Constructed wetland for the treatment of wastewater of zootechnic activities

Current laws concerning water protection from pollution due to wastewater, represented in Italy by the law n. 152 of 11 may 1999 and its integrative law n. 258 of 18 august 2000, have individuated two typologies of wastewater: civil wastewater produced by urban areas or similar, and industrial wastewater. Inside of this typology, particularly important is wastewater produced by productive centres like milkhouses, cheese factories, dairy and swine industries. All these activities produce wastewater that is so polluted that a particular attention is necessary in the individuation of the most adequate approach for a correct depuration.

Wastewater of a cheese factory, for example, contain high concentrations of solids, grease and nitrogen and phosphorus compounds; these substances can result in eutrophication of downstream water bodies, damaging and altering aquatic ecosystems. It has been observed that this kind of wastewater present average values of BOD5 ranging from 1500 to 8000 mg/l, that is 30 times the concentration of a typical civil wastewater. Swine industry produce wastewater characterized essentially by blood, faeces, urinals, stomach content, and other various residuals of washing water of working rooms and machinery; moreover, there is a strong presence of solid materials constituted by residuals of furs, rush, food and many greasy substances. For this kind of activity, it has been observed wastewater with average values of BOD5 ranging from 2500 to 3000 mg/l, that is 10 times the concentration of a typical civil wastewater.

Another important characteristic that matter with depuration of wastewater of zootechnic industry is represented by the fact that the working activity is seasonal, for it is connected to productive period of animals, and that the operations are limited to a part of the day, generally 8-12 hours, and don't take place every day of the week. The characteristics of the wastewater, therefore, tend to vary in its quantity and quality following productive phases. Principal problems connected to the production of wastewater of zootechnic industry are the presence of pathogens, odors, high concentrations of nutrients and lack of adequate on site treatments.

The traditional method of waste management used for century in zootechnic industry is waste application to land: it is the primary source for cropland fertilization, but it has problems as high solids content, high nutrient concentrations, limited pumping distances. Alternatively, in the biggest activities, it is possible to find technological plants for wastewater treatment.

Constructed wetlands compared to conventional systems have less construction, operation and energy costs plus more flexibility in pollutant loading. It has been showed that constructed wetlands significantly reduce concentrations of nutrients, solids, and oxygen demanding substances in zootechnic wastewater. It has to be noted that a good solid removal prior wetland treatment it is necessary to assure a long term functionality.

It is interesting to observe that, in case of productive activities situated in isolated places not connected to the net, the utilization of phytodepuration systems represent an optimal solution also for the treatment of wastewater produced by residential houses present in the factory.