June 2009, Vol. 21, No.6

Plant Profile

City of Zanesville Wastewater Treatment Plant

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Location: Zanesville, Ohio
Startup date: 1959
Service population: 58,000
Number of employees: 31
Design flow: 7.75 mgd (29,300 m3/d)
Average daily flow: 7.49 mgd (28,400 m3/d)
Peak flow: 18 mgd (68,000 m3/d)
Annual operating cost: $7 million

It’s always good to have options. When its current upgrade is complete, the City of Zanesville (Ohio) Wastewater Treatment Plant will have options for handling normal and peak flows, as well as waste activated sludge processing and biosolids disposal.

Currently the Zanesville plant can provide full secondary treatment for up to 18 mgd (68,000 m3/d) and primary treatment for up to 24 mgd (91,000 m3/d). However after the upgrades, primary and secondary treatment capacity will be increased greatly.

Growing Capacity
Increasing the plant’s capacity is a priority because in addition to its 11,000 residential customers, 1000 commercial dischargers, and 36 industrial users, the plant receives flow from the area’s combined-sewer system. Rainfall greatly increases the amount of flow to the plant.

Through a two-phase upgrade, Zanesville will increase its design flow from 7.75 to 11 mgd (29,300 to 42,000 m3/d). The plant’s maximum flow will be increased to 36 mgd (136,000 m3/d).

Phase 1, which was completed in July 2007, included adding specific features, such as a new gravity thickener, a septic receiving station, and a primary pump station with a mechanical bar screen that removes items as small as cinders and cigarette butts. In July 2007, Phase 2 began. The addition of a supervisory control and data acquisition system, a third primary clarifier, a new grit-removal system, a fourth secondary clarifier, and a solids contact stabilization tank will increase the plant’s total primary treatment capacity to 36 mgd (136,000 m3/d) and its secondary treatment capacity to 27 mgd (102,000 m3/d).

In addition to adding the third primary clarifier, the two existing primary clarifiers will be stripped and restructured to match the third. All three will be able to operate as high-rate clarifiers during high flows. A new polymer and ferric chloride addition system will enable chemical addition to help increase settling capabilities during high flows. During low or average flows, the clarifiers will be used without chemical additions.

During Phase 2, the plant’s three trickling filters, which can treat 18 mgd (68,000 m3/d), will be stripped bare. The old plastic media will be replaced with new plastic media, and the structure will be resealed.

The new solids contact stabilization tank adds a new treatment process for 27 of the 36 mgd (102,000 of the 136,000 m3/d) that the plant will be able to treat. The solids contact stabilization tank will use air to oxidize ammonia and inactivate pathogens.

Biosolids Trio
The solids-handling side of the plant also benefited from the ongoing upgrade. One of the largest components to Phase 1 was the construction of a belt press and microwave dryer to add a biosolids disposal option, making it the plant’s third option. Liquid biosolids can be land-applied as a Class B product, belt-filter-pressed solids can be sent to a landfill, or the microwave-dried solids can be land-applied as a Class A product.

 Primary and waste activated sludges are pumped from their respective clarifiers and fed to a gravity thickener en route to the first of the plant’s two high-rate, two-stage anaerobic digesters. Septage from the Lakeside receiving station also is added to the digester. The first digester heats and mixes the sludge, while the second digester is used primarily as a sludge and gas holding tank.

The settled solids from the secondary digester meet Class B standards for reduction of volatile solids, pathogens, and vector attraction reduction factors. Additionally, the plant monitors biosolids for nine metals listed in the 40 CFR 503 regulations and has not had any metal problems.

Belt filter pressing is the plant’s second biosolids management option. While operators can process a large amount of digested sludge quickly, this option is costly, and the resulting solids must be sent to a landfill.

Using microwaves to dry solids, Zanesville’s third option, makes the plant a pioneer in United States, as it is one of the first installations of a microwave solids dryer. Extensive testing has shown that the resulting product, which is dry and resembles tree bark mixed with soil, is a Class A product. The microwave also greatly reduces the volume of the material it treats. The biosolids are stored onsite until several truckloads are produced — about 2 to 3 days — then land-applied. In the near future, Zanesville plans to entertain bids from companies willing to market its product for purchase.

Sloppy Sludge

 

As with any new technology, the operators have had to make some adjustments to get the best results from the microwave. Initially, following microwave drying, the solids resembled dry and brittle sheets of tree bark. Then they passed through a series of screw conveyors to be broken into small pieces and moved to the storage area.

However, upon exiting the conveyors, the solids were wet and dense, resembling cow manure. This thicker consistency plugged the conveyor screws.Operators soon discovered that the turning blades breaking up the barklike solids released heat and moisture, which was trapped inside the enclosed conveyor. The resulting condensation rewetted the dried solids.

The solution was to modify the conveyors. The plant maintenance staff replaced the solid top covers with an open grating, which allowed the heat and moisture to escape. This simple change did the trick and led to a dry product.

Operators also have noticed that the longer the solids sit in storage, the further they break down, as in a compost pile.