A Simple Insert Can Save Your System

Inserts are a low-cost, high-efficiency way to reduce inflow through manholes.

A Simple Insert Can Save Your System

Tests have shown that up to 45 gallons of water per minute can enter a collections system through a single standard 24-inch-diameter manhole cover with two 1-inch-diameter pick holes.

Inflow is the first “I” in I&I, but it regularly takes a back seat to infiltration.

The inflow of stormwater into collections systems is often the result of illegal or improper connections, but excess flow entering the system through manhole covers can also pose significant threats. It’s a problem that plagues communities throughout the U.S.

When rainfall or snowmelt cause unwanted water to enter the collections system through manhole covers, the amount of actual flow can far exceed the design capacity of the wastewater treatment plant. It strikes quickly and can carry dirt, debris, sand, grit, oils and other pollutants that also end up in the collections system. One of the most efficient and cost-effective ways to reduce inflow through manhole covers is to install manhole inserts. The units are lightweight and can be installed under existing manhole covers by one person in just a few minutes. Removing the units to access manholes is just as easy by using the attached lifting strap. 

Manhole inserts have been around since the late 1970s, yet can go unnoticed when it comes to solutions for addressing inflow and infiltration in manholes. The focus is usually toward the infiltration side of the equation, which is certainly important in its own right, but inserts play an important role in the overall picture of combating inflow. Each time it rains, unwanted clean water is entering sanitary collections systems by passing through manhole cover pick holes or the area between the frame and cover, ultimately ending up at the treatment plant. Manhole inserts are designed to drastically reduce that unwanted inflow and also contain the dirt, grit, sand, oils and pollutants that come with it.  

Inflow in Ohio

Greenville is a small town in west-central Ohio with a population of about 13,000. Its heritage goes back to the early days of statehood and is best known for being the place where the Treaty of Greenville was signed in 1795, opening up exploration to the northwest. As with many small older communities across the country, they reached a point in the early 2000s where nearly every rain event was overwhelming the sewer plant and causing basement backups. Any significant rain would take one to two days for the system to recover. Something had to be done. 

A flow study was completed in 2004 and a plan was put together to try to eliminate some of the easiest inflow and infiltration. Greenville Creek runs through the town and the study identified the south side of the creek as the area where most of the inflow and infiltration initiated. 

The town has approximately 880 manholes, many of them with covers that had vent or pick holes in them. In 2005, then street supervisor Tim Harless decided to install polyethylene manhole inserts into about 400 manholes on the south side. After several months and many rain events he noticed that basement backups, which had been a common occurrence during rainfall events, had virtually disappeared. It was a welcome end result that he attributed primarily to the inserts. He was so satisfied with the outcome that the decision was made to install inserts in the remaining manholes the following year. 

Today, wastewater supervisor Vaughn Downey reports that the rain event surge to the treatment plant has been reduced by a conservative estimate of 20 to 25 percent over the last 10 years, and the recovery rate has decreased from one to two days to about one hour. He attributes a good portion of this success to the installation of manhole inserts and also to the diligence of the street department in identifying and fixing problems as they are found. 

Covered, not sealed

Several in-house tests conducted by manufacturers of both manhole inserts and manhole covers have shown that up to 45 gpm of water could potentially enter the collections system through a single 24-inch-diameter manhole cover with two 1-inch-diameter pick holes and only 1 inch of water over the cover. A cover without pick holes can still allow up to 25 gpm with that same 1 inch of water over it because the water can penetrate through the gap between the outside edge of the cover and the inside portion of the frame, which is typically about 1/8-inch wide. Anyone who has had water leaking into their home knows all too well how water can find its way through the smallest of openings. Keep in mind that these tests were performed under controlled conditions using clean manhole frames and covers.  

One municipality performed its own testing in actual field conditions using from 1 to 10 inches of water over the manhole cover while developing three different sets of circumstances. For the first test the bearing surface was sealed, to simulate dirt and grit deposits between the frame and cover, and the pick holes were left open. Test two had a clean frame and cover, but the pick holes were plugged, and for the third test, the ring and cover were clean. Results with 1 inch of water over the cover ranged from 12 gpm for test one to 15 gpm for test two and 27 gpm for test three. Moving to 5 inches of water produced rates of 20, 24 and 42 gpm, respectively. At the 10-inch mark, inflow increased to 25, 29 and 47 gpm. 

In order to look at this problem from a dollars and cents standpoint, the cost to treat all that water entering the WWTP must be established. If an amount of $2 per 1,000 gallons is used, with the best-case scenario of 12 gpm of inflow from the municipal field test, a total cost per manhole per day can be calculated. At 12 gpm of inflow x 60 minutes (720 gallons per hour x 24 hours), 17,280 gallons per day is entering the WWTP from a single manhole. Dividing the total inflow per hour by 1,000 = 17.28 and then multiplying that number by the treatment cost of $2 = $34.56 per day to treat surface water at 1 inch deep over just one manhole. 

It’s easy to see the impact that even 100 manholes can have on WWTP flows during a rain event. Manhole inserts should be considered an investment, rather than a cost, since the price of the insert can be recouped in as little as 24 hours of rain.

Craig S. Gaul is president of Parson Environmental Products, and Bob O’Connor is president of Municipal & Contractor Sealing Products.



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