Reusing treated wastewater for beneficial purposes, such as agricultural and landscape irrigation, industrial processes, toilet flushing, or groundwater basin replenishment, is growing in response to environmental and economic concerns. One of the key factors involved in recycling wastewater treatment plant (WWTP) effluent for another use is the need to reduce total dissolved solids (TDS). This is often done by using a reverse osmosis (RO) system, which relies on pressure differential to force a solution (in this case, water) through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. While extremely effective on biologically treated wastewater, RO systems need to be coupled with an effective pretreatment system to avoid common issues that can result in system failure, including plugging, fouling, and scaling. One of the most effective pretreatment options for wastewater applications is membrane bioreactor (MBR) technology, in which a membrane process like ultrafiltration (UF) or microfiltration is combined with a suspended growth bioreactor. MBR provides high quality feed water to the RO, minimizes footprint and the cost of civil works, and reduces treatment plant downtime, thereby reducing operating costs. Koch Membrane Systems’ PURON® submerged MBR technology has successfully been used as the pretreatment option for challenging industrial and municipal water reuse RO systems, and can help make water recycling technologies more cost-effective. Reverse Osmosis Systems Present Challenges for Water Reuse
Pretreatment methods are critical when designing RO systems. For example, RO membranes used for most water reuse applications contain a brine spacer, typically made of low density polyethylene mesh netting. If there is a high level of suspended solids in the feed water, this brine spacer can become plugged. Another issue is the high levels of organics contained in many biologically treated wastewaters, which are rejected by the RO membrane and progressively concentrated as the water flows across the membranes. This concentration of organics can foul the membrane, especially towards the RO system outlet. Biofouling can also occur, because the organics in wastewater make an excellent food source for microorganisms. Also, some treated wastewaters contain high levels of bacteria, so biogrowth may occur quickly even if RO feed water is disinfected. Finally, calcium phosphate scaling can cause problems with RO systems operating on some wastewaters. The scaling can be mitigated by operating at lower water recovery, using acid or other antiscalant to minimize scaling, or modifying the operating conditions of the WWTP to reduce the amount of phosphate in the RO feed. These plugging, fouling, and scaling issues mean that the RO system needs to be operated at higher pressures, leading to increased power consumption, increased chemical costs for cleaning, and a shorter membrane life. How can these challenges be minimized and overall water reuse system lifecycle costs reduced? Effective pretreatment of the feedwater before it flows through the RO system is the answer, provided that the pretreatment steps are chosen carefully to ensure that the RO system can work as intended. Reverse Osmosis Pretreatment Options
There are many different pretreatment options, and the best for a particular process depends on power, chemical, labor and land costs, wastewater source, and the existing wastewater treatment system. Conventional Pretreatment
The conventional effluent pretreatment scheme might be primary treatment, biological treatment and, the most crucial part of the process, solids-liquid separation using secondary clarification. The conventional sedimentation process often doesn’t remove enough bacteria and suspended solids, so sand filtration may be added to improve the solids-liquid separation and provide higher quality water to feed...