Wastewater operators face fog in two distinct ways. They might have to drive through thick banks of fog on their way to work only to face FOG (fats, oils and grease) again at the treatment plant.
Once the Lab Detective made it through dense fog to the treatment facility, he found the operators working alongside the collection system crew members, who were running a vacuum truck, normally used to clean sewer lines. They had the suction piping extended to its highest elevation and the flexible hose up over the wall of the aeration tank.
Joe, the chief plant operator, described the work being performed and the viscous foam that was covering the aeration basin and entering the clarifiers (Figure 1). The foam had formed a thick scum on the clarifiers, covering the entire surface and flowing into the scum boxes, which pumped to the aerobic digesters.
The foam was also exiting the treatment plant in the effluent, creating a large chlorine demand and elevated effluent TSS values. Joe had asked the detective to come to the treatment facility to help troubleshoot the problems and come up with a plan to rid the plant of the excessive scum and foam.
The detective collected data about the plant, including lab results, O&M manuals and process control data, and also interviewed the plant operators. The facility was a 5.0 mgd (design) conventional activated sludge plant with three aeration tanks of about 1.7 million gallons each and four secondary clarifiers.
The plant influent flow was just below half the permitted capacity, there were no primary clarifiers, and the facility discharged to surface water, requiring effluent chlorination and dechlorination. There were three shifts with two plant operators per shift, along with five maintenance mechanics and two utility workers.
Under the microscope
The detective performed a microscopic exam of the foam and the mixed liquor suspended solids (MLSS), and the results indicated Nocardia as the culprit causing the foam. Nocardia is one of several fungi that cause foaming in activated sludge facilities. It is relatively easy to identify, since it is a short, truly branched filament that is gram positive and mostly neisser negative (Figure 2).
It is hydrophobic (doesn’t like water) and loves oil and grease as a food source. Nocardia amarae and similar actinomycete are found in soil, water, and the human digestive tract. Other foaming bacteria include Type 1851, Microthrix parvicella and Type 0092. Some Nocardia species are pathogenic, causing tuberculosis-like symptoms. Caution should be taken when working around the aerosol generated by aeration equipment when Nocardia is present.
Nocardia and nocardioform fungi (now reclassified as Gordona) are commonly found in activated sludge mixed liquor when there is an abundance of FOG in the influent, when the water is warm, and at older sludge ages.
Joe’s treatment plant fit this description well: the influent temperature was about 25 degrees C, the facility was run at a 17-day sludge retention time (SRT), and there were many restaurants in town with minimal grease traps, if any at all. There was no enforcement of local ordinances that prohibit dumping waste grease and oil down the drains.
Plan of attack
There are many opinions on how to get rid of scum and foam, and Joe’s operators had been trying anything and everything. In fact, there was much disagreement about the best way to correct the problems. Some operators thought the waste rate should be increased, while others thought the waste sludge flow was already too high. Some thought the return activated sludge (RAS) flow should be decreased, and still others thought the dissolved oxygen was too high. It seemed the three shifts operated independently, each running the plant according to their liking.
After gathering and reviewing the plant data and talking with the shift operators, the detective formulated an action plan. First, he conducted a plant operations staff meeting to review his findings and ensure that all operators were on the same page. Second on the list was to attack the Nocardia on several battlefronts.
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First, it is imperative to keep Nocardia from recycling back into the treatment plant through plant drains or digester supernatant or by hosing down of the foam on the tank surfaces. All operators were made aware of this.
Sludge wasting was increased to reduce the SRT. The sludge went into the digesters but was quickly hauled to a residual management facility (RMF), meeting EPA biosolids requirements without recycling Nocardia back into the plant.
Foam and scum removed from the tanks’ surfaces with the vacuum truck were applied to existing sludge drying beds, allowed to dewater, and then also hauled to the RMF. High-test hypochlorite tablets were placed into the drain lines to chlorinate the drying bed filtrate.
A chlorine feed system was established to feed about five pounds of gaseous chlorine per day to each 1,000 pounds of mixed liquor volatile suspended solids. This dose was applied into the RAS wet well to give maximum detention time with the highest concentration of solids.
An industrial/commercial pretreatment department was formed to begin enforcement of the existing FOG ordinance. The department received authority to inspect grease traps, monitor pumping of the traps and enforce the city codes when sewer users did not follow the guidelines.
Foam trapping is an issue in many plants that have compartmentalized tanks with submerged inlets and outlets. When foam gets trapped in treatment tanks, it can reseed itself when conditions are right.
Preventing foam trapping is a method of foam control. Some facilities have been able to use a physical barrier or baffle to remove the foam from the tanks, using the natural current of the tank or clarifier. In Figure 3, a 4-inch PVC pipe, capped on both ends and supported by chains, floats on the oxidation ditch surface near the tank’s outlet weir. This barrier corrals the foam to the weir, where the foam then flows into the clarifier.
Once the foam is in the clarifier, the surface sweep arms push it into the scum troughs. The foam then goes to digestion and out with the biosolids. This method has been very successful at the treatment plant described here.
The Nocardia foam and scum gradually decreased, and with about two weeks of effort, the facility was back into compliance with its permit. After about one month, the Nocardia was almost entirely gone from the clarifier surfaces. The staff received training on how to identify Nocardia with the microscope. Now, when they see even a little bit, they sound the alarm and take preventive measures.
Power of pretreatment
One of the greatest achievements that came from this event was the formation of the city’s industrial/commercial pretreatment program. This department has since made a continuous effort to monitor and enforce the sewer use ordinance that prohibits pouring FOG and other nasty substances down the drain. Pollutant metals have decreased in the plant biosolids, and Nocardia has not revisited the plant.
FOG continues to be one of the biggest problems faced by operators of today’s treatment plants and collection systems. It contributes to sanitary sewer overflows (SSOs), sewer backups and lift station failures. Facilities that have challenges with FOG can find many solutions online, in workshops at wastewater association conferences, by contacting local Department of Health offices, by networking with neighboring utilities, or contacting me at the University of Florida TREEO Center.
About the author
Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at email@example.com.