The secondary treatment process has three 105-foot diameter trickling filters each containing 16 vertical feet of plastic cross-flow media. A rotary 4-arm distributor sprays primary effluent over the media at a rate of approximately 1.7 gpm/sf. The trickling filters are elevated above the plant hydraulic gradient to allow the effluent to flow to the solids contact tanks by gravity.
The filters are covered with aluminum domes to contain foul air. A forced air ventilation system transfers foul air exhausted from the headworks building into the space below the dome enclosures. Air moves downward, concurrent with the wastewater flow through the trickling filter media to provide oxygen for the biological process. Foul air collected from the plenum area below the media is discharged to a chemical scrubber system to remove odorous compounds prior to discharge to the atmosphere.
Five variable speed 24 mgd vertical column pumps are available to lift primary effluent to the top of the trickling filter. Trickling filter effluent is recycled through the pump station to maintain minimum wetting for the fixed-film process. Electric variable speed drives and motor control centers for the trickling filter pumps are housed inside the Trickle Filter Service Building.
The Return Aeration/Solids Contact Tanks (RA/SCT) is an integral part of the Trickling Filter/Solids Contact (TF/SC) biological treatment process. In this process the Trickling Filter Effluent (TFE) is mixed with activated sludge in the SCTs to improve sludge settleability and to oxidize any Carbonaceous Biochemical Oxygen Demand (cBOD5) remaining in the TFE or in any Primary Effluent (PE) that is sent to the system. The RA Tank serves to reaerate the Return Secondary Sludge (RSS) coming from the final clarifiers before it is mixed with the TFE in the SCT tanks.
There are six tanks in the system; RA1, RA/SCT2, SCT 3-6.
Each of the six tanks measures 108 feet long by 49 feet wide by 12.8 feet deep and has an approximate volume of 500,000 gallons.
Reaeration Tanks RA1 and RA/SCT2 are designed as the reaeration tanks. Only one of these tanks will be in service as an RA tank at any given time. The Reaeration tank receives all of the RSS from the final clarifiers. The tank is equipped with fine bubble diffusers and an automatic air control valve. This tank is designed to provide retention time for reaeration of the solids from the seven final clarifiers. The Hydraulic Retention Time (HRT) for this tank will vary with flow from about 29 minutes at 25 MGD RSS flow to about 14 minutes at 50 MGD RSS flow. RA/SCT2 serves as a backup RA tank for use when RA1 is out of service or as a SCT.
The SCTs receive the reaerated RSS from the RA tank plus TFE and PE. These tanks are equipped with fine bubble diffusers and automatic air control valves. These tanks are designed to condition the solids to improve settling in the final clarifiers and to oxidize any cBOD5 remaining in the influent.
The HRT for the SCT tanks is designed to be about 29 minutes at the maximum design flow of 100 MGD (50 MGD TFE+PE + 50 MGD RSS) with four SCT on line. At lower flows fewer SCTs may be in service. To maintain the 29 minute HRT at 50 MGD (25 MGD TFE+PE + 25 MGD RSS) only two SCT are needed.
The Formation of Floc
As bacteria begin growing, they generally develop into small chains or clumps. They are very active and motile and it is difficult for them to settle. They have not yet developed the slime layer which aids in their sticking together. So, when mixing occurs, the small chains or clumps are broken up and the bugs are dispersed, and they will not flocculate or settle. As the sludge is allowed to age, the bugs lose their motility and accumulate more slime. Then the clumps and chains are better able to stick together. The clumps grow bigger and bigger until they form a floc. If the organisms are allowed to develop properly, under the right conditions, the floc get large and compact and begin to settle. The mixing in the aeration tank tends to keep the floc small since, even though the bugs are sticky, the bond formed holding the organisms together is not very strong. This is good because it allows the cells, food, and oxygen to contact each other.
The growth curve characteristics of bacteria.
•Lag-phase - During this phase bacteria become acclimated to their new surroundings. They are digesting food, developing enzymes and other things required for growth.
•Accelerated Growth phase - The bacteria are growing as fast as they can, since there is an excess of food. The cells are mostly dispersed, not sticking together.
•Declining Growth phase - Reproduction slows down because there is not an excess of food. A lot of food has been eaten and there are now a large number of bacteria to compete for remaining food, so the bacteria do not have enough remaining food to keep the growth rate at a maximum.
•Stationary phase - The number of bacteria is the highest possible, but not much food is left, so the bacteria cannot increase in number. There is some reproduction, but some cells are also dying, so the number of bacteria remain relatively constant. The bacteria have now lost their flagella and have a sticky substance covering the outside of the cell, allowing them to agglomerate into floc. In fact, the floc get big enough that if aeration and mixing were stopped, the floc could settle to the bottom.
•Death-phase - The death rate increases with very little if any growth occurring. Therefore, the total number of living bacteria keeps reducing. The bacteria are just trying to keep alive.
The settled solids form a sludge "blanket" on the bottom of the clarifier that is collected using a rotating suction header which is connected to the 24" line that travels to the secondary sludge wetwell. The suction in the 24" clarifier-RSS wetwell pipe is created by the pumping of RAS from the return secondary sludge wetwell to the reaeration basin. The suction header spans the diameter of the bottom of the clarifier and has holes on the bottom which then collect the settled sludge as it is rotating and from the siphon created in line from the RSS pumping activities. If the RAS pumps are not running, the siphon is not created thus the sludge blanket can build up to high levels which could possibly jeopardize water quality.
Less dense and colloidal solids that do not settle and accumulate on the surface of the clarifiers are skimmed by a traveling arm. These skimmed solids are collected in the scum box and flow by gravity to the headworks of the facility.
Clarifiers 1 - 5 hold a volume of 200,394 cubic feet (135' diameter, 14' SWD) and a surface area of 14,314 square feet. Clarifiers 6 and 7 hold a volume of 286,278 cubic feet (135' diameter, 20' SWD) with the surface area being the same as the other clarifiers.
Five large multi-stage centrifugal blowers and one small blower, discharging into a common header are staged to provide airflow to the RA/SCTs for mixing and oxygenation. The inlet valves to the blowers can be adjusted to maintain pressure on the air header. A modulating blow-off valve will prevent the blowers from entering a surge condition and provide additional capacity adjustment.
Total blower capacity for the five blowers is 40,500 scfm, 8,100 each. The capacity of the smaller blower is 3,210 scfm. The average air flow demand is expected to be 15,600 scfm (two blowers) and the peak air flow demand is expected to be 34,500 scfm (4 blowers).
Each blower is equipped with an equipment protective package to provide shutdown and alarming for surge, vibration, and temperature
Oxygen is required by these bugs to metabolize food for cell maintenance and growth. Although the bugs need oxygen, some bugs can get along with less oxygen than others. Each bug must have a dissolved oxygen of at least from 0.1-0.3 mg/L to function properly. So, it is important to maintain about 2 mg/L of D.O. in the activated sludge so that the bacteria that are contained in the floc can get oxygen. If the DO is less than 2 mg/L, the bugs on the outside of the floc use the DO before it can get to the center of the floc. If this happens, the bugs in the center may die causing the floc to break up.
The Effects of Mixing
Mixing is required to bring organisms, oxygen, and nutrients together, and to remove metabolic waste products. If there is not enough mixing, proper treatment will not take place because of lack of contact between the bugs, their food and oxygen. If too much mixing is provided, it can cause break up of floc or formation of unstable floc particles.