March 2010, Vol. 22, No.3

Safety Corner

Seeing Through the Fog

How covering trickling biofilters can be dangerous to your health

Zoeanne Tafolla

You know how sometimes fixing one problem just creates another? This proved true for the Vallejo (Calif.) Sanitation and Flood Control District (VSFCD) and its odor-control project. A cracked distributor arm led to a mysterious fog, which led to a new set of rules for entering the plant’s covered trickling biofilter structures.

In 2005, VSFCD began a project to cover the two 24-ft-high by 100-ft-wide (7.3-m-high by 30-m-wide) biofilters, as well as two aeration basins and the primary and secondary clarifiers’ launders in the secondary portion of its treatment plant. The new system has a flow of 12,000 ft3/min (340 m3/min) through each biofilter, recirculating 5000 ft3/min (142 m3/min) of foul air from the aeration basins and secondary clarifiers, and sending 7000 ft3/min (198 m3/min) to a soil filter for scrubbing. Additionally, each biofilter is equipped with one large standby fan for redundancy. The design also included a fresh-air intake designed to be used while in maintenance mode for entry into the biofilters.

During the project, the hydraulically powered existing mild-steel distributor arms were replaced with motorized stainless steel arms because the environment under the cover could lead to catastrophic corrosion of the mild steel.

The new distributor setup meant operators would need to make several entries into a confined space to maintain and repair the motorized distributor arm — something that hadn’t been considered initially.

Once the project was completed in 2006, plant staff entered the biofilters infrequently. Periodically, a biofilter would be shut down to perform routine maintenance — oil changes and inspections. The biofilters were characterized as confined spaces even though the atmosphere was stable, as indicated by our four-way gas monitors (oxygen, hydrogen sulfide, lower explosive limit, and carbon monoxide).

Maintenance staff members wore the proper personal protective equipment — harness, gloves, boots, and safety glasses — while performing work in the biofilter. Staff members were never in the space for more than 6 to 8 hours at a time — until April 3, 2008, that is.

A Cracked Arm

During a routine oil change and inspection, one of the mechanics noticed a crack in the drive-ring weld. Plant staff shut down the unit, took pictures, and called the manufacturer (the unit was still under warranty).

The manufacturer sent a technician the next day to inspect the unit. Fearing that the distributor arm would break and drop onto the media, plant staff decided to leave the unit off with no flow until the technician arrived to assess the damage.

The foul-air system remained on throughout the shutdown to keep the air flowing and ensure adequate ventilation for entering the structure on the following day, April 3. That day was a warm, sunny day in Northern California, with temperatures in the upper 70s.

A Mysterious Fog

When the manufacturer’s technician arrived, plant staff discovered that the dome had filled with a misty fog and a strange odor that was strong at first and immediately went away. Accordingly, the VSFCD plant decided to follow its Declassified Confined Space Procedure and tested the atmosphere for oxygen, methane, lower explosive limit, and hydrogen sulfide using a combustible-gas meter. The meter indicated that it was safe to enter the structure.

The technician entered the structure, made it inside a few feet, and was overwhelmed and nauseated. Plant staff later confirmed the fog to be ammonia gas.

The plant had never seen this problem before, because after the covers had been installed, the biofilters had never been out of service for more than 8 hours at a time.

When the atmosphere was tested using a Draeger tube through a crack under the door, it was estimated that the ammonia level in the structure was well above 100 ppm. The short-term exposure limit, according to the U.S. Occupational Safety and Health Administration is 35 ppm. This means exposure to ammonia is to be limited to a maximum 35 ppm in a 15-minute period. The levels in the biofilter structure were three times that amount.

Slow Dispersion

The plant decided not to allow the contractor to enter the biofilters to fix the problem until the atmosphere was safe. The ammonia levels dropped slowly during the next 7 days. On April 15, it was determined that the atmosphere was safe; ammonia levels at the center of the structure measured 25 ppm.

The distributor-arm technician inspected the units, and repairs were conducted the following day. The levels of ammonia had dropped off from 15 ppm in the morning to 9 ppm when the job was completed 7 hours later. Once repaired, the unit was put back into service and recovered to full treatment in about 5 weeks.

A Second Chance

Another mechanical failure occurred in the biofilters just before Thanksgiving 2008. Given the previous experience, plant staff decided to stabilize the arms, leaving them stationary, and to continue running water through them in an effort to keep the filter from going belly up as before. After the 4-day holiday weekend, technicians entered the structure and inspected and repaired the unit. This time, the ammonia levels never rose above 2 ppm.

Solving the Mystery

Following this incident, staff conducted surveys and questioned experts in the field about their experiences with covered biofilters. No one in the nearby area had experienced anything like this. Most were not even considering the biofilters to be a confined space nor testing the atmosphere.

However, plant staff discovered similar experiences were being reported during the update of a design manual for aerobic fixed-film reactors. In several cases, it was reported that shutting down biofilters enabled ammonia levels and heat to rise. One agency reported that during two different emergency shutdowns, each about 8 hours long, ammonia levels reached 80 ppm, and temperatures ranged from 100°F (38°C) to 140°F (60°C).

Many reported that they created special procedures for short-term (2 to 8 hours) and long-term shutdowns (8 hours and greater) to minimize ammonia production. The case studies indicated that air alone was not enough to cool the temperatures. Without water as a cooling agent, the “compost effect” created by the decomposition of the bacteria raised the internal temperatures in excess of 150°F (66°C) and began displacing oxygen with ammonia. Depending on the weather, this reaction can take several weeks or even a month to occur.

New Procedures

These experiences mirrored VSFCD’s biofilter incidents. The plant has now changed its procedures when entering for short-term (maintenance and inspection) events and for long-term (repair) events. In both instances, the air is monitored continually, and the person entering the structure is required to wear a supplied-air breathing system.

Because of the ammonia potential, VSFCD changed all of its four-way gas monitors to five-way gas monitors and now monitor ammonia for all confined-space entries. A characterization of the biofilters was conducted, and the spaces have been reclassified as a permit-required confined space. VSFCD also has determined that it would be prudent to establish and appropriately equip an in-house rescue team to perform rescues immediately.

VSFCD also has purchased a heat monitor for employees to wear; employees are now required to monitor their heat index and take frequent cooling-down breaks when ambient temperature reaches 85°F (29°C).

When the biofilters are taken off-line for maintenance or repairs, water is still circulated over the media to keep growth alive. The plant has a portable pump with perforated lay-flat hose that disperses approximately 300 gal/min (1140 L/min) of water over the media if the distributor arms are out of commission.

VSFCD was truly fortunate not to have had a disaster occur during these incidents and now understands that conditions can change very fast in a living process, such as biofiltration. Thus, plant staff members always treat the atmosphere inside a biofilter as a permit-required confined space, reviewing the potential exposures with everyone involved, and establishing a plan to rescue anyone who enters the space.


Zoeanne Tafolla is director of safety–risk manager for the Vallejo (Calif.) Sanitation and Flood Control District.


©2010 Water Environment Federation. All rights reserved.