碳中和：一個(gè)面向可持續(xù)污水處理廠可以實(shí)現(xiàn)的目標(biāo)
碳中和是實(shí)現(xiàn)全球可持續(xù)污水處理廠的一項(xiàng)關(guān)鍵指標(biāo)。幾年前，歐洲和美國(guó)一些污水處理廠便開始了它們面向碳中和運(yùn)行的腳步，并建議到2030年時(shí)實(shí)現(xiàn)各自碳中和運(yùn)行。例如，荷蘭STOWA（應(yīng)用水研究基金組織）早在2008年對(duì)其污水處理廠回收資源與能源便便制定了路線圖，并為此提出了面上未來污水處理廠的NEWs（營(yíng)養(yǎng)物+能源+再生水工廠）概念。許多研究與工程試驗(yàn)已被用于探知從污水中回收能源，以滿足污水處理運(yùn)行現(xiàn)場(chǎng)能量自給自足的可行性；這些舉措亦支持減少污水處理廠全生命周期溫室氣體排放的相關(guān)目標(biāo)。一些能量中和運(yùn)行的污水處理廠已在一些歐美國(guó)家出現(xiàn)，但是，面向碳中和運(yùn)行目標(biāo)的發(fā)展進(jìn)程仍未很好建立。
實(shí)際上，碳中和常常與能量中和等同起來。從污水中回收資源以及低能耗污水處理的技術(shù)研發(fā)具有寬廣的范圍，包括從進(jìn)水有機(jī)物及剩余污泥中回收能源、基質(zhì)共消化、熱量回收、污泥焚燒等等。然而，除了能量之外，污水處理廠也常常從處理工藝本身和對(duì)資源消耗中（如，化學(xué)藥劑、混凝土等）誘發(fā)更多溫室氣體問題（例如，難以捕捉的N2O 或CH4）。因此，有關(guān)能源消耗、能量回收/生產(chǎn)、以及溫室氣體直接排放與間接排放的一系列解決方案均需研發(fā)，以建立污水處理廠作為碳中和運(yùn)行的實(shí)體形式存在。
在此情形下，《Water Reaserch》編委會(huì)于2014年夏天決定出版一期與碳中和運(yùn)行相關(guān)的專刊。本�？荚谟懻撔吕砟�、新思想，以此推動(dòng)研發(fā)節(jié)能與能量回收為目的的污水處理技術(shù)并運(yùn)行污水出來廠。從大約50篇特邀與開放投稿中，我們根據(jù)同行評(píng)審結(jié)果篩選出13篇論文，涵蓋面向能量回收潛能、基質(zhì)共消化目標(biāo)的新工藝、新方法研發(fā)，以定量、定向多尺度范圍內(nèi)的可持續(xù)性平衡。
Potentials of energy recovery from wastewater treatment and/or wastewaterheat
從污水處理或污水熱量中回收能源潛能
Excess sludge is definitely animportant energy source to be recovered via anaerobic digestion. However, theamount of excess sludge depends heavily on the influent organic (carbon source:COD) concentrations. In some cases, carbon sources are insufficient,and barelymeet the needs of nutrient removal, and thus energy neutrality cannot beachieved, or is incompatible with conventional nutrient removal. Anaerobicdigesters generally have surplus capacity (about 20% in Germany), which isavailable for co-substrate digestion along with excess sludge. This ishighlighted by a full-scale study in Austria demonstrating the use of existinginfrastructure by addition of organic wastes (organic fraction of municipal waste) to anaerobic digesters to improve the energybalance of a WWTP substantially, resulting in “1+1>2” in terms of biogas production and solidsreduction (Aichinger et al.). The results reveal that organic co-substrateaddition of up to 94% of the organic sludge load resulted in tripling thebiogas production and that at an organic co-substrate addition of up to 25% nosignificant increase in cake production was observed and only a minor increasein ammonia release of about 20% was detected. The case studies fullydemonstrated co-digestion for maximizing synergy as a step towards energyefficiency and ultimately towards carbon neutrality.
