970 the commission assigned the construction of the new wastewater plant to Ditta Soc. Passavant Resine Impianti di Novate Milanese, with the obligation to provide a further self-funded installation of a pilot plant for experimental researches about biological treatment processes.

The project approved, included only a physico-chemical treatment of tannery wastewater and, on December 17th,1971, the job was contractually assigned to Ditta Passavant. Works finished on 27th February 1974.


1974, Physico-chemical plant

The plant was designed for a maximum hydraulic capcity of 13.000 cubic meters per day, it worked from the end of August 1974 to January 1975 and occasionally until the end of April 1975.  The supervision of the plant was assigned to Ditta Passavant which not only gave technical assitency, but also conducted experimental activities with the biological pilot plant.

On April 1975 the technical management felt under the control of municpal administration but, at the end of August 1975, it had been necessary to stop the plant activities because of problems related to disposal of sludge produced by the wastewater treatment process. The first plant, which became the nucleous of the following enlargements, was made of :

A) one grit for the wastewater enetering the homogenization step

B) one homogenization basin with a volume of 7.000 cubic meters

C) one rectangular tank for the chemical oxidation reaction

D) two sedimentation basins

E) one lifting station

F) one building including : offices, an internal analytical laboratory, a pump station, and the hygienic facilities

Here below, on the plant layout, the main part of the plant and the extension of the WWTP in 1974 are highlighted. In 1975 the first sludge thickener was built. Incoming wastewater flow rate was about 20.000 cubic meters per day, from monday to saturday. At that time, there wasn't any distinction between industrial and municipal wastewater due to the absence of two distinc sewer system, so these two type of wastewater were gathered in only one stream flow and treated as they arrived to the plant.

The plant treatment capacity was only 8.000 cubic meters and the untreated exceeding wastewater was discharged into the Antifosso channel, while the treated wastewater flew into the Usciana channel. The pollution degree of raw tannery wastewater was very high, with a COD (Chemical Oxygen Demand) value of about 6.000mgO2/L, and the treated flow stream still had about 1.700-1.800mgO2/L as COD.

Regione Toscana, with the law n. 22 of 22th May 1974, funded the construction of a sewer system for the collection and transfer of tannery wastewater from Fucecchio and Castelfranco di Sotto to the Santa Croce sull'Arno WWTP. Furthermore, the sludge treatment was also financed with the same law. The two new sewage flow rates were, repsectively, 5.000 cubic meters per day from Fucecchio and 2.000 cubic meters per day from Castelfranco di sotto.

It is important to note that, in 1974, the WWTP in Santa Croce sull'Arno was the only one tannery WWTP in Italy. According to this, there were not any other cases study for the evaluation of the  treatment performances; so all research studies as well as the laboratory analyses provided until then,  represented only useful hints but not reliable outcomes because of the lack of comparisons.

The Tuscan Leather District has always been characterized by an heterogeneous distribution of a large number of factories (ca. 300) as well as different production processes, these features affected the wastewater depuration, unable to ensure a continuous and regular treatment of raw sewage.

Thanks to the Regione Toscana financing support, Santa Croce sull'Arno municipality approved two different projects, one started on December 6th,1974 and the other on April 28th, 1975. Consequently, the following works took place:

a) First lot, 700.000.000 Lire. Sewer connection between Santa Croce sull'Arno WWTP, Fucecchio and Castelfranco di Sotto, for the treatment of the tannery wastewater (excluding the flow stream from Ponte a Cappiano), construction of a pump station and a storage tub for the raw sewage. Reorganisation of the main building and installation of new filter-press, hygienic services, locker rooms and water-closes.

b) Second lot, 800.000.000 Lire. Acquisition and start-up of a new filter-press (140 plates 1.500 x 1.500 mm) and its electro-mechanical equipments, for the dewatering of tannery sludge.

These works ended on June 1977. The Whole plant management costs were about Lire in 1979 and it was entirely payed by tanners belonging to Consorzio Depuratore.

Even tough all these works contributed to improve plant performances, the purification issues were still hard and difficult to solve.

The scenario was very complex and many problems needed a solution yet, problems such as hydraulic issues, depuration of raw sewage and the last but not the least , treatment and disposal of sludge.

Hydraulic problems arose from the industrial sewer system, incomplete and capable to collect only a minor portion of industrial wastewater (40%),  furthermore the existing collecting system showed problems related to the high solid content of industrial sewage.

Regarding to the treatment process issues, in those years only the first stage had been made. As a consequence of this, neither all incoming wastewater was treated, nor the requirements provided by Table C of Merli's law were achieved, even if the more restrictive limits of Table A were required.

