Energy Management & Operational Efficiency

Water and wastewater utilities use substantial amounts of energy to pump, treat, aerate, mix, heat, ventilate, and move water and wastewater. Energy management is the systematic process of understanding where that energy is used, identifying avoidable waste, improving operating efficiency, and verifying that improvements produce real savings without reducing treatment performance or reliability.

Operators play an important role because many energy costs are directly affected by everyday decisions involving pumps, blowers, aerators, process control, storage, chemical feed, and equipment maintenance.

Why Energy Management Matters

Energy can represent one of the largest controllable operating costs for a water or wastewater utility.

Improving energy efficiency can help a utility:

• reduce operating cost;
• reduce unnecessary equipment run time;
• extend equipment life;
• reduce peak electrical demand;
• improve process control;
• identify failing equipment;
• support long-term financial planning;
• reduce greenhouse-gas emissions.

Energy Management Begins With a Baseline

Before a utility can improve energy performance, it must understand current energy use.

A baseline establishes a reference point for comparison.

Baseline information can include:

• monthly electricity use;
• monthly electrical cost;
• peak demand;
• water produced;
• wastewater treated;
• pump run time;
• blower run time;
• process load;
• seasonal conditions.

Energy Use Versus Energy Cost

Energy use and energy cost are related but not identical.

A utility can use the same number of kilowatt-hours in two months but pay different amounts because of:

• rate changes;
• time-of-use pricing;
• demand charges;
• seasonal rate structures;
• power-factor penalties;
• other utility tariff provisions.

Kilowatt and Kilowatt-Hour

Operators should distinguish power from energy.

Kilowatt, or kW, is a measure of power.

Kilowatt-hour, or kWh, is a measure of energy used over time.

Basic Energy Calculation

The basic relationship is:

Energy = Power × Time

For electrical use:

kWh = kW × hours

Example

A 50-kW motor operates for 10 hours.

Energy Use = 50 kW × 10 hr

Energy Use = 500 kWh

Energy Cost Calculation

If electricity costs $0.12 per kWh:

500 kWh × $0.12/kWh = $60

This example considers only the energy charge. Actual electric bills can also include demand and other charges.

Energy Intensity

Energy intensity relates energy use to the amount of service provided.

For drinking-water production:

Energy Intensity = kWh ÷ Million Gallons Produced

For wastewater treatment:

Energy Intensity = kWh ÷ Million Gallons Treated

Example of Energy Intensity

A drinking-water plant uses 240,000 kWh during a month and produces 120 MG.

240,000 kWh ÷ 120 MG = 2,000 kWh/MG

This value becomes useful when tracked consistently over time.

Benchmarking

Benchmarking compares energy performance against:

• the facility's previous performance;
• similar facilities;
• established performance targets.

Benchmarking helps determine whether energy performance is improving or deteriorating.

Compare Similar Operating Conditions

Energy use can change because of legitimate operating differences.

Examples include:

• higher flow;
• higher pumping head;
• higher organic loading;
• colder wastewater;
• more stringent treatment requirements;
• seasonal source-water changes.

An increase in energy use is not automatically evidence of inefficiency.

Major Energy Users

Two of the most important energy-consuming processes in water and wastewater utilities are:

• pumping;
• aeration.

Other significant uses can include:

• mixing;
• solids handling;
• dewatering;
• heating;
• ventilation;
• UV disinfection;
• compressed air;
• building systems.

Pumping Energy

Pumping energy depends on several variables, including:

• flow;
• total dynamic head;
• pump efficiency;
• motor efficiency;
• operating point;
• run time.

Total Dynamic Head

Total dynamic head includes the head the pump must overcome.

Important components can include:

• static head;
• pressure head;
• friction loss;
• minor losses.

Higher required head generally increases pumping energy.

Pump Efficiency

A pump converts mechanical energy into hydraulic energy.

Pump efficiency can be expressed as:

Pump Efficiency = Hydraulic Power Output ÷ Mechanical Power Input × 100

Operating far from the pump's efficient range can increase energy use and mechanical stress.

