CONSTRUCTED WETLANDS: A Critical Review of Wetland Treatment Processes

Transcription

1 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, PE Frank M. Tejeda Center New Mexico State University September 2004 University Curriculum Development for Decentralized Wastewater Management

2 NDWRCDP Disclaimer This work was supported by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) with funding provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR ) 0) with Washington University in St. Louis. These materials have not been reviewed by the U.S. Environmental Protection Agency. These materials have been reviewed by representatives of the NDWRCDP. The contents of these materials do not necessarily reflect the views and policies of the NDWRCDP, Washington University, or the U.S. Environmental Protection Agency, nor does the mention of trade names or commercial products constitute their endorsement or recommendation for use.

3 CIDWT/University Disclaimer These materials are the collective effort of individuals from academic, regulatory, and private sectors of the onsite/decentralized wastewater industry. These materials have been peer-reviewed reviewed and represent the current state of knowledge/science in this field. They were developed through a series of writing and review meetings with the goal of formulating a consensus on the materials presented. These materials do not necessarily reflect the views and policies of University of Arkansas, and/or the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT). The mention of trade names or commercial products does not constitute an endorsement or recommendation for use from these individuals or entities, nor does it constitute criticism for similar ones not mentioned.

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

5 CONSTRUCTED WETLANDS I. ARTIFICIAL WASTEWATER TREATMENT SYSTEMS CONSISTING OF SHALLOW PONDS OR CHANNELS WHICH HAVE BEEN PLANTED WITH AQUATIC PLANTS TO TREAT WASTEWATER. RELY UPON NATURAL PROCESSES. A. MICROBIAL B. BIOLOGICAL C. PHYSICAL D. CHEMICAL

6 II. USUALLY HAVE ENGINEERED STRUCTURES A. CONTROL DIRECTION OF FLOW B. REGULATE WATER LEVEL C. REGULATE DETENTION TIME D. IMPERVIOUS CLAY OR SYNTHETIC LINER E. MAY CONTAIN INERT POROUS MEDIA (ROCK, GRAVEL OR SAND)

7 III. IV. REQUIRES PRE-TREATMENT (SEPTIC TANK) TREATS SEPTIC TANK EFFLUENT TO SECONDARY EFFLUENT STANDARDS (PROBLEMATICAL) WASTEWATER TREATMENT TRAINS FOR CONSTRUCTED WETLANDS AND LARGE POLISHING WETLANDS

8 V. CLASSIFIED INTO TWO PASSIVE TYPES OF TREATMENT THAT SHARE MANY CHARACTERISTICS, BUT ARE DISTINGUISHED BY THE LOCATION OF THE WATER LEVEL. VI. FREE WATER SURFACE (FWS). WETLANDS HAVE OPEN WATER AREAS WITH A COMBINATION OF FLOATING AND EMERGENT AQUATIC PLANTS ROOTED IN THE SOIL BOTTOM. THE WATER FLOWS THROUGH THE LEAVES AND STEMS OF PLANTS AND HAVE THREE ZONES: A. FULLY VEGETATED B. OPEN-WATER SURFACE C. FULLY VEGETATED

9 D. INLET AND OUTLET STRUCTURES AND BERMS THAT CONTROL THE FLOW E. SOME HUMAN HEALTH CONCERNS F. MOSQUITO CONTROL MAY BE REQUIRED

10 FREE WATER SURFACE (FWS) CONSTRUCTED WETLAND

11 FREE WATER SURFACE (FWS) CONSTRUCTED WETLAND CROSS SECTION

12 VII. VEGETATED SUBMERGED BED (VSB) WETLANDS CONSIST OF GRAVEL BEDS PLANTED WITH WETLAND VEGETATION, CONTINUALLY SUBMERGED. WASTEWATER FLOWS THROUGH THE GRAVEL, EMERGENT PLANTS GROW IN THE GRAVEL. A. HAS MANY OF THE SAME FEATURES AS FWS B. DISTINGUISHED BY ITS SUBSURFACE HYDRAULIC GRADELINE C. WASTEWATER STAYS BELOW SURFACE OF MEDIA D. ACTS AS A HORIZONTAL FILTER E. PERFORMS AS WELL WITH OR WITHOUT PLANTS

