Physical Unit Operations in Water, Wastewater Treatment

Mostly physical unit operations are used in preliminary and primary treatment steps. Screening, Mixing, Flocculation, sedimentation, gravity separations are some physical unit operations in wastewater treatment. In this tutorial, we are going to learn physical unit operations that engage in water or wastewater treatment processes. In a previous tutorial, we have learnt basic concepts of wastewater treatment.

Written by: Eng (Ms). K. Chathurika Jayanthi, Bsc. Chemical Process Engineering, University of Peradeniya

Physical Unit Operations

Different physical unit operations are applied to stop the materials such as sand, stones, plastic bottles, etc. If these kind of materials reached towards to the rest of treatment process, those processes or equipment will be disturbed. DIfferent physical unit operations are mentioned below.

  • Screening
  • Mixing and flocculation
  • Gravity separation
  • Sedimentation

Screening in wastewater treatment process

Screening is used to remove large particles in a wastewater stream. This helps to eliminate the damage that can happen to the process equipment due to the course parcels in wastewater (for example large particles can damage pumps which used to transfer wastewater). And it improves the process reliability and the effectiveness of process treatment.

As screens remove large partials, we need to transport and properly dispose of them. If not, it will lead to another problem because these particles may be contaminated with pathogenic microorganisms, insects like mosquitos will privilege and odor or toxic gases will form, etc.

Screens can be divided into three categories and they are mentioned with their sizes.

  1. Coarse screens (> 6mm)
  2. Fine Screens (0.5 mm to 6 mm)
  3. Micro screens (<0.5mm)

Coarse screens

Coarse screens eliminate to enter the particles larger than 6mm such as Plastic bottles, rocks, branches, leaves, etc. Quantity and the characteristic of the waste collected on screens depend on the size of the screen opening, type of the bar screening, the geographic location, the sewer system and the volume of waste received. Typical information about the screening removed from wastewater is shown in table 1.

Table 1 Typical information of waste removed by coarse screening (Reference: Metcalf & Eddy, The fourth edition of "Wastewater Treatment Engineering and Reuse")

Typical information of waste removed by coarse screening

There are two types of screens according to the cleaning as manual cleaned and mechanical cleaned. Manually operated course screens are used in small to medium wastewater flow rates. As well as manually operated screens are used for stand-by screens when used mechanically cleaned screen to be repaired or at a power failure. The screen can be consisting of parallel bars, rods, wire mesh.

Mechanically cleaned screens can be categorized as chain-driven screens (figure 1), reciprocating rake screens (Climber), Catenary screen and Continuous belt screen. Mechanically cleaned screen to reduce labor requirements and eliminate the flooding due to uneven cleaning durations.

diagram of a chain-driven coarse screens

Figure 1: A diagram of a chain-driven coarse screens

Fine screens

Fine screen openings are in the range of 0.2 mm to 6mm and are used for primary, preliminary and combined sewer overflows. Rotary drum wedge wire, static wedge wire screen, step type screen are the different types of fine screen are used in wastewater treatments.

Micro screens

Micro screens have a range of openings. Typical suspended solids removal is around 55%. Suitable for low flow rate streams. It is not advisable to use micro screen when solid load fluctuations are possible. These screens are used to remove suspended solids secondary effluents and stabilization pond. Step type screens have self-cleaning when it works while static wedge and rotary drum screens need to clean with a water jet or steam jet.

Mixing and flocculation

Mixing is very important in many steps of wastewater treatment. Mixing is used to mix or blend chemicals with wastewater, flocculation of particles and for good heat transfer. Inline mixers, high-speed in-line mixers, water jets paddle mixers and turbine mixers are some of the mixers used in wastewater treatments.

Flocculation is a process used to form large particles that can be removed in settling or filtration from small destabilized particles. (To destabilized particles in wastewater, some chemicals are used and this process is known as coagulation. Coagulation is a chemical process; hence we will learn it under chemical unit operations) When mixing these small destabilized particles has random movements. Therefore, those particles impacts with each other and stick and form large particles, this is known as micro-flocculation (Figure 2).



Furthermore, in gravity settling large particles has high settling velocity then they overtake small particles and there may colloid and stick and make larger particles, this is known as macro-flocculation.

Different types of mixers can use for flocculation. Over and under baffle reactors are one of the most used methods.

Gravity separation

Gravity separation is the most widely used process in wastewater. In settling tanks, clarifiers and sedimentation tanks are used the gravity separation technique to separate suspended solids that have higher density than water. Gravity separation can occur in 4 ways as below.

Discrete particles

This is applied to remove grids and sand. There is no interaction in between the partials therefore particles settle individually.

Flocculant settling

This is used to remove suspended solids in primary and secondary settling tanks. Chemical flogs are also removed using this method. Particles stick to each other during the setting process hence, increase particles mass. When a particle mass increased it deposits faster.

Adding blasting chemicals and polymers to the waste stream the flocculation process can accelerate to happen rapid settling. This is known as blasted flocculant settling. This method is used to reduce BOD and phosphorous in wastewater.

Hindered (zone)

When partials settle the concentration of the particles increased in the bottom and middle height of the tank, then interparticle forces build with close particles. Due to these interaction forces, particles settle. This kind of settling happens in the secondary settling tanks and clarifiers.


This is occurred at the bottom of the settling tanks due to the weight of the particles. When particles are settling the particles are continually deposited then due to the mass of these solid particles, earlier deposited waste will be compressed.

Gravity settling is used in plate and tube settlers, lamella plate settlers, etc. A schematic diagram of solid settling of inclined plates shows in figure 3. The settling efficacy of an inclined plate depends on the inline plate angle, fluid velocity, properties of solids in water, etc.

Schematic diagram of Inline plate settler

Figure 3: Schematic diagram of Inline plate settler

Inline plate settler gravity settling

Figure 4: Inline plate settler

Sedimentation in wastewater treatment

Sedimentation is used to remove settleable solids from the wastewater. It reduces TDS and BOD of water. This can be a circular or rectangular tank. A rectangular sedimentation tank is shown in figure 5. Most circular settling tanks are designed with central feed and the effluent take came out through weirs. The efficiency of the sedimentation tank can be affected by eddy current formation, thermal convection current or density current formation due to a temperature difference of flow streams, current due to wind effect, etc. these effects may reduce sedimentation efficiency.

Rectangular sedimentation in a small water treatment plant

Figure 5: Rectangular sedimentation in a small water treatment plant