Credit: Schneider Electric

Pump stations are the greatest source of untapped savings for wastewater utilities. This may be because they’re individually small and, thus, not a focal point for life-cycle improvements or due to misconceptions that improving efficiency will compromise reliability. But neither is the case.

Forty percent of pumping costs is related to the energy required to move sewage. Another 38% is related to operations, maintenance, and downtime. Yet many of the energy-saving technologies — particularly variable-frequency drives (VFDs) — utilities have deployed to lower treatment costs aren’t used in collection assets like lift stations.

VFDs at lift stations?

In the past, this was understandable. To cope with reliability, bypass starters were usually provided, which could double control complexity and complicate interactions with related systems. There was a day when lighting could knock out every drive at a pump station simultaneously because the VFD’s control power was derived from incoming AC power protected by surge protectors.

Building an air-conditioned enclosure to protect drives from extreme temperatures, contaminates, and corrosive gases was cost-prohibitive except for large stations. Standby generators to maintain operations during a power outage increased lift station size, complexity, and cost.

Modern VFDs don’t suffer from these limitations. Bypass starters are necessary only in extreme situations and the drives are more immune to surges. Components have matured, so drives have fewer parts, are easier and quicker to repair, and have a much better mean time between failures.

VFDs are now enclosed to protect against dust, humidity, biological contaminates, and chemical gases such as hydrogen sulfide. A National Electrical Manufacturers Association (NEMA) 3R drive rated at 122° F, for example, pushes filtered air past heat sinks and conformal-coated power components and houses sensitive control electronics within a separate compartment.

Finally, there are “smart” VFDs that use algorithms to estimate flow and whether the pump is running within its preferred operating region (POR). When combined with remote telemetry and big data, these units provide key performance data utility managers can use to lower lift station operational costs and extend asset life.

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