Pretreatment units for municipal wastewater may include: coarse screening (bar screen), medium screening, crushing, flow measurement, pump lifting, slag removal, pre-aeration, flotation, flocculation, and chemical treatment.
Domestic wastewater treatment generally does not use flotation, flocculation, or chemical treatment. The adoption of these methods sometimes depends on the industrial wastewater in the municipal wastewater. Flotation is used to remove fine suspended solids, grease, and fat, either in a separate unit or in a pre-aeration tank for grease removal and sometimes slag removal. If the petroleum industry and meat processing plants have appropriate pretreatment, municipal treatment plants may not need a flotation unit. High-intensity municipal wastewater may use flocculation with or without chemical agents to improve primary treatment efficiency and prevent overloading of secondary treatment processes. Sometimes, chlorination is added to the raw wastewater to control odor and improve sedimentation properties. Variations in the layout of pretreatment units depend on the characteristics of the raw wastewater, the subsequent treatment process, and the pretreatment units used. There are some general principles that are frequently applied to unit layout. The grit chamber is used to protect the pumps and prevent scale buildup in the sedimentation tank or metering tank. Small treatment plants typically place a Parshall metering tank before the constant-speed lift pump. In large treatment plants or where variable-speed pumps are used, the metering tank can be placed after the pump. In most independent domestic wastewater treatment plants, the sedimentation tank is placed after the lift pump, but when a large sludge load is anticipated, the sedimentation tank should be placed before the pump.
Primary Treatment Unit: Primary treatment is sedimentation; however, the commonly accepted term "primary treatment" includes pretreatment processes. All large city treatment plants use raw wastewater sedimentation, which must be placed before conventional biological filters. Unsedimented raw wastewater can be treated using the fully mixed activated sludge process; however, due to sludge disposal and operating costs, this process is only used in small towns.
Secondary Treatment Unit: Biological secondary treatment uses the activated sludge process, biological filters, or stabilization ponds. In new wastewater treatment plant designs, high-load biological filters have widely replaced low-load biological filters, and the fully mixed sludge process is replacing the conventional activated sludge process. Stabilization ponds are generally limited to use in small towns. In large-scale treatment plants, high-load biofiltration and completely mixed activated sludge are currently the two most common methods used for secondary treatment. Biofilters are advantageous because they are easy to operate and can withstand sudden load changes and overloads without complete failure. Completely mixed activated sludge processes can withstand sudden load changes but will fail under prolonged overload. For example, when the BOD load of a biofilter increases from the design load of 45 psi (45 lb/1000 ft³/day) (R=1) to 90 lb/1000 ft³/day (R=2), the efficiency decreases from 77% to 70%. An activated sludge unit subjected to the same level of overload will fail due to sludge bulking and loss of activated sludge solids in the effluent. The BOD removal efficiency drops from 90% to below 50%.
The advantages of completely mixed activated sludge processes are high BOD removal efficiency, the ability to treat high-intensity wastewater, and adaptability to future transitions to advanced treatment processes. For the secondary treatment of a strong wastewater with a settleable BOD of 300 mg/L, activated sludge secondary treatment can remove at least 90% of the BOD, resulting in an effluent BOD of 30 mg/L or less. A single-stage high-load biofilter can only remove 77% or less of the BOD, resulting in an effluent BOD of approximately 70 mg/L. To achieve BOD removal efficiency comparable to the activated sludge process, two-stage filtration is required.
Sludge Disposal: Primary sedimentation and secondary bioflocculation concentrate the organic matter in the wastewater into a sludge volume much smaller than the volume of wastewater being treated. However, the disposal of accumulated sludge is a major economic factor in wastewater treatment. The initial investment in sludge processing equipment is approximately one-third of the total investment in the treatment plant. The precipitated solids from the clarified effluent of the biofilter, or excess activated sludge, are often returned to the head of the treatment plant for removal along with the primary sludge. Raw sludge can be stored at the bottom of the primary sedimentation tank awaiting treatment or pumped to a regulating tank for storage. The pumped sludge can be concentrated in a thickening tank, typically a gravity-fed unit, placed before sludge processing. Excess activated sludge is mixed with the pumped-out primary sludge. In system layouts, a regulating tank is usually connected to a sludge thickening tank. Excess activated sludge can be thickened separately or using a mixed sludge thickening method before processing.
Various methods for processing and disposing of sludge: Common sludge processing methods include anaerobic digestion and vacuum filtration, often using centrifugation and wet incineration. Conventional disposal methods include landfill, incineration, manufacturing soil conditioners, and marine dumping. In coastal cities, marine dumping is often the most economical, while landfill is the preferred method when land is available. Incineration, although more expensive, is often the only feasible disposal method within urban areas.
A municipal treatment plant must carefully consider all possible sludge disposal processes. Choosing the best method should involve the most economical and environmentally friendly process. Factors such as the future use of processed sludge through residential areas and landfills, groundwater pollution, air pollution, other potential public health hazards, and landscape issues must be considered.
Chlorination is a common practice for disinfecting effluent from secondary treatment plants in tailrace waters used for recreation or water supply. Several states initially mandated disinfection of treatment plant effluent only during the recreation season, but now require year-round disinfection. Some states have no such regulations.
Regarding phosphorus and nitrogen removal, much research has been conducted in recent years on developing feasible phosphorus removal methods for wastewater treatment plants. Research has also been done on developing methods for nitrogen removal and complete water recovery. Several pilot phosphorus removal plants and small-scale production plants are in operation, but empirical data is still limited as precedents for designing large-scale equipment. Although water quality standards specify limits for phosphorus and nitrogen, large-scale application of nutrient removal methods is still a matter for the future.
For small towns, the size of the town plays a dominant role in the selection of wastewater treatment processes, particularly in the management, control, and sludge disposal. Methods that do not require sludge disposal (stabilization ponds) or only require occasional sludge removal (delayed aeration) are superior for small villages and districts. Larger towns often employ systems requiring more control and maintenance, such as contact stabilization and oxidation ditch treatment plants. Many existing treatment systems are no longer preferred, such as concealed ponds and other types chosen based on unique local conditions.
The treatment unit series in a biological filter treatment plant includes a hopper-bottom settling tank with an independent washing trough, a primary sedimentation tank, a biological filter, a final sedimentation tank with gravity return to the original wastewater wet well circulation pipeline, a single-stage digester for sludge treatment, and a drying bed. The wastewater returned to the original wastewater wet well includes: the sludge from the washing trough, the return sludge from the final sedimentation tank, the discharge from the drying field, and the clarified liquid from the digester. The treatment plant may have an inlet inspection well or a bypass pipe after the biological filter.
The activated sludge plant treatment unit includes a series of mechanically cleaned grids and a crusher that returns shredded solids to the raw wastewater. It also features constant-speed and variable-speed booster pumps with backup gas generators, a Parshall flume, a clarifier-type aerated sludge settling tank with an independent sludge washer, a primary sedimentation tank, a fully mixed activated sludge secondary treatment tank, and a gravity return pipeline for excess sludge to the wet well.
Additionally, there is a vacuum filter for raw sludge from the sludge storage tank and a landfill for the treated filter cake. The filtrate from the vacuum filter is returned to the wet well. Due to the depth of the wastewater pipe, the raw wastewater cannot bypass the wet well by gravity flow. A backup gas generator is provided for the two-sided, three-blade booster pumps, which can operate in case of a power outage. Overpass pipelines are located after the booster pump station and after the primary sedimentation tank.
