Rehoboth replacing old mixers at wastewater treatment plant

COVID-19 has shifted most municipal work for Rehoboth Beach to a virtual format, but there’s nothing virtual about infrastructure construction. Rehoboth commissioners on May 4 awarded a $1.4 million contract to build three of four mixers used in the wastewater treatment plant’s oxidation ditch.

The contract was awarded to Pennsylvania-based Philadelphia Mixing Solutions, the only company to submit a bid, which came in roughly $20,000 less than engineering estimates.

During the commissioner meeting, Public Works Director Kevin Williams said he wasn’t surprised only one bid came in because fabricating the mixers is a specialized project.

In an email May 14, Williams said fabrication of the mixers will begin immediately, with the equipment expected to be delivered to the wastewater treatment plant in November. He said the plan is to have the mixers installed and operational before the 2021 peak-flow season.

The oxidation tank has four mixers, but the city already replaced a malfunctioning mixer over the winter. The tank is where raw wastewater begins treatment before ultimately being pumped out into the ocean through the outfall built two years ago. The mixers incorporate air into the slurry, which helps to feed bacteria that digest the solids in the slurry.

Last summer, Williams said one of the mixers began knocking, indicating a problem. To avoid a mechanical failure of the entire system, the speed of the mixer was reduced until the knocking stopped, but that also reduced the amount of air being pumped into the ditch.

Williams said the city made an additional air line diffuser system, which worked until the city was able to replace the mixer this past winter.

Because of the threat of failure in the other mixers, and the urgent need to get them replaced, fabrication of the remaining three mixers was broken out from the original Phase 3 project, so they would be operational for the 2021 summer. Otherwise, he said, they would not have been replaced until the 2022 summer.

“We felt we could not wait,” said Williams, describing the replacement of the mixers as very critical. “These four mixers are all in use constantly during the summer season.”

Williams said the city is expected to award the second part of Phase 3 contract sometime in late 2020, which will coincide with the delivery of the new equipment. This will allow for installation prior to the 2021 summer season, he said.

Williams said funding for fabrication of the mixers is coming from $12 million in revolving loan funds the city secured for various plant upgrades. About $9.5 million remained in the fund before this contract was awarded.

The city also has a second revolving loan fund of $40.5 million, which was secured to complete the ocean outfall project. Williams said this fund has been completely spent, and the city is now repaying it.

Phase 2 of upgrades to the treatment plant is well underway. This work, Williams said, is primarily servicing and installing new electrical equipment throughout the plant. He said work completed to date includes a new duct bank along Roosevelt Road, which is the main electric feeder into the plant.

Funding for the $6 million phase 2 is split 50/50 between the city and Sussex County, which has an agreement with the city to use up to 42 percent of the treatment plant’s capacity. The county is putting up the funding to complete this phase now; county and city officials agreed to split the costs, with the county providing a 10-year, zero-percent loan to the city for its $3 million half of the project.

Williams said phase 2 also calls for approximately $3 million of emergency work at the headworks building – the replacement of screens, pumps and piping. The county is also funding this work, but at a 58/42 split, which means it will cost the city about $1.74 million.

Williams said the expected completion date for all phase 2 work is April 2022.