Liquids Treatment Process
Headworks (Schematic #1, 2, 4)
Every day 50 to 60 million gallons of wastewater are conveyed into the Central Valley Water Reclamation Facility (CVWRF) through an extensive collection system of underground pipes. The influent wastewater enters the plant through the headworks which is at the beginning of the treatment system and includes the preliminary treatment processes. In the preliminary treatment phase, bar screens catch and remove large impurities such as rags, disposable wipes, sticks, rocks, plastic materials, and a multitude of other debris that could harm downstream equipment. This material, called screenings, is washed to remove organic material and then dewatered, compressed, and transported to a landfill for final disposal.
The CVWRF’s headworks can treat a total system capacity of 200 mgd with four bar screens, five pumps, and four aerated grit tanks. The system removes 3,000 to 5,000 pounds of dewatered grit and 3,000 to 5,000 pounds of dewatered screenings per day from the wastewater entering the CVWRF.
Large pumps lift the influent wastewater into the aerated grit tanks where inert materials such as fine grit, sand, gravel, eggs shells, and seeds are removed from the water. If grit is not removed, it accumulates in the anaerobic digester tanks, thereby reducing the effective treatment and storage volume for organic solids. The grit also accelerates wear on downstream mechanical equipment such as pumps.
Aeration within the aerated grit tanks causes the light organic solids to remain in suspension, while the denser inert matter, or grit, settles to the bottom of the tank and is removed by specially designed grit pumps with hardened wear resistant components. The pumps deliver the grit slurry to grit hydrocyclones/classifiers where residual organic material is washed from the grit and the grit is dewatered. The dewatered grit is combined with the dewatered screenings in a roll-off dumpster and transported to a landfill for final disposal.
Primary Clarification (Schematic #5)
Wastewater from the grit chambers flows to the primary clarifiers which are large circular tanks with conical bottoms and internal rotating scraper mechanisms. The purpose of the primary clarifiers is to separate solid particles in the wastewater mixture from the liquid. The quiescent flow conditions and long hydraulic retention time in the primary clarifiers allows the suspended organic solids to settle to the bottom by gravity. Light materials including fats, oils, and grease (FOG) float to the surface in the clarifiers. This material is referred to as scum. Since the differences in specific gravity or density of the solid materials and water are used to separate the solids from the liquids, primary treatment is classified as a physical treatment process.
The heavier settled solids referred to as primary sludge is scraped from the bottom of the primary clarifiers to a central sump where the solids are mechanically pumped to the anaerobic digesters for processing. Scum that floats to the surface is skimmed off and also pumped to the anaerobic digesters. Before reaching the digesters, both the solids and scum are pumped through sludge strainers. The strainers have small 4 millimeter perforations to remove residual trash that has passed through the 3/8-inch openings of the bar screens in the headworks. Like the screenings removed by the bar screens, this material is dewatered and sent to a landfill for disposal.
The CVWRF’s primary treatment system has 10 primary clarifiers that are each 110 feet in diameter and 12 feet deep . Each primary clarifier has its own sludge pump and separate scum pump. There are two sludge strainers that are connected to the piping from the 20 sludge and scum pumps. In 2016, the primary clarifiers removed approximately 60,000 pounds (dry weight) of primary solids per day from the wastewater.
After passing through the primary clarifiers, approximately 65 percent of the settleable solids and 40 percent of the BOD has been removed and the water continues on to the secondary treatment process for further treatment.
Secondary treatment is not just a second step of primary treatment. It differs significantly in the type of treatment provided. Whereas primary treatment consists of the separation of solids and liquids through a physical process driven by gravity, secondary treatment consists of a biological process to degrade the remaining organic materials in the wastewater. The CVWRF’s secondary treatment process is called the trickling filter/solids contact process (TF/SC). This process consists of three main treatment components: 1) trickling filters; 2) solids contact aeration basins; and 3) secondary clarifiers.
Trickling Filters (Schematic #6)
Wastewater from the primary clarifiers is pumped to the top of trickling filters. The trickling filters are large tanks filled with a porous plastic media. The wastewater is distributed over the top of the plastic media by a rotating distributor and flows down and over the surface of the sheets of plastic media.
A natural growth of bacteria, protozoa, and fungi called biofilm grows on the surface of the media. As the wastewater flows over the biofilm layer, the microorganisms in the biofilm absorb the pollutants as nutrients and the biofilm grows thicker. Eventually, the biofilm grows too thick to allow oxygen to diffuse into the lower layers and the biofilm sloughs off. The sloughed biofilm is washed from the trickling filter and flows with the wastewater to the downstream solids contact aeration basins.
