IWA Resource Recovery Cluster 污水资源回收最新汇编报告(3)
RECOVERY OF RESOURCES FROM WATER
第三部分 资源成分
Components
Many components can be recovered during the treatment process of used water and fromresiduals from water treatment, such asnutrients, metals and biodegradable plastic. Some examples of recovered components are provided below.
1. Nutrients – Phosphoru s and Nitrogen
The two most prominent nutrients that are discussed in terms of resource recovery are phosphorus and nitrogen. These are both critical components to the agricultural system worldwide. While removal of the components from the liquid stream is standard and a widely implemented practice; recovery of the components vary in terms of scope and stage of development. Nutrient recovery can be divided into three sections, namely accumulation, release and extraction in which nutrient products are recovered in the last step. The main focus on nutrient recovery has been on chemical phosphorus products.
1.1 Phosphorus
Since the1950s, several techniques have been investigated for phosphorus recovery fromused water and other aqueous solutions. 22 different P-recovery technologies have been distinguished by Sartorius et al, ranging from lab-scale to full-scale applications. Sources from which phosphorus can be recovered include used water, urine, ash and sewage sludge.
There are two main possibilities of recovering phosphorus from municipal used water, namely recovery from used water treatment and recovery from produced sludge. Recovery from sewage sludge results in, for example, magnesium ammonium phosphate(MAP), calcium phosphate and iron phosphate. MAP is more commonly referred to as struvite and can easily be separated from used water due to its specific gravity. A method in which phosphorus can be recovered from sludge is through supercritical water oxidation (SCWO), a technique which is growing in terms of practice and commercialization. The process destructs organics in the sewage sludge and leaves a slurry of in organic ash in a water phase free from organic contaminants. Components, such as phosphorus and coagulants, can then easily be recovered from the residual ash. As such, phosphorus removal therefore depends on the production of biomass and precipitated sludge.
The majority of the processes involved in recovering a phosphorus product need chemical consumption. Crystallizationhas been proven to be the established technology with the highest percentage of recovered resource for phosphorus, with a recovery rate exceeding 90%.
1.2 Nitrogen
While many multiple technologies remove nitrogen, not as many can recover the resource. Nitrogen removal can be done either biologically or physico-chemically. The selection of the method is based on the concentration of nitrogen in used water. In order to efficiently recover nitrogen from used water, techniques typically require concentrations above 1,000 mg NH4/L.
Nitrogenous materials present in the sewage or paper mill effluents can be removed from sewage effluent and converted into biomass through activated secondary treatment processes. A technology of protein-based wood adhesives sourced from secondary sludge is further currently being investigated.
Fertilizer grade ammonium sulphate can be produced from the high ammonia-nitrogen concentration side streams from sludge digestion processes by stripping and adsorption. This stream can also be treated biologically by nitration and anammox, the latter being autotrophic denitrification. While not resulting in nutrient recovery, this approach significantly reduces energy requirements compared to the conventional nitrification/denitrification processand eliminates the carbon requirement for heterotrophic denitrification. Stripped ammonia can be recovered via condensation, absorption or oxidation, resulting in a concentrated fertilizer product. Nitrate/nitrite species can further be recovered through using liquid-liquid extraction technologies. This method is based upon the technique of separating components based on relative solubility in two immiscible liquids. The end result is a concentrated nutrient solution which can be stripped from the organic phase.
Another way in which ammonium can be removed from the stream of used water is through electrodialysis (see Figure 1). The approach is to first concentrate nutrients into appropriate dialytic leaves in an overall electrodialysis cell and subsequently recover through a range of technologies, including precipitation, adsorption, desorption and air stripping. The technology is founded on the method of using an electrical current in which anions and cations are separated across ion exchange membranes. Multiple nutrients can be recovered through this process but it is most suitable for nitrogen and potassium.
Figure 1 Electrodialysis
BP: biopolar membrane; A: anion-selective membrane, C: cation-selective membrane; M+: cation; X-: anion; H+: hydrogenion; OH-: hydroxideion; CH3O-: methoxideion.
As with phosphorus, WERF states that among the established techniques, crystallizationis a technique with a high percentage of recovery efficiency. The proces s of obtaining struvite is such an example, in which nitrogen is recovered in addition to phosphorus.
2 Metals
There are certain factors that need to be considered when recovering metals. Such features include initial concentrations of all metals,origin of used water, identification of metals to be recovered and the choice between recovering one specific metal opposed to a group of metals. Furthermore, different removal technologies have different benefits. Some have short processing time while others have cheap and easy monitoring systems. Several techniques have a complete removal of metals from water while others have partial removal of some particular metals from the residual ash. As such, phosphorus removal therefore depends on the production of biomass and precipitated sludge.
Used water content from industries such as mining, electrical and electroplating can contain traces of heavy metals such as cadmium, copper, zinc, gold, magnesium, silver and calcium. There are many elaborated techniques for how metals, with a focus on heavy metals, can be removed. Common methods of removing metals involve physiochemical techniques such as filtration, chemical filtration and solvent extraction. Removal can also be performed through adsorption, electrodialysis and through biological and membrane processes; the latter which are becoming more widely accepted. Chemical precipitation is most extensively used formetal removal from inorganic effluents. Drawbacks to the method include a slow metal precipitation and excessive sludge production that requires further treatment. Depending on size of particle that is to be retained, various types of membrane filtration, for example ultrafiltration, nanofiltration and reverse osmosis, can be employed for metal recovery from used water. Membrane bioreactors (MBR), which combines a membrane with a bioreactor, has received increased attention both academically and commercially.
In comparison to the number of removal techniques, there is less emphasis on how metals can be recovered. However, there are a couple of different heavy metals recovery technologies (HMRT), for instance ion exchange, leaching, adsorption, magnetic nanoparticles and foam fractionation which recover different types of metals. Electrolytic recovery is, for example, a method that uses electricity to leave a metal deposit behind which then can be recovered.
