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Inflatable gates improve storage, reduce CSO.

Combined sewer overflows (CSOs) are a serious concern to cities all over North America. Very few municipalities have enough excess capacity in their treatment plants to handle the extra inflow caused by a general rain event. Fewer still can handle the heavy rains that most communities receive several times each year.

Some sewer districts have built storage tanks or deep, large-diameter tunnels to temporarily store heavy combined flow until it can be treated. Others have installed automated gates and dams in their large-diameter pipelines to take advantage of the inherent storage capacity of their combined sewer collection to help them meet the 1994 EPA CSO regulations.

For the past 20 years, Northeast Ohio Regional Sewer District (NEORSD), which serves the Cleveland, Ohio, metropolitan area, has achieved significant reductions in overflows to the environment without building extra storage capacity. The district has used a sophisticated data acquisition and management system to automatically control a series of gates installed in outfalls and in active sewers, which are used for in-line storage. The NEORSD system, which includes 18 inflatable rubber gates (the largest installation of such gates in the country), has reduced discharge on the order of 0.8 billion gallons per year at low construction and operating cost.

INITIAL INSTALLATIONS

NEORSD installed the 18 inflatable rubber gates between 1974 and 1981. Three of the gates were designed for in-line storage and 15 for outfall control. Eleven of the original gates are still in service. The other seven gates failed because of an apparent flaw in the fabrication of joints in the rubber body.

NEORSD originally considered replacing the seven gates that failed with traditional steel gates, but aside from the manufacturing defect, the inflatable gates had performed according to specification. They had not experienced any environmental degradation, and needed minimal maintenance. The anticipated advantage of using low pressure air to control the gates instead of a hydraulic system had been realized.

In addition, construction considerations were important. Inflatable gates do not require equipment vaults to be immediately adjacent to the gate. The operating mechanisms may be up to 1,000 ft away, and the components needed to install an inflatable gate could pass through a standard manhole. Given the advantages, the decision was made to continue with inflatable gates, provided an acceptable manufacturer could be found.

NEORSD was aware that Bridgestone/Firestone (Nashville, Tennessee) had introduced ExpanGate, a product similar to the original inflatable rubber gates, and that two had been installed in Detroit in 1987. After a thorough investigation, NEORSD decided to use ExpanGate. Seven new gates have been installed over the past six years.

The body of a Bridgestone gate is made of a rubber fabric between 0.375 and 0.688 in. thick, based upon the height of the gate. The internal operating pressure of the gate increases, up to a maximum of 6 psi, as the height of the gate increases. As a result, the thickness of the rubber fabric increases to maintain a minimum safety factor of 3.5. The fabric is reinforced by multiple layers of special nylon mesh that results in a design elongation of 30 percent. Chemical additives are also incorporated into the rubber fabric to enhance its resistance to ozone and other harmful substances. As a result, the gate has a design life of at least 30 years.

The gate is installed in a tunnel by a simple mechanical clamp, without adhesives. The metal components can be either stainless steel or galvanized. NEORSD chose stainless steel bolts and galvanized plates, a decision based on economics. The plates are not difficult to replace, should deterioration occur, but after six years of service, no deterioration is apparent. NEORSD required that many of the anchor bolts be pull-tested to the full design load during installation. The gate has been successfully anchored in typical brick-lined sewers.

The control vaults for the gates are generally within 50 ft of each gate. The connection between a gate and vault is typically two air pipes. Usually, one is either two or three inches in diameter and the other is two inches or less. The larger pipe is used to inflate or deflate the gate while the smaller pipe carries an independent internal air pressure signal. In addition, water level signals need to feed to the vault.

NEORSD contracted with local firms for the gate installation. The installations have gone smoothly with no unforeseen problems.

AUTOMATED CONTROL SYSTEM

The operation of the automated regulators, remote flow and level monitors, and rain gauges is monitored and coordinated from NEORSD's Central Sewer Control Facility. The control system can monitor water levels upstream, downstream, or any combination of the two, and will automatically inflate or deflate each ExpanGate to maintain target wastewater levels in the pipelines.

All control equipment for each gate is contained in the adjacent underground vault. The vault contains control and support equipment for the automated regulator, including level and flow sensing systems, pneumatic systems, motor control panels, uninterruptable power supplies, and a programmable logic controller (PLC).

The PLC can accept analog and digital inputs from remote sensors, and can perform complex math functions. based on the input from these sensors and target levels programmed into the controller, each gate will be raised, lowered, or left alone.

For in-line storage, the gates inflate when the water level in the downstream main reaches a preset height. The gate would remain in the inflated position until either the downstream water level is greater than a preselected setpoint, or until the upstream water level rose to a preselected height. In either of these cases, the dam would be regulated to maintain either or both setpoints.

The setpoints are calculated to prevent flooding of the treatment plant without backing wastewater into customer service lines. To assure correct operation, each controller has redundant level indicators and uninterrupted power supply. In addition, all instrumentation and controls are conservative. All safety devices are set to deflate the dam in an emergency situation.

The standard ExpanGate design results in an inflation time of about 15 minutes. This proved to be too long for NEORSD, so the first gates installed are kept inflated to one psi for four hours, then completely deflated for one hour. This cycle is repeated continuously. The partial inflation significantly shortens the time required for full inflation, and the periods of complete deflation release wastewater that accumulates behind the partially inflated dam before it can become septic. Grit does sometimes accumulate behind the gates during periods of normal flow, but that is quickly scoured away during each rain event. Subsequently, inflation times in the range of six to seven minutes have been specified. Even faster times can be accomplished by increasing the size of the air pipes and the capacity of the blowers.

Some of the gates are designed to completely block the sewer at full inflation pressure, even with a substantial upstream surcharge. Other gates are designed to leave some of the tunnel open when fully inflated. So far, NEORSD has not operated its gates in the fully closed position even though both types are in the system. This does sacrifice some storage.

ECONOMICS

The capital cost of NEORSD's data acquisition and control system, gate procurement, and installation is estimated at $10 million. This provides CSO control of 0.8 billion gallons per year for a capital cost of $0.0125 per gallon. Maintenance costs of the system have been minimal.

The district's real-time control system for CSOs has proven to be efficient in achieving significant reductions in overflow. Higher levels of storage have been attained by replacing fixed weirs with gates and inflatable gates. And, interceptor capacities have been maximized by using in-line storage. In-line storage has reduced both BOD and TSS discharges to the environment and has proven to be cost-effective through the operation of automated regulators. Problems with the initial design and construction have now been identified and rectified. The district has a reliable, functional, and affordable system.

Mr. Hudson is Manager, Systems Operations & Maintenance, Northeast Ohio Regional Sewer District.
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Title Annotation:Combined Sewer Overflows
Author:Hudson, Daniel M.
Publication:Public Works
Date:Feb 1, 1998
Words:1338
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