The CVWRF has six large trickling filters; each is 170 feet in diameter with 14 feet of media. The total volume of media in all six filters is approximately 1.91 million cubic feet. With each cubic foot of media having 30 square feet of surface area, the total amount of surface area available for treatment is approximately 57.20 million square feet. There are seven trickling filter pumps that pump water to the top of the filters. Each 300-horsepower pump has a flow capacity of 25 mgd.
Solids Contact Aeration Basins (Schematic #7)
Wastewater from the trickling filters flows by gravity to the solids contact aeration basins, also known as aeration tanks. The solids contact basins are large and deep channel tanks that contain thousands of porous air diffusers that are mounted on the tank floor. A continuous air supply is delivered through the diffusers and the fine bubbles rise through the liquid, transferring oxygen to the water.
The combination of sloughed biofilm from the trickling filters, residual organic pollutants, dissolved oxygen, and gentle mixing provided by the aeration encourages the growth of additional microorganisms which convert the remaining pollutants into bacterial solids and carbon dioxide. The microorganisms that grow in the solids contact tanks excrete a biopolymer on their cell wall that helps congeal the solids together into large colonies. The microorganisms that grow in the solids contact tanks are referred to as mixed liquor suspended solids (MLSS) and the large suspended colonies are called floc. Since the floc are large particles, they can be readily removed by settling in the downstream secondary clarifiers.
Secondary Clarification (Schematic #8)
Both wastewater and mixed liquor suspended solids (MLSS) flow to the secondary clarifiers. The secondary clarifiers are similar to the primary clarifiers in that they are large circular settling tanks that provide quiescent conditions and hydraulic residence time to allow the floc particles to settle. However, the secondary clarifiers have several important differences. The secondary clarifiers have large central baffles to provide a flocculation zone where the delicate flocs are allowed to interact, attach to each other and grow larger for better settling. The solids removal mechanism vacuums the settled solids from the tank floor instead of scraping the solids to a central sump.
Most of the solids removed from the bottom of the secondary clarifiers are pumped back to the influent end of the solids contact tanks to provide the MLSS culture necessary for treating the incoming wastewater in the solids contact process. These solids are called return activated sludge (RAS). The remainder of the solids that are settled in the clarifiers are wasted from the system and are called waste activated sludge (WAS). The WAS is pumped to the solids thickening process and then to the anaerobic digesters. By now the water is over 95-percent free of all pollutants and is sent to a disinfection process.
The CVWRF’s secondary treatment system has 10 clarifiers that are each 125 feet in diameter; two of them are 15 feet deep and eight are 18 feet deep. Each clarifier has a capacity of 20 mgd and has its own return sludge pump. Each 30-horsepower return sludge pump has a capacity of 2,500 gpm.
Disinfection (Schematic #9)
After the biological treatment in the trickling filters, solids contact tanks, and secondary clarification, the water enters an ultraviolet light disinfection process. This process consists of flowing through four 10-foot-wide by 7-foot-deep channels each containing eight modules of 36 ultraviolet light bulbs (288 bulbs/channel) housed in vertically oriented clear quartz glass sleeves. As the water flows through the channels and in and around the bulbs, the intense UV light damages DNA and deactivates harmful bacteria, protozoa, and viruses.
Post Aeration (Schematic #10)
Following disinfection, the water flows through two long channels where air is bubbled through to increase the dissolved oxygen in the water prior to discharge. The additional dissolved oxygen assists the receiving stream to rapidly assimilate the treated flow and ensures aquatic organisms have plenty of oxygen to thrive. At this point, the liquid treatment process is complete and the water reenters the environment. At the discharge point, the flow is measured and recorded and a composite sampler collects samples 24 hours per day for laboratory analysis to ensure compliance with the CVWRF’s UPDES permit limits.
Recycled/Reuse Water (Schematic #17)
Effluent from the treatment plant can be reused for irrigation of grass and landscaping to supplement existing water supplies if it receives additional treatment. Sand filters or membrane micro-filtration plus additional disinfection are common methods used to prepare the water for recycling. Once treated using these methods and tested to meet Utah Division of Water Quality Standards, the water is designated Type I reuse water.
The CVWRF produces approximately 700 gallons per minute (1 mgd) of Type I reuse water during the summer months. This water is used to fill decorative ponds and water the Central Valley Golf Course and Golf-In-The-Round Driving Range. Some of the disinfected effluent water is also reused within the CVWRF for plant water needs. The plant water is strained through automatic backwashing strainers and the irrigation water is filtered through an upflow sand filter. The CVWRF’s reuse treatment process consists of the following three steps:
- Chlorine disinfection using sodium hypochlorite (bleach),
- Three upflow continuous backwashing deep bed sand filters,
- Ultraviolet Light Disinfection.