Sewage Treatment Plants

---

Membrane bioreactors are combinations of some membrane processes like microfiltration or ultrafiltration with a biological wastewater treatment process, the activated sludge process. These technologies are now widely used for municipal and industrial wastewater treatment. The two basic membrane bioreactor configurations are the submerged membrane bioreactor and the side stream membrane bioreactor. In the submerged configuration, the membrane is located inside the biological reactor and submerged in the wastewater, while in a side stream membrane bioreactor, the membrane is located outside the reactor as an additional step after biological treatment.

The sequencing batch reactor (SBR) is a fill-and-draw activated sludge system for wastewater treatment. In this system, wastewater is added to a single “batch” reactor, treated to remove undesirable components, and then discharged. Equalization, aeration, and clarification can all be achieved using a single batch reactor. To optimize the performance of the system, two or more batch reactors are used in a predetermined sequence of operations. SBR systems have been successfully used to treat both municipal and industrial wastewater. They are uniquely suited for wastewater treatment applications characterized by low or intermittent flow conditions.

Moving Bed Biofilm Reactor (MBBR) represents an advanced iteration of Fixed Film Bio Reactors such as Surface Area Fixed Filmed Technology (SAFF) or Rotating Bio Reactors (RBC), distinguished by the unique characteristic of its carrier media being freely floating in the sewage or effluent. In MBBR technology, thousands of polyethylene biofilm carriers move in a mixed motion within an aerated wastewater treatment basin. The reactor consists of a tank housing submerged but floating plastic media, typically made of HDPE, polyethylene, or polypropylene, with a specific gravity less than 1.0. The extensive surface area of these plastic carriers facilitates substantial bacterial growth, with biomass forming a thin film ranging from 50 to 300 microns in thickness. Each bio carrier enhances productivity by providing a protected surface area for the growth of heterotrophic and autotrophic bacteria. This dense bacterial population achieves high-rate biodegradation, ensuring system efficiency, reliability, and operational simplicity. Medium or coarse bubble diffusers positioned at the tank's bottom maintain a dissolved oxygen concentration of > 2.5-3 mg/L for BOD removal, with even higher concentrations for nitrification. Screens on downstream walls retain the media within the tank, while a clarifier or Dissolved Air Flotation (DAF) unit downstream separates biomass and solids from the wastewater. Notably, no sludge recycle is required in this process, highlighting its efficiency and self-sufficiency.

Package plants find application in rural areas, highway rest stops, and trailer parks catering to populations ranging from 10 to approximately 100 people. Among the systems employed for secondary treatment and settlement, the cyclic activated sludge stands out. In this process, activated sludge is blended with incoming sewage, undergoes aeration, and the settled sludge is subsequently run off and re-aerated before a portion is returned to the head works. While effective, the drawback lies in the requirement for precise control of timing, mixing, and aeration. This precision is achieved through computer controls integrated with sensors. However, the complexity and delicacy of this system make it less suitable for locations with unreliable or poorly maintained controls, or where the power supply may be intermittent. As an alternative, Sequential Batch Reactor (SBR) plants are gaining global deployment, offering a more robust solution to wastewater treatment challenges.

The Submerged Aerobic Fixed Film Reactor (SAFF) stands out as a cost-effective wastewater treatment and sewage sanitation method predominantly applied in residential and commercial complexes. This technology is structured around three key stages: Primary Settlement, Secondary Treatment, and Final Settlement/Clarification.

Within the wastewater industry, SAFF Technology is recognized for its simplicity and cost-effectiveness in commercial and residential sewage sanitation and wastewater treatment, especially in scenarios where available land is limited, and continuous operational manning would be economically impractical. A well-designed Submerged Aerated Filter plant distinguishes itself by the absence of moving parts within its process zones. Any serviceable components are strategically positioned for easy access without disrupting the ongoing treatment process. This not only enhances the efficiency of the system but also contributes to its ease of maintenance and overall reliability.

The Rotating Biological Contractor (RBC) is a versatile treatment system designed to provide either secondary or advanced levels of wastewater treatment. In applications geared towards secondary treatment, the effluent biochemical oxygen demand (BOD) characteristics are comparable to well-operated Activated Sludge Process (ASP). For scenarios requiring nitrified effluent, RBC systems can be employed to achieve combined treatment for BOD and ammonia nitrogen or to separately accomplish nitrification of secondary effluent.

