Charles City: Reed Beds Used for Dewatering and Sludge Storage

Charles City’s Water Resource Recovery Facility (WRRF) plant has begun construction of a major upgrade of their wastewater treatment system. As part of the design, the city wanted to reduce their sludge handling costs and improve their ability to deal with seasonal requirements for land application. For this plant, FOX Engineering recommended using reed beds for dewatering and storage of the sludge.

The existing sludge digestion system stabilizes solids from primary clarifiers and trickling filters with two aerobic digesters with coarse bubble diffusers. A sludge holding tank with a floating mixer provides about six (6) months of storage. Most of the time, this system provides adequate treatment and sufficient storage for the plant. Periodically, land would not be available when the sludge holding tank was nearly full requiring the plant staff to scramble to find additional land application sites.

The new plant will use an oxidation ditch activated sludge system designed for nutrient removal. This type of system will produce more biosolids than their previous system with a much lower solids concentration. Even with decanting, the existing system will not provide sufficient treatment for the new treatment process due to inadequate digestion time. To deal with the increased sludge treatment demands, the sludge holding tank is being converted into a third aerobic digester. The treated sludge is then to be sent to a four (4) cell reed bed system.

Reed beds use a sand bed with a particular species of reeds to dewater treated sludge. The reed’s root structures prevent compaction of the sand media and a flow path for drainage. Combined with the respiration of the reeds to remove water, this greatly improves the operation of the system compared to a conventional sludge drying bed. An influent header distributes the sludge from the digesters across each bed. The solids accumulate on top of the sand while the water drains through the sand and into a layer of graded gravel. A perforated PVC underdrain system collects the water from the bottom of each cell. The reed bed drainage water is sent back to the head of the plant for treatment.

Each bed has about 24,000 square feet of total area. The volume is enough to provide 7 to 10 years of sludge storage. To save construction costs, the perimeter reed bed wall is earthen basin construction with a membrane liner. The divider walls are concrete. A concrete ramp down to each cell will allow a loader into the cell without damaging the membrane or the berms.

Since there are no moving parts, maintenance of the reed beds involves annual harvesting or burning of the reeds and removing the dewatered sludge after 7 to 10 years. After planting, the most common issues with the reed bed plants involve dealing with pests such as aphids and invading weeds. These can be dealt with via selective pesticides and herbicides, but care must be taken during application as overapplication will go to the treatment process and potentially harm the activated sludge process. Preferably, ladybugs can be used for dealing with aphids and weeds can be manually removed. Once the reeds are established, weeds are not usually a problem due to the strong competitive nature of the reeds. The reeds will need to be harvested or burned annually to prevent an accumulation of biomass which may lead to rotting in the beds. The reeds regrow from rhizomes in the root structure.

Once a cell is full, the operator will stop feeding sludge to that cell to allow the biosolids to dry as much as possible. Once the sludge is about the consistency of compost, it can be removed with a front-end loader and handled as a dry sludge cake. Typically, a layer of sludge is left to protect the sand layer and the root structure of the reeds.

The new sludge treatment system will provide the City of Charles City with a flexible system with low maintenance requirements. The new plant is currently under construction and will be fully commissioned in 2022.  The reed beds were planted in December 2020 and the first sludge application applied.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.