The used, are costly to construct and most important,

The aim of the current work was to
investigate the removal efficiency of a lab-scale hollow fiber submerged MBR
system, treating a low strength wastewater, which was derived from a chemically
enhanced primary treatment pilot-scale unit, such as the reuse possibility of
the treated water.

A MBR system includes a bioreactor with
membrane, where all biological processes for nutrients removal are taken place.
Inside the bioreactor, the separation of activated sludge from the treated
water is achieved through membrane filtration, without the need for
sedimentation tank, which is required in conventional activated sludge systems
(Judd and Judd 2006). The membrane can serve as a porous barrier, which
selectively allows the passage of certain mixed liquor components and retaining
others. The design and application of a MBR system is depending on the
requirements of the wastewater treatment, the type of membrane, the land
availability and functional characteristics of the unit (Pellegrin et al.
2017). According to Van der Roest et al. (2002), such bioreactor systems could
be set up and function like external cross-flow MBR, Internally Submerged MBR
or Externally Submerged MBR.

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Due to human overpopulation,
there is the necessity
to improve wastewater treatment plants capabilities. Activated sludge systems,
which are commonly used, are costly to construct and most important, have high
carbon footprint demands. Many alternative methods to treat wastewater have
been developed, including membrane bioreactor (MBR) technology. The advantages
of MBR are the excellent effluent quality, the lower carbon footprint and the system
operating flexibility (Melin et al. 2006; Akhondi et al. 2017). Their main
disadvantage is the membrane fouling, which depends on operating features and
biological issues. EPS and SMP are the main biological factors affecting
fouling. EPS are microbially produced extracellular polymers, lysis and
hydrolysis by-products (Judd and Judd 2006), whereas SMP are soluble microbial
products that are released into the activate sludge mixed liquor under steady state
or stressed conditions (Laspidou and Rittmann 2002).

External carbon source addition is
the most popular method to improve C/N ratio, having although the disadvantage
of higher operating cost (Zhang et al. 2016). There is a broad range of external
carbon sources used, all of them having high efficiency (Xu et al. 2016). For instance,
methanol and ethanol were used in MBR systems (Ahmed et al. 2008; Hagman et al.
2008), while molasses was used in a SBR system (Quan et al. 2005), resulting in
high nutrients removal efficiencies. In another research work (Bodik et al.
2009), industrial wastewater was used as the external carbon source in order to
increase organic carbon content, while food waste fermentation liquor was used for
enhancing nitrogen removal in a low C/N domestic wastewater (Tang et al. 2017)

The main factor to classify a
wastewater as high, medium or low strength is the C/N ratio (Carrera et al.
2004; Ryu and Lee 2009). Microbial growth in wastewater treatment plants is influenced
by various nutrient requirements and operating conditions, where organic content
is considered as a principal growth factor (Ma et al. 2009). In order to
achieve efficient removal of nutrients in an activated sludge system, the C/N ratio
should be ranged between 8-11 (Chiu et al. 2007; Zhu et al. 2010). Therefore,
an efficient sewage treatment process should be applied in order to achieve
high nutrient removal efficiencies from a wastewater with low C/N ratio (Wang
et al. 2015).

Industrialization and rapid
urbanization of society have result in high use of fertilizers for intensive
agricultural production. Moreover, water bodies have affected by excessive
nutrient inputs as the result of enhanced human activity and extended wastewater
generation. This is causing water eutrophication, which is a critical issue throughout
the world (Bloem et al. 2017). It is seriously affecting water quality, thus
resulting in restriction of human drinking water supplies and deteriorating
dissolved oxygen in water bodies, which is a danger for aquatic organisms (Yang
et al. 2008). Nitrogen and phosphorus are mainly responsible for causing
eutrophication in aquatic ecosystems, and eventually leading to aquatic
environment degradation (Boeykens et al. 2017). They are important factors in managing
phytoplankton biomass and thus affecting ecosystem, so their concentration needs
to be controlled in order to avoid eutrophication and to secure water quality.

In order to protect the environment
and subsequently the human health, every country enforces criteria for the
quality of the treated wastewater. Regulations on wastewater reclamation and
reuse are essential, because they protect public health, increase water
availability by saving water resources, prevent coastal pollution and deal with
water drought problems (Kunz et al. 2016; Fang et al. 2016). Wastewater reuse in
Greece complies with the disposal limits set by the Joint Ministerial Decree
145116/2011 (JMD 2011).