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Report An Evaluation Of Low-Energy Treatment Schemes For Water Reuse Carollo Engineers led a team of researchers to determine which of the latest technologies for reclaimed water is most effective and most ready for market. by Erin Mackey and Andrew Salveson he production of reclaimed water from municipal wastewater is now standard practice and almost universally accomplished using primary treatment (discretionary), secondary treatment with activated sludge, filtration, and disinfection with chlorine, sodium hypochlorite, or ultraviolet light (UV) disinfection. Sludge produced by liquid treatment is often stabilized with aerobic digestion. These con- ventional processes are ener- gy-intensive. Lower-energy treatment alternatives exist in varying degrees of "market readiness." Collectively, these represent part of the future of wastewater and reclaimed water treatment. T There are a number of dif- ferent process schemes and technologies that can meet the "lower-energy" reuse label, ranging from anaerobic mem- brane bioreactors to aerobic forward osmosis membrane bioreactors, to non-biological treatment and nutrient and energy-recovery systems, among many others. WateReuse Research Foundation (WRRF) project 10-06, cosponsored by the Water Environment Research Foundation (WERF), represents part of the initial effort (Phase I) to find and dem- onstrate the low energy treatment plant of the future. Project Objectives WRRF 10-06 had two main objectives: Identify emerging low-energy treatment technologies, ranging from alternatives to existing treatment processes to innovations in energy recovery systems. Evaluate the market readiness of these new ideas and products to identify the most promising ideas to bring for- ward for Phase II testing. These ideas would have the high- est potential to save energy and cost and would be close to being ready for use at full-scale. 22 Water Online The Magazine, Wastewater Edition ■ wateronline.com Work Summary Figure 1. Relative energy use at a WWTP – Assumption of high energy use for pumping and disinfection (adapted from WEF 2009, EPRI 2002, and internal operational databases). Step 1: Establishing A Baseline — The first step identified an energy baseline against which new low-energy approach- es could be compared. The baseline energy tabulations were segregated into wastewater treatment plants (WWTPs) with "high" and "low" relative energy use. The high power demand WWTPs saw the largest power uses for pumping, activated sludge aeration, and disinfection (16%, 27%, and 34%, respectively). See Figure 1 for more detail. The same was true for WWTPs with lower relative power demands (22%, 38%, and 25% for pump- ing, activated sludge aeration, and disinfection, respectively). In either situation, energy use at a WWTP is dominated by pumping, activated sludge aer- ation, and disinfection. Both ultraviolet light (UV) and sodi- um hypochlorite use a similar amount of energy once the energy required to generate sodium hypochlorite is account- ed for (see Figure 2 on page 24). Pumping energy was not a focus of this project and so was not pursued further. Since the largest energy use is in the secondary and dis- infection processes, improvement of these processes would yield the largest gain in energy efficiency. This work focused on documenting ways to decrease energy use in the second- ary and disinfection process and on ways to increase methane gas generation and thus increase energy recovery. Table 1 on page 24 summarizes these three key energy components for different wastewater qualities for a "benchmark"/"baseline" plant treating approximately 12 – 15 mgd. Step 2: Identifying New Low-Energy Technologies — In advance of a workshop, project team members assembled a list of technologies to be reviewed and considered for the workshop. This list included both proprietary and non-