美国著名Carollo 环境工程公司专家Rod Reardon 展望：污水处理当前及未来发展趋势

The wastewater industry faces many new challenges that complicate near- and long-term planning decisions. Increasing energy costs, trace organic compounds, finite resources, water conservation, and inexorably more stringent regulations, must all be considered before investing in major facility improvements. While the future is never certain, inclusion of strategic exercises like scenario planning and future mapping during the planning process can help to define the boundaries of what the future might bring to treatment facilities.

Futurists point out that the important trends in the future have their seeds in the present. On this basis, treatment technologies will evolve to address five major trends in wastewater treatment:

未来污水处理发展的五个趋势：

1) nutrient removal and recovery,

营养盐去除剂回收技术

2) trace organic compounds,

微量有机污染物；

3) energy conservation and production,

能量转换和产生

4) sustainability, and

可持续性

5) community engagement.

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The water industry has historically taken far longer than other business sectors to develop and implement new technologies. However, many innovations are now under development with benefits that could be compelling enough to shorten the length of the technology life cycle in the water sector. Implementation of these technologies would radically alter wastewater treatment plants in the future.

Current trends and highlights of some of today’s technical innovations, including nutrient removal and recovery, fine sieves, nitritation- Anammox processes, anaerobic treatment, sludge pre-treatment, and thermal conversions, are discussed.

Background

Speculation on the future of wastewater treatment continues to be a recurring theme in the water industry. Predictably, the future will be shaped by events that cannot be predicted and that will influence the future in ways that are impossible to foresee. However, studying the trends and forces shaping current events, and using this knowledge to develop possible boundaries for future conditions, can result in better insights into what might occur.

Strategic Planning

When the future is assumed to be like the past, forecasts can be made by simple, linear extrapolations. However, with greater degrees of uncertainty that conditions will continue as they are, forecasting becomes less useful. One structured method for evaluating these uncertainties is known as scenario planning, scenario thinking, or scenario analysis. With scenario planning, flexible plans for the future are prepared by evaluating alternative scenarios that could exist in the future. Future mapping is a more visually-based variation on scenario planning that attempts to examine a range of possible futures. Neither process attempts to predict the future, but rather develops an understanding of the forces and their relationships that could shape future conditions.

By creating several plausible, but distinctly different sets of future conditions, an organization can test the viability of current strategies under new circumstances. Ultimately, the goal is to be able to make better planning decisions that provide the flexibility to adapt to future changes.

Global Trends

Current trends (patterns of gradual change) often become the starting point for assessments of possible future conditions. Progressive changes in aspects of our society, businesses, and environment can be discerned and used to foresee the ultimate results of these changes over time. Past experience shows that most significant trends derive from underlying socio-cultural, economical, political, technical, ecological, demographic, organizational, and risk factors. Trends occur at all levels, with The largest, global changes affecting nearly everything, while localized trends will only affect specific regions, locations, or industries.

Key global trends with implications for the water industry include changes in population and demographics, increased urbanization, increasing living standards, climate change, and a scarcity of resources needed to sustain life, including land, water, and phosphorus. Regardless of the scale, utilities can benefit by being aware of the forces at work, and by being prepared to adapt to opportunities and threats that could significantly affect them.

A number of individuals and organizations have explored trends in the water industry including the Water Environment Research Foundation, or WERF (Crawford, G., 2010; Henderson, D., 2011), STOWA, the Dutch acronym for Stichting Toegepast Onderzoek Waterbeheer or Foundation for Applied Water Research (2010), the Water Research Foundation (Means, E.G., III et al., 2006), and the European Commission (Segrave, A. et al., 2007; Zuleeg, S. et al., 2006; and Rosén, L. and Lindhe–Chalmers, A., 2007). These different groups have expressed widely divergent views, as evidenced by the summary of selected studies in Table 1, although there is some commonality. Even though many of these studies were done within the context of potable water supplies, most of the identified trends apply equally to wastewater.

Wastewater Trends

From the perspective of the wastewater industry, five major trends, that encompass some of those in Table 1, are evident. These include nutrient removal and recovery, energy conservation and production, sustainability, treatment for non-traditional contaminants, and community engagement.