On the otherhand, thermal energy in wastewater could be converted into heat to balance theenergy deficit towards carbon neutrality. An evaluation study on the energybalance of WWTPs (generally COD=200-400 mg/L) reveals that anaerobic digestionof excess sludge only provides some 50% of the total amount of energy consumptionin China (Hao et al.). The study furtherindicates that water source heat pumps (WSHP) could effectively convert thethermal energy contained in wastewater to heat WWTPs and neighborhoodbuildings, which could supply a net electrical equivalency of 0.26 kWh/m3× ℃ when 1 m3 of the effluent is cooled by 1 ℃. Overall,therefore, organic and thermal energy sources could effectively supply enoughelectrical equivalency for China to reach to its target with regards tocarbon-neutral operations.
Co-substratedigestion of both organics and inorganics (CO2)
有機(jī)物與無機(jī)物(CO2) 基質(zhì)共消化
As mentioned above, co-digestion of excesssludge with external solid/liquid organics is a potential approach to carbonneutrality. CO2 addition was also tested to stimulate methaneproduction in digestion. A bench-scale investigation proved that high methaneproduction was achievable with the addition of concentrated external organicwastes to municipal digesters, at acceptably higher levels of digester organicloadings and with lower retention times. This allowed the effectiveimplementation of combined heat and power (CHP) programs at municipalwastewater treatment plants, with significant cost savings (Tandukar andPavlostathis). Industrial liquid waste obtained from a chewing gummanufacturing plant (GW) and dewatered fat-oil-grease (FOG) were chosen as theexternal organics, and co-digestion of excess sludge (primary + secondary at40:60 w/w TS basis) with GW, FOG or both was evaluated using four bench-scale,mesophilic (35 oC) digesters. The results show that biogasproduction increased significantly and additional degradation of the excesssludge between 1.1 and 30.7% was observed. Both biogas and methane productionswere very close to the target levels necessary to close the energy deficit.Furthermore, co-digestion resulted in an effluent quality similar to that ofthe control digester fed only with the excess sludge, indicating thatco-digestion had no adverse effects.、
Co-digestionof other organics than excess sludge has identical mechanisms and is often thebasis of co-digestion of excess sludge. A bench-scale study on co-digestion ofdairy manure (MN) with external organics (food waste - FW, alkaline hydrolysate– AH and crude glycerol - GY) evaluated the long-term stability of anaerobicdigesters compared to mono-digestion. Microbiome succession and time-scalevariability was also 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 g VS/L·d;MN:FW = 51:49 and MN:AH = 75:25) ; FW caused no reduction in performance orstability, but AH caused free ammonia concentration at levels previouslyreported as inhibitory, and may have led to the observed accumulation ofvolatile fatty acids at higher loading rates (Usackand 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 alongwith methane production during digestion. On the other hand, CO2enrichment of anaerobic digesters (AD) was previously identified as a potentialon-site carbon revalorization strategy. Two pilot-scale ADs treating food wastewere monitored for 225 d, with the test unit being periodically injected withCO2 using a bubble column (Fernández et al.). The test ADmaintained a CH4 production rate of 0.56±0.13 m3 CH4/kg VS×d (vs 0.45±0.05 in the control) while maintaining a CH4concentration in biogas of 68%. An additional uptake of 0.55 kg of exogenous CO2. A 2.5fold 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 from wastewater
從污水有機(jī)物中轉(zhuǎn)化能源新工藝
High-rate activated sludge (HRAS)processes (like the A stage in the A/B process) are often used to sequesterorganics from wastewater for energy generation in an efficient manner. A HRASpilot plant at psychrophilic temperatures was operated under controlledconditions. This enabled concentration of influent particulate, colloidal, andsoluble COD to a waste solids stream with minimal energy input, by maximizingsludge production, 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. Underthese operating conditions, the HRAS systems required almost 60% less aerationenergy to remove a large fraction of the influent COD (50-80%) when compared toa conventional HRAS process.
