Life Cycle Assessment of a Cluster System Constructed Treatment Wetland. Matt Seib

Transcription

1 Life Cycle Assessment of a Cluster System Constructed Treatment Wetland Matt Seib April 21, 2009

2 Table of Contents Part A: Research Description, Results, and Conclusions... 3 Part B: Reciprocal Mentoring Experience Description References Appendix A Appendix B

3 Part A Research Results, Descriptions, and Conclusions 3

4 Abstract In today s environmentally conscious world, treatment wetlands are starting to gain attention as a sustainable way to treat wastewater. This report is an investigation into the life cycle analysis of a treatment wetland designed for the secondary treatment of wastewater. Results show that the most significant impact categories include respiratory inorganics and fossil fuel use. The major contributors to these categories include materials for construction and their transportation. These results will allow future designers to consider alternative materials, configurations, and management options for the wetland design in order to minimize impacts. The results can also be compared against other wastewater treatment technologies so that decision makers can evaluate the environmental impacts of potential design options. 4

5 Introduction A current trend in today s world is the demand for sustainable products and services. This means developing solutions that satisfy a triple bottom line that considers the environment, society, and industry. The goal is to produce solutions that will satisfy the needs of the present without compromising the needs of the future. Wastewater treatment is one of the most basic needs in society and is a service that can benefit from the implementation of more sustainable design. Conventional methods require large amounts of materials, energy, and maintenance for wastewater treatment, which result in high operating costs. Conventional methods are considered unsustainable because they require continuous external inputs in order to function. (Muga, p. 440) Some examples of conventional treatment include: grit removal, sedimentation, digestion, and chlorine or ultraviolet disinfection. (Metcalf, p. 13) Sustainable alternatives to conventional treatment methods should incorporate technology that use little or no energy, do not need chemical addition or other resource inputs, rely on natural biological processes to treat wastewater, and require little maintenance. (WERF, p. 1-1) Some examples of these technologies are lagoons, evapotranspiration beds, and treatment wetlands, which are considered terrestrial methods for wastewater treatment. (Muga, p. 440) These solutions are intended to serve small communities, and in the case of treatment wetlands, can be used for individual households as well. The research performed is a continuation of work done by Valerie Fuchs on the life cycle assessment (LCA) of constructed wetlands for wastewater treatment. This report is the end product of research performed through reciprocal mentoring for the Michigan Tech S-STEM Program Project. Objectives The purpose of the research was to perform a LCA on a vertical upflow treatment wetland designed for a small community. The primary focus was to be on elements of design that contribute most to land use and air emissions (particularly NOx). The overall goals were to: Compare impacts of operating unharvested wetlands to biomass harvesting (mowing). Prioritize means to reduce environmental impacts in the constructed wetland lifecycle. Provide insight into the impacts of watsewater constructed wetlands on nitrogen cycling. This research is to be used in doctoral research on treatment wetlands being conducted by Valerie Fuchs. (Fuchs, 2008 p. 1) While the majority of the proposed objectives of this research have been met, some had to be excluded. In particular, the analyses of NOx emissions and nitrogen cycling were not done because these items could not be incorporated into LCA due to lack of information and software capabilities. Methodologies and Data The goal, scope, and functional unit for this LCA vary from those used by Fuchs. The primary difference is that the LCA done by Fuchs considered a treatment wetland designed to be used by 5

6 an individual household, while this LCA considers a treatment wetlands designed as a cluster system for a small community. Additionally, the LCA done by Fuchs includes water use through the treatment system, whereas this LCA does not include water use. A specific breakdown in the differences between this LCA and the one conducted by Fuchs is located in Appendix A. Goal and Scope The goal of this LCA is to determine which elements included in the life of a constructed wetland designed to provide secondary treatment of wastewater for 100 US households have the highest environmental impact. The scope will take into account all aspects of a treatment wetland and septic system including materials and processes for construction, operation, and disposal. The system boundaries are shown in Figure 1. Figure 1. System boundaries *Adapted from Fuchs (2006) Functional Unit The functional unit is defined as a constructed wetland designed to achieve secondary treatment of wastewater for 100 US households for 50 years. The average US household is assumed to use 400 gallons of water per day (Crites, p. 170). The water use per day is considered high, but was chosen to be conservative. All water is assumed to flow through a septic tank and wetland system without losses. Water use is not included directly as part of the LCA, but is used to size the wetland and septic system. 6

