Unleash the SBR’s full potential

A Sequencing Batch Reactor (SBR) is a fill-and-draw type of conventional activated sludge treatment system.  This approach was developed over a century ago and has proven to reliably treat municipal and industrial wastewater.  More recent developments in wastewater treatment technology have made the SBR even more of an advantageous treatment method. With a variety of technologies available, it’s important to understand both attributes and challenges presented by the SBR to ensure the best possible outcome for a project.

Fill-and-draw batch processes have been applied in wastewater treatment since the early 1900’s when Ardern, Lockett, and Fowler developed the activated sludge processes (ASP) process in their laboratories in Manchester, UK (Ardern and Lockett 1914, Ardern and Lockett 2015). Since the development of the ASP, continuous flow ASP dominated over the fill-and-draw batch processes due to the lack of automation that we have today.  In years past, manually adjusting valves, switching on and off pumps and the lack of level controls etc. made the process onerous.  However, the development of control systems and mechanical equipment in the 1960’s, 1970s, and 1980’s (Pasveer 1958, Irvine 1971, Wilderer and Schroeder 1986) laid the foundation for reviving the SBR process as a solution for municipal and industrial wastewater treatment. While the SBR process was quickly becoming more popular in small and medium sized plants, another approach to the SBR process was developed: the cyclic activated sludge process (Goronszy 1979, 1985, Demoulin and Goronszy 1997, Demoulin et. al 1999). It was developed and applied in larger wastewater treatment plants using the process names CASS¹ and ICEAS². Similarly, more recently developed processes  incorporate cyclic activated sludge reactors which are continuously fed with fresh wastewater while treated water is intermittently decanted from the reactor.

In more recent years was the development of a new SBR process, the Aerobic Granular Sludge (AGS) Process Technology, (de Bruin et. al 2004, Prout et. al 2015, Prout et. al 2017) which has received a lot of attention. An advantage of this process is the production of larger sludge granules than that of conventional activated sludge flocs.  Larger flocs mean faster sludge settling. This larger floc size also allows for an anaerobic core, an anoxic mid layer and an aerobic outer layer in a single sludge granule compiling anaerobic, anoxic and aerobic driven process steps (e.g. P‐Removal, Denitrification, and Nitrification) (see Figure 1).

Figure 1: Aerobic Granular Sludge Floc

If properly designed , AGS applications offer the potential for achieving:

• Higher MLSS without the use of plastic carriers
• Better settleability
• Reduced reactor volumes
• Smaller footprints
• Shorter cycle times
• Lower capital investments
• Higher process safety
• No need of internal re‐circulation pumps
• Less sludge production
• Decreased energy consumption
• Reduced operating costs

This is why few commercial wastewater treatment system providers, other than the Dutch pioneer Royal Haskoning DMV, have attempted to develop, establish and market their own adaptions of the AGS. All current AGS advancements are based on the SBR principle of using one or more complete mixed reactor(s) where the AGS  is supposed to develop. This means to unleash the potential advantages of the AGS process, challenges which come along with the SBR concept, listed below, must be mastered.

1. Precise hydraulic and load equalization of the flow is required and usually leads to large upstream mixing and equalization
2. It is necessary to overdesign the mechanical equipment because of limited runtime per process
3. Accurately scaling up from lab‐scale and/or pilot scale to life-size and large-scale plants is challenging and requires in-depth understanding of fluid mechanics to be more reproducible.
4. The prerequisite of obtaining completely mixed reactors can significantly limit the reactor, process, and  equipment

Some technologies require precise equalization of hydraulics and loading.  Controlled anaerobic feeding of raw wastewater into the settled sludge blanket is crucial for the kick‐off of granular sludge formations. To achieve this, the most common approach is to feed the reactor through a bottom feeder with flow throughout the bottom of the entire reactor and install a static decanter at the water surface as illustrated in Figure 2.

Figure 2: Typical AGS Reactor configuration with bottom feeder and static surface decanter

This approach works best in smaller circular or rectangular tanks with pipe works providing equal distribution of the raw wastewater throughout the bottom of reactor with the use of a  static decanter at the top and when the required piping  does not become too complex and expensive and does not result in too much of a hydraulic loss. The approach limits the volume per reactor module to smaller volumes and is the reason why these plants require large equalization basins upstream of the biological reactors, because an equal flow distribution at the bottom can only be achieved for the one exact hydraulic flow to the reactor for which the feed systems was designed for. The slightest deviations from the designed influent flow will lead to non‐uniform feed and distribution of raw wastewater in the sludge blanket, improper functioning of the static decanter and the operation of the plant as a whole. A further disadvantage of the bottom feeder system is the high hydraulic loss which is required to achieve equal flow distribution across the bottom of the reactor. Treatment dependence on precise flow makes treatment performance sensitive to minor flow fluctuations which can be caused by something as simple as sludge clogging  the feeder piping.

