SBR process optimization: incorporation of FBR technology as part of an SBR process. PROCESS SIGMA SBR – FBR

1. Introducing Aguasigma

In Agua SIGMA Suministros y Gestiones Medioambientales S.L., The main objective is to provide innovative solutions to mechanical, physical-chemical, biological, filtration and water disinfection processes to cover the various needs for wastewater treatment in the industrial field.

Agua SIGMA belongs to an environment and a culture where relationships with customers and users are established and maintained long term, thanks to effort, service and continuous improvement.

Agua SIGMA equipment are designed with the most advanced water treatment technologies and always with the highest treatment yields while offering the lowest possible ownership and operating costs.

Wastewater treatment engineering

Agua SIGMA manufactures, with its own engineering and design and in its own facilities, the constituent equipment of most of its technologies and processes, such as:

• Coarse solids separation equipment, sieves, and screens.
• Dissolved air flotation (DAF) and induced air flotation (CAF) equipment.
• Aeration systems.
• Press filters.
• Ultrafiltration plants for wastewater and process water, Reverse Osmosis, and Nanofiltration.
• Tanks in AISI304-AISI316 steel.
• Biological aeration and anaerobic reactors.
• High-performance biological processes FBR, SBR, and MBR.
• Disinfection processes using UV, O3, etc. technologies.

The knowledge of Agua SIGMA in the wastewater treatment processes guarantees the most appropriate process design for each application, type of industry and wastewater, its pollutant load and the purpose after its treatment (discharge to the network, discharge to the natural media or reuse).

In addition, Agua SIGMA can provide Process and Product Guarantee, since these processes, equipment and systems have been developed individually for each specific application.

Agua SIGMA also provides project engineering, equipment and turnkey water reuse systems in the form of a pilot plant, with rental or temporary loan and the most appropriate financial solutions for each case. We provide the user with the highest degree of reliability and availability on the market.

Once an installation has been delivered, Agua SIGMA makes its technical assistance service available to the customer, which allows guaranteeing the maximum performance of the installation and optimizing its resources and costs

Department of Technological Development of SIGMA

A highly qualified multidisciplinary team is in charge of the development of each project. From the reception and collection of data to the final delivery, going through all the phases of design, engineering, construction, execution and commissioning.

Technological Development of SIGMA is formed by a team of engineers and experts:

2. Description of the proposed technologies

Among the various technologies currently used for the treatment of industrial wastewater, two advanced processes stand out within BIOLOGICAL TREATMENTS, which are especially effective in removing organic matter (measured as COD) and nitrogen. These processes are as follows:

‘SEQUENCING BATCH REACTOR – SBR’: biological reactor that is operated in batch.

The influent is introduced into a reactor, then the reaction phase occurs, generally with aeration and anoxia stages, in which the biological elimination of pollutants is carried out through the action of microorganisms, later the aeration is stopped and the reactor enters in the sedimentation phase of the sludge and subsequent emptying of the treated effluent.

The separation of the clarified effluent and the biomass is carried out in the same unit, not being necessary to install a clarifier after the SBR reactor.

and

‘FLOTATION BIO-REACTOR – FBR’: this process is an optimization of the conventional activated sludge process, designed and implemented by SIGMA.

The process of a reactor, generally aeration, where the biological treatment is carried out for the elimination of organic matter and nutrients contained in the wastewater, through the action of microorganisms in the presence of oxygen, then the separation of biomass by flocculation and clarification by DAF flotation (dissolved air flotation) takes place.

In this process, biomass flocs are formed that will be separated by flotation with air micro-bubbles in a SIGMA BIODAF equipment. With these special equipment, sludge with a dry solids content of up to 5% is obtained, a value much higher than any conventional system.

After years of experience in the design and operation of conventional SBR plants and FBR plants, the SIGMA Technological Development Department has designed the OPTIMIZATION OF THE SBR CONVENTIONAL PROCESS THROUGH THE INCLUSION OF THE SIGMA-FBR UNIQUE TECHNOLOGY.

The following sections describe each process independently and its unification as the SIGMA SBR - FBR Process for its optimization.

2.1 The SBR Process

Treatment plants with an SBR biological process are ideal for treating wastewater with a discontinuous flow rate and discharge regime and/or with significant variations in pollutant load.

