Filtri a Tela Pile per l'Impianto di Depurazione di Lodi

Pile Cloth Filters for the Lodi Wastewater Treatment Plant

The Lodi sewage treatment plant, in operation since the 1980s, needed upgrading and the addition of a tertiary filtration system: for the latter objective, between 2015 and 2017 MITA Water Technologies supplied 3 cloth filters of 28 discs each (in two successive batches). The purification system, located at Cascina Maldotta, was built starting in 1978 and became operational in 1982: today it is operated by SAL Società Acqua Lodigiana S.r.l. It is a biological activated sludge type plant, with a treatment capacity of civil and industrial wastewater initially equal to 35,000 population equivalents (increased to 45,000 in 2005).

  • Improved performance for Lodi sewage treatment plant through tertiary filtration
  • 60,000 population equivalent
  • Supply of 3 pile cloth filters of 28 discs each, 140m2 filter area per disc
SAL srl Società Acqua Lodigiana
Lodi (Italy)
Interlocutor type
Public participated company
Scope of application
Municipal Wastewater
Application field
Municipal Utilities
Story in a nutshell
In 2015, through a request from the contractor, 2 cloth filters model MSF 28/140 PPC were supplied, then supplemented in 2017 by an additional system with MSF 28/140 PPC fleece filters.
List of customer requirements
Suspended solids reduction by effluent refinement before discharge to surface water
Input flow rate
- Average flow rate: 1,110 m3/h

- Black peak flow rate: 1,710.00 m3/h

- Maximum flow rate: 1,875.00 m3/h
Input SST
- Average TSS:35 mg/l

- Maximum TSS: 50.00 mg/l
Upstream treatment chain
- Screening

- Primary pretreatment: desanding and oil separation

- Secondary treatment denitrification, oxidation and settling
Technologies supplied
No. 3 MSF 28/140 PPC pile cloth filters: 28 discs, 140 m2 filter area
Output TSS
<10 mg/l
Resource usage
Under average conditions, the expected electricity consumption is about 37 kWh/g
Transportation, start-up and testing assistance
Key to Success
High performance with low power consumption

1. Needs identified

The depuration chain of the Lodi plant consisted, until 2017, of the stages of:

  • water pre-treatment (screening);
  • primary treatment (desanding and de-oiling);
  • secondary treatment (denitrification, oxidation, settling);

Two main objectives were set with the last intervention:

  • To bring the plant to a capacity of 60,000 population equivalents;
  • and to ensure compliance with the legal discharge limit through refinement of suspended solids removal.

In particular, in order to achieve the latter objective, it was necessary to add, downstream of the secondary treatment and before the disinfection process, a tertiary filtration stage.

The contractor in charge of the work therefore planned to install a cloth filtration system from MITA Water Technologies.

2. Adopted solution: tertiary cloth filtration

After an analysis of the data and characteristics of the plant, the pile cloth filters to be installed were identified, which are of the MSF 28/140 PPC FHD type.

Each unit supplied consists of filter discs, each with a filtering surface area of 5 m², and mounted on a hollow central shaft. Filtration takes place by gravity with the machine stopped, taking advantage of the level difference between inlet and outlet. The discs are completely submerged in the water to be treated. Feeding is from the outside of the equipment to the inside, with passage through the filter panels and discharge through the central pipe.

In this case, the disc filter type MSF 28/140 PPC FHD is intended for installation in concrete tank, has a total filtering area of 140 m2 and consists of:

  • 28 filtering discs mounting Polstoff cloths;
  • central shaft supporting and evacuating clarified water, equipped with support wheels and guide at the ends. Material AISI 304 with plastic accessories;
  • motorization consisting of a 0.75 kW submersible type gear motor, a double chain drive system made of plastic material, a mobile gear motor support with chain tensioning screws;
  • 7 countercurrent cloth washing devices, each arranged for cleaning 4 discs;
  • 4 submersible pumps (2 + 2 in-line backup) for backwashing of filter cloths and one submersible pump for bottom sludge purging and service tank emptying.

The carpentry, metal parts and hardware are made of AISI 304 stainless steel. The double drive chain, drive sprockets and disc frame are made of plastic material.

Total weight of the MSF 28/140 PPC filter: about 2,500 kg.

3. Advantages of the Polstoff (pile) cloth filter system

ess than or equal to 10 mgSS/l.

In addition, the free Polstoff fiber used is designed for specific application on tertiary final finishing treatments or for reuse for irrigation and industrial purposes. Due to the tortuous path of water through the cloth, the same advantages of sand filtration are achieved. This results in high filtration performance with high sludge loading capacity. As a consequence, there is a low backwash frequency.

The energy consumption required is extremely competitive with other filtration systems. In fact, the machines are normally stopped and rotation occurs only in conjunction with backwashing. Even during this phase, the maximum value of power absorbed during the backwash cycle is always about 4.0 kW (gearbox plus 2 backwash pumps). This is due to the wire cleaning system, which is achieved by means of vacuum, with far lower energy consumption than pressurized water systems.
Further advantages of the system:

  • great flexibility and ease of management due to complete automation. In particular, the backwash frequency is self-adjusting according to the degree of filter clogging;
  • maintenance is simplified and easy thanks to easy access to wear parts. In addition, the suction cloth washing technology avoids mechanical stress on the cloth, contributing substantially to the durability of the filter material;
  • the useful filter area of 140 m² for each unit is 100% usable because it is totally immersed;
  • the civil works, necessary for the containment of the equipment, are particularly small in size and do not require covering, any aerosol phenomenon being absent.
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