Batch wastewater treatment often suffers from discontinuous operation of main equipment such as pumps, mixers, aeration systems, blowers, etc. resulting in over design of equipment Instead of selecting and designing these expensive components for continuous operation, the equipment, depending on the cycle strategy needs to be selected to pump, mix, aerate and decant the wastewater in a fraction of a 24 hour day.

An example of this for the aeration system and blowers is  provided in Figure 3, below. In a continuous flow process the aeration system design is made based on daily actual oxygen requirement (AOR). Since the aeration system must continuously supply the oxygen to the  constant inflowing raw wastewater, the required hourly capacity of the aeration system can be calculated by dividing the daily AOR value by 24. This value is the benchmark which is set to 100. Peaking factors are neglected since they apply in both cases. Typical SBR cycles can vary from 3 to 6 hours resulting in 8 to 4 cycles per day respectively.

The cycle incorporates the following phases: fresh feed of wastewater, mixing, aeration, settling, and decanting.   Assuming that only 3h per cycle can be used for aeration and we use a cycle strategy with 4 cycles a day the total aeration time per day is only 12h instead of 24h. Therefore, the installed aeration system capacity needs to be 2 times for batch processes than what is required by continuous flow reactors with the same capacity. For example, if 2,000 diffuser elements are required for a continuous flow system project, 4,000 are required for a batch process with the same aeration depth.  This applies to other equipment as well (e.g. 6 vs 12 blowers or 450 kW vs 900 kW respectively).  All of which must be purchased, installed, and maintained resulting in greater capital and operations and maintenance costs.

Some SBR processes’ aeration phases overlap with the fill phases resulting in lowered water depth in the reactor reducing the overall aeration efficiency causing the aeration system design has to compensate. This applies to other equipment such as pumps and piping.  This example demonstrates the necessity of investing in processes alternatives with holistic designs which consider processes, reactor, and other equipment.

Precise up-scaling of processes and reactor designs from lab‐scale to pilot scale and from pilot scale to large scale; and possibly from large scale to even larger scale can be extremely challenging. Scale‐up ratios of 1:10 are usually straight forward, but the potential to make mistakes is plentiful. To successfully up-scale  not only geometrical similarity is important, but also reactor behavior and retention time must to be maintained. If not all scale‐up targets can be met at the same time and an experienced engineer  must decide which parameters are most important.

An example is that lab‐scale basic principles are often examined in circular vessels of a dimension of 200 – 300 mm and volumes are measured in liters. Equipment for mixing, aeration and pumping comes from the laboratory and/or a local aquarium shop. This setup usually is sufficient to prove feasibility and to determine basic parameters such as air flow and biological reduction rates. However, because of the size of the reactors, mixing is usually not the limiting process due to the oversized air bubbles providing an abundance of mixing along with the oxygen transfer. This step up from lab‐scale to pilot scale is the reasonably poses the first challenge because aquarium and lab equipment usually is not available on a larger scale and industrial-sized equipment usually is not available in pilot scale.  Therefore large scale equipment is often used in pilot‐scale and the similarity of lab‐scale and pilot‐scale very often cannot be truly reproducible. This is not necessarily a problem as long as the process works in pilot scale and the process parameters can be adjusted accurately. The transfer from pilot scale to large scale (e.g. 20m³ to 1,500m³)  is more challenging, but manageable.  This step is not successfully achieved due to the limited availability of large scale basins and because of the challenge of achieving geometric similarity of the large scale basins.  For aeration system design, the floor coverage is almost never similar and feed piping usually has to follow real live availability and budget restrictions. The list of challenges associated with up-scaling go on, but with a thorough understanding of fluid dynamics and wastewater treatment this process can be more accurate and reproducible.

Figure 3: iC³ – Reactor Module

This process leverages the unique features of the HYPERCLASSIC^®-Mixing and Aeration System (Hoefken et. al. 1991, Hoefken et. al. 1993, Hoefken 1994, Hoefken et. al. 2001, Hoefken et. al. 2004) which is ideally suited for intermittent processes

The primary feature of the INVENT SBR is characterized by the iC³ reactor module which is a drawn‐out rectangular basin with multiple HYPERCLASSIC^®-Mixing and Aeration Systems mounted in series to create individual complete mixed zones (cascaded).Wastewater continuously enters through a specially designed inflow distributor at one end and clear treated water intermittently leaves the reactor through operated decanting systems on the opposite end. This design makes it possible develop and operate an optimized SBR cycle strategy which can vary from zone to zone. Figure 3 shows an iC³ Reactor module in a schematic representation.

Summary

This article summarizes the history of SBRs and the development of the INVENT iSBR^®/iGSR^® process. Over the course of 30 years, various challenges related to the overall design and scaling up of the processes have been overcome to achieve groundbreaking results with modern SBR systems. These features include:

1. Continuous feed with intermittent decanting
2. Multiple cascaded full-mix reactors per SBR module, and Holistic process, reactor, and plant design.

Authors: Dr.-Ing. Marcus Hoefken, Megan House, Dr. rer. nat. Peter Huber, and Dipl.-Ing. Walter Steidl