Nutrient Removal: Many Options Available for Wastewater Treatment

Nutrient Removal: Many Options Available for Wastewater Treatment
The MBBR process uses thousands of polyethylene biofilm carriers operating in mixed motion within an aerated wastewater treatment basin.

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Today’s emphasis on sustainability, together with more stringent discharge limits, has made producing quality effluent a top priority for treatment plant operators. To achieve these goals, many plants are focused on removing nutrients by lowering nitrogen and phosphorous levels through a range of advanced wastewater technologies. 

What follows is a look at some processes being used in nutrient removal efforts. 

1. Moving Bed Biofilm Reactor (MBBR) Technology 

The MBBR process uses thousands of polyethylene biofilm carriers operating in mixed motion within an aerated wastewater treatment basin. Each biocarrier provides a protected surface area to support the growth of bacteria; the high-density bacteria population, in turn, helps achieve high-rate biodegradation within the system, thus supporting BOD reduction, nitrification and total nitrogen removal. 

MBBR technology offers flexible, cost-effective treatment with minimal maintenance. As such, MBBR improves system reliability, simplifies operation and requires substantially less space than traditional wastewater treatment systems. 

2. Integrated Fixed-Film Activated Sludge (IFAS) 

IFAS, a variation of the MBBR process, gets its name from the integration of biofilm carrier technology within conventional activated sludge. IFAS technology is the first process specifically designed for ideal operation in municipal wastewater treatment/activated sludge processes. Typically, IFAS is a retrofit solution for conventional activated sludge systems that are at or beyond capacity, providing minimal plant downtime, zero facility construction and optimization of existing equipment. 

The hybrid process enables activated sludge systems to achieve dramatic gains in volume productivity without increasing mixed liquor suspended solids (MLSS). IFAS processes are designed for complete compatibility with fine-bubble aeration systems, providing demonstrated long-term operation cost savings. 

3. Membrane Bioreactor (MBR) Systems

Membrane bioreactors are a unique wastewater treatment process designed for municipal and industrial uses. They are a combination of a membrane process, like microfiltration or ultrafiltration, with a suspended-growth bioreactor. MBRs can be used in applications ranging from water reuse and new housing developments to resorts, retrofits and turnkey projects. 

There are two types of MBRs: internal/submerged, where the membranes are immersed in the biological reactor, and external/sidestream, where membranes are a separate unit that requires an intermediate pumping step. Membrane bioreactors can be used to reduce the footprint of an activated sludge sewage treatment system. MBR treatment plants can produce a consistent, high-quality effluent that is excellent for public access reuse, urban irrigation and other reclaimed water uses. It is also suitable for discharge to coastal, surface or brackish waterways.

4. Sequencing Batch Reactors (SBR)

An SBR treats a portion of the day’s total wastewater flow in a batch-type process. There are usually at least two SBR tanks on site and are identical in structure and equipment. The interior of an SBR tank includes aeration diffusers, submerged mixing devices, influent and effluent valves, effluent decant withdrawal piping, waste pumps and level sensors or floats.

An SBR tank performs all the work of previously discussed processes, but does this all in one tank. The SBR tank receiving raw wastewater goes through a series of selective processes like anaerobic, anoxic or aerobic time cycles. These cycles are controlled by a computer system that the operator programs to achieve desired effluent treatment goals. While the influent wastewater flows into one SBR tank, the other is not receiving influent and is performing final aeration (react), settle, decant or wasting.

After the SBR tank has decanted a portion of its total liquid volume, it is ready to treat another batch of influent wastewater. An SBR treatment facility does not have secondary clarifiers or RAS pump systems, and can meet very strict effluent limitations on a small land footprint.

5. Modified Ludzack-Ettinger Process (MLE) 

The MLE process consists of modifying a conventional activated sludge process by creating or adding an anoxic zone upstream of the aerobic zone. An internal recycle pump system returns nitrate-rich mixed liquor created in the nitrifying aerobic zone to be mixed with the influent in the anoxic zone. The amount of nitrates potentially removed in the anoxic zone depends on the recycle flow and availability of influent BOD. 

6. Bardenpho Processes 

Bardenpho processes are used in municipal wastewater treatment and are either four stage or five stage. The four-stage Bardenpho process is all about nitrogen removal. In the four-stage process, the stages (or selectors) are: first anoxic basin, aerobic basin, second anoxic then a small reaeration (aerobic) phase. 

The first anoxic is the main denitrification zone where recycled MLSS from the aeration tank and RAS from the clarifiers is returned and mixed with the incoming influent waste stream. Denitrification takes place first, then nitrification occurs in the aerobic zone. This is very similar to the MLE process at this point. 

The second anoxic zone is used mainly as a polishing denitrification stage where any nitrate left from the first two zones is allowed to denitrify. Carbon sources such as methanol, acetic acid or glycerin can be added to enhance denitrification. The last, or fourth zone, is the reaeration tank where aeration is done to help release any nitrogen gas bound in the MLSS, which aids in settling in the clarifiers, and adds DO to the MLSS. 

The five-stage Bardenpho process uses the exact same layout, but adds an anaerobic zone (fermentation tank) in front of the four-stage system. In the anaerobic zone, only RAS from the clarifiers and influent wastewater are mixed, not aerated. Bacteria called phosphate-accumulating organisms (PAOs) compete for the available carbon, but must release their molecules of polyphosphate from their own cells to allow for the food to be taken into the cell. The PAOs do this in the anaerobic zone and the amount of orthophosphate present in the MLSS can double or triple what it was in the influent wastewater. 

Once the PAOs enter a truly aerobic environment, they need to re-establish the polyphosphate molecules in their cells so they can reproduce. They collect the original phosphorus amounts they once had and excess phosphorus. The extra phosphorus is what was originally in the influent wastewater, so in the end they have biologically removed the phosphorus from the wastewater. The PAO bacteria are now laden with phosphorus and settle in the clarifier with the rest of the bugs. In the five-stage Bardenpho process, the last reaeration tank is critical in maintaining some DO in the MLSS. The five-stage process is very efficient at biologically removing nitrogen and phosphorous and the waste sludge will contain high amounts of phosphorous. 

Another option for nutrient removal are biologically active filters (BAF), also known as denitrification filters, which treat effluent wastewater and provide suspended solids removal under either aerobic or anoxic conditions. In a BAF, the media acts simultaneously to support the growth of biomass and as a filtration medium to retain filtered solids. Denitrification is allowed to occur in the media, possibly with the addition of a carbon source like methanol. Accumulated solids are removed from the BAF through backwashing, and nitrogen gas is released to the atmosphere. 

It’s clear that regardless of a treatment plant’s size and capabilities, there are a variety of innovative technologies operators can employ to meet today’s stringent discharge regulations and society’s increasing demands for greater sustainability. 

For more information about emerging and innovative technologies in wastewater treatment, visit http://water.epa.gov/scitech/wastetech/upload/Emerging-Technologies-Report-2.pdf. 



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