Wet-Oxidation Waste Management System for CELSS Yukio Takahashi Department of CIVIIEnglneerlng N~igata University Nilgata, Japan Haruhiko Ohya Department of Chemlcal Engineer~ng Yokohama National Unlvers~ty Yokohama.Japan ABSTRACT compounds (and elements). A wet oxidation system will be But it seems that the discussions on useful in CELSS as a facility to treat the applicability of wet oxidation to organic wastes sr;d to redistribute CELSS does not fully reflect the inorganic compounds and elements. However exper~ences and results obtained in th~s at rather hlgher temperatures needed in field. Thls paper will discuss: the this reaction, for instance, at 260 OC, usefulness and the unsolved problems of only 80:; of organic carbon in a raw wet oxidation in CELSS on the basis of r~~aterial can be oxidized, and 20% of it authors' ten-year research .on wet will remain ln the llquid mainly as oxidation of sewage sludge. acetic acid, which is virtual 1 y non- combustible. Furthermore, nitrogen is EXPERIMENTAL METHODS transformed to ammonium ions which normally cannot be absorbed by plants. To Wet oridation is a reaction in which resolve these problems, it becomes organic or reductive compounds are necessary to use catalysts. Noble metals oxidized in the presence of liquid water. such 3s Pu, Rh and jo on have proved to This reaction occurs between 100°C and be pnrtiall~ effective as these catalysts. 374°C which is the critical temperature That is, oxidation does not occur of water, so ,the reaction should be compl etel y, and the unexpected carried out in a pressure vessel denitrification, instead of the expected (autoclave) to prevent evaporation of nitrification, occurs. Sotit is essential water. to develop the catalysts which are able A1 1 data shown later are the results to realize the cnmplete oxidation and the of autoclave tests. The pressure In the nitrification. autoclave at a given reaction temperature is determined as a sum of the pressures MANY RESEARCH EFFORTS have been funded to of water and gases which are introduced make wet oxidation systems commercially into the reactor at a room temperature available for the purpose of treating before the beginning of the experiment. organic wastes, particularly sewage Therefore, it is impossible to keep the sludge, and since 1957, the Zimmermann reaction pressure below a saturated vapor Process (Zimpro) has been successfully pressure of water at the given reaction l~sed in the USA, Japan and other temperature. countries. This system seems useful in a Before an experiment,it is necessary Closed Ecological Life Support System to introduce enough oxygen gas into the (CELSS) to treat and reduce organic reactor so as to accomplish the intended components of wastewater from plants, oxidation. The rate of oxygen gas in the human beings and so on, into inorganic reactor to the chemical oxygen demand Reprinted with permission @ 1985 Society of Automotive Engineers, Inc. .* --''r .'I ' \ ,2 (COD) of a raw materia'lt ?$sa:<al3ed. -:addex: filtrable organic carbonCTOC(fi1t)l and oxygen index(R)'. In the case of R=l, it cr~nvertedrcarbon quantities of acetic means that the exact quantity of oxygen acid, C2-C6 organic acids(the sum of has been added to oxidize the raw acetic, propionic, i-butyric, n-butyric, mhterials completely. i-valeric, n-valeric, 1 -capr-ic, n-capric Pure oxygen gas or a mixture of acids) in oxit-f~red1 iquors are shown in oxygen and nitrogen gases has been fig.1 as the percentage of carbon used. Even after the determination of quantities of the raw material versus the I?, it is possible to exchange the gas respective temperature. ratio of oxygen to nitrcgen at will. You The behaviour of TOC(mix) curves in have only to increase or decrease the the figure shows that the oxidation c~olumeof the raw material. Sewage sludge occurs when the temperature is higher IS used as a raw material. than lCO°C, and this process becomes more , active at the higher temperature. However- EFFECTS OF THE REACTION TEMPERATURE when the temperature exceeds 260 OC, the reaction will not proceed any more. The CARBON - Fig.1 shows the relation value of TOC(fi1t) increases with the between the reaction temperature and increase of temperature and passes total organic carbon(TOC).The temperature through the highest value in the range of is increased from llO°C to 310°C at an 160 OC to 210 'C, then it decreases and interva1 of 50 OC. Pressures are a1 1 set approaches to the curve of TOC(mix) after at 75kgf/cmA2 except in the case of 310 210°C. "C, where the pressure is controlled at In the case where the raw material 115kgf/cmA2.The reason why high pressures is sewage s 1 udge , so 1 ub i 1 izat ion of are applied at lower temperatures of 11C suspended organ i c compounds occurs first OC or 160°C will be discussed later. The at a temperature less than 160 OC, ther reaction time, after the temperature of both snlubilizat~onand oxidation proceeo the reactor reached the designed in between 160 OC and 210 OC, and after temperature, is 30 minutes. Added oxygen 210°C oxidation only occurs. i~dex(R>is unity and pure oxygen is used With the increase of reaction temperature, amount of C -C organic as an oxygen source. 