#68: Designing Natural Wetlands for Urban Sewage Treatment based on pollutants
With A Focus on plant selection based on Sewage Quality and economic benefits to enhance the nutrient spread and restore nutrient cycles
In our ongoing exploration of urban ecology, we've delved into topics like salt marshes (Drop #66), wastewater management strategies (Drop #67), and the principles of wetland design. Today, in Drop #68, we'll focus on the specific roles of different wetland zones, the best plant choices for maximum pollutant removal, and the economic potential of these natural filtration systems.
Constructed wetlands offer a sustainable solution to urban wastewater challenges by mimicking the processes of natural wetlands. They can be engineered to remove pollutants like zinc, cadmium, and mercury. In this discussion, we'll cover strategic plant selection for targeted pollutant removal, the different zones within a wetland system, and how to incorporate economically beneficial plants to enhance viability.
A constructed wetland typically consists of several zones, each playing a crucial role in the overall treatment process. Understanding these zones is key to optimizing the wetland's effectiveness. We'll delve into these zones and explore how a careful selection of plants – both native and potentially invasive (with appropriate controls) – can maximize both wastewater treatment and economic returns.
Let’s dive into the design of a constructed wetland and compare it to a simpler natural ecosystems design. The intent here is not to pitch one approach vs. the other, both approaches have their merits and demerits, one may select any of the two approaches or a hybrid depending on the resources, scale and economic consideration.
Design of a Constructed Wetland Based on Sven Erik Jørgensen and Marcos von Sperling’s work
The design proposed by Sven Erik Jørgensen and Marcos von Sperling in their work on ecological engineering for wastewater treatment involves a sophisticated and technologically enhanced approach to constructed wetlands. This design incorporates multiple stages and zones, each equipped with specific engineering controls to optimize the treatment of urban wastewater.
Here’s a brief outline of their design:
Influent Zone: This initial zone is responsible for receiving and pre-treating the wastewater. It typically includes mechanisms such as screens for removing large solids and a primary clarifier to settle particulate matter before the water enters subsequent treatment zones.
Nutrient Removal Zones: These zones are specifically engineered to remove nutrients such as nitrogen and phosphorus. They often incorporate constructed wetland cells filled with a substrate (e.g., gravel, sand, or soil) that supports the growth of nutrient-accumulating plants or algae. Controlled water flow through these zones ensures optimal residence time for maximum nutrient uptake.
Aeration Zones: To enhance the breakdown of organic matter and increase microbial activity, these zones are equipped with aerators or mechanical agitators that inject air into the water, promoting aerobic decomposition processes.
Polishing Zone: Serving as the final treatment stage, this zone utilizes a combination of emergent and terrestrial plants to further refine the water quality. Natural processes like sedimentation, filtration, and additional microbial activity here help remove the last traces of contaminants.
Effluent Collection and Discharge: After treatment, the water is collected in an effluent basin or channel, where it is monitored for quality before being discharged into natural water bodies or reused.
Analysis of the Approach
High Dependency on Technology: Their model relies heavily on mechanical and engineered systems like pumps and aerators, which require continuous maintenance and energy inputs, potentially increasing the operational costs.
Complex Management: The need for precise control over flow rates, aeration, and nutrient removal adds layers of complexity in management, demanding specialized skills and constant monitoring.
Scalability Issues: While the system is designed to be modular, the complexity and cost may limit scalability, particularly in regions with limited resources.
Simpler Natural Wetland Approach
Following the analysis of the high-tech approach, a simpler and more natural wetland design might be proposed, consisting of five zones:
Sedimentation Zone: Acts as the first line of defense, capturing sediment and large particles from incoming wastewater, reducing the load on subsequent zones.
High Nutrient Uptake Zone:
Algal Ponds: Utilize microalgae to rapidly absorb dissolved nutrients such as nitrogen and phosphorus from the water.
Macrophyte Subzone: Follows the algal ponds, using plants like water hyacinth and duckweed, which are capable of high nutrient uptake and also begin addressing heavy metal removal.
Secondary Uptake Zone: Features plants that can take up remaining nutrients and contaminants. This might include deeper-rooted plants capable of accessing contaminants that float past the initial zones.
Polishing Zone: Uses a combination of aquatic and marginal plants to fine-tune water quality, ensuring compliance with discharge standards.
Advanced Aeration Zone: Natural cascades or low-energy water movements increase oxygen levels, promoting aerobic microbial activity without the need for mechanical aeration.
This simpler approach reduces reliance on mechanical systems, focusing instead on maximizing the ecological processes inherent to wetlands. It's generally less costly to implement and maintain, offers greater resilience to fluctuations in water quality and volume, and can be more easily adapted and scaled to meet local conditions. This natural system not only effectively treats wastewater but also enhances local biodiversity, providing habitat and promoting ecological balance.
Building on the design of the natural wetland system for effective wastewater treatment, here are specific plant species recommended for each zone within the system. These plants were selected for their efficacy in treating wastewater and their additional ecological and economic benefits.
