IEEE Power & Energy Magazine - January/February 2017 - 42

In WWT processes, energy-efficiency measures include
retrofitting plants with high-efficiency pumps, variablespeed drives, and advanced monitoring and process control for the biological reactor. Traditionally, WWT plants
are fitted with oversized pumps to cope with hydraulic load
fluctuations; as such, these pumps are inherently inefficient.
Thus, there is a need for real-time data analysis and forecasting systems that will inform process-control strategies.
The benefits of such approaches include cost savings, flexibility, and improved control and decision support for planning plant upgrades.

Reconceptualizing the WWT Plant as an
Energy- and Resource-Generation Facility
Future WWT plants will no longer be just pure waste management facilities but rather recovery systems for clean
water, energy, and minerals. The chemical energy in wastewater is in the form of biodegradable and inert chemical
oxygen demand and reduced nitrogen (NH4+ or organic N).
Large-scale WWT plants recover 5-15% of this energy
through anaerobic digestion. However, because other important parts of the treatment plants are energy-intensive (aerobic biological oxidation requires 0.40-0.65 kWh/m3), it
follows that most treatment plants are net users of energy.
With the advent of new and improved treatment technologies, a net-energy-positive WWT plant is now considered
achievable. Emerging process technologies, including anammox, bubbleless aeration, and aerobic granular sludge, can
deliver significant savings in energy demand.
Biogas is central to energy neutrality in WWT. Biogas
can be produced continuously, and there are several possible
uses: on-site electricity generation, combustion for thermal processes (e.g., Cambi), or direct injection into the gas
grid; however, there are upsides and downsides with each
(for example, for injection into the grid, the biogas needs
to be pretreated). Currently, large-scale WWT plants typically recover only 5-20% of the chemical energy through
anaerobic digestion (the production of biogas and cogeneration). Codigestion of excess sludge with external solid/liquid
organics is a potential approach toward carbon neutrality.
Additionally, the transformation of WWT plants can be
seen with the emergence of decentralized, small-scale setups. By reducing the scale of and decentralizing some water
and wastewater operations, it is possible to lower costs and
improve efficiency.
✔✔ Decentralized WWT lowers the cost for pumping
wastewater to central stations for treatment, then back
to communities for reuse.
✔✔ Not all water uses require potable water quality: residential wastewater can be treated to use for local irrigation, flushing toilets, etc.
✔✔ Decentralization can also have benefits in minimizing
the need to increase infrastructure for high-density infill projects. Building codes could minimize runoff;
storm water reuse reduces drain flows.
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✔✔ Forward osmosis and other emerging low-energy

technologies are proving beneficial as ways to treat
water for potable use locally.

Conclusions
Transitioning toward an integrated energy network requires
many drivers. Important players for implementing and
accelerating this change in resource management include
new regulations from resources agencies, economic incentives for end users, and commercialization of new technologies with improved performance and lower costs. When
rallying the diverse stakeholders toward the triple bottom
line of efficiency, the dialog will revolve around the advantages of the system integration approach for the energy-
water nexus. The emerging system-integration case studies presented here in terms of DR, efficiency approaches,
and reconceptualizing the WWT plant as an energy- and
resource-generation facility all work to enable the goals
of efficiency, reliability, security, flexibility, collaborative
excellence, and technology leadership for both the smart
energy grid and the water network.

Acknowledgment
This publication has emanated from research conducted
with the financial support of the Science Foundation Ireland
(SFI) under the SFI Strategic Partnership Programme Grant
Number SFI/15/SPP/E3125.

For Further Reading
R. Hamilton, B. Braun, R. Dare, B. Koopman, and S. A. Svoronos, "Control issues and challenges in wastewater treatment plants," IEEE Control Syst. Mag., vol. 26, no. 4, pp.
63-69, 2006.
B. Sparn and R. Hunsberger, "Opportunities and challenges for water and wastewater industries to provide exchangeable services," National Renewable Energy Laboratory, Golden, CO, Tech. Rep. NREL/TP-5500-63931,
2015.
S. Pabi, L. Reekie, A. Amarnath, and R. Goldstein.
"Electricity use and management in the municipal water
supply and wastewater industries," Electric Power Research
Institute. Palo Alto, CA, EPRI Rep. 3002001433, 2013.

Biographies
Eoin Casey is with University College Dublin, Ireland.
Sara Beaini is with the Electric Power Research Institute, Palo Alto, California.
Sudeshna Pabi is with the Electric Power Research Institute, Palo Alto, California.
Kent Zammit is with the Electric Power Research Institute, Palo Alto, California.
Ammi Amarnath is with the Electric Power Research Institute, Palo Alto, California.
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