Assessment of Small Mechanical Wastewater Treatment Plants: Relative Life Cycle Environmental Impacts of Construction and Operations
Sussan Moussavi
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04/04/2021
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A life cycle assessment was conducted to understand the environmental impacts of small wastewater treatment plants in Nebraska. This study intends to provide stakeholders with guidance as to what may result in a more environmentally sustainable small wastewater treatment plant.
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- [00:00:01.110]Hi.
- [00:00:01.943]My name is Sussan Moussavi and I'm a PhD student
- [00:00:04.020]in Civil and Environmental Engineering.
- [00:00:06.380]Today I will be presenting a study
- [00:00:07.880]that looked at the life cycle environmental impacts
- [00:00:10.270]of small wastewater treatment plants in Nebraska.
- [00:00:16.000]Many slow-growing, shrinking rural communities
- [00:00:18.710]serving less than 10,000 people
- [00:00:20.490]struggle with aging
- [00:00:21.590]or inadequate wastewater treatment plants,
- [00:00:23.920]and face challenges in constructing
- [00:00:25.540]and operating these facilities.
- [00:00:27.610]Loan programs are available for small communities,
- [00:00:30.010]which often includes sustainability requirements.
- [00:00:32.900]However, there's little guidance as to what may constitute
- [00:00:35.600]a more or less sustainable wastewater treatment plant
- [00:00:38.510]especially regarding the construction of these plants.
- [00:00:41.800]Sustainability assessments have been conducted
- [00:00:44.050]to try to understand the environmental impacts
- [00:00:46.140]of a large wastewater treatment plant.
- [00:00:48.240]But these studies generally found
- [00:00:49.760]the environmental impacts associated with construction
- [00:00:52.470]are small relative to operations.
- [00:00:55.760]However, some studies of large plants did find
- [00:00:58.540]that the environmental impacts
- [00:00:59.850]of the construction phase are important.
- [00:01:02.330]The study theorizes that the environmental impacts
- [00:01:04.710]associated with construction may be even more important
- [00:01:07.670]for a small wastewater treatment plant
- [00:01:09.650]where the initial construction can be a larger share
- [00:01:12.070]of the total life cycle environmental impact.
- [00:01:17.410]The Project goals
- [00:01:18.470]were to first quantify the environmental impacts
- [00:01:21.020]of 16 small wastewater treatment plant case studies
- [00:01:24.050]from a life cycle perspective
- [00:01:25.830]using a detailed site-specific inventory.
- [00:01:29.000]Next, we wanted to identify which factors
- [00:01:31.260]influence any variability seen among the case studies.
- [00:01:35.540]Lastly, we wanted to highlight
- [00:01:37.030]where construction practices could be modified
- [00:01:39.760]to reduce the environmental impact
- [00:01:41.530]of small wastewater treatment plant.
- [00:01:47.910]To conduct this study, we first identified 16
- [00:01:50.760]small mechanical wastewater treatment plants in Nebraska
- [00:01:53.660]to be our case studies.
- [00:01:55.500]The detail inventories were then built using the energy data
- [00:01:58.550]that was collected from community utility bills,
- [00:02:01.330]water and soil characteristics that were collected
- [00:02:03.610]from online databases,
- [00:02:05.410]construction design documents that were also collected
- [00:02:08.040]from communities,
- [00:02:09.430]and air emissions that were estimated based on literature.
- [00:02:12.980]Conventional life cycle assessment framework was followed.
- [00:02:15.630]So a functional unit of one meter cubed
- [00:02:17.830]of treated wastewater was defined
- [00:02:19.590]in order to analyze the results fairly.
- [00:02:22.820]A plant design life of 20 years was assumed,
- [00:02:25.530]a life cycle assessment was completed using various software
- [00:02:29.130]and a multiple regression analysis was performed
- [00:02:31.680]to further interpret the results.
- [00:02:39.740]The results of this study show that both the construction
- [00:02:42.200]and operation phases are important phases
- [00:02:44.470]of a small wastewater treatment plant.
- [00:02:46.680]As you can see here in this figure,
- [00:02:49.070]operating energy dominates
- [00:02:50.530]most of the environmental impact categories
- [00:02:52.820]shown on the X axis,
- [00:02:54.610]although construction,
- [00:02:55.730]which includes both the civil works and the equipment,
- [00:02:58.500]contributes than 5% of the relative environmental impact
- [00:03:02.040]for most of the impact categories as well.
- [00:03:08.760]So first I wanna look at operating energy
- [00:03:11.010]since it's the dominating contributor
- [00:03:12.780]to most environmental impact categories.
