前沿关注碳中和：污水处理技术及运行在污水厂节能与能量回收应用

查看全部 · 发表于 2021-5-29 17:02:37

碳中和是实现全球可持续污水处理厂的一项关键指标。几年前，欧洲和美国一些污水处理厂便开始了它们面向碳中和运行的脚步，并建议到2030年时实现各自碳中和运行。例如，荷兰STOWA（应用水研究基金组织）早在2008年对其污水处理厂回收资源与能源便便制定了路线图，并为此提出了面上未来污水处理厂的NEWs（营养物+能源+再生水工厂）概念。许多研究与工程试验已被用于探知从污水中回收能源，以满足污水处理运行现场能量自给自足的可行性；这些举措亦支持减少污水处理厂全生命周期温室气体排放的相关目标。一些能量中和运行的污水处理厂已在一些欧美国家出现，但是，面向碳中和运行目标的发展进程仍未很好建立。

实际上，碳中和常常与能量中和等同起来。从污水中回收资源以及低能耗污水处理的技术研发具有宽广的范围，包括从进水有机物及剩余污泥中回收能源、基质共消化、热量回收、污泥焚烧等等。然而，除了能量之外，污水处理厂也常常从处理工艺本身和对资源消耗中（如，化学药剂、混凝土等）诱发更多温室气体问题（例如，N2O 或CH4）。因此，有关能源消耗、能量回收/生产、以及温室气体直接排放与间接排放的一系列解决方案均需研发，以建立污水处理厂作为碳中和运行的实体形式存在。

在此情形下，《Water Reaserch》编委会于2014年夏天决定出版一期与碳中和运行相关的专刊。本专刊旨在讨论新理念、新思想，以此推动研发节能与能量回收为目的的污水处理技术并运行污水出来厂。从大约50篇特邀与开放投稿中，我们根据同行评审结果筛选出13篇论文，涵盖面向能量回收潜能、基质共消化目标的新工艺、新方法研发，以定量、定向多尺度范围内的可持续性平衡。

Potentials of energy recovery from wastewater treatmentand/or wastewater heat

从污水处理或污水热量中回收能源潜能

Excess sludge isdefinitely an important energy source to be recovered via anaerobic digestion.However, the amount of excess sludge depends heavily on the influent organic(carbon source: COD) concentrations. In some cases, carbon sources are insufficient,and barely meetthe needs of nutrient removal, and thus energy neutrality cannot be achieved, oris incompatible with conventional nutrient removal. Anaerobic digesters generallyhave surplus capacity (about 20% inorganic wastes (organicfraction of municipal waste) to anaerobic digesters toimprove the energy balance of a WWTP substantially, resulting in “1+1>2” in terms of biogasproduction and solids reduction (Aichinger et al.). The results reveal that organic co-substrate addition ofup to 94% of the organic sludge load resulted in tripling the biogas productionand that at an organic co-substrate addition of up to 25% no significantincrease in cake production was observed and only a minor increase in ammoniarelease of about 20% was detected. The case studies fully demonstratedco-digestion for maximizing synergy as a step towards energy efficiency andultimately towards carbon neutrality.

On the other hand, thermal energy in wastewater could be converted intoheat to balance the energy deficit towards carbon neutrality. An evaluationstudy on the energy balance of WWTPs (generally COD=200-400 mg/L) reveals that anaerobicdigestion of excess sludge only provides some 50% of the total amount of energyconsumption in3×℃when 1 m3 of the effluent is cooled by 1 ℃. Overall, therefore, organic andthermal energy sources could effectively supply enough electrical equivalencyforChinato reach to its target with regards to carbon-neutral operations.

Co-substrate digestion of both organics and inorganics (CO2)

有机物与无机物(CO2) 基质共消化

As mentioned above,co-digestion of excess sludge with external solid/liquid organics is apotential approach to carbon neutrality. CO2 addition was also testedto stimulate methane production in digestion. A bench-scale investigationproved that high methane production was achievable with the addition ofconcentrated external organic wastes to municipal digesters, at acceptablyhigher levels of digester organic loadings and with lower retention times. Thisallowed the effective implementation of combined heat and power (CHP) programsat municipal wastewater treatment plants, with significant cost savings (Tandukar and Pavlostathis). Industrial liquidwaste obtained from a chewing gum manufacturing plant (GW) and dewateredfat-oil-grease (FOG) were chosen as the external organics, and co-digestion ofexcess sludge (primary + secondary at 40:60 w/w TS basis) with GW, FOG or bothwas evaluated using four bench-scale, mesophilic (35 oC) digesters.The results show that biogas production increased significantly and additionaldegradation of the excess sludge between 1.1 and 30.7% was observed. Both biogasand methane productions were very close to the target levels necessary to closethe energy deficit. Furthermore, co-digestion resulted in an effluent qualitysimilar to that of the control digester fed only with the excess sludge,indicating that co-digestion had no adverse effects.