13 VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLAND

14 VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLAND CROSS SECTION

15 VIII. PRINCIPAL REMOVAL AND TRANSFORMATION MECHANISMS IN (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 BY PLANTS J. AMMONIFICATION, NITRIFICATION/ DENITRIFICATION (LIMITED) (ammonification) organic nitrogen NH 4 + (nitrification) NH H + + H 2 O + NO 2 - (nitrification) NO NO 3 - (denitrification) (carbon source) + NO 3- N 2 and N 2 O

16 IX. ANAEROBIC PROCESSES DOMINATE IN BOTH FWS AND VSB X. NITROGEN REMOVAL UNLIKELY IN VSB XI. XII. XIII. XIV. XV. XVI. PHOSPHORUS REMOVAL INSIGNIFICANT GENERALLY REQUIRE POST- TREATMENT PROCESSES HAVE INHERENT AESTHETIC APPEAL TO GENERAL PUBLIC NOT UNIFORMLY ACCEPTED BY STATE REGULATORS DESIGN PROCESS IS EMPIRICAL MAY BE AN APPROPRIATE TECHNOLOGY WHERE LAND IS INEXPENSIVE

17 XVII. CONTAINMENT STRUCTURES A. BERMS, DIKES AND BOTTOM LINERS PROVIDE WATER- TIGHT INTEGRITY EXAMPLES OF CONSTRUCTED WETLAND BERMS

18 B. INLET DESIGNS EXAMPLES OF CONSTRUCTED WETLANDS INLET DESIGNS

19 C. OUTLET DESIGNS EXAMPLES OF OUTLET DEVICES

20 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

21 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

22 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 conditions of sunlight, temperature, wind and wastewater strength may remove significant amounts of nitrogen and from 2 to 3 logs of coliforms.

23 XX. CONCLUSIONS A. MAY BE APPROPRIATE FOR SMALL COMMUNITIES WHERE INEXPENSIVE LAND IS AVAILABLE, AND NITROGEN AND PHOSPHOROUS REMOVAL ARE NOT REQUIREMENTS B. HAVE INHERENT AESTHETIC APPEAL TO GENERAL PUBLIC C. DESIGNER MAY HAVE TO CONVINCE PUBLIC THEY ARE NOT A VIABLE OPTION D. DESIGN STILL BASED UPON EMPIRICAL DATA

24 CONSTRUCTED WETLANDS - COMMON MISCONCEPTIONS 1. WETLAND DESIGN IS BASED UPON WELL CHARACTERIZED PUBLISHED DESIGN EQUATIONS. Not so: Due to lack of data, designers have been forced to derive design parameters from limited and unreliable information. 2. VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLANDS HAVE AEROBIC AS WELL AS ANAEROBIC TREATMENT ZONES. Not so: The ability of emergent wetland plants to transfer oxygen to their roots has been over estimated. The amount of oxygen leaked from plant roots is insignificant compared to the oxygen demand of septic tank effluent.

25 3. VEGETATED SUBMERGED BED (VSB) CONSTRUCTED WETLANDS CAN REMOVE SIGNIFICANT AMOUNTS OF NITROGEN. Not so: Uptake of nitrogen by plants must be followed by harvesting and removal of plants from the wetland, which generally is not feasible. Also because anaerobic processes dominate in VSBs and in the vegetated portions of the FWS, nitrification of ammonia is unlikely to occur. 4. CONSTRUCTED WETLANDS CAN REMOVE SIGNIFICANT AMOUNTS OF PHOSPHOROUS. Not so: Phosphorous removal is limited to seasonal uptake by plants, which is very minor compared to the phosphorous load in septic tank effluent and is negated during the plants senescence. Sorption to solids in the wastewater, soils, and plant detritus is temporary and of limited capacity.