CROSS-FLOW MICROFILTRATION OF BIOLOGICALLY TREATED WASTE-WATER

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 th e City of Santa Cruz for the irrigation of banana and tomato crops. Co nsidering the hard competition with South American producers, the prog ramme intends to generate water completely bacteria free. Microfiltrat ion meeting the required standards, this study was devoted to prelimin ary results obtained with cross-flow filtering through a 0.14 mu m ino rganic composite membrane, i.e., Carbosep M14, which indeed was a tota l barrier for suspended solids, total coliform, fecal coliform and fec al streptococci. The removal of turbidity and total COD was also signi ficant at about 93% and 60%. There was no rejection of the soluble fra ction of a size lower than 0.01 mu m. Some 45% abatement of phosphorus was also obtained. The microfiltered water was therefore perfectly ad apted to irrigation. In spite of a fouling mechanism difficult to iden tify, a critical flux of 100 l/m(2) h was obtained at 1 bar driving pr essure and 3 m/s cross-flow velocity, and this value was close to the permeation rate for tap water. A phenomenological approach of the oper ation allowed us to define two dimensionless groups: the shear stress number and the fouling number. These numbers allowed to display all th e experimental results in only one curve.