Energy Benchmarking of Sewage Treatment Works: kWh per Population Equivalent, Guide and Target Values, and Where the Energy Goes

The right metric: kWh per population equivalent per year

Energy per cubic metre treated is the number most often quoted and the least useful. It rewards dilute sewage, infiltration and wet weather, and penalises a works on a tight, separate system that receives strong sewage. What a works must spend is set by the load it removes, not the water it passes, so the comparable basis is energy per population equivalent of load, usually PE_BOD60 (60 g BOD₅ per day) or PE_COD120, measured at the inlet rather than taken from the design population. Germany's benchmarking scheme, in use for two decades, is built on this basis and is the reference set below.

A second number anchors the discussion. Raw sewage contains about 155 kWh per PE per year of chemical energy in its organic content (14 kJ per gram of COD at 110 g COD per PE per day), of which anaerobic digestion with combined heat and power can recover a substantial part. A works that consumes 30 kWh/PE/y and generates 20 from biogas is not far from energy neutrality; the best-performing European works have reached it.

Guide and target values

The German benchmarks distinguish a guide value (the average of well-run plants) from a target value (the best quartile), by process and by size class, for plants with full nitrification and, for the larger classes, nutrient removal. Values are in kWh per PE_BOD60 per year for total works electricity:

Within an activated sludge works with digestion the benchmarks split further: aeration about 18 (guide) and 10 (target) kWh/PE/y, the whole digester and sludge line about 30 to 35 (guide) and 18 (target), biogas yield of 20 (guide) and 30 (target) litres per PE per day, and electricity self-supply of 60 percent (guide) rising to 100 percent (target). Pumping is benchmarked per cubic metre and metre of lift, about 6 Wh/m³/m against a target of 4. All assume a plant within normal conditions: sewage volume up to about 250 L per PE per day, inlet pumping lift under 3 m and an influent N:BOD ratio below 0.2. A works outside those conditions is benchmarked with an adjustment or against its own history.

Across the table the pattern is the message: small works use two to three times the energy per PE of large ones, mainly because aeration cannot be turned down efficiently and fixed loads (pumps, mixers, controls, buildings) are spread over fewer people; and processes without separate sludge stabilisation pay for it in aeration energy.

Where the energy goes

At a nitrifying activated sludge works, aeration takes 50 to 65 percent of the electricity. The rest is inlet and return sludge pumping (10 to 20 percent), sludge thickening, dewatering and digester mixing (10 to 15 percent), mixers in anoxic zones and tanks, and buildings and controls. Benchmark comparison therefore starts with aeration and asks three questions: how much oxygen is actually required (the load and the degree of nitrification and denitrification, since denitrification recovers about 2.9 kg of oxygen equivalent per kg of nitrate-N reduced), how efficiently it is transferred (fine-bubble diffusers at 3 to 6 percent standard transfer per metre of depth against 1 to 2 percent for coarse bubble and mechanical aerators, degraded by the alpha factor of the mixed liquor), and how the blowers follow the demand (dissolved oxygen control, ammonia-based control, turndown range and the efficiency of the blower type at part load).

A simple and revealing calculation is the aeration energy per kilogram of oxygen delivered. Design values for fine-bubble systems with efficient blowers are 0.4 to 0.6 kWh per kg O₂ in the field; measured values at under-performing works are often 1 kWh/kg or more, because diffusers have fouled, dissolved oxygen is held at 3 mg/L instead of 1.5, and the blowers run on inlet throttling.

Worked assessment: a 50,000 PE works

An activated sludge works with anaerobic digestion serves 50,000 PE_BOD60 as measured at the inlet, and its metered electricity is 2.0 GWh per year, with 0.6 GWh generated by a biogas engine.

• Specific consumption: 2,000,000 / 50,000 = 40 kWh/PE/y, against a guide value of 30 and a target of 18 for its class. This works is 10 kWh/PE/y above average practice and 22 above best practice: 500 MWh/y against the guide and 1.1 GWh/y against the target, worth roughly £100,000 to £200,000 a year at typical industrial tariffs and several hundred tonnes of CO₂.
• Aeration sub-meter: 1.2 GWh, so 24 kWh/PE/y against a guide of 18 and a target of 10. Aeration alone explains more than half the gap.
• Biogas: 0.6 GWh generated is 30 percent self-supply against a guide of 60 percent; at a typical engine efficiency of 35 percent that implies biogas energy of 1.7 GWh, which at about 6 kWh per m³ of biogas is 285,000 m³ a year, about 16 L/PE/d of gas against a guide of 20 and target of 30. So the digester is under-producing, which points to feed solids, retention time, mixing or a gas leak to flare.

One meter reading has been turned into two prioritised investigations, aeration efficiency and digester performance, each with a quantified prize. That is what benchmarking is for; it does not say what is wrong, it says where to look and how much it is worth.

Closing the gap

Measures, roughly in order of cost-effectiveness from utility programmes in Europe, Australia and the UK:

1. Control. Dissolved oxygen set point reduction and ammonia-based aeration control; intermittent aeration or anoxic zones to recover oxygen through denitrification; typically 10 to 25 percent of aeration energy for little capital.
2. Diffuser maintenance and replacement. Fouled or torn diffusers can halve transfer efficiency; cleaning and timely replacement, and a move from coarse to fine bubble, are the next step.
3. Blower technology. Turbo or high-efficiency screw blowers with wide turndown in place of inlet-throttled positive displacement or multistage centrifugal units; the gain is largest at part load, which is most of the year.
4. Sludge line. Thickening before digestion to raise retention and gas yield; digester mixing optimisation; thermal hydrolysis where the works is large enough, which raises gas yield and dewaterability (see the thermal hydrolysis guide); engine efficiency and heat use.
5. Pumping. Impeller trimming, variable speed on return sludge and inlet pumps, and removing throttled valves; pumping benchmarks per cubic metre and metre of lift make the losses visible.
6. Process change. At the largest works, mainstream or sidestream anammox, primary enhancement to divert carbon to digestion, and co-digestion of imported organics push a works toward energy neutrality; these are capital programmes justified by the benchmark gap.

Benchmarking has to be repeated: an annual energy balance per PE for every works, with sub-metering of aeration, sludge and pumping, is the minimum instrumentation, and it is also what the net zero commitments of the UK water sector will require as evidence. The design-stage equivalent is to specify the aeration and sludge systems against the target value rather than the guide value, which is where process design with energy as a design parameter pays back over the life of the works. Specific energy of other unit processes, membranes in particular, is discussed in the MBR guide.

How to benchmark the energy use of a sewage works

1. Measure the load. Annual average inlet BOD or COD load from sampling, converted to population equivalents at 60 g BOD or 120 g COD per PE per day; do not use design population.
2. Meter the electricity. Total imported and generated electricity for the year, with sub-meters or estimates for aeration, sludge treatment and pumping.
3. Compute specific values. kWh per PE per year for the works and for each sub-system; biogas litres per PE per day; self-supply percentage; pumping Wh per cubic metre per metre of lift.
4. Compare with guide and target. By process type and size class; check the plant is within the benchmark conditions for flow per PE, inlet lift and nitrogen ratio.
5. Quantify the gap. MWh per year and cost against guide and target for each sub-system; rank by prize.
6. Investigate and act. Detailed energy audit of the largest gaps, implement control and maintenance measures first, then capital measures; repeat the benchmark annually.