December 2012, Vol. 24, No.12

Plant Profile (cont.)

City of Dayton Wastewater Treatment Plant

Profile 2 map 

Location: Dayton, Ohio
Startup date: 1929
Service population: 340,000
Number of employees: 70
Design flow: 272,500 m3/d (72 mgd)
Average daily flow: 189,250 m3/d (50 mgd)
Annual operating cost: $16 million
 

Since 1929, the City of Dayton (Ohio) Division of Wastewater Treatment has worked around the clock, 24 hours a day, 365 days a year to protect the water quality of the Great Miami River. The division serves the city of Dayton and the region, including a large part of Montgomery County, which includes Trotwood, Northridge, Riverside, Harrison Township, parts of Randolph Township, Oakwood, Kettering, Greene County, Moraine, and Wright Patterson Air Force Base.

The 272,500-m3/d (72-mgd) plant uses two secondary processes in series, as well as effluent filtration and post-aeration, to produce excellent quality effluent. This level of treatment is required because at the plant’s outfall, effluent can contribute up to 50% of the total river flow during drought conditions. This large contribution to river flow requires the wastewater discharged to meet the cleanest water quality standards to protect warm-water aquatic animals and plants, as well as allow for such designated uses as fishing, boating, and skiing.

 

Double duty

Following screening, grit removal, and primary settling, influent to the Dayton plant undergoes a two-stage biological treatment system — trickling filters followed by activated sludge. First, the plant’s 20 high-rate trickling filters remove mainly 5-day carbonaceous biochemical oxygen demand (CBOD) and suspended solids (SS). Next, the eight activated sludge tanks work to nitrify the ammonia.

The two stages are separated so nitrifiers can do their job more effectively. When nitrifiers are grown in the same aeration basin with microorganisms that remove CBOD, they tend to have trouble growing to sufficient numbers to convert ammonia. By removing a large portion of the CBOD in the trickling filters, the nitrifiers do not have trouble growing to the numbers necessary in the activated sludge tanks to achieve the required levels of nitrification.

The circular high-rate trickling filters contain graded slag or limestone rock media. In 2001, Dayton upgraded the distribution arms and wells from hydraulic-driven to electrically driven drives and installed fans to ensure airflow.

After the trickling filters, the flow is pumped up to 10 secondary settling tanks, where sloughed-off solids from the trickling filters accumulate. Then, the supernatant from the secondary settling tanks again is pumped to the activated sludge system, which consists of aeration basins, final settling basins, and return activated sludge and waste activated sludge pumping.

While the activated sludge system requires reduced levels of CBOD to enhance the growth of the nitrifiers, some CBOD and SS are required to produce an activated sludge that settles properly. So, Dayton has a provision to add additional CBOD and SS to the activated sludge system if settling problems occur. A structure called the spiking channel, located adjacent to the first-stage trickling filters, enables operators, if necessary, to send a controlled volume of primary effluent directly to the activated sludge system.

 

Finishing touches

Effluent from the final settling basins proceeds by gravity to the effluent filters, which remove even more CBOD and SS to ensure reaching National Pollutant Discharge Elimination System permit limits, especially during summer.

The final settling-basin effluent flows downward by gravity through a relatively deep bed — 762 mm (30 in.) — of granular anthracite coal filter media. The effluent filters at the Dayton treatment plant are designed as variable declining rate filters, which simplify the operation and control of this process. Dayton’s filters use a simultaneous air-scouring during backwashing to enhance removal of solids from the filter media.

Before the final effluent is discharged into the Great Miami River, it is disinfected with a sodium hypochlorite solution. The effluent also is dechlorinated with sodium bisulfite to eliminate the residual chlorine that may be toxic to aquatic plants, fish, and wildlife in the receiving stream.

Finally, a post-aeration step increases the dissolved-oxygen level of the effluent. This is accomplished in the final pass through the chlorine contact basins using coarse-air diffusers, which are designed to increase the dissolved-oxygen concentration to 5.0 mg/L or higher.

 

Splitting up solids duties

The Dayton facility anaerobically digests all of its solids to produce Class B biosolids and biogas for onsite use, but it contracts out thickening and dewatering, as well as final disposal. A daily average of 1.5 million L (386,000 gal) of solids are anaerobically digested. The system operates as a single-stage system consisting of eight primary digester tanks — four have an operational capacity of 5.5 million L (1.46 million gal), and four have an operational capacity of 4.8 million L (1.27 million gal). It is a standard rate system, with loadings less than 1.6 kg/m3•d (0.10 lb/ft3•d) of volatile suspended solids.

Mixing is automated using an 11-point mixing multiple-point sludge-recirculation system sized to turn over the digester contents at least once every 24 hours. The conditions within the digesters are maintained at approximately 36.7°C at a pH between 7.0 and 7.5. The annual solids loading of the digesters average 848 kg of volatile solids per 1000 m3 (53 lb of volatile solids per 1000 ft3) of operating capacity.

Dayton captures and uses the biogas produced during digestion. So far in 2012, Dayton’s digesters have averaged a total of 17,000 m3/d (600,000 ft3/d). The gas is used to fuel a 2.2-MW power plant cogeneration facility as a second source of electric power for treatment and provide a means to reduce utility bills in addition to fueling plant boilers and hot-water loops for heating, ventilation, and air conditioning and solids heating. The cogeneration facility can produce 9600 MWh of electric power per year and 45 billion Btu of heat per year. The cogeneration facility is operated in a peak-shaving mode to accrue the best cost savings in avoided electric utility costs.

 

Biosolids processing and disposal

Following digestion, the biosolids are pumped to a dewatering facility at about 3.5% solids. The city’s biosolids contractor currently owns and operates the privatized facility on the treatment plant’s grounds. The contractor first uses polymer and two gravity-belt thickeners to reach 8% to 10% solids concentration. Then, more polymer is injected, and the solids enter two centrifuges, which increase the solids content to approximately 25% to 30%.

The plant produces about five truckloads of Ohio Class B biosolids every day. The contractor recycles these biosolids on more than 8500 ha (21,000 ac) of farmland in southwestern Ohio to help grow such crops as
corn, soybeans, wheat, and hay.

 

©2012 Water Environment Federation. All rights reserved.