Digging for Answers

The Lab Detective helps an SBR plant’s lead operator figure out why the system is struggling with nitrification and devises a remedy.
Digging for Answers
Figure 1: A positive-displacement blower (Dresser/Roots [GE Energy]) provides aeration for the plant’s sequencing batch reactor.

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Joe, the chief plant operator at the city’s wastewater treatment plant, was perplexed. After numerous adjustments to the aeration system and batch times, the facility continued to struggle with nitrifying.

Since the nitrification process was inhibited, the facility was not meeting its state effluent total nitrogen permit limit of 3.0 mg/L. Joe decided to ask other surrounding operators for their opinion and advice. One of these operators recommended Joe contact the Lab Detective.

Place to start

The detective’s first communication with Joe was in a short phone conversation about the plant and its location. It seemed the facility was not far from where the detective was at the time, and he could pay a visit. During the drive, he thought of possible issues that cause poor nitrification:

  • Low dissolved oxygen (DO) levels
  • Low alkalinity and pH levels
  • Too short of an oxic retention time
  • Too low MLSS amounts and short sludge age
  • Liquid temperatures too high or too low
  • Rapid liquid temperature change (temperature shock)
  • High influent ammonia/ammonium concentration
  • Toxic material in the influent

Remembering that nitrification is the conversion (or oxidation) of influent ammonia/ammonium to nitrite, then to nitrate by nitrifying bacteria with sufficient dissolved oxygen and alkalinity, this list seemed like a pretty good place to start once he arrived.

To remove the oxidized nitrate nitrogen from the wastewater, facultative anaerobic bacteria are used to convert the nitrate to nitrogen gas in a reduction process where no free dissolved oxygen is present, normally in an anoxic basin or anoxic time cycle during a sequencing batch reactor (SBR) treatment batch. The facility seemed to be having trouble in the oxidizing part of nitrogen removal. The detective also went over the plant information Joe had given him:

  • A two-tank SBR, design flow 1.1 mgd
  • Three positive-displacement blowers (Figure 1)
  • Influent ammonia concentration of 30 mg/L as N
  • Influent alkalinity of 270 mg/L as CaCO3
  • Influent pH of 7.5
  • Mostly domestic wastewater from residential users

Surprise on the way

All this information seemed pretty normal for wastewater — nothing seemed out of the ordinary. However, the Lab Detective should have expected the unexpected.

Once the detective arrived at the plant site, Joe gave a brief tour. The SBR plant was about 15 years old — an above-ground system made of porcelain-coated metal plates bolted together. Each SBR tank could hold 550,000 gallons and operated on a timed batch cycle, each cycle being 6.0 hours long. Out of the total batch time, more than 2.0 hours were being devoted to aeration (oxic time). Since the plant had strict total nitrogen permit requirements, complete biological nitrification and denitrification were essential.

Joe and the Lab Detective looked over the last year’s lab data, process control testing results and design drawings. Joe described the operational difficulties during the past year to year and a half. It seemed that as time went on, he couldn’t get enough air as dissolved oxygen (DO) into the basins. Trending data confirmed that over time, the total standard cubic feet per minute (SCFM) output from the blowers had increased, yet the actual DO seemed to be staying constant. Joe estimated that he had tripled the react fill time over the last few months, and only then would he see the DO increasing near the end of the 180 minutes of total aeration.

Checking the air

So, the emphasis of the troubleshooting was placed on the aeration system. Joe and the detective went over the aeration system thoroughly, checking blower maintenance records, SCFM gauge accuracy, actual blower RPM (using a tachometer), and bearing heat signature (using infrared thermography). Joe’s maintenance of the facility was exceptional, and that included the aeration system. Nothing seemed to be amiss. Even the DO meters used for process control testing were examined as a possible cause for under-aerating.

Attention next pointed to the influent wastewater stream and its oxygen-demanding components. Joe’s regulatory permit required 24-hour composite sampling of the influent and effluent every week. The automatic composite samplers (Figure 2) normally initiated the sampling events at midnight of each Tuesday, ending that Tuesday evening at 11:59 p.m.

Several times, the sampling event would be disrupted, and Joe would collect the samples on a Wednesday or Thursday. These additional sampling days provided useful data, since the Lab Detective could see what the influent characteristics were like on days other than every Tuesday. Again, he struck out — there was nothing unusual in the incoming waste stream:

  • Average BOD5: 120 mg/L
  • Average TSS: 135 mg/L
  • Average ammonia (NH3) as nitrogen: 30 mg/L
  • Average nitrite/nitrate nitrogen: 0-2 mg/L

Answers in the earth

Joe and the Lab Detective took a break and chatted about the long winter they had just come through and how springtime was progressing. The trees were full of budding leaves, flowers were blooming, the grass was sprouting and giving the plant grounds a nice green color.

Joe mentioned that it takes a lot of work to keep the plant grounds looking good, and that an aesthetically pleasing plant is a sign of a well-run plant, at least in the eyes of the state inspectors. One of the shift operators, Bill, had joined the conversation. As he poured himself a cup of hot coffee, Bill remarked that the grass always grows well around the plant, except for the patches behind the SBR tanks. Joe and Bill spoke for a moment about getting some sod to fill in the dead spots.

The imaginary light bulb that appeared over the Lab Detective’s head was enough to light the whole room! “What exactly are you talking about?” he asked. Joe explained that for the last several years, they’d had trouble with what he thought were nematodes or some other soil organism eating the roots of the grass in places around the SBR tanks.

“Darndest thing,” Joe said. “They appear about every 20 feet or so around the back side of the plant. Can’t get the grass to grow there anymore.” Joe and the detective took a walk to get some fresh air and check out the bare spots in the yard. The detective stepped away for a moment to get a better look from above, climbing the plant stairs and accessing the catwalk that circled the round SBR tanks.

From above, he could plainly see the brown patches of dirt that Joe said would not support grass. He quickly rejoined Joe on the ground and asked, “Have you ever experienced problems with the aeration header piping?”

Joe thought for a moment, then recalled about one to two years ago he had to replace a few gaskets in the piping joints that dry-rotted and were leaking air near the blowers. At once, he understood why the detective asked the question: “Well I’ll be! Let me get a shovel!”

Making the fix

Joe, Bill and the detective began digging in one of the brown patches, noticing that the soil was warm to the touch in the cool spring air. After digging about 3 feet down, as they neared the 10-inch ductile iron pipe, the hissing sound of escaping air was apparent.

Joe was relieved and excited to finally locate the source of the missing cubic feet of air. Joe and Bill used the infrared thermography tool to get some temperature readings of the soil along the route of the underground aeration header pipe. Sure enough, there was a marked increase in the soil temperature every 20 feet.

Joe immediately initiated repairs to the piping by excavating the places where the joints were located, replacing the joints’ dried-out rubber-gasket mechanical joints with pipe repair clamps. Joe knew this was a temporary fix, but due to the depth of the pipe, the manpower and available parts, he chose the repair clamp method. He also made long-term plans to replace the clamps or to modify the entire piping system in the future. There were four excavations to be made, two for each SBR tank. After several days, the repairs were complete.

Once the repairs were made to the aeration piping, the plant’s nitrogen removal ability was restored. Joe was able to reduce the aeration times of each SBR batch while still maintaining regulatory compliance for biological nitrogen removal — and, in the bargain, save a few dollars on the energy bill at the end of the month!

About the author

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.


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