Wastewater Treatment through Constructed Wetlands: Florence, Italy

Wastewater Treatment through Constructed Wetlands: Florence, Italy (Masi & Martinuzzi, 2007)

“Constructed wetlands”, or “CWs”, have become a new vogue in wastewater treatment techniques of late. Archetypically implemented across the European continent and replicated around the globe, these systems that upscale the natural principles of soil substrate filtration have been manipulated to suit a variety of needs. Among them are the treatment of black water, grey water, rainwater, landfill leachate, and sludge contaminated by point-source pollution, as well as the treatment of water that has been diffusely polluted by various run-off sources. This particular paper cited largely successful CW applications in France, Syria, and Morocco specifically; however many other regions were reported to have followed suit due to the systems’ low maintenance requirements and prospective potential to recycle water resources.

There are two overarching divisions of the CW concept: surface-flow (FWS) or subsurface-flow/ “reed bed” (RBTS) treatment systems. In the surface-flow variety, most often chosen for tertiary treatment, lesser investment cost, or heightened wildlife habitat value, the wastewater is directed through a “shallow basin with emergent and submerged macrophytes” (Masi & Martinuzzi, 2007) in order to produce an acceptably decontaminated effluent. Subsurface or “reed bed” wetlands are further divided into Horizontal Flow (HF) and Vertical Flow (VF) varieties based on their direction of water flow. These systems consist of gravel-type substrates permeated by the roots of reeds or other appropriate plant life, making them the more effective option for filtering solids from wastewater.

The “optimized” constructed wetland design discussed in this Florence, Italy study is a combination of HF and VF subsurface wetlands, known as a “hybrid system”. As illustrated in the schematic above, mixed grey and black water from the 140 p.e. load of this “medium scale tourist facility” (Masi & Martinuzzi, 2007) is fed into the initial HF bed by a loading-control pump to account for fluctuating wastewater production, and from there proceeds via two independent siphons to a secondary VF bed lined with HDPE geo-membrane. This was accomplished within a design that minimizes both operational issues like clogging and evapotranspirational water loss within the relatively low dimensional requirements of 0.9 m maximum depth and respective 160 m² and 180 m² surface areas for the HF and VF flow beds.

After extensive sampling and analysis, the final effluent of this hybrid system has exhibited mean overall removal rates of 84% TSS, 94% COD and BOD, 86% NH₄⁺, 60% total nitrogen, 94% total phosphorous, and a 99.93 – 99.99% removal rate for total coliforms, faecal coliforms, faecal streptococci, and E. coli. This substantial level of purification has recently earned the hotel permission to redirect the hybrid effluent into irrigation for its gardens, with the only additional recommendation being to install a UV lamp in the “reuse tank” for ensured E. coli control.

The Florence hybrid system study presents a few key aspects of wastewater treatment systems that promise potential in our own project:

• The effectiveness of this reasonably straight-forward system in wastewater filtration is encouraging both as a direct option for Monwabisi Park and in the affirmation that systems with a clever use of low technology can still produce satisfactory results.
• The hybrid system “could be adaptable to hot climate countries” (Masi & Martinuzzi, 2007), particularly due to its control of evapotraspiration through minimized surface area. North African applications of hybrid systems have already been explored according to the paper, and appear to be attractive wastewater treatment options for the area. The intra-continental conversion of the system to South Africa, then, would be expected to entail fewer adaptations and complications than directly imitating the Italian example.
• The study provides methodical sampling and analysis procedures for determining the effectiveness of wastewater treatment systems that could be applied to the system options outlined within our own project.
• The approximately 15mg/L average remaining nitrogen content annually of the final effluent “provides a useful nitrate concentration in the outlet for reuse in irrigation” (Masi & Martinuzzi, 2007) that is within the WHO and Italian regulations. Grey water reuse in agriculture is one avenue of resource conservation our project aimed to explore, and the sanctioned example of this study provided a prime launching point for our understanding of how this capability is applied and approved by the necessary institutions.