Nutrient Removal and Recovery – Nutrient removal to reduce nitrogen and phosphorous has been a reality in central Florida since the 1980s. In the future, nearly all treatment facilities will provide some nutrient reduction. Much of the near-term focus will be on meeting lower numeric limits; however, recovery and reuse of materials, initially phosphorus, will likely become mandatory at larger facilities over time. Taking a tiered approach to nutrient limits is likely the best long-term strategy, because the tiers allow flexibility to tailor effluent quality to a variety of reuse applications, thus providing the ability to maximize reuse while minimizing costs. One advantage to lower nutrient effluent limits is that treatment to meet lower effluent limits concentrates nutrients in the solids, where it may be more economical to recover and reuse Energy Management – Rising energy costs paired with restrictions on greenhouse gases will provide the impetus to institute more effective energy management and alternative energy strategies. These trends are raising the bar for wastewater utilities toward being energy neutral or energy positive, whereby energy is not just managed, but instead recovered and reused. Current initiatives to increase biogas production, manage oxygen demand, and control equipment for efficient power use will move the industry in the right direction. A fundamental change in the use of aerobic biological treatment may be required to complete the transition from energy user to energy supplier.

Future treatment plants may incorporate additional anaerobic processes, or chemical and physical barriers, to remove pollutants without aerobic bacteria thus creating energy rather than using energy. However, there are limits to the ability to increase the energy efficiency of existing processes, and there are budgetary limits for implementing new processes and technologies that help achieve an energy neutral target. A prudent strategy dictates that utilities work to achieve the energy neutral goal incrementally.

Toward that end, there are five key components that can frame energy optimization strategies including:

1) maximize efficiency;

2) provide more treatment for less power;

3) consider technologies to reduce or produce energy;

4) generate renewable power; and

5) evaluate the plant carbon footprint.

Sustainability – Better management of natural, human, social, manufactured, and intellectual capital to maintain a sustainable existence will become essential in the future. At wastewater treatment facilities, this will mean reduced consumption of resources and increased recycling and reuse of water, nutrients, and other materials contained in wastewater. In some areas, the need to increase reuse will require some decentralization with construction of satellite treatment plants. Caps on greenhouse gas emissions will affect the selection of treatment technologies and operating strategies particularly for sludge. Increased water conservation will alter both the flows and pollutant concentrations in raw wastewater, potentially leading to new challenges and opportunities.

Treatment for Non-Traditional Constituents – Public concerns over the presence of trace organic chemicals in water will accelerate the application of advanced treatment technologies to remove objectionable compounds from wastewater. Although there is reasonable certainty that removal of trace organic compounds will be needed, the timing, the specific compounds or classes of compounds that will require removal, and the technologies that will be needed, are unknown. Planning strategies might include leaving space on the plant site and in the hydraulic profile based on the technologies that we now know can remove some trace organics, including advanced oxidation processes and biological nutrient removal.

Community Engagement – The current trend for increased stakeholder involvement in utility decisions that affect neighbors of wastewater facilities or the cost of service should continue. Utilities can expect that their communities will demand to be part of the planning process for facility improvements, and that community enhancements be incorporated into utility projects.

Technical Innovations

The pace of innovation in the wastewater industry appears to be increasing, with every year bringing significant new concepts and technologies. Not all the technologies will succeed in the marketplace; however, some will. The following is a quick overview of a few promising wastewater treatment technologies that might be part of the treatment plant of the future.

Anaerobic Treatment – Anaerobic treatment of municipal wastewater is an attractive option for secondary wastewater treatment. The high costs of aeration and sludge handling associated with aerobic sewage treatment are dramatically lower with an anaerobic process as no oxygen is required for removal of carbonaceous oxygen demand and sludge production is reduced dramatically. Historically, however, anaerobic processes have not been feasible for carbonaceous BOD5 removal in municipal wastewater because of relatively low concentrations, the slow growth rate of anaerobic microbes, poor settleability of anaerobic sludge, and the potential for odors.

Phosphorus Recovery - Projections for the exhaustion of the world’s phosphorus reserves vary from less than 100 to over 300 years. More importantly; however, only eight countries contain over 90 percent of the known phosphate rock reserves, and just three (China, the United States, and Morocco/Western Sahara) have the bulk of the commercial reserves. Various predictions have the United States running out of phosphate rock within 25 to 30 years, although some of these predictions are at least that old. In some countries without phosphate rock reserves, the capture and recycling of phosphorus from wastewater has already become a major endeavor as a means to increase the security of their food supply.

