Select Page

Image via Pixabay

Wastewater is an important consideration for business owners and facility managers right across the USA. Management teams need to be able to guarantee the quality of the effluent they leave behind, as well as protect public and environmental safety. With this in mind, where does the preliminary treatment of wastewater fit into the process?

Read on to discover more about the first steps that need to be taken on the wastewater journey.

The Need for Preliminary Treatment

Preliminary treatment is not as thorough nor as complete as primary, secondary, and tertiary wastewater treatment processes, so why do facilities bother with this step at all? If all contaminants and debris will be removed as part of the main treatment phases, why is preliminary treatment necessary in the first place?

To understand preliminary wastewater treatment, we need to shift our focus. Preliminary treatment is not exclusively about protecting public health and ecosystems — this is what the main phases are designed for. Instead, this preliminary phase is intended to protect the water treatment equipment and infrastructure, extending their lifespan and ensuring they stay viable and effective in the long term.

In a sewage treatment plant, there are tanks, filters, and other pieces of crucial equipment. Debris in the wastewater can rapidly erode these pieces of equipment due to friction over time or direct impact in the case of larger objects. As wear and tear build up over the full phase of operation, this infrastructure will need to be replaced. When debris is filtered and removed from the process before the main phases of treatment, the equipment is protected from harm, and the main phases of treatment become more effective.

Types of Preliminary Wastewater Treatment

Preliminary wastewater treatment can be divided into three main categories. These are screening and filtering, grit removal, and large object removal.

  • Screening and Filtering

Screening and filtering refer to the removal of coarse solids that may be floating or suspended within the wastewater sample. There are many different types of solids that may be found in wastewater, and the specificity of these solids will depend on the type of wastewater being filtered. Typically, these solids will consist of plastics and rubber, as well as paper, fabric materials, and vegetable matter.

The screening and filtering phase is a simple, mechanical process. The wastewater passes through a filtration medium — generally a grill or a set of bars. The water particles and many other contaminants are small enough to pass through this filter, but many of the suspended solids are not. These solids are trapped in the filter, where they can be removed.

In a sewage treatment plant, the bars of the filter will typically be spaced between 15 and 25 mm apart. The filters themselves may be cleaned manually by operating teams, or cleaning may be carried out via an automated process.

  • Grit Removal

Grit removal works similarly to screening and filtering, removing small solid particles from the wastewater mixture. Generally speaking, this process of removal is designed to target smaller particles of heavy solids that may be suspended in the wastewater — such as sand or grit. This is what gives the treatment process its name.

The water is allowed to settle, and small particles with a high mass relative to the particles around them are gravity-sorted — i.e., gravity causes them to sink to the bottom of the mixture. Here, grit chambers are deployed to collect the particles in a specific location, where they can be safely and effectively removed. Grit removal will be handled by automated machinery in larger water treatment facilities, although smaller plants may implement this process manually.

  • Large Object Removal

In some cases, large objects with significant mass can cause damage to filters and other treatment media, even in the preliminary phases of treatment. If this is a risk, these large objects must be effectively and systematically removed from the wastewater mixture or neutralized before they come into contact with filtration media.

This may be achieved via a process of comminution — breaking down particles of high average size and mass into particles of lower average size and mass. Comminution requires high levels of energy to complete effectively, so this process is not advised in any case where it is not completely necessary. However, in certain applications, comminution is required to extend the life of other preliminary treatment mechanisms within the system.

Alternatively, large objects can be removed from the mixture directly — perhaps caught in larger, heavier-duty grills before they reach more sensitive filtration media.

The Next Steps — Following on from Preliminary Water Treatment

Once preliminary treatment is complete, the main phase of the wastewater treatment and purification can begin.

  • First comes the primary wastewater treatment phase, in which solid waste within the water is sedimented into sludge and then separated from the rest of the water, ready for processing.
  • Next comes the secondary stage of the wastewater treatment, in which biofiltration and oxidation are used to purify the water left behind after the primary phase of the treatment. Aeration may also be used to accelerate and optimize the process.
  • In the third and final — tertiary — stage of treatment, phosphates and nitrates are removed from the remaining wastewater, neutralizing the effluent and protecting local communities and ecosystems.

The success of each of these three main stages — as well as the longevity of the equipment — depends upon the successful completion of the preliminary processes.

Preliminary Treatments Are Key to the Entire Process

Preliminary wastewater treatment might not take center stage during the treatment process, but it would be impossible to properly treat water without these initial procedures. The preliminary treatment of wastewater not only ensures effective results from the main phases of the treatment process; it also helps treatment equipment and infrastructure last longer. In turn, treatment facilities can achieve efficient processes that reduce cost and effort while also safeguarding the long-term viability of the equipment and infrastructure.