CONSTRUCTED WETLANDS:

A Critical Review of Wetland Treatment Processes

Robert W. Seabloom, P.E.Department of Civil & Environmental Engineering

University of Washington

Adrian T. Hanson, PEFrank M. Tejeda Center

New Mexico State University

September 2004

University Curriculum Development forDecentralized Wastewater

Management

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CitationCitation

Seabloom, R.W. and A. Hanson. Seabloom, R.W. and A. Hanson. 2005. Constructed Wetlands: A 2005. Constructed Wetlands: A Critical Review Critical Review -- PowerPoint PowerPoint Presentation. Presentation. inin (M.A. Gross and (M.A. Gross and N.E. Deal, eds.) University N.E. Deal, eds.) University Curriculum Development for Curriculum Development for Decentralized Wastewater Decentralized Wastewater Management. National Management. National Decentralized Water Resources Decentralized Water Resources Capacity Development Project. Capacity Development Project. University of Arkansas, Fayetteville, University of Arkansas, Fayetteville, AR.AR.

CONSTRUCTED WETLANDS

I. ARTIFICIAL WASTEWATERTREATMENT SYSTEMSCONSISTING OF SHALLOW PONDS OR CHANNELS WHICH HAVE BEEN PLANTED WITHAQUATIC PLANTS TO TREAT WASTEWATER. RELY UPON

NATURAL PROCESSES.

A. MICROBIAL

B. BIOLOGICAL

C. PHYSICAL

D. CHEMICAL

II. USUALLY HAVE ENGINEEREDSTRUCTURES

A. CONTROL DIRECTION OF FLOW

B. REGULATE WATER LEVEL

C. REGULATE DETENTION TIME

D. IMPERVIOUS CLAY OR SYNTHETIC LINER

E. MAY CONTAIN INERT POROUSMEDIA (ROCK, GRAVEL ORSAND)

III. REQUIRES PRE-TREATMENT(SEPTIC TANK)

IV. TREATS SEPTIC TANK EFFLUENT TO SECONDARY EFFLUENT STANDARDS (PROBLEMATICAL)

WASTEWATER TREATMENT TRAINS FOR CONSTRUCTED WETLANDS AND

LARGE POLISHING WETLANDS

V. CLASSIFIED INTO TWO PASSIVETYPES OF TREATMENT THAT SHAREMANY CHARACTERISTICS, BUT ARE DISTINGUISHED BY THE LOCATION OF THE WATER LEVEL.

VI. FREE WATER SURFACE (FWS). WETLANDS HAVE OPEN WATER AREAS WITH A COMBINATION OFFLOATING AND EMERGENT AQUATIC PLANTS ROOTED IN THE SOILBOTTOM. THE WATER FLOWSTHROUGH THE LEAVES AND STEMSOF PLANTS AND HAVE THREE ZONES:

A. FULLY VEGETATED

B. OPEN-WATER SURFACE

C. FULLY VEGETATED

D. INLET AND OUTLET STRUCTURESAND BERMS THAT CONTROL THEFLOW

E. SOME HUMAN HEALTH CONCERNS

F. MOSQUITO CONTROL MAY BEREQUIRED

FREE WATER SURFACE (FWS) CONSTRUCTED WETLAND

FREE WATER SURFACE (FWS) CONSTRUCTED WETLAND CROSS SECTION

VII. VEGETATED SUBMERGED BED (VSB)WETLANDS CONSIST OF GRAVEL BEDS PLANTED WITH WETLAND VEGETATION, CONTINUALLY SUBMERGED. WASTEWATER FLOWSTHROUGH THE GRAVEL, EMERGENTPLANTS GROW IN THE GRAVEL.