7 Septic System & Treatment Wetland Design This LCA assumes that all wastewater is conveyed through a collection system to a single septic system. The septic system consists of two tanks of equal volume running parallel and was sized with a pump-out interval of five years. (Crites, p. 321) The septic system is made out of reinforced concrete and uses a longitudinal baffle in each chamber. Based on the assumed daily flow, the total septic system volume is 29,000 ft 3 with dimensions of 54 ft. x 36 ft. x 15 ft. The treatment wetland is a sub-surface flow wetland that uses a medium grade sand as fill material. The wetland uses a synthetic liner and has dimensions of 20 ft x 1300 ft x 3 ft. The unusual shape is due to the requirements for cross-sectional area based on the hydraulic conductivity of the sand used in the design. (See design calculations in Appendix B) Design guidelines in Crites and Tchobanoglous (Crites, p. 607) along with data in Fuchs (2006 p ) were used to properly size the treatment wetland and determine materials for construction. The design only takes BOD removal into account and neglects requirements for nitrogen reduction. This is because the size of the treatment wetland becomes unrealistic without additional systems to provide nitrogen removal. Therefore, nitrogen removal was neglected, which prohibited any satisfaction of the objectives concerning nitrogen cycling or NOx emissions. Figure 2 shows a conceptual layout of the septic system and treatment wetland. Figure 2: Conceptual system layout 7

8 Compiling the LCA SimaPro software licensed to Michigan Tech was used for the LCA. In order to conduct a LCA in SimaPro, each aspect of design has to be inputted in the form of an ecoprofile. An ecoprofile accounts for the environmental and social impacts of each item included in the analysis. To utilize certain ecoprofiles in SimaPro some assumptions/simplifications had to be made. For example, ecoprofiles for sand and gravel do not state a porosity or density, so it is assumed that each ecoprofile is appropriate for the design. Also, there is no ecoprofile for plants typically used in wetlands, so the ecoprofile for organic wheat straw is assumed to be the closest ecoprofile. All transportation distances were assumed to be 20 km (Fuchs, 2006 p. 12). After the useful life of the wetland and septic system expires, several options are used for disposal. All plants, sand and gravel used in the wetland are taken out and larndfarmed. The wetland is then filled in with new sand. The wetland liner, pipe, and pump are landfilled. The septic system is assumed abandoned. A transport distance of 20 km was used for all material assigned to be landfilled or landfarmed. An ecoprofile for abandonment was not available, so material for the septic system was included in the landfill ecoprofile but was not considered transported. Table 1 lists the inputs used for the LCA. The Stage column refers to various stages of time that comprise the total life cycle of the wetland and septic system. The Material/Assembly and Processes columns are sub-categories within each stage and the entries in each column are the individual ecoprofiles that were used for the LCA. The Treatment Wetland and Septic System stages include all processes for construction along with materials for construction and disposal. The System Disposal stage identifies how materials from the wetland and septic system will be disposed and included processes for providing disposal. The Life Cycle stage is a combination of all other stages (System Assembly) and also includes processes required over the lifetime of the system. The Life Cycle stage is what SimaPro evaluates to compute the LCA. Figure 3 shows the lifecycle timetable for the wetland and septic system. Table 1: Input Data for SimaPro Stage Material/Assembly Amount Unit Processes Amount Unit Treatment Polyethylene Liner 2880 kg Transport, 40 ton truck 209,328,777 kgkm Wetland PVC Pipe, Schedule kg Transport, 16 ton truck 95,463 kgkm Wheat straw 875 kg Hydraulic digger 2,400 m 3 Sand kg Skid steer loader 2,400 m 3 Gravel kg Septic Pumps 1000 USD Transport, 40 ton truck 11,602,571 kgkm System PVC Pipe, Schedule kg Hydraulic digger 3,311 m 3 Concrete 6783 ft 3 Steel 5481 kg Gravel kg System Hydraulic digger 2,216 m 3 Disposal Skid steer loader 2,216 m 3 Transport, 40 ton truck 71,002,443 kgkm Life Cycle System Assembly 1 unit Transport, 40 ton truck 165,889,148 kgkm Mowing, motor mower 2.35 ha Electricity from coal 500 kwh 8