Physicallysequestering organics (fine sieved fraction – FSF: mainly toilet paper) from wastewater is being proposed for energy generation. Abench-scale SBR study on digesting FSF from the influent of a municipal WWTP inthermophilic (55 °C) andmesophilic (35 °C)digesters demonstrated that FSF is a readily digestable solids stream. Decreasingthe AD batch cycle period resulted in improved digester performances,particularly with regard to the thermophilic digester, i.e. shortened lagphases and reduced VFAs’ peaks (Ghasimi et al.). Moreover, the two digesters harbored very differentbacterial and archaeal communities, with OP9 lineage and Methanothermobacterbeing pre-dominant in the thermophilic digester and Bacteroides and Methanosaetadominating the mesophilic digester.
Methane production via digestion is highly temperature dependent. Instead,sequestering organics via conventional primary clarification could directly beintegrated with psychrophilic anaerobic digestion for methane production. Apilot-scale anaerobic baffled reactor (ABR) was operated for more than twoyears to treat raw wastewater at water temperatures ranging from 12 to 23 ˚C (Hahn andFigueroa). The ABR not only exceeded the goal of meeting conventionalprimary clarification (TSS=83±10%, COD=43±15% and BOD5=47±15%), butalso enabled direct capture of the biogas (average 0.45 kWh/m3).Moreover, no settled sludge was wasted from the reactor in over two years ofoperation. Thus, an ABR can be implemented in place of a primary clarifier withmesophilic anaerobic digestion and achieve the same treatment outcomes in asingle unit process at ambient temperature, which does not require input ofenergy or chemical treatment. This paper also extensively assessed thepotential of methane to supersaturate.
Stimulating methane production in digestion could also be enhanced bysome innovative technologies such as microbial electrolysis cells (MEC). Anelectrically-assisted digester (EAD: equipped with a MEC bioanode and cathode)and a control digester were applied to treat waste activated sludge from amunicipal WWTP under ambient temperature conditions (22-23 °C) and three SRTs (7, 10 and 14 d) (Asztalos andKim).The EAD showed reduced concentration of acetic acid, propionic acid, n-butyricacid and iso-butyric acid, thought to be due to direct oxidation of theshort-chain fatty acids at the bioanode as well as an indirect contribution oflow acetic acid concentration to enhancing beta-oxidation. The VSS and CODremoval was consistently higher in the EAD by 5-10%, compared to the controldigester for all conditions. Furthermore, the magnitude of electrical currentin the EAD was governed by the organic loading rate while conductivity andacetic acid concentration showed negligible effects on current generation.
Different routesto carbon neutrality and sustainability
面向碳中和與可持續(xù)性的不同路徑
As mentioned above, carbon neutrality isoften referred to energy neutrality. There are however, manyother routes to carbon neutrality. This includes the management of heat resourcesand nutrient recovery from urine, as the greatest potential for reduction ofgreenhouse gas emissions is at the household level (i.e. decentralized systems),and thus robust wastewater management must be able to cope with the possibilityof a temperature decrease as a result (Larsen). In WWTPs, there is substantial potential for energyoptimization, both from improving electromechanical devices and sludgetreatment as well as through the implementation of more energy-efficientprocesses such as mainstream Anammox process or nutrient recovery from urine. Whethercarbon neutrality can be achieved depends not only on actual net electricityproduction, but also on the type of electricity replaced: the cleaner themarginal electricity, the more difficult to compensate for direct emissions,which can be substantial, depending on the stability of the biologicalprocesses. It is possible, for example, to combine heat recovery and nutrientrecovery from urine at the household level, both of which have considerable potentialto improve the climate friendliness of wastewater management.
