Biological Conversion of Light Energy to the Chemical Energyof Methane

C. G. GOLUEKE AND W. J. OSwVALD

Sanitary Engineering Research Laboratory, Department of Engineerinq, University of California, Berkeley, California

Received for publication Januiaryr 27, 1959

Civilization has progressed at a rate corresponding toman's ability to find and employ new and more extensivesources of energy. Although we are standing on thethreshold of a new era characterized by a potentialsupply of energy far surpassing that available in thepast, namely nuclear power, our present technology isbuilt around the discovery, production, transportation,and utilization of fossil fuels, coal, petroleum, andnatural gas.

Recently, however, much has been written to showthat the abundance of man's known resources of eco-nomical energy is not as great as his predicted energydemands (Ayres and Scarlott, 1952; Putnam, 1953;Furnas, 1954; Meiers, 1956). After the fossil fuels areexhausted, the most abundanit potential sources ofenergy are nuclear fission and nuclear fusion, and solarenergy. Although certainly not as soon as with fossilfuels, the supply of raw materials for nuclear fissionwill eventually become exhausted. However, the nonre-newable feature of fission eniergy is not the major prob-lem with nuclear fission but rather the disposal of radio-active wastes. According to lost (1957), radioactivefission products equal to those produced by 200,000atomic bombs would have to be cared for if presentneeds for energy in the United States were met byfission reactors. Hopes for finiding an inexpensive andyet adequate method of disposal for such wastes areexceedingly dim at presenit. Should fusion power be-come available, it promises to solve the earth's energyproblems for many thousands of years. Approximately8 years of intensive research have failed to reveal anyfundamental obstacle to the ultimate success of nuclearfusion, but neither have they brought forth any proventechnique for power production (Spitzer, 1958). Shouldfusion power prove unattainable for some reason un-known at present, or should its cost prove to be morethan we can optimistically hope, processes involvingsolar energy will be the only remaining alternative.Each year solar energy equal to more than 25,000

times the world's present annual power demand im-pinges upon the earth's surface. However, researchershave struggled for 100 years with but rare success inseeking some economical method for fixing solarenergy. The inability of most of the solar energy systemsto meet the requirements for the economical productionof power apparently stems from certain disadvrantages

219

characteristic of solar energy, chief of which are: (a)intermittency of the supply of energy; (b) the lowconcentration of energy; (c) its limited utility in theavailable form; and (d) the problem of storage. Withinthe past few years hypothetical systems have beenproposed, which if workable would eliminate these dis-advantages (Burlew, 1953; Meiers, 1956). The systems,as proposed, would involve the transformation of solarenergy into the cellular energy of minute algal plants;the cellular energy of the algae would in turn be con-verted to the chemical energy of methane through theanaerobic fermentation of algae by bacteria. Themethanie could either be burned in a gas turbine-gener-ator system to produce electricity or, through suitablecatalytic processes, converted to hydrocarbon fuel.The establishment of the process of transforming solar

energy to methane by the digestion of algae as a sourceof energy was made a reality with the development inour laboratory of a workable process for growing algaeon the end products of algal methane fermentation. Theprocess which is presently in operation may be sum-marized as follows. Organic wastes enter an oxygenationchamber containing a mixed culture of algae and bac-teria where they are decomposed by aerobic bacteria tocarbon dioxide, ammonia, and other nutrients. To-gether with light, these decomposition products furnishthe raw material for algal photosynthesis, with theresult that light energy is fixed in algal cells. The algaeare then concentrated and introduced into a digesterwhere they undergo methane fermentation. As themethane, carbon dioxide, ammonia, and other gases areproduced, they are removed from the generator and arepassed into a collector unit where the carbon dioxideand ammonia are again available for photosynthesis;while the supernatant liquor and incompletely digestedsolids are added to the algal-bacterial culture for whichthey serve as nutrienit. Thus, the elements of organicmatter are repeatedly broken down and resynthesized.The net effect is that the diffuse eniergy of light is con-verted to the high energy of methane.Although the system as described above does not in-

volve any new biological processes, its operation as anintegrated unit constituting a closed nutritional systemhas never been reported previously and, consequently,many unknown factors required investigation. It wasessential to demonstate that integration of the digestion

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and of the oxygenation (aerobic-bacterial algal) phasesin a continuous energy conversion system is operableand to show that algae would grow in sludge super-natant. Inasmuch as previous work had shown that thebreakdown of algal cell material is incomplete in di-gestion (Golueke et al., 1957), it was necessary todemonstrate that the breakdown would be completedeither in the oxygenation chamber or in the collectorunit, so that a sustained build-up of inert organic ma-terial would not occur. Determinations of the net pro-duction of methane from such a system, as well as ofthe photosynthetic efficiency in the collection unit andin the over-all procedure, were required for arriving atconclusions as to the potentialities of the system.