Conventional systems for wastewater treatment use several tanks, each with its specific function of reaction and separation of the solid fraction or sludge, operating simultaneously and in a continuous regime. The SBR processes can perform these tasks sequentially in a single reactor that goes through different phases in time.

The complete process of removal of organic matter and nutrients takes place in five different phases: filling or feeding the rector, aeration, anoxic phase, settling and discharge.

The SBR process generally uses two or more reactors working in parallel, in this way, a cycle can be designed combining the phases in the reactors to be able to treat the entire wastewater flow and obtain a constant effluent. This conventional SBR operating cycle is shown in Figure 1.

1. Filling: introduction of wastewater into the reactor.

The wastewater influent is first retained in a homogenization tank before being pumped to one of the SBR reactors during its filling phase.

This filling of the reactor is carried out without mixing or aeration. The water is introduced near the bottom of the tank, which causes the water to pass through the layer of sludge in a preliminary contact with the microorganisms and mix in a homogeneous way.

In the event of detected phosphate increases in the wastewater, the filling phase is subdivided into a short phase without aeration and another aerated phase. In the phase without aeration, anaerobic conditions prevail, which allows the biological elimination of phosphate.

2. Aeration: aerobic phase, batch period with air supply for biological removal of organic matter.

Once the reactor has been filled, the aeration phase begins, which allows the biological elimination of organic matter by oxidation and nitrification by means of which the ammonium NH4+ is oxidized to nitrate NO3-.

The duration of the aeration phase depends on the composition of the wastewater and the levels of pollutant removal required. Aeration is controlled by oxygen concentration and redox potential.

3. Anoxic phase: denitrification phase, batch period with stirring, but no air supply, for biological nitrate removal.

After aeration, the liquor inside the reactor is mixed by stirring, but without the supply of air. This way, anoxic conditions are caused, allowing the reduction of nitrate NO3- to nitrogen gas N2 and therefore its elimination from the water. It is also possible to degrade the organic matter at the same time with the supplementary addition in short periods of additional feeding.

If needed, a final aeration period is established to finish digesting the organic matter.

4. Settling: batch period without agitation nor aeration in which the sludge settles to the bottom of the reactor.

After the previous reaction phases, sedimentation of the sludge and its separation from the clarified phase is carried out. The clarified phase occupies about one third of the upper part of the reactor volume. Settling takes place under completely static conditions: absence of aeration and agitation.

5. Discharge: extraction of part of the clarified effluent down to the minimum reserve volume designed for the reactor.

The last phase of the SBR reactor cycle is the discharge of the clarified water from the top of the reactor. In general, this discharge is carried out by means of a device that is floating on the surface of the reactor, commonly called 'decanter'. Once the evacuation of a certain amount of water, the 'decanter' stops its operation and the excess sludge can be extracted from the bottom of the reactor, which contains a very high concentration of solids.

Figure 1. Operation cycle of a conventional SBR process.

Figure 2. SBR treatment plant installed by SIGMA for the treatment of wastewater from a malt processing industry.

The most restrictive discharge limits are achievable with an SBR process. The design and adjustment of the duration of each phase allows the system to adapt to changes in the flow and composition of the wastewater.

Other advantages of this process are related to the particular conditions for the biomass present, which allows a natural selection of microorganisms adapted to aerobic degradation in symbiosis with those adapted to anoxic degradation, which has a positive impact on performance and efficiency of the process, and an improvement of the settleability and thickening properties of the sludge.

In an SBR process, the recirculation of sludge is not necessary since the concentration of biomass within the reactor is conserved in the operating cycles.

An SBR reactor is designed from two main points of view:

i)  Control of aeration by oxygen concentration and redox potential.

ii) Division of the times of each phase as a function of a hydraulic balance (flow rates to be treated and available space, which will also lead to the design of the reactor volume) and organic matter removal requirements (design of the total cell residence time and aeration for removal of both COD and nitrogen).

Depending on the case and the needs, an SBR process can be designed in a single reactor (batch work, suitable for very low or discontinuous flow rates, requires reserve tanks to store the residual water that has not yet been introduced into the reactor) or with two or more reactors working in parallel and alternately (allows treating the entire flow continuously, the waste water enters one reactor while the reaction and/or sedimentation and/or discharge is carried out in another or others).