Certain techniques can be chosen for specific metals and for recovering metals from specific materials. Cation-exchange capability using synthetic zeolites is, for example, currently being looked into and investigated for their effectiveness of recovering metals through modified natural material. While metal sulphides can be recovered using sulphate reducing bacteria (SRB), electrodialysis can recover metals such as Cr and Cu. Photocatalysis, which is a technique using low-energy ultraviolet light with semiconductor particles, can recover noble metals from industrial waste effluents. Deposited metals can be extracted from slurry by mech anical and/or chemicalmeans. Mercury(II), chromium(VI), silver(I) and iron(III) ions can be recovered using this relatively new technique. Even a by-product such as ash can be chemically modified in order to recover metals from used water.
3 Other Components
3.1 Biodegradable Polymer
One non-traditional technology under development is the production of a biodegradable plastic. Polyhydroxyalkanoates (PHA) are a type of biodegradable polymer, plastic resins, which many types of bacteria synthesize to store energy. PHA are formed when bacteria are introduced to harsh growth conditions due to limited resources of phosphorous and nitrogen for example, and when there is an excess of a carbon sources such as glucose and proteins.
There are currently some small-scale projects that are researching the possibility of producing PHA using biosolids from used water treatment plants. These biosolids represent an ample carbon source that is available at no cost. An advantage of the produced plastic is its lifespan of months which can be compared to the centuries needed to break down petroleum based plastics. A suitable environment is necessary for the bacteria’s growth and there are multiple mature examples of technologies that perform this on scale already. Figure 2 illustrates a prototype used by a start-upcompany for manufacturing such biodegradable plastics using biosolids.
Figure 2 Start-up company Micromidas' concept for manufacturing biodegradable plastics.
3.2 Methane, Carboxylic Acids and Hydrogen
The organic materials in used water can be converted in anaerobic fermentation processes with a mixed community. The mature technology of anaerobic digestion is best known to produce methane, but since the conversion is channelled through carboxylic acids, including volatile fatty acids and hydrogen gas, these products can also be produced when methane production is inhibited. The separation of carboxylic acids is difficult. One method that is currently under investigation is chain elongation within mixed cultures, which produces longer-chain carboxylic acidssuch as caproic acid with six carbons, which can be easier extracted. As such, products typically requiring methane for its production can beproduced even when methane is not accessible.
3.3 Industrial Chemicals
Other products from recovered resources include industrial chemicals such as hydrogen, hydrogen peroxide and caustic solutions. Such products can be produced using microbial electrochemical technologies (MET), for example microbial electrodialysis cells (MEC). These technologies have yet to be scaled-up to full-scale applications. There are further alternate an aerobic processes that result in industrial chemicals.
Sulphate, which is a common chemical in industry, can further be recovered from water and used water. One way of recovering sulphate is through a two-staged process in which sulphate is converted into elemental sulphur (S). In the first step the sulphate is converted into dissolved sulphide in high-rate bioreactors. The sulphide is then oxidized to elemental sulphur by mixing with air and separating it from the liquid. The process further recovers metals such as copper, nickel and zinc as marketable metal sulphides.
4 New Trends
Many of the above described technologies can be classified as new trends in the field of resource recovery. Some of the replies from IWA survey respondents regarding novel developments include recovery of nitrous oxide and ammonium as well as Zero Liquid Discharge. Co-digestion from mainstream deammonification is another prediction of a future trend. Another new innovation that was mentioned in IWA’s survey is nitrogen recovery via ion exchange membranes.
To recover cellulose from used water is another phenomenon that is currently be inginvestigated. Cellulose fibres have recently been highlighted as a potential resource that can be recovered from used water. Although toilet paper is, and has been, a major constituent of used water, there are hitherto few studies that have investigated the behaviour of cellulose fibres in the activated sludge process. Before cellulose is metabolised, it needs to be hydrolysed and this process, to a large extent, depends on temperature and sludge retention time. In the survey, one IWA researcher respondent noted that realization of cellulose production has been one of the biggest developments within his/her research field of resource recovery.
5 Summary
Numerous techniques have been developed that can be used to recover water, energy and different value-added components. Some techniques have been applied since centuries back while others are new or involve new ideas applied to old applications. Concentrations of different types of substrates, quality of resource and internal and external cost are some crucial factors that need to be taken into account when making decisions on what technologies to use and which end product will evolve. As such, there is a need for a systematic approach when considering technologies for recovering different resources. A trade-off will always be seen. For example, techniques recovering nutrients can be efficient but may require a huge quantity of water and energy. Further, some effective technologies can have negative environmental impacts, such as biosolids incineration which releases persistent environmental pollutants.
Factors can also been seen to be influencing each other. Power generation from biogas is for example particularly attractive in areas with high electricity rates. In the United Kingdom when energy prices doubled during 2003-2006, on-site generation of energy from sludge was increased. WERF endorses this by stating that maximum energy available in sewage and sludge is taken advantage of as energy prices rise.
It should be acknowledged that there is no single technology that is perfectly suited for complete nutrient recovery for all scenarios.The same can be said for water and energy. In all these sectors new trends and technological innovations are continuously emerging. In order for such technologies to be implemented and used on full-scale several factors such as demand, policies and social acceptance need to be taken into consideration.
Authors: Katrin Eitrem Holmgren, Hong Li, Willy Verstraete and Peter Cornel
本文节选自IWA Resource Recovery Cluster的最新汇编报告 State of the Art Compendium Report on Resource Recovery from Water。更多内容，请访问IWA网站: http://www.iwa-network.org/cluster/resource-recovery-from-water-cluster
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