9 Figure 3: Lifecycle timetable *Adapted from Fuchs (2006) Impact Assessment The Eco-indicator 99 method was used for the impact assessment. This method was chosen because it includes the widest range of environmental impacts of any impact assessment method and it also provides a single score evaluation. The single score evaluation is used to compare different impact categories to each other and the valuation of each category is determined by survey. (Fuchs, 2006 p. 14) The hierarchist with average weighting perspective (H/A) was used from within Eco-indicator 99. This perspective was chosen because it uses a moderate approach. The other possible perspective options, individualist or egalitarian, represent the ends of the spectrum used for evaluation. Table 2 provides a brief explanation of each perspective. Results from each life-cycle stage were then compared to determine the areas that have the greatest impact on the environment (Fuchs, 2006 p. 13). Table 2: Perspective descriptions Perspective Time View Manageability Level of Evidence hierarchist blance between proper policy can inclusion based on short and long term avoid many consensus problems individualist short term technology can only proven effects avoid many problems egalitarian very long term problems can lead to catastrophe all possible effects *taken from PRe' 9

10 Results & Discussion The final analysis included a comparison of two treatment wetland designs. The first system is the one described in the Methodologies & Data section and is the primary focus of this report. The second system is identical to the first with the exception that the synthetic liner was replaced with a one-foot thick clay liner. This was done to provide insight into the affects of one possible alteration to the design of the wetland. Synthetic Liner Design Results The most significant impact categories, as shown in Figure 4, are fossil fuels and respiratory inorganics. Other noticeable categories include carcinogens, land use, and climate change. The single score breakdown in Figure 5 shows how each stage of the life cycle contributes to each impact category. It is clear from this figure that most of the impacts are coming from construction stage of the wetland and septic system compared to operation or disposal. Since Figure 5 shows that mowing has a negligible impact, no work was done to determine the impacts of not mowing. Figure 6 is a single score characterization that provides a proportional composition of each ecoprofile used in the LCA. Figure 6 shows that the majority of the impacts are coming from only a few components including sand, transportation, concrete, and the synthetic liner, which are all included in the construction stage. Figure 7 is a network flow diagram showing the flow and level of impact for major components of the LCA. Note that the y-axis on the graphs is an arbitrary point scale and is used to represent proportional relationships between categories. Figure 4: Weighted single score results 10

11 Figure 5: Single score results by stage Figure 6: Single score characterization 11

12 Figure 7: Network flow diagram Synthetic & Clay Liner Comparison A comparison between treatment wetland designs using synthetic and clay liners revealed very little difference with respect to total environmental impacts (Figure 8). This can be attributed to the fact that only small portions of each design differ (Figure 9). The resulting differences are manifested primarily in the fossil fuel and respiratory inorganics impact categories (Figure 10). The synthetic liner design uses more fossil fuels and the clay liner design produces more respiratory inorganics. Essentially, there is a tradeoff between the two designs, but the final result is the same. The tradeoff occurs due to two major factors including transportation and construction materials (Figure 9). In terms of construction materials, the synthetic liner design uses more sand and a polyethylene liner, which results in higher fossil fuel consumption compared to the clay liner design (Figure 11). Regarding transportation, the clay liner design requires more truck and machine operation to deliver and place the clay, which results in higher respiratory inorganics compared to the synthetic liner design (Figure 12). When using average hierarchist weighting scores, the difference between total environmental impacts is negligible (Figure 8). This is important because the results indicate that choosing between a clay or synthetic liner is not a significant decision overall. However, if decision makers are particularly concerned with environmental impacts concerning respiratory inorganics or fossil fuels, these results may help in choosing a final solution. 12

