Selecting Treatment Processes: Comparing the Energy Consumption of 2 Plants

Last Modified: Sep 01, 2007


  • Eileen M. Feldman, P.E., Hazen and Sawyer
  • Richard A. Ruge, Westchester Joint Water Works
  • George Logan, Aquarion Water Company, Bridgeport, CT
  • Ian A. Crossley, C. Eng., Hazen and Sawyer
  • Richard E. Peters, P.E., Hazen and Sawyer

Drinking water treatment plants of all process types consume a significant amount of energy. When planning for future treatment plants, operational costs must be factored into the treatment process selection. In this paper, the energy usage of an immersed membrane treatment plant is compared to that of a dissolved air flotation and filtration (DAFF) treatment plant.

This paper describes how and where energy is used in typical immersed membrane and dissolved air flotation/filtration plants. The energy usage comparison evaluates major consumers of power including differential head, pumps, compressors, blowers, mixers and flocculators.

The evaluation compares the 20-mgd Rye Lake immersed membrane plant, currently being constructed for the Westchester Joint Water Works (WJWW), with Aquarion Water Company’s 30-mgd Stamford WTP, an operational dissolved air flotation and integral filter (DAFF) facility. Both plants are designed meet similar water quality goals, although the source water quality of the membrane plant is superior to that of the DAF plant. The energy usage data has been normalized (on a unit cost basis) to enable a meaningful comparison between the differently-sized plants.

The Rye Lake WTP’s immersed membrane system was designed with several energy-saving techniques and control strategies. These include a siphon system instead of dedicated permeate pumps for filtered water; a control system for blowers that incorporates two modes of operation that save power (air cycling and intermittent air operation); and variable frequency drives (VFD) control all major pumps.

The Stamford WTP’s DAF system was designed with maximum nozzle efficiency to regulate the mass flow and pressure of dissolved air entering the system, thereby decreasing the energy used at the facility. VFDs were included for all major pumps to maximize their efficiency.

The energy-efficiency enhancing features of both plants are described. This comparison of the energy usage at two types of treatment plants provides insight as to selecting cost-effective treatment processes, considering life-cycle operating costs.

For a copy of the full paper, please contact the author at

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