The final water filtration that comes from the purification plants offers many advantages and compensates for the lack of possible process.
High values of Suspended Solids (SS) always involve negative changes even of other parameters.

Requirements such as improving final effluent quality, adapting plants to new operating conditions and recovering water for industrial or agricultural use are the new needs that present themselves to operators of wastewater treatment plants. Corrective interventions are often conditioned by the lack of space.

MITA Water Technologies proposes a simple and effective cloth filtration system to solve these problems. This process is similar to sand filtration because of the results attained. Furthermore, the cloth filters manufacturing concept permits backwashing during the operation without stopping the machine and then the filtration; it allows for minimum head loss (max 0.5 m c.l.), reduced energy consumption and minimum space requirements.

The cloth filters are made of a support structure (drum or disc), coated with a special polstoff filtration fabric consisting of synthetic fibers having a thickness of 4 – 5 mm.

The filters are equipped with a cloth backwashing device commanded by level sensors. Numerous models are available with a working capacity ranging from 10 m³/h up to over 800 m³/h for a single filtration unit.

The technology and the application experience of MITA Water Technologies offer high reliability machines that are simple to use, with very low investment, operation and maintenance costs.

Functioning description

The cloth filter is made up of a drum or alternatively of filter discs mounted on a central hollow shaft. The filtration is done by gravity with an idle machine and uses the level difference between the input and output of the machine. The discs (or the drum) run completely immersed in the water to be treated.

During the filtration phase, the solids are held back by the special polstoff cloth. The minimum water level in the tank containing the filter is controlled by a weir placed on the side of the clean water outlet. With the increase of deposit of solids on the cloth, the hydraulic resistance increases in passing and therefore the difference between the levels of entry and exit.

When a difference of 15 ÷ 20 cm between these levels is reached, it automatically activates the cleaning process of the cloth.

To clean the cloth which is necessary to restore its filtering capacity, the discs (or drum) are rotated slowly. The solids are removed as a result of the backwash passage through the cloth, the water already filtered by a suction pump; this pump is connected to special suction nozzles, which by adhering to the walls of each disc or drum causes the free fiber of the cloth filter to stretch facilitating the release of the sludge particles retained therein.

The contact of the suction nozzle with the cloth filter generates the recompression of the free fibers and the consequent formation of an even filtration panel.

The cloth filter of the drum or disc filters is made of polstoff free fibers (fiber length from 10 to 12 mm and a diameter of 13 to 27 µ fastened to a large weave fabric that serves as support). The type of filtration obtained is comparable to the deep filtration of sand filters and not the surface filtration of the monofilament cloth filters.

In the TF – TV VM (VM = metal tank) series the cloth support is made of a perforated metal sheet drum. This model is used for wastewater treatment plants of small working capacity (up to 2.000 PE) or as a secondary (more…)

In the PEC – PEC VM (VM = metal tank) series, cloth washing is done with pumps, each of which is served by two or more discs. The pumps are actuated in sequence in order not to disrupt the functionality of the system.

The PPC – PPC VM (VM = metal tank) series was developed for plants with medium-large potential.

For filters in this range, cloth washing is done with two pumps only (of which one backup pump) which serveall the discs.

The selection of discs on which to conduct the backwash (in sequence) is carried out by using double-acting valves with pneumatic control.

The tertiary filtration is strongly influenced by the hydraulic profile of the plant, which often cannot be changed. In the PECV VM (VM = metal tank) disc filters series the collection shaft is placed vertically. The goal is to feed equipment completely submerged even with very low hydraulic profiles.

System advantages

• Gravity filtration with limited head loss
• Continuous filtration without standby units for the backwash phases
• Cloth filter in polstoff free fibre provided with high mechanical strength
• High filtration rate (concentrations of TSS < 5 mg/l at discharge)
• Cloth backwash with very low energy consumption
• No aerosols and noise pollution
• Flexibility in case of overloading and self-regulating with regard to the input parameters
• Minimum dimensions
• Reduced electrical consumption (max. 1.9 kW per equipment)
• Low backwash water volumes
• Simple operation, in monobloc or concrete tanks models
• Limited maintenance
• Direct access to all utilities
• Limited environmental impact

Application fields

• Tertiary treatment: the cloth filters are used with great success for tertiary filtration downstream of secondary settling tanks in biological wastewater treatment plants, both civil and industrial. Their application was brought forth from the need to have an increasingly high water quality at the inflow point of the receiving water body. The rate of removal of suspended solids is very high and the concentration upstream of the filter is of < 5 mg/l. The MITA Water Technologies cloth filters often perform an important barrier function to light sludge spillage that would otherwise exceed the limit of suspended solids concentration in the discharge. The simple construction and operation of these machines are fundamental, as well as theminimum need for routine and supplementary maintenance.
• Phosphorus reduction: in purification plants, the reduction of phosphorus concentration in wastewater discharged into bodies of water can be done with two types of treatment: simultaneous phosphorus precipitation in an activated sludge oxidation tank, with the right chemical dosage; further phosphorus precipitation in a solution in dedicated tanks called “post-precipitation” downstream to the secondary settling, again with the right chemical. In both cases the cloth filterinserted as the last filtration step resolves the key function of sludge separation contained in the phosphorous. Given the very high rate of solids removal( TSS < 5 mg/l), very low concentrations of this element are obtained.
• Upstream pre-filtration disinfection with UV: the disinfection of purified wastewater before the inflow of receiving water bodies is increasingly done with UV systems. These systems for reasons of UV functioning efficiency, reducing energy consumption and fouling on the surfaces of UV lamps require that water to be treated should have a very low content of suspended solids; in general a concentration under 10 mg/l is requested. The cloth filter can be perfectly included in the upstream UV treatment, generously ensuring the quality of the water requested and therefore the efficiency of the disinfection system.
• Downstream filtration of chemical-physical treatments: some production processes in the industry field lead to the production of polluted water from metals for example, in ionic-soluble form that can be separated in the form of sludge flakes due to the dosage of the right chemical compounds. The separation of this sludge may be obtained by using the cloth filters, which cloth also adapts to the type of chemical sludge.
• Secondary sedimentation downstream of immobilized biomass processes: the cloth filter is used to replace the secondary settling only in cases in which the biological treatment is carried out with technologies such as immobilized biomass, biodiscs and percolating filters. In these cases, the wastewater from the biological stage has such a high sludge concentration that it can be treated directly with a cloth filter (without a secondary sedimentation stage), which ensures that the discharge has a content of suspended solids surely less than 35 mg/l and almost always less than 10 mg/l. In this regard, MITA Water Technologies developed acompact biocombi plant which combines biorulli^® and the secondary sedimentation with the cloth filter, which represents the application of above examples.
• Upstream membrane pre-filtration processes: in the case of very thorough civil or industrial wastewater treatments with the purpose of reusing the same water in the industrial and at times in the civil fields, ultrafiltration and reverse osmosis technologies are used with the membranes. The use of reverse osmosis requires a water supply with a very low content of suspended solids in order to protect the membranes from fouling as much as possible thus limiting the frequency of necessary washes to maintain the performance of the system. Even in this case the cloth filter is an optimum filtration system for an upstream treatment with reverse osmosis membranes.
• Pre-filtration of superficial waters: superficial waters, often used for the supply of drinking water undergo a pre-filtration, as first treatment, suitable for the separation of solid particles in the form of silt, microalgae and others.The cloth filter is a useful filtration system to for this purpose with or without an upstream flocculation treatment.