Trattamenti Terziari Acque Reflue

Tertiary Treatments of Wastewater

The extra gear in suspended solids removal

A fining process, which refines the removal of nutrients in the effluent: tertiary wastewater treatments are particularly important processes, as they really ensure the user meets stringent regulations in terms of spills into water bodies. Many technologies in the field, including deep filtration of wastewater: high treatment yields, low resource utilization.

The purpose of tertiary treatments

By tertiary treatments we refer to a series of processes that, in civil sewage treatment plants, result from the roughing out of primary treatments and the biological-type stage of secondary treatments. The purpose is the removal to higher levels of total suspended solids and other pollutants. A not insignificant effect is also the abatement of nutrient odors.

An additional function of tertiary treatments is also not emphasized enough: that of remedying any inadequacies in the primary and secondary processes. In fact, it may happen that upstream equipment carries more load to the tertiary stage technologies than expected (due to lack of maintenance, increase in unplanned effluent, design errors, etc.). Therefore, the solutions installed in this phase must be particularly efficient.

The allowable output is in the range of a few tens of mg/l total suspended solids (and consequent, additional phosphorus removal). Wastewater quality of this kind makes it possible to meet even the strictest regulations regarding spills into water bodies: to potabilize the water, however, additional processes with UV, ozonation or chlorination will be needed.

In any case, there are many areas where water reuse is possible following the tertiary phase of treatment: industry, agriculture (except for vegetable growing), and reuse for civilian use. And precisely for the purpose of reuse, the interest of tertiary wastewater treatment processes is growing all over the world: the purpose is, of course, the continuous optimization of resources and rational water management to counter certain phenomena, such as drought.

Trattamenti Primari Acque Reflue

The main processes for tertiary treatments

We are talking about heterogeneous methodologies, all pertaining to tertiary treatments but with their own technical and functional peculiarities: in fact, not infrequently, solutions adopting these processes are integrated with each other by technical studies.

Here is a first list of processes.

  • Cloth filtration. Involves the use of special fabrics to filter out the smallest suspended particles in water. There are basically two types of technologies available: micro-mesh filters (with ply passages consisting of defined, almost predominantly semi-submerged, size passages) and free-fiber cloth filters (with variable passages and “broken-down” threads, so to speak, for depth filtration, with filter volume always completely submerged).
  • Sand filtration. The filter is equipped with a system that evenly distributes raw feed water to the base of the sandy bed by means of an annular distributor: the water passes through the sand from the bottom to the top, and then escapes filtered by a surface weir.
  • Pressure filtration. In contrast to cloth filtration, in this case the filtration process is forced: the water is actually passed through a filter medium, trapping particles and impurities.
  • Adsorption. This is a physical process: a solid material, such as activated carbon, traps pollutants in the effluent. In the context of tertiary wastewater treatment, the adsorption process is used to remove specific pollutants that may still be present after primary and secondary treatments.

Focus: depth cloth filtration

Depth filtration is one of the tertiary treatments for wastewater finishing.However, unlike micro-mesh filters, the medium for carrying out the process is a cloth with free fibers: thus, filtration takes place not on a “flat” surface but on the entire volume developed by the structure of the cloth (12 mm long threads). This allows high removal of suspended solids (and other pollutants present in the effluent).

In addition, the filtration process takes place passively: the use of electricity to operate the machine is therefore very low, being limited to the backwashing phase of the cloths in which the system moreover continues to operate.

MITA Water Technologies solutions for tertiary treatment

For tertiary treatment of civil wastewater, MITA Water Technologies integrates traditional user-friendly solutions (the sand filters) and advanced filtration of free-fiber cloth technologies into its portfolio.

  • Free-fiber cloth filters. The secret of MITA’s solution is the fleece or microfiber filter cloth: thus always consisting of 12-mm free-pile filaments, for far more efficient depth filtration than surface filtration: this cloth is supported by discs, or a drum, mounted on a hollow shaft. In addition, MITA cloth filters work by gravity: the flow of water to be purified follows a hydraulic out-in profile, from the discs (or drum) to the hollow shaft. Thus, no pumps are needed for wastewater filtration, only for the backwashing phase of the cloths using the freshly filtered water-an incredible saving of energy and the blue resource. Not to mention that the high efficiency makes this solution a very useful support to upstream solutions, handling higher than average loads, and also downstream: for example, avoiding fouling of membrane systems (again tertiary filtration) or better efficiency of UV lamps (disinfection).
  • Biocombi compact systems. They are an integration of free-fiber cloth filters and rotating biological contactors (biodiscs): they represent advanced solutions for wastewater treatment in small communities. In fact, they synthesize secondary (biological) and tertiary (free-fiber cloth filtration) treatments into a single solution of very small size. Minimal energy use, ease of operation, small footprint specially with limiting space configurations.
  • Filtrasand sand filters. The simplicity and reliability of a traditional solution for final removal of suspended solids: high efficiency, low investment and operating costs.

Benefits of MITA solutions

  • High filtration yields (TSS concentrations < 10 mg/l expected at discharge).
  • Countercurrent cloth washing with very low energy use (for cloth filters and Biocombi).
  • Gravity filtration with limited pressure drop (for cloth filters and Biocombi).
  • Compact solution (Biocombi) integrating secondary and tertiary treatments.
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Case studies of tertiary treatments

Biodiscs and Sand Filters for Coastal Depots of Esso

MITA Water Technologies provided ESSO Italiana with wastewater treatment solutions at the coastal depots in Naples and Palermo. Through the use of continuous regeneration sand filters and Biorulli(r) biodiscs, significant results were achieved, such as reduced energy consumption and maintenance costs, plant modularity, and compliance with current regulations.

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