How Wastewater Works
How does wastewater work? Below are descriptions of the different steps in the processes that will be utilized at the Tomahawk Creek Wastewater Treatment facility from the beginning to safe water released into the environment. Each month another process will be added.
11. Disk Filters
13. PEW Pump Station
While not a part of wastewater treatment, the overflow channel is an integral part of the Tomahawk Creek site. The overflow channel provides a means for removing excess stormwater from rains or flooding. It prevents flooding of the Wastewater Facility while maintaining current flood levels upstream of the Facility.
The old Tomahawk Creek Wastewater site had areas that were below the 100 year flood elevation. In order meet regulations for the facility it must be fully protected from a 100-year flood event and remain operational during a 500-year event.
Flood protection will be achieved by raising the entire site above the 100-year elevation through placement of fill material. The effects of the additional fill will be countered by the creation of the Overflow Channel. The overflow channel will be utilized only when Indian Creek overflows its banks and will drain after the flood abates.
Influent Pump Station
The primary purpose of the Influent Pump Station is to receive raw wastewater from the collection system, remove large trash and debris, and lift the wastewater to the beginning of the treatment process.
The Flow will enter the existing Influent Pump Station through the screenings area where three bar screens will keep the wastewater free of large trash and debris that could damage downstream equipment. After passing through the screens, wastewater will flow to the Dry Weather Pump Station. If flow increases due to wet weather conditions, the level in the wet well will rise until the water spills over two weirs into the Wet Weather Pump Station.
Flow from both pump stations is pumped to Headworks (#4) to begin the treatment process.
Flow from the Wet Weather Pump Station can also be directed to the Filter Complex (#11) during wet weather events.
Peak Flow Pump Station
Due to spikes in flow during major storms, a Peak Flow Pump Station is needed to provide additional flow through the Facility. This pump station will be engaged when flows exceed the capacity of the Influent Pump Station (#2).
Excess flow enters the Peak Flow Pump Station by flowing over a set of weir gates. Once in the pump station the wastewater flows through two multi-rake screens that will remove large debris and rags.
After screening, the flow enters the wet well that contains four submersible pumps. The diluted wastewater is pumped to the Filter Disinfection Complex (#11) to provide filtration and treatment along with the wastewater that has flowed through the main process.
As a backup to the Influent Pump Station (#2) during normal flows, the Peak Flow Pump Station can also be directed to the Headworks Building (#4).
Headworks of a wastewater treatment facility is the first stage in the treatment process. It is designed to reduce the trash, rags, and grit in the influent (raw) wastewater to protect downstream equipment and processes.
Flow will enter Headworks from the Influent Pump Station (#2).
Three perforated plate fine screens will be capable of screening the dry weather peak flow. Solid waste materials (e.g. rags, wipes, solid waste, etc.) collected from each screen will be reduced in volume through washer-compactor units and taken to the landfill.
Two free vortex units will be used for grit removal. Grit is anything small that settles quickly and can cause damage to equipment downstream. Examples are rocks, sand, egg shells, coffee grounds, etc. The grit will be washed and taken to the landfill.
Following grit removal, the main liquid flow will be sent to the next process: primary clarifiers (#5).
A splitter structure located on the end of the Headworks Building (4) will direct screened and de-gritted wastewater to the three Primary Clarifiers.
Wastewater enters the center of each basin and flows radially outward. The clarifiers slow down the flow to allow solid particles to sink to the bottom, i.e. primary sludge, and grease to rise to the surface. During high flows, ferric chloride will be added to enhance settling.
The primary sludge and scum are pumped out of the clarifiers to the solids treatment train on the east side of the site for further treatment.
After primary solids removal, the clarified wastewater flows into the Biological Nutrient Removal (BNR) Basins (6) for secondary treatment.
Biological Nutrient Removal (BNR)
Biological Nutrient Removal (BNR) is a secondary treatment technique that uses bacteria to break down and reduce the biological components of wastewater. BNR reduces the concentrations of organic matter, ammonia, nitrogen, and phosphorus.
The BNR basins provide the right conditions for the bacteria. The basins are split into different zones that will vary in oxygen concentration, nutrients, and recycle flows to provide the desired responses from the bacteria.
There are four BNR basin trains that include mixers, recycle pumps, and air diffusers. The BNR basins work together with the Final Clarifiers (8) to provide secondary treatment.
Basin Blower Building (BBB) and Chemical Feed
The Basin Blower Building (BBB) houses five blowers that supply air to the Biological Nutrient Removal (BNR) basins (6).
Air is extremely important to the BNR process. The bacteria in the oxic zones need air to reduce organic pollutants and convert ammonia into nitrate.
Blowers typically account for 40%-60% of the energy demand at a WWTF. These blowers are high efficiency gearless turbo blowers and will help control electricity costs.
