37.1 INTRODUCTION
Although use of intermittent operation reactors (sequencing batch reactors – SBR) started many decades ago, it was from the early 1980s that this technology became more widespread and used in the treatment of a larger diversity of effluents. This is partially due to a better knowledge of the system, to the use of more reliable effluent withdrawal devices, to the development of a more robust instrumentation and to the use of automated control by microprocessors. In the past few years, in view of the growing concern with the discharge of nutrients in watercourses, sequencing batch reactors have been modified to accomplish nitrification, denitrification and biological phosphorus removal.
37.2 PRINCIPLES OF THE PROCESS
The principle of the intermittent operation activated sludge process consists in the incorporation of all the unit operation and processes usually associated with the conventional treatment by activated sludge (primary sedimentation, biological oxidation and secondary sedimentation) in a single tank. Using a single tank, these processes and operations simply become sequences in time, and not separate units as in the conventional continuous-flow processes. The intermittent flow activated sludge process can also be used in the extended aeration mode, in which the single tank also incorporates sludge digestion.
C 2005 IWA Publishing. Biological Wastewater Treatment in Warm Climate Regions by Marcos von Sperling and Carlos Augusto de Lemos Chernicharo. ISBN: 1 84339 002 7. Published by IWA Publishing, London, UK.
1024 Activated sludge
Table 37.1. Stages in a typical operational cycle of sequencing batch reactor for carbon removal
Stage Scheme Aeration Description
Fill on/off
• The fill operation consists of the addition of sewage and substrate for microbial activity.
• The fill cycle can be controlled by float valves to a pre-established volume or by timers for systems with more than one reactor. A simple method that is ordinarily applied to control the fill cycle is based on the volume of the reactor, resulting in fill times inversely related to the influent flow.
• The fill phase can include several operational phases, and is subject to several control modes, named static fill, fill with mixing, and fill with reaction.
• The static fill involves the introduction of the influent without mixing or aeration. This type of filling is more common in plants for nutrient removal. In these applications, the static fill is followed by a fill with mixing, so that the microorganisms are exposed to a sufficient amount of substrate, while anoxic or anaerobic conditions are maintained. Both mixing and aeration are performed in the fill with reaction stage.
• The system can alternate among static fill, fill with mixing and fill with reaction throughout the operational cycle.
React on
• The objective of the reaction stage is to complete the reactions started during the fill stage.
• The reaction stage can comprise mixing, aeration or both. As in the case of the fill phase, the desired processes can require alternated aeration cycles.
• The duration of the reaction phase can be controlled by timers, by the level of the liquid or by the degree of treatment, through the
monitoring of the reactor.
• Depending on the amount and duration of the aeration during the fill phase, there may or may not be a dedicated reaction phase.
Settle off
• The solids–liquid separation occurs during the sedimentation phase, similar to the operation of a secondary sedimentation tank in a conventional plant.
• The sedimentation in an intermittent system can be more efficient than in a continuous-flow sedimentation tank, due to more quiescent conditions of the liquid in a sequencing batch tank, with no interference of liquids entering and leaving.
Table 37.1 (Continued )
Stage Scheme Aeration Description
Draw off
• The clarified effluent (supernatant) is removed during the draw phase.
• Drawing can be carried out by several
mechanisms, the most frequently used ones being floating or adjustable weirs.
Idle on/off
• The final phase is named idle, and is only used in applications with several tanks.
• The main objective is to adjust the operational cycle of one reactor with the operational cycle of another reactor.
• The time intended for the idle phase depends on the time required by the preceding tank to complete its cycle.
• Wastage of excess sludge usually happens in this phase.
Source: Adapted from EPA (1993)
The process consists of complete-mix reactors where all treatment stages occur.
This is attained by the establishment of operational cycles with defined duration.
The biological mass remains in the reactor during all the cycles, thus eliminat-ing the need for separate sedimentation tanks and sludge recirculation pumpeliminat-ing stations. This is the essence of a sequencing batch reactor: biomass retention with-out the need for sludge recirculation by pumping. By preserving the biomass in the system, the sludge age becomes higher than the hydraulic detention time, which is a fundamental feature of the activated sludge process. The usual stages in the treatment cycle are summarised in Table 37.1.
The usual duration of each cycle can be altered in view of the variations of the influent flow, the treatment requirements and the characteristics of the sewage and biomass in the system.
The excess sludge is generally wasted during the last phase (idle). However, since this phase is optional, because its purpose is to allow an adjustment among the operational cycles of each reactor, the wastage may occur in other phases of the pro-cess. The quantity and frequency of the sludge wastage are established according to the performance requirements, as in conventional continuous-flow processes.
The plant usually has two or more sequencing batch reactors operating in paral-lel, each one in different stages of the operational cycle. This need is compulsory in systems that receive inflow during all day (such as domestic sewage), because a reactor in the sedimentation stage, for example, is not able to receive influent. At this time, the influent is being directed to another reactor, which is in the fill phase.
In plants receiving wastewater intermittently, such as in industries that work only 8 hours per day, there may be just one reactor, that works in fill (and possibly react) phase for 8 hours, and carries out the other stages of the cycle in the subsequent 16 hours. Figure 37.1 shows schematically a plant with three sequencing batch reactors in parallel.
1026 Activated sludge
Figure 37.1. Arrangement with three sequencing batch reactors in parallel
37.3 PROCESS VARIANTS
Several modifications have been incorporated into the process, in order to achieve different objectives in the wastewater treatment. These changes refer both to the form of operation of the system (continuous feeding and discontinuous drawing) and to the sequence and duration of the cycles associated with each phase of the process. The variations presented can also be used for the treatment of industrial effluents (Goronszy, 1997). Examples of process variants are presented next, some of them being protected by patent.
(a) Sequencing batch reactor for biological nitrogen removal
Biological nitrogen removal can be reached by the incorporation of an anoxic stage after the aerobic reaction stage (Figure 37.2). In this case, there is a post-denitrification stage, which can be easily accomplished, although it occurs under endogenous respiration conditions, that is, at lower denitrification rates, due to the smaller availability of organic carbon.
If very low nitrogen values are not required, then a post-anoxic stage will not be necessary. In this case, a substantial amount of nitrate can be removed in a