Sollevamento grigliatura e dissabbiaturaDesolforazione e omogeneizzazioneI Stadio BiologicoDigestori AerobiciSedimentatori PrimariDenitrificazioneIspessitori fanghiII Stadio biologicoSedimentatori II stadio bioTrattamento terziarioChiariflocculatoreIspessitori rilancio EcoespansoScarico finaleLaboratori e uffici tecnici

The centralized WWTP in Sant Croce sull'Arno receives wastewater produced by the whole industrial area on the right bank of the river Arno within the Tuscan Leather District. This implies that 95% of incoming flow is tannery wastewater while the remaining 5% arises from minor manufacturing processes or extra-fluxes which arrive by tankers. That condition makes the industrial stream flow  very similar to a tannery wastewater which is known to be very difficult to cleanse.

The intrinsic characteristics of industrial wastewater, present several unfavorable issues for conventional activated sludge (CAS) process. Howerer, CAS was the selected process for many years in the past in combination with a heavy physico-chemical treatment which produced huge quantities of excess sludge.

Since 2002, thanks to the "Tutto Biologico" process and the structural interventions for the construction of a second biological oxidation stage, it has been possible to limit an untenable situation such as the tannery sludge issue. At present time the industrial section of the plant has got  total volume of about 135.000mspreads through 34 tanks with different inner volumes that could vary from 100 to 15.000m3. This arrangement allows an internal wastewater residence time of about one week (seven days) ensuring the compliance with the legal limitations for the final discharge.

The critical issues related to well known purification recalcitrance of tannery wastewater, required special solutions which have continuously been  studied and optimized over years. Here below it is reported how these arrangements have been implemented in the present plant.

1) Primary lifting, fine screening and grit removal

Incoming industrial wastewater arrives directly in the lifting basin, so called as by there, eight pumps lift the sewage to the physical pre-treatment situated about 6 meters over, this, in order to exlploit the gravity flow down through the first treatment stages.

The fine screening removes all solids and hair bigger than 1,5mm or unable to pass through the 6 rotating cilindrical screens having this ports sizes.

The screened wastewater moves, then, to the 2 cyclonic grit removal systems, where residual inert solids and sands are settled and removed due to the exploiting of both gravity and centrifugal forces at the same time. 

3) Biological oxidation - First stage

The first biological oxidation stage premises were built in 2001, specially for the conversion to the "Tutto Biologico" process. The conventional activated sludge (CAS) process exploits biological activity of spontaneous microorganisms (activated sludge) which, due to their adaptation and selectivity for the substrate,  is able to degrade the pollutant substances dispersed among the sewage (substrate). In order to mantain the efficiency and the effectiveness of the biological process, it is mandatory to pay attention to biomass activity, strictly controlling main sewage parameters, in order to avoid the death of micro-organisms.

The main advantage of this kind of process is that smaller amount of sludge is produced than old physical-chemical process, because of the low dosage of chemicals.. On the other hand, large amount of air /oxigen needs to be blown through the sewage in order to maintain a high rate of oxigenation inside basins, this is the main requisite for aerobic biomass, which, as well as humans, requires oxygen to perform its metabolic pathways for the pollutants demolition.

Although the biological process requires less chemicals dosage, a strict control of the main parameters has to be performed. Characteristics of sewage are monitored more frequently than in a chemico-physical process, with the purpose to ensure the maximum efficiency of the activated sludge that, due to its nature, is susceptible to the toxicity of certain substances and to strong concentration flutuations, even in case of nutrients.

The first biological oxidation step of Consorzio Aquarno consists in two large rectangular basins of 15.000m3 each, that are identical both for structure and equipments. The aereation of the mixed liquor is ensured by two beds with 6.600 microbubbles diffusers each and feeded by 4 variable flow compressors.

Besides the several measuring systems present on field, the mixed liquor is regularly analyzed by the internal analytical laboratory in order to go over the parameters and work on the flows management. There are many possible actions to provide a precise control of the process,  like the regulation of the activated sludge recycle from the sedimentation section, whose flow rate could influence both the treatment efficiency and the excess sludge production.  

2) Desulphurization and pre-oxidation

The raw sewage sulphides content varies during the day, from the highest concentrations in the morning to the lower ones in the evening. Such fluctuation requires an homogeneization, preferably combined with a pre-treatments. The plant has two twins big circular basins of about 10.000m³ capacity placed alongside each other and where occour, respectively, the sulphur oxidation in the first and the pre-oxidation in the other; the last one is preparatory to the folowing biological oxidation step.

Desulphurization has a dual objective: on one hand it prevents the hydrogen sulphide generation(H2S), which is an extremely toxic and malodorous gas, on the other it provides the abatement of the sewage toxicity caused by the presence of high concenttration of sulphides. The sulphides removal is accomplished by dissolving pure oxygen in the sewage and  maintaining the pH value above 9. .

Once desulphurised, the wastewaters are transferred in the second basin that could be used as storage and recycle of oxigenated mixed liquor among the first biological oxidation step, otherwise it serves as pre-oxidation step. In this case pure oxygen is dissolved in the sewage in order to promote the biological activity of the biomass  withdrawn by the following biological sedimentation tanks. 

The oxygen dissolved among these two first basins is almost totally produced by the two oxygen generators whose total capacity is about 60 ton/giorno.

4) Primary biological sludge sedimentation

The aerated mixed liquor of the first biological oxidation step contains large amount of suspended  activated sludge, which needs to be settled in order to clarify the wastewater before proceeding over to the following purification treatments.

In January 2015, 6 new basins with an individual capacity of about 1.000m3 were unveiled and started. Simply gravitational settlement occurs in each tank, where a system of travelling cranes equipped with sludge scraper rubber blades, shifts the settled sludge to the hoppers.