MATERIAL AND METHODSAn apparatus designed to make possible the trans-

formation of light energy to the chemical energy ofmethane in a completely closed system was set up inour laboratory approximately 2 years ago. It consistedof three principal parts: an algal growth unit (oxygena-tion chamber), an "activated sludge" unit (aerobic-bacterial growth unit), and a digester. Each part is con-nected to the other by suitable tubing and accessories inthe manner indicated in figure 1. The temperature of theculture in the algal growth unit was maintained at26 to 27 C; in the digester at 45 C; while the "activatedsludge" unit was kept at room temperature, 18 to 25 C.The algal growth unit was illuminated by a bank ofnine 30-watt "warm-white" fluorescent lamps. Lightintensity at the face of the growth unit varied from1820 ft candles at the brightest point to 450 ft candles

at the darkest point. The total amount of energy avail-able to the culture was 121,000 calories per day.The daily procedure in operating the sections of the

unit involved the following steps, each performed inthe order given: Step 1 An aliquot of algal suspensionwas withdrawn from the algal growth unit and centri-fuged in 100-ml Goetz tubes at 500 X g for 10 min. Thepacked volume of the concentrate was noted andrecorded, and the supernatant was decanted andreserved. Step 2: An aliquot was then removed fromthe digester. Step 3: The algal solids were added to thedigester, after they had been diluted with a part ofthe reserved algal culture supernatant to a volumeequal to that of the aliquot removed from the digesterin step 2. Step 4: A sample equivalent in volume tothat of the aliquot from the digester was removed fromthe "activated-sludge" unit. Step 5: The aliquot fromthe digester was added to the "activated-sludge" unit.Step 6: The sample obtained from the "activated-sludge" unit was diluted with the remainder of thesupernatant of the aliquot from the algal culture andwas then added to the algal growth unit. The size ofthe aliquots removed from each of the units was de-termined by the length of the detention period atwhich the unit was operating. For example, when thealgal growth unit was operating on a 6-day detentionperiod, the aliquot removed was equal to one sixth ofthe volume of the culture. The "activated-sludge"unit was discontinued on the 110th day, and duringthe remainder of the study the digesting sludge wasadded directly to the growth unit together with thesupernatant obtained from the growth unit. The entire

Figure 1. Schematic drawing of the energy conversion unit

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exchange of materials was accomplished in all units withthe admission of very little, if any, outside gas bysimultaneously opening both the inlet and samplingports so that, as the feed entered the top of the unit,an equivalent amount was being withdrawn from thebottom of the unit. Gas pressures in the units wereaffected by the removal and admission of liquids insuch a manner that no direct contact with the outsideatmosphere was necessary.

Daily gas production was determined by adjustingthe telescoping tube in the gas collector so that theinside and outside levels of the mercury were identical,and then reading the graduation on the tube. The gaspressure in the unit was kept at approximately at-mospheric pressure by the counterweight attached tothe telescoping tube. After recording the volume ofthe gas produced, it was wasted as often as was neces-sary. Occasional analyses to determine the compositionof the gas were made by depressing the telescoping tubeand expelling the escaping gas into a Fisher' gasanalyzer. The gas was analyzed directly for carbondioxide and methane; hydrogen and nitrogen and othergases were determined by difference. Although itwould have been advantageous to burn the methaneproduced by the digester and to return the resultingcarbon dioxide to the converter, this was not donebecause of the danger of an explosion.

