In addition to biological parameters, regulations often include chemical parameters in order to protect human and environmental health, but also to provide for long-term soil productivity and functioning of irrigation schemes. Table 2 gives some examples of wastewater components’ impacts in irrigational use.

In developing countries, salinity is usually the dividing line between water suited or unsuited for irrigation uses. High salt concentrations are an indication of highly concentrated wastewater, a factor typical for arid countries. Heavy metal concentrations are as a rule still relatively low in developing countries and are not yet responsible for any major problems. High salt concentrations in irrigation water hamper the water intake of crops and lead to yield losses for many crops. In addition, high sodium contents in loamy soils lower their permeability for water, which results in lower soil aeration. The consequences of these effects are also yield losses. In the case of high subsoil permeability, there is an additional risk of groundwater salinisation.

Another important aspect is wastewater nutrient content. Raw wastewater contains nitrogen, phosphate, and potassium in concentrations sufficient to cover or even exceed overall plant fertilization needs. The presence of trace elements and organic matter also favors plant growth and raises soil humus levels. These substantial advantages for farmers are offset in part by environmental risks consisting in the danger of nitrate-leaching. Other agro-biological risks are bound up with the fact that nitrogen can, in later phases of growth, have negative effects on plant growth. The nitrogen, however, stimulates undesirable algae growth on cultivated soils. Appropriate management methods are called for here. In table 2 the most important water quality parameters and their significance are listed.

Table 2: Physico-chemical parameters, their significance and approximate ranges for treated wastewater [SAR= Sodium adsorption ratio]