Too much of a good thing can create new problems as quickly as it resolves old ones. Maintaining a proper balance of antifoaming agents and polymer additives is critical for sustaining the appropriate air bubble size and distribution needed to achieve dissolved oxygen (DO) requirements for neutralizing high biochemical oxygen demand (BOD) in many industrial and food processing wastewater applications.
Know The Signs Of Trouble
Foam on the surface of an aeration basin can become a nuisance if it starts blowing around within or beyond the wastewater treatment plant grounds. For this reason, some industrial wastewater applications use defoamer agents to break down existing foam or antifoam agents to prevent foam formation in the first place.
In each case, these surfactants work by reducing the surface tension of the liquid to prevent the formation of small bubbles. Unfortunately, the goal of wastewater aeration is to form small bubbles that maximize the surface area of the air-to-water boundary to support the growth of microbes, which consume the BOD in the wastewater. That leaves the two factors working at cross purposes.
Because bubble size has an important effect on aeration efficiency for neutralizing BOD, anything that disturbs the size and distribution of aeration bubbles in an aeration basin can have a negative impact on wastewater treatment efficiency. The following conditions can be signs of potential problems with aeration bubble size impacted by additives to the process stream or wastewater aeration basin:
- Dropping DO Levels. Steadily decreasing DO levels can be indicative of a variety of problems — from a clogged aeration system, to a sudden spike in BOD loading of the incoming wastewater stream, to poor oxygen exchange due to the reduced surface area of fewer larger bubbles vs. many smaller bubbles. While antifoam additives are but one of many causes for decreasing DO levels, investigating any downward trend in DO is important because it can result in the eventual shutdown of the aeration basin if left unattended.
- Changing Aeration Patterns. A smoothly running aeration pattern should exhibit an even distribution of fine bubble aeration on the surface of the basin. When bubble size is compromised by surfactants or by clogged aeration diffusers, jets, or nozzles, resulting in fewer and larger bubbles, it will typically manifest itself as more concentrated boils in just limited areas of the surface, usually right above the aeration headers (Figure 1).
Figure 1. Observing a coarse boiling effect in concentrated areas of an aeration basin can be an indication of compromised bubble formation resulting from excess antifoaming agent being added during either production processes or wastewater treatment processes.
- Excessive Energy Use. When poor bubble formation compromises oxygen-transfer efficiency, aeration blowers must run harder than normal to keep up with oxygen demand. Therefore, seeing a sharp spike in energy consumption for aeration can be another indicator of poor oxygen transfer. If the system must work progressively harder to achieve the needed DO levels, it can eventually max out to the point of being unable to keep up with BOD neutralization in the basin.
Lessons Learned
For applications where foam formation is a serious problem, wastewater treatment professionals should discuss their concerns with their aeration suppliers. Doing so can identify treatments that have been proven effective at maintaining oxygen transfer capabilities while minimizing foaming issues, based on controlled evaluations in aeration testing tanks. Here are some guidelines on causes of foam treatment problems, plus some strategies for fighting them without exacerbating the wastewater treatment situation:
- Know The Chemistry. Antifoam agents are available in different types of chemistries — each with its own characteristics and impacts on wastewater treatment. Silicone-based and oil-based antifoam chemistries — whether vegetable oil- or hydrocarbon-based — are some of the most problematic. On the other hand, some alcohol-based polyether polyol treatments have shown promise for either having less problematic oxygen transfer impact or, in some cases, promoting good oxygen transfer rates even while achieving antifoam goals (Figure 2).
Figure 2. Applications that require antifoaming agents in the production process should be monitored to ensure that they do not create negative impacts on the wastewater treatment process.
- Communicate Beyond The Aeration Basin. Some industries use antifoaming agents in their production processes because of the nature of their product (e.g., starchy potatoes are a notorious foam producer, as are some juice processing plants and some chemical, pharmaceutical, and pulp and paper operations). In instances where process engineers and wastewater operators need to achieve objectives that seem to be at cross purposes, good communication among them and a knowledgeable wastewater aeration professional can help all parties achieve their needs with the least disruption to the organization’s overall productivity and bottom line. Review water treatments and water chemistry from source water all the way through the production cycle to the final wastewater treatment process to appreciate exactly what impacts are involved.
- Remember, A Little Goes A Long Way. Even at extremely low concentrations — as little as 2.5 ppm — certain antifoaming agents can cause aeration bubbles to go from an average of 2 mm to 3 mm in size up to 25 mm in size!
- Don’t Compound The Problem. In applications where antifoaming agents cause DO levels to remain low for a long time, the biology in the aeration basin can change to the point that downstream solids separation becomes difficult. When that condition occurs, adding polymers to aid in separating the solids from the liquid can further complicate aeration basin chemistry and — if added in excess to MBR processes — may increase the viscosity of the biomass, further deteriorating oxygen transfer. Slot Injector™ aeration systems use both VFD-controlled pumps and blowers and, in some cases, it is possible to control foaming events successfully by shifting the air/liquid delivery flow rates while also reducing overall energy usage. Some plants have reported a significant reduction in chemical costs by operating in this mode.
- When In Doubt, Ask. Whether there are concerns about existing wastewater operations or a need to prepare for changes in production processes or wastewater volumes, it can pay to consult with a wastewater aeration expert experienced in both the chemistry and physics of wastewater aeration across a wide range of industry applications. Testing new chemical additives ahead of time can prevent surprises and emergencies in wastewater treatment as processes change.
