A Sanitary sewer is a system of gravity forced drainage pipes which carry raw sewage and grey water from residences and businesses. Ultimately a localities sewer system ends up in a centralized location referred to as a wastewater treatment plant. Here waste water is treated enough to be suitable for release back into oceans, rivers, streams, or as groundwater. Water must be mixed with sewage and greywater (i.e. sinks, showers, washing machines) to essentially ‘lubricate’ the waste so it can move freely at the rate engineered for the sloped gravity forced system. Sanitary sewers systems however, are much more sensitive and thus highly regulated from unnecessary water entering the sewer system between the home/business and the wastewater treatment plant. One reason for this is the high cost of wastewater treatment at the plant. Also when excessive rains occur, combined systems can fill up quickly and overflow into streams and groundwater, which is a considerable public health hazard. It’s for these reasons that the old way of having combined storm and sewer systems is becoming obsolete.
The most commonly used plastic sewer main pipe is made of PVC and is referred to as SDR-35 pipe, which stands for ‘Standard Dimension Ratio’. SDR is the inside diameter of the pipe divided by the wall thickness. Sanitary sewers are generally (but not always) deeper than their (often adjacent) storm sewer and water main counterparts. Their depth is typically between 6-20’ or sometimes even deeper. The most common depth is arguably 8-18’. The primary reason sanitary sewers are deeper is the fact that so many residences/business are part of the same gravity fed system typically leading to one particular outlet area. Compared to a storm sewer system which can often be broken up into many different isolated sections of pipe draining to different rivers, streams, oceans, or ponds. A sanitary sewer system however typically has many residence/business services ‘tapped’ into the same main and all the waste is travelling to one outlet location. One of the more common areas for a sanitary sewer main to be installed is somewhere underneath the road, though it may be found on the edges of or outside of a road.
The first consideration when it comes to installing plastic sewer main pipe is unloading the material from the truck. The pipe will typically come on a flatbed semi truck, bundled together in groups. The bundles should be lifted off the truck with a forklift, crane, or excavator strong enough to lift the weight of the bundles. If the pipes are delivered loose they will have to be removed one by one using caution not to scratch the pipe or lining. This is often done with textile slings strung through the center of the pipe. The pipes should be stored on a flat surface. A typical sanitary sewer pipe crew will consist of 2 operators, 2 laborers, and a foreman. The equipment required will be an excavator or backhoe and a loader. For shallow sewer mains, the crew size may be dropped to 4 laborers in some instances. For deeper sewers (15-20′ or more) with premium backfill required, a sixth worker often an operator, may be required with an additional excavator or backhoe, so one machine can be dedicated to excavation, and another compaction efforts.
Once the trench is dug, pipe bedding must be placed to receive the plastic water main pipe. There are typically 4 main reasons bedding is required for sewer mains: to provide for consistent support under the pipe bending lengthwise, to increase the loading strength of the pipe, spread loading pressure away from the joint and out more evenly among the length of each piece, and lastly to provide a platform for lining up and leveling the pipe. The most common materials used to bed plastic sanitary sewer main pipe is course uniformly graded aggregate. The typical gradation for course aggregate is: 100% passing 1/2” sieve, 85-100% passing 3/8” sieve, 10-30% passing 4.75mm sieve, 0-10% passing 2.36mm sieve, 0-5% passing 1.18mm sieve. Once the bedding is in place, the next process is to ‘string out’ the pipe along the sides of the trench. This process expedites installation once the actual pipe installation begins. The need to cut pipes may arise. If so, an accurate line must be traced around the pipe to provide a guide for the cutting tool. A good way to check that your cut marks are square is by measuring from the nearest bell or spigot end to several points along the mark assuring the distances are all equal. Always note the minimum pipe length requirements from the pipe manufacturer. PVC SDR35 sewer main pipe can be cut with a hacksaw, handsaw, or cut-off saw with an abrasive or steel blade. After the cut takes place, the rough ends should be smoothed/ground out particularly for connections. As sanitary sewer trenches are often quite deep, anything deeper than 5-6 feet will typically require a trench box. See trench box description for a full explanation. This is particularly true for sandy or gravelly soils which are likely to cave in as the trench becomes deep, especially with heavy equipment often operating directly above, and with wet soils. Also be prepared for the possibility of encountering known or unknown whole or fractured rock when excavating deep sanitary sewer trenches. Because of its toxicity, sewage containing sanitary sewer pipe will typically need to be horizontally 10’ away and vertically 18” away from water mains. If this is not possible, sometimes a carrier pipe, PVC pressure pipe, or concrete encasement may be utilized for a proper separation barrier.
Now the pipe laying can begin. Once a sufficient or desired amount of pipe is strung out along the trench, the excavator will begin lifting pipe with use of ropes, chains, or slings into the trench. Pipe layers or laborers will direct the operator from outside of and inside of the trench on where and when to place the pipe. SDR pipe is not as heavy as C-900 or concrete pipe so depending on the size, it may be placed into the trench without use of equipment. But considering the depth of sanitary sewer trenches and the weight of 8” and larger sdr-35 pipe, equipment is commonly used. SDR35 pipe is manufactured with a gasket already installed in the bell end of the pipe. The pipe being actively laid is then lifted again and the spigot end is slowly moved toward the bell end of the other piece by way of the operator in the excavator. The pipe layers will continue to direct the machine operator on moving the pipe up, down, left, and right to attempt to push the pipe home. If the pipes cannot be connected by hand strength and guidance alone, the spigot end of the pipe being laid can be gently pushed (with a protective barrier in between) with the excavator until the pipes are appropriately connected. Once the stretch of pipe run for that day is placed, it is typically backfilled that same day. Backfill is particularly important for sanitary sewer pipe trenches not only because the excavations are so deep but also because the majority of most sanitary sewer main runs are near or under roads where settlement can become a very big problem. At various locations along the sanitary sewer, tees or wyes receive service laterals from residences and businesses into the sewer main. Wyes are more common with sanitary sewers as they direct the flow of the service lateral with the flow of the main, easing the wastewater into the flow slowly and more gradually.