Last year, we worked with global ceiling tile manufacturer, Armstrong World Industries, to solve a problem they were having in their ceiling tile plants: butyric acid in the wastewater which was causing unwanted odors in their ceiling tiles. However, this year, we broadened our scope, investigating a solution to potentially include the ceiling tile industry as a whole. We probed deeper into the problem, examining raw and finished recycled material inputs, feedback processes in the manufacturing plant, patterns of the butyric acid problem and its locations, and studied the whole physical plant, covering much more than the secondary wastewater treatment plant.
Ceiling Tile Manufacturing Plant Process
A quick overview of the wet-pressed ceiling tile manufacturing process is presented in the Armstrong video. The ceiling tiles’ raw materials consist of a combination of mineral wool, recycled paper, clay, plant starch as a binder, fiberglass, and discarded ceiling tiles. The schematic in Figure 1 captures our iGEM team’s understanding of the manufacturing processes and is based on our technical communications with Armstrong and visits to their Marietta ceiling tile plant and the headquarters’ model plant. Further details on the plant can be found in the Industry Communications section of the wiki. Similar materials and processes are used by other ceiling tile manufacturers.
Schematic of Ceiling Tile manufacturing Plant Processes. (Schematic created by BroadRun iGEM)Plant operations have two main parts; a board mill and a wastewater treatment plant. Inside the board mill, where production of the tiles takes place, fresh stocks of raw ingredients mix with recycled manufacturing waste (wet and dry broke). The manufacturing waste, which are scraps, trimmings, and dust from previous batches of wet boards and dry boards, is mixed with water. From a constant level box, the partially liquid mixture pours onto a moving tray from which water is drawn out (into the 50,000 gallon white water chest) to make the wet boards, which are subsequently dried in high powered dryers before being resized, finished, packaged and then sold. Outside the board mill, water is processed in a primary clarifier and thickener and recycled back into the board mill. In the secondary water treatment plant, the aeration basins serve to remove soluble organic compounds through aerobic bacterial activity. Process water from the aeration basin flows to a secondary clarifier where biomass settles and the activated sludge is largely returned to the aeration basin. The treated water from the secondary clarifier goes to a storage tank from which water is drawn into the board mill. Ceiling tile plants have minimal idle time, operating nearly continuously year round. The plant shuts down only for major holidays and scheduled maintenance.
The Butyric Acid Problem
Working towards our goal of understanding the problem from a systems perspective, we considered the following.
- Plant conditions that give rise to butyric acid and the locations vulnerable to butyric acid problem- High strength organic wastewater as a cause of the problem given that butyric acid is a product of anaerobic starch hydrolysis- Why the problem persists when starch, a necessary binding ingredient, and not a waste material, is added in a controlled manner to the fresh stock - If during normal operations, there were time periods of excess starch that contributed to the problem- If these starch variations stem from 1) recycled waste products in the fresh stock (recycled paper, discarded tiles) and/or 2) manufacturing scrap, wet and dry broke, being recycled back into the plant - Plant locations affected with the butyric acid problem during shutdowns
The main points of our findings and interpretations from examining plant schematics, lab water testing results (dissolved oxygen, butyric acid), notes taken from the manufacturing plant visit, and correspondence with technical staff are summarized below.
- Production is not a linear process going from a beginning-phase to an end-phase; product waste (wet and dry broke) is recycled back into the tile-making board mill for for highly efficient utilization of raw materials. Likewise, water is recycled throughout the plant. Consequently, a problem arising in one process impacts not only downstream operations but also upstream processes. - Biocides are used in several locations inside the board mill and in the wastewater treatment plant on a regular basis. These levels are varied to compensate for butyric acid spikes occurring during normal operations. - Biocide usage is controlled and minimized much as possible, due to the high financial costs, dangers to the environment, and risk of the microbes developing resistance to the biocides. - To reduce butyric acid levels, a newer secondary wastewater treatment plant is in operation with two aeration basins and two secondary clarifiers. It has allowed for lower baseline butyric acid levels at the aeration basin, but still sees intermittent fluctuations during normal operations. Also, a new problem of slime buildup occurs, presumably by higher dissolved oxygen levels. This slime buildup that disrupts sludge settling is managed with different biocides. - Butyric acid levels inside the board mill’s dry broke and wet broke tanks where the recycled waste is held appears to be a source for higher vulnerability to the butyric acid problems.- During plant shutdowns, for holidays and maintenance, the problems worsen; high levels of butyric acid fluctuations after plant startup can last for durations exceeding a week, even with high loads of biocide applications during and post- shutdowns.- Given the nature of the ceiling tile manufacturing process and the well known industry wide use of biocides, both global and domestic, the above conditions are believed to be common to this industry.
Current Solution
As aforementioned, Armstrong currently uses biocides to manage the butyric acid problem. However, biocides have many risks and associated costs.
- Biocides are dangerous to the environment; contamination can damage surrounding ecosystems. These harsh chemical have environmental dangers, and have to be closely monitored and controlled. Contamination of surrounding ecosystems with biocides could have potentially damaging consequences on surrounding ecosystems. - Biocides are expensive, adding to the production cost.- Biocides are a short term solution. Over time the unwanted microbes will grow back and the problem will re-emerge. Thus, these expensive biocides must often be re-applied.- Microorganisms can develop resistance to the biocides, which would make them ineffective. To avoid resistance, manufacturers rotate the biocides used, which also increases costs.
Source of the Problem
A more nuanced understanding of the manufacturing process as it pertains to the use of waste materials and the recycling of water and product between the board mill and the wastewater treatment plant, informs us that starch circulates throughout the plant in varying degrees. We hypothesize that excessive starch concentrations occur during normal operations, at certain vulnerable locations; and, when plant conditions favor anaerobic bacterial metabolism, butyric acid is produced. The anaerobic butyric acid pathway.
