Pretreatment of Secondary Effluent for Drip Irrigation

A pilot‐scale treatment system, for developing drip‐irrigation pretreatment criteria, was designed, installed, and operated intermittently over 5,300 h using activated sludge secondary effluent. Principal unit processes studied were granular‐medium filtration and screen filters for removal of suspended solids, and chlorination treatment at various injection frequencies and concentrations to prevent biofilm formation in emitters and distribution lines. Two types of 4 L/h (1 gal./h) drip emitters were tested—automatic self‐flushing and tortuous‐path turbulent‐flow emitters. Adequate filtration reduced both the required frequency of chlorination and lateral flushing. Intermittent chlorination with 2 mg/L free residual chlorine during the last hour of an irrigation cycle was found as effective as continuous chlorination with 0.4 mg/L free residual chlorine in preventing biofilm formation in emitters and distribution networks. Pretreatment recommendations for these types of emitter designs are presented.