2. WHY?
•An increase might change water quality or food webs.
•Excess algae: scum, noxious blue-greens, taste/odor/smell
•O2 depletion; loss of fish habitat
•Loss of clarity; aesthetic loss
•Excess macrophyte (“weed”) growth- loss of open water;
•Favors exotic species (EWM); sediment destabilization
•Lower bottom O2: increased sediment nutrient release: loss of fish habitat
•Loss of native macrophytes from algal shading; loss of fish & water fowl habitat
and food; reduced shoreline & bottom
•Stabilization, increased erosion
•Excess organic matter: smothers eggs and bugs
3. SO??
• Stringent measure have to be taken to counter these.
• This calls for advanced treatment methods which are also cost effective
which can help us conform with the strict norms.
4. Existing technologies
Physical:
1. Filtration for particulate phosphorus
2. Membrane technologies
Chemical:
1. Precipitation
2. Other (mainly physical-chemical adsorption)
Biological
1. Assimilation
2. Enhanced biological phosphorus removal (EBPR)
The greatest interest and most recent progress has been made in EBPR,
which has the potential to remove P down to very low levels at relatively
lower costs. Membrane technologies are also receiving increased
attention, although their use for P removal has been more limited to date.
5. Some problems
Physicochemical Precipitation
• 25% sludge production increase
• High level of chemical consumption resulting in high costs.
• Salinity increase of the effluent
• Potential detrimental impact on the biological nitrification, due to
resulting low alkalinity and extreme pH.
•High level of metallic impurities.
6. Why not just bio process??
• Build up of phosphates in the system.
• The recycled activated sludge is treated which results in release of
phosphorus taken up during the process.
• This results in reduced efficiency and increased phosphorus content in the
effluent.
• Limitations imposed by settling characteristics of sludge in clarifier (high
process solid levels or high SRT not possible).
7. What is membrane enhanced….??
3 zones
1. Anaerobic zone - an anaerobic mixed liquor has organisms which release
phosphorous into the anaerobic mixed liquor and store volatile fatty
acids from the anaerobic mixed liquor.
2. Anoxic zone - an anoxic mixed liquor has organisms which metabolize
stored volatile fatty acids, uptake phosphorous and denitrify the anoxic
mixed liquor.
3. Aerobic zone - an aerobic mixed liquor has organisms which metabolize
stored volatile fatty acids, uptake phosphorous and nitrify the aerobic
mixed liquor.
8. Schematic diagram of the process
Influent water anaerobic anoxic
membrane
aerobic
Effluent water
reject
9. Processes
Anaerobic site - fermentive bacteria convert BOD into volatile fatty acids. Bio-P
organisms use the volatile fatty acids as a carbon source. In doing so, they
release phosphorus into the liquor, and store volatile fatty acids.
In the anoxic and aerobic zones, the Bio-P organisms metabolize the stored
volatile fatty acids and uptake phosphates from the liquor. The recycle
between the anoxic and anaerobic zones allows the process to operate
substantially continuously.
The stream exiting the aerobic zone passes through the membrane filter. In the
membrane filter, phosphorus-rich activated sludge, finely suspended colloidal
phosphorus, bacteria, and other cellular debris are rejected by the membrane.
A phosphorous lean effluent is produced at the permeate side of the
membrane filter. The effluent is also reduced in nitrogen as a result of
the anoxic and aerobic zones and the recycle between them.
10. Processes
Phosphorus uptake
Influent water Phosphorus released
Denitrification of
mixed liquor
membrane
Phosphorus uptake
Denitrification of
mixed liquor Effluent water
reject
11. How better??
Membrane Bio Reactors-Advantages of membrane filtration
• Complete solid-liquid separation
• Prevents failure of biological systems due to biomass loss or bulking
• Maintains high Mixed Liquor Suspended Solids (MLSS) in the reactor.
• In addition to removing the P in the TSS, membranes also can remove dissolved
P. Membrane bioreactors (MBRs, which incorporate membrane technology in a
suspended growth secondary treatment process), tertiary membrane filtration
(after secondary treatment), and reverse osmosis (RO) systems have all been
used in full-scale plants with good results.
12. Advantages- EBPR
•Enhanced Biological Phosphorous removal (EBPR) process is cost-effective
than co-precipitation.
•This comparison is favored by high phosphorus content in the raw water
(disadvantage for co-precipitation as much coagulant is required to achieve
the discharge criteria), or by high COD/P ratio (advantage for EBPR as more
phosphorus can be biosorbed).
13. Until recently not much effort was made to adapt EBPR processes to the MBR
technology.
• Given the high solid retention times (SRTs) of the early MBR systems –up to
50 days–, it was considered that EBPR could not be efficient and cost-effective
compared to co-precipitation.
