Emergency Diesel Generators for Water and Wastewater Treatment Plants

Which loads are critical

At a drinking water plant, the critical loads are usually raw water intake pumps, treatment process equipment, chemical dosing, high-lift pumps and controls. At a wastewater plant, inlet pumping, screening, aeration blowers and the controls come first. Many plants also back up laboratory, lighting and security systems.

List these loads with their ratings and starting methods. Not every pump must run at once; the operators can say what the minimum safe operation looks like.

Pump and blower starting decides the size

Large induction motors draw several times their full-load current when started direct-on-line. A generator that easily carries the running load may still collapse in voltage when a big pump starts. Variable speed drives and soft starters reduce the starting demand, but drives also create harmonics that the generator must tolerate.

A proper sizing study models the starting sequence, the voltage dip on each step and the harmonic content. Our generator sizing chart gives the basic kW, kVA and amps relationships.

Runtime and fuel

Outages at water plants can last days after storms or floods, and fuel deliveries may be difficult in exactly those conditions. Many utilities choose longer on-site fuel storage than a commercial building would, plus a priority resupply agreement.

Flood risk should also shape where the generators and tanks sit; raising them above the flood level is common practice.

Transfer scheme and controls

Larger plants often have several incoming feeders and several generators connected to a main switchboard. The transfer scheme must restart the critical pumps in the right order after power returns, so that the generators are not overloaded by everything starting at once.

Integration with the plant SCADA lets operators see generator status, fuel level and alarms alongside the process.

Small pumping stations and lift stations

Water networks also include many small pumping and lift stations spread across a town. Each one may only need a modest generator, but an overflow at any of them can cause environmental damage. Options include permanent sets at the most critical stations and a plan to bring towable sets to the others.

Standardising the connection points, cable sizes and phase rotation at every station makes it much faster to connect a towable set during an outage. Clear labelling and a printed connection procedure at each station help crews who may be working at night in bad weather.

Testing and readiness

Weekly or monthly exercising, annual load tests and a full test with the plant on generator power confirm readiness. A test that actually transfers the process loads is the only way to prove the motor starting sequence works. Our page on emergency generator installation covers the commissioning steps.