One city in the Great Lakes Region had to remedy a consent order filed for its untreated combined sewer overflow (CSO) discharges. Officials originally planned to install a 3.8 MLD stormwater storage tank, but were approached by Aqua-Aerobic representatives with a pilot test proposal utilizing a new technology.
The pilot proposal featured the AquaPrime® Cloth Media Filter utilizing 5-micron microfibre cloth media, which would be tested during five wet weather events. The successful pilot test prompted the city to request a design for an AquaPrime filtration system, which could treat both dry and wet weather conditions, and allow alum coagulant to be injected upstream of the filters to meet future effluent phosphorus limits and eliminate fine CSO particles.
Startup of the 14-disk AquaPrime filtration system began in July 2017 with a design average flow rate of 3.8 MLD in dry conditions and a peak wet weather flow rate of 48 MLD. Each cloth media disk is approximately two metres in diameter and provides an effective filtration area of 5 m2, for a total filtration area of 140 m2. The filters were retrofitted into the plant’s existing, abandoned sand media filter structures, saving the city considerable capital costs. In addition, the new filtration system was $1 million less than the original, conventional storage tank design.
The new tertiary/wet weather filtration system also included replacement of an existing gas chlorine disinfection system with a UV disinfection system. The UV system was installed in the existing tank, which also provided significant project cost savings.
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The AquaPrime filter process
The AquaPrime filter features a disk configuration and an outside-in flow path, which allows for three zones of solids removal. These zones are especially critical in wet weather applications due to the high solids typically associated with the first flush after wet weather events.
The top zone is the “floatable zone” where surface materials such as fats, oils and grease are allowed to collect on the water surface. Solids are removed from this zone by allowing floating material to overflow a scum weir a couple of times each day. The middle zone is the “filtration zone” where solids are removed through filtration. Here, solids deposit on the outside of the cloth media, forming a mat as filtrate flows through the media.
This buildup of solids on the media creates hydraulic resistance to flow through the media and causes the water level in the tank to rise. Once a predetermined liquid level or time setting is attained, the disks begin to rotate and the backwash pump starts. This draws filtered water from the inside of the disk through the media and removes solids from the filter media’s surface. This process fluidizes fibres to provide an efficient release of stored solids deep within the fibre.
The bottom or “solids zone” permits heavier solids to settle to the bottom of the tank for intermittent removal. Solids are evacuated from the hopper through collection laterals using the solids/backwash pump.
Design characteristics
In order to remove 85% or more of TSS and CBOD₅ in CSO discharges, and comply with future permit requirements for phosphorus, the AquaPrime filters are designed for upstream alum addition.
The system is also designed to treat for the duration of a wet weather event, which can last for several days. This ability to provide continuous filtration capacity prevents overflow occurrences.
Aqua-Aerobic Systems Inc. is represented in Ontario and eastern Canada by ACG-Envirocan. This article appears in ES&E Magazine’s February 2019 issue.
