Can microfiltration of treated wastewater produce suitable water for irrigation?

Water requirement for irrigation dramatically exceeds the traditional resou rces of Tenerife island that are becoming more and more brackish. An import ant programme of wastewater recovery is actually implemented; it is focused on the reuse of the secondary treated wastewater of the city of Santa Cruz for the irrigation of banana and tomato crops. Considering the hard compet ition with South American producers. the programme demands water completely disinfected. Microfiltration meeting the required standards, this study wa s then devoted to preliminary results obtained by cross-flow filtering thro ugh a 0.14 mm inorganic composite membrane, i.e. Carbosep M14, which was in deed a total barrier for suspended solids, total coliform, fecal coliform a nd fecal streptococci. The removal of turbidity and total COD were also sig nificant, 93% and 60%. There was no rejection of the soluble fraction of si ze lower than 0.01 mm. Some 45% abatement of phosphorus was also obtained. The microfiltered water was therefore perfectly adapted for irrigation. In spite of a fouling mechanism difficult to identify, a critical flux of 100 l/m2 h was obtained at 1 bar driving pressure and 3 m/s cross-flow velocity and this value was close to the permeation rate for tap water. A phenomeno logical approach of the operation allowed us to define two dimensionless gr oups: the shear stress number and the fouling number. These numbers allowed us to display all the experimental results in only one curve. (C) 1998. Pu blished by Elsevier Science Ltd. All rights reserved.