Primary Treatment for Wastewater – Process and Details

Primary treatment for wastewater involves temporarily storing sewage in a calm basin where heavy materials sink and oil, grease, and lighter solids float to the top. Wastewater reaching a treatment plant through pipes first undergoes primary treatment irrespective of its source.

In the previous blog, Wastewater Treatment- Stages and Process full details, I had given an overview of the wastewater treatment process. In this blog, we go on a trip with wastewater entering the primary treatment plant. Let’s dive deep into primary treatment for wastewater and closely observe each of the processes.

Screening – Primary treatment for waste water

The first process in Primary Treatment for Wastewater is screening. I will show you the screening process and different types of screens used in primary wastewater treatment.

In addition to dissolved and suspended impurities, stone, rocks, and even dead animals are among the components of the treatment plant’s wastewater. These solid materials, which make up about a third of the wastewater can erode the pumps and obstruct the flow in pipelines. As a result, eliminating solid waste at the initial stage makes subsequent treatment procedures easier.

Screens and settling tanks remove the majority of the floating materials from the wastewater. Wastewater passes through bar screens consisting of parallel metal bars, wires or grating kept across the flow inclined at 30^o-60^o. According to the method of cleaning, the screens can be manually cleaned or mechanically cleaned screens. For manually cleaned racks the aperture size ranges from 25-50 mm and 5 to 40 mm for mechanically cleaned racks.

Based on the size of the screen opening, we have 3 types of screens as follows:

Coarse screens (≥ 50 mm)
Medium screens (25-50 mm)
Fine screens (10-25 mm).

Normally, domestic wastewater treatment uses medium screens. The channel approach velocities fall in the range of 0.3 to 0.6 m/s for manually cleaned racks and from 0.6 to 1.0 m/s for mechanically cleaned racks.

The wastewater moves to comminutors after screening.

Comminutors

Comminutors reduce bigger suspended particles to smaller sizes by cutting and grinding action. Large plants frequently employ comminutors. It consists of a fixed screen with a rotating or oscillating cutter, or a curved screen with a rotating or oscillating cutter. They are of considerable importance in treatment plants located in cold areas since they eliminate the trapping of waste on freezing screens.

Next, we are going to see grit chambers and their functions.

Grit Chambers

The wastewater after screening enters a grit chamber to settle the grit particles like sand, pebbles etc.
Grit chambers are long, narrow tanks that reduce the flow of water to allow particles like sand, stones, and eggshells to settle out of it.
They are highly relevant in places with combined sewer systems, which carry a significant amount of silt, sand, and gravel washed off roadways or land during a storm.
They protect pumps and pipelines from abrasion and prevents the deposition of grit in pipes and channels.
There are two common types of grit chambers – Horizontal flow and Aerated.
Horizontal Flow Grit chambers permit a velocity of about 0.3 m/s to settle the grit material while allowing the organic impurities to flow through the chamber.
The aerated grit chamber constitutes a spiral flow aeration tank. Wastewater takes a spiral path through the aeration tank. This spiralling action throws away the grit particles into a hopper located underneath.
Scrappers remove the grit for disposal.

Aerated Grit Chamber in Primary Wastewater Treatment

Flow Equalisation – Primary treatment for waste water

Under uniform flow rates, clarifiers and mechanised secondary treatment are more efficient.
Equalization basins store diurnal or wet-weather flow peaks temporarily and make the water flow rate uniform.
Basins serve as a temporary holding area for the incoming wastewater during temporary plant shut down and maintenance.
It acts as a means of diluting and distributing hazardous or high-strength waste into batches.
Flow equalisation basins require variable discharge control which features bypass and cleaning options as well.
Cleaning is easier if the basin is downstream of screening and grit removal.

Also read : What are Water Pollutants? – Definition, Sources and Types

Sedimentation – Primary treatment for wastewater

The wastewater, then moves to sedimentation ponds, settling tanks, or clarifiers after the removal of settled grit. The sedimentation process removes the settleable solids by gravitational settling under quiescent conditions.

On proper adjustment of water flow in the sedimentation tank, the suspended particles begin to fall to the bottom and form a solid mass. Raw primary biosolids, also known as sludge, is the solid mass formed out of the particles. This sludge is removed by vacuum suction or raking it to a discharge point.

Types of Primary Sedimentation Tanks

Rectangular Horizontal Flow Tank
Circular Radial Flow Tank
Up Flow Tanks

Rectangular Horizontal Flow Tank

Feed enters at one end along the width of the horizontal tank.
They can be economically built side-by-side with common walls.
Length ranges from15 to 100m and width ranges from 3 to 24m (length/ width ratio 3:1 to 5:1).
In rectangular tanks, the flow occurs in a horizontal, lengthwise direction.
Rectangular tanks, sometimes use baffle walls to prevent short-circuiting.
Rectangular sedimentation tanks provide reduced maintenance expenses.

Circular Radial Flow Tank

In circular radial flow tanks, influent is fed through a central pipe of the tank and radial flow happens.
They have diameters ranging from 3 to 60 metres (side water depth range from 3 to 5m).
Mechanical sludge scrapers gather the sludge, and a sludge pipe transports it to the bottom.
Circular tanks are more expensive than rectangular tanks, but they have a higher clarification efficiency.

Circular Sedimentation Tanks in Primary Treatment for Wastewater

Up Flow Tanks

Up Flow tanks find application in small treatment plants.
Feed enters through openings along the bottom side of the tank and the effluent after clarification collects at the top.
The flow takes place in a vertical direction.
A sludge blanket in the lower part of the tank acts as a filter for small particles.

The next stage is flocculation which removes the remaining suspended solids.

Flocculation

Flocculation is a water treatment process to remove small suspended solids which don’t settle in the sedimentation tank. In this process solids form larger clusters, or flocs on the addition of a flocculent like aluminium sulphate.

The coagulant molecules have a positive charge. Hence, they can neutralize the negatively charged solid particles that are suspended in the water. Neutralization of the particles initiates the flocculation process. The individual suspended particles come together to join and form a larger mass called a floc.

At the onset of flocculation, we add a chemical polymer. It acts as a bridge between micro and macro flocculants, increasing the mass of particles aggregating together. It also bonds the accumulated material together, preventing it from dissolving even when the water is stirred slightly. After the flocculation, the solid masses are removed either through settling or through the use of filters.

Sludge Removal

In the sedimentation tanks, sludge (the organic component of the sewage) settles out of the wastewater. Mechanical scrapers in the tank’s base continuously move accumulated sludge to a hopper, where it is pumped to sludge treatment facilities. The thickening step removes some of the water before processing the sludge in digesters.

Also read : Activated Sludge Process – Stages and Process Control

Scum Removal

Lighter materials rise to the surface as sludge settles to the bottom of the sedimentation tanks. The constituents of ‘scum’ are grease, oils, plastics, and soap. Scum is skimmed off the surface of the wastewater by slow-moving rakes. Scum is thickened before being poured into the digesters with the sludge.

Primary treatment removes about 60% of the total suspended solids and nearly 35% of BOD. It doesn’t remove the dissolved impurities. The waste must undergo secondary treatment in order to be completely free of toxic substances.