If your ideal is small-town living with lots of skiing, it’s hard to find a better place than the Town of Carbondale, Colorado. Located in a valley below 13,000-foot Mount Sopris, only about 7,000 people live within 2 square miles. But it’s also a bedroom community for the City of Aspen, a popular resort area. Driven by second-home construction, arts and recreational amenities, and tourism, growth is challenging Carbondale’s infrastructure managers to devise solutions to issues – such as wastewater treatment plant odor – that threaten quality of life.
Located right in the heart of town, the wastewater plant receives sewage from 22 miles of collection lines. The site can’t expand because it’s surrounded by homes and businesses. Instead, Utilities Department managers had to find more effective and more efficient treatment methods using existing tanks and existing retention volumes.
Controlling Odor a Challenge
Typically, the most cost-effective way to improve performance without increasing tank size is to promote growth of facultative anaerobe bacteria, which can exist in conditions of low dissolved oxygen and in processes where the oxygen supply is varied. They’re the most stable available in nature for wastewater treatment and, when in high populations, don’t produce hydrogen sulfide gas.
Cycling air and oxygen supplies to the process helps create an environment that promotes facultative anaerobe bacteria growth while also reducing undesirable, odor-causing bacteria. Department managers wanted to operate aerators to digester tanks in an on/off cycling mode to enhance development of beneficial bacteria. Using a sludge holding tank allowed for side-stream treatment and enhancement without affecting the main treatment train, which was the primary odor source. As a bonus, this process fluid could be used as a bio-seeder of facultative anaerobes in the main treatment process stream as returned sludge.
Their goal was to design a mixing system for 100% operational time and then field-adjust to the cycling mode with valve actuators and timers for development of facultative bacteria.
Simple Solution to Complex Problem
A key equipment requirement for this process is an air diffusion system that allows for on/off blower cycling. Conventional coarse bubble diffusers usually clog with sludge when airflow is interrupted due to blower shutdown or power failure. To avert that potential problem, the Utilities Department selected a Tideflex Technologies in-tank valve-and-diffuser nozzle system that isolates air manifold and related equipment and instrumentation from surrounding wastewater and solids.
An elastomer duckbill check valve allows flow in only one direction and closes when external hydrostatic pressure exceeds internal pipe pressure. A coarse bubble diffuser contains two thin-gauge stainless steel plates, which are embedded on both sides of the flat “bill” closure area. When the diffuser is operating (flowing open), the internal closure plates become energized. The plates force the bill area closed when flow stops. This feature is significant when operating in fluids of 1% solids or greater.
A series of these diffusers is installed in a pipe suspended 3 inches to 4 inches above the tank floor so that, when air is discharged, it mixes wastewater and solids without letting any into the diffusers or piping. For sludge storage processes, this allows for decanting and thickening of residuals as well as denitrification.
System Performance: Exceeding Expectations
Plant operators used oxygen reduction potential (ORP) probes to monitor the environmental state of the process fluid and adjust air on/off times accordingly through their computer monitoring and distributed control system. They were able to develop a high-density facultative bacteria population in a tank that eliminated odors and lowered energy consumption 40%. (Most Tideflex alternating systems reduce blower operating costs by at least 35%.)
Since then, the department has installed the system in adjacent sludge tanks and is using ORP monitoring to minimize blower operational run times. The systems have provided maintenance-free operation and, considering energy reduction and savings along with elimination of diffuser maintenance costs, a 36-month return on investment.