Membrane Bioreactors & the Sequencing Batch Reactor, Improve Secondary WWT in the Food Industry

Wastewater treatment (WWT) uses many different processes to achieve the ultimate goal of discharging wastewater that meets all applicable regulations. In the Food and Beverage industry the composition of wastewater streams can be very complex, and highly variable, making the treatment of wastewater quite challenging, especially in the Secondary WWT phase. The demands made on the food and beverage industry are numerous and range from health and financial issues to environmental concerns.

membrane potentiometer

Two additional factors are adding to this challenge. First, environmental discharge regulations continue to tighten, making it difficult for some facilities to consistently operate without NPDES violations. Second, climate change, drought conditions and the trend towards water conservation are forcing some WWT facilities to recycle a portion of their water for in-plant use. The end-use of this recycled water may dictate an even higher quality standard than what is required for discharge.

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Wastewater treatment can be divided into five processes: pre-treatment, primary, secondary, and tertiary treatment then finally disinfection.

Secondary WWT is arguably the most important of the processes used in the treatment of waste in the Food & Beverage industry, due to the high and varying levels of soluble and suspended organic matter in the wastewater. Unfortunately, it is also arguably the most complex of the WWT processes, and can therefore create many challenges from an operations perspective.

The Activated Sludge Process

The first major development in Secondary WWT was the introduction of the activated sludge process in England in 1913. The activated sludge process combines sewage, a concentrated mass of microbes, and high levels of dissolved oxygen to promote the consumption of organic content.

The activated sludge process is still very widely used, and has evolved into many different variations, depending upon specific waste treatment requirements.

The activated sludge process can be quite daunting to operate properly. Loss of control by WWT plant operators, can result in loss of the activated sludge, decimation of the microorganism population, and ultimately in non-compliance.

It takes experience and expertise to operate an activated sludge facility in the event of upset conditions. Another issue with most traditional activated sludge processes is the large footprint and the associated high initial capital costs.

As a result of these issues with the activated sludge process, newer technologies have been developed over the past few years. The Membrane Bioreactor (MBR) and the Sequencing Batch Reactor (SBR) processes are two such technologies.

The use of MBR and SBR has become widespread in the Food and Beverage industry, due to the typical wastewater composition, a general tightening of discharge regulations, and water shortages. MBR and SBR treated wastewater is much better suited for reuse or recycle than activated sludge treated effluent.

The MBR process combines activated sludge treatment with a membrane for liquid-solid separation. While MBR can achieve almost complete separation of suspended solids and dramatic reduction in contaminants, it is prone to membrane fouling. Fortunately, the recent advent of PTFE membranes and improved system designs have minimized the need for membrane maintenance to prevent fouling.

A SBR process typically consists of at least two identically equipped reactors with a common inlet, valved to direct flow to one reactor or the other. As the name implies, the reactors work as batch operations and by nature are easier to operate than continuous flow systems since every batch can be treated and controlled separately. In addition, SBR systems often have a smaller footprint and capital cost and are easier to operate than other types of systems. On the downside, SBR may be limited to smaller wastewater flows.

Membrane Bioreactors & the Sequencing Batch Reactor, Improve Secondary WWT in the Food Industry

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The Sequencing Batch Reactor & Membrane Bioreactors Help Evolve Secondary WWT in the Food Industry

While activated sludge processes are still very widely used, they can be quite daunting to operate properly. Loss of control can result in loss of the activated sludge, decimation of the microorganism population, and non-compliance with permits and regulations. Traditional activated sludge processes require a large footprint and high initial capital costs.

membrane switch

As a result of these issues with the activated sludge process, newer technologies have been developed over the past few years. The Sequencing Batch Reactor (SBR) and Membrane Bioreactors (MBR) processes are two such technologies.

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The use of SBR and MBR have become widespread in the Food and Beverage industry, due to the typical wastewater composition, a general tightening of discharge regulations, and water shortages. MBR and SBR treated wastewater is much better suited for reuse or recycle than activated sludge treated effluent.

Sequencing Batch Reactor (SBR)

A SBR typically consists of at least two identically equipped reactors with a common inlet, valved to direct flow to one reactor or the other.

As the name implies the reactors are designed to work as batch operations, thus the need for two or more in parallel in order to handle the influent.

While many SBR configurations are possible depending upon the specific application the basic process follows these five stages:

fill react settle decant idle/waste sludge

Typically one or more reactors will be in the settle/decant stage while one or more reactors will be either aerating and or filling.

The fill stage will either be anoxic or aerated. The anoxic environment removes nitrate, allows growth of bacteria, controls aerobic filamentous organisms and the design time is a function of BOD and TKN loads, BOD:P ratio, temperature and effluent requirements. Aerated fill treats and removes BOD, allows for nitrification/denitrification and the design time is also a function of the same parameters during anoxic fill. In the reaction stage the activated sludge is mixed and aerated to remove BOD, achieve nitrification, enhance phosphorous uptake, and to denitrify with anoxic/aerobic react for low effluent nitrate requirements. The react phase is followed by the settling stage during which time suspended solids settle to the bottom of the reactor for removal.

In the decanting process stage the clarified water is drawn off for re-use, discharge, or further treatment.

SBR treatment systems by nature are easier to operate than continuous flow systems since every batch can be treated and controlled separately.

High quality effluent can consistently be achieved and no sludge recycling decreases capital and operation and maintenance costs compared to a conventional system.

Microorganism selection minimizes sludge bulking and controls filaments while providing biological phosphorous removal. The reactor design allows for quiescent settling prior to decanting, reduces space requirements, and provides for operations flexibility. The process inherently is capable of biological nutrient removal, reduces operational costs through automated controls and equipment, and reduces power savings due to lower oxygen requirements.

Membrane Bioreactors (MBR)

In the MBR process, the system combines activated sludge treatment with a membrane liquid-solid separation process. The membrane component uses low pressure microfiltration or ultra filtration membranes and eliminates the need for clarification and tertiary filtration. The membranes can be physically installed in the bioreactor tank, or in a separate tank. For most processes submerging the membranes in the bioreactor tank proves to provide the most efficient and cost effective solution.

The membranes used in the MBR process have very small pore sizes (typically 0.04 - 0.4 microns). Almost complete separation of suspended solids from the mixed liquor can be achieved. This fact, along with its basic design results in dramatic reductions in contaminants.

Still, MBR is not without its drawbacks, the largest of which is membrane fouling-no surprise given the operating conditions to which the membranes are exposed. Fouling gradually reduces process efficiency causing cross-membrane pressures to increase or permeate flows to decrease depending whether the process is operated under constant pressure or constant flux conditions respectively. While automated cleaning regimens minimize the impact of membrane fouling, the cleaning and replacement must still be analyzed and factored into the overall analysis of MBR viability for any project. 

The Sequencing Batch Reactor & Membrane Bioreactors Help Evolve Secondary WWT in the Food Industry

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