1.5: Primary Treatment
Learning Outcomes
- Describe different methods to measure wastewater flow
- Examine the purpose of primary treatment
- Compare and contrast conventional primary treatment with chemically enhanced primary treatment
- Assess how specific gravity and density relates to the removal of solids able to settle
Flow measurement
After preliminary treatment, and all of the large debris and inorganic solids are removed, it’s vital to measure the amount of wastewater flowing into the treatment facility. This flow rate will be an important number for process control. Many wastewater treatment plants have a typical diurnal flow pattern as flow into the wastewater treatment plant is not constant. Over a 24 hour period, the flow can fluctuate significantly. There are usually two peaks during the day, typically in the late morning and evening. In the morning, the population that the treatment facility serves is waking up, taking showers, making coffee, cooking breakfast, and using the restroom. The flow tends to lessen in the late afternoon and then increases again in the evening. In the evening, a majority of the population is cooking dinner, washing dishes, doing laundry, and taking showers. In the middle of the night, most people are sleeping and not sending water down the drain.
There are several ways that flow can be measured. A simple way to measure flow is by using a weir. Weirs can either be square or V-notched, but the principle behind them is the same. As the flow of wastewater travels through a channel, the weir is placed in its path. The wastewater is then forced to move over the crest of the weir. Based on the dimension of the square or v-notched section in the weir, the flow can be determined by the height of the wastewater over the weir crest. If there is a significant amount of flow, then the height of wastewater will also be large. When the flow decreases, the height of wastewater over the weir will be lessened.
A Parshall Flume is another way to determine the flow rate entering or leaving the treatment facility. It works similarly to a weir in that the water is forced through a restriction in the channel and the height of water before and after the restriction is used to calculate a flow rate. All Parshall Flumes have a similar shape. There is a converging section, a throat section, and a diverging section. The converging section is where the flow is channeled and forced through a narrow section. The throat section is where the wastewater flow is restricted by traveling through a slimmer channel. This restriction will increase the velocity of the wastewater as it passes through the throat section and cause the level to rise in the converging section. Just like in a weir, higher flow rates will cause the wastewater level to increase and lower flow rates will show a decrease in the wastewater level. Many treatment plants have markings in the converging section to indicate the wastewater level. Modern treatment plants will have ultrasonic level transmitters that will record the level precisely. This data can be automatically transferred to a computer system where the flow rate is instantly calculated and recorded. A benefit of using the Parshall Flume instead of a weir is that the restriction in the channel will not cause any solids to settle out. In a weir, as water builds up behind it, the velocity is reduced and solids can settle out. Those solids will have to be removed periodically or odors can become problematic.
The most modern method of monitoring flow throughout a treatment facility is a magnetic flow meter, or “mag meter” for short. Mag meters work by having the wastewater flow through a pipe. The mag meter is connected to the pipe either by strapping the device to the outside of the pipe or more permanently, by bolting it between two sections of pipe. The device creates a magnetic field inside the pipe and as the water moves through, it creates a disturbance that is measured by the meter. Higher flow rates cause more of a disturbance. The meter can very accurately measure the flow of wastewater moving through the pipe. The benefit to mag meters is that there are no moving parts inside the pipe. So influent wastewater with lots of solids and organics will not interfere with the flow measurement. Mag meters are also very reliable and require little maintenance.
Some treatment facilities may have flow equalization. Recall that the incoming wastewater has an inconsistent flow rate that fluctuates drastically throughout a 24-hour period. This change in flow will also result in changing detention times which many of the treatment processes are dependent on. A treatment plant that utilizes flow equalization will attempt to even out the flow so it is consistent throughout the treatment plant all the time. By looking at historical data the plant operators will have a good idea of what the average flow rate for the plant is. This average flow rate is the target of the flow equalization. When the incoming wastewater is greater than the average flow, the excess amount will be directed towards a holding basin. When the flow is less than the average, typically in the middle of the night, the wastewater is pumped from the holding tank into the treatment facility. This can also reduce energy cost as day time pumping will be reduced. Energy costs are usually higher during peak periods. Some treatment facilities have flow equalization at the end of the treatment process. They may have restrictions in their discharge permit which may limit the flow rate leaving the facility. Other treatment plants don't have any flow equalization.
Sedimentation
After the incoming flow rate is measured the next step in the treatment process is primary sedimentation. 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 BOD5. Recall that settleable solids are the large solids in the wastewater and are measured by an Imhoff cone in the laboratory. The sedimentation process works because these solids are heavier, relative to the wastewater, and will, therefore, settle to the bottom of the tank. These tanks are referred to as Primary Clarifiers or Primary Sedimentation Basins. A clarifier is a tank or basin where the sedimentation process will occur. The rate at which the solids will settle is determined by Stokes Law, which takes into account the size of the solid particle, the specific gravity of the particle, and the specific gravity of the liquid. Specific gravity is a unitless number and is a measure of density relative to a reference liquid. For this book when talking about specific gravity, we will assume water is the reference liquid and it has a specific gravity of 1. When determining how quickly solids will settle in the sedimentation tank, the specific gravity of the solids will be a significant factor. If the solids have a specific gravity less than 1, they will not settle at all, in fact, they will float on the water. If the specific gravity is only slightly greater than one, the solids will settle but at a much slower rate than another particle with a larger specific gravity. This is fairly intuitive. Clearly the heavier the solids the quicker they will settle.
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.
Flotation
As discussed earlier, the specific gravity of the material in the wastewater will determine how it interacts with the wastewater in the sedimentation tank. In addition to settling solids, primary tanks will also remove floatables. Typically, these floatables are classified as fats, oils, and grease (FOG). The specific gravity of these materials is less than 1 so they will rise to the top of the sedimentation tank and be removed. 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.
Sludge Removal
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 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.
The stator/rotor pump has a stationary part, the stator, and a rotating element, the rotor. Both have a corkscrew type shape and the rotor moves within the stator. As it rotates, it is passing the mixture of solids and water progressively through the pump. These pumps are designed to be able to handle the abrasive nature of the solids and not clog up. The solids are sent to an anaerobic digester where they are further treated and stabilized. Eventually, they will be dewatered and sent to a landfill for disposal.
Chemical Addition
To further enhance the sedimentation process, chemicals can be added. These chemicals will adhere to the solids and increase their specific gravity. With a higher specific gravity, the solids will settle more quickly. This can either be done to increase the amounts of solids and BOD5 or can be used to achieve average results in a smaller footprint. This process is referred to as chemically enhanced primary treatment (CEPT). Typically, ferrous or ferric chloride is used as the chemical aide. Other chemicals such as polymers may also be used. Polymers make the solids clump together which subsequently results in quicker settling in the sedimentation tank.
This process is not used as often anymore due to increasing regulatory requirements. Early in the wastewater industry, a treatment plant could have removed enough solids and BOD5 using the CEPT process. But as regulations became more stringent, many utilities upgraded to secondary treatment systems. This secondary biological treatment process is dependent on the BOD5. If too much is removed in the primary process, it will adversely affect treatment.