Detailed Plant Species Recommendations for Each Zone
1. Sedimentation Zone
Function: Traps and settles out sediments and particulate matter.
Recommended Plants:
Common Rush (Juncus effusus): Tolerant of waterlogged conditions and effective in trapping particulates.
Sweet Flag (Acorus calamus): Stabilizes sediments with its dense rhizome networks.
Soft Rush (Juncus articulatus): Thrives in wet, muddy conditions, aiding sediment capture.
Creeping Bur Reed (Sparganium americanum): Effective in stabilizing sediments and supporting diverse aquatic life.
Blue Flag Iris (Iris versicolor): Not only helps with sediment trapping but also adds aesthetic value to the wetland.
2. High Nutrient Uptake Zone
Algal Ponds
Recommended Algae:
Spirulina (Arthrospira platensis): High nutrient uptake, used in dietary supplements.
Chlorella (Chlorella vulgaris): Effective in removing nitrogen and phosphorus.
Macrophyte Subzone
Recommended Plants:
Water Hyacinth (Eichhornia crassipes): Rapid growth and high nutrient uptake capability.
Duckweed (Lemna minor): Efficient at absorbing excess nutrients and easy to harvest.
Water Lettuce (Pistia stratiotes): Effective in nutrient reduction and provides surface coverage.
3. Secondary Uptake Zone
Function: Additional removal of nutrients and fine particulates.
Recommended Plants:
Cattail (Typha spp.): Excellent for nutrient uptake and provides habitat for wildlife.
Reed Canary Grass (Phalaris arundinacea): Aggressive grower, good for nutrient absorption.
Horsetail (Equisetum spp.): Effective in removing heavy metals from water.
Pickerelweed (Pontederia cordata): Enhances aesthetics and supports biodiversity.
Bulrush (Schoenoplectus spp.): Effective in final nutrient polishing and stabilization.
4. Polishing Zone
Function: Final refinement of water quality.
Recommended Plants:
Marsh Marigold (Caltha palustris): Enhances microbial activity and water clarity.
Swamp Milkweed (Asclepias incarnata): Attracts pollinators and removes residual nutrients.
Arrowhead (Sagittaria latifolia): Effective in final nutrient and particulate removal.
Blue Vervain (Verbena hastata): Contributes to the biodiversity and aesthetic value.
Joe-Pye Weed (Eutrochium purpureum): Supports wildlife and helps in removing trace contaminants.
5. Advanced Aeration Zone
Function: Natural aeration to support aerobic microbial processes.
Design Features: Incorporates cascades and gentle slopes to increase water oxygenation.
Recommended Plants: No specific plants are needed for aeration itself, but surrounding areas can be landscaped with native grasses and flowering plants to enhance the ecosystem's aesthetic and biodiversity aspects.
Implementation and Monitoring
For successful implementation:
Carefully plan the spatial arrangement of plants to ensure optimal growth conditions and water treatment capabilities.
Regularly monitor plant health and water quality to assess the efficacy of the treatment zones and make adjustments as needed.
Now, let’s move onto a Tailored mix.
Optimized Plant Selection for Multi-functional Wetlands
The design of the wetland is organized into zones, each with specific plants chosen for their ability to remove or stabilize different metals from the water.
Here is a detailed look at the plant selection for each zone:
1. High Nutrient and Heavy Metal Uptake Zone:
Algal Ponds: Target the removal of nutrients and lighter metals.
Spirulina (Arthrospira platensis): Besides nutrient uptake, it shows potential in binding with heavy metals.
Chlorella (Chlorella vulgaris): Known for its efficiency in absorbing mercury and cadmium from water.
Macrophyte Subzone: Focuses on higher biomass producers to sequester heavy metals.
Water Hyacinth (Eichhornia crassipes): This plant is known for its rapid uptake of nutrients and some heavy metals, and its biomass is useful for producing biofuel and biodegradable products. Highly effective in removing zinc and cadmium.
Duckweed (Lemna minor): Efficient in accumulating iron and other metals. Highly effective at removing zinc and cadmium, with high protein content that makes it suitable for animal feed and fertilizer if within safe limits.
Ricinus communis (Castor Plant): Added for its ability to absorb various pollutants including soil contaminants. The seeds produce castor oil, which has applications ranging from lubrication to pharmaceuticals, adding significant economic value.
2. Secondary Treatment Zone for Additional Heavy Metals and Aromatics:
This zone leverages the abilities of deeper-rooted plants to continue the removal process and stabilize contaminants.
Cattail (Typha spp.): Known for its capability to remove and stabilize cadmium and zinc.
Reed Canary Grass (Phalaris arundinacea): Assists in the uptake of iron and provides substantial biomass.
Willow (Salix spp.): Excellent for phytoextraction of heavy metals like zinc and cadmium. The quick growth of willows provides ample biomass for crafting and construction.
