Overview of Tertiary Filtration for Industrial Water

Industrial water filtration systems are useful when it is necessary to be compliant with increasingly restrictive environmental regulations and to obtain a high-quality final effluent. Tertiary filtration is particularly convenient when space availability is limited. Industries often leverage on it when wastewater reuse and recycle within the production cycle is needed.

The primary objective and main application of industrial water tertiary filtration systems is the reduction of total suspended solids (TSS). This is usually accompanied by organic pollutant removal (BOD/COD) and phosphorus removal.

1. Tertiary industrial water filtration systems: application

Cloth filters are efficient tertiary filtration systems for industrial water. They are used downstream of secondary decanters and in biological wastewater treatment plants for industrial and civil wastewater.

Their application arises from the need to have an increasingly high-quality water for discharge or for reuse in production lines. The removal rate of suspended solids is very high and the expected concentration downstream of the filter is less than 5 mg/l.

2. Why is industrial water filtration important?

Now let’s see why industrial water filtration is important.

Water finds various applications in most industrial activities: to cite a few examples, it is used:

• as an “ingredient” in the preparation of finished products,
• for equipment and surface washing,
• inside cooling towers
• or as a “means of transport” for products within the factories.

Both waste and process industrial water treatment are therefore an integral part of the production cycles and require ever greater attention. This is particularly true when water quality influences that of the finished products and it is necessary to preserve it: for instance, in areas where this resource is scarce or, generically speaking,  for environmental protection purposes.

One of the most valid alternatives offered in the field of industrial water filtration systems is represented by disc filters: they are tertiary filtration systems allowing companies to obtain superior-quality treated water, suitable for reuse and recycling. Water recovery makes it possible to preserve a precious resource which would be discharged otherwise. Moreover, it is economically advantageous as it allows cost reduction associated with water collection or disposal (sewer, water reservoirs, treatment on behalf of third parties).

3. Disc filters in industrial applications

Disc filters are widely used in industrial water treatment plants, in response to different needs:

• Sludge separation generated by chemical-physical treatments.
• Phosphorus removal after chemical precipitation treatments.
• Pre-filtration upstream of membrane processes (ultrafiltration or reverse osmosis) with the aim of safeguarding the membranes from fouling and limiting the frequency of chemical washing.
• Pre-filtration of surface water for the removal of solid particles present in the form of silt, microalgae and more.
• Removal of specific pollutants.
• Water treatment of cooling systems.
• Improved efficiency of UV disinfection

4. Tertiary filtration process of industrial waters

The industrial water filtration process commonly makes use of cloth filters: that is, a piece of equipment consisting of disc or drum filters, mounted on a central hollow shaft. Filtration takes place by gravity while the machine is stopped. It takes advantage of the difference in level between the machine inlet and outlet. Discs or drums work completely immersed in the water to be treated.

During the filtration phase, the water to be treated passes through the cloth. Solids contained in it are retained by the special free-fiber cloth. As solid deposit on the cloth increases, the hydraulic resistance to the passage of water increases like the difference between the inlet and outlet levels.

When a certain difference between the aforementioned levels is reached, the cloth washing process is automatically activated: in this phase,  discs (or drum) are rotated slowly. Solids are removed when the already filtered water (pumped in by a pump through special nozzles) passes through the cloth in a counter-current way. Flow inversion causes the relaxation of the free fibers of cloth filter, favouring the release of the retained sludge particles.

The contact of the suction nozzle with the filter cloth generates the recompaction of the free fibers. Consequently, a uniform filter panel is formed.

5. Types of filters for industrial water treatment

The MITA Water Technologies free-fiber cloth disc filter is a valid alternative to semi-submerged disc filtration systems using microscreen (made of metal wire or other materials). Indeed, it offers the following advantages.

• Filtration from the outside (Out-in): filtered materials remain outside the filtering sectors and do not accumulate inside them. Foreign bodies carried by the water flow are easily removed as they are deposited outside the filtering units and can be quickly removed from the tank.
• Superior mechanical resistance of the cloth: the free-fiber cloth is more resistant than microscreen one when an impact with external solid bodies occurs. This is true thanks to the moderate tension of the cloth on its plastic support and to its greater thickness.
• Greater sludge accumulation capacity: free-fiber cloth allows the formation of a thick filter panel made up of intertwined fibres. The high sludge accumulation capacity slows down the obstruction of the free passages available to the flow. Consequently, degradation of filtration performance occurs.
• Absence of fouling from precipitates: free-fiber cloth is not subject to the problems associated with progressive fouling due to the precipitation of compounds (such as calcium carbonate) which requires frequent chemical washing to restore.
• Reduction of energy consumption: the installed powers are very limited, and the utilities operate only during the washing cycle. The absence of nozzles for pressure washing, replaced by the washing system in slight depression, guarantees a substantial reduction in energy consumption.
• Ease of management and maintenance: the completely submerged installation of the MITA cloth filter avoids the need for tank covers. All components are easily accessible thanks to the service level inside the tank for any maintenance work. Filters are always visible allowing for quick and easy inspection by operators.

6. Downstream of industrial filtration: UV disinfection

The disinfection of treated wastewater is increasingly carried out with systems that use UV rays. Water which gets treated by these systems must have a very low content of suspended solids to allow efficient operations, reduction of energy consumption and to avoid dirtiness on the surface of the lamps. Cloth filters fit perfectly into the treatment chain, upstream of the UVs: they guarantee the required water quality and therefore the effectiveness of the disinfection system.