LuminUltra – Waste Water Bioreactor
Bulking Prediction in a Bioreactor
Problem: A pulp and paper wastewater treatment plant that experiences frequent bulking investigated strategies for early detection of bulking conditions
Facility: Conventional activated sludge plant in eastern Canada receiving pulp and paper mill effluent
Size: 6.8 MLD
Investment: Routine testing (5 times per week) of active biomass (QG21W) and floc-bulking ATP (QG21Wa)
Economic Analysis: Implementing fbATP as a bulking indicator had a calculated potential savings of $25,946 over a 5-month period
Synopsis: A five-month trial was conducted at a conventional activated sludge wastewater treatment plant in early 2007 to evaluate the use of floc-bulking adenosine triphosphate (fbATP) as a method for early detection of bulking events. The wastewater plant, which treats effluent from a nearby pulp & paper mill, had previously experienced frequent bulking events. Traditionally, the plant used stirred sludge volume index (SSVI) to indicate bulking but were looking for a parameter that could provide earlier detection and faster responses to corrective actions. During the trial a three-month bulking event was observed. fbATP signaled that the bulking event had started 2 week earlier than SSVI, allowing valuable time for troubleshooting and potential corrective actions. fbATP was also a much more responsive parameter to chlorination treatment strategies. Following an extended period of chlorination, fbATP dropped then subsequently increased, indicating that the corrective action was ineffective. During chlorination SSVI showed no response. The total cost of chemicals used in corrective actions was approximately $25,946. It is expected that if trouble shooting had occurred during the two-week window following the fbATP alarm, the bulking event could have been prevented.
In early 2007, a conventional activated sludge wastewater treatment plant in eastern Canada conducted a trial monitoring program aimed at early detection of floc bulking. The wastewater plant, which treats effluent from a nearby pulp & paper mill, had previously experienced frequent bulking events. A basic block flow diagram for the plant is shown in Figure 1. Bulking was monitored using floc-bulking adenosine triphosphate (fbATP), 30-minute stirred sludge volume index (SSVI) and microscopic filament counts. fbATP is an advanced 2nd Generation ATP protocol which measures the quantity of ATP exhibiting bulking tendencies. A full suite of other operational parameters, including influent loading, pH, temperature, and chemical dosages, was collected.
When bulking occurs in a wastewater treatment system, various chemical-based strategies can be attempted to rectify the problem. Chemical strategies can include chlorinating the return activated sludge to reduce the filamentous population or adding coagulants/settling aids to the secondary clarifier to reduce sludge carry over. However, these strategies often don’t address the root cause of bulking and instead act as a temporary solution. Some of these chemical strategies also have further disadvantages. Chlorination can harm some of the beneficial organisms while attempting to destroy the filamentous organisms. While coagulant usage can increase sludge generation rates leading to increased wasting. The wastewater treatment plant studied utilized both chlorination using hypochlorite and coagulation using ferric sulphate. It was anticipated that the trial monitoring plan would allow them to identify and implement new strategies to identify bulking events in the early stages of growth, allowing for preventative maintenance and troubleshooting rather than expensive and often ineffective corrective actions.
Results: SSVI was traditionally what the plant used as a bulking indicator. The alarm setting for SSVI in this specific plant is 250 mL/g. Figure 2 shows the SSVI throughout the trial period along with a 7-day average.
During the 5-month trial, a significant bulking event was witnessed. SSVI signaled a bulking event on February 12, 2007. The SSVI then continued to increase for approximately one-month before stabilizing and finally decreased below the alarm value on May 8, 2007.
Well before the bulking event was signaled by SSVI, conditions within the reactor began to favor the growth of filamentous microorganisms. fbATP began to show a change from typical baseline conditions and exceeded the alarm value of 50% on January 29th, 2007, two weeks before the SSVI alarm. The SSVI and fbATP are shown below in Figure 3.
As illustrated in Figure 3, the fbATP is a much more responsive parameter compared to SSVI. It signaled a bulking event 2 weeks before the standard SSVI measurement. In response to the bulking event, two different chemical strategies were used: chlorination using hypochlorite and coagulation using ferric sulphate. Figure 4 shows the time periods for each strategy as well as the response in fbATP and SSVI.
Hypochlorite was added to the plant on four occasions. The chlorination feed rate for each period were:
- January 6-8 (3 days): 8 kg hypochlorite/metric ton of bioreactor solids
- February 2-5 (4 days) : 8 kg hypochlorite/metric ton of bioreactor solids
- a) February 19-20 (2 days): 8 kg hypochlorite/metric ton of bioreactor solidsb) February 21-25 (5 days): 10 kg hypochlorite/metric ton of bioreactor solids
- April 2-6 (5 days): 9 kg hypochlorite/metric ton of bioreactor solids
fbATP decreased during the longer, higher feed rate chlorination events (February 19-25 and April 2-6) while SSVI showed no response. During the shorter, lower feed rate chlorination events (January 6-8 and February 2-5) no response was seen to fbATP or SSVI. Ultimately, fbATP increased after the chlorination events halted, signally that the root cause of the filamentous organism was persisting. A ferric sulphate coagulant was added for 39 days (March 19-April 27) to reduce floc carryover from the secondary clarifier. Coagulant addition did not affect SSVI or fbATP.
After further investigation, it was determined that the root cause of the filaments was from excess concentrations of volatile fatty acids (VFAs) entering the plant. Certain filaments, such as Microthrix, readily consume VFAs resulting in increased growth. The VFAs in this case were being formed in a whitewater accumulator that had turned anaerobic. By implementing strategies to reduce VFA concentrations the plant was able to stop the bulking event. Through the addition of 2nd Generation ATP monitoring, the facility would have had a 2-week warning period adding valuable troubleshooting time before the full bulking event occurred.
The main expenses related to the bulking event was the dosing of hypochlorite and ferric sulphate:
- Over the four hypochlorite dosing periods, a total of 14,884 kg of hypochlorite was added. At a unit cost of $1.67/ kg, this equates to $24,856.
- A total of 4,194 kg of ferric sulphate solution was added. At a unit cost of $0.26/kg this equates to $1,090.
In terms of chemicals, the total cost of the event was $25,946. It is anticipated that this cost could have been mitigated through early detection and the implementation of preventative actions, rather than waiting for a fully developed bulking problem to present itself then taking corrective actions.
The ability of fbATP measurements to predict the onset of bulking conditions is highly significant because it allows operators to:
✓ Identify the causes of sludge bulking by knowing when the instigating conditions occur.
✓ Take preventative action to mitigate the onset of bulking in the early stages of growth.
✓ Save on costly or ineffective corrective actions by making an immediate evaluation (i.e. Chlorine)
Focus on developing meaningful, proactive solutions to prevent or eliminate bulking conditions.