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Aluminum domes for biosolids tanks

Project type:

Wastewater treatment

Services:

Designs, specifications, contract administration, site supervision

Construction cost:

CDN $5 million

An innovative solution for storing biosolids

Regional Municipality of Halton / Oakville, Ontario

Opportunity

In the Regional Municipality of Halton, biosolids from seven wastewater treatment plants are spread over farmland as fertilizer. Applicators mounted on farm tractors inject the material 10 to 15 centimeters (4 to 6 inches) into the soil. This reduces the change of runoff and delivers the material to a depth where crop roots can use it efficiently.

During the winter and during wet weather, biosolids cannot be applied to farmland, and storage is needed. In 1986, the W.A. Bill Johnson Biosolids Management Centre began serving as a central receiving point. Tanker trucks deliver the material to the facility, where it is stored in concrete storage tanks.

The facility was originally built with eight tanks. The floor of each was lined with a high-density polyethylene membrane over a base of mortar sand. To control odor and prevent rainwater from entering the tanks, tanks were covered with high-density floating polyethylene discs with foam inserts and a flexible side skirting. Over time, however, friction broke down the floating discs, leading to costly repairs.

Solution

When two additional tanks were added in 1998, Mott MacDonald designed aluminum covers for them, constructed as fixed geodesic domes. The success of the new covers led the Municipality to install them on the remaining eight tanks. We prepared design specifications for the new domes, reviewed shop drawing submittals, and supervised the construction of the covers.

The first few domes were constructed in a field and lowered into position by crane. When space became unavailable, an innovative method was used to build the domes from the inside of the tanks. To avoid damaging the tank liners, the tanks were filled with water and the domes were constructed from a floating barge.

In 2006, Mott MacDonald was retained to provide all consulting services required for a series of upgrades to the facility. We were responsible for the complete project, including operator training after construction. Upgrades included the following:

  • New building to house the raw water storage tank, dry polymer storage, and solution makeup system
  • Improved instrumentation and monitoring of the biosolids tank levels and status of the biosolids pumps and surface water diversion valve
  • Improved monitoring of the stormwater quality management pond and pumping system
  • New instrumentation systems for the existing PLCs, networked to SCADA computers in the administration center

Outcome

Upgrades to the Biosolids Management Centre have made it possible to handle greater demand for efficiently, and to continue the sustainable practice of recycling biosolids as fertilizer.

The aluminum dome covers make it easier to thoroughly clean each tank with high-pressure hoses once it has been emptied. The tank floor can be accessed when required and the contents of the tank are completely sheltered from precipitation.

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