REDUCE - RECYCLE - REUSE
The City of Davenport Water Pollution Control Plant is one of the largest Wastewater Treatment plants in the State of Iowa. As a Grade IV activated sludge plant, it consists of preliminary, primary and secondary treatment of wastewater with digestion and composting of the biosolids that are produced. It was opened in January of 1977, at an original cost of $18,000,000, and, has an annual budget of $3.2 million dollars with a service rate of $1.48/100 cubic ft. (750 gallons) of water. It serves the cities of Davenport, Bettendorf, Panorama Park and Riverdale, Iowa.
REDUCE - Reducing massive quantities of sewage into usable by-products
GENERAL:
Raw sewage, or wastewater, is delivered to the Water Pollution Control Plant by gravity, through a network of hundreds of miles of sewer pipe. The incoming wastewater, which is usually gray in color, looks like dirty dish water. It is approximately 99.5% water and 0.5% pollutants.
The Federal Clean Water Act gives authority to the Iowa Department of Natural Resources to regulate wastewater treatment discharge into natural waterways. The IDNR in turn issues a National Pollutant Discharge Elimination Permit (NPDES) for each treatment plant. This permit sets measurable parameters and compliance limitations on the amounts of pollutants that a treatment plant can discharge. The permit issued to the City of Davenport allows this plant to discharge treated wastewater to the Mississippi River. It states that there must be at least an 85% removal of the Total Suspended Solids (TSS) and Carbonaceous Biochemical Oxygen Demand (CBOD) that is contained in the plant influent.
The goal of this plant is to see that our effluent meets the permit requirements. Being a continuous operation plant, flow and treatment are monitored 24 hours per day, seven days per week.
The average wastewater flow through the plant in 1998 was 29.2 million gallons per day. (During heavy rains or flooding situations the flow may reach 60 MGD.) The incoming wastewater contained an average of 181.9 mg/l of CBOD and 167.2 mg/l of TSS. The outgoing effluent averaged 11.6 mg/l of CBOD and 11.5 mg/l of TSS. During this time period the removal rate for CBOD and TSS was above 93%.
PRELIMINARY TREATMENT: SCREENING
The cities' network of sewers terminates in a 96-inch diameter pipe that is big enough to drive a small car through. Forty-five feet below ground level, the pipe delivers wastewater to the Wet Well, which is the start of this plant. The initial treatment, the removal of solids, begins here. A bar screen traps the larger solids that flow in with the sewage (plastics, sticks, cans, etc.). Giant rakes clean the bar screen and the solids are deposited in hoppers for removal by truck to the landfill.
The wastewater is then pumped up to ground level by huge centrifugal pumps using 300 horsepower motors. It flows through smaller traveling bar screens and a Parshall flume where flow is measured, then on to the grit removal system. Here, baffles slow down the flow so that gravel, sand and other heavy particles can settle out and be removed.
PRIMARY TREATMENT: SETTLING/ FLOATING
After the grit tanks, the wastewater enters one of the 4 round primary clarifier tanks. Each of these tanks holds approximately 1.3 million gallons. Depending on the rate of flow and the operating conditions, the wastewater will stay in these tanks for 2 - 6 hours. This allows heavier solids to settle and be removed from the cone-shaped bottom of the tank. It also allows grease and other floatable materials to rise to the top of the tank where they can be skimmed off. About 65% of the organic solids are removed here.
SECONDARY TREATMENT: ACTIVATED SLUDGE
From the primary tanks, the flow moves to the activated sludge area. Activated sludge is a biological treatment process. In eight large rectangular tanks, which hold 750,000 gallons each, the nutrient-rich wastewater is mixed with air to provide oxygen for a previously developed, very dense population of microorganisms. Activated sludge is made up primarily of bacteria, fungi, and protozoa. In this oxygen- and nutrient-rich environment, the microorganisms carry on life functions at an accelerated rate. In approximately 7 hours, they consume the pollutants, reproduce, and generate a waste byproduct of their own.
