Water reuse is also known as water recycling or water reclamation. It involves reclaiming water from various sources, treating the water, and reusing it for beneficial purposes. Possible water sources for reuse are municipal wastewater, water used for industrial cooling purposes, and runoff water from agricultural activities. Some uses for recycled water could be agricultural, industrial, or environmental.
Water reuse may be planned or unplanned. An example of unplanned water reuse would be a community using river water partially composed of treated wastewater from upstream communities. Planned water reuse comprises a system with specific fit-for-purpose specification targets for recycled water. Here we will look at the processes involved in treating water for reuse.
Factors that can influence the design of a water treatment process would include the characteristics of the source water supply, operations and maintenance requirements of the system, and the specific application or applications for which the reclaimed water will be used. The current water recycling process design goals would also include limiting or eliminating greenhouse gas emissions and energy conservation. There are essentially four main water treatment processes: preliminary, primary, secondary, and advance.
Preliminary treatment involves measuring the amount of water to be treated and removing large particles and grit from the water. Both primary treatment and secondary treatment involve sludge processing and disposal. Sludge is composed of organic and inorganic material and is a byproduct of the water treatment process.
Sludge processing has four steps. The first step is called thickening. Thickening involves reducing the volume of the sludge for easier handling. Two options for the thickening process are gravity thickeners and the application of dissolved air flotation. The digestion process involves decomposing the organic component of the sludge.
The next process is dewatering. This process involves drying and solidifying the sludge. Options for this process include the use of sludge-drying beds, solid-liquid separation devices, or centrifugation. The last step is disposal which involves burying in a landfill or using the sludge as fertilizer.
Secondary treatment also removes dissolved organic matter and suspended solids and nutrients from the water being recycled. This process uses equipment such as sludge basins and fixed-media filters. Final solids separation during this process uses either settling or membrane filtration or separation. A high rate process uses membrane bioreactors (MBR) as an alternative to sludge processing during secondary treatment. Using MBR, raw wastewater goes into a piece of equipment called a bioreactor composed of numerous microfiltration and ultrafiltration membranes. MBR technology results in solids removal, organic and nutrient removal, as well as disinfection.
Advance treatments involve the further removal of organic waste and nutrients in the water, reducing suspended solids, and the treatment of pathogens. Pathogens are disease-causing microorganisms such as bacterias and viruses. Nutrient removal is crucial if the recycled water is going to be reused as groundwater. Some treatment processes in aid of nutrient removal are gas stripping, breakpoint chlorination, and chemical precipitation.
Filtration is used to reduce suspended solids, with one of the most popular processes being depth filtration, which removes suspended solids and prepares the water for disinfection. Disinfection is the treatment process that removes pathogens by using chlorine and ultraviolet (UV) radiation.
