1.7: Biological Treatment Methods
Learning Outcomes
- Describe different methods to biologically treat wastewater
- Compare and contrast fixed film treatment methods with other biological treatment methods
- Understand the operation of a trickling filter and how to control it
Fixed Film Processes
A fixed film treatment process is still a biological treatment method and utilizes the same type of bacteria as discussed in Chapter 6. The difference is that instead of the bacteria floating near the bottom of a pond, they are fixed to some type of medium. The media can either be natural stone, synthetic plastics, or large rotating disks.
Rotating Biological Contactors
Rotating biological contactors (RBCs) utilize aerobic bacteria to decompose the incoming BOD5. They are composed of a series of closely spaced circular disks. The disks are connected to a shaft that is coupled to a motor. The set of disks lay horizontally in a tank and rotate at a slow speed. The incoming wastewater enters the tank and submerges approximately 40% of the disks. The water moves through the tank and comes in contact with the aerobic bacteria that are growing on the disks. When the disks are submerged in the wastewater, the bacteria have access to the BOD5 and will consume it. As the disks rotate out of the liquid in the tank, they are exposed to the atmosphere where the bacteria can utilize oxygen for the aerobic decomposition of the BOD5.
The disks are typically 12 feet in diameter and multiple disks are combined together to make a long cylinder. Common lengths are 25 feet. The motor spins the disks at approximately 1.5 rpm. It’s not uncommon to see multiple RBCs being used in both parallel and series configurations. As the bacteria thrive in this environment, eventually they will build up on the disks and start to slough off. The effluent of the RBC is sent to a finishing pond or clarifier to further treat these solids.
Each of the RBCs is covered for several reasons. Disks are commonly made out of plastic so covers will protect the disks from becoming brittle due to sun damage. Foul odors from the wastewater and H2S gasses will be present in the RBCs. Covering them will reduce these odors and prevent nuisance complaints from the community. Lastly, covers will protect the bacteria from being washed off the disks during rain events. Fiberglass covers are commonly used and are more cost-effective than building the RBCs in a building. The humidity and wastewater gases can be corrosive to cement, metals, and other building materials.
Trickling Filters
Trickling filters are another form of fixed film biological treatment. They are comprised of circular tanks filled with stone, lava rock, ceramic, or synthetic material all of which are referred to as media. Standard media is approximately 1 inch to 4 inches in diameter. The bacteria will become fixed to this media and aerobic decomposition will occur as the incoming wastewater comes in contact with the bacteria. While trickling filters can be relatively deep, around 3 to 8 feet, there is ample airflow in the voids of the media to allow for aerobic bacteria to thrive. Trickling filters often have ventilation ports near the bottom to ensure there is sufficient airflow to the bacteria. The wastewater enters the trickling filter at the top of the filter. Then gravity takes over and the wastewater trickles down through the media to the bottom of the tank where it is captured in an underdrain system.
The wastewater is conveyed from the primary sedimentation tanks to the trickling filters. This is either done by gravity or by using a pump. Either way, a trickling filter uses this pressure in the water to evenly distribute the wastewater to the trickling filter. There is a distributor arm that expands the diameter of the circular tank. The wastewater is forced out through small outlets on the distributor arm that will cause the arm to rotate.
The wastewater trickles through the media and comes in contact with the bacteria fixed to media. The bacteria are now in a sufficient environment where aerobic decomposition will occur and reduce the BOD5 of the wastewater. Over time the bacteria will build up on the media and eventually will fall or slough off. These solids will be captured in the underdrain system and sent to the secondary clarifier. In the clarifier, sedimentation will occur and the solids will settle to the bottom of the clarifier and be removed.
Recirculation
Recirculation of the clarified effluent is a key operational component of a trickling filter. Water from the secondary clarifiers has already gone through the biological treatment process of the trickling filter and has a lower amount of BOD5. By recirculating this treated water with the incoming wastewater, it will dilute the incoming BOD5. Recirculation will also be able to control the dissolved oxygen level in the trickling filters. Increasing the recirculation rate will cause the dissolved oxygen to increase and lowering the recirculation rate will cause it to decrease. An optimal concentration of dissolved oxygen in the trickling filter is 1.5 mg/L to 2.0 mg/L.
Organic Loading
Trickling filters can be further classified by how much BOD5 is being sent to the treatment unit. Low rate trickling filters receive up to 25 lbs of BOD5/1000 cubic feet/day. At this lower organic load, an operator can expect to achieve 80% - 90% BOD5 removal. Intermediate filters can handle up to 40 lbs of BOD5/1000 cubic feet/day but will see lower removal efficiencies of BOD5. High rate and roughing filters will have BOD5 loading in excess of 50 lbs of BOD5/1000 cubic feet/day. At these higher loadings, BOD5 removal significantly drops and will not meet current regulatory requirements. High rate and roughing filters are often used as a preliminary step and are combined with other forms of treatment to ensure regulatory compliance is achieved.