Production of algae was judged on the basis of thepacked volume of the algal cells obtained by centri-fuging the algal suspension. Determinations of totaland of volatile suspended solids were made from timeto time to relate packed volume and dry weight. Theexperimental design precluded regular determinationsof dry weights of solids, inasmuch as such determina-tions would have resulted in the reduction and conse-quent loss of solids from the system. On the average, itwas found that a packed volume of 5 ml per 100 ml ofculture corresponded to a total suspended solids con-centration of approximately 3 g dry weight per L or avolatile suspended solids concentration of 2.7 g per L.This relationship between packed volume and dryweight changed but little with variations in relativeamounts of algal and nonalgal solids concentration.The efficiency of light energy conversion was deter-mined according to Oswald's method (Oswald, 1957),in which the energy content of the culture, as deter-mined by its heat of combustion, is divided by theamount of light energy absorbed by the culture.Performance of the digester was judged solely on the

basis of gas production per unit of volatile matterintroduced into it. Total gas production, as measuredin the gas collector, was a function not only of the per-formance of the digester, but also of that of the growthunit and of the "activated-sludge" unit. The methanecontent of the gas was a valid indicator of the activity

of the digester, inasmuch as the digester was the onlysource of the methane. The amount of carbon dioxideproduced by the digester was estimated on the basisof the per cent methane and the actual amount of gasproduced, as the ratio of methane to carbon dioxideproduction is known to be relatively constant withinthe range 2:1 (Golueke et al., 1957). Having deter-mined the amount of gas produced by the digester, itwas possible to estimate the amount of gas producedper unit of volatile matter introduced by relating thevolume of gas produced to the amount of volatilematter fed the digester. The amount of volatile matterpresent in the aliquot of algae fed the digester each daycould be estimated from the packed volume of the algalsample. Determination of the volatile solids of thesludge from the digester could not be made because itwas required as nutrient for the algal growth unit, andhence no mass-balance could be formulated. However,by comparing average solids fed to the digester withthe volume and composition of the gas produced, itwas possible to arrive at an approximation of thevolatile matter reduced and the gas produced per unitweight of volatile matter introduced.The algal growth unit was operated on a detention

period of 4 days for the first 20 days; of 5 days untilthe 70th day; of 6 days until the 280th day; and of5/1 days until the termination of the experiment.The "activated-sludge" unit and the digester were eachoperated on a 20-day detention period.

Because preliminary experiments had demonstratedthe advisability of beginning a run with a high concen-tration of algae, the starting culture in the algal growthunit was formed by adding centrifuged algae fromexisting outdoor pond cultures to 1 L of raw sewageuntil a concentration of algae equivalent to a packedvolume of 5.5 ml per 100 ml of suspension was at-tained (3.4 g per L). The same preliminary experimentalso demonstrated the need for a functioning digesterat the beginning of the run; therefore, 500 ml of digest-ing algal sludge obtained from a digester which hadbeen fed algae exclusively over a period of 2 years wasadded to the digester flask. The initial concentrationof the sludge in the unit was 6.5 per cent total solids(5.3 per cent volatile solids). The "activated-sludge"unit was started with 500 ml of "activated-sludge"culture.

Algal groups were classified on the basis of morpho-logical characteristics determined by direct microscopicexamination of culture samples. Cells of small size (2to 6 , in diameter), spherical to broadly ellipsoidal inshape, and having a parietal chloroplast, which may beeither cup-shaped or a curved band, were identified asChlorella spp. Ellipsoidal to fusiform cells occurringsingly or in series with their long axes parallel to oneanother and with a single longitudinal laminate chloro-plast usually containing one pyrenoid were considered

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members of the genus Scenedesmus. Euglena were identi-fied by their single, bifurcate flagellum, spindle shape,and eyespot at the anterior end. Filamentous blue-greenalgae characterized by unbranched, cylindrical, un-sheathed trichomes were classified as Oscillatoria spp.

RESULTS

Algal growth unit. Variations in the concentrationof suspended solids and of algal solids from the 20thday until the end of the experiment are shown in figure2. The values plotted in the curves represent the aver-age packed volumes of total suspended solids and of thealgal fraction for each 10-day period ending on theday indicated. The graph begins at 20 days, becausebefore this time the unit presumably had not reachedequilibrium.At the end of 40 days the concentration of the algal

culture had decreased to 2.2 ml per 100 ml, of which80 per cent were algae. Because of the rapid declinein concentration, the detention period was then in-creased from 4 to 5 days. The increase in detentionperiod decreased the rate of decline in concentrationto some extent, as is shown in figure 2. During thisperiod the population was divided as follows: Chlorellaspp., 75 per cent; Scenedesmus spp., 23 per cent; andEuglena spp., 1.5 per cent. A few isolated filaments ofOscillatoria were also noted.On the 51st day, a few specimens of a small single-

celled blue-green alga were observed, which were1.5 to 2.0,u in diameter and were identified as Synecho-cystis sp. (Sauvageau, 1892) according to the descrip-tion given by Smith (1933) and by Prescott (1954).The concentration of the alga increased rapidly whilethat of the green species remained constant until the56th day. The conceintration of all of the algae was

