Raw sewage enters the recovery facilities through influent pumping stations. At the raw wastewater pumping stations large mechanically cleaned screens with vertically placed bars remove larger debris like sticks, paper, and hygienic products. After being screened, the wastewater is pumped to the grit basins. Grit basins are designed to remove heavy particles, like sand, gravel, vegetable seeds, and egg shells. Material collected from the bar screens and the grit basins are hauled to the sanitary landfill (Bluff Road).
After wastewater passes the grit basins, it goes to the primary clarifiers. Primary clarifiers are designed to remove settleable solids which tend to be more organic in nature and settle at a much slower rate. In addition, primary clarifiers have skimmer arms which collect floating materials, like grease. It takes approximately two-to-four hours for wastewater to travel through the primary clarifiers.
Once the settleable solids have been removed, the wastewater enters a biological reactor or aeration basin, which is called the biological treatment phase. This treatment phase removes dissolved pollutants within the wastewater, utilizing a diverse population of microorganisms like bacteria, protozoa, stalked ciliates, and rotifers that live within these biological reactors. In order for these organisms to live and reproduce, they must have adequate oxygen and an adequate food supply. These small organisms, which can only be seen through a microscope, use the wastewater as their food source. They feed on the organic matter found in wastewater. There are numerous methods of aerating wastewater for the organisms use. Recovery facilities use blowers and submerged diffusers, surface mixers, or cascade the wastewater over rock, plastic, or wood media. The by-product of their metabolism is carbon dioxide, nitrogen, and water, which leads to a purified and treated effluent. To optimize the metabolism of these microorganisms in treating wastewater, the aeration basins must have adequate oxygen and mixing so that these organisms are in constant contact with their organic food source. Wastewater is typically held within the biological reactors for four-to-eight hours.
After wastewater leaves the biological reactors, it is sent to the secondary clarifiers. Secondary clarifiers look just like the primary clarifiers but serve a different
purpose. Due to the feeding requirements of the microorganisms, the wastewater now has a much smaller fraction of organic waste but a significant volume of the wastewater now contains the suspended microorganisms themselves. This combination of treated wastewater and suspended microorganisms is called "mixed liquor". Once the mixed liquor reaches the secondary clarifiers, the microorganisms settle to the bottom of the tank. The secondary clarifier is designed to separate the treated wastewater from the microorganism population and return most of the microorganisms back to the biological reactor.
After the treated wastewater leaves the secondary clarifiers the final treatment step, prior to discharging to Salt Creek, is disinfection. Some viruses and bacteria can pass through the treatment process and remain viable. To prevent these organisms from harming human beings, they must be destroyed. In the disinfection process the treated wastewater passes through two effluent channels with lamps that emit ultraviolet light. As the water is subjected to the disinfecting properties of the ultraviolet light, the harmful organisms are destroyed. Just a few seconds of intense ultraviolet light is effective in disinfecting the wastewater.
Ultraviolet Disinfection Channel
Organic solids collected from the primary clarifiers and waste solids from the secondary clarifiers are pumped to large tanks called anaerobic digesters where microorganisms break down the solids into a more stable form. The anaerobic digestion process is used at both recovery facilities to stabilize organic solids. One beneficial by-product of the digestion process is methane gas. At the Theresa Street facility this gas is used by two enginators as fuel to generate electrical power and provide heat for the digestion process. The enginators can produce as much as 900 kilowatts of power while maintaining a temperature of 98 degrees Fahrenheit within the three egg-shaped digesters. This benefits the environment by reducing the need for electricity produced from fossil fuels and utilizes the methane gas which would otherwise be wasted.
Following 18-22 days of digestion at the Theresa Street facility (25-30 at the Northeast facility), the treated solids are suitable for dewatering and eventual agricultural land application as a soil amendment and fertilizer. Dewatering is a process that removes excess water and reduces the total volume of biosolid waste that must be transported to the land application area. Digested solids from the anaerobic digesters are fed into three belt filter presses which squeeze out excess water between porous belts at the Theresa Street facility. Once the excess water is removed, the biosolids, a "cake", is loaded into large trucks which haul the dewatered solids to local farm ground in Lancaster County for land application. The Northeast facility does not perform dewatering.
Belt Filter Press Dewatering
Following digestion and dewatering the treated solids are suitable for agricultural land application as a soil amendment and fertilizer. From the Theresa Street facility the digested solids are hauled to local farmers who apply the material with their spreading equipment.
At the Northeast recovery facility the digested sludge, which is in liquid form, is injected beneath the ground surface on agricultural land owned and farmed by the City of Lincoln. Crops grown on this site, located near I-80 and North 70th Street, are sold and the revenue is used to offset the costs of water resource recovery.
Test plots and on-going crop research using the digested sludge (biosolids) has been helpful in demonstrating the beneficial uses of biosolids materials on cropland. The City of Lincoln is working with the University of Nebraska-Lincoln Extension in Lancaster County to promote the biosolids program to local farming interests.
Subsurface Biosolids Injection