Flexibility and adaptability: essential elements of the WRRF of the future

Water management requirements, and the supporting technologies, are changing rapidly. Principal drivers for these changes include population and economic growth, which are pushing global water demand beyond the sustainable supply level (OECD 2012), and climate change, which is changing rainfall patterns to create periods of increased and more prolonged drought, along with periods of more frequent and intense rainfall (Rockström et al. 2009; Hoekstra & Mekonnen 2012; Hoekstra & Wiedmann 2014). Indeed, the economic, social, and political impacts of these drivers are increasingly being recognized. For example, the World Economic Forum (2015) has ranked water crisis as the single most significant risk currently facing the world. The negative effects of historical water management practices, including groundwater depletion and environmental destruction, are also becoming more widely recognized. This is resulting in the development and implementation of a water management paradigm based on efficient water use, which also significantly reduces water management energy requirements, and the recovery of water and other resources from the water cycle. It is in this context that the Water and Resource Recovery Facility (WRRF, formerly known as a wastewater treatment plant or WWTP) of the future must be evaluated (Daigger 2007, 2009, 2010, 2012a, 2012b; Grant et al. 2012; Hering et al. 2013).

The WRRF of the future must be able to meet a variety of functional requirements – see Table 1. In fact, WRRFs may become integral components of a bio-based economy. The requirements are likely to vary over time, in nature as well as magnitude, and the facility must adapt to them. As an example, a centralized WRRF may evolve from its initial function producing principally water, to become largely a resource recovery facility as smaller, distributed facilities increasingly reclaim and recycle water upstream within the plant ‘sewershed’ (Daigger 2009, 2010, 2012a, 2012b).

WRRF technologies are evolving rapidly. The discovery of anaerobic ammonium oxidation (anammox), and technologies based on its development and commercialization are evolving rapidly (O'Shaughnessy 2015). Renewed emphasis on energy recovery from the used water (formerly wastewater) cycle is driving further development of anaerobic treatment technologies (Tarallo et al. 2015). Membrane technology (Judd 2011), along with other advanced water treatment technologies – e.g., ultraviolet (UV) treatment, advanced oxidation, and biological activated carbon (BAC) – is facilitating increased water reclamation (NRC 2012; Zodrow et al. 2017). Phosphorus recovery from used water is now a demonstrated technology, and work is on-going on bio- and electro- chemical technologies for further resource recovery. These trends are expected to continue, supplemented by further advances in bio-technology and materials science (nanotechnology). Consequently, the WRRF of the future must not only adapt to evolving functional requirements but also to changing technologies.

The WRRF of the future should not, therefore, be conceived or designed to accomplish a fixed set of functional requirements, based on a fixed set of technologies. Rather, flexibility to adjust to changing requirements, and adapt to and incorporate evolving technologies becomes the principal design consideration. How can a facility be designed for uncertain requirements using evolving technologies, including some not yet invented? This seems impossible, but history demonstrates that it is not. Many WRRFs have been in service for decades and been adapted to a range of evolving requirements and new technologies. Thus, a relatively robust experience base exists to inform decisions on WRRF layout and configuration, when a new facility is being conceived or significant modifications are being evaluated. That experience base is used here.

Experience shows that facilities can be adapted significantly over time (Gujer 2011), even when such adaptations are not anticipated and/or differ from those anticipated. Consideration of future possibilities assists such adaptation, however. Functional adaptations include hydraulic expansion – e.g., adding associated treatment units – treatment process upgrades (e.g., to meet more stringent water quality requirements), new organic matter (‘sludge’ or ‘biosolids’) processing and resource recovery technologies/opportunities, and plant aesthetics – e.g., mitigation of external impacts like odor, road traffic and/or lighting.

Hydraulic capacity is the influent flowrate to be accommodated by the WRRF. Typical hydraulic conditions range from extreme low flow, which might occur seasonally but includes extended dry periods when groundwater infiltration is minimized (WEF 2010), through ‘normal’ flow (with diurnal high and low flowrates), to peak flow, often associated with weather events and/or high groundwater, both of which can increase flows into the collection system). This range of flows must be accommodated throughout the facility's life. Thus, peak flows must be accommodated, generally with at least some treatment units out of service, while avoiding inadequate flow velocities during persistent low flow conditions. The latter can lead to solids settling in pipes and channels, so a range of flows (minimum to maximum) must be specified for any pipe or channel not provided with mixing. The main hydraulic network must often be expanded and treatment units added over time, as the WRRF's service area grows. Flows may also decrease over time because of water conservation and upstream water reuse. It is possible to avoid developing plants comprised of a series of parallel and independent treatment units, which are complex to operate and offer reduced levels of reliability.