For each oxidation basin three sedimentation tanks were provided and they are feeded simultanously by gravity, thanks to one opening on the walls of each  basin. Settled activated sludge  can be sent to the thickening steps or it can be recirculated amont the biological oxidation.

Clarified wastewater drains out by the overflow channel and is transferred to the denitrification.

5) Denitrification 

Denitrifying biomass converts the nitric nitrogen excess dossolved in the incoming mixed liquor, into  gaseous nitrogen. Not only a potential harmful substance is removed, but also a further rate of pollutants is degraded.

Oxigenation of the sewage is not required in this stage, because the denitrfying biomass exploits the molecular oxygen of the nitric group (NO3-) as primary source, so additional oxygen is not needed.

Denitrification basins usually require more depth than oxidations ones and slow internal mixing, with the aim to avoid aeration. Longitudinal slow mixers and absence of air diffusers. The WWTP in Santa Croce sull'Arno has 2 Carousel-flow basins with an individual volume of 4.500m3, equipped with a total of 8 longitudinal slow mixers that ensure the sewage flow around the central separating wall of each basin. No air diffusers are installed and mixers allow a slow movement of sewage avoiding any sort of aeration. 

6) Biological oxidation - Second Stage

Once denitrified, wastewater, together with activated sludge, moves directly to the second biological oxidation stage. A cataracts system splits and distributes the denitrification overflow upon 3 identical oxidation basins, which have the same dimensions, volumes, flow circulation and aeration systems. Each tank has a geometry rather similar to the denitrification basins but larger and with different proportions between depth and width. Every basin has a volume of about 10.500m3 and the aeration system is similar to the first biological oxidation step, however, the 4 variable flow rate compressors feed three, instead two, different diffusion systems. Every tank is equipped with its own aeration system made of two beds (one for each Carousel lane) with 4.800 diffusers each. The inner flow circulation is ensured by 8 longitudinal immersed mixers.

This oxidation plays an important role for the whole process's success because inside its basins a more bio-recalcitrant pollutants rate is removed, which neither the first biological step nor the deitrification have demolished. Due to implementation of modern regulation and measurement systems, as well as to the analytical control accomplished by the internal laboratory, the maximum efficiency of this purification stage is allowed.

7) Secondary biological sludge sedimentation 

The aerated mix outcoming the oxidative process is transferred to a separation step, where a gravitational sedimentation occurs as well as previously seen for  the primary biological sludge sedimentation. Two circular sedimentation tanks of about 4.000m3 each, are dedicated to the sludge settlement. They are placed alongside each other and the aerated mix is fed by a gravimetric splitting system with a cataracts system.

The mixed liquor flows out by the overflow channel situated all around the basins circumferences, and then it moves to the tertiary physico-chemical treatment which is placed exactly between the two sedimentation tanks.

The sludge is withdrawn by the bottom of each tank due to an aspitation system placed on a moving crane, then it can be conveyed to the thickening phase or it can be recycled into the second biological oxidation stage.

9) Sludge thickening and transfer to Ecoespanso

Excess sludge is exctracted from different sedimentation tanks within the plant, but it is very diluited and  requires a preliminary thickening phase before to be conveyed to Ecoespanso plant for its final treatment. The thickening phase has a fundamental role and a great economic impact on the following sludge treatment. The aim of this operation is the concentration of the sludge and, inside the WWTP, it occours mainly due to gravimentric force. This solution allows to reduce the overall volumes of sludge by the elimination of big volumes of mixed liquors which are recycled to the denitrificatin tanks, while the thickened sludge, on the contrary, can be transferred to the following treatments.

The WWTP in Santa Croce sull'Arno has 3 thickeners (circular tanks ) of about 2.800m3 each.

Thickened sludge is regularly transferred to a fourth accumulation tank, where it is collected and pumped to Ecoespanso via a dedicated "sludge-pipe".

8) Tertiary physico-chemical treatment and chiariflocculation

The tertiary treatment is a combination of Fenton's process and subsequent chiariflocculation of treated mixed liquor.

Fenton's process requires the use of Ferrous Salts (such as FeCl2), Hydrogen Peroxyde (H2O2), Hydrochloric Acid (HCl 37%) and lime (Ca(OH)2 solution 20%). This kind of process is an AOP (Advanced Oxidation Process) and it can be tuned on the basis of the  reference parametersvalues that are monitored in the incoming stream flow.  When it is possible, an alternative, weaker treatment is adopted simply excluding the Fenton's process and conserving the chiariflocculation phase only, which requires the use of less chemicals than Fenton AOP,  such as Ferric salts (FeCl3) and lime.

The tertiary treatment tank is located between the two biological sediemntation tanks and from there the mixed liquor is transferred to the chiariflocculator, a big sedimentation basin of about 8.000m3 where the separation of physico-chemical sludge occurs. A cationic polyelectrolite solution is dosed directly in the transfer line, in order to facilitate mixed liquor clarification.

The purified wastewater comes out by the overflow channel and pours into a chlorination tank where Sodium Hypochlorite is dosed when necessary, before to be discharged in the receiving body.

10) Final discharge

Industrial and municipal wastewater, once purified, pours to the final discharge which has to comply with the legal limitations imposed by Italian Law, specially reported in Table 3 of the D.Lgs. 152 of the 3rd  of April 2006.

A sampling station owned by A.R.P.A.T. (Agenzia Regionale per la Protezione Ambientale della Toscana), has been installed in correspondance of the final discharge. Regular periodic controls are performed by the Agency which not only analyses the samples withdrawn by the station but also  ones sampled during in-situ controls.