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3.8 Land Treatment Using Trees

Technical Description

Land treatment is defined as the controlled application of wastewaters to the land surface to achieve a specified degree of treatment through natural physical, chemical, and biological processes within the soil-plant-water matrix. This method includes the utilization of nutrients in the wastewater for wood production and groundwater recharge. In Hungary, the most extensively used irrigation method is the "slow rate" process using poplar trees, while, in Poland, willow trees are used. Both trees use the nutrients and evapotranspirate the wastewater very efficiency. The average annual loading rate of the slow rate process is 2 to 2.5 m/year. Soils ranging from clay loams to sandy loams are suitable for irrigation. Soil depths should be at least 0.3 m of homogeneous material. If the site drainage is poor, underdrains may be required. Wastewater is discharged into small flooding basins or irrigation furrows, which are located between two rows of trees, by various means including gravity flow and pumped flow from primary treatment plants. The primary treatment is provided by bar screens and calcium-hydrate dosing.

Figure 5. Flow chart of the land treatment plant.

Extent of Use

This technology is extensively used in Hungary, and is coming into use in Poland at a few sites.

Operation and Maintenance

The system does not require highly trained personnel for maintenance. Maintenance services include cutting the old trees, plowing, flooding the basins, etc.

Level of Involvement

This technology is implemented at the local administration and household levels.

Costs

Given the need for pre-treatment of the effluent applied to the forest plantations, the cost of a 500 m³/d capacity plant is approximately $200 000. Wood sales should offset a portion of this initial investment cost.

Effectiveness of the Technology

The technology can produce an expected average quality effluent with less than 2 mg/l BOD5, 1 mg/l suspended solids, 0.5 mg/l ammonium-nitrogen, 3 mg/l total nitrogen, 0.1 mg/l total phosphorus, and no faecal coliforms. The treatment efficiency is controlled by the application rate, and the process should be monitored by sampling and analysing the surface drainage and well waters.

Suitability

This technology is suitable for treating municipal wastewater, wastewater from solid waste disposal sites, wastewater plants, and contaminated floodwaters. This method is also acceptable for use by small settlements (to treat night soil and sewage water) and for treatment of agricultural wastes (e.g., liquid manure, wastewater from the food processing industry, etc.)

Advantages

This technology not only results in water savings, by replacing freshwater used to irrigate tree plantations, but also produces an economic return on the marketable woods. It has a low energy requirement and does not require sludge. The tree systems are easy to create and inexpensive to maintain.

Disadvantages

Wastewater applications may result in possible groundwater contamination. The ability to irrigate the wastewater depends on both soil and climate conditions, and may require additional storage to be provided during winter and periods of cold weather. The technology requires a large area of land, which may need to be buffered from surrounding land uses by a protective zone due to odour emissions and similar environmental impacts.

Cultural Acceptability

This is an acceptable wastewater treatment technology. However, it may not be acceptable for people whose estates border upon treatment area.

Further Development of the Technology

This is a fully developed technology.

Information Sources

Petèr Kovac and Dr Kornèlia H. Kocsis, Felsö - Tisza - Vidèki Környezetvèdelmi Felügyelösèg, 4400 Nyiregyhàza, Szèchenyi u.19, Hungary, Tel. (36-42) 310 155, fax: (36-42) 310 713.

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• International Year of Forests
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• World Environment Day
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• UNEP Campaign
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