Water quality requirements change over time, and drive treatment technology selection, as well as affecting plant configuration and layout significantly. Table 2 summarizes potential WRRF treatment objectives and requirements, and corresponding general technology choices.

The capture of influent organic matter in order to minimize gross pollution of receiving waters has been, and continues to be, a principal used water treatment objective. It can be accomplished in the main biological treatment unit, which also provides other functions, such as nutrient removal and sludge stabilization. Since a significant portion of the influent organic matter is oxidized when this is done, there are significant impacts on further organic matter (sludge) processing with related increases plant energy requirements. WRRFs are increasingly designed to maximize organic matter capture, rather than oxidizing it, however, to reduce process oxygen requirements and so that the organic material can be digested anaerobically to produce biogas. Primary treatment is typically used to capture organic matter and divert it to anaerobic treatment at present, but there is increasing interest in returning to the historical practice of high-rate aerobic biological treatment, to maximize organic matter capture and minimize sludge mineralization in the liquid stream. Interest in direct (mainstream) anaerobic treatment is also increasing. The choice between these options will dramatically affect required facilities and plant layout.

Effluent nutrient discharge standards are becoming increasingly stringent. Ecoregion-based ambient water quality standards for nutrients (USEPA 2015), which are often between 0.01 and 0.1 mg/L for total phosphorus (TP), and 1 and 3 mg/L for total nitrogen (TN), provide some indication of future, potential effluent requirements. Biological and chemical phosphorus removal technologies can achieve these levels. Biological nitrogen removal has historically used heterotrophic denitrification which requires use of a significant fraction of the influent biodegradable organic carbon. The discovery of autotrophic anammox denitrifiers makes it possible to achieve significant nitrogen removal using less of the influent biodegradable carbon, freeing it for capture and use for energy (biogas) production.

Increasingly, effective disinfection is required to protect public health when treated effluent is discharged, especially when it is reclaimed for reuse. Filtration to remove particles that interfere with effective disinfection becomes essential, and multiple disinfection barriers can be required for various types of reuse (USEPA 2012). Various technologies are available to remove many trace constituents, as required for discharge and/or reuse. Retrofitting technologies like these to existing WRRFs is relatively common as effluent discharge quality requirements become tighter. Basic treatment may be adequate early in a WRRF's life, but more stringent requirements, added over time, will often require the addition of facilities to achieve many, if not all, of the treatment objectives listed in Table 1. A strategy enabling significant upgrades in effluent quality over time, should be incorporated into the planning and layout of any WRRF.

Technology changes can also be made to improve treatment capacity and/or as a result of changing economics. As noted in Table 3 (Daigger 2011), concrete water holding structures and pipes/channels are typically the longest-lived WRRF components. Rotating mechanical equipment – e.g., pumps and blowers – often has a longer useful life than the specific treatment technology. This suggests that facility layout and configuration should focus more on providing process tankage adaptable to various purposes and treatment technologies over time, rather than being ‘fine-tuned’ to any specific technology.

Good practice when laying out new or expanded WRRFs is to consider the full range of potential product water requirements, and the ultimate hydraulic and organic loading capacity requirements, and allow space for the facilities potentially needed. Space allocations based on existing technologies are often sufficient, as more efficient technologies, which will generally require less space, are likely to be available in the future should more stringent product water quality requirements be imposed. Current technologies can be used as surrogates for future technologies when making space allocations. Allowances can also be made in the plant's hydraulic profile for future additions. For example, sufficient head can be allowed in the hydraulic profile for the addition of something like primary treatment if it might be desirable in the future although not at the start. This can eliminate the need for intermediate pumping in future upgrades. Future capacity requirements can be allowed for by allocating space for additional treatment units, while increasing the capacity of the plant's existing hydraulic ‘backbone’ by installing parallel pipes and/or channels.

Facilities would generally be configured as desired on the blocks shown. While rectangular blocks are shown, this does not mean that rectangular tankage is necessary, various configurations (rectangular and/or circular) could be used, as appropriate. Although not shown, common ‘mixing points’ between various treatment stages – e.g., primary influent, primary effluent, biological treatment effluent – would facilitate sampling and analysis, and avoid multiple independent treatment trains, each requiring individual sampling and analysis for normal process control. The objective is to have ‘one plant’ to ensure, for example, that all primary effluent is collected at one point so that an influent with a single quality receives biological treatment downstream. A further benefit is that a single sample, representing primary treatment performance and downstream process loadings, can be collected. Corridors for utilities (piping, electrical, communications, odor ducting, etc.) would be provided with sufficient space for the ultimate facility.

