A balanced wastewater process is a beautiful thing — plenty of biochemical oxygen demand (BOD), plenty of O2, and all those hungry little microbes are fat and happy. But what can be done when dissolved oxygen (DO) takes a nosedive and there’s not enough O2 to satisfy industrial and food processing BOD? Here are some preparations to consider before wastewater treatment goes completely out of whack.
Living Between A Rock And A Hard Place
DO emergencies can come about for a variety of reasons — blocked aeration headers, screen breakthroughs, water treatment additives that upset the balance of the process, an overload of BOD due to process expansions, low oxygen solubility due to elevated wastewater temperatures, etc. Wastewater from highly organic food processing and from beverage applications like breweries or distilleries can place high BOD demands on treatment facilities (upwards of 50,000 mg/L), especially when there is a filter breakthrough or a bad batch to be dumped. Also, protein extraction processes designed to recoup value from food byproducts and convert them for use in animal feeds can easily overwhelm resource recovery or treatment systems. Whatever the cause of the DO deficit, it can be difficult to find cost-acceptable alternatives for bringing processes back into balance.
- Shut Down The Process. In some industrial or food processes, it might be physically possible to shift production to other facilities, upgrade the aeration system, then restart production. More often than not, however, the impact on the bottom line makes that unacceptable. The best defense against this type of situation is to stay on top of monitoring the processes to keep systems in balance before a DO deficit gets out of hand.
- Buy Pure Oxygen. Buying bulk truckloads of pure oxygen to meter into the wastewater treatment system as a temporary solution can be a relatively quick fix in a pinch, but typically is not a cost-effective solution for the long term.
- Try a Supplemental System. While installing a new system is not an overnight solution, being able to insert skid-mounted supplemental jet or Slot Injector™ aeration headers in a fully operational tank is a viable long-term fix for bringing DO levels back to desirable levels without having to take down the entire basin (Figure 1).
Figure 1. This skid-mounted jet aeration package with submersible pump makes an easy drop-in upgrade to existing aeration basins struggling to keep up with DO demands.
Regardless of which approach is most appropriate for the given situation, it is a good idea to have contingency plans on hand in time to put them into action should a DO emergency arise. Those contingency plans can involve add-on remedial approaches, or they can be incorporated into new wastewater basin designs in anticipation of future problems, as outlined in the “Ounce Of Prevention” case study described at the end of this article.
How Retrofit Opportunities Fill DO Needs
One of the most favorable aspects of a skid-mounted supplemental aeration header is the ability to deliver long-term production volume with a normal cost of operation after the initial capital investment. Adding a drop-in skid-mounted Slot Injector™ aeration system to an existing aeration basin is a great long-term solution, with each unit delivering between 200 and 500 lbs. of oxygen per hour to a struggling wastewater treatment process.
- Submersible Pumps. One way to power a supplemental aeration system is to include a submersible pump as part of the skid-mounted aeration header installed inside the existing basin. This cost-saving configuration is a good option when the area immediately outside the basin is extremely limited in space, but it can be a little more labor-intensive by requiring periodic removal of the pump for maintenance of the mechanical seals.
- Externally Mounted Aeration Ports. Where space is available adjacent to the aeration basin, an alternate configuration is to have the pumps and blowers mounted on the outside of the basin for easier maintenance access. In that configuration, the blowers can be mounted with the backflush system outside the tank. While this configuration requires a slightly higher capital investment up front, the way it simplifies maintenance can reduce downtime and labor costs over the 20-year life expectancy of the system (Figure 2).
Figure 2. Adding a supplemental aeration system with the pump mounted outside the aeration basin provides better access for quicker maintenance and lower labor costs over the life of the system.
An Ounce Of Prevention
Scrambling to evaluate the best maintenance or retrofit response is not the only way to deal with a DO emergency. New jet aeration and Slot Injector aeration system designs offer the ability to incorporate problem-solving solutions into an original aeration system design as a built-in resource to respond to emergency clog situations. Here is how one food processor overcame a near total loss of DO residual with the built-in backup capabilities designed right into their slot-injection aeration system. Those capabilities were able to take the 4.2-million-gallon lagoon from near 0 mg/L DO conditions to normal DO concentrations over the course of a single work shift.