Best Efficiency Point

A centrifugal pump has a Best Efficiency Point, commonly called BEP.

Operating reasonably near the intended efficient range can improve:

• energy performance;
• bearing life;
• seal life;
• hydraulic stability;
• overall reliability.

Oversized Pumps

An oversized pump can waste energy when the required system flow is much lower than the pump's preferred operating range.

Common symptoms can include:

• frequent throttling;
• short cycling;
• operating far from BEP;
• high energy use;
• excessive wear.

Throttling

Partially closing a discharge valve increases system resistance and reduces flow.

This can be operationally necessary in some situations, but continuously wasting head across a throttled valve can be inefficient.

Variable Frequency Drives

A Variable Frequency Drive, or VFD, changes motor speed.

For suitable variable-flow pumping applications, speed control can reduce energy use compared with continuously operating at full speed and throttling flow.

VFDs Are Not Automatically the Best Solution

A VFD should be evaluated against:

• system curve;
• static head;
• required flow range;
• pump curve;
• minimum pump speed;
• motor suitability;
• harmonic and electrical considerations.

Pump Scheduling

Utilities with multiple pumps can reduce energy cost by selecting efficient pump combinations.

Examples include:

• operating one efficient pump instead of two inefficiently loaded pumps;
• avoiding unnecessary simultaneous pump operation;
• using storage to shift pumping when operationally appropriate;
• matching pump capacity to actual demand.

Avoid Excessive Cycling

Frequent pump starts can:

• increase mechanical wear;
• increase electrical stress;
• reduce equipment life;
• create hydraulic transients.

Energy management must be balanced with equipment reliability.

Storage and Energy Management

Drinking-water storage can sometimes provide flexibility in pump scheduling.

For example, a system may be able to pump more water during periods of lower electrical cost and use stored water during higher-cost periods.

However, operators must also protect:

• water age;
• disinfectant residual;
• pressure;
• fire-flow capacity;
• required storage levels.

Demand Charges

Many electrical tariffs include a demand charge based on the highest electrical demand during a billing interval or period.

Therefore, utility cost can increase sharply when several large motors operate simultaneously even if total monthly kWh does not change greatly.

Example of Peak Demand

Suppose three 100-kW pumps start and operate at the same time.

The combined load is approximately:

300 kW

If operating needs allow pumps to be scheduled differently, the utility might reduce peak demand.

Do Not Reduce Demand at the Expense of Service

Demand-charge management must never compromise:

• minimum pressure;
• fire protection;
• treatment capacity;
• wastewater conveyance;
• permit compliance;
• process stability.

Aeration Energy

Aeration is often one of the largest energy users at activated-sludge wastewater treatment plants.

Energy is required to supply oxygen and provide mixing.

Aeration systems can include:

• centrifugal blowers;
• positive-displacement blowers;
• turbo blowers;
• diffused aeration;
• mechanical surface aerators.

Over-Aeration

Providing more air than the biological process requires can waste substantial energy.

Possible indicators of excessive aeration include:

• consistently high dissolved oxygen;
• blower operation above process need;
• high airflow during low-load periods.

Under-Aeration

Reducing airflow too aggressively can cause:

• low dissolved oxygen;
• poor treatment;
• odor;
• loss of nitrification;
• settling problems;
• permit violations.

Energy optimization means meeting process oxygen demand efficiently, not simply minimizing blower operation.

Dissolved Oxygen Control

Automated dissolved-oxygen control can adjust airflow to process demand.

Properly designed control can help reduce unnecessary aeration while maintaining biological treatment.

Clean Diffusers

Fouled diffusers increase resistance and can reduce oxygen-transfer efficiency.

Cleaning or maintaining diffusers can reduce blower pressure requirements and improve energy performance.

Process Optimization

Energy efficiency can often be improved through operational changes before major equipment replacement is required.

Examples include:

• optimizing pump scheduling;
• optimizing aeration;
• reducing unnecessary mixing;
• adjusting operating setpoints;
• repairing leaks;
• reducing excessive pressure;
• improving preventive maintenance;
• reducing unnecessary equipment run time.

