Mechanical Wastewater Treatments

Physical principles as the only leverage for removing suspended solids

Low energy use, usually low acquisition and operating costs: these are some of the features of mechanical wastewater treatments, which remove suspended solids of various kinds without the input of chemicals or biological processes. Some examples of these systems, also proposed by MITA Water Technologies: lamellar pack settlers and decanters, flocculators, free-fiber cloth filters.

What are mechanical treatments

We can speak of mechanical wastewater treatments when we refer to a series of processes to remove suspended substances based solely on the use of physical principles. Thus, they do not involve the use of chemicals or biological processes to remove pollutants.

Here are some examples of mechanical processes for wastewater treatment.

  • Screening. This is the method by which coarser solids, such as plastic objects, twigs and debris, are removed by metal grates usually placed upstream of treatment plants.
  • Desanding. Aims to remove by gravity small but high specific weight elements, such as sand and crushed stone. Removal of oils and grease, which are collected on the surface of the tank, usually also takes place at the same time.
  • Sedimentation. This takes advantage of the higher specific gravity of the substances to be removed compared to water: in this way, pollutants accumulate at the bottom of the tank and can be easily separated from the liquid. It differs from desanding in that it is aimed at removing solids with a slightly higher specific gravity than water, such as biological sludge or particulate organic matter.
  • Flotation. Systems that fall into this category take advantage of the formation of microbubbles within the volume of water: these bind to sludge and other light suspended particles that are thus brought to the surface of the water and then removed by a radial collector.
  • Final filtration. A process that may involve different filter media: sand, micro-mesh (surface) or free-fiber (depth) cloth, semipermeable membranes. The purpose is the removal of suspended solids. The amount of organic matter, nitrogen and phosphorus, present in the effluent, also often tends to drop as a result of these purification processes.

Despite this heterogeneity of types, mechanical treatments have some common advantages:

  • they have low energy consumption,
  • they are simple to operate,
  • and are generally cost-effective.
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Mechanical treatments for civil and industrial wastewater

According to European Union environmental regulations, civil sewage treatment plant processes are divided into primary, secondary, and tertiary treatments.

Several mechanical treatments are found in the first and third of these categories.

  • Primary treatments: aim to remove large suspended substances. The most common primary mechanical treatments are screening desanding and primary sedimentation.
  • Secondary treatments: processes for biological degradation of organic matter belong to this category: they therefore do not include independent mechanical treatments, with the exclusion of secondary separation, or alternatively membranes in MBR reactors, which are, however, an inseparable part of biological treatment.
  • Tertiary treatments: aim to remove unwanted substances that were not separated from water during the first two stages. The most common tertiary mechanical treatments are filtration, reverse osmosis, and disinfection.

In industrial wastewater treatment, this division is less clear-cut: in fact, each process line has specific needs and often requires only some of these treatments. Particularly appreciated in this area is the convenience of mechanical treatments in terms of low acquisition costs, efficiency, and ease of installation and operation.

Mechanical treatments proposed by MITA Water Technologies

Here are the efficient, low life-cycle cost technologies proposed by MITA Water Technologies for mechanical wastewater treatment.

  • Lamella-pack decanters and oil separators. For the separation of suspended solids and oils: they have an internal structure with inclined parallel channels to increase the useful surface area for treatment.
  • Dissolved air floaton units. A highly efficient system resulting from the combined system of uniform hydrodynamics and microbubble air formation.
  • Continuous sand filters. A classic filtration system that evenly distributes raw feed water to the base of the sand bed by means of an annular distributor, enhanced by a continuous sand washing system.
  • Free-fiber cloth filters. Depth filtration, similar to that of sand filters, combined with the compact mechanics of cloth filters, to achieve total suspended solids at the outlet down to 5 mg/l, with continuous feeding and thus no process interruption during backwashing.
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