13 Figure 8: Single score somparison Figure 9: Comparison of inputs 13

14 Figure 10: Comparison of weighted single score results Figure 11: Comparison of contributions to fossil fuel impacts 14

15 Figure 12: Comparison of contributions to respiratory inorganics impacts Discussion One useful purpose of conducting a LCA is to identify which areas of a product or process possess the greatest environmental impacts. Once this has been done, designers can alter portions of the design in an effort to find ways to reduce those impacts. The difficulty in doing this is that SimaPro does not indicate which design elements should be changed and does not offer any suggestions alternatives. This LCA provides easily discernable results when evaluating significant environmental impacts. It is clear to see that the majority of the impacts are manifested in fossil fuels and respiratory inorganics and that the design elements responsible for these impacts are coming from the transportation and construction materials categories. What is not clear, however, is how these impacts will change if portions of the design or assumed construction processes are altered. Design alterations may not have a significant affect on environmental impacts, as is the case with changing the liner from polyethylene to clay. Therefore, prioritizing means to reduce impacts can be suggested, but the specific implications of those changes cannot be known unless additional iterations of the LCA are performed. Based on the results, it appears that first priorities to reduce impacts should be focused on transportation and the fill material used for the wetland. It was assumed that all materials would be transported a distance of 20 km to the construction site and that no in-situ materials were used. Significant impact reductions appear to be possible if in-situ fill material for the wetland could be used instead of bringing sand to the site. This would greatly reduce impacts because it 15

16 would eliminate using sand and would greatly reduce trucking of materials to the site and excavated cut material from the site. Other possible areas of improvement appear to be options for disposal and assumptions for water use. Concerning disposal, the wetland is assumed to be removed and the materials disposed of off site while new material is brought in to fill in the wetland. A potential way to reduce transportation and materials would be to simply abandon the wetland, thus eliminating the need for fill materials and transportation. Regarding water use, if a less conservative per household water usage value were used, the total system would be smaller, thus reducing almost all inputs to the LCA. Altogether, this LCA provides useful insight into the environmental impacts generated from a wastewater treatment wetland system designed to serve a small community. This information can be used by decision makers when deciding upon a solution for wastewater treatment in an environmentally sensitive area or where there are concerns about particular environmental factors. In the future it would be appropriate to consider the requirements for nitrogen removal and to compare this LCA with a LCA for a similar alternative for wastewater treatment. Conclusions The results from this LCA show that the major impacts categories include fossil fuels and respiratory inorganics. The majority of these impacts come from only a few sources including transportation and sand for wetland construction. These results can be used to help improve design by doing things such as utilizing different materials in construction, modifying construction techniques, or by using materials found onsite instead of importing materials to the site. Suggestions for future work include considering the requirements for nitrogen removal and comparing the LCA to a LCA for a similar alternative for wastewater treatment. Outcomes The anticipated outcomes for this project were that the major environmental impacts would come from transportation of materials and water use. The prediction that transportation would be a major factor was correct while that for water use has been voided as water use was eliminated from the LCA. Additionally, the LCA showed that the materials, particularly sand, made significant contribution to environmental impacts. The projects also provided the researcher with experience using SimaPro software, further understanding of treatment wetland design, and greater insight into environmental issues within topics of sustainability. 16

17 References Crites, & Tchobanoglous. (1998). Small and Decentralized Wastewater Management Systems. WCB/McGraw-Hill. Fuchs, V. J. (2006). Life Cycle Assessment of wastewater treatment constructed wetland for design, construction and operation improvements. Michigan Technological University, Civil and Environmental Engineering, Houghton. Fuchs, V. J. (n.d.). Research Projects for S-STEM Grant. Retrieved Nov 30, 2008, from Michigan Tech Sustainable Futures Institute: Metcalf, & Eddy. (2003). Wastewater Engineering Treatment and Reuse. McGraw-Hill. Muga, & Mihelcic. (2008). Sustainability of wastewater treatment technologies. Journal of Environmental Management, Pre`. Eco-indicator 99. Retrieved Mar 12, 2009, from Pre`: WERF. (2006). Small-Scale Constructed Wetland Treatment Systems. London: IWA Publishing. 20

