What are the stages of wastewater treatment?
Stages of Wastewater Treatment
Collections System
All waste generated from society, such as garbage and recycled goods, is collected and managed with various processes to protect the environment. Wastewater is no different. A wastewater collection system is a series of pipes and lift stations that bring used water from houses, businesses, or industries to the wastewater treatment facility. Civil engineers design collection systems in a way that allows gravity to do the work, using the laws of physics to direct water to the wastewater treatment facility. However, in many instances, lift stations are installed in strategic areas to pump water to a higher elevation to support this gravity-led process. Depending on the design, municipalities can either be sanitary sewer systems that transport strictly wastewater or combined sewer systems that collect both stormwater and wastewater. Combined sewer systems often bring additional difficulties, such as high flows, to wastewater treatment.
Headworks
The collection system brings wastewater to the beginning of the treatment facility, also known as the headworks. The headworks are responsible for removing large debris and heavy solids from the water, such as wood, rags, plastics, sand, gravel, grease, and personal hygiene products. This stage prepares the wastewater for the downstream stages by removing solids that may damage pumps and other equipment. Specialized pumps bring raw wastewater from influent wells up to a series of bar screens and grit removal chambers. These two common pieces of equipment will help remove a portion of the incoming solids (total suspended solids, TSS) and organic material (biochemical oxygen demand, BOD). Sensors for TSS, TDS (total dissolved solids), turbidity, and pH can be used to monitor incoming wastewater for EPA violations and loads that may be dangerous to downstream biological processes.
Primary Treatment
After the larger debris is removed, the organic and inorganic solids can be removed in a process called primary treatment, reducing influent BOD by 25-40% and influent TSS by 50-70%. Circular or rectangular clarifiers are most often used to separate solids from the water via gravity separation. The speed of flow in these tanks allows solids to settle at the bottom of the tank to form sludge, which is then pumped to another section of the treatment facility for biosolids treatment. The separated water then flows over the weirs at the surface of the clarifiers and onto the next treatment step. The biosolids collected from both primary and secondary treatment are collected, reduced in volume, and then either disposed of or created into fertilizers. Before the Clean Water Act, many wastewater treatments plants only utilized primary treatment. However, secondary treatment has become a requirement with the increase in industry and city population.
Secondary Treatment
With endless tank configurations and the utilization of live microorganisms, secondary treatment is perhaps the most complex stage of wastewater treatment. In this stage, dissolved pollutants like organic matter or nutrients are removed using the biological processes of certain microorganisms (bacteria, protozoa, etc.). Different environments can be created within large tanks with high solids (i.e., activated sludge basins) to accumulate the desired type of microorganism. Aeration is added to some tanks to provide oxygen and grow bacteria which remove organics and ammonia, while aeration can be purposefully limited to assist with nitrogen or phosphorus removal. The number and order of these tanks are configured to meet goals specific to the facility. After the activated sludge basins, another set of clarifiers is used to separate the activated solids from the water via secondary clarification.
This location utilizes the most online sensors to monitor the activated sludge system. Sensors for DO (dissolved oxygen), TSS, ammonium, nitrate, ORP (oxidation-reduction potential), pH, and orthophosphate monitor the conditions within each tank. Whether the environment is aerobic (oxygen-rich), anoxic (low oxygen, nitrate available), or anaerobic (no oxygen, no nitrate), these parameters are maintained at optimum levels. In addition, sensors are used to automatically control pumps, aeration, and chemical dosing. DO and ammonium sensors can control aeration to provide precise treatment. Similarly, online measurements of TSS, nitrate, and orthophosphate are used to control pumps and chemical dosing to assist with solids control and nutrient removal. Carbon parameters, such as BOD, COD, and TOC are commonly monitored as they are quantifications of the organic material in wastewater. BOD (biochemical oxygen demand) is the current NPDES standard for this measurement, however, COD (chemical oxygen demand) and TOC (total organic carbon) can be substituted for BOD, if a long-term correlation with BOD (COD:BOD or TOC:BOD) has been established.
Tertiary Treatment
At this point, solids and dissolved pollutants are mostly removed from the water. However, a final treatment step called tertiary treatment can include several processes to disinfect the water or further remove pollutants to achieve very stringent treatment goals. Depending on the body of water, wastewater facilities may have strict discharge limits to further protect the body of water. For example, discharge limits near freshwater lakes have very low total phosphorus (TP) limits to limit eutrophication in freshwater systems, while coastal wastewater facilities will have low total nitrogen (TN) limits in a saltwater system.
Disinfection is an important step to kill or deactivate pathogenic organisms before they return to the environment. Chlorine and UV disinfection are the most common methods, both of which damage the cell walls and eliminate the ability of pathogens to reproduce. Chlorine disinfection has been used for decades to disinfect wastewater. Many forms of chlorine can be dosed at the beginning of contact tanks, providing enough time for the chlorine to kill bacteria and dissipate or dechlorinate before the final effluent. Chlorine analyzers monitor chlorine levels after the contact tanks to ensure enough chlorine is dosed, but also to ensure low amounts of chorine are discharged. UV disinfection systems are becoming more prevalent as they do not require storing and dosing hazardous chlorine chemicals. UV banks emit light in a series of channels to deactivate bacteria before discharge. UVT% sensors can control the UV banks to dose the exact amount of light required to achieve the required disinfection.
Tertiary treatment can also include another filtration step utilizing processes like sand filtration, membrane filters, or disk filters, which can remove solids and, thus, other pollutants down to extremely low levels.