The wastewater is then sent to four secondary clarifiers. These tanks are similar to the primary clarifiers. It takes about 5.5 hours for the flow to make its way through the secondary clarifiers. Movement is again slowed and the activated sludge is allowed to settle and is then returned to the aeration tanks. (A relatively small amount of this settled activated sludge is "wasted" to a thickener tank and then to 1 of 4 anaerobic digesters. A skimmer also removes any floating material from the tanks. The treated effluent water, which is now clear, flows through a tunnel to the Mississippi River. Many wastewater treatment plants are required to chlorinate their effluent, but because of the high quality of our effluent and the size of our receiving stream, we are not required to do so.
RECYCLE - By-products are changed to more usable products
DIGESTION:
The settled material that is removed from the primary tanks is further treated in anaerobic digesters. "Waste" activated sludge also is sent to these digesters. Through the digestion process there is a reduction in odors and pathogenic (harmful) bacteria levels, as well as a reduction of the overall sludge volume. During digestion the sludge is heated and mixed. Complex organic materials break down into simpler compounds, then acid-forming organisms convert these organic compounds into volatile organic acids. These acids are changed into methane and carbon dioxide by other bacteria. The production of these substances brings about a decrease in the amount of solids that need further processing. Depending on the volatility of the solids, this solids reduction can range from 30-40%. Biosolids leave the digester as a dark black liquid containing about 2% solids and 98% water.
FILTERING:
After digestion, the biosolids are mixed with a polymer, then put through belt filters for dewatering. Polymer makes it easier to separate the solids and water. The biosolids are spread in a thin layer on the belt filter where the water is pressed out and can drain away. Once through the filters, most of the water has been extracted, leaving biosolids the consistency of soft mud. This drier by-product (about 25 tons per day) is then trucked to the Compost Facility.
COMPOSTING:
Composting is an aerobic biological process designed to reduce organic materials, reclaim nutrients, and eliminate pathogenic organisms. The aerated static pile method of composting is used here in Davenport.
At the compost facility, digested biosolids are mixed with a bulking agent. Wood chips, tire chips, and yardwaste are used for this purpose. Each of these are byproducts of other recycling efforts. The wood and tire chips are bought from other recyclers and the yardwaste comes from grinding up all the leaves, tree trimmings and used Christmas trees that are brought here for disposal during the year. Adding a bulking agent allows more air to circulate through the mixture. Oxygen in the air promotes aerobic bacteria to finish the job of stabilizing the biosolids.
The mixture is put into long piles in the compost barn. Large fans draw air through these which helps the bacteria finish the job of decomposing the organic matter. The compost piles heat up because of all the biological activity. A temperature of about 55 degrees C. is maintained for at least three days. This further reduces the pathogens, helps with odor control and reduces the volatile organic compounds.
To maintain control of the odors that are produced with the compost process, the air that is circulated in the compost barn and drawn through the piles is put through large underground biofilters. These filters naturally scrub the air, eliminating odors.
The completed compost piles are then screened to remove the bulking agents. These bulking agents can be used over again. The finished compost is a relatively dry, biologically stable, odor-free material that can be used as a soil conditioner and fertilizer.
REUSE - Using the products that are produced
CLEAN WATER:
The water that is here on Earth now is all we will ever have. Since time began, water has been recycled. Most of the time nature herself has done the recycling, but with the growing population and so many pollutants, nature just can not keep up. People have come to realize that we all live downstream. Water that we use today has been used by many people before us. We must clean the water we use because someone else will need to use it again. The treated water from this plant is returned to the Mississippi River. The Mississippi is used for fishing and recreation as well as a source of drinking water for the cities that border it.
METHANE:
This plant produces about 354,000 cubic feet per day of methane gas.
This gas contains approximately 213 million BTU's of energy, which is used to power two Caterpillar engines. These engines can provide enough electricity to run various parts of the plant and control the temperature in the main building. The amount of energy produced by the methane considerably reduces electricity costs and provides a means of keeping the plant running in the event of a major power outage.
COMPOST:
The compost that is produced from the wastewater sludge is sold to anyone needing a good quality soil conditioner. The composted biosolids can be used in more ways than digested biosolids. It can be used on farm fields, vegetable or flower gardens, golf courses or any number of other places that need a soil conditioner with good nutrient capabilities.
