Goulburn Valley Water in Australia has undertaken upgrade work to the 266-million-gallon (50-megaliter) High Rate Anaerobic Lagoon (HRAL) located at their Daldy Road, Shepparton, Victoria, waste management facility. The lagoon has an area of 538,196 square feet (50,000 m2). The upgrade work includes removing the existing cover, which has reached the end of its service life; desludging the lagoon; remediating related infrastructure; and installing a new cover. The HRAL cover allows the anaerobic processing of effluent, manages odors and allows the produced methane gas to be beneficially used, thereby reducing greenhouse gas emissions and providing energy from waste.
The reactor has a unique combination of residential inflows with a high proportion of industrial wastewater resulting in an inflow equivalent to that of a population of 1.1 million. To cater to this, the Shepparton wastewater flows are processed by the HRAL.
The lagoon is an earthen basin with a geosynthetic cover, which allows Goulburn Valley Water to control odor, and enables the harvesting and storage of methane gas, which is either combusted to reduce greenhouse emissions or sold to a third party and used to generate electricity. The geosynthetic cover provides a durable gas seal at a lower cost than rigid structure alternatives.
The cover must manage and cope with repeated pressurization/depressurization, ultraviolet (UV) radiation, high temperatures, contact with by-products of the treated waste, and attack from wildlife, such as corellas, a type of bird related to cockatoos. A severe windstorm in 2015 also contributed to the development of a large, 656-foot (200-m) tear in the cover. These assets have a finite life, and, following several maintenance activities, it was deemed to be at the end of its service life. There are few anaerobic covers of this size globally, and they present a significant challenge to remove and replace. With the interval between cover replacement cycles it is also good practice to incorporate in the replacement project new design and materials developments to optimize performance of the replacement cover. The task to replace the existing cover commenced in March 2020 by Fabtech Australia Pty Ltd.
The company started the design work in January 2020. The concept design was developed into a detailed design by March 2020. The original cover was a polypropylene material, and several candidate materials were considered for the new cover. Following extensive analysis, high-performance 78-mil (2-mm) thick high-density polyethylene (HDPE) material was selected. HDPE has excellent chemical resistance characteristics, which is essential in this application, combined with excellent UV aging characteristics and is one of the most robust materials in the industry for this particular use. Fabtech undertook six months of rigorous accelerated laboratory testing to demonstrate that the proposed material would exceed the required design life in this application. With these tests and laboratory studies, the company was able to predict the material life expectancy, giving confidence to Goulburn Valley Water that the proposed material was suitable in this application and fit their requirements.
While the original cover was installed onto an inert lagoon, the replacement cover had to be installed on the active anaerobic reactor. This presented a number of design, construction and safety challenges. As part of Fabtech’s proposal due to the size of the lagoon (486 feet x 738 feet [148 m x 225 m]), a moving work platform was designed and fabricated to facilitate the overall cover installation process. The moving work platform comprises 48 individual elements, each being 20 feet x 20 feet (6 m x 6 m) in size and weighing almost 1.1 ton (1 tonne) each.
Once mobilized to site, Fabtech commenced remediation of the concrete edge beam. In the original design, the concrete-anchoring beam was unprotected and had suffered substantial chemical degradation, and, hence, it required replacement or remediation, as necessary. The new edge beam design is generally identical to the old design and was constructed adjacent to the out wall of the existing beam, between April and July 2020.
The construction sequence allowed the new anchoring-beam elements to be constructed before the existing cover was removed.
Minimizing gas emissions while removing the old cover and installing the new cover was a key objective for the project. Several project activities were overlapped to compress the “cover-off” duration. As the new ring beam works came close to conclusion, Fabtech commenced removal of the old cover to allow desludging of the reactor lagoon.
Desludging barges were installed in the lagoon, achieving on an average of >55 dry tons (>50 dry tonnes) of sludge/scum per day. These works extended over a period of four weeks with a total of >1,323 dry tons (>1,200 dry tonnes) removed from the basin.
Once desludging had been completed, installation of the new cover started in early October. The design of the new cover divides the lagoon into two zones, namely “reactor” and “settlement” zones. These two zones of the lagoon are separated by a gas curtain, allowing them to operate independently of each other. This allowed the construction program to be structured in two phases so that the reactor zone could be completed and commissioned while the remainder of the cover is being installed. This allowed gas harvesting from the active reactor zone, which reduced odor emissions at an early stage during construction.
The reactor zone cover has been commissioned and is now operation. Construction of the settlement area cover is progressing.