Generally, the bottleneck in the design of an SBR reactor are the settling and discharge phases, since the appropriate time of these phases must be found based on the settleability of the sludge (factor V30) to maintain an adequate biomass concentration, a fixed volume that allows to house this biomass and make possible the treatment of the entire wastewater flow.

SIGMA designs and has installed conventional SBR processes efficiently.

2.2 The FBR Process

The FBR system or flotation bio-reactor is a biological system that allows the removal of pollutants in a continuous and compact way. These systems are generally more efficient in terms of removal of organic matter and other biodegradable contaminants, footprint, and sludge generation than conventional activated sludge systems.

An FBR system consists of a biological reactor where the microorganisms consume the organic matter and nitrogen is removed: the wastewater is pumped to an aeration reactor where the biological treatment is carried out which consists of the removal of the organic matter and nutrients contained in wastewater through the action of microorganisms in the presence of oxygen.

Next, the separation of the biomass is carried out by flocculation and secondary clarification in a clarifier specially designed to separate the biomass from the treated water with dissolved air flotation DAF technology. The separation is generally assisted by the use of polyelectrolyte as a flocculating agent.

In this process, biomass flocs are formed and will be separated by flotation with air micro-bubbles in a SIGMA BIODAF equipment specially designed for this application. Clarification using SIGMA DAF equipment allows to obtain a much higher concentration of separated sludge than with conventional settlers, reaching a dry matter content of up to 5%.

In the SIGMA BIODAF flotation equipment, perfectly clarified water is obtained that can be discharged meeting the discharge requirements and a sludge that will be partly recirculated to the biological reactor to maintain a stable biomass concentration and partly extracted as a purge.

The addition of polyelectrolyte to the clarification system allows the generation of easily separable biomass flocs in addition to providing a high concentration of biomass within the reactor, up to 9000 mg/L of MLSS, and therefore a higher yield than in other conventional biological suspended biomass systems.

This particularity is very interesting in industrial wastewater treatment processes with high biodegradability but that give rise to a spongy sludge with low settleability, an effect known as bulking, which makes it difficult to separate it in typical decanters.

The FBR process is also applied when it is necessary to increase the treatment capacity without the need to increase the volume of the biological reactor or to install new biological reactors.

Figure 3 shows a basic diagram of an FBR process.

Figure 3. Diagram of an FBR process.

Figure 4. FBR treatment plant installed by SIGMA for the treatment of wastewater from a frozen vegetable processing industry.

All the information on the FBR technology designed by SIGMA can be consulted in the following links:

• FBR Bioflotación
• Industrial wastewater treatment applying high efficiency biological process FBR

FBR System Data Sheet

2.3. Optimization using the SIGMA SBR – FBR Process

SBR and FBR are two widely known, applied, robust and reliable processes, the union of both for the removal of organic pollutants and nitrogen through nitrification/denitrification is presented as an effective solution that allows to overcome the two of the most notable difficulties of industrial wastewater treatment:

• With the SIGMA SBR - FBR Process, stop and idle times (settling and discharge) of an SBR process are eliminated, allowing a NON-STOP treatment of waste water while maintaining the advantages of an SBR process.
• The incorporation of a SIGMA DAF clarifier as part of the system allows a VERY EFFICIENT SOLID SEPARATION, reaching sludge concentrations of up to 5%, which means a reduction in sludge generation compared to the sedimentation carried out in the SBR reactor itself and reach up to 9000 mg/L of MLSS biomass concentration inside the reactor.

The optimization model can be applied when there are TWO OR MORE TANKS in which the biological treatment is carried out. It is shown as a case example in this article how the design of a SIGMA SBR - FBR Process is executed for a system of TWO reactors and ONE DAF working in parallel and continuous flow.

This process can be applied to:

• New construction of the treatment plant.
• Remodelling and improvement of existing plants continuously.
• Remodelling and improvement of plants with an SBR process that is not working properly or optimization of sludge sedimentation is desired.

Basis of the process are the following:

i) The filling and reaction phase is simultaneous, and the reaction consists of ANOXIC and AEROBIC phases.

ii) The sedimentation phase is not carried out inside the reactor, but is carried out in a SIGMA DAF equipment, therefore, within the reactor only the filling and reaction phases (ANOXIC and AEROBIC combination) and discharge (can be designed in AEROBIC conditions as required by the case) of effluent to DAF.