EMPLOYEES - the people who work here make it all happen
If a wastewater treatment plant discharges too many pollutants, the results can be harmful to the receiving stream. It takes a concentrated effort from all employees to make sure that we do not exceed our permit limits.
PRETREATMENT:
The City of Davenport has a Wastewater Ordinance that states what can and cannot be put into the sewer. The Pretreatment Coordinator works closely with local industries and private wastehaulers to ensure that they do not put anything in the sewer that could harm the delicate balance of microorganisms that are necessary for proper wastewater treatment. Non-compliance with the Ordinance can result in fines, or in severe cases, being shut off from using the sewer until they are in compliance. By negotiating agreements, inspecting industries and monitoring the results of testing, the Pretreatment Coordinator safeguards the flow coming into the plant.
OPERATIONS:
The State of Iowa has four certification levels for wastewater treatment plant operators. Tests are given for each grade level and an appropriate number of continuing education units must be earned during every renewal period to maintain certification. Since this is a grade IV plant, the responsible person in charge must have a grade IV certification. The lead person on each shift must have a grade 11 certification and all other operators are required to obtain grade I certification. Many of our operators maintain a higher certification than is required in their jobs.
Variations occur in the amount as well as the composition of the wastewater flow. Throughout the plant, the wastewater treatment is constantly monitored. Operators keep equipment running, take readings, and make process control adjustments based on their findings. They also take samples for immediate testing and send others to the lab. Solids from the bar screens and grit removal systems have to be hauled to the landfill and residue from the belt filter needs to be trucked to the Compost Facility. Operators are also in charge of all general housekeeping duties as well as routine preventative maintenance on much of the equipment.
MAINTENANCE:
The maintenance department is in charge of keeping everything running. With the large amount of equipment that is necessary for the operation of the plant, the maintenance personnel have to deal with all kinds of mechanical, electrical, design and operational problems. Because this is a continual operation plant, many of the processes are run in duplicate so that if equipment needs to be shut down for repair, flows can be temporarily diverted.
COMPOST:
All the yard waste (leaves, sticks, brush, Christmas trees, etc.) that is brought to the Compost Facility is run through large grinders to create a more uniform bulking agent. The different bulking agents are mixed with the biosolids. Then the operators use heavy equipment to form the compost windrows. The operators monitor the moisture and temperature as the compost cures. When it is finished, they dismantle the windrows and screen out the bulking agents so they can be used again. Much of the compost is then trucked to where it will be used.
LABORATORY:
The laboratory runs about 50,000 tests a year. Many tests are run as indicators of how well the treatment process is working. Plant equipment and flow adjustments are made according to the results of these tests. Other tests are run because they are required by law. Our NPDES permit lists which tests are to be run, the sample location, and the frequency of testing.
About twenty industries that are required to comply with the City Wastewater Ordinance are sampled and tested each month. The frequency of testing depends on the types of processes they use and the amount of wastewater flow they discharge. A calculation, using the results of these tests combined with the water company records of the amount of water they used, determines their monthly sewer bill.
Blackhawk Creek, Goose Creek and Duck Creek are monitored at least three times each year. Several times, sewer line breaks were detected. This also builds a database for diverting problems during flooding situations.
The Mississippi River is sampled once each month from May until October. Nineteen points are sampled across the river above and below where the plant discharges its effluent. Tests are run to determine what, if any, impact the plant effluent has on the river. Results indicate trends that are associated with high and low river stages and temperature variations.
Tests are also run on samples from other treatment plants that are not equipped to run them, other City departments that need periodic testing, and on occasion, for students who are running experiments on water quality.
OVERVIEW:
All aspects of the Water Pollution Control Plant work together to produce the best quality effluent possible. The by-product of the treatment is further processed into compost and methane. These are not only produced in an environmentally sound manner, but are also helping to defray the cost of plant operation.
The Water Pollution Control Plant is a division of the Public Works Department of the City of Davenport.