Treatment Process

Mandan's wastewater treatment process consists of several steps:

Pretreatment

All of the city's raw wastewater and onsite sanitary sewage enters the pretreatment building collection vault. The wastewater from the city is pumped to the site via two 16-inch asbestos cement pipe (ACP) force mains and one 12-inch ACP force main. Each force main can be shut off using plug valves. Flow can be diverted from the pretreatment building to the selector tank, aeration basin or to cell two if needed. The sewage is collected into a channel that flows into the pretreatment building.

Here the wastewater is directed to either the automatic rotary fine screen, micro strainer or both. The rotary fine screen can be lifted from the channel for maintenance and the micro strainer can be used while the fine screen is out of service. Screenings are deposited in a dumpster for routine disposal.

After the screening process, the plant flow enters the vortex grit chamber. The settled grit is removed, dewatered, and deposited in a dumpster for routine disposal.

Upon completion of grit removal a sampler extracts samples for influent stream monitoring. As the flow exits the pretreatment building, it passes through a 12-inch Parshall flume used to measure plant flow and then enters the selector basin.

The average daily flow is 1.6 million gallons a day.

 

Odor Control - Scrubber Building & Biofilter

The TPS BiofiltAerTM Biofilter is a two-stage odor control system. The first step is off-gas scrubber for humidification and temperature adjustment. This is followed by adsorption and biological treatment in a fixed growth bed Biofilter. The supporting equipment for this part of the process includes a fan, media, plenum, boiler and water panel. Instrumentation includes measurements for pH, humidity, temperature and pressure.

The Biofilter is a Mesophilic fixed film biological air filter. Its function is to remove unwanted odor constituents before off-gas is discharged into the atmosphere. This is accomplished by creating the proper environment for the microbes to grow on the media located within Biofilter bed.

The Biofilter consists of two layers of media one being inorganic and the other organic. The bottom layer is inorganic Haydite. The iron content of the Haydite provides essential compounds necessary for the growth of microorganisms that are essential for the treatment of Hydrogen Sulfide (H2S) and mercaptins. The upper media layer is organic wood chips. This layer is comprised of trunk and root wood typically from hard wood trees. This organic media also provides a slow release carbon source, essential nutrients (N, P, and K) and micronutrients (Mn, S, and Se), making the media an ideal site for naturally occurring bacteria to breakdown any odor constituents.

 

Selector Basin

The first step in the treatment process is mixing the influent (raw wastewater) with return activated sludge (microorganisms that treat wastewater) in the selector basin.

As the influent flows by gravity into the selector basin, return activated sludge is pumped into the basin from the final clarifier and the two are mixed within the basin (~4.3 hours).

Substantial treatment is taking place in the selector basin.

  • About a 87% reduction in the organics (Biochemical Oxygen Demand)
  • About a 63% reduction in Ammonia

This mixed liquor then leaves the selector basin and enters the aeration basin.

 

Biological Nutrient Removal - The aeration basin and the WaveOx process

The mixed liquor enters the extended aeration basin through two 20-inch ductile iron pipes and one 15-inch square opening from the selector tank. The aeration system consists of floating aeration chains fixed at both ends with a special molded down comer fitting. These chains are then connected to the air supply by using a flexible hose.

From the floating lateral pipe, fine bubble diffusers are suspended above the basin floor. Air is introduced at one end of the aeration chain and then enters the wastewater through the diffusers. The air both mixes and oxygenates the basin contents. Because the aeration chains are flexible, the air released from the diffusers cause the aeration chains to oscillate back and forth in a regular pattern. The chains are installed so that the entire basin floor is covered by these patterns.

Oxygen levels in the aeration basin are very critical to the quality and efficiency of wastewater treatment. With the aeration basin set at full automatic mode, the various aeration zones alternate between an aerobic and an anaerobic environment. This is known as the WaveOx Process. The principal advantage of the WaveOx Process is its effect on ammonia removal via the nitrification/denitrification process.

When the zones are aerobic (DO between 1 to 2 mg/l), nitrification takes place and ammonia is broken down into nitrites and nitrates. When denitrification becomes necessary, the aeration chains within the zones are turned off and the basin turns anaerobic and nitrogen gases off.

Plant operators control nitrification/denitrification by determining the number and sequence of aeration zones turned on and off. Plant loadings and weather conditions also play a part in determining the number of aeration chains required to carry on the specific nitrification/denitrification process.

Plant operators also have the ability to place the aeration chains in a manual mode of operation to facilitate equipment repairs or to more efficiently treat slug loadings.

Oxygen levels are monitored continuously by a dissolved oxygen probe located in the aeration basin. The oxygen probe sends a reading to the programmable logistic control (PLC) network which will automatically adjust the number of blowers running at once to maintain adequate oxygen. Air is supplied to the wastewater treatment plant by two 200-horsepower Aerzen turbo blowers and three 60-horsepower Sutorbilt rotary positive displacement blowers.

 

Final Clarifier

As flow enters the final clarifier, sludge settles and is removed using four airlifts. Here waste activated sludge (WAS) is pumped to a Facultative Sludge Basin (FSB) located south of the final clarifier and Return Activate Sludge (RAS) is pumped back to the Selector Basin.

Meanwhile, effluent is collected from the top of the clarifier through v-notch weirs. A 24-inch ductile iron pipe collects the effluent from the weirs and transports it through a post aeration tank prior to exiting the final clarifier.

Once outside the final clarifier, the flow can be sent to cell two, cell three, or to ultra-violet disinfection. Flow would be diverted to cell two or cell three during an untreatable slug load, hydraulic overload or plant maintenance of the EAASTS. The plant can discharge to the Missouri River from cell two or cell three provided the discharge limits are met. Under normal operation however, the effluent enters the ultra-violet disinfection and then is discharged.

 

Disinfection & Discharge

Prior to discharging the effluent to the Missouri River, the effluent is disinfected using ultra-violet light (UV). The UV channel depth is controlled by a weighted gate downstream from the UV equipment to maintain the proper water level. It is important for the UV light tubes to be entirely submerged while in service. Upon completion of disinfection, the flow is measured by a 9-inch Parshall flume as it exits the UV room.

At this point in the treatment process samples are collected for permit purposes.

Flow exiting the UV disinfection room generally gravity flows to the Missouri River. If the river is above critical stage, the flow will be directed to the effluent lift station where it can be pumped to provide sufficient head pressure to push the flow to the river through the same discharge piping.