ch2mbr

Combined Nitrogen and Phosphorus Removal

1. A2-O Process

1-1 Process Description

A²/O process (Anaerobic/Anoxic/Oxic) incorporates an anoxic zone between the anaerobic selector and oxic zone for nitrate reduction.
An internal recycle pump returns nitrate rich mixed liquor from the oxic zone to the anoxic zone.

For more stringent TN limits, secondary anoxic zones can be added after the oxic stage
for additional de-nitrification producing effluent with TN of less than 3 mg/L.

1-2 Design Parameter

·  SRT, 5 ~ 25 days
·  MLSS, 3,000 ~ 4,000 mg/l
·  Hydraulic Retention Time (HRT)
    - Anaerobic zone, 0.5 ~ 1.5 h
    - Anoxic zone, 0.5~1 h
    - Aerobic zone, 4 ~ 8 h
·  RAS (% of influent), 25 ~100
·  Internal recycle (% of influent), 100~400

2. Modified 5-Stage Bardenpho Process

2-1 Process Description

The Bardenpho process is a popular choice for biological nutrient removal (BNR) of phosphorus and nitrogen,
but it requires tankage that not all wastewater treatment facilities can accommodate.

The modified, five-stage Bardenpho process is an expansion of the
classic four-stage approach, with an anaerobic tank added on the front.

The five zones ? anaerobic, anoxic, aerobic, anoxic, and aerobic ? each require dedicated tank space,
which is the critical component and cost of the Bardenpho process. Municipalities
that experience high flows due to storm events and inflow and infiltration (I&I) require even further capacity.

In addition to tanks and aeration blowers, anoxic mixers are necessary for the zones that aren’t aerated,
and a recycle pump is needed to return the mixed liquor back to the front of the cycle for de-nitrification, as shown in the diagram below.

2-2 Design Parameter

·  SRT, 10 ~20 days
·  MLSS, 3,000 ~ 4,000 mg/l
·  Hydraulic Retention Time (HRT)
    - Anaerobic zone, 0.5 ~ 1.5 h
    - Anoxic zone, 1~3 h (1st stage), 2~4 h (2nd stage)
    - Aerobic zone, 4 ~ 12 h (1st stage), 0.5~1 h (2nd stage)
·  RAS (% of influent), 50 ~100
·  Internal recycle (% of influent), 200~400

3. Modified UCT Process

3-1 Process Description

Modified UCT process is virtually same as Virginia Initiative Plant (VIP) process,
but typically UCT process is designed for the SRT of 13-25 days while UCT process is designed
for 5-10days. But, when it comes to MBR, the difference between the two processes does not exist.
The major benefit of UCT process is that it does not directly return the nitrate bearing mixed liquor to anaerobic tank,
thereby ORP remains low enough to maximize phosphate release from phosphorus accumulation organisms (PAO).

However, the cascade type mixed liquor recycle to anaerobic tank causes diluted MLSS in anaerobic tank.
To compensate the MLSS dilution in anaerobic tank, tank size needs to be increased.

3-2 Design Parameter

·  SRT, 10 ~ 25 days
·  MLSS, 3,000 ~ 4,000 mg/l
·  Hydraulic Retention Time (HRT)
    -Anaerobic zone, 1 ~ 2 h
    -Anoxic zone, 2~4 h
    -Aerobic zone, 4 ~ 12 h
·  RAS (% of influent), 80 ~100
·  Internal recycle (% of influent), 200~400 (anoxic)

VIP Process

4-1 Process Description

The VIP process stands for the Virginia Initiative Plant (Daigger et al., 1988). The VIP process is similar
to the A2O and UCT processes except for the methods used for recycle systems. In the VIP process,
all the zones are staged consisting of at least two completely mixed cells in series.
The return activated sludge is discharged to the inlet of the anoxic zone along with nitrified recycle
from the aerobic zone. The mixed liquor from the anoxic zone is returned to the head end of the anaerobic zone.
The VIP process is also designed as a hign-rate system, operating with much shorter SRTs, which maximize biological
phosphorus removal efficiency. The combined SRT of the anaerobic and anoxic zones is generally 1.5 to 3d,
while the anaerobic and anoxic HRT values are typically 60 to 90 min. each. The aeration zone is designed for nitrification.

