In 2005, the seaside town of Lewes, Del., began completely upgrading and expanding the Howard H. Seymour Water Reclamation Facility. The result — in addition to doubling the facility’s capacity from 0.75 to 1.5 mgd (2800 to 5700 m3/d) — was the first membrane bioreactor (MBR) facility of its kind in Delaware. Certain properties of the new plant also make it especially well suited to handling seasonal flows in a beach area.
The MBR facility uses a biological nutrient removal (BNR) activated sludge to reduce nitrogen and phosphorus in its effluent. The facility operates two identical BNR process trains. Each train has four zones. Flow first enters an anoxic zone, followed by two aerobic zones and a final anoxic zone. Following the final anoxic zone, 12 cassettes of membranes filter the water. After the membranes, the plant disinfects the flow with ultraviolet light.
The solids side of the facility also uses membranes. Waste activated sludge from the BNR trains is sent to a dual-zone aerobic digester for stabilization. The digested solids are thickened using a membrane system that draws clean water out of the digester. Then, the thickened solids are pumped to either sand drying beds or a vacuum press for dewatering. The dewatered solids are shipped to an offsite contractor for composting and disposal.
Seasonal Flows No Problem
Switching to an MBR system helped the plant handle the variable flows of a beach community. Because the MBR enables the plant to operate with a high mixed liquor suspended solids concentration — greater than 10,000 mg/L — swings in flow strength can be handled pretty easily, because there are always enough microorganisms present, said plant superintendent Walter Baumer.
Year-round, effluent concentration of biochemical oxygen demand and total suspended solids average less than 1 mg/L, and fecal coliform concentrations are less than 1 CFU/100 mL, Baumer said. To put this into perspective, the facility’s permitted levels for total suspended solids and biochemical oxygen demand are about 16 mg/L.
The facility discharges its effluent into the Lewes/Rehoboth Canal, which feeds into the Delaware River and then the Atlantic Ocean.
With an MBR system, preliminary treatment to remove debris and grit is especially important to protect the membranes. Before entering the BNR trains at the Howard Seymour facility, influent passes through 5-mm screens, then a grit-removal process, then an equalization tank, and a set of 2-mm screens.
Even with sand from the beaches entering the collection system, the plant doesn’t have trouble with sand and grit reaching the MBR, Baumer said. The plant’s grit system works well to remove all of the grit that comes down the line, he said.
The facility is owned by the Town of Lewes and since 2002 has been managed by Severn Trent Services (Fort Washington, Pa.). The company’s team of technical specialists was especially useful beginning in 2004, when it helped the town’s board of public works find a compromise solution to an otherwise expensive regulatory decision.
When Delaware issued a total maximum daily load for nitrogen and phosphorus for Rehoboth Bay and other inland waterways, it called for the systematic elimination of all point sources from inland waterways.
That decision meant a switch from liquid discharge to spray irrigation for the plant’s final effluent. Where the plant discharges, a majority of the flow enters the Delaware River and flows to the Atlantic, but some could back up and go into Rehoboth Bay.
To avoid the switch, which would have cost millions, the facility conducted a dye study and found that only 3% of its effluent flow makes it back into the bay. With such little flow entering the bay, and considering the high quality of the effluent from the upgraded treatment plant, the state allowed the plant to continue its liquid discharge. To offset the nutrients in the 3% of flow that does enter the bay, the plant purchases nutrient credits.
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