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8th IWA SPECIALIST GROUP CONFERENCE ON WASTESTABILIZATION PONDS
Conceptual analysis of the
UASB/ Polishing Pond system regarding theremoval of specific constituents and control
of gaseous emissions
Belo Horizonte, April 2009
C.A.L. Chernicharo*, S.F. Aquino**, M.V. Sperling*, R. M.
Stuetz***, L.V. Santos*, M.O.A. Mabub*, M.A. Moreira**,O.M.S.R. Vasconcelos* and R.M. Glria*
*Federal University of Minas Gerais Belo Horizonte Brazil
**Federal University of Ouro Preto Ouro Preto - Brazil
***University of New South Wales Sydney - Australia
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Introduction
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Quick background on UASB reactors
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Main characteristics Advantages Applicability
Large urban
areas
Small
decentralized
systems
Small
communities
No oxygen consumption
Low sludge production
Sludge is more concentrated
and easy to dewater
Biogas production
Simple to operate
Low O&M costs
Low construction costs
Possibili ty of energy
recovery
Main Features of UASB reactors
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Constituents
of interest
Potential
problems
Possible
improvements
BiogasH2S
Odouremission
Corrosion
ToxicityBiogasflare
Biogastreatment
EnergyrecoveryCH4
GHGemission
Explosionrisk
WastegasH2S
Odouremission
Corrosion
ToxicityWastegas collection
Wastegastreatment
CH4 GHGemission
Effluent
Carbon DOdepletion Post-treatment
Nutrient
Eutrophication
Toxicity
Agricultural reuse
Post-treatment
Pathogen HealthrisksDisinfection
Sub-superficial irrigation
Aquifer recharge
SurfactantToxicity
Foam
Aerobicpost-treatment
Reduceturbulences
MicropollutantsToxicity
Healthrisks
Increasesludgeage
Post-treatment
Effluent recycle
DissolvedH2SOdouremission
Corrosion
Toxicity
Aerobicpost-treatment
Gasstripping/treatment
Micro-aerationusingbiogas
DissolvedCH4 GHGemissionGasstripping/treatment
Micro-aerationusingbiogas
Biologicaloxidation
ScumOrganics
DebrisBlockage
Management
Improvepre-treatment
Eliminatebaffle(1)
Maintainbaffle(2)
ScumOrganics
DebrisBlockage
Management
Improvepre-treatment
Removal/disposal
Increasedegradation(3)
Tri-phase
separator-
Gasleakage
Structurefailure
Corrosion
Useofpropermaterials
Useof proper linings
Improvedesign
Cover - CorrosionUseofpropermaterials
Useof proper linings
Feedingsystem -Blockage
Badmixing
Improvepre-treatment
Increaseno ofdistributors
Sludge
Nutrient
Pathogen
Sand
Debris
Sludgebuildup
Healthrisks
Dewatering
Improvedesign
Improveoperation
Improvepre-treatment
Hygienization
Pre-treatment
Pumpingstation
DissolvedH2S
Odouremission
Passageofdebris
Passageofoil/grease
Flowratevariation
Wastegas collection
Wastegastreatment
Useofsieves
O&Gremoval
Control cross-connectionsMinimumof twopumps
Useofvariablespeedpump
Overflow structure
Useof holding tank
Summary of potential problems and possible improvements inthe design, construction and operation of UASB treatment plants
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Topics of interest for possible
improvements in UASB reactorsBiogas and
Waste gas
- Odour emission
- GHG emission
- Energy recovery
- Corrosion
- Toxicity
Liquid
effluent
- Residual carbon
- Nutrient
- Pathogen
- Micropollutants
- Surfactants
- Odour emission- GHG emission
Reactor
Scum accumulation -
Corrosion -
Feeding system -
Tri-phase separator -
Pre-treatment and
pumping station
Odour emission -
Flowrate variation -Passage of debris -
Passage of oil and grease -
Sludge
- Nutrient recovery
- Pathogen elimination
- Presence of sand and debris
- Dewatering
All topics are currently under investigation. Thispaper focuses on liquid effluent and gaseous
emissions
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UASB technology: summary
Great advantages and broad application, but
operational limitations are still present
Known limitations regarding pathogen, nitrogen
and organic characteristics
Other concerns in relation to the removal of
surfactants, micropollutants and sulfides
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Polishing Ponds
Are basically maturation ponds conceived to polish
the effluent from anaerobic reactors
The main role is pathogen removal, but also some
improvement in terms of ammonia and organic
mater can be expected.
