Wastewater from the canning industry is not harmful to the agricultural soil

Irrigation with wastewater from the canning industry is not harmful to the quality of agricultural soil and may even, in some cases, improve it. This is the conclusion of Iñigo Abdón Virto Quecedo in his PhD thesis defended at the Public University of Navarre.

Permanent and rotational crops

The vegetable canning industries, by the very nature of its processes, produce a considerable volume of low-contaminant effluents.

A research project began in 1996 to determine the viability of agricultural irrigation as an alternative to the dumping of this wastewater. To this end, three trials were carried out, two in Villafranca and one in Valtierra, locations in the Ebro river basin region of Navarre with vegetable canning plants located on three agricultural soils representatives of the zone.

Iñigo Virto’s research project involved the characterisation of the soils at the three trial sites in order to subsequently evaluate the effect of irrigation with waste waters from industrial canneries, as a function of the doses received and the type of handling/ kind of irrigation undertaken for the crop.

The viability of these kinds of systems depends fundamentally on the nature of the soil under use, the irrigation system and the type of crop planted. These are the factors that have to be considered when installing irrigation systems using wastewater.

Thus, in soils on which there is permanent meadowland, such as alfalfa or
ray grass meadows and where sprinkler irrigation has taken place, a “positive evolution” of the soil was observed, given that “this water is not contaminant but provides a significant injection of organic material and other types of elements that can be positive for the development of the soil”. Nevertheless, on these types of soils any kind of irrigated meadow crop enhances the soil, independently of the type of water used for the irrigation.

Also, on the soils where rotation crops were planted (in general, maize, wheat and sunflower) and were flood-irrigated, soil evolution was different due to this handling factor. In short, this investigation showed that soil evolution was not as linked to wastewater irrigation as to the handling and quality of the soil.

So, if the crop type is suitably selected and the irrigation system is perfectly viable with this type of water, from the point of view of the soil.

Trials in Valtierra and Villafranca

The trials were undertaken on three stretches of land attached to their corresponding canning factories. Each estate was divided into smaller plots which were irrigated with differing doses of waste water, although each of the three pieces of ground was received the same total amount of water, i.e. within each of the three trial estates, the proportion of waste water and clean, normal water for irrigation as well as the type of crop and irrigation system varied. Moreover, each estate was representative of different characteristic soils of the Ribera (Ebro river basin) area.

Firstly a prospection of the three soils was carried out. After the four-year trial, new profiles were excavated at each of the three areas of land in question. Moreover, over the trial period, samples were taken in order to study the development of their physical properties over time (such as texture, structural stability, permeability, infiltrability and capacity for retention of water at high potentials) and their chemical properties (organic material content, phosphorous, acidity and electrical conductivity, amongst others) and which might have been affected by irrigation with waste water.

The most complex task was finding parameters to be used to monitor the evolution of the soil and its quality, given that “reliable and exact indicators of soil quality were needed”. In this way, after verifying this precision and representativity, the study of naturally dispersed granulometry was chosen in order to evaluate the stability of the structure, and a new method was designed for the physical fractioning in order to separate free organic material, “widely recognised as an early indicator of changes in the soil”.

This indicator measures, in the organic material within the soil, “the fraction most directly related to fertility and to the stability of the soil structure”. The data obtained with these indicators was subsequently corroborated by means of a micromorphological study of thin sheets of soils, including an evaluation of porosity and microstructure though digital image analysis.