70

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then 1.4 ml per 100 ml, approximately 55 per cent ofwhich consisted of Synechocystis sp. No further changein concentration of cells was noted until the 58th day,when the heating element of the temperature regulatingequipment failed and the temperature of the culturedropped to 13 C. By the end of the 58th day, almostall of the blue-green algae were dead, leaving a vividlycolored orange debris. A determination of the packedvolume of the culture showed that it was 1.8 ml per100 ml, but that approximately 1.1 ml per 100 ml of itwas debris. A curve indicating the rapid growth anddecline of the first bloom of Synechocystis is shown infigure 3.

Despite restoration of the temperature to 25 C, thepacked volume of the suspended solids and of the algaecontinued to decline until an average of 1.0 ml per 100ml (0.95 ml algae per 100 ml) was reached. To counter-act the decline in concentration, a 6-day detentionperiod was begun during the 70- to 80-day period. Asshown in figure 2, the concentration of the culture in-creased with the initiation of the 6-day period.During the 70- to 80-day interval, Synechocystis

again made its appearance, increasing from a fewisolated cells to a number sufficiently great as to increasethe packed volume of the algal fraction of the suspendedsolids to 2.5 ml per 100 ml (figure 3). On the 79th day,all of the blue-green algae suddenly died, again leavingan orange debris (total packed volume 3.2 ml per 100ml; orange debris, 2.5 ml per 100 ml). No equipmentfailure occurred at this time, although the pH of theculture declined from 9.2 at the onset of the develop-ment of the blue-green algae to 6.6 at the time of theirdeath.As was observed previously, the orange debris

gradually disappeared and the green algae increased

420

Figure 2. Packed volume of the algal-bacterial culture in the algal growth unit. Each point in the curves represents the average

packed volume of total suspended solids and of the algal fraction for each 10-day period ending on the day indicated.

-0-- TOTAL SUSPENDED SOLIDS---A--- ALGAL SUSPENDED SOUDS

Digester failed

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Begin 6day detention period * a

60 l00 i40 180 220 260 300 340 380DAYS AFTER START OF EXPERIMENT

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in numbers until the packed volume of the algal frac-tion was 3.0 ml per 100 ml during the 80- to 100-dayinterval. At this time, representatives of the genusChlorella constituted the greater portion of the algae.On the 110th day, the "activated-sludge" unit was

discontinued, and thereafter sludge from the digesterwas fed directly to the algal growth unit. Althoughthe daily sample of digester sludge was dark blackish-green when added to the growth unit, the black colorusually disappeared by the succeeding day, as wasdetermined by observing the centrifuged sample fromthe growth unit. The addition of digester solids directlyto the growth unit resulted in no appreciable increasein the proportion of nonalgal to algal material.

Synechocystis made its third and final appearanceduring the 130- to 140-day interval, again increasingfrom a few cells to an amount sufficient to bring theaverage algal concentration to 3.75 ml per 100 ml(figure 3) for the interval. At this time, Scenedesmusspp. constituted approximately 23 per cent of thealgal population; Chlorella spp., 12 per cent; andSynechocystis sp., 65 per cent. Figure 3 shows that,during this "bloom" of Synechocystis, the total packedvolume of all of the algae gradually increased to amaximum of about 4 ml per 100 ml and then declined,reaching 3.70 ml per 100 ml on the 10th day after thebeginning of the bloom. The concentration of the blue-green alga remained the same until the 12th day, atwhich time the cooling equipment failed and the tem-perature increased to 40 C. As a result of this disastrousincrease, approximately 97 per cent of the algae werekilled, leaving a concentration of 0.1 ml per 100 ml

