Author’s note: The terms mill and plant are used interchangeably in this article to describe an industrial manufacturing unit, such as a Pulp & Paper Mill, Petroleum Refinery, Chemical Processing Plant, Meat & Poultry Processing, etc.
Plant (or mill) outages come in two types – scheduled and unplanned. Scheduled outages are generally annual affairs at roughly the same time each year and are used to conduct maintenance or other work that cannot be performed while the plant is in full operation. These outages may involve a shutdown of some or all of the production and ancillary operating equipment. Sometimes a plant will be required to have a cold shutdown, meaning all power or steam generation equipment will also be down for a portion of the outage. Planned outages, as the name implies, are heavily monitored, controlled, and scheduled in terms of timing and duration. Despite the best planning, however, inadvertent losses during shutdown and restart can create issues and potential permit violations in the wastewater treatment system. Another area of importance is the duration of the outage, as biomass activity and viability decrease the longer the bugs go without food. The other type of outage is the unplanned shutdown due to some operational or equipment problem in the plant. Unplanned outages can wreak havoc on the wastewater system, increasing the risk of serious system upsets and high effluent BOD numbers.
The key to minimizing the risks associated with either type of outage is to be prepared by developing and implementing action plans for all likely scenarios. The goal is to stay ahead of potential issues to reduce the impact on the biological population. EBS has spent many years working with mill personnel before, during, and after outages to reduce this risk and its associated cost.
Planning and preparation are key components to minimizing the risk of an effluent excursion during a scheduled outage. Staggering machine shutdowns and tank cleaning dramatically control both hydraulic and BOD loading to the wastewater system, avoiding most serious problems during this part of the outage. But even with the best controls, BOD loading nearly always increases and can overload the existing biological population. This makes it important to proactively increase nutrient feed rates so the bacteria can continue reproducing in sufficient quantities to handle the load. And, the amount of time needed to increase the biological population to sufficient levels through reproduction within the system may determine whether bioaugmentation products are needed to increase the population quickly. During this part of an outage, EBS often provides increased system monitoring. Particularly important parameters to start or increase monitoring frequency of include pH, oxygen uptake rate, and nutrient residuals. Because the bacterial population will have diminished during the “unfed” portion of the outage, increase the nutrient feed rate and add higher than normal amounts of bioaugmentation product several days prior to the startup of boilers and paper machines.
For extended outages lasting more than two weeks, EBS recommends adding a third component to the program, a supplemental source of BOD. A supplemental BOD source, such as EBS MicroCarb™, is often recommended because it contains a very high BOD content, is easily degraded and allows for rapid biological reproduction. We have found that replacing about 10 – 20% of the typical BOD loading for 3 to 5 days prior to restarting greatly reduces the potential for upsets during mill startup.
For unplanned outages, obviously, the situation is different. This is where the EBS Core Value of “Respond at the Speed of Sound” is showcased. EBS is staffed and managed such that we can have resources on plant site within 24 hours of contact, often even the same day. The focus of production personnel during an unplanned outage is getting production back on line while the wastewater personnel are focused on assessing and minimizing the impact of any spills or other actions, such as loss of aeration, etc. For these situations, EBS strongly recommends our clients have contingency plans in place as part of the Standard Operating Procedures or Best Management Practices, including an initial inventory of any response chemicals or bacterial products that may be required. Since plant production problems during startup can unexpectedly send high BOD containing, inhibitory, or toxic water to the sewer, increased monitoring during this period is extremely important. Our “Simple, yet Profound” Core Value leads us to focus on the critical few and put together a straightforward, cost-effective, and technically sound strategy to address both scheduled and unplanned outages.