While activated sludge processes are still widely used, they can be quite daunting to operate properly. Loss of control can lead to loss of activated sludge, destruction of the microorganism population, and noncompliance with permits and regulations. Traditional activated sludge processes require a large footprint and high initial capital costs.

As a result of these problems with the activated sludge process, new technologies have been developed in recent years. Sequencing Batch Reactor (SBR) and Membrane Bioreactors (MBR) processes are two of these technologies.

The use of SBR and MBR has become widespread in the food and beverage industry, due to the typical composition of wastewater, the general tightening of discharge regulations and the scarcity of water. Wastewater treated with MBR and SBR is much more suitable for reuse or recycling than effluent treated with activated sludge.

Sequential Batch Reactor (SBR)

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

As the name implies, the reactors are designed to run as batch operations, so two or more are needed in parallel to handle the influent.

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

• daughter
• react
• settle
• decant
• idle/waste sludge

Typically, one or more reactors will be in the settling/settling stage, while one or more reactors will be aerating or packing.

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

In the decantation process stage, the clarified water is extracted for reuse, discharge or further treatment.

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

Consistently high quality effluent can be achieved and sludge recycling lowers capital and O&M costs compared to a conventional system.

Microorganism selection minimizes sediment build-up and controls filaments while providing biological phosphorous removal. The reactor design allows for settling at rest prior to decanting, reduces space requirements and provides flexibility in operations. The process is inherently capable of removing biological nutrients, reduces operating costs through automated equipment and controls, and reduces energy savings due to lower oxygen requirements.

Membrane bioreactors (MBR)

In the MBR process, the system combines activated sludge treatment with a liquid-solid membrane separation process. The membrane component uses low pressure microfiltration or ultrafiltration 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, immersing the membranes in the bioreactor tank provides 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 pollutants.

Still, MBR is not without its drawbacks, the biggest of which is membrane fouling, which is not surprising given the operating conditions membranes are exposed to. Fouling gradually reduces the efficiency of the process, causing pressures across the membrane to increase or permeate fluxes to decrease, depending on whether the process is operated under constant pressure or constant flow conditions, respectively. While automated cleaning regimens minimize the impact of membrane fouling, cleaning and replacement still needs to be analyzed and factored into the overall analysis of MBR feasibility for any project.