After reading this article, you should understand:
- Functions and processes of a decentralized PWWT system;
- Design considerations of a PWWT system.
'Clean water will be the oil of the 21st Century.' The accuracy and magnitude of this statement is resonating worldwide. There is no more important endeavor by governments and citizens than the protection and conservation of all water resources.
A critical component to the protection of these water resources is the treatment and subsequent disposal of wastewater. Wastewater treatment is commonly performed at a centralized wastewater treatment facility serving thousands and sometimes millions of residential, commercial, and industrial customers. The wastewater from these facilities is typically discharged directly to a local or regional watershed; therefore, the characteristics of the wastewater effluent have a direct effect on the quality of the potential potable water source for those localities. To protect the environment and its populations, it is critical that wastewater facilities provide dependable and sustainable solutions that incorporate technologies capable of significantly extending treatment facility service life cycles while also maintaining fiscal responsibility.
There is an emerging trend toward utilizing decentralized packaged wastewater treatment (PWWT) systems for treatment needs from various industry segments. These segments include residential, commercial, industrial, and agricultural customers who have been required to divert and pretreat their wastewater influent; or have found that connecting to the traditional centralized system is economically constraining. As recently stated in a CE News article, 'Dual focus on wastewater' (Drake, 2011), economics and the environment are the leading drivers in technology adoption.
Many new residential developments are utilizing a 'cluster treatment' approach in their total wastewater treatment designs. Separate PWWT's are utilized congruent with the phases of the development's expansion. For industrial and commercial wastewater generators, a driving factor for implementing a PWWT pretreatment system is a simple economic return-on-investment analysis as compared with current and future wastewater fees. PWWT systems can provide an advanced level of wastewater treatment by incorporating nutrient-removal capabilities and microfiltration processes that allow direct discharge or reuse purposes. Decentralized PWWT technologies are used most effectively when treating up to approximately 500,000 gallons per day (gpd).
PWWT system designs are chosen based on initial installed capital cost, life cycle cost, tankage requirements, equipment, project schedule, sustainability, operating expense, and maintenance costs. Various technologies exist, but can be categorized by the tanking systems used: concrete, steel, fiberglass, and steelreinforced polyethylene (SRPE). Concrete systems are typically the only material used for flows nearing 300,000 gpd and larger. SRPE systems can be designed to treat up to 250,000 gpd and will be used in this article as the example when describing process and compartments.
Types of wastewater streams— Design considerations begin with understanding the source of the wastewater stream, as it is critical to the design for all contingencies that could be encountered during the life cycle of the system and also regulatory compliance. For typical decentralized PWWT systems, the categories include:
Clearly identifying all possible sources of the wastewater effluent will allow the designer to properly design for worst-case-scenarios that could create adverse conditions for effective treatment. For industrial wastewater streams, further considerations should be noted in order to properly design for treatment requirements. Industrial streams for decentralized packaged wastewater treatment plant design can be separated into the following categories:
- Light metals
- Heavy metals
Basic wastewater data to begin design — After considering the challenges from the source, the PWWT designer must understand the influent flow needing treatment, as described in peak hourly and daily average gallons per day. Next, an understanding of both the influent and effluent levels associated with the wastewater treatment system is to be reached for the following required treatment parameters:
- Biological oxygen demand (BOD5)
- Chemical oxygen demand (COD)
- Total suspended solids (TSS)
- Total dissolved solids (TDS)
- Fats, oils, and grease (FOG)
- Alkalinity (pH)
- Ammonia (NH3)
- Total Kjedhal nitrogen (TKN)
- Total nitrogen (TN)
- Total phosphorus (P)
- Disinfection (coliform count)
- Dissolved oxygen (DO)
Typical PWWT process
A PWWT process can be divided into three main categories: primary treatment, secondary treatment, and tertiary treatment. Primary treatment provides flow equalization and first-level trash, grease, and grit removal. Secondary treatment denotes aerobic and/or anoxic biological treatment processes. Tertiary treatment can be considered the polishing process where continued clarification and disinfection can be designed for desired effluent limits.
