City Meets New Wastewater Compliance Regulations, With Exceptionally Low Output Of Nitrates And Phosphorous, While Coping With Weekend 90+% Influent Drop From Poultry Plant
The public works manager for the City of Decatur, AR reports that replacement of their aerated pond system wastewater treatment plant (WWTP) with a special activated sludge process WWTP has provided for compliance with a more stringent National Pollutant Discharge Elimination System (NPDES) permit, while coping with a regular 90-95% weekend decrease in wastewater input from a poultry plant, as well as major seasonal variations in ambient temperature.
“The regulators did not wait on us,” recalled James Boston, the City’s public works manager. “The new instantaneous 10 mg/L nitrate limit had spelled the beginning of the end for the pond system……and our discharge goes to losing stream, which means possible entry to aquifer, so we went to drinking water standard for nitrates.”
“Meanwhile, reduced phosphorous was being staged in--- from reportable, to 3 mg/L, to 2 mg/L, to 1 mg/L…,” he continued. “And whatever we were going to do had to also be able to cope with the routine weekend shutdown of a local poultry processing plant that contributes 1.4-1.6 MGD of pre-treated flow on weekdays….”
“We’ve been amazed to get the numbers we got with this new plant,” he concluded, “especially with the poultry plant as such a major contributor to our input. It’s like having cheeseburgers during the week, going to salad for the weekend, and then back to cheeseburgers the following week. And it hasn’t mattered if they had any bumps or hiccups in their pretreatment operation.”
“We’ve been consistently getting 96-98% biological reduction of phosphorous. We’ve also been seeing at least 50% reduction of sludge during the process, before it goes to the holding tank and sludge press. It was great to see a much greater reduction there than we had calculated.”
Boston reports NH3 now at < 0.1 mg/L, vs. permit level of 5-7 mg/L seasonal; CBOD at < 2 mg/L vs. 10 mg/L; TSS at < 2 mg/L vs. 10 mg/L; nitrates at 6-8 mg/L vs. 10 mg/L instantaneous; and phosphorous at 0.2 mg/L vs. 1.0 mg/L.
The poultry plant is a kill, process, marinate, and pack operation, though is not a renderer, or “protein plant”. It has dissolved air flotation (DAF) installed as pretreatment for its wastewater before discharge to the city.
Under new management since 2009, the poultry plant’s TKN output is now about 50-65 mg/L and its phosphorous 6-12 mg/L. The domestic input from the city for TKN is 35-45 mg/L, and phosphorous 3-6 mg/L.
The poultry plant’s weekend steep volume reduction originally challenged the city from Saturday, 4:00-5:00 am to just after midnight Sunday, and is now from Friday, 9:00-10:00 pm to Sunday, 8:00 pm.
‘We had a major challenge to be able to deal with that change in the time for the volume interruption, as well as changes in nature of that plant’s output to us while we were getting our new plant installed,” recalled Boston. “TKN had been 60-65 mg/L, and was now 50-65 mg/L. Phosphorous was down from 10-15 mg/L to 6-12 mg/L. The hardest was BOD --- we had designed the new plant for 300-350 mg/L, expecting 275 mg/L --- and we got 175 mg/L.”
The low BOD-to-TKN ratio impacts denitrification and nitrates levels. However, the plant is able to stay well below the nitrate limit without the use of supplemental carbon.
New treatment plant options for the city had included alternative oxidation ditch systems, as well as alternative sequencing batch reactors (SBR’s), as finalists that were deemed capable of handling the BOD, phosphorous, and nitrates requirements.
“OEM costs were the major consideration, and we also didn’t like any surface aeration option, with the cooling of our water during the winter months….,” Boston recalled. “… Another issue was dissolved oxygen. If oxygen is over 0.5 mg/l, the bacteria won’t work to pull it off the nitrates…. With the Fluidyne plant option that we selected, they have a stage of the process that handles all that.”
Boston noted that oxidation ditches were regarded as too capital-intensive…. and not offering as much control of wastewater as SBR options did. He said the Fluidyne SBR-type system selected had the smallest footprint, and also one of the lowest electricity costs --- by an estimated 50-75%. It was also appealing that all pumps in that plant were the same brand.
For morer information, visit www.fluidynecorp.com.
SOURCE: Fluidyne Corporation