Wastewater treatment plant revamping works Papiro Sarda S.r.l. – Assemini (CA)

The “main topic” dealt with by the company is the purification of waste water and everything that embraces this topic starting from feasibility studies up to the executive design, also including the drafting of technical / economic documents for participation in design tenders and / or works inherent in the treatment of civil or industrial wastewater and sewer works. In some cases, AGquadro also took care of the construction aspect of the works. Particular attention has always been paid to innovative techniques that embrace new technologies for the biological treatment of wastewater such as systems with alternating cycles of anoxic and oxidative phases, Membranes (MBR) or mobile bed systems (MBBR). The Papiro Sarda plant is an example of an MBBR plant with mud recirculation, therefore to be classified as HYBRID.

CLIENT:
Papiro Sarda Srl

CATEGORIES AND AMOUNTS OF WORKS:
IA.01. The final amount of the work is € 169,400.00

OBJECT OF THE INTERVENTION:

Construction of expansion works of the purification plant for Cartiera Papiro Sarda Srl

BRIEF DESCRIPTION OF THE INTERVENTION:

The wastewater treatment plant produced by the paper mill is located inside the square, south-east side, in a dedicated space, in the immediate vicinity of the production facility located in Macchiareddu, in Assemini (CA). It was designed according to an adhered biomass process with oxidation sludge recirculation known as “hybrid MBBR” or as “IFAS” – Integrated Fixed-film Activated Sludge System. The works and installations involved various sections of the plant, some have maintained their functionality, others have been completely adequate, and still others have been built from scratch. In particular, the flow rate at the entrance to the plant was completely revised and automated, the air blowing system was completely revised, the rectangular sedimenter with lamellar packs was replaced with a circular one with a diameter of 4.5 meters in steel. A degassing tower was also inserted downstream of the biological compartment and a sludge incubator called EBF Reactor designed and built by AGquadro was implemented in the layout. In this reactor, operating in batch mode, both the biomass taken from the biological cycle and some selected nutrients are introduced. By intervening on the nutritional elements of the wastewater, the native biomass, favored by the operating conditions created inside the reactor, is allowed to grow and proliferate. The cell culture thus biostimulated is inoculated in the oxidative compartment, inevitably leading to an acceleration of the purification processes and a complete consumption of the nutritional substrate, with consequent reduction of the polluting load. The new works were made entirely in Aisi 304.

OBJECTIVES ACHIEVED:

1) The purification performance of the plant has been significantly improved also thanks to the installation of the EBF reactor.
2) an efficient exchange of oxygen with the wastewater was ensured thanks to a fine bubble air diffusion system;
3) the sedimentation phase has been improved both by installing a degassing tower and by creating a new sedimenter with a larger surface;
4) maintenance interventions on the dissolved oxygen measurement probe have been reduced to a minimum;
5) a precise and punctual control of the QIN is allowed

1. ANTE-OPERAM PLANT STATUS

The WWTP is located inside the square, south-east side, in a dedicated space, in the neighborhood of the manufactory located in Macchiareddu, Grogastu – Decima strada – Assemini (CA), as shown in the image below

The wastewater treatment plant was recently built in 2016, it has been designed as an IFAS process Integrated Fixed-film Activated Sludge System that is to say an hybrid MBBR.

Within the reactor there is the simultaneous presence of suspended biomass and biomass fixed to the supports (carriers) loaded inside the oxidation tank. The sludge separated in the final sedimentation tank is recirculated in the biological reactor where active sludge develops.

The main issues found in the purification process which required the interventions described below, were the following:

– high variability of the hydraulic and organic loads, in particular the section dissolved air flotation (DAF);

– Very low reduction yields (close to 40%). This was determined by an initial input load recorded of about 5500 mg / l obtaining a discharge with an average COD of 3300 mg / l.

– A large bubble air diffusion network: Hollow bubble diffusers with large bubbles, easily conceivable above, were present in the MBBR tank.

The reduced contact surface obtained with large bubbles, minimize the exchange of oxygen with the bacterial flora fixed on the carrier.

– The presence of a lamella clarifier that did not guarantee the correct separation between clarified and sludge due to a huge packing phenomenon.

– A significant hydraulic jump which caused turbulence at the exit of the MBBR tank and disturbed the
subsequent sedimentation phase.

The revamping works gave greater stability to the reactions of the process and therefore make it compliant with the limits of current regulations.

2. POST OPERAM PLANT STATUS

The works and installations involved several sections of the plant, some have maintained the their functionality and have simply been located in different points of the system (for example the two blowers), other sections have been revamped or completely replaced because they were no longer performing.

The planned works were:

1. The installation of a new high performance submerged mixer inside the primary tank (V ≈ 200 mc) with the aim of mixing and homogenizing the wastewater from the bottom to avoid sedimentation phenomena The particular shape and type of mixer has been chosen to mainly avoid clogging and blockage of the mechanisms due to wastewater containing long and solid fibers such as the production waste of the paper mill.

2. The installation of equipment and valves along the process lines (electromagnetic flow meters, spindle valves with automatic actuator….) Through which it was possible to guarantee a certain regulation and constant control of the variability of hydraulic and organic loads sent for treatment;

Valvola a fuso

Misuratori di portata

3. Substantial improvements have been made to the biological sector by installing a fine bubble diffusion network (made ad-hoc) on the bottom of the oxidation tank. The choice of fine bubble diffusers was necessarily dictated by the type of process , which is an hybrid MBBR . The presence of active biomass suspended in the aerated mixture together with the biomass fixed to the carriers requires an exchange of oxygen with a very high yield, an exchange that can only be ensured by having bubbles of small dimensions.
A wastewater correction was also made by installing an EBF Reactor model, with all the consequent advantages in terms of biodegradability and sedimentation of the sludge.

Sezione vasca MBBR

Particolare rete di diffusione aria

Posa in opera del

sedimentatore circolare

Torrino degasatore

4. Due to continuous sludge sedimentation problems, a device was created and installed whose function is exclusively to create an hydraulic discontinuity between the MBBR tank and the clarifier, which highlights the presence of a hydraulic jump of about 3 meters .
That jump cause turbulence and rising air trapped between the sludge flakes which negatively influence the sedimentation of the corpuscular part.

5. After the degassing phase, the wastewater is sent to the subsequent sedimentation phase carried out by placing a circular clarifier which ensures larger surfaces with better separation efficiency and reduced maintenance. The clarifier is entirely made of stainless- steel.

With the first tranche of works, it was possible to overcome most of the critical issues that had been
highlighted in the preliminary investigation phase:

• the mixing process within the pimary tank has been significantly improved;

• An efficient exchange of oxygen with the wastewater has been ensured thanks to a system of high performance fine bubble air diffusion;

• The sedimentation process has been significantly improved by replacing the old one Lamella clarifier with a radial flow settler and placing between the MBBR tank and the clarifier itself, a degassing tower that acts as an energy sink;

• maintenance works on the dissolved oxygen measuring probe has been reduced to the minimum necessary as it has been protected by a perforated tube which allows contact with the effluents but protects the lens from continuous impacts with the floating carriers;

• precise and punctual control was allowed, and therefore easier management of the process, through the installation of two automated spindle valves.

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