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Cloth Media Filters

The effluent from municipal and industrial wastewater treatment plants that employ biological secondary treatment can contain high levels of total suspended solids (TSS), that could impact downstream treatment processes and the environment into which it is discharged. Filtration is typically used to remove the TSS and improve the effluent quality. Traditionally conventional sand or media filters have been used for tertiary filtration of secondary treated effluent. However, sand filters typically require a large footprint, require standby filtration capacity with ancillary equipment (including filtered water storage and compressed air systems for air scouring), to allow for backwashing of duty filters, and can be difficult to operate with high concentrations of TSS and where variable influent TSS levels a highly variable.

MITA Water Technologies offers a simple and effective cloth media filtration system for tertiary filtration applications that overcomes the problems typically experienced with sand media filters. Cloth media filtration achieves similar performance in terms of effluent quality (with typical effluent TSS < 5 mg/L) when compared to sand filtration. In addition, cloth media filters provide forward filtration while backwashing occur; this means there is no need for standby filters to take a filter offline for backwashing. Cloth media filters require low headloss (max 500 mm), have reduced energy consumption, and have minimal space requirements.

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

The cloth media filters are equipped with an onboard cloth backwashing system. Numerous cloth filter 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 highly reliability equipment that is simple to use, with very low investment, operation and maintenance costs.

Functioning description

Polstoff for cloth filters

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. During filtration operation the discs (or the drum) are stationary, and only rotate when backwashing (or cleaning of cloth media) is required. The level difference between the inlet and outlet  of the filter activates a backwash sequence. The discs (or the drum) are completely immersed in the water to be treated.

During the filtration phase, the solids are held back or removed by the special polstoff cloth. The minimum water level in the tank containing the filter is controlled by a weir placed on the filtered water outlet. As deposition of solids on the cloth increases, the hydraulic resistance increases in passing liquid through the cloth, which results in a water level differential.

When a differential of about 200 mm is reached, it automatically activates the cleaning process of the cloth to restore its filtering capacity, called a backwash sequence.

During the backwash process, the discs (or drum) are rotated slowly. Solids are removed by passing filtered water as backwash through the cloth. A backwash pump, connected to special suction nozzles that contact the cloth media, provides suction that causes the free fibers of the cloth filter to straighten to release the solids particles retained therein.

As the discs (or drum) rotate, the contact of the suction nozzles with the cloth filter generates the recompression of the free fibers and the consequent formation of an even filtration panel.

The cloth media 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 µm) that are fastened to a large weave fabric that serves as support. The type of filtration obtained is comparable to the deep bed filtration of sand filters and not the surface filtration of the monofilament cloth or screen-type filters.

 

Cloth filters: advantages and application

 

System advantages

  • Available as packaged filters in steel tanks, or fitted into concrete tanks
  • Gravity filtration with low headloss
  • Continuous filtration without the need for standby units for backwashing
  • Cloth media manufactured in polstoff free fibre, with high mechanical strength
  • Operational flexibility, tolerating high variability in hydraulic and solids loading, and self-regulating with regard to the input parameters
  • Cloth media backwash requiring very low energy consumption
  • High filtration rate
  • No aerosols and noise pollution
  • Small footprint
  • Low electrical installed power draw
  • Low backwash water volumes
  • Simple operation, fully automated
  • Very low maintenance
  • Direct access to all utilities

Clean water flowing over effluent wier 

Application fields

  • TERTIARY TREATMENT:  Cloth media filters are used with great success for tertiary filtration downstream of secondary settling tanks in biological wastewater treatment plants, both municipal and industrial. Through reduction of total suspended solids and associated BOD, cloth media filters greatly enhance effluent quality to the receiving water body. MITA Water Technologies cloth media filters often perform an important barrier function to prevent discharging sludge into the effluent when poor sludge settling occurs in the upstream treatment processes, that would otherwise exceed the limit of suspended solids concentration in the discharge. The design and construction of these machines are robust, operation is simple, and routine and supplementary maintenance requirements are very low.
  • PHOSPHORUS REDUCTION: In purification plants, the chemical reduction of phosphorus concentration from the effluent, can be achieved in several ways: simultaneous phosphorus precipitation in an activated sludge process, further phosphorus precipitation prior to the secondary settling, and flocculation & coagulation upstream of the filtration process. In all cases cloth media filters provide the last filtration to separate solids containing chemically bound  phosphorous. Given the very high rate of solids removal( TSS < 5 mg/L), very low effluent phosphorous concentrations are obtained.
  • UPSTREAM PRE-FILTRATION DISINFECTION WITH UV: Disinfection of treated effluent prior to reuse or discharge to receiving water bodies is increasingly done with UV systems. Removal of TSS from the effluent is required for efficient UV disinfection,  to reduce energy consumption and limit fouling of UV lamp sleeves; typically TSS concentration of less than 10 mg/L is required. The cloth media filter can be incorporated readily upstream of UV systems, to ensure effective UV disinfection.
  • DOWNSTREAM FILTRATION OF CHEMICAL-PHYSICAL TREATMENTS: Treated effluent from some industrial processes that contain metals (for example) can lead to environmental pollution. Through chemical addition such metals can be converted into insoluble form, that can be separated from the liquid. Cloth media filters can be used to separate such chemical solids from the effluent prior to environmental discharge.
  • SECONDARY SEDIMENTATION DOWNSTREAM OF attached growth BIOMASS PROCESSES: The cloth media filter can be used to replace secondary settling only in cases in which the biological treatment is carried out with attached growth technologies such as immobilized biomass, Rotating Biological Contactors (RBC) and percolating filters. In these cases, the wastewater from the biological stage has such a high solids concentration that it can be treated directly with a cloth filter (without a secondary sedimentation stage), to ensure the filtered effluent has a TSS content of less than 35 mg/L and almost always less than 10 mg/L. MITA Water Technologies has developed a compact Biocombi package plant which combines biorulli (RBC) for biological treatment and providing secondary sedimentation with the cloth filter.
  • PRE-FILTRATION for membrane PROCESSES: In municipal and industrial applications where high level of treatment is required for the purpose of reusing the water, ultrafiltration (UF) and reverse osmosis (RO) technologies are used with the membranes. The use of UF and RO requires a water supply with a very low content of suspended solids in order to protect the membranes from fouling, thus limiting the frequency of cleaning 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 UF and RO membranes.
  • PRE-FILTRATION OF surface WATER: Surface water, often used for the supply of drinking water often requires pre-filtration for the removal of solid particles in the form of silt, micro-algae and others. The cloth filter is a useful pre-filtration system for this purpose, with or without an upstream flocculation treatment.