Tannery sludge produced by the wastewater treatment plant was stored along 350 meters of a collecting channel between Santa Croce sull'Arno and Fucecchio. Despite of the first plant enlargement and the further installation of the first filter-press, sludge production still was a serious problem and so far from its solution. In fact, both the transport and the dispose of such a waste were very difficult because of the inefficient dewatering system which didn't allow to achieve the required threshold of 40-45% as solid substance. Two were the possible solutions: landfill disposal or incineration. At that time any other solution, including recycle, were excluded due to both the sludge characteristics and the lack of researches about a controlled landfill.

1982, Physico-chemical plant

On 10th of April 1979, Santa Croce sull'Arno city council entrusted Studio C.P.C. from Padova, to provide a WWTP expansion project in order to increase the treatment capacity and to ensure compliance with limits of Table A  of the Law n.319 of 10th May 1976 (Merli's Law). The project should had also checked and updated available data about the wastewater quality. In fact, from 1969 to 1979 the scenario had changed due to the  increase of factory number and the diffusion of chrome tanning process.

The project was submitted and approved by the city council of Santa Croce sull'Arno on 22th of February 1980. The operative plan included the construction of the new areas as four different lots for a total value of 16.575.000.000 Lire.

The municipal administration of Santa Croce sull'Arno entrusted Consorzio Depuratore of Santa Croce sull'Arno a company founded and funded by tanners, who carried out the interventions, thanks to which the following sections were built or installed:

N. 1 self cleaning coarse screen

N. 1 self cleaning fine screen

N. 2 circular homogenization basins with a storage volume of 10.000mc each one and 50m circumference

N. 2 rectangular primary sedimentation basins with a storage volume of 1.150mc each one and equipped with sludge hoppers

N. 2 sedimentation basins identical to the existing ones with a storage volume of 960 mc each one

N. 1 circular sludge thickener with a circumference of 18 m.

N. 1 circular sludge thickener with a circumference of 22 m.

N. 1 filter-press indentical to the existing one (140 plates 1500 x 1500mm each)

N. 1 filter-press with bigger plates ( 2000 x 2000 mm)

Either the filter-presses were equipped with necessary electro-mechanical instruments.

The data about the incoming wastewater for the period corresponding to the  first working month of the new sections (from 10thJune to 10th July,  1982) are reported below :

Flow rate mc.        working days   holidays

Santa Croce               16.000                3.000

Fucecchio                     2.000                  500

Castelfranco                 2.000                  300

Total                            20.000              3.800

Average quality of treated wastewater:

                                                Incoming   Outcoming

pH                                                8,5               8,2

COD (mgO2/l)                             8.000           2.000

BOD (mgO2/l)                             4.500           1.000

Solidi sospesi (mg/l)                   3.000              50

Solfuri (mg/l)                                280              inf. a 2

Cr III(mg/l)                                   120               inf. a 2

The new areas and the new plant extension in 1982 are both highlighted on the planimetry.

On 13th  May 1982 Consorzio Depuratore presented another expansion plan that was shortly approved by the municipal council of Santa Croce sull'Arno. The new project provided the introduction of one biological treatment stage and the increase of treatment capacity up to 30.000mc per day  of  industrial and municipal wastewater. Three  municipalities were served by the WWTP: Santa Croce sull'Arno, Fucecchio and Castelfranco di Sotto. Studio. C.P.C. from Padova, the project producer, had provided two different steps for the execution of expansion works, for a total cost of about 12.400.000.000 Lire.

a) First step - achieving compliance with limits of Table  "C"  "Merli's Law" before 31.12.83;

b) Second step - achieving compliance with limits of Table "A" "Merli's Law" no later than 1985

Throughout the working period of the third batch, the first biological section was built and noticeable interventions were made, even more than those stipulated, with the aim to ensure the compliance with Table "C".

Works started on 30th September 1982 and finished on 20th October 1983, after first tests the start-up occurred at the end of the year 1983.

1983, Physico-chemical plant with the first biological stage

Main works:

N.2 biological oxidation basins

N.2 final sediemntation tanks

N.1 storage and lift system for treated water

N.1 final  discharge structure

N.1 distribution frame for final biological sedimentation

N.1 lifting plant for excess and recycle sludge

N.2 sludge thickeners

N.1 electric power station SS - 4

Purchase of two filter-presses

From 31st December 1983  the wastewater treatment plant was able to compliance limits of Tabella "C"  of Merli's Law (COD< 500mgO2//L) with a monthly average value of COD of 250-300 mgO2/L, only Chlorides and Sulphates were off-limits.

December 31st was also the deadline prescribed by Regione Toscana authority for the achievement of that important target.

Other two important issues, such as the sludge dewatering and disposal was partially solved due to the purchase of two new filter-presses: Tahnks to their big dimensions and capacity (140 plates 2000 x 2000mm each and 1000mc for each cycle), a great improvement to sludge treatment was achieved and the R.U. average value of 60-65% made easier their landfill disposal.

