One such city, with a 9.5 million gallon per day peak capacity wastewater treatment plant, receives daily flows exceeding 50 million gallons when rainfall exceeds 1 in. Another city, with a 6 million gallon per day peak capacity wastewater treatment plant, tries to cope with 11 million gallons per day during heavy rains.
The city of Bryan, Ohio had similar troubles. Then, two items costing less than $250 per manhole were installed by city personnel, resulting in more than a 50 percent reduction in combined I & I from the designated test sections.
One of these items was a manhole chimney seal placed inside the chimney; and the other was a plastic dish fitted below the manhole cover.
The Good and the Bad
The City of Bryan, population 8,350, is located in the northwest corner of Ohio. In 1993, it was selected as one of the 100 most livable cities in America.One reason for this distinction is its modern sewer system. Most lines were built to carry sanitary sewerage only. Previously combined lines were separated over a 13-year program, successfully completed in 1988 when a new wastewater treatment plant was also placed in service.
After the completion of the separation projects, about 20 percent of the once-combined sewers remain in service as strictly sanitary sewers; the remaining 80 percent are in service as strictly storm sewers.
During the first year of its operation, WWTP influent records began to establish a pattern that showed a rapid increase in influent flow following a rainfall event. Despite all the work, it was evident that inflow problems still existed in the separated sewer system. Such inflow sources as residential, commercial and industrial buildings with down spouts connected into the sanitary sewer system, manhole tops with holes, leaking manhole risers, cross-connections with catch basins or storm sewers were considered the main probable trouble areas.
In 1992, the city decided to be pro-active about the problem and hired Finkbeiner, Pettis & Strout, Inc. to assist them in a detailed flow study.
Sanitary Sewer Study
Since every sanitary sewer in Bryan eventually flows into the Union Street trunk sewer, flowmeters were placed in its various tributary sewers.During three study months (April, May and June 1992) each rain event caused correspondingly large spikes in the influent flow rate. The most dramatic increase occurred on June 17 following a rainfall of 0.95 in. in which the WWTP flow increased from 2.0 mgd to 8.5 mgd in less than one hour.
Large rapid increases such as this one made a serious impact on the city's plant. Generally, the first flush of I & I through a sanitary sewer system washes many materials through the lines that, under normal flow conditions, has accumulated there and in the manholes. This material can cause shock loadings of solids and BOD that often disrupt biological processes at the WWTP and increase the amount of sludge produced. As the I & I increases, the WWTP may become hydraulically overloaded, adversely affecting the treatment process. Detention time in the primary and final settling tanks is reduced, causing the settling of solids to be less efficient. Increased solids, then discharged in the effluent, can in turn, cause possible violations of the WWTP's NPDES permit.
In addition, higher flows increase pumping needs, power consumption and wear. This is particularly true for primary and return sludge pumps. The initial BOD shock load can lower the D.O. in the wastewater, thereby increasing the demand on aeration basin blowers and reducing the effectiveness of the plant to reduce BOD. Under the stress of decreased oxygen, good bacteria can die and harmful bacteria can flourish, thus causing even greater problems.
Bryan's WWTP has a design average daily flow of 3.1 mgd and a peak capacity of 8.0 mgd. A sluice gate at its influent enables the flow rate to be throttled if it begins to exceed 6.0 mgd. The surcharge is pumped to two equalization ponds, with a combined capacity of 3.0 million gallons. Once the flow subsides, the ponded wastewater is released to the WWTP for treatment. However, if the influent flow rate does not return to normal after a short period of time, the equalization ponds overflow directly to Prairie Creek without benefit of any secondary treatment.
Manhole Measurements
In addition, during periods of high flow, manholes on or near the WWTP site surcharge to the point of dislodging their covers. Wastewater then pours out onto the surrounding ground.The thinking was that a reduction in the amount of I & I entering the sewers would help alleviate these problems, resulting in reduced power consumption, lower operation and maintenance costs, and increased overall effectiveness of the WWTP.
On the basis of the flow data gathered, on-site observations and conversations with City personnel, a ranking for areas contributing to the manholes investigated were developed. For each of the manholes, the volume of I & I resulting from a 1 in. rain was estimated. Recommendations for further investigations included smoke and dye testing and videotaping. Flowmeters were installed in 29 different manholes.
One key manhole turned out to be South Beech Street No. 13. Located in an easement, flow was measured from September 15 to October 5. Three small rainfalls did not seem to affect its flow. However, a 1.56 in. rain event did. Occurring on September 21, this storm caused the rate to soar over 1300 percent - from 60,000 gpd to 850,000 gpd. Prior to this date, city personnel did not suspect this sewer was the cause of any of their I & I problems.
Solutions
At this point, 13 years of studies were reviewed. These studies revealed that nationally, manholes generally account for more than half of the total I & I entering into any sanitary system.Typically, about 30 percent of the inflow enters through the body of the manhole. Solutions include replacing the manhole (which may be cost prohibitive), or rehabilitating the existing structure. Rehabilitating a manhole costs about $60 per vertical foot when done by a contractor using quality materials.
It was also found that another 10 percent of outside water enters through holes in the manhole cover and around the lid seat. One solution is to install new watertight frames and covers. However, the solution chosen in Bryan was to place a plastic dish under the cover and seal the seat area. The dishes selected provide a watertight barrier that releases pressure from the manhole when needed, and controls drip-back so water does not lie in the dish.
The largest amount of outside water, 60 percent, enters through the manhole chimney. There are a number of causes, and the volume can be sizable. When it rains, inflow at this point often increases to 50 or more gallons per minute. A seepage of just 1 gallon per minute, allowed to continue for a year, adds up to a total of over 525,000 gallons of inflow.
One of the main causes of the problem is movement. Frost heave in grassy areas can total 3 or 4 inches. Manhole frames can also be loosened by being hit by lawn maintenance equipment. Voids, created between the casting and manhole chimney, allow water to enter the sewer line after flowing along the lawn surface and the underlying soil. Location of the manhole in natural drainage swales, as was the case in the area studied, multiplies the problem. So does time. The wash of water carries an increasing amount of soil into the manhole, creating bigger and bigger flow channels.
After chimney seals manufactured by Cretex Specialty Products, Waukesha, Wis., were installed in each of the 30 manholes servicing the section of low-lying feeder line that seemed to be causing most of the inflow in the study area (and a no flow dish was placed under each of the covers), total I & I was reduced by 63 percent at Manhole #13 and 57 percent at Manhole #17. Relatively new, these manholes and their chimneys were all precast. All were in backyard easements.
City personnel were able to perform all the work. The 30 installations took two laborers less than 20 minutes each.
Made of a high-quality rubber compound, the seals were locked in place with rust-free stainless steel bands. Where manholes are located in grassy areas, the rubber pleats withstand repeated movement of the frame caused by shifts in the surrounding soil, frost heave action and lawn equipment impacts. Where manholes are located in streets, the seals stop the inflow of water that enters the subgrade through cracks in the pavement.
Once each seal was locked in position, rubber tight against the chimney wall, city crews checked its integrity by pouring water behind the top band. No leaks were noted at any location.