Lavender (Lavandula angustifolia): Produces essential oils, offering commercial avenues through aromatherapy products, and contributes to air purification by emitting aromatic compounds.
Eucalyptus (Eucalyptus globulus): Known for its rapid growth and ability to purify air, eucalyptus trees can absorb a range of pollutants including mercury. The wood and essential oils are economically valuable.
3. Polishing Zone for Final Treatment and Aesthetic Value
The final refinement of water, focusing on removing trace amounts of metals and improving water clarity
Alpine Pennycress (Thlaspi caerulescens): A hyperaccumulator of zinc and cadmium, potentially useful in remediation of contaminated soils.
Horsetail (Equisetum spp.): Effective in refining the removal of iron and other residual metals.
Bamboo (Various species): Effective in stabilizing banks and absorbing residual pollutants, with a fast-growing nature that makes it an excellent resource for sustainable construction and paper industries.
Reed (Phragmites australis): Useful in final pollutant uptake and soil stabilization. Reeds can be harvested for use in thatching, paper making, and as biomass for energy production.
Mint (Mentha spp.): Enhances air quality with its aromatic properties and provides raw materials for culinary and herbal medicine industries.
Implementation and Monitoring
For successful implementation:
Plant Placement: Careful placement of plants to ensure optimal growth conditions and water treatment capabilities. Ricinus communis should be strategically placed to maximize its pollutant uptake without competing excessively with other plants for resources.
Harvesting Strategy: Establish a harvesting strategy for Ricinus communis to safely extract and process castor seeds, considering their toxic properties. Similarly, manage harvesting of other biomass like water hyacinth and bamboo to sustain the system's health and productivity.
Regular Monitoring: Monitor the health of the wetland and the efficacy of pollutant removal regularly. This involves checking the concentrations of heavy metals in the water and the biomass growth rates of the plants.
This tailored approach not only addresses the primary concern of heavy metal removal but also maximizes the economic outputs of the wetland system through the production of valuable by-products like biofuels, essential oils, and castor oil. By leveraging these multifunctional capabilities, the wetland serves as a sustainable solution for urban wastewater management while contributing to local economic development and ecological sustainability.
Conclusion and Future Considerations for Wetland Design
As we conclude our discussion on the tailored plant mix for treating heavy metal pollutants in urban wastewater using constructed wetlands, it's clear that this approach not only addresses environmental concerns but also enhances biodiversity and provides economic benefits. The integration of plants like Ricinus communis and systems such as the Advanced Aeration Zone highlights the potential of these natural systems to efficiently manage urban wastewater in a sustainable manner.
Exploring Future Possibilities in Wetland Design
Enhanced Aeration Zone Innovations
Natural Cascades and Unpowered Pumping Solutions: What if we could design a wetland system where water is elevated through natural landforms or inventive, unpowered solutions, creating cascades that aerate the water without the need for electricity? Such systems could mimic natural stream processes, enhancing oxygenation and facilitating microbial breakdown of pollutants more effectively.
Integration with Freshwater Mangrove Areas
Flood Management and Closed Loop Systems: What if these wetland areas could also manage flood overloads? Designing them to temporarily hold and then slowly release floodwaters back into the main treatment zones could create a resilient, closed-loop system that handles extreme weather events. Introducing freshwater mangroves could add additional resilience and treatment capabilities, as these plants are adept at water filtration and heavy metal uptake.
Bioindicators and Ecological Monitoring
Use of Sensitive Plants and Organisms: What if we included plants known for their sensitivity to specific pollutants, which change color or form in response to environmental changes? For instance, certain fern species can indicate arsenic presence. Additionally, incorporating organisms like mussels or certain fish species, which react visibly to pollutants, could provide ongoing monitoring of water quality without continuous mechanical testing.
Microplastic Removal Capabilities
Specialized Plant and Microbial Systems: What if the wetland system could also target the removal of microplastics? Research into plants and associated microbial communities that can break down or accumulate microplastics could be integrated into the wetland design. For example, fungi known for their ability to degrade plastics could be a focal point of research for integration into these systems.
These enhancements aim to boost both the functionality and sustainability of constructed wetlands, while also prompting us to consider their integration into urban settings more effectively. By exploring these "what if" scenarios, we aim to push ecological engineering further, developing systems that not only treat wastewater efficiently but also address wider environmental challenges such as flood management, pollution monitoring, and microplastic removal. As we refine these systems, the potential for dramatically positive impacts on urban environmental management increases, promising a brighter future for cities facing complex pollution issues.
In our ongoing exploration of natural wetlands for sewage treatment and nutrient cycle restoration, a pivotal question emerges: Could mining phosphorus from sewage water simultaneously manage wastewater and alleviate phosphorus scarcity? In my next post, I'll dive into this exciting possibility, exploring the potential for integrating phosphorus recovery into wetland management practices. Stay tuned as we investigate whether this innovative approach can benefit both our environment and agriculture. Thank you for your time.