March 2011, Vol. 23, No.3

Plant Profile

Coffee Creek Wastewater Treatment Plant


Location: Edmond, Okla.
Startup date: 1972
Service population: 80,000
Number of employees: 5
Design flow: 34,000 m3/d (9 mgd)
Average daily flow: 26,500 m3/d (7 mgd)
Peak flow: 51,000 m3/d (13.5 mgd)
Annual operating cost: $1.8 million

At the Coffee Creek Wastewater Treatment Plant in Edmond, Okla., the operators focus on continuous improvement. Examples of this frame of mind range from simple fixes, such as replacing handrails that need annual painting with galvanized ones that don’t, to the in-house rehabilitation of a lift station that led to a $50,000 savings. The operators say even the small things over time add up to big improvements.

In 2005 and 2010, the Oklahoma Water and Pollution Control Association (Woodward) recognized the plant’s achievements with its Outstanding Large Wastewater Plant of the Year award. (That’s not a 5-year gap; winners must wait 5 years until they are eligible again.)


Incremental growth
Since opening in the early 1970s, the Coffee Creek plant has undergone two major expansion projects, one each in the 1980s and 1990s. Today, that translates into three separate secondary treatment trains for the continuous-flow activated sludge process. (The plant skips primary treatment.)

After bar screening and fine screening, influent flows through a splitter box containing valves that control flow volume to a 3800-m3/d (1-mgd) two-stage nitrification system or one of two 11,400-m3/d (3-mgd) oxidation-ditch trains. The two-stage nitrification system includes two primary and two secondary aeration basins with clarifiers after each step. Each oxidation-ditch train has two ditches, followed by two clarifiers.

The flows recombine for a polishing step through two banks of sand and anthracite filters. Each bank has four filters. Finally, chlorine gas disinfects the effluent, and sulfur dioxide removes excess chlorine.

Interestingly, the plant’s permit only requires disinfection from May through September. These are the months when the receiving stream, Coffee Creek, is used for recreation.

Solids wasted from all three trains are sent to facultative lagoons for treatment before land application. The plant has three treatment lagoons, and a fourth is used to decant water from the treatment lagoons.

Each treatment lagoon is dosed, goes through an isolation period, and then has accumulated sludge removed. A complete facultative treatment cycle extends 2 to 3 years. During any particular year, one lagoon receives waste activated sludge, another is in the treatment mode and does not receive sludge, and the third is cleaned. The average solids retention time in the lagoons is 1 to 2 years. Prior to land application, a lagoon will be isolated for a minimum of 6 months, during which the ambient air temperature is above 4°C.

The three lagoons provide treatment capacity for 1052, 1153, and 1513 dry Mg (1160, 1271, and 1668 dry ton), respectively. Capacity is based on maintaining a 1-m (3-ft) freeboard and a 1-m (3-ft) water blanket in each lagoon. Accumulated sludge averages 4% total solids.

The treated sludge meets Class B biosolids requirements. Coffee Creek ensures compliance by monitoring indicator organisms. Seven samples are analyzed for the number of colony-forming units of fecal coliform per gram of biosolids.

Once a year, beginning in July and wrapping up in early August, the biosolids are land-applied to local farmland to grow second-generation food crops, such as hay for animal feed. In 2010, the plant hauled out 23 million L (6 million gal).


Preventive maintenance
The Coffee Creek operators work to prevent equipment failures instead of waiting to put out fires, according to chief plant operator Kris Neifing. For example, after 20 years of operation, the operators decided the plant’s first oxidation-ditch train could use refurbishing. So they painted clarifier structural steel, replaced weirs, painted rotors, replaced diversion-box valves, and rebuilt gear boxes before they failed. They are currently planning the refurbishment of the other oxidation-ditch train, which came on-line in the 1990s.

In 2005, the plant began using a computerized maintenance management system to better track its maintenance efforts and spot problem areas. For example, in 2009, the computerized maintenance management system helped flag the lift-station sump pumps as a concern. (In addition to operating and maintaining the plant, the Coffee Creek operators also look after Edmond’s nine lift stations.)

Operators were spending a lot of time working on and paying for pump rebuilds. To break that cycle, the plant evaluated several other pumps with different configurations to replace the maintenance hogs. New pumps were selected and installed and to date have not failed again.


Quick reaction
Sometimes, though, emergency repairs are needed, and the Coffee Creek operators are ready. In November 2009, a leaking force main threatened the operation of the plant’s lift station, which carries half of the influent to the plant. To make matters worse, a major snowstorm was on the way.

The force main is 5 m (15 ft) deep and located beneath a conduit bank containing 15 conduits ranging from 19 to 50 mm (0.75 to 2 in.), as well as two junction boxes. Digging to expose the main began at 11 a.m. and took more than 10 hours.

Upon inspection, operators discovered the leak on the lift station’s discharge pipe, just outside the lift-station wall. The 400-mm (16-in.) leaking pipe was fabricated from steel, but 450-mm (18-in.) ductile iron pipe had been used upstream and downstream of the leaking steel pipe.

With the problem unearthed, the operators discovered there were no clamps in the city that could fit the leaking pipe, so they improvised and welded a patch onto the steel pipe as a temporary fix. The temporary fix was covered at 11 p.m., and the force main and lift station were back in action.

Operators got to work locating and ordering the right parts to fix the force main properly. As soon as the parts arrived — 2 weeks later — the hole was uncovered again. To ensure a permanent fix, operators installed new ductile-iron piping through the wall of the lift station and used corrosion-resistant appurtenances to help mitigate future issues.


Future savings
In addition to focusing on excellent maintenance, Coffee Creek also is working to increase its energy efficiency. The plant received a grant from the Oklahoma Municipal Power Authority to help switch its motors to more premium-efficient motors. The plant replaced 14 motors ranging from 19 to 75 kW (25 to 100 hp). The changes will pay for themselves in power savings in about 3 years and keep power consumption low during peak periods.

In another energy-conscious change, the entire Water and Wastewater Utilities Department in Edmond switched to wind-generated electricity in 2009. This change includes the Coffee Creek plant and its associated lift stations and reduces the plant’s carbon footprint to near zero. The change also insulates the plant from fuel cost adjustments.


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