Cross-flow microfiltration of biologically treated wastewater

Water requirement for irrigation dramatically exceeds the traditional resources of Tenerife island that are becoming more and more brackish. An important 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 competition with South American producers, the programme intends to generate water completely bacteria free. Microfiltration meeting the required standards, this study was devoted to preliminary results obtained with cross-flow filtering through a 0.14 mu m inorganic composite membrane, i.e., Carbosep M14, which indeed was a total barrier for suspended solids, total coliform, fecal coliform and fecal streptococci. The removal of turbidity and total COD was also significant at about 93% and 60%. There was no rejection of the soluble fraction of a size lower than 0.01 mu m. Some 45% abatement of phosphorus was also obtained. The microfiltered water was therefore perfectly adapted to irrigation. In spite of a fouling mechanism difficult to identify, a critical flux of 100 l/m(2) 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 phenomenological approach of the operation allowed us to define two dimensionless groups: the shear stress number and the fouling number. These numbers allowed to display all the experimental results in only one curve.