The Cleaning Process
A state-of-the-art facility, Central Valley treats the waste using a biological process called the trickling filter/solids contact process. It employs primary sedimentation tanks, trickling filters, aeration tanks, secondary sedimentation tanks and, finally, a disinfection tank to meet the safe water standards for discharge into the Jordan River. Every day, millions of gallons of wastewater are collected and piped into the facility. Every day, those same millions of gallons of water are processed, separated from impurities, and treated so that only clean water is returned to the Jordan River system.
Organic material is the major pollutant of wastewater. Beneficial bacteria is grown and nurtured to consume most of this organic material. This bacteria requires oxygen to live and to accomplish the task of destroying the pollutants. The more pollutants, the more oxygen is required to sustain the bacterial organisms needed to perform the task. Biochemical Oxygen Demand (BOD) is a measure of the oxygen required to destroy the organic pollution. Before the water is released back into the environment, it must comply with strict discharge standards. The organic material must be removed and the BOD in the water must be reduced to a level safe enough to sustain the aquatic life of the receiving system.
Central Valley also treats the waste materials removed from the water using a digester which reduces the solids by converting them to water, methane gas and a residual we call bio-solids. The water is pressed out of the bio-solids and retuend to the liquid portion of the plant for treatment. The bio-solids are taken and applied to the ground for beneficial agricultural use. The methane gas is used to fuel five engine generators to power the plant.
It is a clean, safe process requiring minimal maintenance and energy to operate, delivering benefits to the ratepayer. Additionally, Central Valley is on of the few Utah facilities to use a digester gas/natural gas mixture to generate 70% of its electricity needs. Waste heat from the engines is also used to heat and air condition the plant.
Central Valley's wastewater treatment process is a natural process. Central Valley's process speeds up the natural purification process by using extensive and complex equipment and technology from many disciplines such as hydrology, biology, chemistry, physics, and all types of engineering.
Sounds complicated? While it might appear to be so, Central Valley can literally turn bad water into clean water within six hours. You could break down the process into 13 steps.
1. Preliminary Treatment
Every day millions of gallons of wastewater are conveyed into the Central Valley Water Reclamation Facility through an extensive collection system of underground pipes. In the preliminary phase, bar screens catch and remove large impurities such as rocks, sticks, rags, plastic cups, and other debris. This trapped material is eventually transported to landfills. Large pumps then lift the incoming sewage flow into the aerated grit chambers where inert materials such as sand, smaller rocks, eggs shells, coffee grounds, and seeds are removed from the water. While organic material remains in suspension, the inert matter, or grit, settles to the bottom of the tank and is removed by pumps.
2. Primary Sedimentation
Wastewater from the grit chambers flows to the sedimentation tanks, where heavier solids are given time to settle to the bottom. Referred to as sludge, these solids are mechanically pumped to sludge digesters for processing. Oils and fats found floating on the surface are skimmed off and pumped to the digesters. By now, almost 60% of the settleable matter has been removed and the water continues on for further treatment.
3. Trickling Filtration
Water from the primary sedimentation tank is pumped to the top of tanks called trickling filters. Water flows down and over thin sheets of plastic media covered with a natural growth of bacteria, protozoa and fungi. This biological growth absorbs the pollutants as nutrients or food as the water passes over the surface. The remaining organic solids are dealt with in the following process.
4. Solids Contact
5. Secondary Sedimentation
Both wastewater and solids flow to the secondary sedimentation tanks. Given time, the solids form, settle to the bottom, and are removed back to the aeration basins, with some wasted to the digesters. By now the water is 95% free of all pollutants.
7. Post Aeration
Disinfected effluent passes to the post aeration tanks where dissolved oxygen is diffused into the flow prior to discharge to Mill Creek. The additional dissolved oxygen assists the receiving stream to rapidly assimilate the treated waste flow.
8. Recycled Wastewater
With the growth in the Salt Lake Valley, water is becoming more scarce. Droughts have shown that new sources of water are needed along with conservation. Water from the plant can receive additional treatment which would make treated water available for safe use on lawns and other outside spray irrigation purposes. Sand filters and micro-filtration are methods used to prepare the water for recycling. Central Valley uses three sand filters to treat the water for use on a golf
9. Sludge Processing
Sludge from the primary sedimentation tanks and thickened sludge from the secondary sedimentation tanks are mixed together. The mixture is strain-pressed to remove inert solids with the remaining bio-solids pumped to the anearobic digesters.
10. Anaerobic Solids Digestion
In the anaerobic (airless) digesters, bacteria reduce the volatile solids making them into a more stabilized material to be used for soil amendments. Other products of the digestive process are water, which is returned to the head of the plant for reprocessing, and methane gas, which is used for electrical energy production and heat recovery.
11. Energy Recovery
Central Valley recovers the methane gas, a low heat value gas, in the anaerobic solids digestion process and mixes it with natural gas. Enginators, using this gas mixture, produce approximately 70% of the electrical power needs of the entire plant. Waste heat from the enginators is used for cooling and heating the plant buildings.
12. Solids Dewatering
Sludge leaving the digester process is 97% water. Organic polymers are added to the sludge to help separate the water from the solid material. Belt filter presses exert high pressure on that material and force the release of water, creating a cake-like substance.
13. Sludge Disposal
Now light weight and compact from the pressure of the belt filter presses, the sludge is trucked out to farm lands for agricultural land application. A portion is composted and made available to residents to use on their lawns and gardens.