Improvingthe energy balance of WWTPs, with the aim of moving towards carbon neutrality,may benefit the environment due to reduced carbon emissions. However, there isalso a need to explore wider economic, environmental and societal impacts, assustainability is a complex, multi-dimensional concept comprising of thesefactors and/or indicators. In this respect, ‘carbon neutrality’ or ‘energyneutrality’ do not necessarily imply sustainable operation as they address onlyone element of sustainability and implementation of low carbon solutions mayhave unintended detrimental effects on other aspects. An evaluation studydemonstrates that reducing energy use and/or increasing energy recovery toreduce net energy can be detrimental to sustainability (Sweetapple etal.).In the study, sustainability indicators including operational costs, net energyand multiple environmental performance measures are calculated. This enables identificationof trade-offs between different components of sustainability, which must beconsidered before implementing energy reduction measures. A major conclusion, highlightedat the end, is that improving the energy balance (as may be considered an approachto achieving carbon reduction) is not a reliable means of reducing totalgreenhouse gas emissions.
Apositive analysis of carbon neutrality towards sustainability illustrates thatthere are design and operational conditions under which submerged anaerobicmembrane bioreactors (AnMBRs) could be net energy positive and contribute tothe pursuit of carbon negative wastewater treatment (Pretel et al.). In this analysis,a quantitative sustainable design process was leveraged to develop a detaileddesign of submerged AnMBR by evaluating the full range of feasible designalternatives using technological, environmental, and economic criteria, whichintegrated steady-state performance modeling across seasonal temperatures(using pilot-scale experimental data and the simulating software DESASS), lifecycle costs (LCC) analysis, and life cycle