By definition, a circular economy is a systemic approach to economic development designed to benefit businesses, society, and the environment. The management of water and wastewater can be viewed as a special business to produce water with sufficient quantity and suitable quality for consumers. Therefore, the principle of circular economy should be applied to water management, especially when we stress Water Sustainability. The value of water to sustain human life has been fully realized for long, but the idea of “Wastewater is a resource” is yet to be widely realized. Taking domestic wastewater as an example, we can say that it is the water polluted by various contaminants entering the water flow during water use. We may also say that as a result of water pollution, the water itself has lost its value and become Wastes. However, the truth is that the process of water use does not bring about any change in water molecules, and they are still there in the wastewater flow. As long as most of the contaminants can be separated from the liquid phase, the water can become useful again. On the other hand, the major part of the contaminants are organic substances and nutrients. They are not much different in their nature from those we are using as resources for generating energy and producing fertilizers. As long as technologically and economically feasible, energy and materials can also be recovered from the wastewater flow. This is the reason that more and more people call for practices of water reclamation and energy/materials recovery from wastewater. Such a topic is within the scope of circular economy and becomes the direction of development toward the future.
Water Reclamation
Water reclamation (also called wastewater reuse, water reuse or water recycling) is the process of converting domestic wastewater (sewage) or industrial wastewater into water that can be reused for a variety of purposes. Types of reuse include: urban reuse, agricultural reuse (irrigation), environmental reuse, industrial reuse, planned potable reuse, wastewater reuse (unplanned potable reuse). Reuse may include irrigation of gardens and agricultural fields or replenishing surface water and groundwater (i.e., groundwater recharge).
Reused water may also be directed toward fulfilling certain needs in residences (e.g., toilet flushing), businesses, and industry, and could even be treated to reach drinking water standards. Treated domestic wastewater reuse for irrigation is a long-established practice, especially in arid countries. Reusing wastewater as part of sustainable water management allows water to remain as an alternative water source for human activities. This can reduce scarcity and alleviate pressures on groundwater and other natural water bodies. There are various technologies used to treat wastewater for reuse. A combination of these technologies can meet strict treatment standards and make sure that the processed water is hygienically safe. These technologies can be classified into the following categories:
Upgrading Conventional Wastewater Treatment is a widely applicable strategy to promote water reclamation. The typical conventional wastewater treatment process is the so-called Secondary Treatment with biological oxidation (e.g., activated sludge process) as the core treatment unit. As stricter regulations have been put forward in many countries and regions on treated water quality even when the main objective of treatment is for discharge to receiving waters, there is growing requirement for upgrading the conventional wastewater treatment plants. There are basically two kinds of technological approaches for treatment process upgrading. One is the enhancement of biological treatment such as by combination of aerobic, anoxic, and anaerobic processes for enhancing nutrient removal, and another is to add additional advanced treatment units after the secondary treatment process, such as chemical oxidation, adsorption, and even membrane filtration. In some countries, the quality requirement for treated effluent is almost equivalent to that for water reuse. In such cases, the treated effluent from the upgraded plants has already become usable for many purposes of reuse. The topic thus becomes how to use the treated effluent wisely for mitigating the envisaged problem of water shortage.
Treatment for Water Reclamation is another type of practice for purposed reclaimed water supply using the treated effluent from conventional wastewater treatment plants as the source water. In such cases, the treatment facilities can be built within the wastewater treatment plant for treating part of the effluent to meet the requirement of reclaimed water supply, or an independent reclaimed water production plant. Regarding water treatment for water reclamation, the process design almost follows the similar principle of drinking water production. The difference is mainly in the source water (wastewater treatment effluent vs natural source water) and the purpose of water use (non-potable use vs potable use in most cases). There are also examples of water reclamation for purposed potable use. What to be paid with special attention in this case may be the impurities, mostly in trace concentration but unignorable health risk, originated from human feces and generated in the biological wastewater treatment process.
Industrial Wastewater Treatment and Reuse is somewhat different from domestic wastewater treatment and reuse. The pollutants contained in the industrial wastewater flow usually depend on the nature of industrial production processes. For food industries, the wastewater generated may contain pollutants very similar to those in domestic wastewater but in some cases with much higher concentrations; for dyeing industries, inorganic and/or organic dyes are the main objective of removal; for chemical industries, the pollutants may be more complicated in terms of variety, concentration, and toxic nature; while for cooling water in many large industries, it may not be very polluted after being used in a single cycle. It is thus suggestable that industrial wastewater treatment and reuse be confined in the industrial plant by using appropriate technologies.
