Operators of the water reclaim plant in a California prison devise a creative solution to provide denitrification and meet effluent standards for off-grounds discharge.
The four inmate operators of a water reclamation plant at the California Correctional Institution in Tehachapi faced a dilemma. Working with plant flows and influent BOD well below the original plant design levels, they could not denitrify well enough to meet a 10 mg/L total nitrogen permit limit for discharge to customers off the facility grounds.
Working together, and under the supervision of three state-employed operators and a contract chief plant operator, they developed a creative solution to the problem that enabled the facility to resume delivering reuse water to customers during a severe drought when demand for reclaimed water was high.
Decline in flow
“We operate a 1.1 mgd Class IV activated sludge water reclamation plant with extended aeration, tertiary treatment and UV disinfection,” reports Keith Fredrickson, lead inmate operator. “Our plant was designed and constructed in 2008, when the facility population was expected to increase over the years.”
However, the state’s initiatives to reduce prison overcrowding resulted in a drastic decline in the inmate population at Tehachapi. As a result, the influent flow dropped to 0.55 to 0.60 mgd, and the BOD loading averaged about 200 mg/L, just one-third of the average for which the plant was designed.
“The significant decrease in the organic load caused us to face a severe dilemma,” says Fredrickson. “Per Title 22 reuse permit requirements, our plant must produce total effluent nitrogen below 10 mg/L in order to discharge off the grounds to customers. This became almost impossible last winter, since our centrifugal blower system was drastically oversized.
“Previous methods to create an anoxic zone for denitrification were no longer effective. Any attempt to run the blowers at the lowest possible airflow resulted in a surge that shut down the blowers due to excessive back pressure. Although shutting off the blowers would lower dissolved oxygen (DO), it would not allow for a sustainable anoxic period, and very little denitrification occurred.
“Keeping the blowers off for too long placed our aerobic bacteria at risk and also created short-circuiting problems by allowing our solids to settle to the bottom of the basin. Our plant’s Parkson Biolac system relies on diffused aeration for both mixing and oxygen transfer.”
The team tried multiple blower operation schemes seeking to achieve the desired results. Making things worse, Tehachapi’s below-freezing winter weather was in full swing, entraining air in the water and making it more difficult to maintain microorganism activity. Reuse water customers were growing frustrated, as California’s drought-driven water restrictions had raised the cost of potable water and the demand for reclaimed water was at an all-time high.
Still, says Fredrickson, “Nobody gave up hope. We continued to work as a team to come up with a solution. After putting our minds together, we came up with a plan. Our single aeration basin used 12 separate diffuser rows, each with 12 arrays. On each array were five separate perforated tubes that created the fine bubbles for oxygen transfer.
“Our plan involved converting some of the 12 diffuser rows into mixers. The theory was that if we removed the perforated tubes from each array of an entire row, that section of the system would mix the basin contents without significantly increasing the DO level. Large bubbles would help mix the contents but would dissipate to the atmosphere before being absorbed in the system.”
The team decided to test the theory with one diffuser row. They tested the DO level in the first section, and then removed all the perforated tubes in the first row. “The first thing we noticed was the increase in mixing,” says Fredrickson. “The row being tested was capable of mixing not only its section but also the section next to it on both sides.
“The best news, however, was when we tested the DO and saw that it had dropped significantly. It worked — we had finally found something that would allow us to control our DO levels despite the oversized blowers.”
Next they had to decide which rows to convert to mixers and how many should be converted. They decided to convert four of the 12 diffuser rows to mixers by removing the perforated tubes. “We created one mixer in between each two diffusers to allow for a uniform balance between mixing and diffused aeration throughout the system,” says Fredrickson.
The Parkson Biolac system then allowed them to control the on/off sequences for the mixers and air diffusers directly from the PLC. The sequence that worked best involved running the eight remaining air diffusers for one hour while the mixers were off, then running all four mixers for one hour while the diffusers were off.
“Running the mixers every hour kept all of our solids in suspension without transferring much additional oxygen to the basin,” says Fredrickson. “This allowed for a time period where the microorganisms could lower the surplus DO while the basin remained uniformly mixed. The increase in kinetic energy created by the mixers also kept biological activity high despite decreasing ambient temperature.
“All the manuals say the operators of an activated sludge plant must have control over three factors: the air rates, the waste activated sludge (WAS) rates and the return activated sludge (RAS) rates,” Fredrickson says. “Although WAS and RAS control was never a problem, we finally found way of controlling the air rates in our basin.
“The new sequences allow us to make adjustments when necessary to maintain continuous control. We are now able to create a complete anoxic zone for total nitrogen removal. We also no longer experience short-circuiting, as the mixers keep all solids in suspension during denitrification. This prevents any plug of raw wastewater from passing through untreated.”
Fredrickson reports that the teamwork paid off: As of last summer, the plant was meeting its requirements for discharge off the grounds and was meeting the needs of water customers. “There is no greater feeling than knowing we were able to come up with a solution together,” Fredrickson says.
“As an inmate, this is the best job I ever had. Most prison jobs involve performing menial tasks, such as preparing food or basic janitorial work. I had a job that allowed me to contribute something significant to the institution and its community. The plant had a legitimate problem. My co-workers and I were asked to think like operators. We poured our heart and soul into finding a solution, and then it finally came.”
Fredrickson notes that he and his fellow inmate operators all expect to be paroled by the end of 2017 or sooner. He says, “We are all excited about the prospect of starting our own careers in the water and wastewater industry.”
The operations team
Keith Fredrickson, the lead inmate operator at the Tehachapi correctional facility who provided most of the information for this story, has a Grade III Wastewater Operator license (second highest) and has passed his Grade IV exam. He also holds T2 Water Treatment and D2 Water Distribution licenses.
Charles Knepper III, senior inmate operator, is also a Grade III Wastewater Operator license holder who has passed his Grade IV exam. He owns a D2 Water Distribution license as well.
Richard Martin, inmate operator, has a Grade I Wastewater license and has passed his Grade II exam. Lemuel Talley, inmate operator-in-training, has passed his Grade I Wastewater Operator license exam.
The inmate team is supervised by Luis Ching, contract chief plant operator; Angel Ribera, drinking water treatment chief plant operator; Carlos Martinez, plant operator; and Trinidad Rodriguez, weekend operator in charge.