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).
The depuration chain of the Lodi plant consisted, until 2017, of the stages of:
Two main objectives were set with the last intervention:
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.
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:
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.
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:
A solution for high filtration yields and efficient handling and backwashing, with only two pumps for the whole system: the PPC and PPC VM disc filters are for concrete or metal tank, respectively. Versatility and little ancillary work required for operation.
In this area, contractors and engineering firms contacted by municipalities face a number of challenges: knowledge of regulations, managing costs, and selecting cost-effective solutions. MITA Water Technologies can support them with its efficient and easy-to-manage solutions.
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