Research into methods of recovering phosphorus from wastewater, originally initiated as a means for controlling magnesium ammonium phosphate (struvite), have accelerated over the last ten years. At present, the most feasible option is to precipitate struvite from side streams from dewatering anaerobically digested sludge. While side stream precipitation of struvite can recover about 40 percent of the influent phosphorus load, combining mainstream phosphorus removal with recovery from the sludge stream can capture up to 90 percent. Processes under development include additional precipitation methods, including one using a waste building material, and wet chemical and thermal methods for recovering phosphorus from sludge and incinerator ash. While phosphorus recovery and recycling may not be economical for some time, some are looking to the water industry to show the way, and to become an incubator for nutrient recovery technologies.

Nitrogen Cycle Revisited – Significant developments over the last 10 to 15 years have led to new processes for removing nitrogen from wastewater, particularly from warm, high-ammonia side streams from dewatering anaerobically digested sludge. Typical nitrogen removal at a wastewater treatment plant is a multi-step process in which a combination of autotrophic and heterotrophic bacteria sequentially converts ammonia to nitrogen gas. The classic nitrification- denitrification process can be managed so that the initial conversion of ammonia by ammonia oxidizing bacteria (AOBs) is stopped at nitrite (nitritation), and then the nitrite is converted to nitrogen gas (denitritation) by normal heterotrophic bacteria, thereby reducing the oxygen and carbon required for nitrogen removal. Coupling nitritation with denitritation provides a 25 percent savings in energy cost over conventional nitrification, and 40 percent savings in methanol cost over conventional denitrification.

Advances in molecular methods, aided by serendipity, have led to the discovery of microorganisms in both natural ecosystems and in biological treatment processes that were unknown less than 20 years ago. We now recognize that many more microorganisms are involved and their interactions are more complex. For example, both archaea and planctomycetes are major players in the nitrogen cycle of the open oceans; both microorganisms were unknown 20 years ago.


Ozone with Granular Activated Carbon (GAC) and Biological Aerated Filter (BAF) – 臭氧-粒状活性炭联用或曝气生物滤池或许可以去除一些难以被活性污泥段去除的微量有机污染物

Conventional treatment does not provide effective removal for all trace organic contaminants (TOrCs), and advanced treatment may be required depending on the compound, concentration, and future regulations. While researchers have shown that ozonation provides excellent removal of numerous TorCs, no single treatment process is capable of removing all TorCs to below sensitive analytical detection limits (Benotti, M.J. et al., 2009; Snyder, S.A. et al., 2007). For example, fire retardants are one group of compounds that are not well removed by ozonation, but are well removed by GAC.

A plant of the future should include process flexibility to implement a multi-barrier approach for TorC removal, where additional advanced treatment processes, such as GAC or BAF, would provide TorC removal for compounds not well removed by ozonation alone.

Thermal Conversion – Recognizing the potential energy content of wastewater residuals, newer technologies are being developed to create energy independent systems. Gasification and pyrolysis are among the most promising of these technologies, which are being increasingly developed, both of which traditionally require sludge to be dried to 90 percent solids. Some new gasification developments appear to show promise at 50% solids or even 10% solids, thus eliminating the energy intensive drying stage. The gasification process heats solids to above 800 oC under oxygen-starved conditions to form syngas, which is mainly composed of hydrogen and carbon monoxide. The energy content of the syngas can be increased by adding steam to the process, a spin-off known as hydrogasification.

Pyrolysis creates syngas similar to gasification, but operates in the 700 oC range and in an oxygen-free environment. Both processes are designed as close-coupled systems, where the syngas is burned to heat flue gas, which is then used as the heat source for the drying process. In both cases, most of the recoverable energy is used to dry the solids, leaving little to produce power. As a result, many close-coupled systems are net-positive energy consumers.

The green energy and cleaner emission potential of gasification and pyrolysis are gaining momentum among alternative thermal treatment technologies. In a two-stage system, syngas can be conditioned for use in cogeneration systems to produce electricity. Newer systems are using the syngas to produce clean diesel or hydrogen. Alternative feedstocks, such as agriculture waste FOG, food waste, green waste, and wood waste, can increase the energy content of the syngas. Rather than using it to produce energy, syngas can be purified and injected into a natural gas grid or purified to create an alternative fuel commodity, essentially eliminating combustion and associated emissions.