If properly designed, the system can lead to
effluents complying with WHO guidelines for
unrestricted irrigation.
Designed as maturation ponds, both as cells in
series or baffled units, with depths ranging from
0.60 to 1.00 m.
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UASB reactor + Polishing Pond system
Very simplified process train:
Preliminary treatment units Anaerobic treatment unit
Single baffled pond or ponds in series
Dewatering unit
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AIM
To present a conceptual and integrated analysis of the
UASB/PP system in order to improve the overall
performance of the wastewater treatment plant, notonly in terms of better effluent quality but also in
relation to the control of gaseous emissions.
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MOTIVATION
Previous studies have reported on the feasibility of PP for
improving the microbiological quality of anaerobic effluents
and also the quality in terms of organic matter and nutrients:
Constituent Effluent concentration (mg/L) Removal efficiency (%)UASB reactor Overall
(UASB/PP)UASB reactor Overall
(UASB/PP)BOD 70 100 40 - 70 60 75 75 85
COD 180 270 100 - 180 55 70 70 83Ammonia-N 30 50 10 - 20 (a) 50 65N total 35 55 15 - 25 5 15 50 65E. coli 106 - 107 102 - 104 1 2 log units 3 5 log unitsHelminth eggs > 1 < 1 70 - 90 ~100
However, very little is known in relation to other
constituents of interest, such as surfactants,
micropollutants (e.g. endocrine disrupters,
pharmaceuticals etc.) and dissolved and gaseous sulfide.
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Flow sheet, sampling points and main characteristics of the UASB/PP system
METHODOLOGY
Removal of specific constituents and control of
gaseous emissions was based on literature review
and on preliminary measurements of surfactants,
micropollutants and sulfide in a small full-scale
UASB/PP system.
UASB reactor Diameter: 2.0 m Height = 4.50 m Volume = 14 m3
HRT = 11 h
Polishing Ponds Length (bottom)=25m (PP1, PP2); 16.6m (PP3) Width (bottom)=5.25m Depth=0.80m (PP1, PP2); 0.60m (PP3) HRT=3 days (PP1, PP2); 2 days (PP3)
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Location: Centre for Research and Training on Sanitation UFMG/COPASA
View of the small full-scale UASB/PP system
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Analysis of Anionic surfactant
The homologues of the anionic surfactant (LAS) were
quantified by high performance liquid chromatography
(HPLC) using UV/Vis detector set at 220 nm after their
separation in a Lichrosorb 10 RP8 column (Chrompack)kept at 35 oC.
Details of mobile phase, filtration and eluation procedures
are described in the paper Three 24-hour composite samples were taken for the raw
wastewater and for the effluent of UASB reactor, while
depth-composite samples were taken for the effluents of allponds.
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Analysis of Micropollutants
Contaminants analysed: diethylphthalate (DEP),
bis(2ethylhexyl)phthalate (BEHP) and bisphenol A
(BPA).
These contaminants have known endocrine disrupting
properties to aquatic fauna, are normally present in
relatively high concentration in surface waters andderive from plastic ware such as PVC and
polycarbonate.
Four 24-hour composite samples were taken for theraw wastewater and for the effluent of UASB reactor,
while depth-composite samples were taken for the
effluents of pond 2.
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Analysis of Micropollutants
300 mL of aqueous samples (raw sewage and
effluents from the UASB reactor and from the first
polishing pond) free of solids were first filtered
through C-18 cartridges (Strata, 500 mg)
Details of eluation , drying and injection procedures
are described in the paper. The organic extracts were analyzed by gas
chromatography coupled to mass spectrometry (GC-
MS) in a Shimadzu QP 2010 equipment. Operatingconditions are described in the paper.
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Analysis of Sulfide
Sulfides were determined using the methylene blue
method, according to Plas et al. (1992).
Triplicate grab samples were collected in four differentdays, being immediately mixed with a solution of zinc
acetate (0.01 M) in order to promote the precipitation
of dissolved sulfide
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Results
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Removal of surfactants
Removal of LAS continued along the treatment system, but theoverall efficiency was limited to around 50%.
The UASB reactor was responsible for a minor reduction (around18% removal), whereas the set of three ponds was responsible fora further 35% removal efficiency.
