University Curriculum Development for Decentralized Wastewater Treatment

Transcription

1 Constructed Wetlands Systems: Design Approaches Wallace Page i University Curriculum Development for Decentralized Wastewater Treatment Constructed Wetland Systems: Design Approaches Suggested Course Materials Scott D. Wallace, P.E. Vice President North American Wetland Engineering P.A.

2 Constructed Wetlands Systems: Design Approaches Wallace Page ii 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 ) 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. 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 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. Acknowledgements The author would like to acknowledge Nancy Deal, Jim Kreissl and Mark Gross for their assistance and support.

3 Constructed Wetlands Systems: Design Approaches Wallace Page iii Citation of Materials The educational materials included in this module should be cited as follows: Wallace, S.D Constructed Wetlands: Design Approaches Text. 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. Wallace, S.D Constructed Wetlands: Design Approaches - 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.

4 Constructed Wetlands Systems: Design Approaches Wallace Page iv Constructed Wetland Systems: Design Approaches Suggested Course Materials Table of Contents Overview...1 Agenda... Outline.. Goals...5 Learning Objectives...6 Prerequisites...7 Evaluation Form...8 Problem Sets...9 Problem Sets with Solutions...10

5 Page 1 Constructed Wetland Systems: Design Approaches Overview This module presents an introduction into the design of constructed wetland treatment systems. This module is aimed at engineering students who have already taken courses in hydraulics and who have been previously introduced to the fundamentals of wastewater treatment. Because constructed wetlands are an evolving technical discipline, many older design methods are outdated and result in unrealistic treatment expectations. Consequently, designers considering the use of treatment wetlands need to understand the strengths and weaknesses of different design methods, and be open to new developments in the field. Upon completing this module, students will have been introduced to the two main types of constructed wetlands (free water surface and vegetated submerged beds), and will have been exposed to five methods of designing wetlands that are in common use today. Comparing and contrasting these design methods provides important insights into the degree of certainty offered by the current level of understanding within the constructed wetland field. Module materials include a text for student use, PowerPoint lecture materials, and problem sets for use in and out of the classroom. Depending on the instructor s and student s level of interest in constructed wetlands, this material may be presented in one or two 50-minute class periods.

6 Page Constructed Wetland Systems: Design Approaches Agenda The materials in this module are intended to be used for one or two lectures as part of a larger overall course on decentralized wastewater management. Depending on the instructor s and student s level of interest in constructed wetlands, the following agendas are suggested, based on a 50-minue class period: 1-Lecture Format Background: Assign module text as reading material Lecture 1: Use PowerPoint Lecture materials for Sections 1, and Problem Set: Assign Wetland Fundamentals questions + one design problem (VSB or FWS) -Lecture Format Background: Assign module text as reading material Lecture 1: Use PowerPoint Lecture materials for Sections 1,, and 5 Problem Set 1: Assign Wetland Fundamentals questions Lecture : Use PowerPoint Lecture materials for Sections 4 and 6; work example problem in class Problem Set : Assign two design problems (one FWS; one VSB)

7 Page Constructed Wetland Systems: Design Approaches Outline I. Introduction A. Types of Constructed Wetlands 1. Free Water Surface (FWS). Vegetated Submerged Bed (VSB). Vertical Flow II. III. IV. Free Water Surface (FWS) Wetland Processes A. Overview of treatment mechanisms B. Flow C. Role of emergent plants in FWS wetlands D. Oxygen transfer in FWS wetlands E. Sedimentation (Suspended Solids) F. Organic matter degradation G. Nitrogen cycling H. Phosphorous cycling I. Pathogen reduction J. Mosquito control K. Water temperature in FWS wetlands Vegetated Submerged Bed (VSB) Wetland Processes A. Overview of VSB treatment mechanisms B. Flow C. Role of plants in VSB wetlands D. Oxygen transfer in VSB wetlands E. Sedimentation (Suspended Solids) F. Organic matter degradation G. Nitrogen cycling H. Phosphorous cycling I. Sulfur cycling J. Pathogen removal K. Water Temperature in VSB Wetlands Wetland Design Methods A. Sources of design methods 1. Crites and Tchobonaglous. EPA. Kadlec and Knight 4. Reed, et.al. 5. TVA B. FWS design example C. VSB design example