4-2 Design Parameter

·  SRT, 5 ~10 days
·  MLSS, 2,000 ~ 4,000 mg/l
·  Hydraulic Retention Time (HRT)
    - Anaerobic zone, 1 ~2 h
    - Anoxic zone, 1~2 h
    - Aerobic zone, 4~ 6 h
·  RAS (% of influent), 80 ~1 00
·  Internal recycle (% of influent), 100~200 (anoxic), 100~300 (aerobic)

5. Sequencing Batch Reactor

5-1 Process Description

The sequencing batch reactor (SBR) process utilizes a fill and draw reactor with complete mixing during
the batchreaction step (after filling) aeration and clarification occur in the same tank.
All SBR systems have five steps in common, which are carried out in sequence as follows: (1) fill, (2) react-aeration, (3)
settle-sedimentation/clarification, (4) draw-decant, (5) idle. For continuous flow applications,
at least two SBR tanks must be provided so that one tank receives flow while the other completes its treatment cycle.

Operational step	Description
Fill	During the fill operation, volume and substrate (raw wastewater or primary effluent) are added to the reactor. The fill process typically allows the liquid level in the reactor to rise from 75% of capacity (at the end of the idle period) to 100%. When two tanks are used, the fill process may last about 50% of the full recycle time. During fill, the reactor may be mixed only or mixed and aerated to promote biological reactions with the influent wastewater.
React	During the react period, the biomass consumes the substrate under controlled environmental conditions.
Settle	Solids are allowed to separate from the liquid under quiescent conditions, resulting in a clarified supernatant that can be discharged as effluent.
Decant	Clarified effluent is removed during the decant period. Many types of decanting mechanisms can be used, with the most popular being floating or adjustable weirs.
Idle	An idle period is used in a multi-tank system to provide time for one reactor to complete its fill phase before switching to another unit. Because idle is not a necessary phase, it is sometimes omitted.

5-2 Design Parameter

·  SRT, 20 ~40 days ·  MLSS, 3,000 ~ 4,000 mg/l ·  Hydraulic Retention Time (HRT)     - Anaerobic zone, 1.5 ~3 h     - Anoxic zone, 1~3 h     - Aerobic zone, 2~ 4 h

5-2 Design Parameter

If sufficient nitrate is removed during the SBR operation, an anaerobic reaction period
can be developed during and after the SBR fill period. An anoxic operating period is used after
a sufficient aerobic time elapses for nitrification and nitrate production. Alternatively cyclic
aerobic and anoxic periods can be used during the react period. The nitrate concentration is thus minimized
before settling, and little nitrate is available to complete for rbCOD in the fill and initial react period.
Thus, anaerobic conditions occur in the fill and initial react period, so that rbCOD uptake and storage by
phosphorus-accumulating bacteria can occur instead of rbCOD consumption by nitrate-reducing bacteria.

6. CH2MBR Process

6-1 Process Description

The CH2MBR process is developed to minimize the effect of nitrate in weaker wastewaters
in entering the second anaerobic tank to avoid additional recycle system.

The amount of nitrate in the second anaerobic tank is critical to the biological phosphorus removal efficiency.

6-2 Design Parameter

·  SRT, 10 ~ 25 days
·  MLSS, 3,000 ~ 4,000 mg/l
·  Hydraulic Retention Time (HRT)
    - 1st Anaerobic zone, 0.5~1 h
    - 2nd Anaerobic zone, 1~2 h
     - Anoxic zone, 2~4 h
    - Aerobic zone, 4 ~ 12 h
·  RAS (% of influent), 80 ~100
·  Internal recycle (% of influent), 200~400 (anoxic)