Energy Use at Water and Wastewater Treatment Plants
Larry W. Moore, Ph.D., P.E.
University of Memphis
Civil Engineering Department
March 13, 2012
Energy Use at WTPs
Energy
• Energy is the ability or capacity to do work.
• The most common form of energy used in water and wastewater treatment plants is electric energy. Energy Usage
• There are approximately 60,000 public community water utilities in the United States.
• 3 to 4% of the nation’s electricity is used for treating and distributing potable water (and treating wastewater).
• Almost 80% of the total energy is used for transporting water.
• The national average of electricity consumption (power) for conventional surface water plants is 700 to 1800 kWh per million gallon of water treated.
• The power consumption for facilities treating groundwater is greater than that for a surface water. • Additional pumping is required to transport water from aquifers to the surface treatment.
• Energy consumption at water facilities varies significantly depending on:
– Location and topography of the service area
– Plant size
– Treatment processes used
– Age and condition of the water transmission system
Energy Usage
• The majority of energy consumption in water facilities is due to the transportation of:
1. Raw water from the source to the plant.
2. Water within the treatment plant (in‐plant pumping).
3. Treated water from the plant to the distribution system (high service pumping).
Energy Conservation Measures (ECMs):
• ECMs are defined as a physical improvement, plant operation, or equipment maintenance practice that results in a reduction in utility or operating cost. Baseline Measurements
• Refer to the analysis of existing energy bills and operating data to identify:
– The current level of consumption
– Peak energy usage
– Costs for an existing WTP process, or system.
• Are made before implementing any ECMs so that the positive effect of each ECM can be measured.
Simple steps for identifying ECMs:
1. Understand and become familiar with the procedures used by the power company to bill for energy used.
2. Analyze utility bills and request time‐of‐day charts for kW and kWh used. Review the trends of demand charges. If possible, change the schedule for elective operations, such as exercising equipment with large motors during periods of off‐peak demand charge.
3. Review historical energy consumption by the WTP as a whole as well as any individually‐
metered components (36 months of data is preferred; 24 months is required as a minimum).
4. Develop a list of significant energy‐
consuming equipment. Then factor into the assessment the amount of time the equipment is operated.
5. Define current process control procedures.
6. Analyze data collected to identify equipment presenting the greatest opportunity for improvement. Research the unit and identify potential ECMs.
• Pumping is the largest energy user in a water treatment system.
• There are numerous operation and maintenance (O&M) practices that can be implemented to either:
‐ Improve pumping system efficiency, or
‐ Restore original efficiency
• Inefficient pumping operations simply waste energy and money.
Relative Distribution of Energy*
Process
Relative Distribution of Energy by Process
Admin., Lab, Maint.
Finished Water Pumping
Clearwell Storage
Cl2 Feed
In‐Plant Pumping
Backwash Pumping
Hydraulic Surface Wash
Gravity Filtration
Sedimentation
Flocculation
Rapid Mix
Polymer Feed
Coagulant Feed
Raw Water Pumping
2.1
67
0
0.4
8.6
0.5
0.3
3
0.3
1.4
4.7
0.4
0.2
11.1
0
10
20
30
40
50
60
% of Total Energy Use
* For a sample 10 MGD surface water treatment plant
70
80
90
100
Energy Use at WWTPs
• Energy use for a given‐size WWTP may vary significantly depending on:
– Location
– Strength of wastewater
– Level of treatment
– In‐plant recovery
– Type of treatment process
– Mode of operation
• Relative distribution of energy use at a secondary wastewater treatment* plant:
Relative Distribution of Energy by Process
Post Aeration/Cl2 Mixer
3.1
Process
Lighting
2.2
Heating
7.1
Solids Dewatering
7.0
Utility Water
3.6
Effluent Filters
0.9
Thickener, P.S.
1.6
Secondary Clarifier RAS
3.7
Activated Sludge
55.6
Primary P.S., Clarifier
10.3
Headworks
0.4
Raw Water P.S.
4.5
0
10
20
30
40
50
60
70
% of Total WWTP Energy Use
* Sample 7.5 MGD WWTP
80
90
100
• Primary energy uses are those directly associated with on‐site electrical power and fuel requirements.
• Secondary energy uses are associated with materials manufacture and transport and manufacture of consumable chemicals used in the treatment process.
• Most often, aeration is the single largest energy user (commonly around 50% of the total), followed by pumping systems.
• The influent wastewater pumping energy requirements alone can represent 15 to 70% of the total WWTP use of electrical energy depending on:
– WWTP site elevation
– Influent sewer elevation
• Energy costs account for:
– 15 to 30% of the operation and maintenance (O&M) budgets at a large WWTP, and
– 30 to 40% of O&M costs at a small WWTP.
Secondary Treatment
• Overall, aeration devices used for the activated sludge system represent the most significant consumers of energy within a WWTP.
• Most aeration systems are classified as:
– Diffused
– Dispersed
– Mechanical
• The ability of any type of equipment to dissolve oxygen within a wastewater treatment system depends on:
– Diffuser device type
– Basin geometry
– Diffuser depth
– Turbulence – Ambient air pressure
– Temperature
– Spacing and placement of the aeration devices
– Diurnal variations in wastewater flow and organic load
• Having high DO (dissolved oxygen) concentrations within aeration tanks is a waste of energy
• If the system uses blowers, an operator should cut back on blowers or blower output.
• If the facility has coarse bubble diffusers, then a fine‐bubble‐diffuser system that is more efficient and uses less energy should be considered.
• If a facility has surface aerators, the submergence on the unit may be decreased, which results in:
– Less DO concentration
– Less amperage load on the motor (less electrical cost)
• If the liquid level of the basin cannot be adjusted, then:
– VFDs should be installed on the aerators, or
– Aerators should automatically start and stop based on time intervals.
• Activated sludge aeration equipment uses large horsepower motors and multiple units such as:
– Centrifugal‐type blowers
– Positive‐displacement blowers
– High‐speed turbine (HST) blowers
– Surface aerators
• Adding excessive air into the aeration tanks will only result in a waste of energy.
• Improving the operation of aeration systems is one of the best ways to reduce the energy costs for wastewater treatment.
• The amount of oxygen required by the activated sludge process depends on:
– Flow rate
– Organic waste load
• DO probes (sensors), in conjunction with online instrumentation systems perform the critical function of measuring DO levels in the aeration process.
• Typically, oxygen requirements vary throughout the day by a factor of 5 to 7 and can be regulated by automatically controlling systems.
• Air flowrates may be automatically adjusted by:
– Changing blower speed
– Adjusting blower inlet guide vanes
– Operating control vanes
• A key to controlling the activated sludge process (and energy use) is matching oxygen supply to oxygen demand.
• Typically, as wastewater flow and oxygen demand increase in the morning based on the diurnal flow of a WWTP, more aeration is needed.
• Conversely, as flow decreases during the night, the air supply should be reduced.
• A general rule has always been to maintain a DO concentration of 2mg/L in the aeration tanks.
• Once the carbonaceous and nitrogenous oxygen demand have been satisfied, maintaining any DO above 2 mg/L is excessive and a waste of energy.
• A number of treatment facilities maintain a DO concentration of 0.5 to 2.0 mg/L at the end of the aeration basin.