CASE STUDY
Lagoon sludge removal provides affordable solution for algae and H2S issues
The challenge:
Algae blooms and corrosive gases caused by sludge accumulation
Sludge accumulation was thought to be causing significant operational challenges for the O’Leary Wastewater Treatment Facility (WWTF), a two-cell lagoon system on the west side of Prince Edward Island, commissioned in the 1990s.
O’Leary’s treatment lagoon serves about 800 residents, several businesses and, for a period of time, was also receiving wastewater from a potato processing plant.
The lagoons were experiencing serious blooms of blue-green algae during the summer months and exceedances of TSS, BOD and coliform bacteria in treated effluent. Hydrogen sulfide (H2S) gas, which was also attributed to the algae and sludge, was building up in a below-grade chamber that housed the plant’s UV disinfection system.
Not only was the gas accumulation a hazard for operators, but it caused corrosion that damaged ductile iron pipes, an electrical panel and the plant’s UV system.
Operators also saw large “islands of sludge” periodically rise out of the water before bursting and releasing clouds of methane and H2S gases.
Since completing the project, operators report that there have been no issues with gas accumulation in the below-grade chamber or blue-green algae growth in the lagoons.
The solution:
Dredging and passive dewatering meets regulatory and budget requirements
In 2019, the Town of O’Leary began working with engineering consultants WSP to repair the damage to the UV system, alleviate the algae blooms and end the release of corrosive gases from the lagoon.
WSP began with a comprehensive sludge survey, obtaining sludge depth measurements from several locations in each of the lagoons.
The results revealed that despite their large area, the lagoons are relatively shallow, with a maximum depth of only 5 ft (1.5 m). Each cell had accumulated about 1.5 to 2 ft (0.45 to 0.6 m) of sludge, which significantly
reduced their ability to remove nutrients and settle out solids.
Sludge maps are an important step in planning and executing lagoon cleanouts. WSP’s map of the O’Leary lagoon revealed significant accumulation.
The team evaluated several options and awarded the project to AJL Contracting and Bishop Water to desludge the lagoon and dewater removed solids with the Bishop Solids Management Solution. This simple, low-energy system uses Geotube® containers, specially selected polymers and gravity to collect, dewater and consolidate dredged sludge.
Dredging the lagoon enabled it to remain in operation while sludge removal occurred and also complied with a provincial regulation that prohibits tracked heavy equipment from operating in lagoons with clay liners.
The project began in the summer of 2020, with the construction of a lined laydown area designed to accommodate a layer of three Geotube containers 85 ft in circumference and 114 ft in length (26 x 34.7m) and a second layer of two 95 x 100 ft (29 x 30.5m) containers that would be laid on top. This stacking arrangement minimizes the footprint and cost of the dewatering cell.
The results:
No algae, no gases, no rush to haul away solids
Dredged sludge was first pumped through a 0.6 inch (15 mm) screen to remove debris, trash and improve the quality of the final dewatered biosolids for land application.
From the screening plant, the sludge was pumped to the Geotube containers. Bishop Water’s Venturi Emulson Polymer Activation System (VEPAS) added and mixed polymer directly in the feed line, eliminating the need for polymer aging and mix tanks.
The Bishop Solids Management Solution accepts and dewaters solids as quickly as the dredge can pump, dramatically outperforming centrifuges or belt presses for speed and energy efficiency. Low-TSS filtrate released from the microscopic pores of the Geotube container was directed by gravity to cell two.
Technicians continually monitored polymer performance and dose rates to ensure optimal dewatering and retention of contaminants. Daily sampling showed that the system was achieving about 20% solids concentration after just a few hours of dewatering.
The Bishop Solids Management Solution used a two-layer, stacked arrangement of Geotube dewatering containers to minimize the footprint and cost of the dewatering cell.
Desludging of cell one was completed in about four weeks. Two more weeks of sludge removal filled the first layer of Geotube containers to capacity and technicians stacked a new container on top of the others.
One more week of dredging and pumping completed the project. The total volume of wet sludge removed was 26,480 m3, which corresponds to 872 bone dry tonnes.
A major advantage of Geotube containment is that the solids can remain at the site for months or even years, allowing ample time for the town to find a suitable location for land application and arrange funding for hauling. Throughout that time, passive dewatering will continue further consolidating the material.
A seasonal freeze-thaw cycle also significantly im-proves dewatering, potentially elevating the solids concentration of the dredged material to 40% or more in this case.
Since completing the project, operators report that there have been no issues with gas accumulation in the below-grade chamber or blue-green algae growth in the lagoons.
