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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


Removal/disposal

Increasedegradation(3)

Tri-phase

separator-

Gasleakage

Structurefailure

Corrosion

Useofpropermaterials

Useof proper linings

Improvedesign

Cover - CorrosionUseofpropermaterials

Useof proper linings

Feedingsystem -Blockage

Badmixing


Increaseno ofdistributors

Sludge

Nutrient

Pathogen

Sand

Debris

Sludgebuildup

Healthrisks

Dewatering

Improvedesign

Improveoperation


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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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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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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