TRAINING

THE DES MOINES METROPOLITAN WRA
WASTEWATER RECLAMATION FACILITY

Construction began in 1985, and was completed in 1993, at a cost of nearly $250 million. These facilities replace the former plant built as a WPA project at this same site. The new plant is designed to treat up to 190 million gallons of wastewater a day. The plant is approximately one mile in length and one-quarter mile in width at its widest point. Approximately 120 professional people operate and maintain the facilities twenty-four hours a day, monitor effluent quality, and manage a budget of $14 million staff it.

The operation of the plant is paid through both domestic and industrial user fees. (Our current 1991 cost of treatment is $1.03/1000 gallons. This is minus administrative and bond reduction cost.) Construction was financed through the cooperative efforts of Federal, State and local governments. The WRA consists of Des Moines, Ankeny, Johnston, Bondurant, Altoona, Urbandale, Clive, Windsor Heights, West Des Moines, Pleasant Hill, Polk and Warren Counties. The average American uses 1,280 gallons of water per day. The treatment plant can serve over 300 million people. Typically wastewater is 99.9% water. The entire treatment process is typically to treat pollution that is less than one tenth of one percent. Our goal, in treating this wastewater, is the protection of public health and the environment by meeting our federal and State discharge permits.

The Des Moines Metropolitan WRA Wastewater Reclamation Facility consists of preliminary and primary treatment, roughing filters, secondary treatment processes with nitrification, disinfection and anaerobic digestion.

Lift Stations

The water provided by the Des Moines Waterworks is used by residents, industries and businesses and is discharged into the sewer system. The treatment plant operates and maintains 26 sanitary lift stations located throughout the collection system. These lift stations pump the wastewater from a lower sewer to a higher sewer so that the flow arrives at the plant essentially by gravity. The lift stations are monitored by a modern sophisticated computer surveillance system. Each station is automatically contacted by radio and its operational status surveyed by radio each minute. In the advent of a failure, an alarm is sounded and recorded at the main control room. A team of highly trained and experienced professionals is then dispatched to the troubled station. Often repairs are made without any disruption of service. In addition to the automatic surveillance, each station is routinely visited by a crew to insure proper and dependable operation. The treatment plant regulates, through the pretreatment program, all of the major industrial effluents into the sewers to assure that toxic and improper wastewater, that could contaminate the river, residual solids or that could interfere with the biological treatment processes, are not discharged into the public sewers.

Due to infiltration and inflow into the collection system during wet weather periods, our effluent flow can reach in excess of 220 MGD. Treatment of this storm water can exceed $150,000 per day. To alleviate these flows, our crews operate and maintain five flow equalization facilities in addition to the lift stations. A flow equalization structure temporally stores excess flow in the collection system for later release. As an example, the SW Equalization Basin at 59th and Winona is designed to hold approximately 10 million gallons of storm water. After a storm and the high flows at the main plant have receded, this temporally stored water is released back into the collection system for treatment. The WRF operates 25 storm water stations. Many of these stations screen the storm water for floating debris prior to discharge directly to the river while others pump water over the levee when the levee gate wells are closed to prevent the river from backing up the storm sewer and into the city. In total, the WRF crews operate, maintain, and monitor 56 lift stations and 285 gates. In addition, WRF personnel operate the Des Moines’ Airport glycol station. This station treats the glycol used in de-icing operations.

The Distributive Control System

The distributive control system consists of 21 separate computers that controls and monitors the facility, thereby providing the precision necessary to maintain the complex processes at the treatment plant. The computer system is a multi-bus 286/386 micro processor system with three command stations and a historical collection system used to collect data and print reports. Seventeen distributive control systems (DCS) collect the data, the operator command stations (OCS) provide the man-machine interface from which the operator can monitor and control the process. The ICS (Information Control Station) provides for the collection of historical data and the production of reports. Computer strategies and automatic interfacing between the computer and field equipment allows the computer to automatically respond to changing conditions without operator input. However, the operator can override the computer and control the process through the keyboard by typing in the correct commands. And should the system fail, the operator can control the plant from the equipment control panels.