assessment (LCA). Ultimately, theauthors 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 articles advance our understanding of howto achieve carbon neutral WWTPs. This laudable goal will undoubtedly require aportfolio of solutions, requiring academia and industry to work together onnumerous fronts to establish WWTPs as not only a protector of the local aquaticenvironment, but also the global environment that we all share.
XiaodiHao*
Beijing University of Civil Engineering and Architecture, China
Damien Batstone
The University of Queensland, Australia
Jeremy S. Guest
University of Illinois at Urbana-Champaign, USA
*Corresponding author.
E-mail address: haoxiaodi@bucea.edu.cn(X.-D. Hao).
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雜志簡(jiǎn)介
《中國(guó)給水排水》是面向全國(guó)給水排水和環(huán)境工程界的專業(yè)性科技期刊，具有較高的理論導(dǎo)向性和較強(qiáng)的工程實(shí)踐性，被稱為中國(guó)水行業(yè)的“首席雜志”、中文核心期刊、“中國(guó)百?gòu)?qiáng)科技期刊”中國(guó)精品科技期刊、中國(guó)科學(xué)引文數(shù)據(jù)庫(kù)來源期刊（CSTP）。
專家觀點(diǎn)︱污水處理碳中和運(yùn)行需要污泥增量
編者按污水處理碳中和運(yùn)行已成為未來污水處理的核心內(nèi)容，這就使得剩余污泥將成為潛在的能源載體物質(zhì)，需要以增量方式去獲得，從而徹底改變污泥是污水處理過程中的一種“負(fù)擔(dān)”、需以減量方式消滅之的現(xiàn)行觀念。為此，歐美等國(guó)家通過COD內(nèi)源截留與外源挖潛方式最大限度地去實(shí)現(xiàn)“污泥增量”。對(duì)我國(guó)市政污水COD普遍偏低的情況，應(yīng)尋求與廚余垃圾等市政有機(jī)固體廢棄物共消化之機(jī)會(huì)方能實(shí)現(xiàn)“污泥”增量的目的。
污水處理朝著碳中和運(yùn)行方向邁進(jìn)早已成為歐美國(guó)家污水處理今后的發(fā)展方向。如果以碳中和運(yùn)行為目標(biāo)，剩余污泥顯然將不再成為污水處理的“負(fù)擔(dān)”，轉(zhuǎn)而變成碳中和運(yùn)行的緊俏原料。為此，污水處理行業(yè)不再需要去一味追求污泥減量化，轉(zhuǎn)而期盼污泥增量化，以增加污水處理能源自給自足的原料份額。然而，剩余污泥的多寡完全取決于進(jìn)水COD負(fù)荷的高低，低的COD負(fù)荷必然導(dǎo)致較少的剩余污泥產(chǎn)量，也就意味碳中和運(yùn)行能量需求可能會(huì)出現(xiàn)赤字。因此，以碳中和運(yùn)行為目標(biāo)的污泥增量近年來已在國(guó)際上悄然興起，在常規(guī)剩余污泥之外還會(huì)尋求內(nèi)源和外源其它途徑的污泥增量，如，前端篩分COD技術(shù)、后端厭氧共消化技術(shù)等。
荷蘭早在2008年便提出了污水處理的NEWs概念，將未來污水處理廠描述為“營(yíng)養(yǎng)物（Nutrient）”、“能源（Energy）”、“再生水（Water）”三廠（Factories）合一的運(yùn)行模式。
日本有關(guān)部門發(fā)布“Sewerage Vision 2100”，指出到本世紀(jì)末將完全實(shí)現(xiàn)污水處理能源自給自足。