8 V. Wetland Vegetation A. Wetland plant hydrology B. Planting stock viability C. Start-up water level management D. Selection of wetland plant species University Curriculum Development for Decentralized Wastewater Treatment Page 4 VI. VII. Constructed Wetlands current level of understanding References

9 Page 5 Constructed Wetland Systems: Design Approaches Goals The goal of this module is to familiarize students with how wetland treatment systems function, and to introduce them to various design methods commonly in use today. Students completing this module should be able to objectively assess the use of constructed wetlands for decentralized wastewater management.

10 Page 6 Fundamental Concepts for Environmental Processes Learning Objectives 1. Upon completing this module, students will have an understanding of the internal mechanisms that operate within wetland treatment systems and how these mechanisms affect pollutant transformations and removal.. Module materials also introduce students to various design methods in common use today. Comparing and contrasting these design methods provides important insights into the degree of certainty offered by the current level of understanding within the constructed wetland field.. After completion of this module, students should be capable of utilizing wetland design manuals and engineering textbooks to size wetland treatment systems, and should be able to critically evaluate the feasibility of wetlands as a treatment technology.

11 Page 7 Constructed Wetland Systems: Design Approaches Prerequisites 1. Freshman chemistry. Introductory course in Hydraulics. Introductory course Environmental Engineering The typical student utilizing these materials will be a junior- or senior-level undergraduate in civil engineering who has an interest in decentralized wastewater management.

12 Page 8 Constructed Wetland Systems: Design Approaches Evaluation Form Name: We are requesting your assistance in reviewing the modules developed through the On-Site Consortium curriculum project. Please complete the following form while reviewing the materials With a rating scale of 1 (Disagree) to 5 (Agree), please respond to the following questions: Review of printed materials: Disagree Agree The text adequately covers the application of constructed wetlands for decentralized wastewater management The text presents information on constructed wetlands in a fair and objective manner Comparing design methods in the text helps students assess the degree of certainty associated with current wetland design Visual materials adequately describe wetland systems Example problems enhance students understanding of wetland design Review of learning objectives: I gained a better understanding of how wetland treatment systems work I gained experience with design methods commonly used to size wetland treatment systems I gained a better understanding of typical applications for different types of wetland treatment systems After completing this module, I would feel confident in being able to evaluate constructed wetlands as a treatment option What specific recommendations would you provide for the text? What specific recommendations would you provide for the visuals? Please give specific constructive comments on the topic/module.

13 Page 9 Constructed Wetland Systems: Design Approaches Problem Sets Wetland Fundamentals 1. Describe the two major types of constructed wetlands in common use today.. Describe typical applications for each wetland type.. Will VSB wetlands provide habitat for mosquitoes? 4. Describe design steps that can be taken to minimize mosquito production in FWS wetlands. 5. Describe the role of plants in FWS wetlands. 6. Summarize maximum recommended organic loading rates for FWS wetlands. What limits these loading rates? 7. Describe how nitrogen will be transformed in FWS wetlands. 8. What role does hydraulic conductivity play in VSB wetland design? 9. Do plants play a significant oxygen transfer role in VSB wetlands? 10. What type of wetland system is most appropriate for cold-climate applications? Why? Design Problems 1. Design a VSB wetland to treat septic tank effluent from a truck stop using the method of Crites & Tchobanoglous and USEPA based on a flow of 8,000 gpd, an influent BOD concentration of 600 mg/l, a permit limit of 0 mg/l, and a minimum water temperature of 59 o F (15 o C). Assume a porosity of 0.5 and a water depth of 1.5 feet. Design a FWS wetland to polish effluent from a poorly maintained package treatment plant, which often discharges partially-treated effluent with 50 mg/l BOD and 50 mg/l TSS. Using the method of Kadlec & Knight and USEPA, design the FWS based on a flow of 5,000 gpd, and a permit limit of 5 mg/l for BOD and TSS, with a minimum water temperature of 54 o F (1 o C).

