91st Avenue Treatment Plant

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The 91st Avenue WWTP Chlorination Improvements project involved the replacement of a 120-ton bulk chlorine system with a system consisting of 32 one-ton chlorine cylinders.  The plant has a treatment capacity exceeding 180 mgd. The existing chlorination equipment and piping was replaced while the plant was in full and normal operating condition. The plant’s treated effluent is used at the Tres Rios Wetlands and as cooling water for the Palo Verde Nuclear Generating Station.

The project also involved the development and coordination of the Maintenance of Plant Operation (MOPO) procedures. The chlorination of the plant effluent had to be maintained at all times during construction. The project also included building structural, electrical, and HVAC modifications. The equipment installed included the following:

  • Five 10,000 lb/day chlorine evaporators
  • Seven 10,000 lb/day chlorinators
  • Five 3” ejectors, and two 2” injectors
  • Sixteen 1-ton cylinder emergency shut off valves
  • Nitrogen purge system
  • Two-trolley monorail and one hoist system
  • Gas monitoring and alarm system
  • Process control system monitoring

The highlight of the project was the discovery of the failure mechanism for the PVC/CPVC pipe material. Initial theories assumed that the periodic failure was due to a combination of sunlight, heat, and age. Our design team performed an infra-red spectroscopy analysis and found that the pipe failure was due to paint coating applied to the pipe surface. Testing showed the virgin PVC pipe had an impact strength of 190 ft-lbs. Upon application of an epoxy coating, the impact strength dropped to around 10 ft-lbs. This project won the ACEC Grand Award.

Wastewater & Stormwater Collection Systems


GHD has the knowledge base and systems to tackle any wastewater transportation problem — for new systems or the upgrading of existing assets.

Having designed more than 1000 sewage pump stations and over 6000km of sewers, our industry professionals challenge conventional approaches to establish new cost-effective benchmarks.

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Wastewater Treatment & Recycling


Because all wastewaters are different, it is necessary to work with a team that can combine comprehensive knowledge with practical experience in the fields of chemistry, biology, hydraulics, mechanical processes/equipment, instrumentation and control, materials handling and plant layout.

GHD can deliver on all the process components that make up a wastewater treatment plant. We have designed and project managed more than 300 wastewater treatment plants (including major municipal facilities processing 500Ml a day and industrial projects).

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