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of suspended solids, of which 85 per cent were Chlorellaspp. and 5 per cent Scenedesmus spp. Apparently noblue-green algae survived.The culture was not disturbed for 4 days, after

which only 50 per cent of the daily sample was removedduring the succeeding 10 days, although the usualamount of sludge was added. The concentration of thealgae gradually increased until a packed volume of 1.6ml per 100 ml was reached during the 170- to 180-dayinterval (figure 2). From the 180th day until the 210thday, the concentration of the culture increased slowlyto an average of 2 ml per 100 ml, of which Chlorellaspp. constituted about 96 per cent and Scenedesmusspp. about 4 per cent. A gradual accumulation ofdebris was noted after the 210th day. This accumula-tion paralleled a decline in the activity of the digesteruntil the latter ceased functioning during the 230- to240-day interval; at which time the packed volume ofthe suspended solids in the growth unit was 5.2 mlper 100 ml, of which living algae constituted about3.3 ml per 100 ml.

After installation of a new digester on the 245thday, the average packed volume of the total suspendedsolids remained approximately 5 ml per 100 ml, al-though a decline in the algal fraction of the solids wasnoted during the first few days. Within a short time,however, the algae fraction began to increase until amaximal concentration of 4.75 ml per 100 ml was ob-tained during the 300- to 310-day interval.On the 311th day, the temperature regulating system

failed and as a consequence the temperature in thegrowth unit rose to 38 C. Although the number of algae

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Figuire 3. Curves indicating the rapid growth and decline of "blooms" of Synechocystis sp.

9--oI-2 3 4 5 6 7 8 9

DAYS AFTER APPEARANCE OF SYNECHOCYSTIS SP

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killed was large, the extent of the destruction was notas great as that which characterized the first rise intemperature, inasmuch as the average packed volumeonly decreased to 1.6 ml per 100 ml in this instance.Chlorella spp. constituted approximately 95 per centof the surviving algal population and Scenedesmusspp. the remainder. The experiments with the energyconverter system were discontinued on the 340th day,although the system has been kept functioning sincethen for demonstration purposes.The efficiency of light energy conversion by the algal

culture averaged approximately 3 per cent when theculture was at a maximal concentration. The value isthat for the algal growth unit as a whole, because thenature of the experiment made it practically impossibleto determine the precise amount of living algae, andthus to distinguish between the amount of light re-ceived by the algae and that absorbed by dead algae ornonalgal material.

Digester unit. The production of gas varied from 75to 250 ml per day depending upon the amount of algaefed it. Gas production per g of volatile matter intro-duced averaged 0.6 ml per mg (9.8 cu ft per lb). Infigure 4 the curve is shown for estimated ml of gasproduced per mg of volatile matter introduced. Eachpoint on the curve represents the average gas produc-tion for a 10-day period ending on the day indicated.As with the growth unit, the values obtained duringthe first 20 days are not indicated. As is shown in thefigure, the yield of gas, per mg of volatile matter intro-duced, increased until the 50th day of the experimentwhen it was 0.8 ml per mg of volatile matter introduced,although the packed volume of the algal culture de-clined during the 40- to 50-day period. Gas production

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remained relatively uniform until the 90- to 100-dayinterval, during which the average gas productiondropped to 0.3 ml per mg (5.0 cu ft per lb) of introducedvolatile matter. No change in pH or operating condi-tions was noted that could account for the drop exceptperhaps the small amount of solids available during thisperiod. Gas production per mg of volatile matter in-creased for a time as more nutrient became available,although a second period of decline was noted duringthe 130- to 150-day interval. After reaching a secondhigh on the 190th day of 0.7 ml gas produced, per mg ofvolatile matter introduced, the yield of gas began todrop rapidly, indicating that the digester was nolonger breaking down the material introduced into it.This was confirmed by the steady accumulation ofsolids in the growth unit. The digester failed completelyduring the 210- to 220-day period.During the normal operation of the digester, the

methane content of the gas varied from 60 to 74 percent, with an average of 71 per cent. The methanecontent of the gas dropped to 40 per cent on the 20thday prior to the failure of the digester, indicating thatthe digester had begun to fail at least 3 weeks beforeits complete stoppage. The carbon dioxide of the mixedgas in the system varied from 2 to 20 per cent, changingwith algal concentration. When the algal concentrationwas at its maximum, as for example during the firstand fifth days, and the packed volume of the algaewas 5.5 ml per 100 ml, the carbon dioxide concentrationwas 3 per cent. On the 56th day, with a packed volumeof algae 1.4 ml per 100 ml, it was 5 per cent; and onthe 110th day, with a packed volume of 2.6 ml per100 ml, it was 3 per cent. On the other hand, duringthe interval between the 150th and 160th day, during

DAYS AFTER START OF THE EXPERIMENT

Figure 4. The estimated amount of volatile matter introduced into the digester and the amount of gas produced. Each point inthe curves represents an average of the daily values for a 10-day period ending on the day indicated.