14 Page 10 Constructed Wetland Systems: Design Approaches Problem Sets With Solutions Wetland Fundamentals 1. Describe the two major types of constructed wetlands in common use today. Free Water Surface (FWS) wetlands have exposed water bodies and are similar to natural marshes in appearance. Vegetated Submerged Bed (VSB) wetlands employ a gravel bed planted with wetland vegetation. The water is kept below the surface of the gravel, and flows horizontally from the inlet to the outlet.

15 Page 11. Describe typical applications for each wetland type. The most common application for FWS wetlands is to polish effluent from a lagoon, trickling filter, activated sludge, or other secondary treatment process. The most common application for VSB wetlands is for onsite wastewater treatment for single-family homes.. Will VSB wetlands provide habitat for mosquitoes? Properly designed and maintained VSB wetlands operate with the water level below the surface of the gravel. Consequently, water is not exposed during the treatment process and there is no suitable mosquito habitat. 4. Describe design steps that can be taken to minimize mosquito production in FWS wetlands. Since FWS wetlands provide suitable habitat for mosquitoes, the wetland should be designed to support predator organisms such as mosquitofish (Gambusia holbrooki) so that mosquito larvae do not survive to become adults. Specific design steps include: Limit organic loading that would result in low dissolved oxygen levels in the water column.

16 Page 1 Provide open water areas for predator organisms and avoid designs where large mats of plant detritus can accumulate (plant bridging). Avoid designs that create isolated areas (such as distribution channels or inlet works) that support mosquitoes but are not accessible by predator organisms. 5. Describe the role of plants in FWS wetlands. Increase sedimentation by reducing water column mixing and resuspension Provide surface area in the water column to increase biofilm biomass and pollutant uptake. Increase the removal of particles from the water column by increasing biofilm and plant surfaces available for particle interception. Provide shade from the plant canopy over the water column to reduce algae growth. Containing and preserving duckweed fronds which greatly limit reiteration and light penetration into the water column. Structurally cause flocculation of smaller colloidal particles into larger, settleable particles. 6. Summarize maximum recommended organic loading rates for FWS wetlands. What limits these loading rates? FWS Type Typical Loading kg/ha d Range kg/ha d Semiplug-flow Semiplug flow with :1 recycle and step feed Semiplug flow with step feed, :1 recycle and supplemental aeration Organic loading rates are limited by oxygen transfer into the water column. Exceeding these rates generally results in odors, plant stress, enhanced mosquito production and poor treatment. 7. Describe how nitrogen will be transformed in FWS wetlands. Wetland areas with emergent vegetation do not have sufficient oxygen transfer to oxidize ammonia to nitrate (nitrification). Generally speaking, only open water areas within the wetland have sufficient oxygen transfer to allow nitrification to occur. Nitrate, if present in the influent or formed within open water areas, can be reduced (denitrified) in emergent vegetation zones. 8. What role does hydraulic conductivity play in VSB wetland design?

17 Flow in a VSB wetland is governed by Darcy s Law: Q= k AS Where: s c s c University Curriculum Development for Decentralized Wastewater Treatment Page 1 Q=average flow through wetland, m k =hydraulic conductivity, m d A =cross-sectional area of bed, m S=slope of hydraulic gradeline, m/m Due to organic loading, only a small fraction of the clean hydraulic conductivity is used for design purposes. USEPA recommends only using 1% of the clean hydraulic conductivity for design purposes. 9. Do plants play a significant oxygen transfer role in VSB wetlands? Oxygen transfer by plants was initially thought to be a dominant mechanism in VSB treatment, but later research has demonstrated that the vast majority of the oxygen translocated by the plant is used for root metabolism, and the amount released to the water column is exceedingly small, about 0.0 g m - d -1. Consequently plants do not provide significant oxygen transfer. 10. What type of wetland system is most appropriate for cold-climate applications? Why? VSB wetlands are typically used for year-round applications in cold climates. VSB wetlands are more thermally efficient because the water is not exposed, and the gravel bed can be covered with an insulating mulch layer. d