2 6 Tota 1 organic carbonCTOC(mix)l, acids increases and their quantity reaches two-thirds of TOCtmix) at temperatures higher than 260°C. On the other hand, the ratio of acetic acid in C,-C organic acids increases with the L 6 increase of the reaction temperature. When it reaches 260 "C, acetic acid occupies nearly the total amount of C -C organic acids, and maintains this am&$ independent 1 y to the increase of temperature thereafter. This does not mean that the reaction had reached equilibrium. Acetlc acid, whose concentration ranges from zero to 16g/l as converted carbon quantities, is not oxidized at all, even if excessive oxygen is added to the reactor. Therefore the main products in wet oxidation is 'acetic acid' in addition to carbon 2 dioxide and water. pressdre : 75~gf/cm (11 5kgf/cm2 for 310*C) NITROGEN - Fig.2 shows the relation Retention Time : 30min. between temperature and nitrogen output. Pure oxygen. (~)=1.0 As the temperature becomes higher, the so 1 ub i1 izat ion of suspended n itrogen Fig.1 Reaction Temperature and TOC compounds occurs, and it is almost completed at 160 "C. But this Phosphorus scllubil ization does not result in the format I or) UF arnrnon i a. Between 160°C and 21O0C, at ithlsh both the solubilization and the oxidation of organic compounds occur-, production of ammonia increases rapidly. Even ~f the solubilization has heen compl eted,sol ubl e n i trogen compounds do not beccme anrmor-~iatotal 1 j at 210 OC, hut 260 OC. in wet oxidation without catalysts, the nitrogen compounds may not be oxidized to nitrite and nitrate. On the other hand, forms of nitrogen usable by plants are n~triteand/or nitrate with the except:ion of some plants such as r ~ce. So i CELSS, ~t is impossible to apply oxidized liquor directly to plant culture in hydroponic solutions. It is Pressure .75kzi/cm 7 (11 5kgf/crn2 For 310'C) Yi trogen 3erent.lon 71me : jOmln. Pure Oxygen. (3)zl.O 1OOr (5) Fig.3 Reaction Temperature and Phosphorus temperature is shown. The ratio of phosphorus in the liquid to the total - phosphorus quantity is remarkably small 50 comparing with the one of nitrogen in the liquid to the total nitrogen quantity, which gives only approximately 10 per cent at best. This may be caused by the fact that sewage sludge contains various i o cations such as calc~um ions which produce unsoluble salts with phosphorus. 3 :10 160 210 260 310 In CELSS, the wet oxidation should Teapi'C) fu~ction to re-supply nutrient sources 2 ?rsssLre : 75ksf/:a For plant culture in hydroponic solutions. ii15xgt/cn2 for 310'C) It is rather desirable for nutritious Retent:on Time : 3Omln. elements such as phosphorus, nitrogen pure oxygen. (?)=I-0 etc. to be dissolved in water. Therefore, F1g.2 Reaction Temperature this phenomenon is not favourable in and Nitrogen making use of wet oxidation in CELSS. The behavior of phosphorus in wet oxidation necessary to convert ammonia to the forms has not been elucidated except in the nitrite and/or nitrate by catalysts or by case of sewage sludge. In this respect, any other means. The decrease of Kjeldahl the study of phosphorus is greatly -nitrogen above 260°C is due to either needed. the denitrification or NOx formation. PHOSPHORUS - Fig.3 shows the EFFECTS OF THE REACTION TIME relation between the temperature and phosphorus output. The percentage of CARBON - Fig.4 shows the status of total and soluble(fllterable> phosphorus TOC along with time. The reaction ln the oxidized liquor to the quantity of temperatures are 210°C nnd 260 OC. In raw material versus the respective either cases, the percentage of TOC(mix> - 851398 and TOC(fi1t) to the quantlty of a raw process. 6ut suspended TOC, i.e. TOC(mix) material is shown in relation with time. - minus TOC(filt), does not disappear even Pr~ssure 1s kept 35kyf;'cm"2 in the case sfter 45 minutes. of 210°C, and 75kgf/cmA2 in the cas-e of TOC(mix),TOC(filt) and the converted 260°C. The added oxygen index(R> is unity, carbon quantltles of acetic acld and C - 2 and pure oxygen is used as a oxygen C6 iargar~lc aclds are showrj in f ig.5 in source.