In the early 90s, new state and federal laws prohibiting “free water” in municipal sludge from being spread in landfills presented problems for New York City, whose 15 wastewater treatment plants generate a combined 70,000 cubic yards of grit and screenings a year.
The Department of Environmental Protection had been collecting the grit and screenings in standard roll-off or lugger containers and then decanting the loose water before loading the containers on trucks for transportation to the landfill. During the trip, however, vibration from the trip tended to compact the material and liberate enough free water to fail a paint filter test.
The city investigated several mechanical means of dewatering the grit to meet the moisture content requirements for landfill dumping, and finally settled on the Container Filter manufactured by Flo Trend™ Systems, Inc.
The container filter consists of a load support basket inside a container. A 30-mesh filter medium lines the inside of the support basket. When grit is deposited in the basket, any free water drains into the space between the bottom of the basket and the floor of the container, and then out through a drain on the container’s side.
Although gravity drainage is adequate in most cases, on occasion a pump is connected to the drainage ports to help speed drainage. When the sludge grit level is above the perforations in the basket, the pump creates a vacuum between the basket and the floor of the container, helping draw water from the grit.
At most of New York City’s treatment plants, Cyclone degritters are used to separate out the grit, which is then fed onto conveyors or deposited directly into the containers. Because the city’s treatment plants vary in size, they use a variety of different size Container Filters, including small satellite boxes that can be handled by a forklift, 6- and 10-yard lugger boxes, and 20- and 25-yard roll-off types.
When a container is full, it is loaded onto a truck and hauled to the landfill. The system results in a 20 percent reduction in grit volume, and a significant savings in hauling costs and landfill fees, which are $60 per ton, a city spokesman said.
Depending on the size of the plant and container, it can take from two hours to a day to fill a container. During wet weather events and after winter street sanding, grit production can increase fivefold.