18 Page 14 Design Problems. Design a VSB wetland to treat septic tank effluent from a truck stop using the method of Crites & Tchobanoglous and USEPA based on a flow of 8,000 gpd, an influent BOD concentration of 600 mg/l, a permit limit of 0 mg/l, and a minimum water temperature of 59 o F (15 o C). Assume a porosity of 0.5 and a water depth of 1.5 feet Crites & Tchobanoglous Method The detention time for BOD removal is calculated by: ln CC t = k Where: t = detention time for BOD removal, d C = influent BOD concentration, mg/l o C = effluent BOD remaining from influent (BOD k apparent apparent o RIW ), mg/l -1-1 =overall BOD removal rate constant, corrected for temperature,d (1.1d recommended) The predicted effluent BOD concentration does not include background BOD from plant decay, which is indicated to be to mg/l (BOD PD ). Once the detention time is calculated, the net area of the wetland can be determined from: Qave t.07 As = η d Where, A = surface area of VSB, ac Q s ave w = average flow through wetland, Mgal/d t = detention time, d η = porosity of gravel bed media d = water depth, ft w The rate constant, 1.1 d -1 should be temperature corrected to the 15 o C water temperature. Assume the temperature correction factor, θ, is 1.06 This is essentially the same method presented in Natural Systems for Waste Management and Treatment (Reed S.C. et al., 1995): ( ) ( ) k apparent,10 = = 0.8 d

19 Page 15 The VSB will produce a background concentration of BOD of about mg/l. Consequently, the design effluent value will be 0- = 7 mg/l. The detention time for BOD removal is calculated by: ln CC ln ( o ) t = = =.8 days k 0.8 apparent The net area of the wetland can be determined from: Qave t.07 As = η d w 8, ,000,000 As = = 0.1 ac 9, 04 ft The required width of the wetland can be calculated using Darcy s Law: Q= k AS Where: Q=average flow through wetland, m k =hydraulic conductivity, m d s A =cross-sectional area of bed, m c s c S=slope of hydraulic gradeline, m/m For flat beds, a water surface gradient, S, of is recommended. Assume the wetland will have a water depth (d w) of 1.5 feet (0.8 meters) and the bed material has a D 10 of 8 mm and a porosity (η) of 0.6 resulting in clean bed hydraulic conductivity of 5,000 m/d (16,400 ft/d). No reductions in hydraulic conductivity are required, consequently: Q 1,070ft d Ac = = = 65. ft k ( 16,400ft s S ) d d = 1.5 ft w 65. ft Width = = 5 ft 1.5 ft 9,04 ft Length = = 17 ft 5 ft 17 Length:Width Ratio = =.:1 5 d

