By Tal Fabian
Water scarcity is a subject of major concern worldwide due to the depletion of clean water sources and growing population concentrated in urban areas. This places wastewater treatment and reuse in the spotlight as a sustainable solution for water reclamation, while protecting water sources from pollution. The need for advanced wastewater treatment schemes is on the rise, due to de facto indirect potable reuse that occurs when treated wastewater effluents are discharged into streams that serve as a water source for a downstream water treatment facility.
Water reuse treatment schemes
There are two main treatment schemes that are frequently applied on the municipal level. In cases where the removal of dissolved solids is required, membrane-based desalination technologies are required. In other cases, where the removal of only organic contaminants is required, a combination of biofiltration and advanced oxidation processes (AOP) are frequently applied.
On the industrial side, the treatment scheme is largely dependent on the industry type. Each industrial sector has different types of streams, therefore requiring tailored solutions to address the various contaminants. The most common process technologies include:
- Pretreatment for removal of suspended solids’ organic/ inorganic components. This treatment is usually achieved using coagulants, clarifiers and filters (sand filters, micro/ultrafiltration).
- Removal of dissolved solids. The main technologies implemented are either membrane-based desalination (RO, nanofiltration) or electrically driven processes such as electrodialysis.
The required final treated water quality depends on the end-user and the cost of brine disposal. In some cases, where brine disposal is costly, a minimal liquid discharge (MLD) solution is needed. In other cases, when there is no option for liquid waste, the brine is minimized to a solid discharge by applying crystallization technologies. This is known as zero liquid discharge (ZLD), which is considered the most advanced type of treatment. To date, only a few industries utilize ZLD due to the high capital and operational expenditures (Capex and Opex) related to this.
Currently, most projects push toward MLD solutions, trying to maximize water recovery without drastically increasing the Capex and Opex. This is a crucial driving force for the implementation by the industry. When designing systems for MLD applications, the existing process technologies are pushed to their limits. This introduces new operational risks for scaling and biofouling, which can complicate the operation of such systems. Traditional strategies for increasing the water recovery of the system introduce proprietary antiscalants and biocides. There is a very clear need for innovative process designs that utilize advanced control methods with minimum requirement for specialty chemicals.
A focus on RO technologies
Standard water reuse schemes typically include ultrafiltration (UF), RO and UV/AOP units. Chloramine compounds, typically dosed to the RO feed stream, help control the biofouling of the RO membranes. Chloramine is a weak oxidant and is permitted by most membrane manufacturers. Chloramine is different from free chlorine (which is a strong oxidant) because it is chemically bound with ammonium and does not oxidize the polyamide membranes. This is practiced in most water reuse facilities, such as the Orange County Groundwater Replenishment System (GWRS), CA (the world’s largest water purification system for indirect potable reuse) and West Basin, CA.
Chloramine, however, is a precursor to the formation of DBPs such as N-Nitrosodimethylamine (NDMA), a dangerous organic contaminant and a suspected carcinogen. DBPs and NDMA are treated and removed in the last stage of the GWRS treatment system by UV/H2O2 on the permeate stream, before discharge.
The conventional RO technology, which has been used for the last 50 years, operates at constant ﬂow and concentration conditions. In the past decade, however, new technologies have been developed, where feed pressure and TDS vary over time. One such technology is pulse flow reverse osmosis (PFRO), an innovative and improved version of the conventional RO desalination process. PFRO addresses the challenges of operating high-recovery RO systems, while avoiding increased Capex and Opex. This can be achieved by operation at higher recovery and ﬂux compared to standard RO. In addition, PFRO does not require chloramine dosage, therefore, avoids the formation of DBPs and leads to reduced overall water cost.
PFRO operation uses alternating hydraulic and osmotic conditions to control the formation of scaling and biofouling. These alternating conditions are created by controlling the brine discharge – by opening and closing a brine valve at specific times. When the brine valve is closed (production cycle), the operation is dead-end filtration, meaning that all the feed flow leaves as permeate. When the brine valve opens (flushing cycle), part of the accumulated brine leaves the pressure vessel, resulting in an immediate decrease in the osmotic pressure and an increase in flow and shear force. The duration of each of these cycles determines the system recovery. The cycles are short, well below the typical induction time needed to form scale.
The outcome of this well-controlled process is a very low tendency for scaling compared to a standard RO process, since crystal formation is disturbed by the changing hydraulic and osmotic conditions. The combination with short periodic cleaning sequences keeps the membranes clean, which minimizes the system downtime. In addition, since microorganisms constantly try to adapt to these changing conditions (which require that they invest their energy in their survival), there is less tendency for the formation of biofilm on the membrane surface. The continuity of brine flow has been an unshakable foundation of the conventional RO process for decades, but the shift to pulse flow can open the door to a plethora of benefits, including higher recovery rates, lower power consumption and minimal biofouling, to name a few.
Water reuse has become critically important worldwide as ground- and surface-water supplies are increasingly depleted. It’s only a matter of time before municipal and industrial demand will surpass water supplied by rain, rivers, lakes and reservoirs. As a result, water reclamation technologies, such as reverse osmosis, must continue to gain acceptance as efficient, economical and environmentally friendly methods for minimizing water demand and securing water resilience for years to come.
About the author
Tal Fabian graduated from the Technion – Israel Institute of Technology, with BSc and MSc degrees in Environmental Engineering specializing in wastewater desalination. He started at IDE Technologies in 2018 as an RO process engineer and was soon promoted to team leader, where he focuses on Industrial and Municipal wastewater desalination plants for recycling and reuse. Previously, Fabian was a process engineer at Tenova Advanced Technologies, which offers process technologies for the chemical and mining industry.