Flow equalization and primary clarification (primary treatment) — Commonly, wastewater from the collection system is gathered into a common point known as the flow equalization and influent pump tank, designed for the ultimate gpd flow. This tank can be considered the first phase of primary clarification, which utilizes screening, grit, and grease removal methods. These primary clarification methods help remove the insoluble/ indigestible solids, grit, trash, floatables, and grease common in wastewater streams before they enter the intricate components of the PWWT system. Large solid debris that are indigestible solids common to a wastewater system can be removed with various methods, including an automatic mechanical screening system with the screenings bagged in an automatic bagger and manually loaded in a dumpster for landfilling. Grit removal is achieved after settlement and is required to eliminate abrasives, which have the potential to reduce treatment plant equipment service life.
In many designs, wastewater flows can be directed from the screening equipment to a flow splitter assembly or directly to the second phase of the primary clarification. A flow splitter assembly can be utilized to alternate and/or balance wastewater flows to different treatment trains in a PWWT system and includes adjustable flow gate assemblies that have manual operation capability.
Secondary treatment (anoxic reactor) — In the case of a SRPE PWWT system, wastewater is directed to an oxygen-depleted anoxic reactor upstream of the bioreactor after primary clarification. Denitrification of the wastewater stream is accomplished by returning activated sludge from the bioreactor to the anoxic tank. Dissolved oxygen levels are approximately 0.2 to 0.5 milligrams per liter (mg/l) in the anoxic reactor, thereby facilitating the denitrification process as anoxic microbes break down existing nitrates for their available oxygen component. The result is the release of inert nitrogen gas into the atmosphere.
Secondary treatment (bioreactor) — After the anoxic reactor, the next phase within the SRPE PWWT is the secondary treatment bioreactor. The most typical bioreactor processes involve Moving Bed Biological Reactor (MBBR), Sequencing Batch Reactor (SBR), or Integrated Fixed-film Activated Sludge (IFAS) designs in conjunction with an extended aeration activated sludge process. Depending on the size of the flows, multiple treatment trains may be needed.
After treatment in the bioreactor, the design process may transfer the mixed liquor (biomass) flows into a post-anoxic basin for further mixing and facultative manipulation of the biomass relative to nutrient removal. The post-anoxic basin permits the biomass to consume available dissolved oxygen before the biomass modifies its metabolism to consume oxygen molecules bound to the nitrite and nitrate compounds generated in the nitrification phase of the treatment process. After post-anoxic treatment, the biomass can be diverted into a post-aerobic zone that provides an oxygen-rich environment where the biomass reverts back to its primary biological function of metabolizing dissolved oxygen and incoming organic carbon. This phase of treatment prepares the biomass for a return trip to the pre-anoxic zone by way of the Return Activated Sludge(RAS) pumping system in the downstream clarifier.
After treatment in the bioreactor, wastewater flows to a final gravity clarifier where the wastewater is stilled and any floating or suspended solids remaining in the flow are settled and then returned upstream in the treatment process. RAS pump assemblies are used to return sludge to the treatment process. The clarifier is equipped with a conventional v-notch weir for discharge of the treated effluent.
Tertiary treatment (clarification and disinfection) — Treated effluent flows can be directed to tertiary filters that provide microfiltration. Various filters can be utilized, including: cloth filters, polymeric membranes, ceramic membranes, and granular/sand filters. The filters are able to remove suspended solids to as small as 0.04 microns. After filtration, the high quality effluent flows can be directed through further tertiary treatments, including ultraviolet (UV) light, chlorine, or ozone disinfection systems. For environmental and economic reasons, UV is fast becoming the typical choice. UV units are capable of providing 99.99-percent pathogen removal rate, which complies with the discharge limits imposed by many regulatory agencies.
After disinfection, the high-quality effluent flows can be sent to a post-aeration basin or directly to the designed outfall. If required, post-aeration is intended to increase dissolved oxygen levels prior to discharge to address receiving water concerns.