• Recent observations indicate that efficient and stable phosphorus removal
could be set up in MBR systems with high SRT due to
the complete retention of solids and biomass through the
membrane,
the absence of sludge bulking or flush-out,
the final aerobic reconditioning of the biomass in the membrane
vessel, instead of anoxic or anaerobic conditions in the clarifier.
14. Two major advantages of MBR on Conventional Wastewater Treatment
Plants (CWWTP) can be noted for EBPR efficiency:
Lower effluent phosphorus concentrations are achieved through
(i) Complete removal of all particles (containing usually up to 0.1mgP/mgTS),
and
(ii) final aeration in the membrane tank which allows to prevent from
phosphate release during the separation phase (settled sludge in clarifiers
is more or less anoxic).
This achieves also a final “reconditioning” of the sludge:
A complete P-uptake and COD degradation before being returned in
the system.
15. Advantages continued..
The membrane filter removes colloidal phosphorus and bacteria which
would normally pass through a clarifier. Although the absolute amount of
colloidal solids is relatively small, the percentage of phosphorus in the
colloids is surprisingly high and its removal results in unexpected low levels
of phosphorus in the effluent. With membrane filters to remove biomass
from the effluent stream, a fine biomass can be maintained in the
anaerobic reactor. This may result in enhanced reaction rates and higher
than anticipated release of phosphorus in the anaerobic reactor, with
resulting higher uptake of phosphorus in the anoxic and aerobic zones.
Further, since the process is not limited by the settling characteristics of the
sludge, the process is able to operate at very high process solid levels,
preferably with an MLSS between 3 and 30 mg/L and short net hydraulic
retention times, preferably between 2 and 12 hours. The short HRT allows
increased throughput of waste water for a given reactor size.
16. …..
In addition, since the design avoids chemical precipitation of phosphates
upstream of the membrane filters, there is reduced membrane fouling which
further enhances the performance of the process. Moreover, contaminants in
the sludge resulting from precipitating chemicals are reduced permitting the
system to operate at a high sludge age. At high sludge retention times,
preferably between 10 and 30 days, an unexpected significant crystalline
phosphorus accumulation occurs in the biomass, effectively removing
phosphorus from the system. As well, there is lower net sludge generation.
17. Results
An experimental reactor was set up. The membrane filter consisted of four
ZEEWEEDTM ZW-10TM modules produced by Zenon Environmental Inc.
having a total of 40 square feet of membrane surface area. A control reactor
was set up using a clarifier instead of the membrane filter , recycling the
clarifier bottoms to the anoxic zone and not using a retentate recycle stream
or nitrified liquor recycle . Both reactors had a volume of 1265 L, the volume
of the clarifier not being counted as reactor volume. Sludge retention
time (SRT) was kept constant at 25 days.
Three experimental runs were conducted with the experimental reactor at
hydraulic retention times (HRTs) of 9 hours, 6 hours and 4.5 hours produced
by varying the feed flow rate. The control reactor was run successfully at a
hydraulic retention time of 9 hours using the same operating parameters as
for the run of the experimental reactor with a 9 hour HRT. Running the
control reactor at a hydraulic retention time of 6 hours was attempted, but
adequate operation could not be achieved (because the clarifier failed), most
conventional processes running at an HRT of about 12. The sizes of the zones
and the HRTs of each zone are summarized in Table 1 below
18.
19. During the first run, the experimental and control reactors were
operated at a 9 hour HRT for 16 weeks. The MLSS concentration
varied between 3-5 g/L during this period. A summary of the
average P and N concentrations for both reactors is shown in
Table. Effluent P was generally below 0.3 mg/L for the
experimental process while effluent P for the control process
varied from 0.2-0.7 mg/L.
20.
21. During the second run, the experimental reactor was operated at a 6
hour HRT for about 14 weeks. The MLSS concentration increased from
about 4 mg/L at the start to about 8 mg/L at the end of the run. By the
end of the run, the experimental process had stabilized in terns of VFA
uptake and phosphorous release in the anaerobic section. There was a
slow and steady improvement in performance as the experimental run
progressed, the monthly average effluent P dropping from 0.178 mg/L
to 0.144 mg/L to 0.085 mg/L over the approximately three months of
the test.
During the third run, the experimental reactor only was run at an HRT
of 4.5 hours. MLSS concentration increased to 15 g/L. Effluent P
concentrations were generally below 0.5 mg/L over a three month
period, still better than the P removal of the control reactor operated
at a 9 hour HRT.
22.
23. References
1. Aquatic Ecosystem P – Prof. S Mohan
2. Biological process for removing phosphorus involving a membrane filter,
United States Patent 6485645 – Husain, Hidayat; Koch, Frederic;
Phagoo, Deonarine
