Two mechanically cleaned bar screens are used to remove plastics, rags, debris, and other materials that could damage downstream equipment. New bar screens were installed in 1999 to replace outdated and worn equipment. In the bar screens the wastewater stream passes through openings that are only 5mm X 20 mm, which are effective at capturing materials as small as cigarette butts. Materials removed by the bar screens are deposited on a conveyor, which transport the screenings to a dumpster. Screenings are hauled to a landfill for disposal.

Grit and other heavy particles that can cause wear, plugging, and other problems in downstream equipment are removed in two aerated grit chambers. Air is used in the grit chambers to keep sludge and other organics from settling. The heavier grit settles to the bottom of the grit chambers, where it is pumped to a cyclone separator. The cyclone separates the grit from the wastewater, and the grit is washed to remove organic material and discharged into a dumpster. Grit is also hauled to a landfill for disposal.

Four primary clarifiers, or settling tanks, are used to remove sludges, floating material, and scum. Primary clarification typically removes 30% of the organic wastes and up to 70% of the settled solids. The primary clarifiers also remove the majority of the oil, grease and other floating materials. The settled solids and the floating scum are both pumped to anaerobic digesters for further treatment.

The equalization basins are used to store peak wastewater flows for later treatment. Typical daily flows range from about 2,400 gallons per minute at night to over 8,500 gallons per minute in the afternoon. The equalization basins absorb the instantaneous peak flows that are common during the afternoon and evening hours and allow the secondary and tertiary processes to treat nearly constant flows. The equalization basins also absorb the peak flows that sometimes occur during rainstorms. This results in substantial process equipment savings, because the equalization basins eliminate the need to oversize downstream units to handle peak flows. In addition, the secondary treatment process, which depends on the activity of biological organisms, behaves in a much more predictable manner with more constant flows.

Over 90% of the treatment process takes place in the aeration basins. The size and capacity of the aeration system was doubled in 1999 to provide reliable year-round complete nitrification of the waste stream. Eight aerated basins are used to grow bacteria that in turn digest waste materials and remove contaminants from the wastewater stream. The treatment process is called “activated sludge”. The aeration basins contain a highly specialized ecosystem, nurtured and maintained just for wastewater treatment. Large centrifugal blowers supply dissolved oxygen to the bacteria and provide mixing. The staff controls the process by monitoring and controlling dissolved oxygen, alkalinity, and the size of the biological population. Usually the activated sludge organisms are retained in the system for 8-15 days before they are removed in a process called “wasting”. The time the organisms spend in the aeration system depends on the time of year and the temperature of the water. VVWRA uses a process referred to as plug flow extended aeration with anoxic selection and alkalinity recovery, although the plant is designed so the Operators can also use step-feed, contact stabilization, or conventional aeration treatment.

Six secondary clarifiers, or settling tanks, are used to separate the activated sludge from the water. After settling, the biological microorganisms and solids are removed, and most of the material is recycled to the aeration basins. A portion of the settled material is pumped, or “wasted”, to the dissolved air flotation thickeners and then to the anaerobic digesters. Sludge “wasting” is used to control the size and the age of the microorganism population. On a typical day over 100,000 gallons of activated sludge are removed, or wasted, from the system. The clarified water that is left over can be discharged directly to percolation ponds, or it can flow to the tertiary filtration and disinfection system for further treatment.

The water that flows from the secondary clarifiers to the tertiary filtration and disinfection system is called secondary effluent. Prior to filtration, secondary effluent is typically treated with a chemical called alum that causes any remaining solids to stick together. Polymer can also be added to assist in the coagulation of the remaining solids. The secondary effluent with alum and polymer is then flocculated, which is a process in which the water is slowly stirred. The flocculated secondary effluent is then ready for tertiary or final filtration in either the traveling bridge filters or in the moving bed filters.

After going through the secondary clarifiers, the treated water is run through Aqua Diamond filters to remove any microscopic particles that remain. The Aqua Diamond combines two proven technologies: traveling bridge and media cloth filters. Our previous traveling bridge filter facility was retrofitted to accommodate theAqua Diamond. Our sand filters were decommissioned.

Secondary effluent can be discharged directly to any one of six percolation ponds, which have a combined surface area of about 13 acres. Discharging to the percolation ponds saves chemical and energy costs when compared to a river discharge. The requirements for discharge to the percolation ponds are less stringent than the requirements for direct river discharge. The percolation ponds are also used during times when conditions are such that the plant cannot achieve the treatment quality required for direct river discharge. The percolation ponds can either be filled individually or in series.

Ultra violet light is used to disinfect the treated water. VVWRA previously used chlorine in this process, but UV light has proven to be just as effective and more environmentally friendly. The disinfected effluent is either reclaimed for irrigation or other beneficial purposes, or it is discharged directly to the Mojave River. The effluent quality requirements for reclamation and direct river discharge are essentially the same.

Much of the the treated and disinfected reclaimed water is released into the Mojave River where it is allowed to sink into large underground aquifers. The treated effluent can also be pumped to other users and other sites for irrigation, industrial process water, and similar beneficial uses.The VVWRA treatment plant uses fully treated reclaimed water for fire protection, wash down hoses, pump seal water, process water, and foam control sprays. Using reclaimed water in the treatment plant reduces our need to pump potable groundwater from wells for such purposes.

Two dissolved air flotation thickeners are used to thicken primary clarifier and waste activated sludges. The sludges are thickened by removing excess water. Water saturated with air under pressure is mixed with the primary and waste activated sludges. When the sludge enters the thickener tank, the pressure is released and the air attaches to the sludge in the form of small bubbles. The small bubbles cause the sludge to float to the surface of the thickener tank. The thickened sludge is skimmed off and pumped to the anaerobic digesters for further treatment. The water that is removed from the sludge is returned back to the beginning of the plant for full treatment.

Three anaerobic digesters are used to reduce the concentration of organic wastes in the thickened sludge. Sludge in each digester is heated to 95 degrees Fahrenheit, and mixed using large pumps. Bacteria that do not need oxygen consume the wastes and in turn produce methane gas. The methane gas is used to fuel engine driven blowers that provide air for the aeration basins. Heat generated by the blower engines is captured and used to heat the sludge in the digesters to the desired temperature. The sludge stays in the digesters for 15-20 days or more. After treatment in the digesters, the sludge is dried on solar drying beds and used for agricultural fertilizer. The sludge can also be composted with green wastes such as lawn clippings and tree limbs and used as a potting soil, a soil amendment, or as a fertilizer.

The VVWRA Treatment Facility utilizes a state-of-the-art system of computers and programmable logic controllers to control pump speeds, chemical addition rates, aeration dissolved oxygen, etc. The system is called “supervisory control and data acquisition”, or SCADA. Plant Operators enter setpoints into the computer and use the SCADA system to stop and start various processes. In addition, the SCADA system automatically calls Operators during times when the Facility is not manned to report problems. The SCADA system generates much of the data that must be reported to regulatory officials. The SCADA system saves the Facility operating costs by maintaining better control over the process, even when the plant is not manned.