Laboratory facilities — Laboratory facilities can be provided within a dedicated building at the wastewater treatment plant site in order to perform the required effluent/ operational testing as listed in the permit. The building can also serve as office space for maintenance and operator personnel. All electrical and equipment control panels are located in the laboratory facility to protect them from the elements and to provide ease of access for maintenance and observation.
Additional project considerations
Many PWWT systems utilize similar processes and designs. Engineers are challenged to design PWWT systems that answer many project needs that are not addressed by the systems' basic process designs. These additional project considerations can be categorized by tank attribute analysis, equipment differences, project schedule, initial capital costs, and total life cycle costs.
Tank attribute analysis — Typical of most wastewater treatment systems, the majority of the existing tanks are open at the top. Designers must plan for security measures as well as safety precautions because of these openings. Some measures may involve protective railings around the perimeter and, in some cases, catwalks to interior locations. This safety requirement must be monitored continuously. Exposure to the atmosphere also permits temperature fluctuations within the wastewater stream. These temperature fluctuations have a direct effect on the micro-organisms necessary for the treatment process and especially for those systems designed for nutrient removal.
PWWT system innovations have provided new designs that allow for direct burial tankage (e.g. SRPE tanks) that can provide a steady temperature state due to the insulating properties of the surrounding earth. This condition permits a controlled temperature for the treatment micro-organisms to efficiently flourish and accomplish the biological treatment process. The buried PWWT tanks are accessed through sealed access hatches that are flush with the grade. This condition also provides protection to the system in a flooded situation. It is common for larger wastewater treatment systems to become inundated during flooded conditions, thereby releasing thousands or millions of gallons of untreated wastewater into the environment. Buried systems can alleviate flood risks if sealed and designed for adequate buoyancy prevention.
Equipment differences — Many of the processes for PWWT systems utilize simple and readily available screens, filters, submersible pumps, diffusers, and blowers. There are a few unique patented mechanical systems (e.g. rotating wheels) available for the bioreactor and clarifier systems, but in general, the tank housing the process is the only difference.
As illustrated in the example system in Figure 1, a readily available MBBR system is utilized in the bioreactor. The MBBR media fosters a dual purpose biological metabolism that is considered a proven biological treatment approach. The MBBR system generates limited waste sludge due to its inherent self-regulating biomechanisms and can self-adjust its process to address hydraulic surges, fluctuating organic loading, and other issues that typically affect conventional wastewater treatment systems. The MBBR system typically has no moving parts other than the floating plastic MBBR media. Aeration for the bioreactors is provided by standard blowers. These blowers are very reliable and easily serviced using standardized parts. The implementation of non-proprietary equipment permits expeditious equipment replacement if necessary.
Various PWWT designs utilize a few basic secondary clarifying concepts and equipment. Some utilize conventional flight and chain assemblies that return settled sludge and surface scum to the treatment process. Others use gravity staged processes that implement hopper bottoms and simple RAS pumps and surface scum removal systems. It is important when considering designs for a PWWT system to compare the life cycle costs of the equipment and its off-the-shelf availability versus proprietary lead times.
Project schedule — PWWT systems, depending on their complexity, can take months to years to design and build. Project delays can come from a variety of sources, including: permitting, funding, politics, weather, and installation. The choice of PWWT system directly affects the installation schedule and possible delays. Good planning and understanding of fabrication lead times and installation rates should be a part of any effective project manager's analysis.
Initial Capital Cost — As equally important as the process attributes of a PWWT system are the corresponding project costs. It is important to note that the cost differences between types of systems tend to increase as the size and complexity of the treatment increase.
Life cycle cost — The most important comparison between two systems is that of life cycle cost. A public utility owner or treatment system owner in general must be concerned with the life cycle of a treatment system and its eventual replacement. Utilities derive income from long-term operations. Although budgets must include annual costs for equipment maintenance and repair, eventual system replacement cost most affects the utilities' bottom line. Industry standard life span estimates for the most widely used materials are as follows:
- Treated steel tanks — 15 to 30 years
- Concrete tanks — 30 to 40 years
- Fiberglass tanks — 40 to 60 years
- SRPE tanks — 50 to 75 years
Sustainability is defined as the 'capacity to endure.' According to Robert Gilman, Ph.D., president of Context Institute (1990), 'As a value, it refers to giving equal weight in your decisions to the future as well as the present.' Decisions made today - whether financial, political, or environmental — will impact future generations. There is a sustainability movement afoot where governments, industry, and individuals are recognizing and acting on green initiatives. A focus on products that use recycled materials, promote low-impact development, protect environmentally sensitive areas, and ensure system longevity is a growing trend in wastewater treatment system design.