This strategy provides significant flexibility over time as the plant is expanded and increasingly stringent effluent requirements are imposed. For example, experience suggests that it can be convenient and practical to avoid primary treatment in smaller plants, to simplify plant operation and the associated solids handling system. Primary treatment often becomes desirable, however, with plant expansions, as the economics of sludge processing and disposal change. This can be accommodated with layouts like that shown in Figure 1. The initial construction phase could exclude primary clarifiers, but more bioreactors would be needed and constructed to treat the higher biological process organic matter loadings that would result. Subsequent expansions could provide primary treatment, and the biological treatment system could be expanded by adding only additional secondary clarifiers as sufficient bioreactors would already be available. This transition would be made possible because space was initially allocated for future primary clarifiers in the plant layout, along with sufficient hydraulic head in the hydraulic profile.

Experience with plant upgrading indicates that difficulties expanding a plant's hydraulic backbone often constrain increases in plant hydraulic capacity using existing treatment units. New technologies often provide increased treatment capacity within existing water holding structures, but taking advantage of the potential cost savings and the possibility of reusing existing plant for other purposes requires that additional flow be directed to the existing, repurposed facilities. Increasing the flow to treatment units can be facilitated by allocating space to add more or larger pipes and channels. The location of major plant conveyance structures within or under water-holding structures, or in conflict with other plant utilities – e.g., electrical and plant communication conduits – gives rise to such constraints. Water-holding structures can also be configured with the potential for future retrofit in mind. Comparison of the life of water-holding treatment structures to treatment technologies (Table 3) suggests that several different technologies may be applied, over time, in the same tanks. While retrofits are always possible, even when significant changes must be made, they can be made less costly and easier if relatively standard rectangular tankage amenable to a wide range of technologies is constructed. Internal process equipment and non-structural walls can be changed more easily than the basic water holding structure. The relative ease with which activated sludge bioreactor tankage can be adapted to various process configurations certainly represents one reason for its long-standing use and the current dominant position of this technology in the industry.

It is assumed above that water-holding tankage will be constructed using material like reinforced concrete which can have a very long service life. Materials, such as steel, which have lower initial costs but also a shorter service life, can and have been used. Reinforced concrete or similar materials have traditionally been used for water-holding structures in WRRFs as their long-term, life cycle costs are generally lower. Whether this remains true should be evaluated based on current and projected costs, and the potential availability of new materials. The general approach of allowing space for expanding treatment capacity and upgrading product water quality, while allocating space for conveyance facilities and utilities would necessarily continue. Increased use of new liquid-solids technologies, such as membranes, which are modular and with shorter service lives, can also affect layout decisions.

Liquid stream processes, such as primary sedimentation and biological treatment, produce residuals that contain carbon-based matter (organic material) as their bulk constituent, along with a wide variety of other components, including nitrogen, phosphorus, etc. The volumes of these organic matter containing streams are much lower than those of the liquid streams, and they are generally pumped. Consequently, they can be re-routed more easily, reducing the importance of permanent allowances for piping corridors based on gravity flow. It would be best to reuse existing pipes where possible, but the cost of re-routing these flows is much lower than that for the main liquid stream flows.

Organic matter processing facilities include mechanical equipment – e.g., for sludge thickening and dewatering, and for biogas processing – which is often housed in buildings. The useful life of this equipment is less than typical water-holding structures, so removal and use of the space provided to install other equipment, even with different functions, is quite practical. Thus, more flexibility generally exists to transition from one organic matter processing technology to another than for liquid stream treatment, as long as sufficient space is allocated to such processing in general. Given sufficient space allocation initially, enough flexibility will exist to change technologies over time. This is reflected in the building block approach – see Figure 1.

Aesthetics refers, here, to the WRRF's external appearance, while mitigation refers to minimizing off-site impacts such as odors, truck traffic, etc. These factors have become increasingly important to WRRF design as: (1) facilities are constructed in urban areas and (2) the public becomes less tolerant of facilities which adversely impact adjacent properties and their market value. Architectural involvement, and extensive gas collection and treatment can add significantly to costs. The energy required for collecting and treating off-gases in some recent plants has, for example, exceeded that for aerobic biological treatment. Public expectations concerning aesthetics and mitigation, and their associated cost, must be balanced. The ‘balance point’ changes over time, however, as urban growth brings people and their associated activities closer to WRRFs, and expectations concerning WRRF external impacts become more stringent. Thus, it is prudent to anticipate that the aesthetic appeal of any WRRF must be increased over time, and that external issues, like odors and truck traffic, will need increasing mitigation. Allowances for such improvements are best incorporated into plant layouts.

A number of practical and moderate cost measures can significantly improve the aesthetic appeal of a WRRF. External screening using either engineered or natural materials can be quite effective at moderate cost. Minimizing the height of tall structures and grouping them to improve the visual appeal, should be considered during facility layout and design. Architects with relevant experience can be quite helpful in this. Modern design techniques can help show how the facility will appear from outside and guide relevant decisions. Too often engineers base the site layout on the site plan, rather than considering views from adjacent properties or elsewhere. Space should be allocated for extensive odor control, even if not initially provided. Lighting technologies are available to minimize external light impacts. The WRRF of the future must not only provide effective used water and resource recovery, but it must also be viewed as an asset to the local community.