Carbon supplement chemical tanks are also located at the BBB. Carbon supplement may be fed at each anaerobic zone and post-anoxic zone in the BNR to help stabilize nutrient removal.
Final Clarifiers with Splitter Structure
After biological treatment (6), the water will pass through the Final Clarifier Splitter Structure and flow to the Final Clarifiers. The Final Clarifier Splitter Structure diverts flow equally between the four Final Clarifiers.
Final Clarifiers function in the same way as the Primary Clarifiers (5). The main difference is that a portion of the settled solids is returned to the BNR Basin. The returned solids contain active bacteria that continue to remove pollutants. The rest of the settled solids will be wasted along with the scum to solids treatment.
If additional phosphorus removal is required, ferric chloride can be injected in the splitter structure to enhance settling in the clarifiers.
The clarified water flows into the Tertiary Pump Station (10).
Final Sludge Pump Station
The Final Sludge Pump Station is located between the Final Clarifiers (8) and will be equipped to remove the sludge and scum from each clarifier.
There are 10 return activated sludge pumps that send solids back to the BNR Basins (6).
There are 6 waste activated sludge pumps and 2 scum pumps that send flow to the solids treatment at the Solids Processing Building (16).
The pumps in the basement can also be used to drain a final clarifier for inspection and maintenance.
A storage room is located on the upper floor for equipment and parts.
Tertiary Pump Station
The Tertiary Pump Station will lift effluent from the Final Clarifiers (8) to the Filter Complex (11).
The pump station is a below grade wetwell with four solids-handling submersible pumps. Each pump is rated for a capacity of 19 million gallons a day (MGD) and the station is rated for 57 MGD.
The wetwell will be designed to allow two of the pumps and half of the wetwell to be drained for maintenance while the other half of the station remains in service.
There will be four pumps total, three of which are expected to be in active duty with the remaining one on standby.
Chlorine Contact Basin
After going through the Disk Filters (11), the water is ready for disinfection. It is important that we remove harmful bacteria and viruses before discharging into public waters.
Disinfection is accomplished by adding sodium hypochlorite (i.e. chlorine) into a chlorine contact basin (CCB). The CCB will include an influent mixing zone, four contact trains, and dechlorination. Dechlorination is required as chlorine is toxic to aquatic life. The CCB is sized for full wet weather flows.
The Disinfection Chemical Building provides the chemicals needed for the disinfection process.
Ferric chloride is also stored here to be used throughout the site.
After the CCB, the water will flow to the reaeration cascade.
The Disk Filters provide additional removal of smaller particles and remaining phosphorus after the Final Clarifiers (8). The Disk Filters have eight “Cells”, each containing 24, 10-ft diameter disks that screen and filter the clarified water.
The Disk Filters use a special cloth media and rely on gravity to convey water through cloth disks to a hollow center tube. Water flows over a weir into the Disinfection Basin (12). Solids are retained on the cloth media disks or settle to the tank bottom. Typically, only two Cells will be in service for normal flows.
The filters serve a critical role during major storms so that all flow can be treated onsite. Wet weather flows can be received from the from the Influent Pump Station (2) and Peak Flow Pump Station (3) and filtered through the disk filters.
In between each Cell is a Filter Pump Station with a backwash pump that is used to vacuum, or clean, the disk when dirty. When the pump is running, the disks rotate ensuring the whole disk is cleaned. Solids are sent back to the Influent Pump Station (2).
PEW Pump Station
In an effort to conserve resources, the reclaimed water at the end of liquid treatment is reused throughout the site.
The Plant Effluent Water (PEW) pump station sits above the Chlorine Contact Basin (12) and sends chlorinated effluent water through a piping network to hoses for wash down, pump seal water, and special equipment needs.
There are four pumps in the PEW pump station that can altogether pump a max of 1,800 gallons per minute.
This reduces the demand on city water and helps to control our operating costs.
Reaeration Cascade & Outfall
After the Chlorine Contact Basin (12), the Reaeration Cascade is the final liquid treatment process before discharging to the creek.
Reaeration is achieved through a simple passive, step type aeration where the flow cascades down steps. The turbulence and fall adds oxygen to the water making it safe for the aquatic life living in the creek.
After the cascade, the water enters the Outfall of the facility where it flows through the Overflow Channel (1) and merges with Indian Creek, which eventually flows into the Missouri River.
This completes the liquids treatment process of Tomahawk Creek WWTF.
As soon as liquid enters the Facility, one of the main goals is to separate the solids from the liquid for further treatment. Solids removed from the Primary Clarifiers (5) are pumped over to the Gravity Thickener.
The Gravity Thickener is a small circular clarifier that also serves as a fermenter to aid in biological liquid treatment. The solids in the Gravity Thickener will reside and thicken in the tank and ferment for 3 days. This helps produce an acid that is a source of energy for bacteria in the BNR Basins (6) associated with phosphorus removal.