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which the packed volume of algae averaged only 0.3ml per 100 ml, the carbon dioxide concentration was

as high as 18 to 20 per cent. The oxygen content of thegas was strictly a function of algal activity and ofexternal gas that may have leaked into the system.

"Activated-sludge" unit. Very little quantitative datacan be given for the "activated-sludge" unit becauseno determinations of solids could be made, inasmuchas they were required as nutrient for the algal growthunit. The daily change in packed volume was too smallto be detected. The dry weight of the solids at thestart of the experiment was 2.02 per cent of the wetweight, of which 1.82 per cent was volatile solids. Thesludge was always dark green in color, very viscous, anddewatered slowly. It was characterized by a rich earthyodor, such as is noticeable in material being decom-posed by actinomycetes. Protozoa and other faunacharacteristic of "activated-sludge" were completelyabsent.

DIscussIoN

Algal culture. The algal culture in the energy con-

version unit was characterized by two outstandingfeatures, the periodic rapid growth and the even more

rapid disappearance of Synechocystis sp. and therhythmic character of the increase and decrease inthe total algal population. The sudden rapid increasein numbers of Synechocystis cannot be explained exceptby some change in the environment of the culture,although the nature of these changes could not bedetermined. The sudden dying of Synechocystis couldbe explained on two occasions; the first, when thetemperature of the culture became excessively low(13 C); and the second, when it became excessivelyhigh (38 C). The killing of the alga at the two tem-perature levels indicated that the permissible range

in which organisms of this type could compete andthrive was restricted to a range between 13 and 38 C.Although a drop in the pH level of the culture from9.3 to 6.6 paralleled the destruction of the third bloomof Synechocystis, it was impossible to determine fromthe data obtained whether or not the change in pHwas a causative factor or merely the result of a changein algal and bacterial activity. There is the possibilitythat, under the conditions prevailing in the growth unitat the time, the organisms may have formed a toxic sub-stance which accumulated as the population increased,until a sufficient amount was present to kill it.The periodic fluctuation in the concentration of all

of the algal groups may have been due to the accumu-

lation of inhibitive by-products of cellular metabolism,which continued until a sufficient quantity was presentto inhibit the growth of the organisms. This was pos-

sible because the culture liquid was retained in thegrowth unit throughout the experiment, only the solidswere removed and fed to the digester. When the algal

population was in its period of decline, daily output ofthe harmful by-product may have been less and bac-teria and other organisms in the culture were then ableto decompose the algae. When the excreted by-productsreached a sufficiently low level, the algal members ofthe culture would again increase. Another explanationmay be found in the complexity of the culture. Inas-much as the sole source of nutrient for the algae wasfound in the decomposition products resulting fromthe activity of bacteria and protozoa in breaking downthe digested algae fed the unit each day, the welfareof these organisms and their consequent degree ofactivity would determine the amount and nature ofthe nutrients available to the algae. Because of thelimitations of the experiment, these phases could notbe investigated. The periodic increase and declinecannot be accounted for by changes in pH, because nocorrelation was noted between maximal populationsize and pH level. With one exception, the pH levelthroughout the experiment ranged from 6.2 to 9.0, arange suited to growth of algae and bacteria.The experiment showed that the temperature and

detention period of the algal culture were of primeimportance in the energy converter. The detentionperiod in the present study was between 4 and 6 days.In energy converter systems the optimal length ofdetention period will be dependent not only upon thecharacteristics of the algal population, but also uponthose of the bacterial and protozoan populations, andupon the effective depth or energy flux per unit of cul-ture volume. Temperature had a direct effect on all ofthe organisms; at temperature levels lower than 19to 20 C, the activity of all of the organisms involvedwas retarded and the culture began to deteriorate.Temperatures in the upper 30's probably were moreharmful to the algae than to the bacteria; at leastuntil an algal population adapted to the high tempera-ture was developed.

