#67: Insights on Sewage Water Handling Metrics in Constructed Wetlands
From the simplest strawbale pit to systems incorporating Serpentine flows, HRT, horizontal and vertical flows, subsurface flows and surface flows.
Welcome back to our in-depth exploration of ecological systems and their practical applications in urban environments. In our last session, Drop #66, we delved into the vital world of salt marshes, uncovering their indispensable roles in sustaining biodiversity, supporting urban resilience, and promoting environmental sustainability. Today, in Drop #67, we transition these ecological insights into actionable strategies for designing urban wetlands tailored for effective wastewater management. I have just mentioned some of the plants, however, I have seen from experience that there are a lot of other plants suited to these areas as well, not covered in this study, for example in Islamabad, sewage water in streams always have guilds comprising of Ricinus communis, Indian Hemp, Wild Fig trees, Mulberry, cattails and some bulrushes.
Bridging Salt Marsh Functionality with Urban Needs
Our journey continues by translating the lessons learned from salt marsh ecosystems into frameworks that cities can use to address their unique challenges. By focusing on the most suitable plant species, their specific roles in habitat restoration, and effective restoration techniques, we're setting the stage for creating urban wetlands that not only manage wastewater efficiently but also enhance urban livability.
Strategic Plant Selection for Urban Wetlands
From the robust Spartina, known for its resilience in salt-laden soils, to the nutrient-hungry Juncus, each plant we studied in salt marshes offers unique benefits that can be adapted for urban settings:
Spartina spp.: Ideal for areas with saline water runoff, it stabilizes sediments and aids in filtering pollutants.
Juncus spp.: Its ability to absorb excess nutrients makes it an excellent choice for nutrient-rich urban wastewater.
Phragmites australis: Despite its reputation as invasive, its high efficacy in pollutant removal makes it a powerful tool under controlled urban conditions.
Designing a Scalable Urban Wetland System
Let's translate these insights into a scalable model for urban applications. Starting with a basic yet effective household system, we can project these concepts onto a larger, city-wide scale:
Household Model: Incorporating a simple septic system followed by a biofilter consisting of strategically chosen wetland plants, set within a small, manageable wetland area.
Area Requirement and Calculations:
For an average household generating approximately 400 gallons of wastewater daily, and using our initial conservative estimate of 5 gallons per square yard per day for treatment, the required area would be:
Expanding to Serve an Urban Population
Scaling this model to accommodate a city of one million inhabitants:
Total Wastewater Volume: Estimating at 44 million gallons per day based on typical water usage,
Total Required Area:
Leveraging Scientific Studies for Enhanced Design
To ensure our urban wetland designs are both scientifically grounded and practically viable, we draw from several pivotal studies. These studies provide detailed insights into the optimization of wetland systems, particularly focusing on HRT management, pollutant removal efficiencies, and design adaptations that enhance treatment capabilities.
A. Study on Hydraulic Retention Time Optimization
Source: "Enhanced Nitrogen Removal in Constructed Wetlands: Effects of Hydraulic Retention Time and Plant Species" (Journal of Environmental Management, 2020).
Key Insights:
HRT Optimization: Increasing HRT from 12 to 24 hours significantly boosts nitrogen removal efficiency.
Plant Impact: Certain species like Phragmites australis greatly influence the efficiency of nutrient removal due to their specific biological traits.
Quantitative Data:
Nitrogen Removal Efficiency increased from 45% to 89% with extended HRT.
Phragmites australis exhibited a 30% higher nitrogen removal efficiency compared to Spartina alterniflora under similar conditions.
B. Study on Wetland Design and Scalability
Source: "Scalability of Constructed Wetlands for Urban Wastewater Treatment: A Meta-Analysis" (Ecological Engineering, 2019).
Key Insights:
Emphasized modular designs for scalability and effectiveness, noting smaller modules manage pollutants more efficiently due to improved oxygenation and root distribution.
Quantitative Data:
Small modules (100-200 square yards each) were found to remove pollutants by approximately 15% more effectively than larger setups.
C. Wetland-Based Economic Approach for Arid Regions
Key Design and Performance Metrics:
Incorporates both horizontal and vertical subsurface flow constructed wetlands (HSSF-CW and VF-CW), demonstrating diverse flow and retention strategies tailored to specific regional needs.
Quantitative Data:
HSSF-CW expected to remove 96.7% BOD and 70% TP.
VF-CW removes 75% BOD and achieves 92.7% fecal coliform removal.
D. Advanced Treatment of Domestic Sewage
Source: Wang et al. (2020).
Experimental Insights:
Utilized a small-scale movable wetland model to simulate and optimize performance dynamically.
Quantitative Data:
Effective reductions in COD, nitrogen, and phosphorus highlight the potential for adaptable, small-scale wetland applications in urban settings.
E. Study on Pollutant Removal Efficiencies
Source: "Comparative Analysis of Pollutant Removal Efficiency in Urban Constructed Wetlands" (Water Research, 2021).
Key Insights:
This study highlighted the importance of wetland depth and flow configuration, advocating for mixed-depth designs to optimize contact time and enhance sedimentation.
Quantitative Data:
Achieved up to 92% phosphorus removal and over 80% heavy metal removal in varied depth zones.
Integrating Study Insights into Urban Wetland Design
By synthesizing these detailed scientific findings, urban wetland designs can be strategically tailored to include:
Multi-Stage Treatment Areas: Segregating the wetland into specific zones that target particular contaminants, optimized by both HRT and the specific plant species known for their efficacy in removing those pollutants.
Adaptive Modular Design: Implementing scalable and modular wetland units ensures high efficiency and adaptability across different urban contexts, enhancing the system’s resilience and effectiveness.
