Constructed Wetlands As A Practical Tertiary Treatment Strategy
Municipal wastewater treatment plants are under growing pressure to produce higher-quality effluent while controlling energy use, chemical demand, and operating complexity. Tertiary treatment, which follows primary and secondary processes, can provide the final polishing needed for reuse, discharge compliance, or protection of sensitive receiving waters.
Constructed wetlands offer a nature-based approach to this polishing step. Carefully designed basins use wetland plants, porous media, microbial communities, and controlled hydraulic flow to remove residual suspended solids, nutrients, pathogens, and organic matter from secondary effluent.
This case study examines a representative Los Angeles Basin application. The facility had reliable secondary treatment but needed an additional barrier before discharge to a nearby surface-water system. The project team evaluated a hybrid constructed wetland as an alternative to expanding conventional filtration and chemical treatment.
Why Tertiary Treatment Was Needed
The wastewater facility served a growing urban service area with limited room for major expansion. Its secondary process consistently reduced biochemical oxygen demand and suspended solids, yet the final effluent still contained measurable concentrations of total nitrogen, phosphorus, fine particles, and indicator organisms.
Seasonal water-quality objectives created additional pressure. During dry weather, the receiving water had low dilution capacity, making small increases in nutrient or pathogen loads more significant. The agency also wanted an approach that could complement future water reuse without committing immediately to a large advanced treatment plant.
The planning team compared a constructed wetland with cloth filtration, disk filtration, tertiary membrane systems, and expanded chemical nutrient removal. Each alternative could meet the design objectives, but the wetland required less mechanical equipment and offered opportunities for habitat creation and public education.
Site And Treatment Concept
The selected site included an underused parcel downstream of secondary clarifiers and upstream of the final discharge point. A preliminary screening system removed coarse material, while a flow-splitting structure directed a controlled portion of the effluent into parallel wetland cells. Parallel cells allowed operators to isolate one basin for maintenance without interrupting treatment.
The treatment train used a hybrid configuration. Horizontal subsurface-flow zones provided contact with gravel and root-zone microorganisms, while shallow free-water cells supported sunlight exposure, sedimentation, plant uptake, and additional biological activity. A final polishing cell improved hydraulic distribution and reduced the risk of short-circuiting.
Native or regionally appropriate vegetation was selected for durability, habitat value, and tolerance of intermittent loading. Planting plans considered cattails, bulrushes, rushes, and other emergent species, while avoiding invasive plants that could spread beyond the treatment area. The design also included access roads, sampling points, mosquito management measures, perimeter fencing, and provisions for eventual expansion.
Hydraulic residence time was a central design variable. Too little detention would limit treatment, while excessive detention could increase land requirements and create stagnant zones. Engineers used flow modeling, tracer testing, and conservative peak-flow assumptions to size the cells and establish operating levels.
Monitoring Results And Treatment Performance
During the first year, the agency monitored influent and effluent from each wetland cell. Sampling included total suspended solids, biochemical oxygen demand, ammonia, nitrate, total nitrogen, total phosphorus, turbidity, E. coli, temperature, dissolved oxygen, pH, and conductivity. Operators also tracked water levels, odors, vegetation coverage, mosquito activity, and sediment accumulation.
The wetland performed best when secondary effluent quality was stable and hydraulic loading remained within the design range. Suspended solids and turbidity declined through settling and filtration. Ammonia conversion benefited from oxygen transfer near plant roots and at the free-water surface, while denitrification occurred in low-oxygen media zones containing available carbon.
Nutrient removal varied more than solids removal. Plant uptake contributed during the growing season, but long-term nitrogen reduction depended primarily on microbial processes and sufficient anoxic volume. Phosphorus removal was strongest early in the operating period, when clean media provided more adsorption capacity. The team therefore treated phosphorus performance as a declining design function rather than a permanent benefit.
