The second stage of wastewater treatment is wastewater treatment. Suspended solids, colloidal particles, oil, and grease are removed during initial treatment. Biological treatment of the wastewater is used in secondary treatment to eliminate any organic particles. Biodegradable soluble pollutants like sugar, fat, detergent, and food waste are consumed by indigenous and aquatic microorganisms such as bacteria and protozoa. These processes are temperature sensitive, and when the temperature rises, the rate of biological reactions rises as well.

There are two different treatment techniques for secondary wastewater treatment:-

1. Aerobic Treatment: Aerobic wastewater treatment is a biological treatment that breaks down organic materials and removes other contaminants such as nitrogen and phosphorus using oxygen. The following are examples of aerobic therapy technologies:

• Activated Sludge Process (ASP) / Extended Aeration System (EAS)
• Sequential Batch Reactor (SBR)
• Moving Bed Bio Film Reactor (MBBR)
• Membrane Bioreactor (MBR)

2. Anaerobic Treatment:Anaerobic treatment is a procedure in which microorganisms break down wastewater or materials without the use of dissolved oxygen. Anaerobic bacteria, on the other hand, can and will use oxygen from the oxides put into the system or from organic material in the effluent. Up-flow Anaerobic Sludge Blanket Reactor is an anaerobic treatment technology (UASB).

Activated Sludge Process/Extended Aeration System

The Extended Aeration process has a similar design and mixing regime to the entirely mixed process.

Low organic loading, lengthy aeration period, high MLSS concentration, and low F/M are all used in the process. The effectiveness of BOD elimination is excellent.

The mixed liquor solids undergo significant endogenous respiration and become well stabilised as a result of their prolonged detention in the aeration tank.

The increasing oxygen demand has an impact on the system's total operating costs.

The operation is simplified, however, because primary settling and separate sludge digestion are no longer required.

Sequential Batch Reactor (SBR)

The Sequential Batch Reactor is a wastewater treatment technology that fills and pulls activated sludge.

A single batch reactor can be used for equalization, aeration, and clarification. For big industrial effluents, the SBR system works well.

A secondary clarifier is not required with the SBR. Although SBR requires more maintenance in operation due to the PLC-based process, it has higher efficiency and requires less space for the ETP.

SBR is a procedure that combines secondary treatment and settlement. The oxygen is bubbled with a mixture of wastewater and activated sludge to remove organic matter (BOD and COD). The treated water can then be discharged into surface water after treatment.

BOD, TSS, and a tiny amount of nitrogen, phosphorus, and heavy metals are removed from wastewater during secondary treatment, which removes 85 to 90% of them. Tertiary wastewater treatment, also known as advanced treatment, is the third step of wastewater treatment. The burden of nitrogen and phosphorus in the water is removed during tertiary treatment. Filtration, ion exchange, activated carbon adsorption, electro dialysis, nitrification, and denitrification are some of the processes involved.

The properties of effluent after secondary treatment and the type of water required at the end of the treatment determine the treatment options in tertiary treatment. For example, if we require drinkable water, wastewater must undergo filtration and disinfection.

Sludge produced by wastewater treatment operations and processes is normally in the form of a liquid or semisolid liquid containing 0.25-12 percent solids by weight, depending on the operations and processes employed. Due to the presence of organic stuff in sludge at the end of biological treatment, its processing and disposal is possibly the most challenging problem. Different treatments on the sludge are done in sludge dewatering to give a somewhat continuous, homogeneous feed to sludge processing facilities.

The following technologies can be used to cure sludge:

• Filter press
• Belt press
• Screw press
• Centrifuge

Filter Press

A dewatering equipment known as a filter press is used to lower the moisture content of sludge and biosolids.

The recessed plate filter press has a set volume and is made up of a series of square plates that are recessed on both sides and are supported face to face in a vertical position on a frame with a fixed and moveable head.

Each plate has a filter cloth hanging or fitted over it. The plates are kept together with enough force to seal them, allowing them to resist the pressure used in the filtration process.

Chemically treated sludge is pumped into the area between the plates, and a pressure of 100 to 225 lbf/in2 is applied for 1 to 3 hours, forcing the liquid through the filter cloth and plate outlet ports. After that, the dishes are separated and the cake is removed. The sludge cake thickness ranges from 25 to 45 mm, while the cake's moisture content ranges from 48 to 70%.

Belt Press

Between two filter belts, a belt filter allows continuous sludge dewatering. It is divided into zones where sludge passes.

Flocculator: The sludge flocculant blend is completed in the flocculator, after which the flocculated sludge is evenly dispersed across the filter belt.

When the input stream enters the clarifier, it is stopped. Hoppers are placed at the bottom of the clarifier to catch solid particles that settle to the bottom while the cleared liquid escapes.

A gravity drainage zone: The sludge is drained on the first belt, with the picket fences laying freely on the belt assisting the drainage. A water line is established, and the majority of the water is removed through flocculation.A water line is established, and the majority of the water is removed through flocculation.

A progressive compression zone: After draining the flocculated water, the sludge is crushed between two filter belts. With the introduction of the upper belt, a gradual increase in pressure occurs:

Low-pressure belt filters can withstand up to 4 bar of pressure.

Medium pressure belt filters can handle up to 5 bars of pressure.

High-pressure belt filters can handle up to 7 bar of pressure.

A cake scraping zone: When the sludge is squeezed, it takes on a more solid appearance. A sludge cake, or simply cake, is what it's called. After that, the cake is scraped from the surface of the two belts that divide at this level.

A high-pressure washing station cleans each belt constantly with a bank of nozzles operating at 7 to 8 bars (100 to 120 PSI).

Screw Press

A screw press is a machine in which the screw drives a ram up and down. It employs a coarse screw to turn the lever's rotation into downward force. Balls are commonly used as flyweights in levers. These weights assist in maintaining the tool's velocity and push, making it easier to operate.