Community Research and Development Information Service - CORDIS

Use of wastewater for irrigation, a global approach blending water treatment, irrigation with various systems on various crops and institutional/organisational aspects

Tertiary wastewater treatment and removal of heavy metals:
Sand filtration: The tertiary treatment by sand filtration is better if the sand layer is thicker. A low flow (6 l/h) is also more favourable than a important flow (9 l/h). The chlorination of the wastewater has a negative influence on the purification efficiency. The choose of sand with particles� diameters lower than 2000 µm is more useful for the filters.

Storage in basins:
The natural decontamination of the treated wastewater in basins is faster if the depth is smaller. By temperatures of 23 to 28 degrees Celsius the reduction of the faecal coliforms by 3 U.log is achieved in 3 days for a depth lower than 1.5m or in 7-10 days if the depth is set to 4m. Storage in basins (2 to 5 months) contributes to the total removal of the faecal contaminants. The storage in basins (more than 2 weeks) gives treated wastewater with a stable bacteriological quality and equivalent to the groundwater.

Heavy metals:
The algae (from an high efficiency algal channel) have a great capacity to fix the heavy metals. This new method of heavy metal removal could be interesting for the biological elimination of the slight trace mineral components and the heavy metals from the wastewater.

The in situ and laboratory tests demonstrate the algal trapping of the toxic metals, particularly Cu and Cd. The trials have demonstrated the negative influence of the heavy metals (Cr > Cd) (in primary treated wastewater) on the plants productions and quality. Also, the soils (particularly, the clayey soils) irrigated with raw wastewater had higher concentrations in heavy metals.

Irrigation by wastewater:
In the framework of the E.C. project AVI-CT94-0002 (final report, 1998, see RCN 13248) an important bibliographical research and trials on cultures fertilization have been described.

Under temperate climate, the fields irrigated by wastewater (coming from a vegetables conditioning industry) have more regular crops than the others and, with a controlled irrigation process, the wastewater have no influence on the nitrogen migration and the leaching. On the contrary, the soils are not enriched in nitrogen and the irrigation enhances the plant uptake.

Under dry climate, irrigations with treated wastewater on citrus trees, hot peppers, egg-plants, carrots, cucumbers, melons and flowers have been tested. The achieved results demonstrate that the crops of citrus fruits, hot peppers and cucumbers are better with treated wastewater than with clear water.

The tests on the melons and flowers were considered satisfactory for the environment preservation by reducing the quantities of wastewater discharged in the streams and by preserving the conventional hydrological resources.

Other trials have been led on greenhouse crops, forest and fodder crops.
Localized irrigation of forest plantations with treated wastewater has led to faster growth and development. This technology is confirmed interesting to contribute to the reforestation and biomass production.

Globally, the qualitative and quantitative results demonstrate the agronomic and economic interests of the treated wastewater reuse. The input of fertilizer is reduced. The qualitative and quantitative yields are better than the yields obtained with clear water complemented with fertilizer.

Moreover, the bacteriological quality is suitable if the irrigation is operated with appropriate methods.

However, (raw or treated) wastewater irrigation cannot be used for crops that are very sensitive to salinity and hydric stress.

The wastewater post-treatments (basins storage, epuvalisation) can be considered as appropriate techniques to optimise the water quality before their use in agriculture : bacteriological quality improvement (and therefore the sanitary quality of the crops) and reduction of nitrates (which may cause water table pollution).

Also the treated wastewater by the filtration-percolation system on sand filters (after an anaerobic sedimentation) are classified Class A (WHO) and can be used for the irrigation of market garden and ornamental crops if appropriate measures and techniques are adopted to preserve the user from contamination. But an appropriate management of the irrigation supply is necessary to avoid salt accumulation in the rhizosphere and the pollution by the nitrates.

The material used for irrigation (sprinklers, drip system) must be correctly selected. The clogging of drippers is high with wastewater treated by the epuvalisation system.

Institutional and organisational aspects of the wastewater reuse :
Analysis of this aspects was elaborated during the project and constitutes the last part of the report (final, 1998).

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