目前，歐美國(guó)家一些污水處理廠以剩余污泥為主要能源載體，同時(shí)結(jié)合前端篩分COD（進(jìn)水COD負(fù)荷高時(shí)）技術(shù)，或后端厭氧共消化（廚余垃圾、食品加工廢料、糞便等）技術(shù)，以最大化“污泥增量”方式從污水或外源有機(jī)物中通過厭氧消化獲取能源（CH4），并已完全或部分實(shí)現(xiàn)碳中和運(yùn)行目標(biāo)。污泥能源轉(zhuǎn)化碳中和運(yùn)行潛力歐美等國(guó)家一些實(shí)施碳中和運(yùn)行目標(biāo)的污水處理廠也大都以剩余污泥厭氧消化轉(zhuǎn)化能源為主要手段。歐洲等國(guó)家因生活習(xí)慣、無化糞池、雨污分流、食物破碎等原因往往會(huì)形成較高進(jìn)水COD濃度（600~1 000 mg/L）。一些歐美以碳中和為運(yùn)行目標(biāo)的污水處理實(shí)例表明，如果進(jìn)水中COD£600 mg/L，采用傳統(tǒng)處理工藝（如A2/O等脫氮除磷工藝）所產(chǎn)生的剩余污泥量通過厭氧消化轉(zhuǎn)化能源很難完全滿足（100%）碳中和運(yùn)行目標(biāo)，一般能達(dá)到70%碳中和運(yùn)行率就已足矣。
我國(guó)污水中有機(jī)物含量較歐美等國(guó)家要低得多，因而僅靠產(chǎn)生的剩余污泥難以實(shí)現(xiàn)碳中和運(yùn)行目標(biāo)。圖1繪出了能量平衡計(jì)算中剩余污泥（初沉+二沉）COD截留率（污泥中總COD與進(jìn)水COD之比）與碳中和率的關(guān)系曲線。圖1趨勢(shì)表明，要想獲得更大的碳中和運(yùn)行率便需要有更多的污泥相對(duì)應(yīng)，即，所謂的“污泥增量”概念。污泥增量從內(nèi)源COD來源角度看，意味著進(jìn)水中的COD除滿足脫氮除磷對(duì)碳源的需求外，應(yīng)避免COD無目的的直接氧化。
早在15年前，針對(duì)定位于能源與磷回收的可持續(xù)市政污水處理，與荷蘭代爾夫特理工大學(xué)（TU Delft）Mark van Loosdrecht教授合作，我們便提出了如圖2所示的概念工藝。為有效截留污水多余（脫氮除磷所需碳源之外）COD并厭氧消化轉(zhuǎn)化為甲烷，利用早年德國(guó)A/B法中的A段用于濃縮懸浮狀與溶解狀COD。與二沉污泥相比，A段截留污泥可消化性較好，可產(chǎn)生甲烷含量較高的生物氣。
為最大程度截留進(jìn)水中COD，歐洲學(xué)者還提出通過絮凝后微濾方式截留膠體狀與溶解狀COD，使之用于厭氧消化轉(zhuǎn)化甲烷的設(shè)想并付諸行動(dòng)。例如，德國(guó)柏林某水務(wù)集團(tuán)融資并聯(lián)合德國(guó)KWB組織已經(jīng)啟動(dòng)旨在回收污水中能源的應(yīng)用研究項(xiàng)目—CARISMO（Carbon is money，即，碳即是錢），工藝流程如圖3所示。
圖3 德國(guó)CARISMO前端COD篩分及后續(xù)污水、污泥處理工藝
污泥共消化發(fā)揮了基質(zhì)間的協(xié)同作用，提高了底物的降解速率和降解程度，使能源轉(zhuǎn)化效率顯著提高。表1列出了幾種不同外源有機(jī)廢棄物與剩余污泥共消化后呈現(xiàn)出的能量轉(zhuǎn)化效果，剩余污泥與其它有機(jī)廢棄物共消化潛力可見一斑。
表1 不同種類/比例外源基質(zhì)與污泥共消化能源轉(zhuǎn)化效果
|
共消化基質(zhì) |
共消化比例 （剩余污泥: 有機(jī)廢物） |
生物氣 增加量 （%） |
參考文獻(xiàn) |
|
餐廚垃圾 |
4:1 |
21 |
[34] |
|
石莼藻 （Ulva sp.） |
17:3 |
26 |
[35] |
|
油脂廢棄物 |
2:3 |
285 |
[36] |
|
滅菌后屠 宰廢水 |
19:1 |
470 |
[37] |
從技術(shù)角度來說，污水處理碳中和運(yùn)行并不存在障礙，主要受限于政府的宏觀環(huán)境政策。只要政府高瞻遠(yuǎn)矚，予以政策支持、甚至是財(cái)政補(bǔ)貼，觸動(dòng)污水處理行業(yè)朝著碳中和方向邁進(jìn)，從而獲得被普遍看好的綜合環(huán)境效益。該文章全文將于近期發(fā)表在《中國(guó)給水排水》雜志的“述評(píng)與討論”欄目。
雜志簡(jiǎn)介
《中國(guó)給水排水》是面向全國(guó)給水排水和環(huán)境工程界的專業(yè)性科技期刊，具有較高的理論導(dǎo)向性和較強(qiáng)的工程實(shí)踐性，被稱為中國(guó)水行業(yè)的“首席雜志”、中文核心期刊、“中國(guó)百?gòu)?qiáng)科技期刊”中國(guó)精品科技期刊、中國(guó)科學(xué)引文數(shù)據(jù)庫(kù)來源期刊（CSTP）。