There are three products associated with the Gravity Thickener: thickened sludge, scum, and supernatant. Thickened Sludge is pumped from the bottom of the tank by three pumps. A portion of the flow is recycled back to the Gravity Thickener and the remainder is pumped to the Sludge Blend Tank in the Solids Processing Building (16) to be co-thickened with other solids.
The scum is any grease or oil that collects on top of the liquid surface. This will be collected in an adjacent wetwell. One scum pump is used to pump to the Sludge Blend Tank.
The liquid, or supernatant, flows into an adjacent wetwell. Three supernatant pumps send the liquid back to the BNR Basin (6) where it combines with the Primary Clarifier (5) effluent and is used for biological treatment and phosphorus removal.
Co-Thickening (Solids Processing Building)
The Solids Processing Building houses three processes: Gravity Thickener (15), Co-Thickening, and Dewatering (17). We are focusing on co-thickening.
All the removed solids from the liquid treatment are sent to the Solids Processing Building for thickening. Thickening increases the solids content in the liquid to about 5%.
Solids from multiple sources are combined in the Sludge Blend Tank.
This includes thickened primary sludge, waste activated sludge, scum, and surface wasting from BNR.
The blended sludge is pumped from the basement to the Rotary Drum Thickeners (RDTs) on the first floor.
An RDT works by rotating the sludge in a hollow cylinder made of a porous screen.
Polymer will be injected into the blended sludge prior to entering the RDT. Polymer promotes adhesion between solids particles as they move through the RDT, improving thickening performance.
The co-thickened sludge drops into a wetwell (Co-Thickened Sludge Wetwell) and is steadily pumped to the Digesters (18).
The water removed from the RDTs is called filtrate and enters a separate wetwell. Filtrate is pumped to the beginning of the BNR Basins (6) for further treatment.
DeWatering (Solids Processing Building)
There is too much liquid in the digested biosolids, so it needs to be dewatered. This makes the biosolids easier to handle and reduces the number of trucks required for transport.
From the Digesters (17), the biosolids will be pumped to three centrifuges located on the second floor of the Solids Processing Building.
A centrifuge is a mechanical separation process where the solids are separated from the water using centrifugal forces. Each centrifuge will be fitted with a screw conveyor that will transport the dewatered solids, i.e. cake, to a trailer below.
Once the trailer is full, a truck will haul it away and pull in a new trailer. The biosolids are hauled off site to be land applied on specialized farm fields as a fertilizer.
Digesters and Biosolids
After passing through the Rotary Drum Thickeners (RDTs), the thickened solids are pumped to the Digesters. Similar to BNR treatment, digesters use bacteria to:
reduce the amount of solids,
destroy pathogens, and
produce methane gas (biogas).
These bacteria operate in the absence of oxygen and like a nice warm environment. We take advantage of the methane that is produced and use it to maintain a temperature of 95 degree Fahrenheit in the digester.
There are three primary digesters, each with a volume of 800,000 gallons. The contents are mixed and spend at least 15 days in these tanks before they are transferred to the secondary digester. At this point the solids are considered biosolids and can be land applied, but they need to be dewatered first.
The secondary digester serves as a holding tank for the biosolids as they are pumped to Dewatering (17).
Biogas & Flare
One of the byproducts of digestion (17) is biogas. Biogas comprises mostly methane followed by carbon dioxide. Small amounts of water vapor, hydrogen sulfide, and other gases may be present.
Methane is not only flammable but is a significant greenhouse gas. To prevent its release into the atmosphere, each Digester will be covered, and the gas will be collected.
The majority of the collected gas will be beneficially reused onsite to heat boilers that are used to keep our Digesters at 95oF.
Surplus gas will be burned off by the gas flare. This reduces the methane into carbon dioxide and helps to eliminate any odors from the biogas.
The gas flare has an enclosed design, which eliminates a visible open flame and reduces hazards.
One of the other byproducts from dewatering biosolids is centrate. Centrate leaves the centrifuges in the Solids Processing Building (17) and flows to sidestream treatment.
Centrate is the liquid separated from the biosolids after dewatering. It carries a heavy ammonia and phosphorus load relative to the raw wastewater.
Sidestream treatment is a newer technology that helps to remove the majority of the ammonia more efficiently than in the BNR Basins (6).
The system consists of a holding tank, feed pumps, boilers, heat exchangers, reactors with specialized bacteria, and BNR feed pumps
The bacteria in the reactors, called annamox, will reside in small plastic wafers. The wafers will be gently mixed inside the reactor and are able to convert the ammonia to nitrogen gas with little carbon or free oxygen.
After sidestream treatment, the treated centrate will be pumped to the head of the BNR basins for secondary treatment.