As a result, a reasonable amount of LAS is released in the finaleffluent (average concentration of 5.9 mg/L). This value does notmeet the Brazilian standard for this parameter which is set at 2mg/L.
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Removal of micropollutants
The anaerobic step has little impact on the biodegradation of phthalates(8 to 28%) whereas bisphenol A is removed by 78%. Most likely this isdue to adsorption onto the sludge due to the high hydrophobicity ofsuch alkyl phenol.
The polishing pond significantly reduced the concentration of phthalates
from the UASB effluent leading to an overall efficiency of phthalatesremoval from 53 to approximately 70% in the UASB/PP system.
As far as the bisphenol is concerned, the results show that the polishingpond was very efficient in removing this alkyl phenol from the UASB
effluent, and this might have occurred due to a combination of the highhydraulic retention time and adsorption to algae mass
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Removal of sulfide
Most of the H2S is removed even before entering the pondsystem, being stripped from the liquid phase due to thepresence of a splitting box located 3.5 meters lower than theeffluent level.
The remaining sulfide concentration is completely removedalready in the first pond, with no trace concentrations beingdetected in ponds 2 and 3.
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Conceptual analysis of H2S removal in liquid-basedsystems
Concept of waste-gas treatment in aerobic liquid-based systemswas originally developed for activated sludge processes AS,being later expanded for other systems, such as RBCs and SABs
In liquid-based systems, the odorous contaminants aretransferred from the gas into the oxygen-containing bulk liquidwhere they are degraded by a suspension of bacteria. Thewaste-gas pollutants are co-degraded with the contaminantsdissolved in the influent wastewater.
Since most volatile organic contaminants have a much higheraffinity for the liquid when compared to oxygen (as evidenced by
lower Henrys coefficients), the liquid-based system is generallyoversized for mass transfer of the odorous contaminants .
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Conceptual analysis of H2S removal in liquid-basedsystems
The expansion of the liquid-based system concept for H2S treatmentin PP systems seems to be a feasible option:
o H2S also has a higher affinity for the liquid phase than oxygen, thereforemass transfer limitation should not be a problem;
o H2S biodegradability is considered high.
However, major differences between PP and other liquid-basedsystems:
o suspended and attached growth biomass are present in much higherconcentrations in AS, RBC and SAB systems;
o mixing/aeration is provided in AS, RBC and SAB systems;
o AS and SAB systems have much higher water depths;
o RBC and SAB are usually enclosed systems.
On the other hand, polishing ponds:
o have much larger areas, which allow a scattered distribution of thegaseous emissions into the liquid phase;
o present much higher pH (usually between 8 and 9), meaning that nearly100% of the H2S will dissociate into the non-odorous HS
- form.
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Conceptual analysis of H2S removal in liquid-basedsystems
Biogas
Flare
Waste gascontaining H2S
Final
Effluent
Biogas
PP 1 PP 2 PP 3
Oxidation of H2S(g)
inside the PPEffluentcontaining
dissolvedH2S
Oxidation of H2S(aq)
inside the PPW
astegasfrompreliminary
treatmentandpumpingstation
UASB
reactor
Schematic representation of combined management of gaseous andliquid emissions in small treatment plants with anaerobic reactors:Biogas flaring, liquid and waste gas treatment in Polishing Ponds
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CONCLUSIONS
These preliminary observations have shown that UASB/PPsystems can also provide for the removal of surfactants,
micropollutants and dissolved and gaseous sulfides.
Results indicated that Polishing Ponds can contribute to theremoval of dissolved and gaseous H2S, although the degree of
bio-chemical oxidation was not measured and this should be
more investigated.
The removal of selected micropollutants and surfactants was
limited and is most likely linked to the characteristics and
operating conditions of the treatment system.
There are also concerns in relation to micropollutants beingadsorbed onto algae and released with the pond effluent. The
removal of these constituents in UASB/PP systems requires
further investigation, in order to evaluate these limitations and
to identify means to improve its performance
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ACKNOWLEDGEMENTS
Our sincere gratitude to Catiane Talita Domingues dos Santos,
Paulo Gustavo Sertrio de Almeida and Valria Martins
Godinho, whose effort in planning, collecting and analysing
samples was essential for the completion of this paper.
FINEP, CNPq, CAPES and FAPEMIG for supporting the research.
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Thanks for your attention
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