18 Appendix A: LCA Comparisons 21

19 Goal, Scope, and Functional Unit Differences between Fuchs and Seib: 22

20 Input differences between Fuchs and Seib: 23

21 Appendix B: Design Calculations 24

22 Design Calculations for Sub-Surface Flow Wetland all calculations taken from Crites and Tchobanoglous (Ch 9), 1998 Step 1 Determine the temperature adjusted rate constants for BOD and N removal (Temperature of 10 o C used to account for cold weather) BOD N k T = k (T "20) = (1.1d "1 )(1.06) (10"20) = 0.61d "1 k T = k (T "20) = (0.107d "1 )(1.15) (10"20) = 0.026d "1 k=rate constant T=temperature ( o C) Step 2 Determine detention time for BOD removal t = " ln(c /C 0) k apparent ln[(10mg /L) /(90mg/L)] = " = 3.60d 0.61d "1 t=time (days) C=effluent BOD concentration (mg/l) C 0 =influent BOD concentration (mg/l) k apparent =temperature adjusted rate constant (d -1 ) Step 3 Determine BOD loading rate (Do not exceed 100 lb BOD/ac!d) L org = (C 0 )(d w )(")(F 1 ) (t)(f 2 ) = (90mg /L)(2.5 ft)(0.30)(8.34) (3.60d)(3.07) = 50.96lbBOD/ac # d L org =BOD loading rate (lb BOD/ac!d) C 0 =influent BOD concentration (mg/l) d w =water depth (ft)!=porosity t=detention time (days) F 1 =conversion factor, 8.34 lb/[mgal(mg/l)] F 2 =conversion factor, 3.07 ac!ft/mgal

23 Step 4 Determine wetland surface area based on BOD removal A s = (Q ave)(t)(f 1 ) (")(d w ) = (0.04 MGD)(3.60d)(3.07) (0.30)(2.5) = 0.589ac A s =surface area (ac) Q ave =average daily flow (Mgal/d) t=detention time (days) F 1 =conversion factor, 8.34 lb/[mgal(mg/l)] d w =water depth (ft)!=porosity Step 5 Determine wetland surface area for N removal A s = (Q ave)[ln(n inf " N eff )] (k apparent )(d w )(#)(F 3 ) = (0.04 MGD)(133,690 ft 3 / Mgal)[ln((20mg/L) " (5mg/L))] =17.049ac (0.026d "1 )(2.5 ft)(0.30)(43,560 ft 2 /ac) A s =surface area (ac) Q ave =average daily flow (ft 3 /d) N inf =influent N concentration (mg/l) N eff =effluent N concentration (mg/l) k apparent =temperature adjusted rate constant (d -1 ) d w =water depth (ft)!=porosity F 3 =conversion factor, 43,560 ft 2 /ac] Step 6 Determine detention time for N removal t = (A s )(d w )(") (Q ave )(F 2 ) = (17.049ac)(2.5 ft)(0.30) (0.04 MGD)(3.07) =104.13d t=detention time (days) A s =surface area (ac) d w =water depth (ft)!=porosity Q ave =average daily flow (Mgal/d) F 2 =conversion factor, 3.07 ac!ft/mgal

24 Step 7 Determine cross-sectional area (use 10% of hydraulic conductivity to be conservative) XA = Q ave (k)(s) = (0.04MGD)(133,690 ft 3 / Mgal) = 3260 ft 2 (1640 ft /d)(10%)(0.01 ft / ft) XA=cross-sectional area (ft 2 ) Q ave =average daily flow (ft 3 /d) k=hydraulic conductivity (ft/d) S=slope (ft/ft) Step 8 Determine wetland dimensions 8a. Determine minimum width w = XA ft = d w 2.5 ft =1304 ft 8b. Determine design width w=wetland width perpendicular to flow (ft) XA=cross-sectional area (ft 2 ) d w =water depth (ft) L = A s w = (0.589ac)(43,560 ft 2 /ac) = 20 ft 1,304 ft Dimensions Length 20 ft Width 1304 ft Depth 3 ft Surface Area ft ha Plants (plant on three-foot centers) Rows 7 Plants per row 435 Total 3045 plants Mass 875 kg (based on Fuchs, 50 kg waste from 290 plants over 30 years)

