SBR and MBR: A sustainable technical alternative

A sequencing batch reactor (SBR) is a biological wastewater treatment system using activated sludge, whose main characteristic is that all processes required for the treatment are carried out sequentially in the same reactor.

This type of process has certain advantages, which is why it has been implemented in many industrial plants for decades. One of the main advantages of these processes is that they do not require high investments and have moderately low operating costs. Yet an SBR has several limitations, such as its discontinuity, requiring a previous storage tank, the in- ability to achieve a sufficiently high effluent quality to conform to discharge limits, and the low stability of operation, which is very sensitive to load and flow variations. Another essential characteristic of this kind of process is the fact that the sludge is separated from the purified water by sedimentation in the reactor itself, so the process depends to a great extent on the sedimentability of the biological sludge. There is thus a high risk of bulking, producing a loss of active biomass and having negative effects on the discharge quality.

In this article, we focus on MBR technologies with external membranes.

The main advantages of the MBR are its compact design, simple and highly automated operation, its small foot- print and the small biological volumes required. The plants offer high process stability, even in the case of load variations, due to highly specialised microorganisms and a high, solid-free discharge quality. The reason for this is the safe retention of the active biomass with the help of ultrafiltration modules.

In contrast, MBR processes usually require higher investments than other water treatment technologies.

When choosing a particular treatment technology, different factors must be taken into account, such as the inlet characteristics of the wastewater, the required discharge quality, the space available and the investment costs the client can accept.

Based on these considerations, each technology has its corresponding applications and market niches. The main technical differences between SBR and MBR, which allow the selection of the appropriate technology, are as follows:

Process characteristics: The SBR system is a discontinuous process (batch process) working with biomass concentrations below 5 g/l MLSS to ensure proper sedimentation and usual tank heights of up to 6 m. The MBR, on the other hand, is a continuous process (operation 24/7) that works with concentrations of up to 25 g/l MLSS and tank heights of up to 10 m, since the separation is carried out by membranes and not by sedimentation. Due to these differences, MBR processes require smaller biological volumes and footprints than SBR processes.

Effluent quality: The effluent of an MBR is completely free of suspended solids since the ultrafiltration represents a physical barrier. Due to this fact, the microorganisms are also more specialised (higher sludge age), making it possible to achieve high chemical oxygen demand (COD) and nitrogen elimination rates. In general, MBR processes achieve significantly higher effluent qualities than SBR processes (COD 50-60% lower than in SBR processes under the same conditions and nitrogen concentrations below 10 mg/l). As for the denitrification rates, although the nitrate concentration of the effluent is comparable in both technologies, the SBR has a higher risk of inhibition of the biological process than the MBR due to the combination of high ammonium concentrations and high pH values during the anoxic denitrification phase.

Production of surplus sludge: Due to the higher biomass concentration and sludge age, the quantity of surplus sludge produced in an MBR is significantly lower than that of an SBR process.

Process stability: SBR processes are much more sensitive to variations in the wastewater composition, which often cause changes in the biomass, affecting sedimentation and consequently leading to a loss of biomass and deterioration of the discharge quality. The MBR biomass separation system with membranes is less influenced by the rheological characteristics of the sludge, allowing a high degree of adaptation to changes in the inlet without affecting the process. On the other hand, with the MBR being a continuous process, the setup of the operating parameters (pH, dissolved oxygen, temperature, chemical dosing) is stable and continuous over time, which makes plant control and operation easier than in an SBR.

Energy consumption: For the same wastewater, the energy consumption of an MBR is generally higher than in an SBR, largely due to the high energy requirements of the MBR ultrafiltra- tion pumps. However, this is partly compensated by the higher energy effi- ciency of the MBR aeration.

Combination with post-treatment steps: Due to the high effluent qual- ity obtained in an MBR and the fact that the effluent is completely free of solids, the combination with refining post-treatment steps (salt reduction, recycling) is simpler and more efficient in the case of an MBR process than in the case of an SBR process.

In some cases, the installed SBR process does not suffice to achieve the required treatment objectives. There are a variety of possible reasons: changes in the characteristics of the actual wastewater compared to those considered in the design (concentrations, flow rate), modification of the discharge requirements, incorrect process design, etc.

In this case, a technological alternative for improvement is the upgrade of an SBR to an MBR, which allows most of the existing plant to be reused while achieving the discharge requirements reliably, quickly and economically.

The technical study of the requirements for the conversion of an SBR into an MBR is specific to each individual plant, but there is a set of general common criteria to be taken into account:

Analysis of actual conditions: the client must provide detailed information about the actual situation at the plant

Detailed characteristics of the existing wastewater present and/or future discharge requirements, specific project limitations (space, heights, location, noise, future change of regulations, other technical considerations)

• Detailed data on the current SBR plant (engineering, design, equipment dimensioning, operating data)

• Reasons why the client thinks that the SBR does not meet expectations and requirements (insufficient treatment capacity, non-compliance with discharge limit values, non-compliance with other specific regulations, operating problems and difficulties, high costs, etc.)

With all data collected, a detailed and justified study must be carried out on the specific technical reasons for the in- adequate functioning of the SBR, which typically include the incorrect design or dimensioning of different units of the system (biological volume, aeration, cooling, nutrients, programming, etc.), the systematic failure to comply with the discharge requirements or operating problems (bulking, foaming, etc.).

Based on the previous analysis, the interventions required for the conversion of an SBR into an MBR reusing most of the existing SBR plant are presented.

One of the essential actions to convert an SBR into an MBR is to install an ultrafiltration unit with external membranes for the separation of activated sludge connected to the existing biological SBR reactor, which will basically be operated continuously as an aerated nitrification reactor for the elimination of COD. Moreover, the adaptation of the rest of the equipment (aeration, cooling, anoxia zones, reagent dosing, post-treatment steps, etc.) to the new process must be analysed and the operation system must be updated with a new PLC/SCADA program.

The external membrane ultrafiltration systems are very compact and usually delivered "fit for purpose", pre-installed on racks or in standard sea freight containers. For this reason, the conversion of an SBR into an MBR is usually a quick and simple project with minimal interface to the existing SBR system.

An external ultrafiltration unit requires very little space. Depending on the application, it is possible to install a complete ultrafiltration system with a hydraulic capacity of up to 500 m3/d in a commercial 40’ container (length 12 m, width 2.5 m).

The conversion of an SBR into an MBR is a sustainable technical alternative, with the following advantages and improvements compared to the existing SBR:

• Reuse of most of the existing SBR system

• Increase of the COD and nitrogen load to be treated by up to five times without having to increase the biological volume

• High effluent quality (free of suspended solids, low COD load, ammonium practically zero) to comply with very strict discharge limits and possibility of water reuse

• Direct, economic and efficient combination with post-treatment steps

• Very compact solutions with very small footprints

• Simple and quick installation with minimal interface between the SBR and the new units

• Economic solution with improved treatment capacity and discharge quality at low investment costs.