1.1: Conventional Treatment Review
Learning Outcomes
- Describe the difference between wastewater, stormwater, and combined collection systems
- Understand the general treatment process of wastewater
- Compare and contrast the different stages of wastewater treatment
Wastewater Collection
The wastewater collection system is the network of pipes that convey the wastewater from households and business to a wastewater treatment facility. Each customer will have a lateral connected to the main sewer lines. In order to keep sediment from settling out in the collection system and causing a blockage, the pipe is sloped to ensure a velocity of 2 ft/sec. A majority of the system will utilize the slope of the pipe and gravity so the water travels downhill. However, that is not always feasible and when needed a lift station will be installed. The lift station is comprised of a wet well where the wastewater is collected and pumps the water to a higher elevation where it can then resume to flow by gravity. The portion of the pressure pipe that is connected to the lift station is called a force main. The force main is always under pressure and the wastewater completely fills the pipe. Where a gravity sewer has minimal pressure and under normal conditions only about ⅓ of the pipe is filled with wastewater.
There are three different types of sewer systems; sanitary, stormwater, and combined. Sanitary sewer systems only convey wastewater that was derived from sanitary sources. This includes 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 direct 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.
Preliminary Treatment
Preliminary treatment is the first step in treating raw wastewater. When the wastewater first enters the wastewater treatment facility there are a lot of nuisance materials that have found their way into the collection system. Items such as large rags, bottles, tree branches, and numerous other nuisance items can be found in the influent to the treatment facility. These large items can cause damage to downstream pumps, take up valuable space in settling tanks, and can be hazardous to other mechanical equipment needed in the treatment process. So the first, or preliminary step is to remove these large items.
Methods used in preliminary treatment include screening, communition, and grit removal. A wastewater treatment facility may use one or all of these methods to handle the large items that may enter the treatment facility. Bar screens are capable of removing items that are larger than the spacing between the bars. For example, a ¾” bar screen will hold back any debris that is larger than ¾” and anything smaller will pass through it. Common items that are removed in this preliminary treatment step are rags, roots, large rocks and aggregate, bottles, cans, and numerous other large objects that can make their way into the wastewater collection system.
An alternative to screen these large debris items is to shred or grind them. A comminutor is a device that sits inside the channel where wastewater is flowing into the treatment facility. The comminutor will grind the large debris items, turning them into smaller items. Communitors are designed to produce a solid size of a certain diameter. By breaking up the debris into smaller diameters, the downstream pumps and equipment will not be as impacted.
Smaller inorganic solids, such as coffee grounds, eggshells, sand, silt, and gravel are collectively called grit. Grit must be removed because it will cause excessive wear on plant equipment such as the impeller of a pump. Also, this inorganic material can settle in the subsequent treatment process and take up valuable space in tanks which decreases plant efficiency and can inhibit further treatment. While most gritty materials are inorganic, large organic solids such as corn kernels and other food waste may also be removed. There are several methods in removing grit. One common method is an aerated grit chamber. Aerated grit chambers will have air piped to diffusers at the bottom of the tank. The addition of air in the tank creates a rolling action of solids which helps keep the lighter organic solids in suspension while the heavier grit material is directed to the bottom of the tank. In aerated grit chambers, the amount of air sent to the chamber is a critical operating parameter. If too much air is supplied, then the grit material will stay in suspension and not be removed. If not enough air is supplied, then the lighter organic material can settle out.
Primary Treatment
The primary goal of sedimentation is to remove the settleable solids. A well-operated primary sedimentation tank can remove around 90% - 95% of settleable solids. There will also be a reduction in total suspended solids and a slight reduction in BOD 5 . The sedimentation process works because these solids are heavier, relative to the wastewater, and will, therefore, settle to the bottom of the tank. Another phenomenon that occurs in the sedimentation process is that as the solids collect at the bottom of the tank, the weight of the solids begin to compact and compress. This causes the solids to be thickened and have slightly less water content. Detention time, or how long the wastewater takes to travel through the tank, is a critical design parameter of primary sedimentation tanks. There needs to be enough time to allow the solids to settle but not so much time that the solids start to decompose. Decomposition will cause gas bubbles to form which can hinder solids settling and create foul odors.
Primary sedimentation tanks can either be circular or rectangular. Regardless of the configuration, the tanks will have similar components. At the inlet structure where the wastewater enters the tank, the velocity is typically high in order to prevent solids settling in the piping network as the wastewater comes from the preliminary unit process to the primary tank. Once in the primary tank, the velocity must be slowed. To accomplish this there will be some type of diffuser at the inlet end that will redirect flow and prevent short-circuiting. The dimensions of the primary sedimentation tank must be able to accommodate the flow of wastewater but must also reduce the velocity.
In addition to settling solids, primary tanks will also remove floatables. Typically, these floatables are classified as fats, oils, and grease (FOG). A rectangular primary tank will have flights that span the width of the tank. They are connected by a chain that is motor driven to slowly move with the flow of wastewater. The flights provide several functions. They prevent short-circuiting of material on the surface, they convey the FOG on the surface to a collection trough at the end of the tank, and they convey the settled solids to a hopper at the beginning of the tank. Circular tanks have a similar mechanism called a swing arm that provides the same functions.
Once the solids have settled to the bottom of the tank, they are conveyed to a hopper in the tank. Circular tanks are typically coned at the bottom so the solids build up in the center. Rectangular tanks have a hopper at the front of the tank and the flights convey the solids there. The solids must be removed from the tank periodically so they do not cause adverse conditions. There is organic matter in the solids and if it starts to decompose, it will create foul odors and gas bubbles that will hinder other solids from settling. Typically incoming wastewater is around 1% solids. Due to the sedimentation process, the percent solids concentration in the sedimentation tanks increases to around 4% to 8%. Due to the high amounts of solids, a standard pump cannot be used. Instead, special pumps including a progressive cavity type pump or a stator/rotor pump are commonly used.
Secondary Treatment
Secondary treatment methods typically involved some form of biological treatment to further reduce the amount of BOD 5 . Trickling filters and activated sludge treatment plants will utilize a secondary clarifier to separate the treated wastewater from the microorganisms of the biological treatment step.
A key component of a trickling filter is the recirculation of the clarified effluent back to the trickling filter. Water from the secondary clarifiers has already gone through the biological treatment process of the trickling filter and has a lower amount of BOD 5 . By recirculating this treated water with the incoming wastewater, it will dilute the incoming BOD 5 . Recirculation will also be able to control the dissolved oxygen level in the trickling filters.
A secondary clarifier is also a key component of the activated sludge system. Not only does it separate out the microorganisms from the now treated wastewater, but it will also concentrate them through the sedimentation process. A secondary clarifier works exactly as discussed previously in primary sedimentation. The only difference is that in a primary sedimentation tank the main goal is to remove unwanted solids. In a secondary sedimentation tank, the goal is to concentrate the Mixed Liquor Suspended Solids(MLSS) so it can be returned to the aeration tanks.