  Co-digestion of other organics than excesssludge has identical mechanisms and is often the basis of co-digestion ofexcess sludge. A bench-scale study on co-digestion of dairy manure (MN) withexternal organics (food waste - FW, alkaline hydrolysate – AH and crudeglycerol - GY) evaluated the long-term stability of anaerobic digesterscompared to mono-digestion. Microbiome succession and time-scale variability wasalso assessed (Usack and Angenent; Regueiro et al.). After operating for 900 d, four mesophilic individualco-digesters demonstrated different behaviors on both specific methane yield(SMY)/produced inhibitory compounds, and links between changing environmentalconditions and the microbiome composition. Among other things, GY co-digestionresulted in an optimum SMY of 549±25 mL CH4/g VS at a total organicloading rate (OLR) of 3.2 gVS/L·d (MN:GY = 62:38); stable digestion beyond this level was restricted by anaccumulation of long-chain fatty acids and foaming (Usackand Angenent). FD and AH co-digestion had the almost SMY (around 300 CH4/gVS at OLD=3.9 and 2.7 gVS/L·d; MN:FW = 51:49 and MN:AH = 75:25) ; FW caused no reduction inperformance or stability, but AH caused free ammonia concentration at levelspreviously reported as inhibitory, and may have led to the observedaccumulation of volatile fatty acids at higher loading rates (Usack and Angenent). Moreover, high throughput 16S rRNA gene sequencing, examiningthe microbiome succession revealed that the AH reactor microbiome shifted andadapted to high concentrations of free ammonia, total volatile fatty acids, andpotassium to maintain its function, and that adding FD and GY as co-substratesalso led to microbiome changes, but to a lesser extent, especially in the caseof the GY digester microbiome (Regueiro et al.).

As is well known, carbon dioxide (CO2)is a product along with methane production during digestion. On the other hand,CO2 enrichment of anaerobic digesters (AD) was previously identifiedas a potential on-site carbon revalorization strategy. Two pilot-scale ADstreating food waste were monitored for 225 d, with the test unit beingperiodically injected with CO2 using a bubble column (Fernández et al.). The test AD maintained a CH4 production rate of0.56±0.13 m3 CH4/kgVS×d (vs 0.45±0.05 in the control) while maintaining aCH4 concentration in biogas of 68%. An additional uptake of 0.55 kg of exogenous CO2. A2.5 fold increase in hydrogen (H2) concentration was observed andattributed to CO2 dissolution and to an alteration of the acidogenesisand acetogenesis pathways.

New processes for organic energy conversion fromwastewater

从污水有机物中转化能源新工艺

High-rate activatedsludge (HRAS) processes (like the A stage in the A/B process) are often used tosequester organics from wastewater for energy generation in an efficient manner.A HRAS pilot plant at psychrophilic temperatures was operated under controlledconditions. This enabled concentration of influent particulate, colloidal, and solubleCOD to a waste solids stream with minimal energy input, by maximizing sludgeproduction, bacterial storage, and bioflocculation (Jimenez et al.). Results indicatethat important design parameters such as SRT, HRT and DO had little impact onthe removal of soluble COD. Therefore, controlling and maximizing removal ofcolloidal and particulate COD while minimizing mineralization and hydrolysis ofthe slowly biodegradable COD is pivotal for carbon redirection. Operating at alow SRT and HRT, the observed yield was near its maximum resulting in optimaluse of COD for biomass production near maximum sludge production rates. Under theseoperating conditions, the HRAS systems required almost 60% less aeration energyto remove a large fraction of the influent COD (50-80%) when compared to aconventional HRAS process.

  Physically sequestering organics (finesieved fraction – FSF: mainly toilet paper) from wastewater is beingproposed for energy generation. A bench-scale SBR study on digesting FSF fromthe influent of a municipal WWTP in thermophilic (55 °C) and mesophilic (35 °C) digesters demonstrated that FSF is a readilydigestable solids stream. Decreasingthe AD batch cycle period resulted in improved digester performances, particularly withregard to the thermophilic digester, i.e. shortened lag phases and reduced VFAs’peaks (Ghasimiet al.). Moreover,the two digesters harbored verydifferent bacterial and archaeal communities, with OP9 lineage and Methanothermobacter beingpre-dominant in the thermophilic digester and Bacteroides and Methanosaetadominating the mesophilic digester.

