Wastewater Characteristics
Wastewater is another word for sewage; it is water that has been used in households, industries, institutions and enterprises which is often collected in a sewage collection or drainage system.
Raw wastewater is 99.9% water and 0.1% contaminants in general.
However, if wastewater is not adequately treated, the contaminants in it can harm our environment, generate odors, and pose severe hazards to human health.
Organic matter accounts for roughly 75% of the pollutants in wastewater; it primarily consists of human and food waste.
Water contains nitrogen, phosphate, and trace amounts of other nutrients.
Nutrients promote plant development, which can result in excessive plant and algae growth in water, which is harmful to the natural ecology.
As a result, surplus nutrients must be removed from the water source before disposal.
Toxic components in industrial effluent must be eliminated before discharge.
Heavy metals, chemical compounds, oils and fats are the primary concerns.
Heavy metals present in wastewater include arsenic, cadmium, cobalt, chromium, copper, iron, lead, manganese, nickel and zinc.
The majority of the metals are removed during the treatment procedure and end up as solids.
As a result, the majority of heavy metal concerns are concerned with the disposal or reuse of sludge.
All wastewater contains microorganisms that are both beneficial and hazardous to wastewater processing.
Aerobic and anaerobic bacteria decompose organic matter into more stable forms that may be disposed of more easily in the environment.
Microorganisms that cause disease in plants, animals and humans are known as pathogens.
Before discharge, the processed water is disinfected to kill bacteria that may be harmful to the ecology.
How is wastewater collected?
The collection of waste streams from residences, companies and industrial complexes is the first step in the wastewater treatment process.
These streams feed into the “collection” or “drainage” system, which delivers wastewater to a wastewater treatment plant for treatment.
Typically, the collection system is run as a separate department inside the municipality.
The collection system is made up of pipes, junction boxes, lift stations and other equipment that transport raw wastewater to the treatment facility.
Storm runoff will be collected by the collecting system in many circumstances.
Combined sewer overflow (CSO) systems are systems that transport storm runoff and waste streams.
Individual residences are linked to the collection system by a lateral sewer to the main sewer line.
A junction box is where sewer lines from several directions meet.
The flow from main lines is combined at junction boxes into a considerably bigger flow that is directed towards the wastewater treatment plant.
Gravity will be used to transport wastewater to the wastewater treatment plant wherever practicable due to the architecture of the collecting system and the location of the facility.
Lift stations will be employed to pump wastewater to the plant where this is not possible due to the municipality’s location, elevation fluctuations and system design.
The maximum predicted flow rates at each station will determine the size and pumping capacity of each lift station.
Lift stations with modest flow rates are typically rather small, with two small submersible pumps to move the stream.
Flow rates will increase as the collection system draws closer to the treatment plant.
Lift stations located closer to the plant can grow rather large, necessitating the use of numerous large-capacity pumps to provide enough flow capacity.
All lift station designs must account for flow rate variations caused by demand variables such as time of day and storm surges.
Showers are taken in the early morning, which corresponds to the peak daily flow rates experienced by a treatment facility.
If the collecting system is set up for CSO, these pumping stations may house several huge auxiliary storm pumps.
The wastewater enters the wastewater treatment plant from the collection system.
What do wastewater treatment plants do?
A Wastewater Treatment Plant (WWTP) is a facility that receives wastewater mostly from home, commercial and industrial sources and removes contaminants that harm water quality and endanger public health and safety when discharged into receiving streams or bodies of water.
To obtain the necessary outcomes, most facilities use a combination of mechanical removal procedures and bacterial breakdown.
Chlorine is frequently added to plant discharges to limit the risk of disease spread caused by the release of dangerous germs.
Lifts or pumping stations transport raw wastewater to the wastewater treatment facility.
When a sewage system serves a neighborhood or location lower than the plant, an uphill shortcut will greatly reduce the total length of pipe necessary to tie into the plant, or existing structures or other constraints demand an uphill approach to the WWTP, lift stations are required.
What are the different stages of wastewater treatment?
The wastewater enters the plant at the headworks where processing starts.
The typical water processing steps include:
_ Preliminary treatment
_ Primary treatment
_ Secondary treatment
_ Tertiary treatment
Preliminary treatment
The headworks include the influent channel, coarse and fine screens and aerated grit chambers where preliminary treatment occurs.