The discharge flow contains the homogeneous mixture of the liquor inside the reactor, as in an SBR conventional process the reactor is not fully emptied, but a fixed volume calculated during the design of the process is maintained.

In this way, a constant and high concentration of biomass is achieved at all times (with MLSS values achievable between 2000 and 9000 mg/L) and DAF is used as a decanter with the advantages that this implies (high concentration of solids compared to a classical sedimentation).

The working principles of the process are as follows:

• The reactors will work alternately and in a constant set: first one of the reactors is filled and the reaction phase is carried out (with ANOXIC - AEROBIC sequence) and its subsequent discharge.
• During the filling and reaction of a tank, the discharge of the second tank is carried out so that the feed to the process and the effluent to the DAF are CONSTANT.
• This work regime does not affect the capacity to accept wastewater, the ENTIRE DESIGN FLOW can be treated continuously. Sequence can be designed for TWO OR MORE tanks.
• The aeration system of each reactor is programmed according to the aeration times and flow rates as described below. In the case of optimization of existing plants, no special equipment construction or reform of existing equipment and tanks is required, except for the reconfiguration of valves and installation of a DAF unit.
• The inclusion of an ANOXIC PHASE in the reaction sequence allows the removal of nitrogen by nitrification/denitrification.
• The AEROBIC PHASE is designed so that the aerobic cell residence time is sufficient for the elimination of COD.

A representative scheme of the reaction regime described for a system with TWO BIOLOGICAL REACTORS and ONE DAF UNIT is shown in Figure 5.

Figure 5. Representative diagram of the sequence of operation of a SIGMA SBR - FBR Process using two reactors and one DAF unit.

3. Case example of the design of a SIGMA SBR – FBR process

Below is a real case example of the design of the SIGMA SBR - FBR Process for the treatment of industrial wastewater with an average flow rate of 83 m³/h with COD of 1500 mg/L and total nitrogen content of 120 mg/L.

3.1. Design parameters

Flowrates and concentrations as design parameters are the following:

Table 1. Design parameters of a case example of industrial wastewater treated with a SIGMA SBR - FBR Process.

The core objective of industrial wastewater treatment is compliance with the following discharge limits. The process in this case example has been designed so that the COD and total nitrogen concentrations in the effluent meet these discharge limits.

Table 2. Discharge limits for industrial wastewater.

The operational parameters of the reactors and the SIGMA DAF unit are as follows:

Table 3. SIGMA SBR - FBR Process operational parameters applied to the case example.

3.2. Operational sequence

The times in which each reactor will work are distributed as follows:

• Total time of ONE CYCLE in ONE TANK is 5.80 hours. During these hours the water flows continuously as one tank is filled and another tank empties alternately and without stopping.
• The filling and reaction phase (ANOXIC - AEROBIC sequence) in ONE TANK are simultaneous and last 2.90 hours. It is carried out AT THE SAME TIME as the discharge of the other tank.
• The discharge phase of ONE TANK lasts 2.90 hours. It takes place the SAME TIME as the filling and reaction of the other tank.
• 4.14 cycles are carried out per day and per tank.

The time series (in hours) of each cycle for the two tanks is shown in Figure 6.

Figure 6. Time series of the SBR cycles of the case example of the SIGMA SBR - FBR Process.

3.3. Air consumption

Air consumption is determined by the oxygen requirements during the aeration phases. The oxygen consumption of a biological process with COD removal and nitrification/denitrification is calculated as the oxygen required by microorganisms to:

• Removal of carbonaceous matter
• Biomass maintenance
• Nitrification
• Excluded released oxygen during denitrification

Knowing the oxygen requirement, the characteristics and limitations of the aeration system and the total hours of aeration established according to the operational sequence selected in point 2.2. the air consumption for the process is calculated.

4. Alternative sequences and individual cases

SIGMA can design alternative sequences by varying the cycle time and performing various reaction phases ANOX-AER-ANOX-AER-ANOX-AER… to meet the required COD and nitrogen removal yields.

Furthermore, in cases where denitrification is not necessary, the SIGMA SBR-FBR Process can be designed using aeration homogeneously distributed throughout the cycle.

The design of these multiple sequences or continuous aeration will depend on the design parameters of each individual case, availability of space, instrumentation or characteristics of the reactors already existing in the plant that need to be optimized.