25 Liner (assume liner is kg/ft 2 ) A liner ft 2 Mass 2880 kg Pipe (assume linear mass is kg/100 ft for 2" PVC) Inlet Outlet Total Mass 1976 ft 1324 ft 3300 ft 1018 kg Sand & Gravel (assume sand is 1602 kg/m 3 ) (assume gravel is 1682 kg/m 3 ) Sand 30" in wetland ft 3 3" under liner 6520 ft 3 Total ft 3 Mass 2031 m kg Sand - Restoration Fill 36" restoration ft 3 sub-total ft 3 Mass Gravel 6" in wetland ft 3 Mass Excavation 2216 m 3 disposal excavation kg 369 m kg (assume in situ soil is clay loam, density 1280 kg/m 3 ) Volume ft 3 Mass 2400 m 3 construction excavation kg

26 Design Calculations for Septic System all calculations taken from Crites and Tchobanoglous (p 321), 1998 Total Volume (use 5 yr pump out interval) V = 3.65(Q ave )(PF) = 3.65(40,000gpd)(1.5) 7.48gal / ft 3 = ft 3 Tank Layout V=volume (ft 3 ) Q ave =average daily flow (gpd) PF=peaking factor *use two tanks for redundancy *tanks use a shared wall *include a longitudinal baffle in each tank *baffle will help treat scum and add structural integrity *use L:W ratio of 3:1 Total Volume ft 3 Each tank ft 3 depth 15 ft length 54.1 ft width 18.0 ft tank footprint ft 2 total footprint ft 2 Concrete (assume 9" thickness all sides) (assume density is kg/ft 3 ) Volume 6783 ft 3 Mass kg Rebar (use #4 bar) (assume linear mass is kg/ft) (assume two layers of rebar per side laid perpendicular) Length Mass ft 5481 kg

27 Pipe (assume 2 inlets and outlets for each tank) (assume linear mass is kg/100 ft for 3" PVC) Inlets Outlets To pumps Total Mass 12 ft 12 ft 55 ft 79 ft 51 kg Pumps Use 2 pumps for redundancy Pump cost = $500 (arbitrary value) Excavation Sub-grade 0.5 ft Tank depth 15 ft Tank length 54.1 ft Tank width 36.1 ft Cut ratio 4:1 Volume ft 3 Gravel Amount of backfill gravel 3311 m 3 (assume gravel is 1682 kg/m 3 ) Volume 2928 ft 3 Mass 83 m kg

28 Design Calculations for System Septic Pumping/Hauling tank volume ft L sludge density 1 kg/l *assume sludge has same density as water sludge weight kg for cluster system, 10 pumps over 50 years sludge weight kg transport distance: 20 km transport value kgkm Electricity Use based on Fuchs, 3 kwh over 30 years for single home this equates to 0.1 kwh per year at 400 gpd for cluster system, 50 year lifetime 40,000 gpd Electricity 500 kwh Mowing wetland area ha for cluster system, 10 mows over 50 years total ha mowed ha

29 Design Calculations for Disposal assume: all wetland gravel and sand is landfarmed septic tank is abandoned pumps, pipe, liner go to landfill Landfarm plants 875 kg wetland sand 2,957,706 kg wetland gravel 591,541 kg sub-total 3,549,247 kg percent of total % Landfill pumps pipe 1,068 kg liner 2,880 kg sub-total 3949 kg percent of total 0.05 % Abandon (landfill) sub-grade wetland sand 295,771 kg sub-grade septic gravel 132,816 kg restoration wetland sand 3,549,247 kg septic concrete 441,782 kg septic rebar 5,481 kg sub-total 4,425,096 kg percent of total % Total 7,979,167 kg