Materials/Resource/Energy Recovery
In addition to reusable water, resource recovery from wastewater facilities in the form of energy, biosolids, and other resources, such as nutrients, represents an economic and financial benefit that contributes to the sustainability of water supply and sanitation systems and the water utilities operating them.
One of the key advantages of adopting circular economy principles in the processing of wastewater is that resource recovery and reuse can transform sanitation from a costly service to one that is self-sustaining and adds value to the economy. Indeed, if financial returns can cover operation and maintenance costs partially or fully, improved wastewater management offers a double value proposition. Recovering energy from wastewater was most prevalent in large-scale plants in the form of biogas and/or electricity generated from sludge. It is reported that the unit energy recovery can reach 1.3 to 2.9 MJ/m3 in the form of biogas, and 0.14 to 0.97 MJ/m3 in the form of electricity. Domestic wastewater also contains about 15 g/m3 of organic nitrogen, 25 g/m3 of ammonia, and 8 g/m3 of phosphorus, which are also very important resources for recovery. The highest level of nutrient recovery (and highest efficiency) is usually achieved at small scale plant using urine source separation where a separation efficiency up to 80% is achievable. The integration of wastewater treatment systems in terms of resource recovered can show significant reductions in costs and environmental impacts. However, several challenges need to be considered for their implementation, including monitoring and management, flowrate reductions, an increased risk for system failures, social acceptance, and regulatory compliance.
Anaerobic Digestion for Energy Recovery is the most common practice in many countries and regions in the whole world. Anaerobic Digestion (AD) process has been developed as environmentally friendly and cost-effective technology for biodegradable materials degradation, sludge stabilization, and biogas production from wastewater resources. Biogas production by chemical energy contained in organic matters from wastewater through the anaerobic digestion of biosolids for electric power and thermal energy generation is one of the most promising applications of on-site energy recovery. By sewage sludge transformation into biogas, a mixture of methane (50%–70%), carbon dioxide (30%–50%), and traces of other gases, such as nitrogen and hydrogen are produced. The produced methane in the treatment plant can be used to feed the gas engines and produce both electrical and thermal energy. Widespread sources such as organic fraction of municipal solid waste, waste activated sludge, animal manures, industrial wastes, energy crops, and algae are used in the anaerobic digestion process. By a series of phases of biochemical reactions in the sequence of hydrolysis, acidogenesis, acetogenesis, and methanogenesis, organic substance in the sewage sludge will be transformed into biogas. It is estimated that wastewater treatment plants with sludge digestion consume about 40% less net energy than those without anaerobic digestion. Also, for improving anaerobic digestion process in WWTPs, thermal hydrolysis technologies are highly recommended, and from economic and environmental point of view, co-digestion of sewage sludge with other biodegradable waste is recommendable.
Nutrients Recovery from Wastewater can be achieved by the integration of multiple nutrient recovery technologies (NRTs) is recommendable. Ammonia stripping, adsorption and struvite precipitation are three of the most dominant physicochemical NRTs. Ammonia stripping could theoretically make up for the limitation of struvite precipitation in terms of nitrogen recovery, given its equimolar recovery on ammonium nitrogen and orthophosphate phosphorous. The optimal pH ranges for struvite precipitation and ammonia stripping are 8.5–10 and > 9.25, respectively. The overlap on their optimal pH ranges makes their integration feasible. So far, the technical integration of these two physicochemical technologies has been applied for source-separated urine, swine wastewater and liquid digestate. And it has been achieved in either “One single integrated reactor” or “Multiple discrete reactors”. The application of struvite precipitation requires a good struvite collection system; otherwise, poor solid–liquid separation could cause the considerable loss of struvite crystals. From this perspective, a crystal size of at least 100 μm is desirable for optimal struvite collection. To enlarge the crystal size, some small particles of adsorbents are integrated with struvite precipitation as seeding materials. In addition to the seeding material, adsorption has also been used to recover the amount of nutrients that cannot precipitate as struvite. Stripping plus adsorption is also a technical integration for nutrients recovery. In this case, stripping is mainly involved into regenerating adsorbents. Zeolite is the most common adsorbent involved, and other adsorbents, such as activated carbon, metal organic frameworks and other nanostructured materials can also be used as the alternative of the acid scrubber following the stripping column for nitrogen recovery.
