Managing the treatment of wastewater — especially industry-generated wastewater that can elevate dissolved oxygen (DO) demands far beyond those of municipal/residential applications — is hard enough. Trying to do so during upset conditions is enough to put any water treatment professional on edge. Here are several preventive and remedial options to keep in mind before or after DO readings go haywire.
Answering The Diverse Nature Of DO Challenges
Compared to typical municipal wastewater applications, many industry-generated waste streams are highly concentrated and create much more challenging oxygen demands under normal operating circumstances. Also, municipal treatment plants that serve a mix of residential customers and small commercial food processing, pharmaceutical, or industrial customers can experience intermittent, sharp rises in biochemical oxygen demand (BOD) that make them candidates for periodic DO stresses and supplemental oxygen needs.
In certain industries — e.g., food and beverage processing — fine screens might be used to keep larger particles out of the wastewater flow for several reasons. One reason is to capture biological components that have commercial value as part of a repurposing application, such as byproducts that can be reused or sold for animal feed. Another reason is to strip out excess BOD and other particles that could clog up equipment — pumps, nozzles, slot injectors, or fine bubble systems. In the event of breakthroughs in those screens or clogging in aeration ports, however, DO levels can plummet quickly.
Reasons For Aeration Stresses
The threats of low DO levels can occur for multiple reasons:
- Degradation Of Oxygen Distribution Capacity. In some cases, a decrease in DO occurs because diffusers, jets, or nozzles become coated or clogged with a variety of process byproducts — including hair or feathers, calcium deposits from calcium-rich wastewater streams, or an accumulation of oils, fats, and grease from the foods being processed.
- Excessive BOD. In other cases, available DO levels that are appropriate at the start of a process can suddenly be overwhelmed by an extreme increase in BOD. WWTP operators must be able to adapt quickly to those changes caused by:
- Increases In Process Throughput. DO demand can increase progressively over time as production capacity ramps up.
- Screen Breakthroughs. Accidental screen breakthroughs due to aging screens or wear and tear from repetitive cleanings can elevate DO demand slowly or quickly, based on the severity of the breakthrough.
- Screen Bypass Events. Whether these are accidental or intentional events, bypass events can quickly create an overwhelming demand for DO.
Whatever the cause, the remedies can vary from more frequent maintenance and troubleshooting, to the addition of supplemental aeration jets, to planning for added treatment basin capacity. Whatever the concern, anticipating maximum needs before a crisis level is reached is the best approach. Several guidelines for responding to changing DO conditions are outlined below.
Spotting Problem Areas In Jet Aeration
Here are several ways to identify aeration changes creeping into the process, even before they show a drastic drop in DO level readings or impact wastewater treatment efficiency.
Figures 1A and 1B. Clogged diffusers, jet nozzles, or Slot Injectors™ can disrupt the broad dispersion of fine bubbles and result in isolated larger bubbles rising directly over the aeration header. The image on the left shows the pronounced ‘boil’ from such an obstructed header, while the image on the right shows the even dispersion of finer bubbles after cleaning of the aeration system.
- Visual Pattern Changes. Accumulation of small particles in an aeration system over time can change the volume of air flowing through it. This is often visible as a change in the aeration mixing pattern on the surface of the basin (Figures 1A and 1B). A knowledgeable supplier can help confirm those conditions based on an emailed short video or even still photo images of the basin.
- Pressure Readings On Pump Discharge. As aeration pumps, diffusers, jets, or nozzles tend to clog with material over time, they will create backpressure, which can be noticeable as an increase in pump discharge pressure. This is typically a sign that a jet or Slot Injector™ has blocked liquid propulsion nozzles, which will markedly reduce the oxygen transfer capacity of the system.
- Rising Energy Consumption. If an aeration system monitors blower run time, an increase in energy use can signal when the system is working harder than normal to meet targeted DO levels.
Tactics For Maximizing DO Availability
The best options for getting DO back to the required levels can depend on whether those low oxygen levels are related to an occasional process BOD overload, a rare summer-heat-related occurrence, or are part of an ongoing pattern related to increasing plant throughput levels. Here are several options:
- Backflushing. A backflush system can go a long way toward regenerating the optimum airflow in jet aeration systems by using a mechanism to reverse the flow of water and air through the jets to dislodge any clogged material. It can be the difference between keeping the basin running and having to take it offline for cleaning — a costly, disruptive process if there is no alternate capacity for treating the wastewater. While it is ideal to add such backflushing capability during the initial jet aeration installation, the capability can also be retrofit into existing installations from outside the basin (Figure 2).
Figure 2. Backflush systems installed outside the aeration basin reverse the flow of air and water through the nozzles to clear debris that can compromise aeration bubble patterns and reduce DO levels.
- Supplemental Air Delivery. Being able to add skid-mounted supplemental air distribution headers to an existing treatment basin (Figure 3) — without having to shut down the process — can be a financial windfall. This can make the difference between meeting discharge requirements without disrupting plant processes vs. having to perform a complete process shutdown with its attendant loss of production revenue. Such skid-mounted systems can also be removed for periodic cleaning if necessary, without having to shut down the entire wastewater treatment operation.
Figure 3. Adding supplemental jet aeration headers and nozzles to an existing aeration basin without a disruptive shutdown of operations is a way to handle increased wastewater treatment capacity due to process growth or reduced efficiency in aging systems.
- Supplemental Oxygen Feed. When all else fails, wastewater operators also have the option of adding liquid oxygen as a way of supplementing base aeration systems that cannot keep up with oxygen demand under particularly stressful circumstances. While this can be a quick solution for dealing with a critical oxygen-starved upset condition, it can be expensive due to the ongoing costs for the liquid oxygen.