Flow Through the Plant

Two sewer lines enter the main pump house. From the east is a 60" sewer line and from the west is the 96" south-side interceptor. The west sewer line flows through a diversionary structure where the amount of flow coming into the plant can be controlled. Excessive flows can be bypassed to the Des Moines River at this point. However, it is important to note that the wastewater will be diluted with large amounts of storm water. The flow from the two sewer lines join and pass through four bar screens. Bar screens are similar to the bars you would imagine in a jail, but closer together. As the sewage passes through the bars, large items (i.e. rags, sticks, paper, etc.) are caught. These bars are mechanically cleaned and the debris is carried up to the first floor. The screenings can be diverted to either an augur or a dumpster. Lime can be applied to the screenings in the dumpster. This alkaline material kills bacteria and controls odors. This is called chemical stabilization. The augur transports the screenings to a washer. The screenings are then dried and can then be bagged and placed into a dumpster. The screenings are taken to the sanitary landfill for burial.

After screening, the influent wastewater enters a wet well. Here, six centrifugal pumps, each driven by a 700 horsepower motor, lift the sewage 54 feet into a channel and a parshall flume where the flow will be measured. Four of these pumps are variable speed, two are constant speed. Each is capable of pumping 38,192 gpm.

Next is another physical treatment process, grit removal. There are six aerated channels that slow the flow to one foot per second. Here, sand, gravel, and similar material drop to the bottom. Later, the overhead gantry crane will be used to remove the grit. The grit is stabilized in building 12, the residuals handling facility.

Now the flow divides and flows down the east and west primary influent channels and through the splitter boxes into the primary clarifiers. This is another physical treatment process in which floatables (grease and scum) and fine solids are removed. Each primary is 130 feet in diameter and approximately 12 feet deep. Each holds 1.19 million gallons. Generally, a retention time of 1 to 2 hours is maintained, allowing the scum to float to the surface where it is removed by a skimmer. The floatables are pumped to building 12 for further processing. The solids, that settle to the bottom, are removed by a bottom scrapper and pumped to the blended sludge wet well. Here, the primary sludge will be mixed with the thickened waste activated sludge and pumped to the anaerobic digesters.

The liquid portion, containing dissolved and suspended organics, overflow the weirs and into the 108" line leading to the roughing filter control chamber. This chamber will allow up to 97 MGD to the roughing filters. Any excess, up to 93 MGD will go directly to the aeration basins. Any flow in excess of 190 MGD will be bypassed directly to the river.

There are twelve roughing filters that are 152 feet in diameter and 7 feet deep. They are filled with rock that are 2 to 5 inches in diameter. On the surface of this rock live millions of microorganisms. These organisms use the dissolved, suspended and colloidal organics in the wastewater as food. This is an aerobic biological and secondary treatment process. As the wastewater splashes on the rock, oxygen is dissolved into the water. The microorganisms use this dissolved oxygen to live. As they grow and reproduce, the organics are removed. When the bacteria slough off the rock, they will eventually be removed in the final clarifiers.

The aeration basins are the structures for the activated sludge process. Activated sludge is also an aerobic biological and secondary treatment process. The microorganisms live on small particles of sludge instead of rock. Because these particles are so much smaller, billions more bacteria can be grown in a smaller space. But they now use oxygen at a much faster rate than can be dissolved naturally into the water. To insure that they have plenty of oxygen, four 2000 horsepower motors blow filtered air through fine diffusers in the bottom of the basins. There are six aeration basins, each with four passes. They are 300 feet long, 140 feet wide, and 20 feet deep. Each basin holds 6.48 million gallons. In the latter passes, nitrification occurs. That is, ammonia is converted to nitrates.

After the activated sludge process, the mixed liquor flows to the final clarifiers. There are twelve clarifiers, each 140 feet in diameter, and 14 feet deep at the sides. Each holds 1.66 million gallons. Here, the activated sludge settles very rapidly to the bottom and is removed. Most of the sludge will be returned to the beginning of the activated sludge system to be used again. Some of it will be wasted to one of three large centrifuges. Here some of the water will be removed. The thickened wasted activated sludge is mixed the primary sludge in the blended sludge wet well and pumped to the digesters.

The liquid flows to the disinfection process to kill most bacteria. The final effluent discharged to the river is now cleaner than the river itself with a BOD₅ less than 5 PPM, ammonia less than 5 PPM, and suspended solids less than 5 PPM. Also, not only as a cost saving measure, but as an important conservation measure, we reuse a portion of the final effluent for process and washing water.