How a Wastewater Treatment plant works
To understand how a wastewater treatment plant (WWTP) works you have to look into all aspects of the water treatment process. The series of processes that occur throughout the wastewaters journey from being untreated influent to it being discharged as a treated effluent often into open water sources. As such below all aspects of a wastewater treatment facility will be discussed.
Pretreatment
The journey from untreated to treated wastewater begins with the pretreatment of the influent. At the inlets of the WWTP the process of screening is undertaken with equipment such as bar screens to remove heavy solids as well as grit chambers that use the flow of the influent to deposit heavier material including grit and stones. During this phase of treatment the removal of heavy solids and grease occurs, this is an important part of the wastewater treatment process as the removal of these materials prevents damage to machinery and equipment that allow a WWTP to work to its maximum capacity. If it were not for this process pumps, pipes and other infrastructure throughout the treatment plant would inevitably become blocked by these larger materials preventing the facility from treating wastewater without large repair works being undertaken. Following the pretreatment process the wastewater travels to Primary wastewater treatment.
Primary treatment
Primary treatment is based around the process of settling through the use of clarifiers to remove the fine organic solids that remained following pretreatment. The initial settling of wastewater can occur through the use of circular sedimentation clarifiers or inclined plate settlers otherwise known as Lamella clarifiers. A circular sedimentation clarifier settles the organic material through the process of time and gravity with the suspended solids settling to the bottom of the clarifier. A mechanical system situated inside the clarifier rotates slowly throughout the day removing the settled primary sludge from the bottom of the clarifier while the primary treated effluent flows out of the clarifier onto secondary treatment. Lamella clarifiers on the other hand use a series of inclined plates where the untreated effluent flows along these plates, which are designed to have a large settlement area, while the fine suspended material settles to the bottom of the plate as there is not enough transportational energy within the wastewater to carry the suspended solids. This is conducted using a series of plates in order to achieve an effluent with a substantial amount of the organic materials and pollutants being removed allowing for secondary treatment to take place.
Although both of these clarifiers use different methods they have the same purpose and that is to reduce the concentration of the solids within the wastewater allowing for an effluent that only contains dissolved and suspended biological matter. The wastewater then continues to secondary wastewater treatment while the primary settled sludge is then removed from the clarifiers.
Secondary treatment
As the effluent is discharged following the initial settlement of the wastewater it enters into the second phase of treatment where the main aspect of biological treatment takes place to remove dissolved and suspended organic material. During secondary treatment the effluent will be treated to a standard where the majority of the remaining suspended solids within are removed allowing for the effluent to potentially be discharged into surface water sources to meet environmental quality standards. In order to complete this the effluent goes through the activated sludge process where the effluent is oxygenated to create biomass from the dissolved and suspended material that is removed as sludge.
There are multiple methods to complete secondary treatment of effluent with each of these methods requiring the process of aeration and settling to remove the sludge. In order for the activated sludge process to occur, a small portion of this organically rich activated sludge that is removed following this secondary settling is deposited back into the aeration stage seeding the tank to promote biological activity aiding in the removal of the dissolved and suspended material. This process can be done using a combination of aeration tanks, Secondary clarifiers and sludge storage as well as through the use of a Sequencing Batch Reactor (SBR). In an SBR the whole process of aeration and settlement to remove the suspended solids occurs within a single reactor designed to complete all stages of secondary treatment. Multiple SBRs are often used to provide adequate treatment at all times, with a larger treatment capacity as each of the SBRs must be filled and treatment must take place before the effluent is discharged and the tank can be filled again. While the use of aeration tanks and clarifiers has a more constant flow of effluent for secondary treatment. Both of these methods achieve the same aim of removing the remaining organic material from the effluent allowing for it to be discharged into surface water at a high environmental standard meaning that it does not negatively impact the environment. This is often the final stage of treatment in a WWTP in many areas around the world, however there are further advanced stages of treatment to remove other environmentally damaging substances from the effluent that are being implemented.
Advanced treatment
Tertiary treatment
Following on from secondary treatment other treatment infrastructure is used to remove nutrients and minerals that are Nitrogen and Phosphorus rich that have the potential to negatively impact the wider environment once discharged. Nitrogen is removed through the use of multiple processes in a reactor that force the nitrogen based compounds to change their form from ammonia to nitrogen gas. This is done through oxidation and reduction creating aerobic and anaerobic conditions causing chemical reactions to occur. While phosphorus can be removed through the process of flocculation using Iron, with the phosphorus reacting and being built into easily removable flocs. In most advanced WWTP around the world this is the final stage of wastewater treatment with the effluent posing minimal concern for environmental health.
Quaternary treatment
Quaternary treatment is not implemented throughout the world as it is often costly. During this stage of treatment effluent is disinfected using UV filtration to remove any bacteria that have remained throughout the entirety of wastewater treatment. This achieves a discharge effluent of high quality and is often implemented in regions to protect biota as well as the use of surface water for other uses.
Sludge removal
As a large mass of sludge is produced during wastewater treatment the disposal of this sludge must be done as the sludge created throughout treatment is harmful to both the environment and humans if poorly dealt with. The disposal of sludge is often conducted through the use of Digesters, large tanks that digest the sludge through mixing to produce biogas and digested material. The Biogas can often be used as a source of energy for WWTPs reducing the energy requirements for the facility creating their own energy while the digested sludge is further treated. To do this the material is sent to a dryer that removes moisture from the sludge at a high temperature, producing a dry material that is safe for use as nutrient rich fertiliser, forming another line of income for the WWTP.