In the past decade, six micro- or ultra-filtration/reverse osmosis (RO) reuse treatment plants, ranging in capacity from 4 to 100 ML/d, have been constructed in South East Queensland, Australia. The plants produce water for a variety of purposes, from industrial use through to indirect potable reuse. The feed water for all these reuse plants is wastewater treated to a secondary biological nutrient removal standard with secondary clarification only.
Two different pretreatment approaches have been adopted ahead of the membrane processes, with varying objectives and success. These pretreatment approaches fall into two broad process types; those that directly feed treated wastewater onto the MF or UF (without pretreatment other than micro-screening), and those with chemical clarification pretreatment prior to MF or UF.
The reuse plants that include pretreatment ahead of MF/RO are designed to maximise the production of reuse water from the available wastewater. As a result, the recovery adopted is 82%, resulting in 83 to 85% recovery being required from the three-stage RO process. In comparison, the reuse plants that have no pretreatment prior to MF/RO generally operate at lower RO recovery of about 75%.
The paper examines the design, performance and operational experience of these two different water reuse treatment plant types. Chemical clarification pretreatment provides stable operation of high recovery RO processes, with low CIP requirements. However, these processes may require design and operation that is outside normal practice. In comparison, the operation of reuse plants without pretreatment has resulted in lower RO and plant recovery rates being adopted, and potentially higher RO CIP requirements. Long-term operation appears possible, with low maintenance requirements for the MF system. Routine chemical cleans of both the MF/UF and RO membranes at these smaller plants, may lead to improved process performance.