After 20 years of operation, the performance of the main anaerobic lagoon at McCain Foods’ processing facility in Ballarat had deteriorated due to a progressive build of sludge and resulting reduction in capacity and effluent residence time. Other issues identified were evidence of leakage of effluent through the liner, a significant floating scum buildup affecting surface water drainage and fugitive leaks in the cover resulting in biogas leakage and increased oxygen levels under the cover which in turn had resulted in flare damage.
During the initial installation of the original liner & cover, effluent circulation pipes and gas extraction pipes had been installed significantly reducing access around all sides of the lagoon. The original common liner & cover anchor trench was also considered very shallow by current standards.
The client’s design brief was for a negative pressure cover with the ability to also inflate as a positive pressure cover if required in an emergency. The cover material was also to be able to withstand the effects of the effluent for a period of 20 years.
While the lagoon was full the existing cover was cut into sections and removed with a long boom excavator. A dredge was placed in the full lagoon before the water level was lowered to facilitate the removal of sludge that had built up in the base of the lagoon. The dredge was then removed from the empty storage with a slewing crane.
The existing liner was cleaned and electronically leak tested using the water lance test method ASTM D7703-15 to locate defects in the liner for patch repair.
The cover material selected was Layfield’s Enviro Liner EL7080. Enviro Liner 7080 is a 2.00mm thick triple layer coextruded polyolefin with fortified HDPE skins coextruded onto a flexible Enviro Liner 6000 core layer. The fortified HDPE skins provided excellent chemical resistance to the effluent and the flexible core allows the cover to flex if required to operate under positive pressure.
A new deeper anchor trench was constructed outside the existing anchor trench to ensure a more robust gas tight seal however this also resulted in access around the lagoon being further restricted. An Enviro Liner EL6060 gas sealing skirt was welded to the exposed edge of the existing liner and temporarily placed into the new anchor trench while the Enviro Liner EL7080 cover was prefabricated and concertina folded at one end of the anaerobic reactor ready for deployment.
Due to the restricted access an innovative deployment method was adopted. Ground anchor screws were installed at the opposite end of the reactor to which snatch blocks were secured. Tow cables were clamped to the leading edge of the cover, passed through the snatch blocks and secured to the “tow” vehicle which then slowly pulled the cover into place within ½ a day.
The cover was then partially folded over at the anchor trench to allow the gas skirt to be also welded to the underside of the gas cover. The cover was then folded into the anchor trench and back filled. As the cover was sealed to the liner via the gas skirt it was not required to seal the cover around the sludge management or inlet/outlet pipes in the anchor trench. Flanged biogas take-off pipes were connected to the cover and the existing gas transfer piping, with the entire system purged with nitrogen before flaring was commenced.
The Enviro Liner EL7080 cover material has provided the ideal solution of a chemically resistant and flexible cover material for this anaerobic lagoon application. The innovative construction methodology employed by the lining contractor, Fabtech Australia, ensured the successful installation with no impact on the owners operations.