TREATMENT AND STORAGE OF WASTEWATER FOR AGRICULTURAL IRRIGATION

Freidler E. and Juanico M. (1996). Treatment and storage of wastewater for agricultural irrigation. Int. Wat. Irrig. Review, 16(4):26-30.

ABSTRACT

The selection of appropriate technology for municipal sewage treatment and disposal not only requires a proper characterisation of the quality of modern municipal sewage, but also a analysis of the effects that the treated wastewater would have both on the agricultural needs and the environment. A wide variety of pollutants appear in modern municipal sewage, the most important of which being: biodegradable organic material (expressed as BOD or COD), pathogens (bacteria and viruses), nutrients (nitrogen and phosphorous), salts, heavy metals and xenobiotic materials.

In order to enable the use of treated wastewater for agricultural irrigation, the wastewater in question has to comply with sanitary, agrotechnic and environmental quality requirements. These requirements do not always coincide, as in the case of nutrients, where from the environmental point of view the concentration in the effluents should be as low as possible, while from the agrotechnic point of view a certain level of nutrients is welcomed since it replaces the need for addition of costly fertilisers. The main requirements of the above three categories are listed herewith:

The primary sanitary needs are low concentrations of pathogen bacteria, parasites and viruses.

The agrotechnic requirements are low salts, relatively low suspended solids concentration (to prevent clogging of the irrigation system), and storage capacity in order to regulate between sewage production (which is relatively steady throughout the year) and demand of treated wastewater for irrigation (which occurs only in certain hours/days/seasons).

The environmental needs are low concentration of heavy metals and xenobiotic materials, controlled level of nutrients and prevention of malodours.

The appropriate wastewater treatment technology should be able to deal with most requirements of the above categories. Intensive sewage treatment systems (such as activated sludge) successfully remove BOD, but fail to remove detergents, heavy metals, xenobiotics, and pathogens as well, nor do they have any significant storage capacity. Thus, these intensive systems alone can not achieve the requirements for agricultural irrigation and have to be combined with extensive treatment units (such as wastewater stabilization reservoirs). However, including extensive units after intensive ones will not be the most cost effective solution. A proper combination for achieving the above requirements may include semi-intensive reactors (such as aerated lagoons) for partial BOD removal followed by extensive units for the removal of refractory pollutants and pathogens and enhanced removal of the remaining BOD. Under different conditions a proper combination might include wastewater reservoirs exclusively, being operated as sequential batch reactors (SBR) in parallel or in series.

It should be noted that no treatment technology apart from desalination (which is expensive) can deal with excess amounts of salts. Thus, high salts concentration should be prevented by separation of salty streams from the main sewage discharge at source.