A. HAS MANY OF THE SAMEFEATURES AS FWS

B. DISTINGUISHED BY ITS SUBSURFACE HYDRAULICGRADELINE

C. WASTEWATER STAYS BELOWSURFACE OF MEDIA

D. ACTS AS A HORIZONTAL FILTER

E. PERFORMS AS WELL WITH OR WITHOUT PLANTS

VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLAND

VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLAND CROSS SECTION

VIII. PRINCIPAL REMOVAL ANDTRANSFORMATION MECHANISMSIN (FWS) AND (VSB)

A. BIOCONVERSION

B. ADSORBTION

C. SEDIMENTATION

D. FILTRATION

E. PLANT UPTAKE

F. VOLATILIZATION

G. PREDATION

H. UV RADIATION

I. EXCRETION OF ANTIBIOTICS BYPLANTS

J. AMMONIFICATION, NITRIFICATION/ DENITRIFICATION (LIMITED)

(ammonification) organic nitrogen → NH4+

(nitrification) NH4+ + 1.502 → 2H+ + H2O + NO2

-

(nitrification) NO2- + 0.502 → NO3

-

(denitrification) (carbon source) + NO3- → N2 and

N2O

IX. ANAEROBIC PROCESSESDOMINATE IN BOTH FWS ANDVSB

X. NITROGEN REMOVAL UNLIKELYIN VSB

XI. PHOSPHORUS REMOVAL INSIGNIFICANT

XII. GENERALLY REQUIRE POST-TREATMENT PROCESSES

XIII. HAVE INHERENT AESTHETICAPPEAL TO GENERAL PUBLIC

XIV. NOT UNIFORMLY ACCEPTED BYSTATE REGULATORS

XV. DESIGN PROCESS IS EMPIRICAL

XVI. MAY BE AN APPROPRIATETECHNOLOGY WHERE LAND ISINEXPENSIVE

XVII. CONTAINMENT STRUCTURES

A. BERMS, DIKES AND BOTTOMLINERS PROVIDE WATER-TIGHT INTEGRITY

EXAMPLES OF CONSTRUCTED WETLAND BERMS

B. INLET DESIGNS

EXAMPLES OF CONSTRUCTED WETLANDS INLET DESIGNS

C. OUTLET DESIGNS

EXAMPLES OF OUTLET DEVICES

XVIII. TYPICAL REMOVALS IN CONSTRUCTED WETLANDS

A. FREE WATER SURFACE (FWS)

1. BOD, 60-80%

2. TOTAL SUSPENDED SOLIDS,50-90%

3. FECAL COLIFORM, 90-99%

4. NITROGEN -NOT SIGNIFICANT

5. PHOSPHOROUS -NOT SIGNIFICANT

6. SULFUR - NOT SIGNIFICANT

B. VEGETATED SUBMERGED BED (VSB)

1. BOD, 60-80%

2. TOTAL SUSPENDED SOLIDS, 50-60%

3. FECAL COLIFORM, 90-99%

4. NITROGEN -NOT SIGNIFICANT

5. PHOSPHOROUS -NOT SIGNIFICANT

6. SULFUR - NOT SIGNIFICANT

XIX. TYPICAL REMOVALS IN CONSTRUCTED WETLANDS

Free Water Surface (FWS)

BOD 60-80%Total Suspended Solids 50-90%Fecal Coliform 90-99%Nitrogen Not significant*Phosphorous Not significant*

Vegetated Submerged Bed (VSB)

BOD 60-80%Total Suspended Solids 50-60%Fecal Coliform 90-99%Nitrogen Not significant*Phosphorous Not significant*

*A properly designed FWS wetland with fortuitous conditionsof sunlight, temperature, wind and wastewater strength mayremove significant amounts of nitrogen and from 2 to 3 logsof coliforms.

XX. CONCLUSIONS

A. MAY BE APPROPRIATE FORSMALL COMMUNITIES WHEREINEXPENSIVE LAND IS AVAILABLE, AND NITROGENAND PHOSPHOROUS REMOVALARE NOT REQUIREMENTS

B. HAVE INHERENT AESTHETIC APPEAL TO GENERAL PUBLIC

C. DESIGNER MAY HAVE TO CONVINCE PUBLIC THEY ARENOT A VIABLE OPTION

D. DESIGN STILL BASED UPONEMPIRICAL DATA

CONSTRUCTED WETLANDS -COMMON MISCONCEPTIONS

1. WETLAND DESIGN IS BASED UPONWELL CHARACTERIZED PUBLISHEDDESIGN EQUATIONS.

Not so: Due to lack of data, designershave been forced to derive designparameters from limited and unreliableinformation.

2. VEGETATED SUBMERGED BED (VSB)CONSTRUCTED WETLANDS HAVE AEROBIC AS WELL AS ANAEROBICTREATMENT ZONES.

Not so: The ability of emergent wetlandplants to transfer oxygen to their rootshas been over estimated. The amount ofoxygen leaked from plant roots isinsignificant compared to the oxygen demand of septic tank effluent.

3. VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLANDS CAN

REMOVE SIGNIFICANT AMOUNTS OF NITROGEN.

Not so: Uptake of nitrogen by plants mustbe followed by harvesting and removal ofplants from the wetland, which generallyis not feasible. Also because anaerobicprocesses dominate in VSBs and in thevegetated portions of the FWS, nitrificationof ammonia is unlikely to occur.

4. CONSTRUCTED WETLANDS CAN REMOVE SIGNIFICANT AMOUNTS OF PHOSPHOROUS.

Not so: Phosphorous removal is limited toseasonal uptake by plants, which is veryminor compared to the phosphorous load in septic tank effluent and is negatedduring the plants’ senescence. Sorptionto solids in the wastewater, soils, and plant detritus is temporary and of limitedcapacity.