High and dry: Leaf River survives drought through planning, teamwork.

Water is the lifeblood of every pulp mill, and a dramatic decrease in water levels can threaten a mill with serious operating problems. That was the situation Georgia-Pacific's Leaf River Pulp Operations in New Augusta, Mississippi, LISA, found itself in two years ago. The mill was faced with a possible shut-down due to a drought-induced drop in water levels. However, through a careful assessment of" the situation, a committed application of best management practices (BMP), and pulping process modifications, the mill was able to continue and even improve operations.

DRY TIMES

The pulp mill sits on the banks of the Leaf River, a relatively small stream with a drainage area of 2542 square miles in fine southeastern corner of the state. During the drier months of the year, typically July through October, the base flow of the river comes from the shallow aquifers that feed into the river. These aquifers recharge mainly during the winter months, November through March.

During the winters of 1997-1998 and 1998-1999, most of Mississippi was in a severe drought. Normal average rainfall during November through March for southeast Mississippi is 26.5 inches, but only 12 inches of rain fell during the winter of 1999-2000. In June 2000, four months prior to the normal low flow period, the Leaf River flow dropped lower than any ever recorded during the mill's 16 years of operation. On August 25 the flow reached an all time low of 357 cubic feet per second ([ft.sup.3]/s), 141 [ft.sup.3]/s below the 7-day, i0-year recurrence interval (Q10). During 2000, the flow rate tot the Leaf River at New Augusta was below the 7-day Q10 for a total of' 130 days.

THE CHALLENGE

The Mississippi Department of Environmental Quality (MDEQ) Office of Land and Water Resources (OLWR) regulates water use in the state. The mill operates under a surface water permit granted by fine OLWR, which allows water withdrawal for beneficial use at a maximum daily volume and rate. This withdrawal, however, is allowed only when the flow rate of the Leaf River exceeds the established minimum flow--the 7-day Q10.

The ever-shrinking biochemical oxygen demand (BOD) limit and the river color limits are the biggest permit challenges for the mill during low river levels. The water quality*based BOD limit ratchets downward to protect the stream's dissolved oxygen levels. During the summer of 2000, the BOD limit dropped to a low of 2500 pounds per day. At 18 million gallons effluent discharge flow, this equates to a BOD concentration limit of less than 17 milligrams per liter (mg/l). The permit establishes color limits as a maximum monthly average increase in the river of 50 platinum cobalt color units (pcus) and a daily maximum of 65 pcus. Therefore, if the effluent flow and color is constant, the increase in color of the river is directly dependent on the river flow. The challenge facing the mill was to continue operating at maximum production while meeting stringent effluent limits and maintaining stream water quality standards.

PREPARING FOR THE WORST

The summer and fall of 1999 gave a small preview of what was to happen in 2000. During a 6-day period in September, the river flow dropped below the 7-day Q10 flow for the first time since the mill started up in 1984. This incident forced a hard look at what to do if the problem reoccurred and became even worse the next year. As a first step, the mill management convened a meeting of operations, maintenance, technical, engineering, and environmental personnel to brainstorm ideas on what could be done if the river reached record lows for extended periods. The group selected a worst-case flow rate of 300 [ft.sup.3]/s, approximately 150 [ft.sup.3]/s lower than what occurred during 1999. The two issues the group focused on were:

* Preventing spills of black liquor or soap, which could effect both color and BOD, and

* Finding ways to reduce effluent color beyond what is normal during good operations.

After several meetings, the group developed a plan it felt would succeed. The first step was for environmental personnel and Production Manager Richard King to share the information with all mill employees. Their joint presentations clearly explained the seriousness of the drought, the importance of following the mill's plan, and the consequences--including shutting the mill down--if the plan was not followed. After the meeting, it was clear that one bobble, only one mistake, would be enough to force a shutdown to prevent BOD and color limit violations. With so much on the line, the employees' response was phenomenal.

BEST MANAGEMENT PRACTICES

The most obvious thing to do was eliminate all spills. The mill operates a very efficient extended aeration, activated sludge wastewater treatment system capable of reducing BOD down to less than 10 mg/l. The key, however, is to avoid overloading or shocking the system. A large volume spill could shock the system, causing the effluent BOD concentration to double or triple, prompting a mill shutdown. In reducing effluent discharge to meet the BOD limit, the mill would no longer De able to meet the use requirement of returning essentially the same amount of water as is withdrawn. This would create a real "Catch 22" situation. Spills would also increase the color of the effluent, thus potentially forcing a reduction in discharge which would also jeopardize meeting the water use restrictions.

Detecting, capturing, controlling and monitoring spills--all part of a BMP plan--truly became an operations survival plan. Sewer conductivity monitoring, spill collection systems, and good communications quickly headed off potential threats to the treatment system. BMP records helped identify the most frequent spill causes and prompted the mill to implement the following procedures:

* Install agitators in the decanters, allowing them to be boiled, and pump the residual material back into the liquor system. This replaced the need to clean tall oil decanters nearly every week and divert the material to the treatment system.

* The sluice tank also had to be frequently washed out due to line plugging, resulting in 70% solids black liquor being diverted to the treatment system. To eliminate this, the mill rerouted the sewer in this area to the evaporator sewer where an existing spill collection sump could pick up the liquor during a tank wash out.

These two changes addressed 62% of the spills recorded over 2000 as shown by Figure 1.