Chlorella and Scenedesmus species probably are moredesirable in an energy converter unit even though theydid not exhibit the rapid rates of increase in populationshown by Synechocystis sp. The former organismsdemonstrated their greater ability to withstand thevariations in conditions encountered in the unit. Despitethe peaks and depressions in their numbers, they werealways present in sufficient quantity to insure thecontinuity of the experiment. However, with more in-formation on the nutrition and physiological charac-teristics of Synechocystis sp., under conditions in whichculture failures would be minimized, it may be possibleto cultivate Synechocystis with great advantage.

Unfortunately, the scope of the experiment did notinclude a study of the taxonomy of bacterial andprotozoan populations in the growth unit. In general,however, excepting for the stalked ciliates such asVorticella, bacteria and protozoa characteristic of typi-

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cal activated-sludges were present in the growth unitculture. The presence of actinomycetes was indicatedby the slightly earthy odor of the culture.

Digester. Characteristics of the anaerobic culture inthe digester were comparable to those encountered inearlier studies on the anaerobic digestion of algal sludge(Golueke et al., 1957). Gas production averaged ap-proximately 10 cu ft per lb of volatile matter intro-duced. Of course, the figure is only approximate, becausethe value for the volatile matter introduced was basedon an estimate made from the amount of the packedvolume of the culture. The avrerage yield for gas pro-duction probably was 1 to 2 eu ft per lb of volatilematter less than the values indicated, if previous workon the anaerobic digestion of algae is used as a basis ofestimation. A high reading could have come fromunderestimating the amount of volatile solids intro-duced. The apparently high rate of gas productionper lb of volatile matter broken down during the30- to 50-day interval probably was due to the largeamount of volatile matter accumulated in the digester.This was possible because the digester at this stagemay as yet have had some of the volatile matteroriginally present in the starting culture; and, in addi-tion, the high concentration of algae in the growth unitat the start of the experiment insured a large dailydosage of algae at that time. The response of the digesterto the abundance of these nutrients was not immediate,since some time was required for adjustment to thenew environment. However, once the adjustmenit wasmade, gas production was high and the values fromthe beginning of the experiment to the 30- to 50-dayinterval exceed those which would normally be ex-pected. The delayed response to the large loadings ofnutrient was characteristic of the digester throughoutthe study, as may be seen upon comparing the curvefor gas production in figure 4 with that for the volatilematter fed the digester. The low production of gasduring the periods of high loading and high productionof gas at low loadings probably were due to a lag inthe breakdown of cells in the digester. Because of thefluctuations in the solids concentration in the growthunit, the digester was not fed a uniform dose of nu-trients each day; consequently the bacterial populationwas not constant, and hence time would elapse betweenthe introduction of a large dose of nutrient and thebuild-up of a population adequate for coping with it.No detectable change either in pH or in temperaturecould account for the eventual failure of the digesterunit. The solids concentration of the growth unit wassufficiently great to supply at least a major part of thenutrient requirements of the culture at the time. Thereis the possibility that a defective flutter-valve may haveallowed gas from the growth unit with its abundance ofoxygen to be forced into the digester. The oxygen

organisms in the digester culture. With the installationof a new digester and a new valve in the gas line be-tween the digester and the remainder of the unit, no

further trouble was had, and the new digester was stillfunctioning long after the termination of the experi-ment. Occasional determinations of gas yield, per unitof volatile matter introduced, showed no increase over

that of the original digester.A preliminary experiment, showed that, unless a

digester is operated at a temperature above the thermaldeath-point of algae, a large number of the algae survivepassage through the digester, and as a result the entireconversion unit becomes less efficient.

"Activated-sludge" unit. The "activated-sludge" unitdid little to advanice the breakdown of organic matterin the system to a point suitable for consumption byalgae. It is true that a large population of actinomycetesdid develop but, except for the removal of ammonia andhydrogen sulfide from the digester sludge, the productobtained from the unit was physically almost the same

as that introduced from the digester. The failure of the"activated-sludge" culture to carry out its intendedfunction probably was due to physical limitations ofthe apparatus. Another explanation may be that a

large portion of the energy content of the algae proc-

essed in the digester was transformed to that ofmethane, while the remaining energy of the digestersolids was tied up in the cell walls and other componentsof the dead algae in a form which was unavailable tomost of the bacteria other than actinomycetes. Never-theless, the elimination of odors and the clarificationof the li(uid phase indicated that some activity didtake place.

