Welcome to the virtual tour of Springfield’s Wastewater Treatment Plant. In addition to this site, the plant proudly gives tours to small groups of students, businesses, organizations, and individuals from around the city. If you would like to schedule a tour of the facility or have questions about the processes on this page, please call the plant at (937) 324-7626.
- 1. Raw Sludge Pumping
- 2. Primary Anaerobic Digesters
- 3. Secondary Digester
- 4. Belt Filter Press
- 5. Biosolids Disposal
Sludge is removed from the primary clarifiers and is pumped to one of three primary anaerobic digesters. Primary clarifier sludge is pumped a minimum of 4 minutes to a maximum of 15 minutes each hour and the start time for each primary clarifier is offset by 20 minutes. There are three raw sludge pumps that are controlled by the SCADA system. The SCADA uses one pump at a time to pump sludge from each one of the primary clarifiers. The pumps are of progressing cavity design and pump at a fixed rate of 140 gpm. Each pump has an in-line grinder before the pump.
Raw sludge is pumped to a primary digester and heated to body temperature (97°F to 99°F). At this temperature bacteria breaks down the organic matter to water and gases. Not all of the organic waste can be broken down, what is left is process more at the Secondary Digester and Belt Filter Press.
The primary digesters are circular with fixed covers and measure 60 feet in diameter and 20 feet in depth. The volume of each primary digester is 422,770 gallons and a total volume for all three primary digesters is 1.27 million gallons. The design detention time for the primary digesters is 18.75 days. The primary digesters are mixed using three individual draft tube mixers. Two of the mixers run in forward and the third mixer runs in reverse. The order is changed every 8 hours. This helps to keep any scum blanket from forming on top of the digester. The sludge is displaced by raw sludge addition and is transferred to the secondary digester through a series of overflows. The primary clarifier sludge gets pumped into digester #1 from 8:00 a.m. to 4:00 p.m., digester #2 from 4:00 p.m. to 12:00 a.m. and digester #3 from 12:00 a.m. to 8:00 a.m. The design loading is 0.21 lbs/ft3/day of dry solids and loading for January 2007 to October 2008 was 0.17 lbs/ft3/day.
Each digester has one heat exchanger rated at 500,000 BTU/hr and one centrifugal sludge-circulating pump pumping at 300 gpm.
Two gas-fired boilers with an output of 2,000,000 BTU/hr provide heat through a series of three-way valves. The boilers are dual fuel units; they normally run on sludge gas but can be changed over to natural gas if the volume of sludge gas drops too low. Sludge gas is produced as part of the sludge digestion. The three primary digesters provide the production of the gas supply. Digester #4 can produce some, but is mainly for storage, and therefore digester #4 is not heated or mixed. The sludge gas is mainly made up of methane and carbon dioxide. The digesters make an average 126,317 cu/ft/day of gas and use 97% of it to run the boilers. The gas system is made up of six-inch pipe connecting all of the four digesters together. The secondary digester has a floating gasholder that provides a means for maintaining even gas pressure as production and consumption of the sludge gas varies. Each primary digester has a pressure/vacuum relief and flame arrester assembly with gate valve at the center of the fixed cover. The pressure and the vacuum relief covers rest on a seat, and have removable weights to adjust settings. The normal range of pressure for the pressure relief is 11 to 13 inches of water column. The total heat exchange capacity for the sludge heating system is 36 million BTU/day with the energy required to raise the digester contents one degree F/day being 10.8 million BTU/day. The digesters are kept at a temperature of 98 degrees Fahrenheit. This is done by continuously pumping the sludge though the heat exchanger with the 3-way valve set to let enough hot boiler water to the heat exchanger to keep the sludge-out temperature at 100 degrees. The three primary digesters are monitored for percent solids, volatile solids, VA/alkalinities and pH on a weekly basis.
The raw sludge volatile solids are an average of 75%, this is reduced to 51% in the primary digesters. The percent of sludge volume reduction from raw to land applied was 67.5% in 2007.
From the primary digesters, the sludge overflows into a circular secondary digester. This is mainly a settling tank that thickens the sludge, removes water and holds biogas to be use later.
This digester has a floating gasholder and is 65-foot in diameter and is 20 feet deep. Normal operation maintains a constant liquid level with supernatant returned to the plant influent. The total volume of the secondary digester is 668,000 gallons (2070 gallons/inch) and the volume of the gasholder is 16,580 cu ft.
The supernatant is manually removed three days a week to coordinate the added plant loading to low loading times. The supernatant is removed by manually opening a valve that drops the liquid level 30 inches, around 62,100 gallons. This is done over a five-hour period of time with a flow rate of 207 gpm.
By slowly mixing with plant influent during low loading times, the high strength supernatant is diluted to a point that it has no impact upon overall plant operations. If the supernatant flow rate is too high the high loading could upset the microbiology of the activated sludge system.
The Belt Filter Press takes sludge from the secondary digester and presses the water out of it. The sludge goes from a 4% solid to a 20% solid. That is done to reduce the volume of the sludge. That in turn reduces the cost of disposal.
The press system works best with a warm sludge, so the sludge is pumped from the secondary digester.
There are three Moyno progressive cavity pumps that can either mix the holding tank sludge or deliver the sludge to the Belt Filter Press. The pumps have VFDs to control the flow rate to the press. A polymer is added to the sludge to promote the separation of liquid from the sludge. This is injected into sludge before reaching the press. We have two Rodiger 2.2 meter press. One press is run five days a week for six hours a day. On average the influent to the press is 3.5% solid and the press produces a 20-25% solid sludge cake.
Polymer is added to the press influent just before the press to help separate water from the sludge. The press has an average sludge feed rate of 125 gpm and a polymer feed rate of 2.25 gph. The sludge cake is pumped, using a progressive cavity pump, to a sludge storage building for later disposal by a contractor. The sludge storage building has space for about six weeks worth of sludge cake.
The Springfield Wastewater Treatment Plant produces an average of about 4700 wet tons of biosolids each year. After the biosolids have been dewatered, they are stored until disposal. Storage is done at the Wastewater plant in a covered 2-cell building parallel to the press building. When weather allows, the biosolids are transported to farm fields throughout Clark and surrounding counties for application. A contracted company is paid (by the wet ton) to remove biosolids from the plant and apply them to farm fields where they act as a nutrient supplement. As the trucks leave the plant, they are weighed by truck scale to ensure proper billing occurs.
At the site of application, a tractor and spreader are loaded and apply the biosolids at the proper rate to ensure even distribution to the land. This process is much like you would see a farmer applying manure to his field or pasture. Application rates ensure that nutrients and other constituents are not overloaded to any portion of the field.
Several constituents are monitored. Cadmium, Chromium, Copper, Lead, Nickel, Silver, Zinc, Selenium, Mercury, Molybdenum and Arsenic are metals monitored in the biosolids to prevent overloading of the soil. Ammonia and Phosphorus are two beneficial constituents also monitored. Each constituent has a maximum amount that can be applied per acre, verified through testing of the removed biosolids.
Biosolids are controlled through regulation 40 CFR 503 (503 regulations) and Ohio Administrative Code 3745-40-05, which state that the biosolids are required to meet Class B pathogen requirements (less than 2,000,000 Colony forming units per gram) and vector attraction requirements (a minimum 38% reduction in volatile solids). These requirements are verified through testing and comparing of the raw sludge entering the digestion system with the biosolids being removed to the fields.