Sewage Treatment
Sewage is wastewater discharged from a home, business, or industry. Sewage is treated to remove or alter contaminants in order to minimize the impact of discharging wastewater into the environment. The operations and processes used in sewage treatment consist of physicochemical and biological systems.
The concerns of those involved in designing sewage treatment systems have changed over the years. Originally, the biochemical oxygen demand (BOD) and total suspended solids (TSS) received most of the attention. This was primarily because excessive BOD and TSS levels could cause severe and readily apparent problems, such as oxygen deficits that led to odors and fish kills, and sludge deposits that suffocated benthic organisms. By removing BOD and TSS, other contaminants were also removed and other benefits were realized; so even today, some discharge permits contain only limits for BOD and TSS. However, many permits now contain limits on other contaminants as well, and these limits, as well as other requirements, are constantly changing.
Among the first contaminants to be added to the requirements for discharge permits were nutrients. The most commonly regulated nutrients are phosphorus and nitrogen. Originally removing phosphorus and nitrogen could only be done through expensive, advanced methods. But scientists have recently discovered ways to accomplish enhanced removals of nutrients in conventional biological treatment plants with relatively minor operational and structural adjustments.
The most recently regulated pollutants are toxicants. There are regulations for specific toxic agents, and there are the generic-type regulations, which specify that the toxicity to certain test organisms should not exceed a certain level. For example, the wastewater discharged from a particular municipality may be restricted from killing more than 50 % of the Ceriodaphnia in an aquatic toxicity test. The municipality would not need to determine what is causing the toxicity, just how to minimize its effects. Efforts to understand the causes of toxicity are referred to as toxicity reduction evaluations. The generic limit can therefore sometimes turn into a more specific standard, in the view of the municipality or industry, when the identity of a toxicant is determined; the general regulatory limit might remain, but treatment personnel are more cognizant of the role that a certain pollutant plays in overall, effluent toxicity.
The systems used to treat sewage can be divided into stages. The first stage is known as preliminary treatment. Preliminary treatment includes such operations as flow equalization, screening, comminution (or grinding), grease removal, flow measurement, and grit removal. Screenings and grit are taken to a landfill. Grease is directed to sludge handling facilities at the plant.
The next stage is primary treatment, which consists of gravity settling to remove suspended solids. Approximately 60 % of the TSS in a domestic wastewater is removed during primary settling. Grease that floats to the surface of the sedimentation tank is skimmed off and handled along with the sludge (known as primary sludge) collected from the bottom of the tank.
The next stage is secondary treatment, which is designed to remove soluble organics from the wastewater. Secondary treatment consists of a biological process and secondary settling. There are a number of biological processes. The most common is activated sludge, a process in which microbes, also known as biomass, are allowed to feed on organic matter in the wastewater. The make-up and dynamics of the biomass population is a function of how the activated sludge system is operated. There are many types of activated sludge systems that differ based on the time wastewater remains in the biological reactor and the time microbes remain there. They also differ depending on whether air or oxygen is introduced, how gas is introduced, and where wastewater enters the biological reactor, as well as the number of tanks and the mixing conditions.
There are also biological treatment systems in which the biomass is attached. Trickling filters and biological towers are examples of systems that contain biomass adsorbed to rocks plastic. Wastewater is sprayed over the top of the rocks or plastic and allowed to trickle down and over the attached biomass, which removes materials from the waste through sorption and biodegradation. A related type of attached-growth system is the rotating biological contactor, where biomass is attached to a series of thin, plastic wheels that rotate the biomass in and out of the wastewater.
It is important to note that each of the above biological systems is aerobic, meaning that oxygen is present for the microbes. Anaerobic biological systems are also available in both attached and suspended growth configurations. Examples of the attached and suspended growth systems are, respectively, anaerobic filters and upflow anaerobic sludge blanket units.
The end-products of aerobic and anaerobic processes are different. Under aerobic conditions, if completely oxidized, organic matter is transformed into products that are not hazardous. But an anaerobic process can produce methane (CH4), which is explosive, and ammonia (NH3) and hydrogen sulfide (H2S), which are toxic. There are thus special design considerations associated with anaerobic systems, though methane can be recovered and used as a source of energy. Some materials are better degraded under anaerobic conditions than under aerobic conditions. In some cases, the combination of anaerobic and aerobic systems in a series provides better and more economical treatment than either system could alone. Many substances are not completely mineralized to the end-products mentioned above, and other types of intermediate metabolites can be considered in selecting a biological process.
Biomass generated during biological treatment is settled in secondary clarifiers. This settled biomass or secondary sludge is then piped to sludge-management systems or returned to the biological reactor in amounts needed to maintain the appropriate biomass level. The hydraulic detention time of secondary clarifiers is generally in the area of two hours.
As mentioned above, biological systems are designed on the basis of hydraulic residence time and sludge age. In a conventional activated-sludge system, sewage is retained in the reactor for about five to seven hours. Biomass, due to the recycling of sludge from the secondary clarifier, remains in the reactor, on average, for about ten days.
Disinfection follows secondary clarification in most treatment plants. Disinfection is normally accomplished with chlorine. Due to the potential environmental impact of chlorine, most plants now dechlorinate wastewater effluents before discharge.
Some facilities use another stage of treatment before disinfection. This stage is referred to as tertiary treatment or advanced treatment. Included among the more commonly used advanced systems are adsorption to activated carbon, filtration through sand and other media, ion exchange, various membrane processes, nitrification-denitrification, coagulation-flocculation, and fine screening.
The treatment systems used for municipal sewage can be different from the systems used by industry, for industrial wastes can pose special problems which require innovative applications of the technologies available. Additionally, industrial wastes are sometimes pretreated before being discharged to a sewer, as opposed to being totally treated for direct discharge to the environment.
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