Copa Hybrid BNR - Suitable for N & P removal

With 600 Installations world wide, the Copa Hybrid BNR Process offers proven economical and reliable treatment of wastewater, particularly where low Nitrogen and Phosphorus limits are required

The process combines the performance benefits of conventional continuous flow processes incorporating anoxic and aerobic reactors, with the simplicity and cost benefits of SBR’s (sequencing batch reactors). This offers operational advantages and significant cost savings over other BNR technologies from the construction phase through to the long term maintenance and operational life of the plant.

Operation Principle

Biological Nutrient Removal (BNR) is achievable via several techniques, but generally involves introducing a bacteria in the presence of oxygen, to biologically degrade the organic component of the incoming effluent. The BNR process involves complex chemistry balancing with reliable process mining and settlement, therefore all BNR systems are different. The copa HBNR system has been optimised over 20 years to produce a very economical and reliable process system.

For the Copa Hybrid BNR Process, the following provides a brief explanation of the setup of the system.

Using a twin circular tank construction offers savings in construction time and cost. Design criteria are generally flexible, sometimes allowing locally available forms to be utilised or in some cases existing tanks may also be used.

The first tank is the Aerobic Anoxic Tank (AAT). It has an anaerobic selector (when configured for bio-P removal), followed by a sequencing aerobic/anoxic zone.

Flows up to ADWF continuously flow into the anaerobic selector along with recycled sludge. The flow then passes into the sequencing aerobic/anoxic zone. Cycles consisting of periods of aeration followed by non-aerated mixing periods occur in this tank. The cycle times are varied depending on the time of day and according to DO levels in the tank.

The second tank is an intermittently operated Clarifier (IOC). The IOC operates as a sequencing aerobic and anoxic reactor as well as a clarifier. Flow enters the IOC from the AAT through an energy dissipating baffled section. As per the AAT the aerobic and anoxic phases of operation are cycled. Sludge is recycled from the IOC to the AAT tank during the Aerobic/Anoxic phase to allow the ‘bugs’ to do it all again.

Aeration is supplied by either patented Sinkair aerators for smaller plants or diffusers for larger plants (or where minimum energy consumption is required). Mechanical surface aerators can also be used.

Fluid in the IOC tank builds to a pre-set level and then the aeration, mixing and sludge return are stopped and the sludge is allowed to settle for a pre-set time. Following this settling time the clear supernatant from the top of the tank is decanted.

Advantages

■ Low footprint
■ Robust process characteristics
■ Lower effluent solids (and BOD, N, P etc.)
■ Low SVIs (sludge volume index)
■ Simple robust decanting system with no moving parts
■ Fully Automated — minimal operator attention needed

A simple but clever patented decanting system called a Gas-Locked Siphon (GLS) is utilised. It has no moving parts and is designed to allow decanting of the fluid at much lower effective weir loading rates than conventional mechanical floating weirs. This minimises the risks of mechanical problems and solids loss in the effluent.

The whole process is controlled by a PLC and SCADA system designed to maximise process performance and minimise operator requirements. Most plants operate unattended.

Features and Benefits

The process design of the Hybrid BNR system has many features that give this process significant advantages over alternate processes.

De-nitrification

All Copa HBNR plants are designed to achieve high levels of de-nitrification, even if regulations don’t require this. This is because de-nitrification:

• Allows for the recovery of alkalinity — avoids pH reduction in low alkalinity waters, which eliminates alkalinity addition and ensures an optimal pH for nitrification
• Recovers oxygen — reduces aeration costs
• PRovides additional process stability — helps prevent nitrogen de-gassing and resultant sludge floatation during the clarification period
• Is essential for efficient biological P removal.

Shallow tank design

The Copa HBNR utilises a shallow tank design rather than deep tanks. This gives improved solids capture due to:

• Lower rise rates — improved settling of solids during the settling and decanting phases
• Lower supernatant velocities over the surface — eliminates the scouring of solids during decanting
• Lower rise rates under peak flow conditions — allows higher peak wet weather flows without degrading the performance.

Gas-Locked Siphon Decanter

The utilisation of the Gas-Locked Siphon Decanter offers several advantages over other decanter designs including:

• Much lower effective weir loading rates 1-2 litres/second/meter of weir compared to 15-20 litres/second/meter for conventional lowering weir systems
• No possibility of mechanical failures due to no moving parts or mechanically complex design — lower operational costs
• Much cheaper construction and installation costs.

High Effluent Quality

The key feature of the design is the attention paid to minimising solids in the effluent and maximising process stability. Because de-nitrification is an important step, even where regulatory consents on effluent quality are not high, the process will still achieve very high removals, particularly of Nitrogen.

Typical effluent quality figures (95th percentile) are presented in the following table. These show the effluent qualities for a standard plant where consent limits are not too stringent, and for a full BNR plant to meet the most stringent of regulatory requirements.

The Gas-Locked Siphon (GLS) Decanter

During normal operation (aerobic/anoxic cycles), the air valve is closed. This traps air (a ‘Gas- Lock’) in the decanter.

At the end of the aerobic/anoxic cycle aeration is turned off and the sludge is allowed to settle.

Sludge settles for a pre- determined time until the top of the sludge blanket is minimum 300mm below the bottom of the GLS nozzles.

The air valve on the GLS opens allowing fluid to displace the trapped air and start discharging into the effluent.

The air valve closes and a siphon forms removing fluid from the IOC rapidly until the bottom water level is reached.

As the bottom water level is reached, the air valve opens to break the siphon and the flow to the effluent stops.

When flow to the effluent stops the air valve closes again creating the gas lock. Aerobic/anoxic cycles are then resumed.