Energy Audit

An energy audit evaluates where energy is used and identifies opportunities for improvement.

An audit can examine:

• utility bills;
• equipment inventory;
• motor sizes;
• run times;
• pump performance;
• blower performance;
• process loads;
• lighting;
• building systems;
• operating schedules.

Walk-Through Audit

A basic walk-through audit can identify obvious problems such as:

• equipment running unnecessarily;
• air leaks;
• poorly maintained motors;
• throttled pumps;
• failed controls;
• excessive ventilation;
• unused equipment left energized.

Detailed Energy Audit

A more detailed audit can include:

• temporary metering;
• pump testing;
• blower testing;
• motor-load measurement;
• power-factor measurement;
• process modeling;
• economic analysis.

Energy Management Cycle

A practical energy-management cycle is:

1. establish baseline energy use;
2. identify major energy users;
3. perform an energy assessment;
4. identify improvement opportunities;
5. prioritize projects;
6. implement improvements;
7. measure results;
8. repeat the process.

Measure Before and After

An energy-efficiency project should be evaluated using comparable before-and-after data.

Possible performance indicators include:

• kWh per MG;
• energy cost per MG;
• kW peak demand;
• blower kWh;
• pump kWh;
• treatment performance;
• maintenance cost.

Normalization

Energy performance should be normalized when operating conditions change substantially.

For example, wastewater aeration energy can increase because influent organic load increased, not because the aeration system became less efficient.

Energy Conservation Measure

An Energy Conservation Measure, or ECM, is a specific action intended to reduce energy use or cost.

Examples include:

• high-efficiency motors;
• pump replacement;
• blower replacement;
• VFD installation;
• lighting upgrades;
• control improvements;
• process optimization.

Simple Payback

Simple payback estimates how long it takes energy-cost savings to recover the initial project cost.

The formula is:

Simple Payback = Project Cost ÷ Annual Savings

Example

An energy project costs $120,000 and is expected to save $30,000 per year.

$120,000 ÷ $30,000/year = 4 years

The simple payback is four years.

Simple Payback Has Limitations

Simple payback does not fully account for:

• equipment life;
• maintenance savings;
• financing;
• interest;
• future energy prices;
• replacement costs;
• residual value.

More complete financial analysis can be appropriate for major projects.

Life-Cycle Cost

Energy-efficient equipment can have a higher purchase price but lower total life-cycle cost.

Life-cycle analysis can consider:

• purchase;
• installation;
• energy;
• maintenance;
• repair;
• replacement;
• disposal.

Maintenance and Energy Efficiency

Poor maintenance can increase energy use.

Examples include:

• worn pump impellers;
• misalignment;
• damaged bearings;
• clogged filters;
• fouled diffusers;
• poor lubrication;
• air leaks;
• failed instrumentation.

Instrumentation

Reliable energy management depends on reliable measurements.

Useful instrumentation can include:

• flow meters;
• pressure transmitters;
• power meters;
• dissolved-oxygen probes;
• level instruments;
• SCADA trend data.

SCADA and Trend Analysis

SCADA can help operators identify:

• unnecessary equipment operation;
• peak-load periods;
• pump efficiency changes;
• abnormal blower operation;
• high-pressure conditions;
• process instability.

Power Factor

Power factor is a measure of how effectively electrical power is converted into useful work.

Poor power factor can increase electrical system loading and, under some utility tariffs, can increase cost.

Power-factor correction should be evaluated by qualified electrical personnel.

Energy Resilience

Energy efficiency and power resilience are related but different.

Energy efficiency reduces unnecessary energy use.

Power resilience focuses on the ability to continue critical operations during:

• grid outages;
• storms;
• equipment failures;
• fuel interruptions.

Backup Power

Generators and other backup systems do not necessarily reduce routine energy use, but they can protect critical treatment and pumping functions during outages.

Energy planning should therefore consider both:

• efficiency;
• reliability.

Renewable Energy

Some utilities use renewable or recovered energy sources such as:

• solar power;
• wind power;
• biogas;
• combined heat and power.

Wastewater plants with anaerobic digestion can sometimes recover methane-rich biogas for useful energy.