Entire system. The maximal efficiency attained bythe algal culture in the growth unit was 3 per cent;and the maximal over-all efficiency of the entire unit,that is, the conversion of light energy to the chemicalenergy of methane, was approximately 2 per cent. Thelow efficiency of the unit as a whole was due chieflyto the low efficiency of the algal culture, inasmuch as

the efficiency of conversion of algal cells to methane bythe digester must have been of the order of 66 per centbecause light to algae was 3 per cent, and light to gas

was 2 per cent. As a result of the low efficiency of thealgal culture, the digester received less than its normalloading, and consequently gas production was low.This fact indicates that higher efficiencies of light con-

version to gas can be attained with increased efficiencyon the part of the growth unit.The experiment showed that light energy could be

converted to the chemical energy of methane by biologi-cal means within a relatively brief time with equip-ment and knowledge already available. It showed thatclosed biological systems of microorganisms can bemaintained for appreciable lengths of time, not only

content may have been great enough to harm the

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BIOLOGICAL CONVERSION OF LIGHT ENERGY

vironment. The importance of this is that the conversionof solar energy into methane by algal and bacterialcultures need not be restricted by limited quantities oforganic wastes available at any one time, because eachday's contribution of nitrogen, phosphorus, and otherelements would be retained in the growth chambersand used repeatedly. By recycling digested algae fromthe digester to the algal culture, in which the digesterorganic matter is further broken down by aerobicbacteria, and the resultant breakdown products serveas nutrient for the algae, digested sludge is upgradedin energy content as a result of the photosyntheticactivity of the algae, and thus new organic matterbecomes available and a new cycle is begun.

SUMMARYA description is given of a series of experiments in

which was developed a closed system of convertinglight energy to the chemical energy of methane throughthe agency of unicellular algae and aerobic and anaer-obic bacteria operating in an integrated unit consistingof an algal growth unit (oxygenation chamber), "acti-vated-sludge" unit, and digester. Decompositionproducts from the breakdown of organic material(algae and dead bacteria) by aerobic and anaerobicbacteria, together with light, furnished the raw materialfor algal photosynthesis, as a result of which lightenergy was fixed in algal cells. Dead algal cells, in turn,served as nutrient for the bacteria. As a result, theelements of organic matter were repeatedly brokendown and resynthesized.Gas production by the digester averaged about 10

eu ft per lb of v,olatile matter introduced. The methane

content of the gas varied from 68 to 74 per cent. Themaximal efficiency attained by the algal culture was 3per cent, whereas the maximal over-all efficiency ofthe entire conversion unit was approximately 2 per cent.

In general, the experiments showed that light energycould be converted to the chemical energy of methaneby biological means within a relatively brief time; andthat closed biological systems of microorganisms, notonly with respect to liquid, but also to gaseous en-vironment, can be maintained for an appreciablelength of time.

REFERENCES

AYRES, E. AND SCARLOTT, C. A. 1952 Energy sources-Thewealth of the world. McGraw-Hill Book Co., Inc., NewYork, New York.

BURLEW, J. S. 1953 Algal culture from laboratory to pilotplant. Carnegie Inst. Wash., Publ. No. 600.

FURNAS, C. C. 1954 Energy sources of the future. Ind.Eng. Chem., 46, 2446-2457.

GOLUEKE, C. G., OSWALD, W. J., AND GOTAAS, H. B. 1957Anaerobic digestion of algae. Appl. Microbiol., 5, 47-55.

MEIERS, R. L. 1956 Science and economic development.John Wiley and Sons, Inc., New York, New York.

OSWALD, W. J. 1957 Light conversion efficiency in photo-synthetic oxygenation. Doctoral Dissertation, SanitaryEngineering Research Laboratory, University of Cali-fornia, Berkeley, California.

POST, R. 1957 Fusion power. Sci. American, 197, 73-89.PRESCOTT, G. W. 1954 How to know fresh-water algae. Wm.

C. Brown Co., Dubuque, Iowa.PUTNAM, P. 1953 Energy in the future. D. Van Nostrand

Co., Inc., New York, New York.SMITH, G. M. 1933 T'he fresh water algae of the United States.

McGraw-Hill Book Co., Inc., New York, New York.SPITZER, L., JR. 1958 The stellerator. Sci. American, 199,

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