Environmental Engineering Reference
In-Depth Information
The purpose of recent research was to stress advantages of SBR in comparison with conventional
flow systems. The principal investigations are publications of Dennis and Irvine (Dennis 1979;
Irvine 1979; Ketchum 1979), in which they studied the effects of
fill/react
ratios. Hoepker et al.
(1979) found out that smaller loading has given better quality of effluent. Ketchum et al. (1979)
studied the possibilities of SBR for tertiary treatment, especially for phosphorus removal. Obaja
and Mace (1985) presented the possibilities of nitrification and denitrification of a given plan and
control of the process.
The majority of advantages of the SBR can be attributed to the flexible nature of operation
parameters (Hvala 2001; Kazmi 2001; Miklos 2001; Morling 2001; Ng 2001; Ruiz 2001; Tilche
2001; Yalmaz 2001; Yoong 2001). A large choice of system parameters can be the consequence
of constant volume, where we can change
fill/react
ratios and the time of aeration. Flexibility of
operation parameters also enables understanding of basic mechanisms of the process and critical
phases that are very important for further application (Wareham 1993; Orhon 1994; Zec 1997;
Paul 1998; Wu 2001). Recently, a unified basis of design for SBRs was prepared mainly covering
practical aspects of SBR technology and emphasizing the need for appropriate design guidelines
(Artan 2001; Artan et al. 2006).
Although there are several configurations of SBRs, the basic process is similar. The installation
consists of at least two identically equipped tanks with a common inlet that can be switched
between them. The tanks have a “flow through” system, with raw wastewater (influent) coming
in at one end and treated water (effluent) flowing out the other. While one tank is in settle/
decant mode, the other is aerating and filling. At the inlet is a section of the tank known as the
bioselector. This consists of a series of walls or baffles that direct the flow either from side to side
of the tank or under and over consecutive baffles. This helps to mix the incoming influent and the
returned activated sludge, beginning the biological digestion process before the liquor enters the
main part of the tank.
There are four stages to treatment: fill, aeration, settling and decanting. The aeration stage
involves adding air to the mixed solids and liquid either by the use of fixed or floating mechanical
pumps or by blowing it into finely perforated membranes fixed to the floor of the tank. During this
period, the inlet valve of the tank is open and a returned activated sludge pump takes mixed liquid
and solids (mixed liquor) from the outlet end of the tank to the inlet. This “seeds” the incoming
sewage with live bacteria.
Aeration times vary according to the plant size and the composition/quantity of the incoming
liquor, but are typically 60-90 minutes. The addition of oxygen to the liquor encourages the
multiplication of aerobic bacteria and they consume the nutrients. This process encourages the
production of nitrogen compounds as the bacteria increase their number, a process known as
nitriication.
To remove phosphorus compounds from the liquor, aluminum sulfate (alum) is often added
during this period. It reacts to form nonsoluble compounds that settle into the sludge in the next
stage.
The settling stage is usually the same length of time as the aeration. During this stage the sludge
formed by the bacteria is allowed to settle to the bottom of the tank. The aerobic bacteria continue
to multiply until the dissolved oxygen is all but used up. Conditions in the tank, especially near the
bottom, are now more suitable for the anaerobic bacteria to flourish. Many of these, and some of the
bacteria that would prefer an oxygen environment, now start to use nitrogen as a base element and
extract it from the compounds in the liquid, using up the nitrogen compounds created in the aeration
stage. This is known as denitrification.
As the bacteria multiply and die, the sludge within the tank increases over time and a waste
activated sludge pump removes some of the sludge during the settle stage to a digester for further
treatment. The quantity or “age” of sludge within the tank is closely monitored, as this can have a
marked effect on the treatment process.
The sludge is allowed to settle until clear water is on the top 20-30% of the tank contents.