In 1984, a total amount of 6.705.411mc of sewage was treated in the WWTP as a mix of industrial, municipal and rain wastewater. Regarding to the industrial sewage (3.603.890mc),  2.943.000mc came from Santa Croce sull'Arno, 349.453mc from Fucecchio and 311.437mc from Castelfranco di Sotto, while the municipal wastewater (912.500mc) had the following distribution: 487.500mc from Santa Croce sull'Arno and 425.000mc from Fucecchio. 

The total amount of tannery sludge produced by the WWTP was 113.000 tons, with an average dry content of 40%. At that moment the incoming wastewater flow stream had an average COD value of 5.500mgO2/L, total suspended solids were 5.803mg/L and Chlorides 3.208mg/L.  In case of heavy rains or downpour, the water exceeding the hydraulic plant limit, was discharged into Antifosso channel. On February 1985 the fifth filter-press was started and then the total amount of excess sludge could be treated.  

After the accomplishment of the first porject's batch and the fulfilment of its requirements, including compliance with Table "C" limits of Merli's Law, the Second step took place in order to comply with the program of the plan. According to this, new extension works needed to be made as well as the complete division between industrial and municipal sewerage, this in order to improve the wastewater treatment performance. As a duty of all tanners, the completion of the second stage of enlargement works took place with a view to comply with the Table "A" limits of the Merli's Law.  With the decision n.135 of 22nd March 1985, the municipal council of Santa Croce sull'Arno approved the excecutive project, for a total cost of 16.800.000.000Lire. Besides  civil and electro-mechanical works for the completion of the bioogical treatment section, that project included also all works related to the sewerage separation in three different typologies: the industrial, the municipal and the rain one.

The Municpial council, with its decision n.296 of 17th April 1986, asked to Regione Toscana to be funded with F.I.O.( Found fo Investments and Occupation). Regione Toscana with deliberation n.4212 of 12th May 1986 approved the project and gave to the Municipality of Santa Croce sull'Arno the leading role for the execution of works, which started the 22th July 1986 and finished on 18th October 1991.

1991, Physico-chemical pant + biological stage + nitrification and denitrification + final clariflocculation

Thanks to that further enlargement all the following premises were implemented:

N.1 biological oxidation basin

N.1 de-gasification tank

N.1 disinfection basin

N.2 denitrification basins

N.1 clariflocculation basin

N.1 pump and dosing station

N.1 building for the storage and dosage of active carbon

N.1 building for micro screening and grit removal

Pure oxygen diffusion system within the homogenization stage

 Coverage of manufacts, initial screening, primary sludge extraction Wells and primary slufge thickeners

Air treatment system

Ferrous Salts dosing system

Electric system upgrade

In July 1987 a new office  building was inaugurated, where offices, hygienic services and the store of SOGES's employees were implemented. In addiction to that, the plant administration offices were placed there too.

Since 1988 different sections of the plant started subsequentially, following the enlargement works, which ended in October 1991. After that date the plant felt under the Check-up Commission control, named by Regione Toscana, which managed the start-up operations and confirmed the previous outcomes obtained by the Operating Society. The plant showed a great treatment capacity, specially referring to biodegradable organic substances and thenitrogenated fraction of wastewater. Much more diffuculties  were encountered for the unbiodegradable COD  ratio, which exceeded the limit of 160mg/L.

Besides those expansion works, as requested by Tuscan C.R.I.T.A., a new excecutive project was carried out in order to accomplish the abatement of unpleasant odours and reduce the sludge production (1989).

At the end of December 1987, was completed the sewerage separation between industrial and mixed (municipal and rain water) water,within the municipality of Santa Croce sull'Arno 

The elimination of the industrial wastewater bypass into Antifosso,  in case of heavy rain, was one of the most relevant outcome and a further sewerage separation between Fucecchio and Aquarno was accomplished in March 1988.

When the works of the last plot ended, the new WWTP had doubled its internal hydraulic volumes, the number of machines, and the automation systems; moreover the filter-press station was completed and it was able to treat all sludge produced.

From 1984 to 1988 the plant dimension were doubled. The energy consumptions passed from 9.000.000 to 20.000.000kwh. Despite that, neither guarantees nor contiuity to compliance with limits were allowed. The COD threshold of 160mg/L provided by Merli's law, was often override. In order to remediate to this, a new action plan was drawn-up by the C.I.S. (Comitato Impianti Smaltimento, private-public society), which assigned the project design to Studio di Ingegneria Cevese. The plan was approved on 30th September 1992 by Santa Croce sull'Arno Municipality and then financed by Regione Toscana thanks to the F.I.O. funds, for a total amount of 2.600.000.000Lire. 

The excecutive project was revised on 10th May 1995 and works started on 26th Ocober 1996 and finished on 26th November 1997.