Sludge Composting is a biological process that uses naturally occurring microorganisms to convert biodegradable organic matter in the sewage sludge into a humus-like product, known as compost, which is a good fertilizer for plants. The composting process destroys pathogens, converts nitrogen from unstable ammonia to stable, organic forms of nitrogen, and reduces the volume of waste. This process is controlled by environmental parameters (temperature, moisture content, pH, and aeration) and substrate properties (C/N ratio, particle size, and nutrient content). Due to high moisture and low carbon contents, sludges must be mixed with dry materials for composting. The materials used as bulking agents when composting waste treatment sludges include the organic fractions of municipal solid waste, sawdust, wood chips, and many other agricultural wastes. Wastes containing lignin, such as plant residues or hulls from agricultural production systems, are difficult to manage and dispose, because they are bulky and have a low commercial value. These materials may be used as regulatory bulking agents to balance the moisture contents of the sludge and increase its porosity to permit airflow. In addition, these materials may be used to balance the C/N ratio and provide additional carbon for improve the microbial activity. Supplementation of bulking agents can also provide optimum free air space and void dispersion in composting, which permit adequate water and gas exchange between gas and solid phases, and prevent excessive compaction of the composting materials.
Energy Saving and Emission Reduction
Energy use can account for as much as 10% of the annual operating budget for public utilities in a city. A significant amount of this municipal energy use occurs at water and wastewater treatment facilities. With pumps, motors, and other equipment operating 24 hours a day, seven days a week, water and wastewater facilities can be among the largest consumers of energy. The major part of the energy consumed is in the form of electricity.
Energy Saving in Water Processing and Distribution is an important topic for water industries due to the fact that electricity consumption accounts for at least more than 2/3 of the operating costs. The objective of energy saving is not merely for a reduction of the operating cost to gain economic benefit but more importantly the reduction of energy and environmental burden of water supply. Large part of the electrical energy consumed in water processing and distribution is for pumps, including those used in the drinking water facilities and those used in the distribution systems. Theoretically, the power consumed to drive pumps depends on the change in total pressure between the inlet and outlet of a pump which relates to the hydraulic head to be reached by water lifting, the volume flow-rate of the water, and pump efficiency. Therefore, there are basically three pathways for energy saving. The first is to decrease the hydraulic head of water lifting, which relies on rational planning and design of the whole water system; the second is to reduce water flow, which relates water saving for various water supplies; and the third is to improve pump efficiency, which relies on selection of energy efficient equipment in system design and upgrading. Many countries are implementing energy efficiency programs in water systems. Very sophisticated measures are often adopted in these programs in the stages of “Plan, Do, Check, and Act”, thus forming systematic frameworks in accordance with practical situation.
Energy Saving in Operating Wastewater Facilities slightly differs water processing and distribution. Although the operation of wastewater treatment plants entails a huge amount of electricity, thermal energy is also required for pre-heating the sludge and sometimes exsiccation of the digested sludge. On the other hand, the entering organic matter contained in the wastewater is a source of energy. The organic matter is firstly concentrated in the separated sludge, and can then be digested in anaerobic digester to produce biogas which is potential energy source. The onsite availability of biogas represents a great opportunity to cover a significant share of electricity and thermal demands for the wastewater treatment. The biogas can be efficiently converted into electrical energy and heat via high temperature fuel cell generators. It has been underlined in many studies that the energy content potentially available in water is higher than standard energy consumption for wastewater treatment. However, through the conventional combination of aerobic water treatment coupled with anaerobic digestion, only a portion of the inlet energy is recovered. There is till a room for technological development and application toward the goal of “energy neutral”, namely a transition of energy use toward self-sufficient wastewater treatment plants.
Emission Reduction in Water Industry targets the reduction of GHG, such as CO2, methane (CH4) and nitrous oxide (N2O) emissions from water industries. For water supply facilities, direct GHG emission may not be a major issue, so attention is mainly paid to GHG emission from wastewater systems, including sewers and wastewater treatment facilities. Studies have indicated that CO2, and CH4 are generated from urban sewers at non-negligible level, and methyl alcohol, methylamine and acetic acids existing in the sewer flow are causing substances. With regard to GHG emission from wastewater treatment plants, the organic carbon of wastewater is either incorporated into biomass or oxidized to CO2. The main sources of CH4 emissions are related to the sludge line units where anaerobic digestion is carried out, the primary sludge thickener, the centrifuge, the exhaust gas of the cogeneration plant, the buffer tank for the digested sludge, and the storage tank for the dewatered sludge. These units contribute to around 72% of methane emissions while the remaining emissions come from the biological reactors and can be mainly attributed to the CH4 dissolved in the wastewater. N2O production occurs mainly in the activated sludge units (90%) while the remaining 10% comes from the grit and sludge storage tanks. N2O gas is formed during denitrification operated at low pH values and toxic compounds or low DO concentrations are present in the media. As most of the technologies available to remove GHG are expensive or even not suitable to be applied to gaseous streams of the wastewater treatment plants, minimization of GHG emissions by a good control of the operational conditions of the biological systems, including the main stream and sludge line, is the major strategy.