Combined Heat and Power

Combined heat and power, or CHP, can use fuel or digester gas to generate electricity while capturing useful heat.

Potential benefits depend on:

• available fuel;
• energy demand;
• equipment efficiency;
• maintenance capability;
• economic conditions.

Energy Efficiency and Treatment Reliability

An energy project is not successful if it causes poor treatment.

Examples of bad energy management include:

• turning off aeration below biological needs;
• reducing pumping below required service;
• allowing low pressure;
• reducing mixing until solids settle improperly;
• disabling ventilation required for safety.

Example: Pump Efficiency

A pump is consuming more power than usual at the same flow and head.

Possible causes can include:

• mechanical wear;
• misalignment;
• impeller damage;
• motor problems;
• measurement error.

An energy trend can therefore reveal an equipment-maintenance problem.

Example: Aeration Optimization

A plant maintains dissolved oxygen at 6 mg/L throughout the aeration basin even though stable treatment can be achieved at a lower approved operating range.

If process conditions allow, improved DO control can reduce blower energy while maintaining treatment performance.

Example: Peak Demand

Several large pumps routinely start during the same electrical demand period.

If hydraulic and operational requirements allow, scheduling some equipment at different times can reduce peak demand charges.

Example: Water-System Pressure

A distribution system consistently operates at pressure far above what is required.

Excessive pressure can increase:

• pumping energy;
• leakage;
• main-break risk.

Any pressure adjustment must still maintain required service and fire-flow conditions.

Example: Equipment Replacement

An old blower has low efficiency and high maintenance cost.

A replacement project should consider:

• energy savings;
• maintenance savings;
• capital cost;
• equipment life;
• process reliability.

Common Exam Mistakes

• Confusing kW with kWh.
• Assuming lower kWh always means lower electric cost.
• Ignoring electrical demand charges.
• Comparing energy use without considering flow or process loading.
• Assuming every pump should operate at full speed.
• Assuming a VFD always saves energy.
• Ignoring pump efficiency and system head.
• Reducing aeration without considering treatment requirements.
• Ignoring maintenance when evaluating energy efficiency.
• Evaluating an energy project only by purchase price.
• Using simple payback as the only financial measure for every project.
• Reducing energy use at the expense of safety, compliance, or reliability.

A Practical Energy Review

1. Collect utility bills and operating data.
2. Establish baseline energy use and cost.
3. Calculate useful energy-intensity metrics.
4. Identify major energy-consuming equipment.
5. Review pumping and aeration performance.
6. Review demand charges and operating schedules.
7. Inspect equipment condition.
8. Identify operational and capital improvement opportunities.
9. Estimate energy and cost savings.
10. Evaluate project cost and payback.
11. Implement selected improvements.
12. Measure performance after implementation.

What to Remember for the Exam

• Energy management begins by establishing a baseline.
• kW measures power; kWh measures energy.
• Energy equals power multiplied by time.
• Energy intensity can be expressed as kWh per million gallons.
• Benchmarking compares energy performance over time or against appropriate reference facilities.
• Pumping and aeration are major energy users in water and wastewater utilities.
• Pumping energy depends on flow, head, efficiency, and run time.
• Operating pumps far from their efficient range can waste energy and increase wear.
• VFDs can improve efficiency in suitable variable-flow applications but are not automatically beneficial in every system.
• Electrical demand charges can be affected by simultaneous operation of large equipment.
• Aeration should meet biological oxygen demand without unnecessary over-aeration.
• Energy audits identify where energy is used and where efficiency improvements may be possible.
• Energy management is a repeating cycle of baseline, assessment, improvement, measurement, and review.
• Simple payback equals project cost divided by annual savings.
• Life-cycle cost provides a broader view than initial purchase price.
• Poor maintenance can increase energy consumption.
• SCADA and instrumentation can help identify energy and process trends.
• Energy efficiency must be balanced with treatment reliability, worker safety, hydraulic requirements, and regulatory compliance.
• Backup power improves resilience but is not the same as energy efficiency.
• Successful energy management reduces waste without sacrificing service or treatment performance.