The National Ground Water Assn. (NGWA) announced that ...
Foreseeing industry growth and overall city expansion, the city of Cedar Rapids, Iowa, Wastewater Treatment Plant sought a new permit that would increase the total allotted daily discharge and allow for the release of higher amounts of regulated compounds. However, the city already was having difficulty meeting some of the required river flow concentration levels set forth by the Iowa Department of Natural Resources. In order to obtain a better permit, the plant needed to significantly reduce the pollutant concentrations being produced by its current level of discharge.
"In the winter, during low river flow, it was hard to meet the required concentrations of certain pollutants, such as ammonia and biological and chemical oxygen demand," said operations specialist Chris Hatch. "We were meeting our water quality regulations but foresaw the increase in city industry would make that an even harder goal to reach in the next 10 years."
The city consulted engineering firm Howard R. Green for a means of lowering dilution-zone pollutant concentrations without further restricting the current discharge operation. The firm concluded the city’s existing outfall line could be redesigned into a multi-port diffuser that would significantly enlarge the dilution zone and mix the effluent more rapidly with the receiving water body. An effluent diffuser would be a more reliable and cost-effective option than altering the treatment process or installing a second outfall line.
The Cedar Rapids diffuser system is composed of a total of 73 ports spaced evenly across the length of the outfall pipe. Attached to each port is a Tideflex® Effluent Diffuser Valve with a Redflex® Rubber Riser and a 45-degree Redflex Rubber Elbow, all provided by Red Valve Company of Carnegie, Pa. The unique duckbill design of the Tideflex creates a pressure drop across the valve – to maximize the velocity of the discharging effluent and increase the speed of mixing, reducing pollutant concentrations at each discharge point.
Red Valve’s complete diffuser package offers several benefits that have maximized the efficiency of the Cedar Rapids system. Because the products are constructed entirely of flexible elastomers, they will not rust, break or erode, problems that are common with metal diffuser products. The duckbill design of the Tideflex prevents backflow into the header pipe as backpressure from the receiving water body ensures drop-tight sealing. The diffusers are constructed of durable, abrasion-resistant elastomers that will not rust or corrode, eliminating the risk of long-term maintenance requirements.
Because Cedar Rapids is a city that is continually expanding, Hatch knew that a system working efficiently at the time of installation would eventually have to be upgraded. Howard R. Green and Red Valve worked together to engineer a diffuser system that would meet the city’s current needs and still allow for future increases in capacity without any major redesign requirements. Red Valve was able to vulcanize closed a portion of the Tideflex Effluent Diffusers, which limited the number of ports in operation to maintain adequate velocities within the line at the plant’s current rate of flow. Once that rate of flow was significantly increased, the Tideflex was easily slit open to increase the overall system capacity.
To test the efficiency of the city’s new system, Hatch took aerial shots of the diffuser zone with an infrared lens to determine the rate of dissipation. Before installation of the system, the heat at the single discharge point did not dissipate for over one mile. After installation, this same heat generally dissipated within 100 yards of each operating discharge port.
With this much more efficient and environmentally friendly means of discharging its wastewater effluent, the city of Cedar Rapids was able to obtain a better permit. "The new permit regulations are much easier to meet," Hatch said, "and they allow the city a lot of future growth."
For further information, phone Red Valve at 412-279-0044.