Rapid response system reduces time, cost, and bod: three case studies, Solutions!, Online Exclusives, October 2004

Recently, three different southeastern pulp and paper mills experienced excessive waste loads in their treatment ponds, with only a short time to remedy them. The causes for the excursions were different in each case but the problem was the same—they all needed to reduce biological oxygen demand (BOD), fast! In all three cases, Praxair Services’ In-Situ Oxygenation (I-SO) emergency service allowed the mills to maintain uninterrupted production.
          Praxair’s emergency oxygenation service provided a rapid, efficient and economical solution for each mill. Each portable I-SO unit can immediately transfer more than four times the oxygen into the water as a traditional aerator, while using half the horsepower (hp). The system includes a liquid oxygen tank, which carries three times as much oxygen as would be possible in gaseous form; a portable vaporizer (the TMVU-1000), which converts the cold liquid oxygen to ambient-temperature gas; and the portable, floating I-SO unit, which injects and then stirs oxygen into the water. One 40hp unit delivers 7,000 pounds of oxygen per day, doing the job of four 75hp air-based units.

Rapid BOD reduction
One urgent BOD-reduction situation occurred at a paper mill that was expanding its sub-surface bubble diffuser system. It had taken its aeration stabilization basin out of service to add aeration arms. Wastewater was being moved, untreated, directly into a 240-acre holding pond. The mill had been putting 20 million treated gallons a day into the holding pond, and releasing the same. To compensate for the increased BOD from the now untreated water, the mill had planned to release less, retaining the excess until the treatment pond was back in service and the holding pond had normalized. However, the project went over schedule by about three weeks. Now the mill was facing a severe retention problem. As the mill’s senior engineering advisor said, “We had a shut-down situation. We needed extra BOD treatment capability, NOW.”
          This mill had already run trials with Praxair to see how the I-SO service might meet its needs should it ever find itself in a bind. The time had come.
          “Praxair Services gave the quickest response and the best service I’ve ever gotten, on a problem that was not straightforward. Within just seven days, they were not only here, but up and operating. It was remarkable. And the equipment did it from there,” reported the engineer.
          Three I-SO units were placed toward the end of the retention pond near the outfall, in channels where the flow occurred inlet-to-discharge. These units, together, added 21,000 pounds of oxygen to the wastewater per day, which allowed the mill to release enough treated water to continue running at full production levels.
          “We kept the units near the outfall for two months, because it took 40 days after the ASB resumed operation before the entire holding pond was back to the required BOD levels. The set up and demobilization costs are built in, so adding extra days on site did not cost much extra,” explained the engineer. He also commented on Praxair’s reliable operations and safety practices.
          “They have their own specific safety procedures, and they teamed with us to meet all of the mill’s safety requirements,” he said.
          The head environmental engineer added, “Whenever Praxair’s people came on site, I pretty much didn’t have to touch anything.”

Simplicity and efficiency
“Not having to touch anything” was one of several reasons why another southern paper mill chose I-SO to handle a weather-related BOD excursion.
          “The I-SO system is just so much simpler and faster,” said that mill’s environmental engineer. “That’s why we chose them—because that equipment can reduce more BOD than conventional aeration, and they can install it within three to four days of when they get the purchase order. I was looking for speed, and they hustled.”
          He explained the situation. “(This mill can) produce 20,000 pounds of BOD a day. When there is an excursion, you have to move fast, or things can snowball. Before, I would rent mechanical aerators. They were expensive and labor-intensive to set up. In this situation, I would have needed 12-13 aerators, at 75 hp each, plus the generators to run them. I would have had to find a place to put the generators, and I would have had to bring in a diesel truck to fill the generators. It would have been two weeks worth of work just to set up. With the Praxair units, I issued a purchase order on Thursday, I was feeding oxygen by the next Thursday, and I was back to a comfortable BOD rating in two weeks.”
          The engineer was also able to use his own substation, reducing power costs overall and completely eliminating generator rental costs.

Even more speed
Pulp and paper mills manage living waste treatment systems that aren’t predictable in their responses to weather and other factors. And, even with well planned-operations, unexpected things can occur. That was the situation for the third mill in this case review.
          “Their response time was tremendous. I called on Thursday, and they were in on Friday. The system was operating by the following Thursday,” said a mill engineer.
          This integrated pulp and paper mill had undergone some changes, and saw an elevated waste load whose cause had not yet been identified. The cooling weather and resulting slowdown in bacterial action were most likely contributing factors. They needed a fast fix while they were diagnosing and remedying the problem.
          “We took two aerators offline and put two I-SO units in a five-acre section of our 1,000 acre wastewater treatment system. With those units, we had seven times as much oxygen going in. We measured the dissolved oxygen (DO) in the pond before we put in the units and it was .50 parts per million. After the I-SO, that five-acre area was saturated, with 10-12 parts DO per million,” reported the mill engineer.
          The mill is back to normal, but the engineer has chosen to keep the I-SO units on standby for a bit longer due to a coming weather transition. He explained, “It can be a bit precarious when the ‘summer bugs’ are dying off and the ‘winter bugs’ are coming in, and with the extra load we recently experienced, it makes sense to keep a rapid, cost-effective solution on hand during the transition.”

          As these three cases demonstrate, rapid response, immediate BOD reduction, simplicity, and cost effectiveness are making In-Situ Oxygenation a preferred solution in paper and pulp mills that want to maintain full, uninterrupted production when experiencing expected or unexpected excursions.

To learn more about I-SO emergency oxygenation service, contact Mindy Strickling, marketing communications manager, at 1-800-394-9929, or visit www.praxair.com/services.