- [00:03:15.650]The environmental impact related to operating energy
- [00:03:18.250]becomes higher when the plant is operating inefficiently
- [00:03:21.630]or in other words has a high energy intensity.
- [00:03:24.900]Operating inefficiencies are highlighted
- [00:03:26.810]in this figure here,
- [00:03:28.610]by the plants where there's a large horizontal distance
- [00:03:31.740]between the actual energy intensity
- [00:03:33.730]and the regression estimate energy intensity.
- [00:03:36.870]These inefficiencies were typically due
- [00:03:38.720]to inadequate screening and lack of plant automation.
- [00:03:42.810]It's important to recognize though,
- [00:03:44.210]that as plants become more operationally efficient,
- [00:03:47.500]the environmental impacts from operations will decrease.
- [00:03:51.330]Now those is good,
- [00:03:52.230]considering the large influence that operating energy has
- [00:03:55.490]on the total environmental impact of a plant.
- [00:03:58.070]We wanna be sure that we're not shifting this burden
- [00:04:00.520]to the construction stage
- [00:04:02.070]which will become relatively more important
- [00:04:04.130]as operating energy impacts decrease.
- [00:04:08.640]So to try to reduce the environmental impacts related
- [00:04:11.270]to construction,
- [00:04:12.570]I wanted to identify non-processed related factors
- [00:04:15.310]that may be driving the variability,
- [00:04:17.090]related to the construction impacts.
- [00:04:19.710]I focused on non-process related factors
- [00:04:22.100]because these are the factors that are not dependent
- [00:04:24.310]on design standards.
- [00:04:26.060]We realized that factors like concrete or reinforcing steel
- [00:04:29.610]are heavily dependent on design flow per design guidelines.
- [00:04:33.180]And so I wanted to identify factors
- [00:04:35.220]that were more variable plant to plant
- [00:04:37.080]and would not interfere with any design guidelines.
- [00:04:40.060]And these could potentially present the greatest opportunity
- [00:04:42.750]for environmental impact reduction
- [00:04:44.790]through design modifications.
- [00:04:47.520]I identified these factors to be design flow
- [00:04:50.160]and average flow,
- [00:04:51.340]which the ratio of these two
- [00:04:52.700]is the capacity utilization ratio
- [00:04:55.460]or essentially how over-designed a plant is
- [00:04:57.580]in terms of flow rate,
- [00:04:58.880]and I also found cast iron and aluminum
- [00:05:01.320]to be important factors.
- [00:05:06.750]So now I want to dive a little bit deeper
- [00:05:08.870]into the capacity utilization ratio
- [00:05:10.910]or the over-design of a plant.
- [00:05:12.930]We can see in this figure here,
- [00:05:15.490]especially when you look at the deep oxidation ditches,
- [00:05:18.470]that a high construction impact
- [00:05:20.210]weakly correlates to a high operating energy impact.
- [00:05:23.780]This emphasizes that the over-designed infrastructure
- [00:05:26.500]may override any intended operational benefits.
- [00:05:30.410]In the case of Deep ODs,
- [00:05:32.170]we actually talked with design engineers
- [00:05:34.320]and found that basins were designed larger and deeper
- [00:05:37.690]with the intention of improving energy efficiency
- [00:05:40.280]and treatment.
- [00:05:41.740]But we see here,
- [00:05:43.090]that even with that higher construction impact,
- [00:05:45.890]so meaning more concrete, more reinforcing steel,
- [00:05:49.610]more construction resources
- [00:05:51.850]and the intention to improve energy efficiency
- [00:05:54.320]and treatment,
- [00:05:55.410]the energy impacts still increased.
- [00:06:01.830]So, in conclusion,
- [00:06:03.090]both the operational and construction impacts
- [00:06:05.460]of a small wastewater treatment plant should be considered
- [00:06:08.580]in an environmental life cycle assessment.
- [00:06:11.210]As wastewater treatment plants become more energy efficient
- [00:06:13.880]in the future,
- [00:06:15.000]whether it's because of renewable energy opportunities
- [00:06:17.680]or more plant automation,
- [00:06:19.670]the construction impact
- [00:06:20.920]will become relatively more important to consider.
- [00:06:24.070]Focusing on factors like the capacity utilization ratio
- [00:06:27.820]or limiting the amount of cast iron and aluminum at a plant,
- [00:06:31.500]may reduce environmental impacts related
- [00:06:33.600]to construction
- [00:06:34.700]while staying in compliance
- [00:06:35.930]with the common design guidelines.
- [00:06:39.150]Lastly, I would like to thank
- [00:06:40.480]the Nebraska Department of Environment, Energy
- [00:06:42.730]and local consulting engineering firms
- [00:06:44.820]for their help with this project.
- [00:06:47.060]Thank you.
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