Green Filter for Wastewater Treatment

Construction of a Green Filter for Wastewater Treatment in the Municipality of San Miguel de Sema, Colombia

Project goals

General objective: Reduction of the domestic wastewater inflow to Lake Fúquene.

Specific objective: Establishment of a green filter as an alternative and sustainable wastewater treatment system in San Miguel de Sema, Colombia.

Background

Lake Fúquene (3,260 ha) is a shallow lake with great endemic biodiversity. It provides water for 200,000 people, livestock, agriculture, in addition to sustaining local fishing activities. The lake is nowadays threatened by the loss of hydraulic storage capacity, the introduction of exotic fish and macrophytes, and by a growing eutrophication.

The main cause of eutrophication is wastewater discharge flowing directly into the Lake. The discharge is constituted essentially of domestic wastewater generated by San Miguel de Sema (4,000 inhabitants) and 14 other municipalities within the Lake Fúquene’s basin.

San Miguel de Sema was selected for this green filter pilot project for many reasons. The pronounced local interest towards the project, the characteristics of the wastewater produced by the municipality, and the availability of an adapted terrain were the main incentives for GNF to launch its green filter pilot project. With the success of this project, San Miguel de Sema will also serve as an incentive to expand green filters in Fúquene’s basin for a better wastewater management.

Technology description

Green filters have the advantage of being a low cost infrastructure, their operation and maintenance costs are also low and they do not require skilled workers for maintenance.

As an alternative system for traditional wastewater treatment plants, green filters have the advantage of consuming no energy or chemical products. In addition of having low negative impacts on the environment, they contribute significantly to enhance the landscape´s aesthetics and they attract species associated with natural wetlands.

The proposed project consists of a grid of channels planted with floating aquatic macrophytes and receiving wastewater after a basic primary treatment. The channel structure ensures a functioning similar to a plug flow reactor with a higher efficiency than a constructed wetland.

Mechanisms of contaminants´ removal are the same that occur in wetlands. These mechanisms are a combination of complementary bacterial metabolic routes which vary in abundance according to the depth of the water column. The roots of the macrophytes function as a substrate for bacteria and constitute the core part of the filtering process. Water hyacinth (Eichhornia crassipes) will be used in this green filter because of its highly developed root system allowing therefore a higher contact between the root surface area and the wastewater

When the green filter is correctly maintained, the efficiency of this type of treatment is equal or superior to those of conventional wastewater treatment techniques. The filtering process can remove 80 to 95 % of the BOD, 30 to 80% of nutrients (P and N) and up to 90 % of all pathogens. Also, a correctly maintained green filter will prevent the development of odors and disease vectors.

In conclusion, green filters are adapted to the socioeconomic conditions in rural areas in general and more specifically to the Fúquene rural basin. This filtering technique gathers all advantages of being environmentally friendly and provides efficient wastewater treatment.

Associated Impacts

Wildlife: Green filters often attract wildlife related to wetland ecosystems having therefore a positive impact on biodiversity.

Flora: The land stripping involves removing the plant cover surrounding the lagoon. However the selected terrain has no value for flora conservation and the green filter will not impact the flora biodiversity. A moderate trimming of the surrounding trees might occur in order to increase the solar radiation on the filter.

Soil matrix: Excavation will be shallow and therefore will not destabilise the slope.

Water component: The topsoil in San Miguel de Sema is constituted of waterproof clay that once compacted will prevent any infiltration of wastewater to the groundwater storage. In addition, the discharge water will have its organic load significantly reduced before entering the stream of Santa Ana and a tributary of Lake Fúquene.

Neighbouring population: Literature review showed no nuisance impacts of green filters operating in tropical areas of Southeast Asia and Central America. Therefore the green filter of San Miguel de Sema is not expected to be a source of odors or disease vectors.

Technical data of the green filter of the municipality of San Miguel de Sema

Inflow: 0.9 l/sec = 3.24 m³/h = 77.76 m³/d

Volume of the green filter: 415 m³

Water retention time in the green filter: 7 days

Processing capacity: 0.15 m³/m²*d

Length of the roots: 60 – 90 cm (approx. 66 % of the water level)

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Construction of a Green Filter for Wastewater Treatment in the Municipality of San Miguel de Sema, Colombia

May 2013 – August 2013

Colombia

Kärcher and SIKA

Fundación Humedales