20 Page 16 USEPA Method The area of the VSB is determined using mass loading rates: Parameter Area Loading Effluent Rate Concentration BOD 6 g/m d 0 mg/l TSS 0 g/m d 0 mg/l The necessary width is calculated using Darcy s Law. Because solids accumulation in the inlet portion of the VSB, a safety factor should be applied to the clean media hydraulic conductivity: Initial 0% of VSB Final 70% of VSB K i = 1% of clean K K f = 10% of clean K 1m d 18,168g 600mg 0.m 1000L 1g L d 1m 1000mg BOD Loading = = 18,168 g d Area = =, 08 m,59 ft 6g d This area should be broken into the primary treatment zone (first 0% of the VSB) and the secondary treatment zone (last 70% of the VSB): Primary Treatment Area, A = 0%,08 m = m si sf Secondary Treatment Area, A = 70%,08 m =,199.6 m The width of the bed should be determined using Darcy s Law and the dirty hydraulic conductivity of the primary treatment area. With a recommended media of 0 0 mm gravel, the clean hydraulic conductivity is 100,000 m/d. The dirty hydraulic conductivity would then be: K i = 1% 100,000 m/d = 1,000 m/d It is recommended that the head loss in the primary treatment zone be limited to 10% of the bed depth. Since the bed depth in this example is 1.5 ft (0.8 m), the allowable head loss would be 10% X 0.8 m = 0.08 m. For Darcy s Law, the gradient is the head loss (change in elevation) divided by the flow length. The flow length in the primary treatment zone is the primary treatment area (A si ) divided by the width of the VSB bed (W). Substituting these terms into Darcy s Law and rearranging the terms yields the following equation:

21 W Where: W = width of VSB bed, m Q = flowrate, m /d A = Primary Treatment Area, m si i w Q Asi = K d h i w University Curriculum Development for Decentralized Wastewater Treatment Page 17 K = "dirty" hydraulic conductivity in Primary Treatment Zone, m/d d = water depth in VSB bed, m h = elevation change of water surface across Primary Treatment Zone, m (10% of d recommended) w For this example, the width of the bed can be calculated as: ( )( ) 0.m d 908.4m Q Asi W = = K d h W = 4.7 m i w ( 1,000m d)( 0.8m)( 0.08m) The calculated width of 4.7 m (14 feet) is less than the maximum recommended width of 0.8 m (68 feet). The length of the primary treatment zone, A si, can be calculated to be m, and the length of the secondary treatment zone, A sf, can be calculated to be 50. m. The total VSB length is then 71.1 m ( ft), and the resulting length/width ratio is.4:1, which is considerably longer than the recommended range of 1:1 to 1:. Consequently, a wider and shorter design should be used. A square cell (length/width ratio of 1:1) would result in a VSB 55 m wide and 55 m long. Summary Table Method VSB Area Length:Width Ratio Crites & Tchobanoglous 9,04 ft.:1 USEPA,59 ft 1:1 In this example, the USEPA VSB design is much larger and will have a lower organic loading across the cross-sectional area of the bed. Consequently, this system is likely to give more stable and robust performance.

22 Page Design a FWS wetland to polish effluent from a poorly maintained package treatment plant, which often discharges partially-treated effluent with 50 mg/l BOD and 50 mg/l TSS. Using the method of Kadlec & Knight and USEPA, design the FWS based on a flow of 5,000 gpd, and a permit limit of 5 mg/l for BOD and TSS, with a minimum water temperature of 54 o F (1 o C). Kadlec & Knight Method The general form of the k-c* model is: * Ce C kat, ln * = Ci C q Where: C = outlet target concentration, mg/l e C = inlet concentration, mg/l i * C = background concentration, mg/l k A,T = temperature dependent first-order areal rate constant, m/yr q= hydraulic loading rate, m/yr Rearrangement and a unit conversion give the area required for a particular pollutant: * Q Ci C A = ln * ka Ce C Where: A = required wetland area, ha Q = water flow rate, m The rate constant, k A, can be temperature corrected: ( T 0) k = k θ AT, A,0 Where: k k o A,T = first-order areal rate constant at temperature t, C o a,0 = first-order areal rate constant at 0 C θ = temperature correction factor T = wetland water temperature, o C d

23 Applicable k-c* design parameters are: University Curriculum Development for Decentralized Wastewater Treatment Page 19 Parameter k A,0 (m/yr) θ C*, mg/l BOD Ci TSS Ci Design for BOD Removal: The rate constant for BOD removal is 4 m/yr and θ is 1.0, so the rate constant does not change for a water temperature of 1 o C. The background concentration is calculated as (Ci), or: * C = + = (50) 6 mg L The required area can be calculated using the influent flow of 5,000 gpd (1.5 m /d) as: A = ln = 0.1ha (0.ac) Design for TSS Removal The rate constant for TSS removal is 1000 m/yr and θ is 1.0, so the rate constant does not change for a water temperature of 1 o C. The background concentration is calculated as (Ci), or: * C = + = (50) 1mg L The required area can be calculated as: A = ln = ha (0.019ac)

