Advanced wastewater treatment for reuse

Changing climates and steady global population growth are placing increased pressure on freshwater supplies, intensifying the need to manage this vital resource. Alongside conservation measures, water reuse processes - in which wastewater is treated to meet quality standards for beneficial use - is emerging as a critical strategy for augmenting freshwater supplies.

For the past decade-plus, common water reuse applications have included industrial cooling processes, toilet flushing and irrigating green spaces. However, in more recent years, advanced treatment methodologies have ushered in the viability and safety of potable reuse - both direct and indirect potable reuse - and this trend is quickly gaining traction in the municipal sphere.

After larger solids and microorganisms are removed from an incoming reused water stream via conventional treatment methods, advanced treatment targets smaller particles and viral contaminants that remain. This requires procedures such as filtration, reverse osmosis and disinfection, which enable achievement of stringent water quality standards for potable reuse.

Filtration is commonly employed to remove suspended solids, bacteria and some larger organic molecules. Filtration materials can include sand, granular media and fine membranes.

Membrane filtration has long been a staple of potable water treatment, used to remove contaminants by pressurizing and pushing water through fine membranes. Membrane pore size varies based on requirements, and often, various membranes are used, with pore size decreasing as the process progresses downstream.

Microfiltration is regularly deployed toward the beginning of tertiary treatment, with pore sizes ranging from 0.1 to 10 microns. Ultrafiltration (UF), with pores ranging in diameter from 0.01 to 0.1 microns, is effective for removing suspended solids, bacteria and many viruses. Both of these membrane types act as physical barriers, trapping contaminants on the membrane surface while allowing clean water to pass through. Because contaminants build up on the membrane surface over time, backwashing is required periodically to maintain efficiency and keep the pores from clogging so water can continue to pass through.

Taking filtration to an even finer level, nanofiltration employs membranes with pores approximately 0.001 microns in diameter. As a result of its fine apertures, nanofiltration is capable of removing a broader spectrum of contaminants, including dissolved organic matter, hardness ions - such as calcium and magnesium - pesticides and some heavy metals. It is regularly used as an intermediary between ultrafiltration and reverse osmosis (RO), providing a high level of purification without the high energy demands required by RO to remove dissolved salts.

When water purity is critical, as in the case of direct potable reuse and many industrial applications, reverse osmosis (RO) membranes can be used to remove the finest of contaminants. These membranes possess pores of approximately 0.0001 microns in diameter, catching salts, minerals, trace pollutants, bacteria and viruses. With such tiny pores and the need to overcome osmotic pressure, RO requires significant energy to pressurize and pump water across the membranes.

Because the pores are so small, RO systems typically leverage cross flow filtration, in which filtered permeate water is routed one way, while contaminated concentrate water takes a different route through the vessel. Cross-flow filtration enables the concentrate stream to sweep away contaminant buildup on the membrane and maintain enough turbulence to keep the surface from clogging.

The concentrate stream consists of brine and other compounds incapable of passing through the membrane. To increase water recovery, RO systems often consist of multiple stages, whereby the concentrate from one stage is pushed through a second-stage RO membrane or recycled back through the initial membrane.

Following filtration, disinfection is typically the final step within tertiary treatment. Common disinfection methods include chlorination and advanced oxidation methods like ultraviolet and ozonation. These procedures inactivate remaining pathogens, including bacteria, viruses and protozoa to ensure stringent water quality requirements are met.

The success of any water reuse program extends beyond technological sophistication, requiring meticulous process control, rigorous feedwater quality monitoring and a robust maintenance regime. Process control systems must be designed to continuously monitor and adjust treatment parameters to ensure consistent product water quality, while environmental management measures must be maintained to prevent the introduction of contaminants that can compromise treatment.

As governments recognize the growing role of water reuse in ensuring water security, the legal landscape is evolving rapidly. Indirect and direct potable reuse permitting procedures, especially, are becoming increasingly common in many regions, outlining requirements for water reuse projects while addressing public concerns about system safety. As treatment technologies advance, regulatory frameworks should continue adapting to foster innovation while safeguarding public health and environmental integrity.

The future of water security hinges on the ability to embrace sustainable practices, and continued investment in research, infrastructure and public education will help ensure sustainable water yield for generations to come.