REDUCING EFFLUENT COLOR

The mill's NPDES permit color restriction is based on limiting the increase of the receiving water color. The permit allows for a 50 pcu color increase as a monthly average with any one day's maximum not to exceed an increase of 65 pcus. When operating with good spill controls, normal water usage of 10,000 gal/ton of pulp, and a normal five-stage bleaching sequence of DEoDED, the softwood effluent color is typically between 1000 and 1200 pcus. To ensure the stream flow did not exceed the permit limit of 50 pcu color increase, it would have to exceed 660 [ft.sup.3]/s, which it historically does at least 99% of the time.

Based on the experience during the summer of 1999, however, the mill anticipated that the flow could fail as low as 300 [ft.sup.3]/s. Assuming an effluent flow of 18 million gal/day, the effluent color would have to be below 690 pcus should the river flow drop to 300 [ft.sup.3]/s. On certain days water usage might exceed 18 million gal/day, so to give a margin of safety the mill decided to reduce effluent color to 500 pcus.

After considering various options, the mill addressed the problem by modifying bleach plant operations. Earlier studies indicated that bleach plant color could be reduced, but whether that would be enough to reduce the effluent color to 500 pcus or less was riot certain. The first step in the process was to add hydrogen peroxide to the two extraction stages. Normally, the mill used hydrogen peroxide for bleaching from time to time based on economic factors. That would change the bleaching sequence to DEpoDEpD.

The next step was changing showers on the first two chlorine dioxide stage washers. Normally, these two washers operate with 1000 gal/min of fresh water and 1000 gal/min of filtrate from the following extraction stage as showers. The change was to replace the fresh water with additional caustic extraction filtrate. This effectively took all of the flow going to the caustic sewer and recycled it back through the extraction stage. Hydrogen peroxide usage was increased to offset the increase in organics entering the extraction stages. This caused peroxide bleaching of the extraction filtrate, producing lower color wastewater entering the treatment system and lower effluent color.

There were, however, two prices to pay for the lower color. First was the expense of' doubling hydrogen peroxide use. The second was increased barium sulfate deposition on the first chlorine dioxide washer. These deposits normally occur at a rate requiring cleaning every four years or so. Under the new plan, deposits were severe enough to require annual cleaning.

RESULTS

The mill's goals were to maintain full operation of its bleached market pulp facility, remain in compliance with NPDES permit limits for BOD and color, and meet the surface water withdrawal requirements of returning as much water to the river as is removed during low flow conditions. In addition, the mill had to maintain stream water quality standards, To accomplish this, Leaf River used best management practices to control spills and prevent a treatment plant upset while also reducing softwood effluent color from between 1000 pcus and 1200 pcus to 500 pcus. The mill met or exceeded all of the goals and established a new monthly production record during August 2000.

Figure 2 shows wastewater treatment system inlet BOD before and during the summer drought period. The key to meeting BOD limits was controlling spills, as shown by the low and consistent loading. Due to the BMP controls and bleach plant changes, effluent BOD concentrations dropped to unheard of levels for this facility. During September, the mill reached a record low BOD discharge of 161 lb/day. The average effluent concentration was 1.1 mg/l, with all but one day's sample measuring 1 mg/l. One theory for the record low concentrations is that organics from the extraction stages were broken into smaller molecules that were removed by the treatment system. Figures 3 and 4 show the effluent BOD and total suspended solids (TSS) prior to and during the drought. When the Leaf River flow dropped below the 7-day Q10, the MDEQ began requiring daily river monitoring for dissolved oxygen. As shown in Fig. 5, the stream dissolved oxygen concentrations remained above the minimum standard of 5 mg/l throughout the entire drought period due to the low effluent BODs.

[FIGURES 2-5 OMITTED]

With good spill control and extraction stage bleach plant recycling, the mill reduced effluent color below 500 pcus, a lower than anticipated figure. Figure 6 shows the color before and during the drought period. Another interesting observation was that the wastewater color entering the treatment system was consistently greater than the effluent color, perhaps due to the biodegrading of some of the smaller molecular weight color molecules.

[FIGURE 6 OMITTED]

A TEAM EFFORT

The Georgia-Pacific Leaf River pulp operations survived the drought of 2000 due to many factors. First and foremost is the talent of its employees. Their knowledge and attention to detail allowed mill operations to do things it had never done before. The mill benefited from having a modern mill with great flexibility and a highly efficient wastewater treatment system. The ability to use hydrogen peroxide and recycle filtrates was integral to the mill's flexibility. Having best management systems to detect, collect, and correct spills gave the facility the opportunity to succeed. The timing of the drought was also important. Had it occurred three years earlier--before the startup of the oxygen delignification system--achieving the required results would have been much more difficult.

The mill continues to face the possibility of future droughts. There is, however, a confidence among the mill's staff, that next summer--even if the river tails to new record lows--with everyone working together, the mill's operation will go on as usual.

Figure 1. Sources of BMP reportable spills

BMP SPILL REPORTS BY AREA

#2 Evaps                    2
Sluice tank                29
Pulp mill/Digester area    36
#1 Evaps                   14
Tall oil/Decanter area     61
Salt cake mix tank          1
Turpenine area              2

Note: Table made from pie chart.

About the authors: R. Acker smith is environmental Manager and Thomas Brent Walley is Senior environmental engineer at Georgia-Pacific Leaf River Pulp Operations, New Augusta, Mississippi, USA. Contac Smith at [e-mail] or Walley at [e-mail]