1.3: Wastewater Collection
Learning Outcomes
- Examine how wastewater is conveyed from the source to a wastewater treatment facility
- Assess the impacts and causes of odor in a collection system and explain how to mitigate the negative impacts for a community
- Analyze how gravity sewer design is used to maintain sufficient velocity
- Evaluate system conditions to determine when a sewer pump station is needed
- Differentiate between the various methods to maintain and clean sewer systems
Gravity Sewers
The main purpose of a wastewater collection system is to convey the wastewater from the source to a treatment facility. Recall from Chapter 1 that the source of wastewater contains anything and everything that goes down the drain. There are residential sources such as homes, apartments, and office buildings. And, there are industrial sources like restaurants and manufacturing processes. These sources will have laterals that are connected to a main sewer line. Laterals are usually privately owned and the maintenance on them is the responsibility of the property owner. The lateral is a direct connection from the source to the main sewer line. The main sewer line is typically owned, operated, and maintained by a public utility. The main line is several sizes larger than the laterals as it will be required to have enough capacity to accommodate all of the laterals that are being discharged into it. In larger cities, there may also be interceptors or trunk lines. These larger trunk lines collect all of the main lines through the city and send the wastewater to the wastewater treatment facility.
Whenever possible the collection system will utilize gravity to send the wastewater from a higher point to a lower point. To achieve gravity flow, the main sewer lines are designed and constructed on a slope. The amount the line is sloped is dependent on the size of the pipe and the expected quantity of wastewater. The collection system must have a minimum wastewater flow velocity of 2 ft/second (fps). At this velocity, solids will be kept in suspension. Slower velocities will allow settling in the piping collection system. If solids settle in the collection system, these depositions will accumulate over time and can cause a blockage. If the blockage becomes too severe, then wastewater will not be able to flow freely. If the blockage is large enough, it can cause the wastewater to back up and overflow onto the streets.
Pressure or Force Mains
Gravity sewer lines cannot be used in every situation. Areas that are very flat or communities that are located in a valley will require a lift or pump station. A lift station uses pumps to lift the wastewater from a lower elevation to a higher elevation. The section of pipe from the discharge to the gravity sewer connection is known as a force main. Unlike a gravity sewer, a force main is completely full and under pressure. The pressure comes from the energy of the pump which is needed to lift the wastewater to a higher elevation.
Different Types of Sewers
There are three different types of sewer systems; sanitary, stormwater, and combined. Sanitary sewer systems only convey wastewater that was derived from sanitary sources. As discussed in Chapter 1, this is wastewater from household toilets, showers, and dishwashers, as well as industrial sources of wastewater from manufacturing processes. Sanitary sewers differ from stormwater sewers in that they contain fecal matter from human waste. It’s paramount that these wastes are conveyed to a wastewater treatment facility so they can be removed and stabilized to protect public health and the environment.
Stormwater sewers are a network of pipes that collect only stormwater runoff and directs the flow to a nearby waterbody or the ocean. While stormwater does have a direct connection to human or animal waste, it is considered less harmful and can be discharged without treatment. However, it’s important to understand that stormwater is by no means “fresh water”. Stormwater can have large amounts of trash, plant material, silt gravel, oil & grease. There are even fairly high amounts of harmful bacteria from animal wastes. The theory is that during storm events there is a significant amount of water flowing through these systems that these contaminants become diluted and are not as concentrated. This theory is constantly being challenged and stormwater is now being seen as another water source that can be treated and even beneficially reused.
A combined sewer is a network of pipes that conveys both sanitary wastes as well as stormwater. This can be beneficial during dry weather flows where there is minimal stormwater. The stormwater that does exist is sent to a wastewater treatment facility where harmful contaminants are removed prior to discharge to a waterbody. However, combined sewer systems can be overwhelmed during storm events. Systems that have older infrastructure which has not been upgraded to deal with larger populations and storm events are especially vulnerable. When this happens, instead of only diluted stormwater being sent to a waterbody, sewage containing high amounts of fecal matter from the sanitary sewer is also discharged. This can cause increased pollution to the waterbody.
Infiltration and Inflow
Collectively referred to as I & I, infiltration and inflow is wastewater that has entered into the collection system unregulated. An inflow is when water other than sanitary wastewater enters the collection system through an illicit connection such as rain gutters, basement drains, or foundation drains. Infiltration is water such as groundwater and stormwater that finds its way into the sewer piping from manhole covers and cracks in the piping.
Odor Control
Sewage can have an extremely foul odor. As the organic material starts to break down in the sewer system, it will generate foul odors. Often, this breakdown of organic material and the biological reduction of sulfates in the wastewater will create hydrogen sulfide (H2S). H2S has a very distinctive rotten egg smell and can cause several problems in a wastewater collection system. H2S is extremely toxic and workers working near or inside a wastewater collection system must continuously monitor the atmospheric conditions and properly ventilate the workspace to ensure that the environment is free of H2S and other hazardous atmospheric conditions. H2S will also breakdown in the collection system forming sulfuric acid which can corrode pipes, manholes, and other parts of the system. Although H2S is heavier than air, it is still able to escape the collection system and cause nuisance odor conditions to the surrounding community.
These sewer gases can be mitigated in several different ways. One way is to add chemicals to the collection system that will prohibit the formation of H2S. For example, iron salts such as ferric chloride will react with the sulfides in the wastewater and do not allow the molecules to form H2S. Another way to remove H2S is to let it form in the sewer system and then have a fan that will pull the foul sewer gases from the sewer and convey the odors to a treatment site. Typically, the air treatment is achieved by carbon adsorption or bio-filtration. Carbon in this filter form is extremely porous when examined under a microscope. When the foul sewer gases are forced through a media of carbon particles, the gasses will adhere to these microscopic pores and be removed from the airflow. In a bio-filtration setup, the sewer gases are conveyed to a large tank where bacteria will metabolize the H2S and thus remove it from the airflow.
Collection System Maintenance
Collection systems, while underground and out of sight from the general public, still require a significant amount of maintenance to ensure that the wastewater flowing through the system stays underground. If the wastewater cannot flow through the network of pipes, it will begin to back up and overflow out of the manhole covers in the streets. When this happens, it is called either a Sanitary Sewer Overflow (SSO) or a Combined Sewer Overflow (CSO) depending on which type of wastewater collection system is used within the community. To prevent this from happening, wastewater professionals have several tools available to them.
Bucketing, rodding, flushing, jetting, and closed-circuit television are the primary means used to maintain a wastewater collection system. Flushing is a hydraulic process that is great for light cleaning of wastewater lines. Flushing moves a high volume of water with low pressure through the wastewater line that needs to be cleaned. Adding this extra water increases the velocity in the line. At high velocities, the flushing water scours the sediments and debris in the line and pushes everything downstream.
Rodding and bucketing are both mechanical mechanisms that use machinery to physically remove any obstructions and debris from the wastewater pipeline. This is typically used when flushing is insufficient to remove the debris. Rodding occurs when a cable with a special attachment at the end of it is sent down the wastewater pipe that needs to be cleaned. The rod is then rotated and the tool can break through the obstruction in the line.
Jetting combines both hydraulic and mechanical tools. Jetting is similar to rodding except a heavy-duty hose that can handle high pressures is used instead of a rod. At the end of the hose, there is a nozzle specifically designed to send concentrated streams of water at high pressures against the interior wall of the pipe to remove any debris or obstructions.