  Methaneproduction via digestion is highly temperature dependent. Instead, sequesteringorganics viaconventional primary clarification could directly be integrated with psychrophilicanaerobic digestion for methane production. A pilot-scale anaerobic baffled reactor (ABR)was operated for more than two years to treatraw wastewater at water temperatures ranging from 12 to 23 ˚C (Hahn and Figueroa).The ABR not only exceeded the goal of meeting conventional primary clarification(TSS=83±10%, COD=43±15% and BOD5=47±15%), but also enabled directcapture of the biogas (average 0.45 kWh/m3). Moreover, no settled sludge was wasted from the reactor inover two years of operation. Thus, an ABR can be implemented in place of a primaryclarifier with mesophilic anaerobic digestion and achieve the same treatmentoutcomes in a single unit process at ambient temperature, which does notrequire input of energy or chemical treatment. This paper also extensivelyassessed the potential of methane to supersaturate.

  Stimulating methane production in digestioncould also be enhanced by some innovative technologies such as microbialelectrolysis cells (MEC). An electrically-assisted digester (EAD: equipped witha MEC bioanode and cathode) and a control digester were applied to treat wasteactivated sludge from a municipal WWTP under ambient temperature conditions (22-23 °C) and three SRTs (7, 10 and 14 d)(Asztalos and Kim). The EAD showed reduced concentration of aceticacid, propionic acid, n-butyric acid and iso-butyric acid, thought to be due todirect oxidation of the short-chain fatty acids at the bioanode as well as anindirect contribution of low acetic acid concentration to enhancingbeta-oxidation. The VSS and COD removal was consistently higher in the EAD by 5-10%,compared to the control digester for all conditions. Furthermore, the magnitudeof electrical current in the EAD was governed by the organic loading rate whileconductivity and acetic acid concentration showed negligible effects on currentgeneration.

Differentroutes to carbon neutrality and sustainability

面向碳中和与可持续性的不同路径

As mentioned above,carbon neutrality is often referred to energy neutrality. There are however, many other routes tocarbon neutrality. This includes the management of heat resources and nutrient recovery from urine, as the greatest potential forreduction of greenhouse gas emissions is at the household level (i.e. decentralizedsystems),and thus robust wastewater management must be able to cope with the possibilityof a temperature decrease as a result (Larsen). In WWTPs, there is substantialpotential for energy optimization, both from improving electromechanicaldevices and sludge treatment as well as through the implementation of moreenergy-efficient processes such as mainstream Anammox process or nutrientrecovery from urine. Whether carbon neutrality can be achieved depends not onlyon actual net electricity production, but also on the type of electricityreplaced: the cleaner the marginal electricity, the more difficult tocompensate for direct emissions, which can be substantial, depending on thestability of the biological processes. It is possible, for example, to combineheat recovery and nutrient recovery from urine at the household level, both of whichhave considerable potential to improve the climate friendliness of wastewatermanagement.

  Improvingthe energy balance of WWTPs, with the aim of moving towards carbon neutrality,may benefit the environment due to reduced carbon emissions. However, there is alsoa need to explore wider economic, environmental and societal impacts, as sustainabilityis a complex, multi-dimensional concept comprising of these factors and/orindicators. In this respect, ‘carbon neutrality’ or ‘energy neutrality’ do notnecessarily imply sustainable operation as they address only one element ofsustainability and implementation of low carbon solutions may have unintendeddetrimental effects on other aspects. An evaluation study demonstrates thatreducing energy use and/or increasing energy recovery to reduce net energy canbe detrimental to sustainability (Sweetapple et al.). In the study, sustainability indicators includingoperational costs, net energy and multiple environmental performance measuresare calculated. This enables identification of trade-offs between different componentsof sustainability, which must be considered before implementing energyreduction measures. A major conclusion, highlighted at the end, is that improvingthe energy balance (as may be considered an approach to achieving carbonreduction) is not a reliable means of reducing total greenhouse gas emissions.

  A positive analysis of carbon neutralitytowards sustainability illustrates that there are design and operationalconditions under which submerged anaerobic membrane bioreactors (AnMBRs) couldbe net energy positive and contribute to the pursuit of carbon negativewastewater treatment (Pretel et al.). In this analysis, a quantitative sustainable design process was leveraged todevelop a detailed design of submerged AnMBR by evaluating the full range offeasible design alternatives using technological, environmental, and economiccriteria, which integrated steady-state performance modeling across seasonaltemperatures (using pilot-scale experimental data and the simulating softwareDESASS), life cycle costs (LCC) analysis, and life cycle assessment (LCA). Ultimately,the authors demonstrate the need to integrate economic and environmentalassessments in decision-making by quantifying how mitigating GHG emissions maytransition from being financially advantageous to prohibitively expensive, evenacross a single design decision.

Taken altogether, these articlesadvance our understanding of how to achieve carbon neutral WWTPs. This laudablegoal will undoubtedly require a portfolio of solutions, requiring academia andindustry to work together on numerous fronts to establish WWTPs as not only aprotector of the local aquatic environment, but also the global environmentthat we all share.

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