Comparing Basic and Optimized Systems:
Basic Pit Wetland: While a basic pit styled wetland (septic tank followed by a simple soil and straw bale setup) offers a straightforward solution for small-scale applications, its efficiency is limited by lack of control over factors like flow rate and pollutant load, typically achieving lower removal efficiencies.
HRT Optimized Wetland: In contrast, an HRT-optimized wetland, informed by the studies mentioned, provides a controlled, efficient, and scientifically validated approach to handling larger volumes of wastewater with higher pollutant removal efficacy.
Simplified Wetland Solutions for Arid Regions and Small Towns
For desert areas or small towns lacking access to expert design resources, implementing a straightforward pit wetland construction provides a feasible solution. This simpler method can be effectively utilized with a fundamental understanding of wetland capacity and local plant life.
Design Simplicity: A pit-style wetland, which might include basic layers of soil and straw bales planted with suitable vegetation, offers a straightforward and low-tech approach that local communities can manage.
Baseline Metrics for Design:
Water Treatment Metric: Using a baseline metric of 5 gallons per square yard of day allows for the estimation of the required wetland area based on daily wastewater output.
Population-Based Metric: Alternatively, a metric based on population, such as designing the wetland to accommodate the waste from one person per X number of square yards, can simplify the scaling of the system to match community size.
Plant Selection: Utilizing plants previously discussed, such as Spartina spp., Juncus spp., and Phragmites australis, supplemented with local native species, can enhance the resilience and ecological integration of the wetland.
Advanced Wetland Designs for Areas with Design Expertise
In regions where there is access to more advanced technological resources and expertise, designing wetlands that incorporate optimized Hydraulic Retention Time (HRT) can significantly enhance treatment efficiency.
Advanced Design Metrics:
High-Efficiency Metric: In settings where HRT can be precisely managed, using an advanced metric of 435 gallons per square yard per day leverages the full potential of the wetland system to handle larger volumes of wastewater efficiently.
Technological Integration: These areas can benefit from more complex designs, such as multi-stage systems that incorporate both horizontal and vertical flow wetlands, as highlighted in the studies, to address a broader spectrum of contaminants effectively.
Strategic Application Across Different Settings
For Arid and Remote Areas: The simpler pit wetland approach not only facilitates easier construction and maintenance but also aids in land rehabilitation by using water treatment as a tool to improve soil conditions and support local vegetation growth.
For Urban and Developed Areas: Where expertise and resources are available, applying scientifically validated designs with precise HRT controls ensures maximum efficiency in pollutant removal and supports higher density urban planning needs.
Conclusion
By adapting the wetland design to fit the local context—whether a small town in a desert region or a technologically advanced urban area—both approaches contribute positively to environmental management and community well-being. This tailored strategy ensures that each system not only meets local wastewater treatment needs but also integrates into the broader ecological and socio-economic landscape, promoting sustainability and resilience.
Exploring the Possibilities
What if we could integrate capillary action materials to create self-sustaining water movement in wetlands without relying on mechanical pumps?
This could enable continuous water flow and nutrient distribution, mimicking natural systems more closely while maintaining urban adaptability.
What if elevated wetlands could be constructed on rooftops or above parking lots, transforming underused urban spaces into productive ecological habitats?
These systems could contribute to biodiversity, manage stormwater, and provide recreational spaces, fundamentally changing the urban landscape.
What if optimal nutrient exchange in these wetlands could be achieved through carefully calibrated flow rates that mimic tidal or riverine systems?
This could maximize the ecological productivity of the wetlands and enhance their capacity to purify water, all while using natural models as a guide.
What if we faced challenges in maintaining consistent aeration in these dynamic systems?
Exploring natural aeration methods or low-energy solutions could provide resilience and sustainability to the system’s design.
What if these innovative wetland systems could be designed to scale based on the specific needs and constraints of various urban environments?
Modular and customizable wetland components could be deployed across diverse urban settings, offering tailored solutions to local ecological and water treatment needs.
What if the introduction of such dynamic wetlands could influence microclimates within urban areas, potentially reducing the urban heat island effect?
These systems could offer cooling effects, increased local humidity, and improved air quality, contributing to more livable urban environments.
What if community involvement in the development and maintenance of these wetlands could lead to stronger connections between urban populations and their natural environments?
This could enhance community well-being, increase environmental awareness, and foster a culture of sustainability and conservation. These questions also set the stage for a conversation I was having with Shashi Bhatnagar from BION Labs, regarding wetlands in cities.
Well that’s it for today, for those interested in seeing the calculations for per person, you can see the appendix. See you next time. Thank you for joining me in the exploration.
Appendix
A conversion from gallons per square yard per day to person per square yard per day with assumptions
Assumptions:
Water Usage per Flush: 3 gallons.
Frequency of Toilet Use:
Pooping: 1 time per day.
Urinating: 4 times per day on average (as a middle value of 3-5 times).
Total Daily Water Usage Per Person for Toilet:
Total flushes per day = 1 (pooping) + 4 (urinating) = 5 flushes.
Total water usage per person per day = 5 flushes/day × 3 gallons/flush = 15 gallons/day.
Revised Conversion Calculations:
For Simple Systems (5 gallons per square yard per day):
Capacity: 5 gallons per square yard per day.
Daily Water Usage per Person: 15 gallons per person per day.
Number of Persons per Square Yard:
Each square yard of a simple wetland system can support approximately 0.33 persons, or roughly one person per three square yards.
For Complex Systems (435 gallons per square yard per day):
Capacity: 435 gallons per square yard per day.
Daily Water Usage per Person: 15 gallons per person per day.
Number of Persons per Square Yard:
Each square yard of a complex wetland system can support about 29 persons.