Flow measurement, screening, pumping and grit removal is the typical step in preliminary treatment.
Wastewater enters the influent channel into the coarse screens.
The screens remove large debris that enters the sewage collection system such as rags, tramp metal, sticks, broken glass, rocks, sand and a vast variety of other materials.
Screens are utilized early in the wastewater treatment process to minimize pump and equipment damage within the facility.
In many wastewater treatment plants, fine screens are utilized to remove smaller debris.
All screened debris is removed and disposed of at a landfill.
The wastewater is then pumped into grit removal chambers.
Air is introduced into the chamber to scour the organic materials from the grit before the grit settles to the bottom of the chamber.
The settled grit or sand is delivered by a screw conveyor to a pit at one end of the chamber.
From there, it is pumped by a grit pump to a grit/water separator.
This debris is also disposed of in landfills. Liquid separated from the grit is returned to the grit chamber.
Wastewater from the grit chamber then flows to the primary clarifiers.
Primary treatment
The primary treatment process reduces the solids content of wastewater through sedimentation.
Wastewater slowly flows into large tanks called primary clarifiers where heavier particles are allowed to settle at the bottom of the clarifier.
Scrapers move the settled solids (primary sludge) to sumps at one end of the clarifier.
From there, the primary sludge is pumped into a holding tank where solids processing commences.
Solids lighter than water float to the top and are skimmed from the top of the primary clarifier and pumped to a thickener for solids processing.
The greases and fats skimmed from the top of the clarifier are called scum.
Primary treatment removes approximately 30 – 50% of the suspended solids.
The remaining clarified liquid, containing mostly dissolved materials, flows to the secondary treatment stage.
Secondary treatment
During secondary treatment, organic material is removed through biological treatment most widely used biological treatment method is the activated sludge process.
The activated sludge process requires an aerated tank containing bacteria that break down the organic materials.
The bacteria use the organic material in the liquid and clump together to form a microbial floc, which is also known as activated sludge
This liquid flows into the secondary clarifiers where the activated sludge is allowed to settle.
In some wastewater treatment plants, ferric chloride is added after biological treatment to cause precipitation of phosphate materials remaining in the liquid.
Flow enters the clarifiers from the bottom of the tank through a pipe located at the center of the tank.
The clarifiers are designed to direct the flow from the center of the clarifier in a downward direction to encourage the solids to settle.
The activated sludge settles at the bottom of the secondary clarifier.
Some of the settled activated sludge is collected and returned to the aeration tank to ensure sufficient bacteria and organic waste supply to maintain the biological process.
This material is called Return Activated Sludge (RAS).
The activated sludge not needed for the biological process is called Waste Activated Sludge (WAS) and will be pumped to the sludge conditioning stage for further processing.
The clarified liquid, with over 95% of the organic materials removed, flows to the tertiary treatment stage.
Scum, formed on the top of secondary clarifiers is sent to a thickener for solids processing.
Tertiary Treatment
The tertiary treatment stage normally starts with the filtering of the clarified liquid that flows from the secondary clarifiers.
The liquid is processed through a bed of sand or another filtering device that removes additional pollutants from the liquid.
The water then moves to the disinfection tank.
Water enters the disinfection tank where chlorine gas or sodium hypochlorite is metered in the tank.
The water slowly moves through the tank to enable the chlorine to kill the microorganisms remaining in the wastewater that may be harmful to fish life.
The disinfected water is then passed on to a dechlorination stage to remove the chlorinated materials that also could be harmful to fish life.
Sulfur dioxide or sodium metabisulfite is the most cost-effective chemical utilized to neutralize chlorine.
Another disinfection method that eliminates a dechlorination stage is called ultraviolet disinfection.
Ultraviolet light sources are submerged in a holding tank.
The ultraviolet lamps emit radiation that penetrates the cell wall of the microorganism and is absorbed by cellular materials, which either prevents replication or causes the death of the cell.
As a result, pathogenic microorganisms are almost entirely inactivated or killed.
The UV light disinfection technology is considered to have no adverse environmental impact.
The water or effluent can now be discharged into the ecosystem.
References
METCALF & EDDY (1991). Wastewater engineering: treatment, disposal and reuse.