A number of ‘underground’, or partially to nearly fully ‘covered’ WRRFs have been built around the world. These approaches can be implemented effectively, but generally at some increase in cost, and with increased long-term operating and maintenance impacts. They may be necessary sometimes, however, to achieve public approval for the WRRF, or they may be cost-effective if land values are quite high. Sufficient aesthetic appeal can often be achieved, however, at lower cost with surface facilities.

Preparing for evolution in product water quality and sludge processing requiring technology additions and changes over time is discussed above. This section covers some specific evolving technologies, and water and resource management paradigms, that are important.

Increased interest in the ‘energy neutral’, or even ‘energy positive’, WRRF is leading to significant re-thinking of WRRF process trains. Energy neutral plants were more of the norm in the past. Typically, they used primary treatment followed by either high-rate activated sludge or trickling filter biological treatment systems to minimize liquid stream energy requirements. Coupled with anaerobic digestion of the primary and secondary sludges, and use of the biogas produced in combined heat and power (CHP) systems, sufficient power was produced to meet plant-wide electrical needs. Acceptable removal of organic matter (five-day biochemical oxygen demand, BOD₅) and total suspended solids (TSS) was also achieved, as long as stringent effluent quality requirements were not imposed. The need to provide consistently higher quality effluent (lower BOD₅ and TSS), especially coupled with nitrogen and phosphorus removal, led to use of lower rate activated sludge systems and the corresponding increased aerobic stabilization of sludge in the biological treatment system. The net result is reduced secondary sludge production, with less biogas for use in CHP systems, and increased energy demands for biological treatment, making WRRFs net energy negative (requiring energy purchase).

A number of factors have combined to renew interest in increased carbon capture from the influent for use for energy production, especially through conversion to biogas via anaerobic processes. The discovery and development of autotrophic anammox-based nitrogen removal processes is enabling extensive innovation in this area. The most practical and cost-effective approaches for doing this are not clear, however, given existing and evolving technology options. Increased interest in phosphorus recovery, rather than simple phosphorus removal, also affects process choices.

CEPT represents one approach to increase the fraction of influent biodegradable organic matter diverted to anaerobic treatment. Substantial phosphorus removal also occurs, but methods for subsequent phosphorus recovery are limited because the removed phosphorus is chemically bound. High rate biological treatment systems (either suspended growth or biofilm-based) and non-nitrifying biological phosphorus removal processes are other options for capturing carbon and diverting it to anaerobic treatment. Biological phosphorus removal processes require lower biological loadings (longer solids residence time, SRTs, to allow the relatively slow-growing phosphorus accumulating organisms to exist in the system (Grady et al. 2011).

In either case, nitrogen removal would be accomplished downstream in an anammox-based biological treatment system. A further option is mainstream anaerobic treatment, followed by a combination of either biological nitrogen and phosphorus removal, or biological nitrogen removal with chemical phosphorus removal. Table 4 summarizes the principal options. Understanding the relative advantages and disadvantages of these options, and the types of applications where each offers most advantages, will develop further over the next few years. Supporting technologies will also evolve. In the meantime, decisions concerning the number of such options to accommodate in plant layouts will require care. They can affect plant layout and costs significantly.

While practical technologies for phosphorus recovery from used water are available, this is not currently true for other significant constituents like nitrogen. Potentially suitable technologies and approaches are being evaluated, however, and may become useful in future. Further development of alternatives to gravity sedimentation for liquid-solids separation, the true ‘workhorse’ of the industry today, continue. Secondary clarifier replacement in activated sludge systems with the membrane bioreactor (MBR) process is one example. An MBR requires much less space than a conventional suspended growth biological treatment system, and the configuration of some tankage – e.g., the membrane tanks – differs considerably. Careful consideration of liquid-solids separation technology use, both now and in future, is needed.

Evolving treatment objectives, product water quality requirements, and technologies require WRRFs to be designed with significant flexibility to adapt and to use technologies that are evolving in parallel. WRRF layouts and process tank configurations should not be based solely on current treatment requirements and technologies, but should anticipate changes and incorporate allowances to facilitate them. Techniques are available to do this. WRRFs can be laid out using a ‘building block’ approach to anticipate future growth in required capacity, treatment and organic matter processing upgrades, and the need for increased attention to aesthetics and mitigation over time. Plant tankage can be configured to accommodate various technologies to facilitate changes. Planning to facilitate change truly leads to implementation of the WRRF of the future.