Blower selection is an important decision when designing a new wastewater treatment plant or upgrading existing facilities. The proper selection of blower equipment for the application, as well as purchasing from a reputable supplier, can save thousands of dollars in energy, repairs and maintenance costs. This article provides an overview of the blower technologies available for wastewater treatment, and discusses the advantages and limitations of each technology in various wastewater treatment applications.
Take a look at a municipal wastewater treatment plant near a textile factory in Asia (Figure 1). Is there any question as to why we treat industrial wastewater? The use of blowers for aeration and mixing accounts for 40 to 75 percent of the total energy consumption in a wastewater treatment plant. For this reason, the specification of the correct technology and size for each blower application is of paramount importance.
There are four stages of wastewater treatment:
- Pretreatment typically involves some type of filtration or screening.
- Primary treatment can be accomplished by either dissolved air flotation (DAF) or sedimentation.
- Secondary treatment is most commonly accomplished by the activated sludge process, and this article covers several other methods of secondary treatment in addition to the activated sludge process.
- Tertiary treatment is not always required, but can be accomplished by additional filtration, advanced oxidation, UV disinfection or a combination of these methods depending on the quality of effluent required.
This article focuses on secondary wastewater treatment methods that utilize blowers in their processes.
Secondary wastewater treatment applications
The most commonly used secondary treatment processes include the activated sludge process, aerated lagoon, membrane bioreactor, moving bed biofilm reactor, sequential batch reactor, and anaerobic digestion.
The Activated Sludge Process. Developed in the United Kingdom in the early 1900s and currently utilized in medium- to large-scale industrial plants, the activated sludge process separates the flocculants (i.e., suspended solids) from the wastewater through sedimentation. Effluent enters the aeration tank or lane, and low pressure air is introduced through a grid of diffusers. Water usually passes through the process in a few hours, while the sludge retention rates vary from a few days in warmer climates, to a few weeks in colder weather.
Lagoon Aeration. Similar to the activated sludge process, lagoon aeration is typically used in rural areas with small- to large-sized plants. A series of shallow earthen basins (lagoons) act as the aeration basins and holding tanks. Although lagoons are often equipped with surface aerators, there are several diffuser systems available specifically for these applications. A well-designed diffused air system uses approximately half as much energy as surface aeration.
Membrane Bioreactor (MBR). The MBR process uses additional filtration to produce a higher quality effluent and is a variation of the activated sludge process. Common in medium- to large-scale plants, the MBR features an ultrafine membrane filter installed at the discharge end of a standard aeration basin. Pumps installed on the membrane filters create a slight vacuum and pull the effluent through the membrane. The ultrafine holes in the membrane do not allow the biological microbes to pass through the membrane, keeping the microbes in the aeration basin. The membrane becomes clogged or fouled in this process and requires more frequent cleaning.
Moving Bed Biofilm Reactor (MBBR). The MBBR is a much more compact method of wastewater treatment that can be scaled to fit any size plant. In this method, the reactor is filled with thousands of biofilm carriers (plastic balls) that protect the bacteria used to breakdown pollutants in the wastewater. A diffuser grid produces the air required to effectively disperse the biofilm carriers throughout the basin, while also providing the necessary aeration for biofilm growth.
Sequencing Batch Reactor (SBR). The sequencing batch reactor (SBR) is also a very compact process typically used in small- to medium-sized plants. A small SBR consists of two equal-sized tanks that alternate between treatment stages. The process begins with the filling of one of the tanks; the wastewater is then mixed during the anaerobic stage using a jet aerator or mechanical pump. During the aerobic stage, the wastewater is aerated using a diffused air system. Next, the wastewater is allowed to settle so that the sludge can be separated from the effluent during the sedimentation stage. Finally, the effluent is drained in the decanting stage and the reactor is in idle. The idle stage can be used to perform cleaning or remove excess sludge from the reactor. As soon as the first tank is filled, wastewater is diverted to the second tank and the process begins again. The multiple tank (reactor) design allows the plant to operate continuously and larger plants use more reactors to accommodate higher loads.
Anaerobic Digestion. Anaerobic digestion is used to treat the sludge created by the wastewater treatment process, as well as other biological waste. By definition, anaerobic digestion does not use air; instead, a series of microorganisms break down solid waste into methane and carbon dioxide gas, in addition to nitrogen, ammonia, and hydrogen sulfide in smaller quantities. This biogas is extracted from the reactor and either flared to reduce greenhouse gas emmissions, or treated in a series of filters before being injected into a generator. Once injected into a generator, the biogas can be used for on-site electricity and heat generation, also known as a cogeneration system. Depending on the composition and amount of waste in the digester, this process can produce a substaintial amount of energy that can be used for plant operations.