Wastewater Treatment
History of Wastewater Treatment
Wastewater is a combination of water-suspended waste originating from many sources within society, including domestic and industrial waste, mixed with surface, ground and storm-water (Araneda et al., 2017; Sonune and Ghate, 2004). Therefore, wastewater contains a wide array of substances including organic matter (OM), nutrients, chemicals and sediments that must be treated to reduce the environmental, social and economic impacts (Araneda et al., 2017; Sonune and Ghate, 2004).
The necessity for adequate sanitation, through wastewater treatment (WWT), has been seen historically, with the Romans developing the Cloaca Maxima, the world’s earliest sewage system in Rome, in ˜600 B.C.E. to reduce the exposure of Ancient Rome to untreated wastewater (Jeppsson, 1996). The negative impacts from lack of sanitation can be seen with the spread of disease in areas throughout history, from the Dark Ages, Bubonic plague, to modern times, Shigellosis and more, being attributed to the poor sanitary conditions (Mara et al., 2010; Jeppsson, 1996). As such, adequate sanitation has been identified as the most important medical breakthrough since 1840 (Mara et al., 2010; Ferriman, 2007).
A key aspect of providing adequate sanitary conditions stems from WWT, as WWT drastically reduces the spread of disease providing a better standard of living for serviced areas (Mara et al., 2010). Therefore, there is a large requirement for WWT to reduce the quantity of these substances entering the wider environment, reducing the negative social and environmental implications associated with untreated wastewater (Salgot et al., 2018). To achieve this, in modern history there have been many developments in WWT, improving treatment and sustainability, seen through reducing the footprint of wastewater treatment plants (WWTPs), for both the spatial and ecological footprints, and increasing treatment efficiency (Salgot et al., 2018; Zhou et al., 2018; Flores-alsina et al., 2010; Yoon et al., 2006).
Conventional WWTPs
WWT has advanced greatly, with the quality of treatment changing in response to developing social needs and environmental standards, outlined in the Water Framework Directive (WFD) and Urban Wastewater Treatment Directive (UWWTD) in the European Union as well as national standards (Commission Directive 98/15/EC, 1998; European Commission 2000/60/EC, 2000). Traditional WWT, treatment is divided up into three main areas they being Pre-treatment, Primary and Secondary treatment (Salgot et al., 2018; van Lier, 2017a; Siemens AG, 2015; Quevauviller et al., 2007a; Paul et al., 2004; Sonune and Ghate, 2004). Pre-treatment consists of influent screening, removing larger material including debris and materials that will affect the overall performance of the WWTP (van Lier, 2017b; Siemens AG, 2015; Sonune and Ghate, 2004). The wastewater passes to primary treatment, which includes the initial settlement of the OM within the wastewater, sludge, using clarifiers that slowly remove this primary sludge (Salgot et al., 2018; van Lier, 2017b; Siemens AG, 2015; Sonune and Ghate, 2004). Once primary sludge settling and removal has occurred the wastewater is transferred to secondary treatment where the biological activity within the sludge is targeted (van Lier, 2017b; Vanrolleghem et al., 2006; Sonune and Ghate, 2004). This phase consists of aeration to facilitate biological activity to breakdown OM before the further settlement of the material to remove surplus sludge (Salgot et al., 2018; Araneda et al., 2017; van Lier, 2017b; Sonune and Ghate, 2004; Jeppsson, 1996).
Contemporary, Advanced Wastewater Treatment
WWT has developed to further improve the efficiency of the treatment methods. Advanced treatment is classified as a process designed to produce higher quality effluent, improve treatment efficiency and reduce the spatial and ecological footprint that would not be achieved during conventional WWT (Sonune and Ghate, 2004).
Activated Sludge Process
The activated sludge (AS) process is an advanced treatment technology for domestic and industrial wastewater that uses aeration and flocculation, consisting of flocs of bacteria and proto-/metazoa (van Lier, 2017b; Pauli et al., 2014). This process conventionally consists of the use of an aeration tank, where air/oxygen is pumped into the mixed liquors (ML), with the combination promoting the oxidation of OM, ammonium and the removal of nutrients such as nitrogen and phosphorus, through biological immobilisation (van Lier, 2017b; Quevauviller et al., 2007b). This is done as the aeration and flocculation leads to the development of a relatively larger sludge blanket allowing for better WWT (van Lier, 2017b; Quevauviller et al., 2007b).
Once an aeration cycle has been completed the wastewater is transferred to a secondary settling clarifier, where the sludge settles from the ML and is removed as surplus activated sludge (SAS), producing a treated effluent that can either be discharged or undergo further treatment (Miklos et al., 2018; Gogate and Pandit, 2004; Sonune and Ghate, 2004). During the removal of SAS, a portion is retained and re-enters at the beginning of the AS process in order to facilitate further biological WWT by seeding the reactors with biological activity (Salgot et al., 2018; van Lier, 2017b; Quevauviller et al., 2007b; Sonune and Ghate, 2004). Seeding reduces the level of OM as the reintroduced organisms within the SAS have a high Biological oxygen demand (BOD), therefore, exposing the biota to the Dissolved oxygen (DO) rich environment facilitates further biological activity (van Lier, 2017b; Quevauviller et al., 2007b; Sonune and Ghate, 2004).