In response to economic and environmental concerns, the use of treated wastewater for useful applications such as industrial processes, agricultural and landscape irrigation, groundwater basin replenishment and toilet flushing is on the rise. The necessity to minimise total dissolved solids is one of the most important aspects of recycling wastewater treatment plant effluent for another purpose. This is frequently accomplished by the use of a reverse osmosis (RO) system, which uses a pressure differential to force a solution through one membrane that holds the solute on one side while allowing the pure solvent to pass through on the other.
Present challenges of RO for wastewater recycle
When developing RO systems, pre-treatment procedures are crucial. A brine spacer, for example, is commonly made of low density polyethylene mesh netting in RO membranes used for most water reuse applications. This brine spacer might become blocked if the feed water has a high level of suspended particles. Another concern is the high quantities of organics in many biologically treated waste waters, which are rejected by the RO membrane and gradually concentrated as the water passes through it. This level of organics can clog the membrane, particularly near the RO systems outlet. Furthermore, calcium phosphate scaling in some wastewaters can pose difficulties with RO systems. Scaling can be reduced by operating at a lower water recovery, employing an anti-scalant such as acid or changing the operating conditions of the wastewater treatment plant to minimize the quantity of phosphate in the RO feed.
Alternatives for Reverse Osmosis Pre-treatment
There are a variety of reverse osmosis pre-treatment options, and the ideal one for a given process is determined by power, labour, chemical and land expenses, as well as the wastewater source and current wastewater treatment system.
Primary treatment, biological treatment, and, most importantly, solids-liquid separation utilising secondary clarification are all examples of traditional effluent pre-treatment schemes. Because the traditional sedimentation method frequently fails to remove enough bacteria and suspended solids, sand filtration can be used to improve solids-liquid separation and provide better water for the RO system. Using ferric chloride in conjunction with sand filtration may help remove more solids and organics.
Ultrafiltration RO pre-treatment
To remove suspended solids, many modern water reuse systems include an ultrafiltration (UF) pre-treatment stage. Hollow fibre UF membranes are commonly used in these systems, and they do an outstanding job of delivering water with low suspended particles to feed the RO system. The UF system, on the other hand, is an additional treatment stage that requires more space and raises operating costs. The UF system may also be vulnerable to disruptions from a traditional wastewater treatment plant, which could raise its operating costs even more.
RO Pre-treatment using a Membrane Bioreactor (MBR)
The UF membranes are submerged in the activated sludge in an MBR, which combines the biological and solid-liquid separation steps into a single process. The membrane serves as a barrier, enhancing effluent quality. The MBR eliminates the secondary clarifier and does not rely on gravity for liquid-solid separation, allowing the activated sludge to operate with a greater mixed liquor suspended solids concentration.
The usage of RO systems to recover and recycle wastewater effluent is on the rise, and Netsol Water MBR technology is increasingly being explored as a pre-treatment alternative for a variety of industrial and municipal reuse applications.