來源：《中國(guó)給水排水》
污水處理碳中和：污泥增量化如何實(shí)現(xiàn)碳中和運(yùn)行
污水處理朝著碳中和運(yùn)行方向邁進(jìn)早已成為歐美國(guó)家污水處理今后的發(fā)展方向。如果以碳中和運(yùn)行為目標(biāo)，剩余污泥顯然將不再成為污水處理的“負(fù)擔(dān)”，轉(zhuǎn)而變成碳中和運(yùn)行的緊俏原料。為此，污水處理行業(yè)不再需要去一味追求污泥減量化，轉(zhuǎn)而期盼污泥增量化，以增加污水處理能源自給自足的原料份額。
然而，剩余污泥的多寡完全取決于進(jìn)水COD負(fù)荷的高低，低的COD負(fù)荷必然導(dǎo)致較少的剩余污泥產(chǎn)量，也就意味碳中和運(yùn)行能量需求可能會(huì)出現(xiàn)赤字。因此，以碳中和運(yùn)行為目標(biāo)的污泥增量近年來已在國(guó)際上悄然興起，在常規(guī)剩余污泥之外還會(huì)尋求內(nèi)源和外源其它途徑的污泥增量，如，前端篩分COD技術(shù)、后端厭氧共消化技術(shù)等。
國(guó)外剩余污泥能源轉(zhuǎn)化現(xiàn)狀
污水處理碳中和運(yùn)行的實(shí)質(zhì)就實(shí)現(xiàn)整個(gè)污水處理過程能源自給自足，為實(shí)現(xiàn)這一目標(biāo)，歐美、甚至周邊一些亞洲國(guó)家相繼頒布了面向21世紀(jì)污水處理碳中和運(yùn)行的路線圖，并付諸實(shí)踐。例如:荷蘭早在2008年便提出了污水處理的NEWs概念，將未來污水處理廠描述為“營(yíng)養(yǎng)物(Nutrient)”、“能源(Energy)”、“再生水(Water)”三廠(Factories)合一的運(yùn)行模式。美國(guó)推行的“Carbon-free Water”，期望實(shí)現(xiàn)在人們對(duì)水的取用、分配、處理、排放全過程達(dá)到碳中和。日本有關(guān)部門發(fā)布“Sewerage Vision 2100”，指出到本世紀(jì)末將完全實(shí)現(xiàn)污水處理能源自給自足。
目前，歐美國(guó)家一些污水處理廠以剩余污泥為主要能源載體，同時(shí)結(jié)合前端篩分COD(進(jìn)水COD負(fù)荷高時(shí))技術(shù)，或后端厭氧共消化(廚余垃圾、食品加工廢料、糞便等)技術(shù)，以最大化“污泥增量”方式從污水或外源有機(jī)物中通過厭氧消化獲取能源(CH4)，并已完全或部分實(shí)現(xiàn)碳中和運(yùn)行目標(biāo)。
污泥能源轉(zhuǎn)化碳中和運(yùn)行潛力
歐美等國(guó)家一些實(shí)施碳中和運(yùn)行目標(biāo)的污水處理廠也大都以剩余污泥厭氧消化轉(zhuǎn)化能源為主要手段。歐洲等國(guó)家因生活習(xí)慣、無化糞池、雨污分流、食物破碎等原因往往會(huì)形成較高進(jìn)水COD濃度(600~1 000 mg/L)。一些歐美以碳中和為運(yùn)行目標(biāo)的污水處理實(shí)例表明，如果進(jìn)水中COD£600 mg/L，采用傳統(tǒng)處理工藝(如A2/O等脫氮除磷工藝)所產(chǎn)生的剩余污泥量通過厭氧消化轉(zhuǎn)化能源很難完全滿足(100%)碳中和運(yùn)行目標(biāo)，一般能達(dá)到70%碳中和運(yùn)行率就已足矣。
我國(guó)污水中有機(jī)物含量較歐美等國(guó)家要低得多，因而僅靠產(chǎn)生的剩余污泥難以實(shí)現(xiàn)碳中和運(yùn)行目標(biāo)。圖1繪出了能量平衡計(jì)算中剩余污泥(初沉+二沉)COD截留率(污泥中總COD與進(jìn)水COD之比)與碳中和率的關(guān)系曲線。趨勢(shì)表明，要想獲得更大的碳中和運(yùn)行率便需要有更多的污泥相對(duì)應(yīng)，即，所謂的“污泥增量”概念。污泥增量從內(nèi)源COD來源角度看，意味著進(jìn)水中的COD除滿足脫氮除磷對(duì)碳源的需求外，應(yīng)避免COD無目的的直接氧化。
污泥增量方法與措施
A/B法A段濃縮COD
早在15年前，針對(duì)定位于能源與磷回收的可持續(xù)市政污水處理，與荷蘭代爾夫特理工大學(xué)(TU Delft)Mark van Loosdrecht教授合作，我們便提出了如圖2所示的概念工藝。為有效截留污水多余(脫氮除磷所需碳源之外)COD并厭氧消化轉(zhuǎn)化為甲烷，利用早年德國(guó)A/B法中的A段用于濃縮懸浮狀與溶解狀COD。與二沉污泥相比，A段截留污泥可消化性較好，可產(chǎn)生甲烷含量較高的生物氣。