24 Page 0 USEPA Method Zone 1 is a densely vegetated region designed for flocculation and sedimentation of influent suspended solids and BOD. Zone is an open water region designed to increase the dissolved oxygen content of the water, allowing for aerobic degradation of soluble BOD and nitrification. Zone is a densely vegetated region similar to Zone 1, which is designed to reduce suspended solids reaching the outlet and provide for denitrification. Area Loading Rates for the -zone FWS Model Parameter Area Loading Effluent Concentration BOD 45 kg/ha d <0 mg/l 60 kg/ha d 0 mg/l TSS 0 kg/ha d <0 mg/l 50 kg/ha d 0 mg/l It is suggested that the detention time in Zone 1 be approximately to days, since flocculation/sedimentation of influent suspended solids is essentially complete at this point. Zone 1 will remove approximately 80% of the influent TSS. Similarly, a to - day detention time is recommended for Zone for removal of suspended solids produced in Zone. Zone is recommended to have a detention time less than - days to avoid algae blooms. If additional detention time is required in Zone, it is recommended to break the open water area into multiple zones (with intervening emergent vegetation zones) to minimize algae production.

25 Page 1 To achieve a 5 mg/l limit for BOD and TSS, loadings must be less than: BOD Loading: TSS Loading: 45 kg/ha d 0 kg/ha d Design for BOD Removal The first step is to calculate the BOD mass loading: 50mg 1000L 1.5m 1g 1kg = 6.6 kg d L 1m d 1000mg 1000g The next step is to determine the required wetland area: 1ha d 6.6kg = 0.15ha (0.7ac) 45kg d Based on the average flow rate, a mean water depth of 0.8m and a mean porosity of 0.8 across the entire wetland (emergent vegetation zones 1 and plus open water zone ), the hydraulic retention time can be calculated: 10,000m 1d 0.15ha 0.8m 0.8 = 7.d ha 1.5m Assuming that the hydraulic retention time in each zone is equal results in 7./ =.4 days per zone. Since this is less than days for the open water area (Zone ), algae blooms should not be a concern. Calculate the area occupied by the open water (Zone ) assuming that this zone has a depth of 1.m and a porosity of 1.0: 1ha 1.5m ,000m d m.4d = 0.07ha The areas of Zones 1 and would then be: = 0.06ha

26 Page Design for TSS Removal: The first step is to calculate the TSS mass loading: 50mg 1000L 1.5m 1g 1kg = 6.6 kg d L 1m d 1000mg 1000g The next step is to determine the required wetland area: 1ha d 6.6kg = 0.ha (0.55ac) 0kg d Based on the average flow rate, a mean water depth of 0.8m and a mean porosity of 0.8 across the entire wetland (emergent vegetation zones 1 and plus open water zone ), the hydraulic retention time can be calculated: 10,000m 1d 0.ha 0.8m 0.8 = 10.6d ha 1.5m Assuming that the hydraulic retention time in each zone is equal results in 10.6/ =.5 days per zone. Since this exceeds days for the open water area (Zone ), algae blooms may occur. In this case, designing Zone with a -day retention time and slightly increasing the size of Zones 1 and appears to be a reasonable design choice. Alternatively, Zone could be broken up into two open water zones. Calculate the area occupied by the open water (Zone ) assuming that this zone has a depth of 1.m and a porosity of 1.0: 1ha 1.5m ,000m d m.0d = 0.0ha The areas of Zones 1 and would then be: Summary Table = 0.09ha Method BOD Removal TSS Removal Kadlec & Knight 0. ac ac USEPA 0.7 ac 0.55 ac