EXCESS SLUDGE TREATMENT

Impianto_Ecoespanso Drying section Dehydration Pyrosyntering Energy recovery and fumes treatment Plastofill production

ECOESPANSO COMPANY

The upgrading of treatment plants and related technologies,  have significantly reduced the amount of excess sludge to dispose of. Throughout the past 20 years the excess sludge production drop from 180.000 ton/y on 1995 to 15.000 ton/y on 2015. Despite of this, the massive flows still remain huge and the landfil disposal has shortly showed objective problems connected, firstly, to the volumes needed by this kind of solution. As a consequence of this, an alternative method has been studied, which would reduce the overall volume of excess sludge and allow its reuse as recover second raw material MPS at the same time, closing, this way, a virtuous recycle started from a waste.

The Ecoespanso s.r.l. plant  was built in 2001 for the purpose to solve the environmental issues related to the excess sludge  landfill disposal, in order to improve the environmental sustainability of the whole tanning process.

Since July 2014 Consorzio Aquarno has been managing the Ecoespanso plant and it has carried out some little rearrangements, also due to new synergies between two plants and their needs.

Ecoespanso premise is almost fully dedicated to the treatment of Consorzio Aquarno's excess sludge, whose production, since June 2013, has been increased significantly, due to  felt of Consorzio Conciatori di Fucecchio under Aquarno control. The final product of Ecoespanso's process is the sintered granules (KEU) which is usually mixed with Calcium Carbonate to make the filler mixes HSC and HBC. The first one is used as component for the production of cement, while HBC could be used for the production of asphalts.


Ecoespanso is authorized to manage a plant able to store, centrifuge and treat by heat  waste sludge produced by the WWTPs on the right bank of the river Arno. Excess sludge is regularly conveyed to the facility by mean of a "sludge-pipe", a special pipeline which links together the two wastewater plants (Fucecchio and Santa Croce sull'Arno), to the final sludge treatment plant in Castelfranco di Sotto.

Ecoespanso has a maximum treatment capacity of about 20.000 ton/y of dry substance which correponds to a sludge flow of more than 500.000m3/y, that is more than currently produced by Consorzio Aquarno.


 

THE PROCESS

A dedicated "sludge-pipe" directly conveys Aquarno's sewerage sludge to the only receiver inside Ecoespanso plant, which, due to its 3.000m3 of volumetric capacity, is able to guarantee the continuity of the whole treatment process, consisting of 5 main sections:
  1. Dehydration (centrifugation) 
  2. Drying
  3. Pyrosintering
  4. Energy recovery and fumes treatment 
  5. Plastofill production (HSC e HBC) containing KEU (sintered granules)


1. DEHYDRATION

Thickened sludge (with about 3% of total solid content) incoming from both Santa Croce sull'Arno and Fucecchio plants, is centrifuged to get the dry content up to 26-30% as TS content.  This way a primary mass and volumetric reduction of about 10 times is obtained, allowing a significant energy saving over the following drying stage. The liquid rate of the sludge is discharged directly by centrifuges and then returned back to the Aquarno  WWTP through a dedicated pipeline.  

2. DRYING
This section aims to dry the sludge up to 80-85% of Total Solid content.
This process takes place inside three ovens equipped with an internal sludge conveyor  and a hot air drying system. This stage of the treatment process is splitted on two subsequent sections: the first with two ovens and the second with only one. 
The heating system consists of several hot air blowers which recycle the steam produced during the drying process, together with the hot air obtained from heat exchangers batteries feeded with saturated steam (250bar, 40atm). This steam flow arises from an heat recovery system located nearby the plant in an Ecoespanso's premise, where four Rolls-Royce engines are used for Electrical Energy production (30 MW) by methane combustion. An external company manages these electric generators inside the premise and it supplies Ecoespanso's steam needs at the same time. According to this, the drying section provides a great energy recovery due to utilization of saturated steam produced by hot exhausted fumes of the electric generators which, otherwise, would be lost. The ovens are close systems splitted into two comunicating sections: the air heating section and the sludge drying tunnel. The hot air is continuously recirculated by blowers from the first to the tunnel and back again, the steam produced by the sludge is partially condensed an partially recycled

3. PYROSINTERING
Dryed sludge coming out from the drying tunnels is then pyrolised and sintered in order to obtain the sintered granules (KEU). These two process phases are accomplished inside two rotatory ovens, physically connected each other for the purpose to allow the continuous sludge transfer from one stage to the other.

Two methane/oxygen burners, one for each oven, supply the thermal energy needed respectively by the pyrolysis and sintering processes. A direct flame heat exchange system is used, adopting stoichiometric oxygen/methane ratio in order to prevent oxidative side reactions.

4. ENERGY RECOVERY AND FUMES TREATMENT

Hot fumes generated by the final two ovens are mixed with the drying section incondensable gases and then conveyed to their treatment and energy recovery. This exhaust gas stream contains a significant percentage of methane and combustible gases produced by the pyrolysis and sintering processes. The mix, thus, is burnt inside the post-combustion system where urea is added for the NOx abatement.

The heat generated in that stage is recovered inside an economizer that produces overheated steam,  which is then recycled within the drying section, reducing, this way, the fresh steam consumption. Subsequently off gases stream passes through a water quencher column to remove dust and control its temperature; after this, a further dust abatement occurs within two different bug filter batteries, before the stream passes through a wet scrubber for the acid gases removal and then the prified exhaust gas is released through the final chimney.

All dust trapped in bug filters as well as those suspended in the quench water, could contain hexavalent Chromium, thus, in order to eliminate this problem, both these stream flows are treated inside three different reactors , where a slution of Ferrous Chloride 30% is dosed to convert Cr(VI) into Cr(III).

5. PLASTOFILL PRODUCTION (HSC E HBC) CONTAINING KEU (sintered granules)
Plastofill HSC and HCB are obtained due to the mixing and subsequent grinding of sintered granules (KEU) with Calcium Carbonate, originating from Campiglia (LI) marble quarry as ballast. The KEU leaving the hot inertisation section is mixed with precises amount of ballast and then finely crushed in order to obtain particles size up to few microns. The produced silico-calcareous filler could be sold as additive for asphalts (HCB: 30% KEU 70% Calcium Carbonate) and for cements (HSC 10% KEU, 90% Calcium Carbonate)