| Castle Rock is evaluating the possibility of combining advanced oxidation with its current membrane treatment process. |
By William Atkinson
The technology and systems required for water and wastewater utilities to work together to provide potable water reuse have been in existence for a long time. Traditionally, however, efforts have primarily focused on indirect potable reuse (IPR).
There are currently two strategies for transforming wastewater into potable water. One of these, as mentioned, is IPR, which involves releasing treated wastewater into groundwater or surface water sources. Subsequently, this released water is then reclaimed and treated to become purified water that meets drinking water standards. The other, and newer, method is direct potable reuse (DPR), which involves purifying wastewater and then introducing the purified water directly into a drinking water supply system.
Several communities are considering DPR --along with IPR -- and at least one has actually implemented DPR technology.
Considering DPR and IPR
One utility with a wastewater-to-potable-water project under consideration is in the city of San Angelo, Texas. The utility already has a water reclamation facility designed to treat wastewater such that it is environmentally safe for agricultural irrigation.
But given the persistent dry conditions in Texas, the utility was prompted to take water reclamation to the next level. "Severe drought in West Texas has brought on the need to look at all water sources," said Ricky Dickson, water utilities director. "San Angelo has predominantly relied on surface water only. We are currently developing a groundwater source and wanted to add reuse to our water supply portfolio."
San Angelo has hired a consulting firm to conduct a feasibility study. "The study will concentrate on finding the best and most beneficial use of the reclaimed water," said Dickson. This could include surface water augmentation (pumping reclaimed water to one of the local reservoirs), using the water for large potable water users for irrigation needs (purple pipe system), and/or DPR (pumping the water directly to the water treatment facility).
Given the environmental considerations, Dickson noted that the Texas Commission on Environmental Quality will be heavily involved throughout the project. And the effort is worth it, as Dickson sees it. "There is a potential of adding an additional 5 to 7 million gallons a day (MGD) of water to our supply," he said. "This could provide as much as one-third of our water needs."
Another utility considering both DPR and IPR is in the town of Castle Rock, Colo. "There was no specific event that triggered our interest in potable water reuse," said Mark Marlowe, utilities director. "Castle Rock has done a very good job of studying water resource needs strategically. We developed a Water Resources Strategic Master Plan in 2005 and have continued to review that plan in light of continued growth of the town over time."
Currently, the city is evaluating both DPR and IPR but has not implemented either one -- not yet. "We have come to realize that indirect reuse of our treated effluent is an excellent resource -- both from efficiency and cost standpoints -- so this is something that we plan to look at going forward," Marlowe said. "However, we are also currently evaluating direct reuse in combination with several options for indirect use."
In terms of engineering, the utility has done some very preliminary work, such as looking at combining advanced oxidation with its current membrane treatment processes. "If we do end up going this route, we estimate that we will be able to meet 40 percent to 50 percent of our current demand with indirect reuse water," said Marlowe.
DPR in Action
While a number of utilities are in the process of considering DPR, the city of Wichita Falls, Tex., has actually implemented it. During its previous drought, which was in the late 1990s, Wichita Falls began evaluating both the DPR and IPR options. "The work that we are currently doing during this drought is a continuation of that previous work," said Daniel Nix, public utilities operations manager. The Texas Commission on Environmental Quality has approved the project, and the utility started discharging the treated DPR water to distribution on July 9, 2014.
The wastewater effluent is being disinfected with chloramines in the pipeline between the wastewater plant and the water plant. At the water plant, it is clarified further, micro-filtered, treated with reverse osmosis (RO), blended on-site with lake water, and then treated another time through a complete conventional plant. "The reason we selected these technologies is that they already existed in the city's treatment plants," said Nix. "This was one of the things that made DPR work for us. We didn't have to construct any treatment facilities. All we had to do was connect the two plants with a pipeline."
