The Challenge of Reed Drying Beds: Preventing Escape

Background Video

eed bed drying technology is ideal for many smaller wastewater treatment plants (Fig.1). Phragmites australis, common reed is planted in the sand beds and biosolids are poured on them periodically (every few weeks during the growing season) Sludge Bedover 4-8 years, depending on the depth of the bed. Reeds accomplish the task of drying biosolids through the use of an energy efficient (plant-assisted solar drying) technique that requires a minimal capital investment in infrastructure and has a low input of labor for day-to-day operations. These characteristics contribute to an ecologically and economically positive outcome and provide an aesthetically pleasing site to the public. However, because Phragmites has a history of being very invasive in many environments, there is apprehension that the reed from drying beds will escape onto farmland and natural areas that are not infested. This could be either via seed from the drying bed or via rhizomes or seed in sludge that is spread on farmland as a fertilizer rather than being hauled to a landfill.

This project focuses on testing a drying bed clean out protocol on two reed beds at the Bridgeville, Delaware Wastewater Treatment Plant. First, the reeds were sprayed with a glyphosate herbicide last summer and fall to kill the majority of plants. This past winter we tested the effectiveness of the herbicide in killing the rhizomes in a greenhouse assay where we planted rhizomes collected from the beds in sand-filled trays. Not surprisingly with this vigorous plant, we found that some rhizomes were still viable (Fig. 2). We also tested the viability of the seed bank in the drying bed during the dormant season and found that some Phragmites seeds were viable from the shallow depths of the bed but not from the deeper zones. Other weed seeds were more dominant in the shallow zones (Fig. 3).

Phrag Trays
Fig. 2 Rhizome viability test.

weed seeds in trays

Fig. 3 Seed bank test.

In May, the two beds were cleaned out by Clean Delaware, Inc. (Figs. 4, 5, 6) and the sludge temporarily stored in Milton. In June, field plots were delineated at a farm in Harbeson, DE (Fig. 7) and sludge from one bed was spread over 12 treatment plots at a rate of 7.5 tons per acre (Fig. 8). The rate of biosolids application was determined by Steve Rohm of the Delaware Department of Natural Resources and Environmental Control Biosolids Program. With the control plots, the study occupied a 7.5 acre area. The field was disked a few days later (Fig. 9) and then planted with glyphosate-resistant soybeans (Figs. 10 & 11). The area has been monitored for any Phragmites growth. Just prior to canopy closure of the crop, traditional weeds and any reeds that may have grown were eliminated using a glyphosate herbicide. Plots will be monitored until soybeans harvest for herbicide effectiveness. Sludge from the second bed was composted for future study (see composting section below).

No Phragmites shoots from either rhizones or seeds have been found in either the plots receiving biosolids or the controls. In a recent survey other weeds averaged 630 per m2,the vast majority of which were very small to small seedlings.

field survey

Fig. 17: Final survey for living Phragmites in late August prior to the application of Roundup. None were found.

bean plant

Fig. 18: Bean plant in late August.

bean plants

Fig. 23: Bean plants as they appeared on 20 September. The plant on left is typical of the plants in the driest part of the field. Plant on right is typical of the better plants. The best plants are about 40% taller. No new pods are being formed at this date but those present are filling out.

Composting Biosolids from a Bridgeville Reedbed:

Fig: 24: Clean Delaware employee David Stout tells University of Delaware interns, Colleen Butler and Michael League, how he will cover the newly created compost pile with wood chips.

Fig. 25: Mixing biosolids dried in reed beds with wood chips and chicken litter before the blend is piled on the wood chip base in which air lines are buried.

standing by pile

Fig. 26: Blower used to move air through the compost pole. Forty-one tons of biosolids.

JJ digging

Fig. 27: Collecting samples from the composted biosolids.

Fig. 28: Samples were taken from two depths under the wood chip cap.

DJ inspecting

Fig. 29: Setting up assays for possible growth of rhizomes and seeds in composted biosolids.

An empty drying bed in Bridgeville will be planted with our variegated variety of Phragmites (Stripes) that has not produced viable seeds in our experiments (see Stripes website). This variety has a DNA marker in addition to the visual one. If the clean out and agronomic protocol described above results in effective control of the Phragmites in the field, the escape issue should no longer be a factor of concern in deciding whether to use reeds in beds for drying biosolids, especially if the Stripes variety of Phragmites is used. Check back to this site for new developments as the season progresses.

Acknowledgment: This research is being conducted in cooperation with Phillip Mowbray at the Bridgeville Wastewater Treatment Plant and Wayne Hudson at Clean Delaware, Inc.

Funding Source: U.S. Environmental Protection Agency


Dr. Denise Seliskar

Halophyte Biotechnology Center


Dr. John Gallagher

This page updated: 17-May-2010