The RBC process typically involves multiple units operated in series, the number of which depends on the treatment objectives. Two to four stages are employed for BOD removal, while six or more stages are utilized for nitrification. These stages can be realized through the use of baffles in a single tank or through the utilization of separate tanks in series. Suppliers of RBC equipment may differ in disk designs, shafts, packing support, and configuration designs. Key elements in the design of an RBC system include the shaft, disk materials and configuration, drive system, enclosures, and settling tanks, with variations in each contributing to the overall efficacy and performance of the treatment system.

The Extended Aeration treatment system operates by creating optimal conditions for aerobic bacteria and other microorganisms. These microorganisms, in turn, facilitate the decomposition of biological contaminants present in raw sewage. The treatment plant establishes an environment with adequate oxygen and other essential elements, enabling bacteria to effectively consume organic matter and thrive within the system. This process allows aerobic bacteria and microbes to break down sewage and waste into a stable, odor-free, and nuisance-free form. By fostering a controlled and supportive environment, the Extended Aeration treatment system ensures the efficient and thorough treatment of wastewater, contributing to the overall purification of the sewage.

Grey water is inherently easier to purify and reuse in comparison to wastewater, primarily due to its lower concentrations of pathogens. As a best practice for sanitation purposes, it is strongly recommended to keep grey water separate from wastewater. Sewage, or black water, results from the combination of grey water and wastewater and necessitates treatment in a sewage treatment facility.

Grey water from household and building appliances typically contains elevated levels of organic substances. To address this, ultrafiltration or other filtration methods are required to remove these contaminants before the grey water is discharged into a treatment system. If the complexity of this purification process proves challenging, an alternative is redirecting the grey water to the sewage system or a nearby sewer for proper treatment and disposal. This strategic separation and treatment of grey water contribute to sustainable water management practices, minimizing environmental impact while ensuring efficient sanitation.

Anaerobic treatment stands apart from conventional aerobic treatment by eschewing the use of aeration. The absence of oxygen facilitates controlled anaerobic conversions of organic pollutants into carbon dioxide and methane, the latter serving as a valuable energy source.

The primary advantages of anaerobic treatment lie in its ability to accommodate very high loading rates, reaching 10 to 20 times those achievable in conventional activated sludge treatment, and its remarkably low operating costs. This treatment method proves cost-effective in reducing discharge levies while concurrently producing reusable energy in the form of biogas. The investment payback period for significant expenditures in anaerobic treatment technologies can be remarkably short, sometimes as low as two years. Particularly in regions where the primary focus in discharge control is the removal of organic pollutants, anaerobic treatment of domestic wastewater emerges as an intriguing and economically viable solution.

Sludge conditioning is a vital process involving the treatment of sludge solids using chemicals or alternative methods to optimize the sludge for subsequent dewatering processes. Chemical conditioning, a common technique in sludge conditioning, enhances the sludge's characteristics for more efficient and cost-effective treatment using vacuum filters or centrifuges. Various chemicals are employed in this process, including sulfuric acid, alum, chlorinated copperas, ferrous sulfate, ferric chloride, and lime, either individually or in combination. The objective is to modify the sludge's physical and chemical properties, facilitating enhanced dewatering performance and the separation of water from the sludge solids.

Thermal drying is a proven technology for processing solids from municipal wastewater treatment plants (WWTPs), initially developed for industrial applications. This method effectively removes water from dewatered solids, reducing both volume and weight. Typically, dewatered solids with 18% to 35% dry solids content undergo thermal drying, resulting in a product with approximately 90% solids. The process involves raising the temperature to evaporate water, and the energy for this can come from various sources, such as combustion of fuels or the reuse of waste heat.

Thermal drying meets EPA standards for pathogen kill and vector attraction reduction, ensuring biosolids are more than 90% solids. The high temperatures used preserve organic matter without causing oxidation. The resulting material, with 90% to 96% dry solids, may take various forms, including dust, flakes, pellets, or larger fragments. Thermal drying typically follows or is done in conjunction with a dewatering process, serving as the final stage in WWTP solids processing. The resulting dried material can be utilized for various purposes due to its reduced volume, weight, and nutrient-rich content.