Another modern day sustainable consideration is the impact of a particular packaged wastewater system and its tankage material on a project's Leadership in Energy and Environmental Design (LEED) credentials. The U.S. Green Building Council's LEED Green Building Rating System is based on points and evaluates the overall performance of a green building project by assessing each of the materials and systems used in aggregate. Typical LEED credit categories that can be submitted for PWWT systems are Sustainable Sites, Water Efficiency, and Materials and Resources.
International markets that previously did not have material access or the financial ability to implement advanced wastewater treatment systems into their societies now have an affordable and sustainable option. Decentralized PWWT systems can be developed anywhere in the world. Solutions for wastewater flows up to 250,000 gpd can now be provided in many types of tankage components that are not limited by traditional materials. There now exists the very real opportunity to drastically and quickly improve and protect the health and well-being of the world's desperately poor populations that lack access to even the most basic of wastewater treatment systems. Sustainable wastewater solutions are essential to the protection and conservation of water resources around the world for future generations.
- What are the current leading drivers in technology adoption within the wastewater industry?
- Economics and politics
- Regulatory requirements and power supply
- Economics and environment
- Zoning and installation rates
- For a residential developer, what is driving the need to utilize a decentralized packaged wastewater treatment plant (PWWT)?
- Economic constraints connecting to centralized networks
- Political pressures
- Availability of effective treatment solutions
- Proximity to effective collection systems
- What is the pathogen removal rate available for PWWT systems using UV light?
- 98.0 percent)
- 99.0 percent
- 99.9 percent
- 99.99 percent
- What tank material is most commonly used for wastewater flows in excess of 250,000 gpd?
- SRPE tanks
- Steel tanks
- Fiberglass tanks
- Concrete tanks
- What are the approximate dissolved oxygen levels within a SRPE PWWT anoxic reactor?
- 1.0 to 1.2 mg/lb
- 0.2 to 0.5 mg/l
- 2.0 to 5.0 mg/l
- 1.2 to 1.5 mg/l
- What is a RAS pump?
- Retractable aeration system pump
- Return aeration system pump
- Raised air service pump
- Return activated sludge pump
- Name two 'project' considerations that should be considered during PWWT design analysis?
- Life cycle cost and tank attribute analysis
- Equipment and sludge storage process
- Process effectiveness and laboratory building options
- System size process and effluent limit
- What are the three main treatment processes within a PWWT design?
- Pretreat, treat, discharge
- Influent, treatment, effluent
- Primary treatment, secondary treatment, tertiary treatment
- Flow, storage, retention time
- What project delay is most dependent on the type of tank system chosen?
- What process stage addresses disinfection needs?
- Effluent discharge
- Anoxic reaction
Andrew M. Jenkins E.I., is the product manager of Packaged Wastewater Treatment and DuroMaxx for CONTCH Construction Products Inc. He holds a Bachelor of Science degree in Engineering from Missouri University of Science and Technology (Rolla). Jenkins' experience comes from 14 years in the petroleum, mining, and civil engineering industries.
Daniel M. Early, P.E., is vice chairman and Chief Technology Officer with AppTech Solutions. He holds a Bachelor of Science degree in civil engineering from Virginia Tech and has 18 years of professional experience in land development design and municipal engineering, with an emphasis on water/wastewater design. He is responsible for new product development and holds professional licensure in North Carolina, Virginia, Maryland, and Pennsylvania.
- Drake, B., 2011, Dual focus on wastewater, CE News, July, pages 54-60.
- Gilman, R., 1990, Sustainability: The State of the Movement, In Context, No. 25, www.context.org/ICLIB/IC25/Gilman.htm..