The following planning results are illustrative ranges for a well-operated hybrid system, rather than a guarantee for every site:
| Parameter | Secondary Effluent | Wetland Effluent | Primary Treatment Mechanism |
|---|---|---|---|
| Total suspended solids | 8–18 mg/L | 2–8 mg/L | Settling and filtration |
| Biochemical oxygen demand | 8–20 mg/L | 3–8 mg/L | Microbial oxidation |
| Ammonia nitrogen | 2–8 mg/L | 0.5–3 mg/L | Nitrification |
| Total nitrogen | 10–22 mg/L | 6–16 mg/L | Nitrification and denitrification |
| Total phosphorus | 1.5–4 mg/L | 1–3.5 mg/L | Media adsorption and uptake |
| Turbidity | 4–12 NTU | 1–5 NTU | Sedimentation and plant-root filtration |
Operations And Maintenance Lessons
The project demonstrated that a constructed wetland is an engineered treatment asset, not a passive landscape feature. Operators inspected inlet structures, water levels, embankments, underdrains, vegetation density, and flow distribution on a defined schedule. Small hydraulic problems were corrected before they affected the entire treatment area.
Vegetation management required seasonal planning. Dead stems were selectively removed to preserve conveyance and maintain access, while excessive growth near inlet zones was cleared to prevent clogging. The agency also maintained an invasive-species response plan and monitored plant health as an early indicator of hydraulic or water-quality stress.
Pretreatment proved essential. Grit, screenings, algae, and excess biological solids can reduce pore space and cause surface ponding. Routine cleaning of screens and upstream channels protected the wetland media. Where phosphorus limits were especially strict, the team retained the option of adding a small chemical polishing step rather than forcing the wetland to carry the entire removal burden.
The facility created standard operating procedures for bypasses, storm events, freezing conditions, pest control, and unexpected toxicity. This gave operators practical control over a treatment process that naturally changes with weather, plant growth, and loading conditions.
Benefits And Tradeoffs
The wetland reduced reliance on blowers, pumps, and chemical feed systems compared with several conventional alternatives. Its low-energy profile supported the agency’s greenhouse-gas reduction goals, while the visible vegetation and open-water features created a platform for environmental education.
Land demand was the clearest disadvantage. Urban agencies may find it difficult to dedicate several acres to treatment, particularly where land values are high. Hydraulic constraints, groundwater conditions, floodplain requirements, and permitting restrictions can further limit site selection.
Performance also changes over time. Media can clog, plants can become overgrown, and phosphorus adsorption capacity can decline. Seasonal temperature changes affect microbial activity, and intense storms can introduce high flows that overwhelm normal detention time. These issues do not eliminate the value of wetlands, but they require realistic performance targets and long-term asset management.
Public acceptance improved when the agency explained that the wetland was part of a monitored treatment train. Interpretive signs, controlled viewing areas, and clear information about reclaimed water and public health helped distinguish the facility from an unmanaged marsh.
Practical Priorities For Future Projects
Agencies considering a similar project should connect the wetland design to a specific water-quality objective instead of treating nature-based treatment as a general solution. A system intended for nitrogen reduction will require different media, oxygen conditions, residence time, and monitoring than one focused on turbidity or pathogen reduction.
The project team also benefited from involving operators during preliminary design. Their experience with seasonal flows, equipment access, sampling routines, and maintenance constraints improved the final layout. Technical exchanges through regional water events can help professionals compare lessons from pilot systems and operating facilities.
Key recommendations include:
- Define effluent limits, seasonal flows, and reliability targets before selecting the wetland configuration.
- Provide robust pretreatment and accessible isolation points to protect media and simplify maintenance.
- Use parallel cells so operators can rest, inspect, or repair one unit while others remain in service.
- Establish a monitoring plan that separates short-term plant uptake from durable microbial treatment.
- Reserve space and hydraulic flexibility for future filtration, disinfection, or nutrient-polishing upgrades.
A pilot or demonstration cell is especially valuable where local data are limited. It can reveal actual hydraulic conductivity, plant survival, mosquito risks, nutrient behavior, and maintenance costs under site-specific conditions.
Building A Reliable Treatment Program
The central lesson from this case study is that constructed wetlands work best as a carefully managed component of a broader treatment strategy. They can polish secondary effluent, reduce energy intensity, improve site aesthetics, and support environmental education, but they should be designed around measurable performance requirements.
For water and wastewater professionals in the Los Angeles Basin, the most useful next step is to bring treatment objectives, operations experience, regulatory expectations, and lifecycle costs into the same conversation. Engineers, operators, consultants, and agency staff can evaluate project concepts through technical workshops, facility tours, and professional development programs. Agencies seeking a focused discussion about a potential application can contact LABS of CWEA to connect with the regional water environment community.
Use the case study as a starting point for a screening evaluation, then verify the concept with site data, pilot testing, lifecycle analysis, and an operations plan that remains practical long after the vegetation is established.