前端篩分COD技術(shù)
為最大程度截留進(jìn)水中COD，歐洲學(xué)者還提出通過絮凝后微濾方式截留膠體狀與溶解狀COD，使之用于厭氧消化轉(zhuǎn)化甲烷的設(shè)想并付諸行動(dòng)。例如，德國(guó)柏林某水務(wù)集團(tuán)融資并聯(lián)合德國(guó)KWB組織已經(jīng)啟動(dòng)旨在回收污水中能源的應(yīng)用研究項(xiàng)目—CARISMO(Carbon is money，即，碳即是錢)，工藝流程如圖3所示。
污泥共消化技術(shù)
污泥共消化發(fā)揮了基質(zhì)間的協(xié)同作用，提高了底物的降解速率和降解程度，使能源轉(zhuǎn)化效率顯著提高。表1列出了幾種不同外源有機(jī)廢棄物與剩余污泥共消化后呈現(xiàn)出的能量轉(zhuǎn)化效果，剩余污泥與其它有機(jī)廢棄物共消化潛力可見一斑。
如果今后能將廚余垃圾、綠化草木、旱廁糞便與剩余污泥一并共消化，將會(huì)形成出現(xiàn)2種以上底物共消化情形。在研究與應(yīng)用實(shí)踐中，³3種有機(jī)底物共消化案例目前還十分罕見。這一課題應(yīng)該成為今后厭氧共消化的研發(fā)方向，不僅可探明多基質(zhì)協(xié)同消化的機(jī)理與作用，而且也為綜合處置市政有機(jī)固體廢棄物開辟一條可持續(xù)發(fā)展之路。
從技術(shù)角度來說，污水處理碳中和運(yùn)行并不存在障礙，主要受限于政府的宏觀環(huán)境政策。只要政府高瞻遠(yuǎn)矚，予以政策支持、甚至是財(cái)政補(bǔ)貼，觸動(dòng)污水處理行業(yè)朝著碳中和方向邁進(jìn)，從而獲得被普遍看好的綜合環(huán)境效益。來源：沈陽(yáng)新華
全球能量自給污水處理廠Top 10
1. 奧地利Strass污水處理廠
處理規(guī)模約3.8萬噸/日，處理工藝采用AB工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)，號(hào)稱是世界第一個(gè)實(shí)現(xiàn)能量自給的污水處理廠，Strass污水廠以實(shí)踐主流厭氧氨氧化而聞名于世。
2.美國(guó)EBMUD污水處理廠
處理規(guī)模65萬噸/日，處理工藝采用二級(jí)處理，污泥厭氧消化并熱電聯(lián)產(chǎn)，該廠是美國(guó)乃至全球污泥協(xié)同厭氧消化的典范。
3. 德國(guó)漢堡污水處理廠
處理規(guī)模44萬噸/日，處理工藝為AB工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)，同時(shí)污泥干化并焚燒。
4. 荷蘭Apeldoorn污水處理廠
處理規(guī)模4.5萬噸/日，處理工藝采用脫氮除磷工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)，該廠在側(cè)流應(yīng)用了DEMON，并進(jìn)一步準(zhǔn)備實(shí)踐污泥熱水解。
5. 丹麥Ejby Molle污水處理廠
處理規(guī)模5萬噸/日，處理工藝采用氧化溝工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)，該廠的曝氣自控非常出色。
6. 美國(guó)Sheboygan污水處理廠
處理規(guī)模4.3萬噸/日，處理工藝為傳統(tǒng)脫氮除磷工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)，該廠的一個(gè)特色是熱電聯(lián)產(chǎn)設(shè)備采用了微型燃?xì)廨啓C(jī)。
7. 德國(guó)Steinhof污水處理廠
處理規(guī)模6萬噸/日，處理工藝為A2O工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)。
8. 匈牙利North Pest污水處理廠
處理規(guī)模13萬噸/日，采用脫氮除磷工藝，污泥厭氧消化并熱電聯(lián)產(chǎn)。
9. 匈牙利South Pest污水處理廠
處理規(guī)模5萬噸/日，生物處理，污泥厭氧消化并熱電聯(lián)產(chǎn)。
10. 美國(guó)Gloversville-Johnstown污水處理廠
處理規(guī)模5.3萬噸/日，污泥厭氧消化并熱電聯(lián)產(chǎn)。
來源：water8848
